Polymers via Chemical Vapor Deposition and
Their Application to Organic Photovoltaics
ARCHIVES
by
Miles Clark Barr
Master of Science, Chemical Engineering Practice
Massachusetts Institute of Technology, Cambridge, MA, 2008
Bachelor of Engineering, Chemical Engineering
Vanderbilt University, Nashville, TN, 2006
Submitted to the Department of Chemical Engineering
in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY IN CHEMICAL ENGINEERING
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2012
C 2012 Massachusetts Institute of Technology. All rights reserved.
. /
A
Signature of Author:
Department of Chemical Engineering
February 14, 2012
Certified by:
/
Karen K. Gleason
Professor of Chemical Engineering
Thesis Supervisor
Accepted by:
William M. Deen
Professor of Chemical Engineering
Chairman, Committee for Graduate Students
I
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Polymers via Chemical Vapor Deposition and
Their Application to Organic Photovoltaics
by
Miles Clark Barr
Submitted to the Department of Chemical Engineering
on February 14, 2012, in partial fulfillment
of the requirement for the degree of
Doctor of Philosophy in Chemical Engineering
Abstract
There is emerging interest in the ability to fabricate organic photovoltaics (OPVs)
on flexible, lightweight substrates, which could lower the cost of installation and enable
new form factors for deployment. However, substrate and material choices are often
limited by compatibility with common processing agents such as heat and solvents. Here,
we explore oxidative chemical vapor deposition (oCVD) as an all-dry, vacuum-based
method for processing conjugated polymer device layers for organic photovoltaics. The
entire process occurs via the vapor phase and under conditions of low temperature and
low energy input, enabling conformal coverage and film deposition on delicate substrates
(e.g., papers, plastics, and textiles). Moreover, oCVD offers the well-cited benefits of
vacuum processing, including parallel and sequential deposition, well-defined thickness
control and uniformity, and inline integration with other standard vacuum processes (e.g.,
vacuum thermal evaporation).
Conductive poly(3,4-ethylenedioxythiophene) (PEDOT) layers deposited by
oCVD are explored for a variety of roles within vacuum-processed OPVs, including as a
transparent anode, as an anode buffer layer on ITO transparent electrodes, and as a
cathode buffer layer in electrically inverted devices. By using in situ shadow masking,
the oCVD PEDOT electrodes are vapor-patterned over large areas to monolithically
fabricate OPV circuits directly on a variety of common paper substrates. The resulting
paper photovoltaic arrays power common electronic displays in ambient indoor lighting
and can be tortuously flexed and folded without loss of function. Further, optically
inverting the device structure (by positioning the oCVD PEDOT electrode on top of the
device and illuminating from above) improves performance with non-transparent
substrates; power conversion efficiencies just under 3% are demonstrated, including up to
2% on common paper substrates. We also show applicability of an oCVD semiconductor,
unsubstituted polythiophene, as the photoactive electron donor in all-vacuum-processed
polymer heterojunction OPVs.
Thesis Supervisor:
Title:
Karen K Gleason
Professor of Chemical Engineering
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Acknowledgments
This thesis and my doctoral degree from MIT would not be possible without the support
of many generous and caring people. I would like to express my deepest thanks to all
who have helped me get to this point. I am truly grateful.
I would like to start by thanking Professor Karen Gleason, who has been an
outstanding thesis advisor. She has provided a perfect balance of guidance, patience, and
creative freedom, and has helped me grow immensely as a scientist and researcher.
Professor Vladimir Bulovid has also been an invaluable mentor during my time at MIT,
providing enthusiastic support and insightful advice, scientifically and beyond. I thank
him for believing in me and generously dedicating so much of his time and effort to my
various endeavors. I would also like to thank Professor Michael Strano for his support
and comments as a member of my thesis committee.
The Gleason research group, past and present, has been an exceptional
environment of support and collaboration. I would like to thank Mahriah Alf, Nathan
Trujillo, and Jingjing Xu for being great classmates, collaborators, and friends. Sung Gap
Im was amazingly helpful in guiding me during my early years in the lab and paving the
way for many of the initial studies presented here. I have also thoroughly enjoyed
collaborating with David Borrelli, Rachel Howden, and Christopher Boyce, and am
excited to include some of our findings in this thesis. Additional thanks go to Ayse
Asatekin, Dhiman Battacharyya, Sal Baxamusa, Anna Coclite, Christy Petruczok, Wyatt
Tenhaeff, Sreeram Vaddiraju, and Rong Yang for valuable discussions and being great
company in the lab.
The success of this thesis has been built on cross-department collaborations, and
the Bulovid research group and the entire organic nanostructured electronics laboratory
have been unconditionally welcoming, giving me a warm home away from home. Special
thanks go to Richard Lunt for his mentorship, scientific support, and incredible passion
over the past year. Jill Rowehl has also been a great collaborator; I have especially
appreciated her help in the lab, thoughtful discussions, and amazing intellectual curiosity.
I also thank Alexi Arango, Patrick Brown, John Ho, Tim Osedach, Annie Wang, and the
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entire Bulovid group for keeping the ONE lab running smoothly and happily dropping
everything to answer my questions or lend a pair of hands.
This research was generously supported by a National Science Foundation
Graduate Research Fellowship and by Eni SpA under the Eni-MIT Solar Frontiers Center.
Through this support, I have had the pleasure of working with many researchers at the
Eni Donegani Insitute in Novara, Italy, which has been very rewarding and a highlight of
my research. I would like to give special thanks to Chiara Carbonera for her friendship
and fruitful collaborations.
Additional thanks go to many wonderful people outside of the lab. To my ChemE
classmates, notably my fellow Practice School team members, for their camaraderie and
many fond memories. To Bart Howe and Erdin Beshimov for their support and
commitment over the past year and for being two of the nicest people I know. And to the
many friends and supporters from Kansas City, Vanderbilt, and around the country, for
believing in me.
I would also thank the many teachers throughout my life who have undoubtedly
shaped the way I think today, notably my undergraduate advisor Professor Kane Jennings
for introducing me to scientific research, Karen Kriens for starting my passion for
chemistry in high school, and Molly Hankins for fostering a spirit of creativity in
everything I do. There are countless others, and my sincerest appreciation goes out to all;
I wouldn't have reached MIT without them.
Finally, I thank my family, particularly my parents, Christopher Barr and Kaysee
Clark, and my brother, Dylan Barr. They have supported me in everything I have done
from the very beginning, instilling in me a passion for creativity, a tireless work ethic,
and a desire to succeed. They have truly made me believe I can accomplish anything, and
I will be forever grateful.
Most importantly, I would like to thank my wife, Lisa Barr, who has been by my
side throughout my entire Ph.D. Her companionship, patience, humor, and love have
undoubtedly helped me survive the past five years, while keeping them fun and exciting.
Earning my degree is even more rewarding knowing that I get to share my achievement
with her. I would also like to thank Lisa's family, Scott, Carolyn, and Matthew Bryington,
for their enthusiasm and support.
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Table of Contents
Abstract ...........................................................................................................................
3
A cknow ledgm ents......................................................................................................
5
Table of Contents .....................................................................................................
7
List of Figures ...............................................................................................................
11
List of Tables.................................................................................................................25
List of V ideos................................................................................................................26
List of Abbreviations..................................................................................................28
1 Introduction.................................................................................................................31
1.1 M otivation ...........................................................................................................
32
1.2
Oxidative chem ical vapor deposition (oCV D)...............................................
34
1.3
Scope of thesis.................................................................................................
41
1.4
References ........................................................................................................
45
2 Transparent PEDOT Bottom Electrodes Deposited by oCVD
for O rganic Photovoltaics. ....................................................................................
Introduction ......................................................................................................
2.1
51
52
2.2
2.3
2.4
2.5
oCV D PED OT transparent electrodes.................................................................52
Photovoltaic device integration ......................................................................
54
M echanical durability ......................................................................................
2.4.1 Flexing .................................................................................................
56
2.4.2
Stretching .............................................................................................
58
2.4.3
Folding ..................................................................................................
59
Photovoltaics on paper substrates....................................................................
2.5.1 D evice perform ance.............................................................................
2.5.2
2.5.3
M onolithic circuit integration .............................................................
Encapsulation and m odule dem onstrations.........................................
56
61
62
65
68
2.6
Conclusion......................................................................................................
71
2.7
Experim ental....................................................................................................
2.7.1 D eposition of oCVD polym er electrodes.............................................
71
2.8
2.9
71
2.7.2
D evice fabrication...............................................................................
72
2.7.3
Thin-film packaging ................................................................................
73
2.7.4
2.7.5
Electrode and substrate characterization..............................................
D evice characterization.........................................................................75
74
Supporting Figures ..........................................................................................
A cknow ledgm ents ..........................................................................................
7
77
84
2.10 References .......................................................................................................
84
3 Top-illuminated Organic Photovoltaics on a Variety of Opaque Substrates
w ith oC V D PED O T Top Electrodes......................................................................
89
3.1 A bstract................................................................................................................90
3.2 Introduction .....................................................................................................
90
3.3 Results and discussion....................................................................................
93
3.3.1 Top-illum inated device structure ........................................................
93
3.3.2 Device perform ance .............................................................................
95
3.3.3 oCVD oxidant exposure to photoactive semiconductor layers............97
3.3.4 D em onstrations on everyday opaque substrates .................................... 100
3.4 Conclusion.........................................................................................................103
3.5 Experim ental......................................................................................................104
3.5.1 Substrate preparation .............................................................................
104
3.5.2 D evice fabrication..................................................................................104
3.5.3 Characterization .....................................................................................
105
3.6 A cknow ledgm ents .............................................................................................
106
3.7 References .........................................................................................................
106
4 Cathode Buffer Layers Based on Vacuum and Solution Deposited PEDOT
for Efficient Inverted O rganic Photovoltaics .........................................................
4.1 Abstract..............................................................................................................112
4.2 Introduction .......................................................................................................
4.3 Experim ental......................................................................................................114
4.4 Results and discussion.......................................................................................115
4.4.1 Low w ork function PED OT...................................................................115
4.4.2 Inverted organic photovoltaics...............................................................117
4.4.3 Optim ization and optical m odeling ........................................................
4.5 Conclusion.........................................................................................................120
4.6 A cknow ledgm ents .............................................................................................
4.7 References .........................................................................................................
5 Unsubstituted Polythiophene Photoactive Layers via oCVD
for Polym er Photovoltaics ........................................................................................
5.1 Abstract..............................................................................................................126
5.2 Introduction .......................................................................................................
5.3 Experim ental......................................................................................................130
5.3.1 Polythiophene depositions .....................................................................
5.3.2 Polym er characterization .......................................................................
5.3.3 D evice fabrication and characterization ................................................
8
111
112
119
121
121
125
126
130
131
132
5.4
5.5
5.6
5.7
Results and discussion.......................................................................................133
133
5.4.1 PT synthesis ...........................................................................................
5.4.2 UV-vis and fourier transform infrared (FTIR) spectroscopy.......135
138
5.4.3 Electrochem ical properties ....................................................................
5.4.4 Photovoltaic device perform ance...........................................................139
Conclusion.....................................................................................................144
145
A cknow ledgm ents .............................................................................................
146
References .........................................................................................................
6 C onclusion..................................................................................................................151
6.1 Sum m ary............................................................................................................152
6.2 Implications .......................................................................................................
6.3 Future directions ................................................................................................
6.3.1 Multilayered CV D device structures .....................................................
6.3.2 Application to other organic electronics ................................................
6.4 Concluding rem arks...........................................................................................161
6.5 References .........................................................................................................
155
156
156
159
161
A Patterned and Functional Grafted Conducting Polymer Nanostructures
163
via oCV D ....................................................................................................................
A .1 Abstract..............................................................................................................164
164
A .2 Introduction .......................................................................................................
A .3 Experim ental......................................................................................................167
A .3.1 Preparation of functionalized PED OT nanobow ls................................167
168
A .3.2 Characterization ....................................................................................
A .4 Results and discussion.......................................................................................169
A .4.1 3D patterning via colloidal lithography ................................................
A .4.2 oCVD PEDOT nanobow ls....................................................................171
A .4.3 Surface functionalization ......................................................................
A .5 Conclusion.........................................................................................................176
A .6 A cknow ledgm ents .............................................................................................
A .7 References .........................................................................................................
169
174
176
177
B Facile Vapor Phase Method to Prepare Dynamically Switchable Membranes...181
B .1 Abstract..............................................................................................................182
182
B.1.1 Introduction ...........................................................................................
B.1.2 D eposition methods...............................................................................185
B.2 Experim ental......................................................................................................187
B.2.1 D eposition conditions............................................................................187
B .2.2 Sam ple preparation................................................................................188
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B .2.3 Sam ple characterization ........................................................................
189
B .2.4 M embrane perm eation ..........................................................................
189
B.3 Results and discussion.......................................................................................190
B .3.1 Proof of concept on flat surfaces...........................................................190
B .3.2 M em brane application...........................................................................193
B .4 Conclusion.........................................................................................................201
B.5g
........................................................................................
202
B .6 References .........................................................................................................
202
C Graphene Organic Solar Cells with oCVD PEDOT Hole-Transporting
Layers.........................................................................................................................207
C .1 A bstract..............................................................................................................208
C .2 Introduction .......................................................................................................
209
C .3 Results and discussion.......................................................................................212
C .3.1 Graphene electrodes ..............................................................................
212
C.3.2 oCV D PED O T layers on graphene electrodes ...................................... 214
C.3.3 Graphene/oCVD PEDOT anodes in OPV devices................................218
C.4 Conclusion.........................................................................................................222
C.5 Experim ental......................................................................................................223
C.5.1 Graphene synthesis (LPCVD , A PCV D ) ............................................... 223
C.5.2 Graphene transfer and anode preparation. ............................................ 223
C .5.3 O PV device fabrication process ............................................................
225
C .5.4 Characterization ....................................................................................
225
C .6 A cknow ledgm ents .............................................................................................
226
C .7 References .........................................................................................................
226
10
List of Figures
Figure 1-1: Schematic representation of the oxidative chemical vapor deposition
(oCVD) process. The lower inset shows polymerization mechanism for PEDOT
including the following steps: (1) oxidation of EDOT to form cation radicals; (2)
dimerization of cation radicals; (3) deprotonation to form conjugation; (4) further
polymerization from n-mer to (n+l)-mer; (5) oxidative p-doping of PEDOT. ........ 35
Figure 1-2: Oxidative Chemical Vapor Deposition (oCVD) reactor built for some of
the work in this thesis. Solid oxidizing agent is delivered through resistively
heated crucible (yellow). Heated liquid monomer vapor flow regulated by needle
valve or mass flow controller (0.1-10 sccm) (green). Substrate is inverted and the
temperature is controlled between 0-150'C. Thickness monitoring is possible by
in situ laser interferometry or quartz crystal microbalance (QCM) (red). Chamber
is evacuated with a mechanical dry pump and optional turbomolecular pump; the
pressure is controlled between 1-500 mTorr (white) ...............................................
36
Figure 1-3: Conductive, 150-nm thick, oCVD PEDOT polymer electrodes (1001000 S-cm-2) are uniformly deposited on ultra-delicate substrates with no pre- or
post-treatment and at identical conditions: ~10-ptm thick SaranTM wrap (a, upper
left), water-soluble rice paper (b, upper middle), and a single ply of bathroom
tissue that is porous and absorbent (c, upper right). In contrast, the conventional
drop-cast conducting polymer solution (Clevios
TM
PH 750) does not wet the
hydrophobic surface (a, lower left), easily dissolves the soluble substrate (b, lower
middle), and soaks through and easily damages the delicate fiber matrix (c, lower
right). Measurement in the lower half of the figure indicates the 2-point film
resistances of 1300, 1200Q, and 5.9 kM, respectively. From supporting materials
in [7 1]............................................................................................................................3
Figure 1-4: Scanning electron microscope image of a 100 nm thick PEDOT
electrode on newsprint. The left side is bare newsprint and the right side is coated
with the oCVD PEDOT layer. The similarity between the uncoated and coated
11
8
sides show that the PEDOT layer conforms to the geometry of the underlying
paper fibers at least to length scale visible here. The interface between the two
sides gives an idea of the resolution achievable by the shadow masking step (~20
ptm). From supporting m aterials in [71]...................................................................
39
Figure 1-5: 200-nm thick, oCVD PEDOT polymer electrodes are vapor-patterned in
situ directly on the substrate of choice. The figure shows examples of PEDOT
printed in 15 pt. bold Verdana font (line width: 0.5-1.0 mm) on 10-mil thick PET,
SaranTM wrap, Tracing Paper, and Tissue Paper. With the chemistry and
conditions used here, the low vapor pressure of the FeCl 3 oxidant at the substrate
temperature (80"C) allows for pseudo-directional flow of this species through the
mask to substrate. This, in combination with its fast reaction rate with the morevolatile EDOT monomer, creates well defined oCVD PEDOT anodes in the
masked pattern and prevents significant mask undercutting. From supporting
materials in [7 1]............................................................................................................4
0
Figure 2-1: Relationship between percent transmittance (550 nm) and sheet
resistance in ohms per square (Q/0) for the vapor-printed oCVD PEDOT used in
the work. The red line is a fit to equation (1) giving adc/aop=9 (the dashed black
line is for reference and corresponds to adc/oop=35, representative for traditional
metal oxide electrodes). Lower inset: sheet resistance plotted versus film
thickness. Upper inset: 200-nm thick PEDOT film vapor printed on tissue paper
in 15 pt. bold V erdana font. ......................................................................................
53
Figure 2-2: Current density-voltage characteristics under illumination (AMI.5, 100
mW-cm-2 ) for organic oCVD PEDOT PVs on glass differing only in anode
structure (the yellow and red curves are for reference and do not include oCVD
PEDOT). The inset shows the device structure used in the work; C60 was utilized
as the acceptor unless otherwise noted......................................................................
Figure 2-3: (a) Conductivity with respect to flexing cycle (radius <5 mm) for an asdeposited 50-nm thick oCVD PEDOT film covalently bonded to a flexible PET
substrate (blue) and commercial ITO-coated PET (red). Insets show surface of the
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54
films after 1000 flexing cycles; the scale bar is 200 pm. (b) J-V characteristics of
an ITO-free OPV on PET before and after flexing cycles (AM 1.5, 100 mW cm-2).
The inset photograph illustrates the orientation of flexing for an array of these
OP V s.............................................................................................................................5
7
Figure 2-4: Conductivity of a 50-nm thick oCVD PEDOT electrode on SaranTM
wrap (~10-[tm thick) as it is anisotropically stretched until substrate failure. The
images below the plot show the surface of the electrode upon stretching; the scale
bar is 10 [Lm. The dashed line in the lower right image is to guide the eyes through
a remaining continuous film path in the view window.............................................59
Figure 2-5: Conductivity of a 100-nm thick oCVD PEDOT electrode on newsprint
(~80
im thick) after repeated folding. The inset shows the crease orientation: -
180' (blue) or +180' (red). The paper was flattened back to 0' between each
iteratio n . ........................................................................................................................
60
Figure 2-6: oCVD-printed PVs on paper substrates. (a) J-V performance
characteristics (AM 1.5, 500 mW cm-2) of OPVs utilizing 100-nm thick oCVD
PEDOT electrodes on common as-purchased (b) tracing paper (~40-pm thick), (c)
tissue paper (-40-pm thick), and (d) printer paper (~120-pm thick). Devices
maintain power generation in air after folding into three-dimensional structures;
(e) shows a paper airplane with integrated photovoltaic wings, folded from a ~50
cm 2 sheet of tracing paper that was first patterned with the oCVD-enabled device
structure (inset) (see Supplementary Video S2-1). Functioning devices are also
made on other common papers, including (f) printed newspaper, without
disturbing the underlying ink, and (g) wax paper, which is resistant to many
solvents, as shown by the droplet of conducting polymer solution (CLEVIOS TM
PH 750) that beads up on the completed device. ....................................................
63
Figure 2-7: Internal and external quantum efficiency comparison for devices on
glass/ITO/PEDOT:PSS (black) and tracing paper/oCVD PEDOT. C60 and PTCBI
were used as the electron acceptor for the left and right graphs, respectively.........64
13
Figure 2-8: Large-area monolithic photovoltaic arrays. (a) Printing schematic for
250-cell, series-integrated monolithic arrays. The photographs show the printed
PEDOT (~50 nm) pattern (left) and a completed array (right) on tracing paper. (b)
Current-voltage performance curves for series-integrated photovoltaic arrays with
vapor-patterned oCVD electrodes on paper (red) and glass (black) under
illumination (AMI.5, 80 mW-cm- 2 ) (bold) and in the dark (thin). (c) Spatial map
of individual cell open-circuit voltages across the respective ~50 cm 2 arrays. The
lower insets show the cumulative fraction of devices producing at or below a
g iv en v o ltag e.................................................................................................................6
7
Figure 2-9: Integrated paper PV demonstrations. (a) Normalized efficiency of thinfilm-packaged and unpackaged arrays (250 series-integrated cells) subjected to
constant illumination (80 mW-cm-2, halogen lamp) in air at 42'C (108'F) as a
function of time. The right photograph shows the laminated paper circuit
powering an LCD display in air with ambient sunlight. The display's circuitry is
powered by the photovoltaic alone and regulates a constant voltage to the display
for changes in light intensity. The demo has maintained function after over 5000
hours of intermittent ("shelf') illumination. (b) A paper array is progressively
folded in air while being tested at AM 1.5, 80 mW-cm-2 . The final folded structure
can be dynamically unfolded and refolded without loss of performance in each
three-dimensional configuration (See Video S2-2). (c) The iCVD-coated array
(28-series integrated cells) is submerged in water during operation (See Video S23). The right inset shows a nearly spherical droplet of water on the surface of the
paper photovoltaic array. The same device also withstood tortuous roll-to-roll
processing by an office laser-jet printer (See Video S2-4). ......................................
Figure S2-1: Optical microscope images after 100 compressive flexing cycles (film
on the inside of the bend) to radius <5 mm for (a) 100-nm thick oCVD PEDOT
on 5-mil PET and (b) commercial 100-nm thick ITO on 5-mil PET. The films
correspond to those shown in Figure 2-3(a). (c) and (d) show similar images for
the cathode surface of the entire devices [Anode/CuPc (20 nm)/C 6o (40 nm)/BCP
(10 nm)/Ag (100 nm)] grown on the same starting surfaces and flexed in the same
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70
way as (a) and (b), respectively. (e) and (f) show 2-dimensional profiles (left-toright) across the surface of the devices in (c) and (d), respectively. The device
imaged in (c) and (e) correspond to the device tested in Figure 2-3(b). ..................
77
Figure S2-2: Specular and diffuse contributions to UV-visible transmission,
reflection, and absorption spectra for various fiber-based papers (tracing paper,
copy paper, newsprint, tissue paper, wax paper, and photo paper). (a) and (b)
show specular transmission (%) and reflection (%), respectively; (c) and (d) show
diffuse transmission (%) and reflection (%), respectively; and (e) and (f) show
total transmission (%) and reflection (%), respectively. The absorption (%) losses
(g) in the paper substrates were calculated as 100-%T-%R using the total
reflection and total transm ission spectra ...................................................................
78
Figure S2-3: Total active layer absorption in CuPc/C 60 heterojunction devices on
(a,b) paper/oCVD PEDOT and (c,d) glass/ITO/PEDOT:PSS. The device structure
is [CuPc (80 nm)/C60 (40 nm)/BCP (10 nm)/Ag (150 nm)] and the oCVD
PEDOT thickness is 50 nm. The active layer absorption (%AACT, red solid) was
estimated as the difference between the percentage of incident light reflected by
the substrate/anode/Ag structure (%RsUBs, black solid) and the percentage
reflected by the complete device structure (%RTOT, black dashed). The experiment
is shown for both total reflection (as measured with an integrating sphere) (a,c)
and specular reflection (b,d) to illustrate the high contribution of scattered light to
active layer absorption in paper-based cells; however, the total absorption spectra
were used to calculate the internal quantum efficiency spectra [Figure 2-7]. ......... 79
Figure S2-4: Total active layer absorption in CuPc/PTCBI heterojunction devices on
(a,b) paper/oCVD PEDOT and (c,d) glass/ITO/PEDOT:PSS. The device structure
is [CuPc (80 nm)/C60 (40 nm)/BCP (10 nm)/Ag (150 nm)] and the oCVD
PEDOT thickness is 50 nm. The active layer absorption (%AACT, red solid) was
estimated as the difference between the percentage of incident light reflected by
the substrate/anode/Ag structure (%RSUBs, black solid) and the percentage
reflected by the complete device structure (%RToT, black dashed). The experiment
15
is shown for both total reflection (as measured with an integrating sphere) (a,c)
and specular reflection (b,d) to illustrate the high contribution of scattered light to
active layer absorption in paper-based cells; however, the total absorption spectra
were used to calculate the internal quantum efficiency spectra [Figure 2-7]. .......... 80
Figure S2-5: Normalized open-circuit voltage (a) and short-circuit current (b) of
thin-film-packaged and unpackaged paper photovoltaic arrays (250 seriesintegrated cells) on paper subjected to constant illumination (80 mW-cm 2 ,
halogen lamp) in air at 42'C (108'F) as a function of time. These lifetime plots
correspond to the arrays tested in Figure 2-9 ............................................................
81
Figure 3-1: Schematics of the device structures and materials used in this report: (a)
Chemical structures of DBP, C6 0 , BCP, and CVD PEDOT polymerized and doped
with FeCl 3. (b) Conventional orientation PV device with transparent ITO anode
(device is illuminated from the substrate side). (c) Top-illuminated orientation
PV device with transparent CVD PEDOT anode (device is illuminated from the
d ev ice sid e)....................................................................................................................9
4
Figure 3-2: (a) Representative J-V performance curves measured under 1.1 sun
illumination and (b) external quantum efficiency spectra, for the conventional
device with ITO anode (dotted blue) and top-illuminated devices with CVD
PEDOT anode, with (solid red) and without (dashed black) MoO 3 as a buffer
layer. All devices are on silver-coated glass substrates. ...........................................
Figure 3-3: UV-visible absorbance spectra for (a) glass/DBP [25 nm]/MoO
3
96
[0 nm
(black) and 20 nm (red)] and (b) glass/C6 0 [40 nm]/MoO 3 [0 nm (black) and 20
nm (red)]. Films were measured before (dashed) and after (solid) exposure to
FeCl 3 under CVD polymerization conditions ..........................................................
Figure 3-4: Performance parameters for top-illuminated cells (solid symbols) with
different MoO 3 buffer layer thicknesses, measured under 1.1 sun illumination: (a)
short-circuit current density (blue diamonds), (b) open-circuit voltage (red
circles), (c) fill factor (green triangles), and (d) power conversion efficiency
16
98
(black squares). The conventional, bottom-illuminated cell with ITO anode is
shown for reference at x=-15 (open symbols). Data points are the average of 3-5
devices measured across each substrate, and error bars represent the maximum
and m inim um values recorded .................................................................................
99
Figure 3-5: (a) Representative J-V curves for top-illuminated OPVs fabricated on
the top side of some common opaque substrates under 1.1 sun illumination,
including photo paper, magazine print, a U.S. First Class stamp, plastic food
packaging, and glass for reference. (b) Photographs of completed 10 device arrays
are also shown. All substrates were used as purchased, so the original surface
images are visible in the spaces below the completed PV devices (i.e., printed
text, Statue of Liberty image, and "Nutritional Facts" text, respectively)..................101
Figure 4-1: (a) Work function of both PEDOT:PSS and CVD PEDOT treated with
TDAE or Cs 2 CO 3 . (b) Schematic of the inverted device architecture with
transparent ITO cathode and low work function PEDOT buffer layer inserted
between ITO/C 60 interface. (c) Flat band energy level diagram for the inverted
device architecture. The top-right inset shows the proposed electron-limiting
Schottky-contact formed at an unbuffered ITO/C6 0 interface. Band energies are
taken from the literature..............................................................................................116
Figure 4-2: J-V curves under AM1.5G simulated solar illumination. (a) Comparison
of the conventional device orientation (ITO/MoO 3 20 nm/DBP 25 nm/C60 40
nm/Ag) (open diamonds) with the same device stack inverted (ITO cathode) with
no buffer layer on the ITO (open squares), and with untreated CVD PEDOT
(filled triangles) and PEDOT:PSS (filled circles) on the ITO. (b) The same
inverted device structure incorporating a LWF PEDOT buffer layer on the ITO
cathode: CVD PEDOT/TDAE (filled triangles), CVD PEDOT/Cs 2 CO 3 (open
triangles), PEDOT:PSS/TDAE (filled circles), and PEDOT:PSS/Cs 2 CO 3 (open
c irc le s).........................................................................................................................1
Figure 4-3: Power conversion efficiency, q, (filled triangles), open-circuit voltage
(filled circles), fill-factor (filled diamonds), and short-circuit current (filled
17
18
squares) as a function of the DBP electron donor layer thickness for the inverted
device with PEDOT:PSS/TDAE buffer layer. The dashed lines show short-circuit
current vs. DBP thickness calculated from optical interference simulations using
DBP exciton diffusion lengths from 5-20 nm (as noted). The control cell in the
conventional orientation [ITO/MoO3 (20 nm)/DBP (25 nm)/C60 (40 nm)/Ag] is
included as x=0 for reference (open symbols). Solid lines are to guide the eyes. ...... 119
Figure 5-1: Properties of materials deposited by various techniques for use in
organic solar cells. Traditionally, the use of polymers is limited to those that can
be dissolved and therefore can be deposited by a solution-based technique. Vapor
deposition is usually limited to molecules with low enough molecular weight to
be thermally evaporated. This leaves a region of vapor-deposited polymers that is
difficult to access by traditional m ethods....................................................................128
Figure 5-2: Processes (1) and (2) occur during the oCVD deposition process, while
process (3) is a post-deposition step. (1) Oxidative polymerization of thiophene to
polythiophene. (2) Oxidation of the polymer chain leads to the formation of
polarons and bipolarons (shown), which are charge balanced by counteranion
dopants. (3) Rinsing the deposited film with methanol reduces it back to neutral
P T ................................................................................................................................
13 4
Figure 5-3: As-deposited oCVD PT film (left) and methanol-rinsed film (right)
uniformly deposited on 25 x 75 mm glass slides. The blue PT film is doped with
FeC13 and has a conductivity between 10 and 20 S cm', whereas the red film is
neutral PT and nonconductive.....................................................................................134
Figure 5-4: Absorption coefficient of doped (- -) and dedoped (-)
oCVD PT films
on quartz. The energy levels of midgap peaks in the doped film suggest that it is
heavily doped, resulting in bipolarons in the film . .....................................................
136
Figure 5-5: FTIR spectra of oCVD PT film before and after MeOH rinse. The
bottom spectrum is a reference spectrum for neutral PT [38]. All spectra are
normalized by the C-H vibrational peak at 790 cm', as indicated by the asterisk.....137
18
Figure 5-6: Cyclic voltammogram of the oCVD PT film deposited onto ITO-coated
glass in an acetonitrile solution of Bu 4NPF 6 (0.1 M) at a scan rate of 100 mV s-.
The Ag/Ag* reference electrode was calibrated using the Fc/Fc redox couple.........138
Figure
5-7:
(a)
J-V
characteristics
of devices
with
structure
ITO/PT
(~30nm)/C 6o/BCP (8 nm)/Ag under 100 mW cm-2 AM1.5G simulated solar
illumination. (b) Performance characteristics of the above devices. Markers and
error bars correspond to the average and maximum and minimum values obtained.
An efficiency maximum is achieved for a 30 nm-thick C60 layer...............................140
Figure 5-8: The thin lines show the EQE spectra (left axis) of the devices in Figure 6
in which the C60 thickness is varied. The bold lines show the absorption
coefficients of C60 (- -) and oCVD PT (-) (right axis). The absorption edge past
600 nm in the EQE suggests that the oCVD PT is functioning as a photoactive
lay e r.............................................................................................................................14
1
Figure 5-9: (a) J-V characteristics of devices with structure with structure
ITO/PT/C60 (30 nm)/BCP (8 nm)/Ag. (b) Performance characteristics of the above
devices. Markers and error bars correspond to the average and maximum and
minimum values obtained. A maximum efficiency of 0.8% was obtained for a 25
nm PT layer w ith 30 nm of C60 ..............................................
.......... .. ......................... 143
Figure 6-1: Cross-sectional representation of an OPV device on a highly non-planar
substrate (e.g., paper), in which the electrode layer (CVD PEDOT) is conformal
over the surface topography, while the active layers and metal electrode are not......158
Figure 6-2: Cross-sectional schematic of a generic organic light emitting diode. ......... 160
Figure A-1: PS beads are self-assembled on a vinyl-treated Si wafer to form a 2-D
HCP colloidal template (a). PEDOT is deposited via oCVD, conformally coating
the void space of the colloidal template with conducting polymer, which grafts to
the surfaces of the PS beads and the vinyl-treated wafer (b). The non-grafted cores
of the PS template are removed via dissolution and rigorous ultrasonication in
19
THF, revealing well-ordered PEDOT nanostructures with a grafted PS surface
lay er.............................................................................................................................17
0
Figure A-2: SEM images show a PEDOT shroud in which the internal PS template
has been dissolved away by brief ultrasonication in THF (a). Rigorous
ultrasonication reveals the hemispherical HCP geometry of the patterned PEDOT
nanostructure below (b). Using FeCl 3 as the oxidant yields a well-defined
PEDOT/PS nanostructure with a very smooth texture (c). When CuCl 2 is used as
the oxidant, hierarchically patterned PEDOT conducting polymer films are
formed with tunable nanoporosity (d). The inset captures the morphology of the
porous PEDOT on an unpatterned substrate...............................................................171
Figure A-3: FT-IR spectra of a blanket oCVD PEDOT conducting polymer film (a),
a 2-D monolayer of 1 gm diameter polystyrene colloidal template (b), and oCVDpatterned conducting polymer nanostructures (c) in which the PS template was
removed by rigorous rinsing and ultrasonication in THE. The observation of PS
peaks in (c) is evidence of grafting between the PEDOT nanobowls and the
surface of the P S beads...............................................................................................173
Figure A-4: Surface amine moieties provide a linkage point between the bulk
conducting polymer nanostructure and various ligands, demonstrated here by
attachment of NHS-tethered fluorescein (a). SEM image of oCVD PEDOT
patterned using 1 [tm amino-functional PS beads (b); the inset captures the
nanobowl shape, which becomes evident at the interface with the unpatterned
bulk PEDOT. Fluorescence microscopy shows fluorescence areas patterned using
amino-functional PS beads (c). The focal plane of the fluorescence microscope
causes the fluorescence signal to appear greatest along the circumference of the
bowls. The inset shows the interface (highlighted by the dotted white line)
between the fluorescent patterned regions and the unpatterned bulk PEDOT,
which does not exhibit fluorescence after the identical functionalization treatment.. 175
Figure B-1: (a) Cross-sectional schematic of samples developed with dynamically
switchable properties
for both
flat glass substrates and
20
a track-etched
polycarbonate (PC) membrane. The deposition steps for pEDOT and pEGDMA
were performed from the top side, while the p(NIPAAm-co-DEGDVE) layer was
deposited from the top side for the glass substrate and the bottom side for the PC
track etched membrane. (b) Schematic of how the temperature-responsive
polymer film will control the water flux through the membrane. Below the LCST,
the polymer swells, narrowing the membrane pores and causing a decrease in
flux, while above the LCST, the polymer deswells against the pore walls, creating
a larger effective pore size and higher flux.................................................................184
Figure B-2: (a) Temperature curves for pEDOT-coated glass substrates with varying
resistances over a range of applied AC voltages. The data points were taken after
allowing 10 min. of temperature equilibration at each voltage step. The dashed
lines represent Equation 1 as fitted to the data using only one fitting parameter (h
= 13 ± 1 W m- K-). (b) Setup used to obtain static contact angle measurements
of the responsive surface using pEDOT coating to provide resistive heating. Static
contact angles for glass slides coated with pEDOT followed by pEGDMA and
p(NIPAAm-co-DEGDVE) both (c) below the film's LCST (~22'C) and (d) above
the film's LCST (~ 35'C) as achieved via application of 15 V AC across the
substrate (RpEDOT = ,200 D). -----
.
.
.
---------.. --------------...............................-
191
Figure B-3: XPS survey scan taken of the top side (as in Figure B-1) of a PC tracketched membrane after (a) pEDOT deposition followed by (b) pEGDMA and (c)
p(NIPAAm-co-DEGDVE). The important elemental peaks are highlighted where
S is present at the surface only after the pEDOT deposition, only C and 0 are
present following the subsequent pEGDMA deposition, and N is present only
after the subsequent p(NIPAAm-co-DEGDVE) deposition. ......................................
195
Figure B-4: SEM images of the top side [as depicted in Figure B-i (b)] of (a) the
blank, 2 pm pore diameter PC track etched membrane used for Sample 1, (b)
Sample 1 after the three deposition steps, (c) Sample I after the membrane
permeation study. The scale bars represent 5 pm . ......................................................
21
197
Figure B-5: (a) Data representing the normalized (to the first low temperature flux,
Sample 1: 2.7 ± 0.1 mL-m-2 s , Sample 2: 3.8 ± 0.1 mLm
2
-s-1) DI water
permeation rate of membrane Sample I (*) and 2 (M). The temperature at which
the measurements were taken are given in red open points on the second axis and
the LCST of the polymer film is represented by the dashed line at 28'C,
illustrating that low temperature measurements were taken below this value and
high temperature measurements above it. (b) Temperature changes as measured
by a thermocouple affixed to the membrane in DI water from the base system
temperature with applied AC voltage for the two different membrane samples.
The dashed (sample 1) and solid (sample 2) lines are included merely as guides.
(c) Transient temperature change profiles for a AC voltage jump of 0 to 12 V on
two different runs (open vs. solid points) for sample 2...............................................199
Figure B-6: Transient temperature change profiles for a AC voltage jump of 0 to 12
V on two different runs (open vs. solid points) for sample 2 with corresponding fit
to Equation SI to determine the time constant of the response (A = 0.60 ±
0.02 C -V-1,k = 12 V , T= 42 ± 9 seconds)..................................................................201
Figure C-1: Optical (a-c, scale bar: 10 gm) and AFM (d-e, scale bar: 1p m) images
of graphene transferred on the SiO 2 (300 nm) substrates synthesized under
different pressure conditions: (a,b,d) APCVD. (c,e) LPCVD images are shown
here as a reference for APCVD.
(a-b) APCVD graphene consists of non-
uniformly distributed multilayer regions on top of the mono-layer background,
which can be clearly identified from the image taken at the edge of the graphene
(b). (b) The dotted area indicates a region of graphene film broken due to the
transfer process, which further confirms the existence of the mono-layer
background.
(d) AFM image which illustrates the non-uniformity of APCVD
graphene. The rms roughness of APCVD graphene is 1.66 nm compared to 1.25
nm for LPCVD graphene in (e)...................................................................................213
Figure C-2: Schematics outlining the fabrication of graphene electrodes, PEDOT
HTLs, and OPV devices. (a) Graphene synthesis and transfer. The last part of the
22
transfer procedure is repeated to prepare 3-layer graphene stacks for LPCVD
graphene. Detailed growth parameters are graphically illustrated in Figure C-7,
below. (b) PEDOT:PSS spin-coating vs. vapor printing of PEDOT deposition.
The spin-casting layer covers the graphene and the surrounding quartz substrate
while the vapor printed patterns align to produce PEDOT only on the graphene
electrodes.
(c)
Graphene
anode
OPV
structure:
Graphene/PEDOT/DBP/C 60 /BCP/Al..........................................................................215
Figure C-3: Transmittance data for the oCVD PEDOT HTL layers, measured using
ultraviolet-visible spectroscopy (UV-Vis) over wavelengths from 350-800 nm.
The oCVD PEDOT layers decrease in transmittance and sheet resistance with
increasing thickness. The three thinnest PEDOT layers (2, 7, and 15 nm) have
high transmittance values (>90% over a majority of the range), which are
preferred for HTL layers. The thickest (40 nm) PEDOT layer has a transmittance
<70% over the wavelength range, which causes performance losses in the devices
due to optical absorption.
The oCVD PEDOT sheet resistance was measured
using a 4-point probe (taking the average of 10 measurements). With increasing
oCVD PEDOT thickness, there are more pathways for charge transfer, so the
sheet resistance (Rsh) decreases. Rsh decreases dramatically from the thinnest (2
nm) to thicker PEDOT layers (7, 14, and 40 nm ). ......................................................
216
Figure C-4: Macroscopic images comparing HTL coverage of actual graphene
devices: (a) A blanket (i.e. unpatterned) PEDOT:PSS spun-coated on the entire
0.5" square substrate. The coating is irregular and most of the PEDOT:PSS layer
is dewetted from the substrate with dark macroscopic defects visible to the naked
eye. (b) In contrast, pure PEDOT, containing no PSS, can readily be confined left
side of the substrate if the area on the right side is shadow masked.
In the
unmasked region on the left side, the oCVD PEDOT appears as a uniform light
blue coating.
(c-d) Optical micrographs of the spin-cast PEDOT:PSS on the
graphene surface illustrating the poor wettability of PEDOT:PSS on the graphene. .217
23
Figure C-5: J-V characteristics of representative graphene (3-layer, LPCVD) OPV
devices (Graphene/PEDOT:PSS (40nm), vapor printed PEDOT/DBP, 25nm/C60 ,
40nm/BCP, 7.5nm/Al, 100nm) under simulated AM 1.5G illumination at 100
mW/cm 2 . (a) Graphene devices with PEDOT:PSS and vapor printed PEDOT
(15nm) HTLs, compared with ITO/PEDOT:PSS reference device.
(b) Energy
level diagram of complete OPV device structure. ......................................................
219
Figure C-6: (a) J-V characteristics of representative graphene (APCVD) OPV
devices (Graphene/vapor printed PEDOT, 15nm/DBP, 25nm/C 6 0 , 40nm/BCP,
7.5nm/Al, 100nm) along with ITO/PEDOT:PSS reference device under simulated
AM 1.5G illumination at 100 mW/cm2 .
(b) Comparison of graphene-based
device performances, where graphene electrodes are prepared under different
conditions, i.e. LPCV D vs. A PCV D ...........................................................................
221
Figure C-7: Illustration of graphene growth process at different stages: (a) LPCVD
and (b) A P C VD ...........................................................................................................
24
224
List of Tables
Table 2-1: Representative solar cell parameters for organic photovoltaics with
different anode layers. Summary of device performance for the devices shown in
Figure 2-2 (AM 1.5, 100 mW-cm 2 ) differing only in anode composition. ........
55
Table 3-1: Summary of performance parameters under 1.1 sun illumination for
devices on glass substrates [Figure 3-2]...................................................................
96
Table 3-2: Summary of performance parameters under 1.1 sun illumination for
devices on common opaque substrates [Figure 3-5]...................................................102
Table 5-1: Atomic ratios in PT films from XPS survey scans after various methanol
5
rin sin g times................................................................................................................13
Table 5-2: Summary of device structures and performance that use PT as the donor
material. The devices using oCVD PT provide comparable or better performance
compared to other PT deposition methods..................................................................144
Table B-1: Details of deposition thickness as measured from profilometry for
pEDOT and VASE for pEGDMA and p(NIPAAm-co-DEGDVE) on reference Si
wafers for the two different 2 Vm pore
diameter track-etched PC membrane
samples used in subsequent permeation studies. ........................................................
193
Table B-2: Expected and actual elemental compositions as calculated from XPS
survey scans on the top side of 2 pam pore size membranes after pEDOT,
subsequent pEGDMA and subsequent p(NIPAAm-co-DEGDVE) depositions.........195
Table C-1: Optical transmittance (%T) and sheet resistance
(Rsh)
of oCVD PEDOT
with varying thicknesses described in Figure 3. .........................................................
25
216
List of Videos
Video S2-1: A paper airplane with thin-film photovoltaic cells integrated into the
wings is folded and tested in air. The final three-dimensional structure is folded
from a flat -50 cm2 sheet of tracing paper that was first patterned with an oCVD
polymer electrode and thermally evaporated active device layers and silver
electrode. The device is illuminated from above with a 20 W desk lamp. The
video segments from 0:14-0:23 and 0:31-0:34 are played at 6 times the original
frame rate. .....................................................................................................................
82
Video S2-2: The folded paper photovoltaic shown in Figure 2-9(c) is dynamically
folded and unfolded while simultaneously being tested at open circuit. The paper
photovoltaic is illuminated from below with simulated solar illumination (80
mW-cm-2 AM 1.5). The video segment from 0:12-0:18 is played at 2 times the
original fram e rate .....................................................................................................
82
Video S2-3: The iCVD-encapsulated circuit shown in Figure 2-9(d) is first folded
into a small ramp and exposed to copious water droplets while simultaneously
being tested at open circuit. A close-up view is shown in the segment from 0:300:44. The circuit is then unfolded and completely submerged in a water bath while
also being tested at open circuit; the submersion of the hydrophobic paper
photovoltaic is aided by physical pressure applied with a glass rod. The paper
photovoltaic array (and water bath) is illuminated from below with simulated
solar illumination (80 mW-cm-2 AM 1.5). The video segment from 0:12-0:28 is
played at 3 tim es the original fram e rate.................................................................
Video S2-4: The same paper photovoltaic shown in Video S2-3 is shown powering
the LCD clock when illuminated. The entire integrated paper photovoltaic is then
fed through a roll-to-roll office laser-jet printer. The resulting ink spells MIT on
the device side of the paper array, which then continues to power the LCD clock.
The clock circuitry is shown at the end of the video, which is powered by the
photovoltaic alone and regulates a constant voltage to the display for variations in
light intensity. The paper photovoltaic array is illuminated from below with
26
83
simulated solar illumination (80 mW-cm-2 AM 1.5). The video segment from
0:04-0:05 and 0:24-0:26 are played at 6 times the original frame rate. ....................
27
83
List of Abbreviations
Alq 3
tris-(8-hydroxyquinoline) aluminum
APCVD
atmospheric pressure chemical vapor deposition
BCP
bathocuproine
C60
fullerene C60
CMOS
complementary metal oxide semiconductor
CuPc
copper phthalocyanine
CVD
chemical vapor deposition
DBP
tetraphenyldibenzoperiflanthene
DVB
divinyl benzene
EDOT
3 ,4-ethylenedioxythiophene
EQE
external quantum efficiency
FF
fill factor
FTIR
Fourier transform infrared spectroscopy
HCP
hexagonal close-packed
HOMO
highest occupied energy level
HOPG
highly ordered pyrolytic graphite
HTL
hole-transporting layer
iCVD
initiated chemical vapor deposition
ITO
indium tin oxide
IQE
internal quantum efficiency
Jsc
short-circuit current density
J-V
current density-voltage
LPCVD
low pressure chemical vapor deposition
LUMO
lowest unoccupied energy level
LWF
low work function
oCVD
oxidative chemical vapor deposition
OLED
organic light emitting diode
OPV
organic photovoltaic
OTFT
organic thin film transistor
P3HT
poly(3-hexylthiophene)
28
PCBM
[6,6]-phenyl C6 i butyric acid methyl ester
PCE
power conversion efficiency
PECVD
plasma-enhanced chemical vapor deposition
PEDOT
poly(3,4-ethylenedioxythiophene)
PET
polyethylene terephthalate
PMMA
poly(methyl methacrylate)
PPFDA
poly(l H, 1H,2H,2H-perfluorodecyl acrylate)
PS
polystyrene
PSS
poly(styrenesulfonate)
PT
polythiophene
PTCBI
3,4,9,10-perylene tetracarboxylic bisbenzimidazole
Rsh
sheet resistance
TBPO
tert-butyl peroxide
TCE
transparent conducting electrode
TCVS
trichlorovinyl silane
TDAE
tetrakis(dimethylamino)ethylene
THF
tetrahydrofuran
TPD
N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine
UPS
ultraviolet photoelectron spectroscopy
UV-Vis
ultraviolet-visible (spectroscopy)
VOC
open-circuit voltage
VPP
vapor-phase polymerization
WF
work function
XPS
X-ray photoelectron spectroscopy
29
30
Chapter 1
Introduction
31
1.1
Motivation
Advancement of low-cost solar cells and other large-area electronics motivates the
development of materials and fabrication processes compatible with inexpensive, flexible
substrates and high-throughput manufacturing. For this reason, organic photovoltaics
(OPVs) have gained significant momentum as a possible low-cost energy source and
recently reached record efficiencies of nearly 10% [1], suggesting near-term commercial
potential. OPVs have the potential for mass deployment in the form of flexible,
lightweight, everyday surfaces, owing to their lightweight and mechanically flexible
device layers [2-4]. Additionally, the organic materials are synthesized from earthabundant elements and typically have low toxicity compared to many inorganic
semiconductors, minimizing concerns over availability, environmental impact, and
disposal [5]. By adding energy-harvesting functionality to surfaces that are already
commonly used in society, consumer adoption is simplified, installation costs can be
significantly reduced, and electronic functionalities can be added to otherwise passive
products, such as wall coverings, product packaging, documents, and apparel.
Much of the recent research and advances in this area have focused on solutionprintable conductors and semiconductors that can be processed on flexible plastic
substrates by methods such as inkjet printing [6-11]. For example, highly conductive
blends of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) have
been developed and incorporated as transparent electrodes on plastic-based OPVs with
comparable performance to devices utilizing the industry standard transparent conductive
electrode indium tin oxide (ITO) [9, 12, 13]. The ability to replace ITO for flexible OPVs
is particularly attractive
due to its brittle nature, high-temperature
32
processing
requirements, limited availability, and high cost [14-16]. There are, however, a variety of
limiting requirements associated with solution deposition techniques including the need
for soluble materials, substrate wettability, solvent compatibility with the substrate and
underlying device layers, and high solvent purity to eliminate introduction of
contaminants and defects. The need for solvent compatibility is obviated when
electrically active thin films are deposited by a physical vapor-phase deposition method;
however, these films are often non-conformal on rough surfaces or require hightemperature processing and are thus incompatible with most plastics, papers, and textiles
[15, 17]. Moreover, the high temperatures necessary to physically deposit polymers by
vacuum thermal evaporation leads to polymer degradation [18], limiting materials to lowmolecular-weight organics.
Chemical vapor deposition (CVD) provides a potential link between many of the
above considerations, which we explore here as an alternative fabrication technique for
depositing polymer thin films for organic photovoltaics. The entire process occurs via the
vapor phase and under conditions of low temperature and low energy input, eliminating
solvent compatibility challenges and enabling more-than-line-of-site coverage and film
deposition on delicate substrates (e.g., papers, plastics, and textiles). Moreover, CVD
offers the well-cited processing benefits of vacuum processing, including parallel and
sequential
deposition, well-defined thickness control and uniformity, and inline
integration with other standard vacuum processes.
33
1.2
Oxidative chemical vapor deposition (oCVD)
Chemical vapor deposition describes processes in which vapor-phase precursors react at a
substrate to form a thin film [19, 20]. Within this framework, oxidative CVD (oCVD) has
proven successful as a polymerization technique for a variety of electrically and optically
active conjugated polymers
including poly(3,4-ethylenedioxythiophene)
(PEDOT),
poly(pyrrole), poly(aniline), and others [19-29]. The conjugated backbone of these
materials allows for high charge mobility due to the delocalized nature of the pi-bonded
electrons, making them attractive for integration within organic electronic devices as
electrode or semiconductor layers.
oCVD is achieved by flowing vapors of a monomer simultaneously with an
oxidizing agent, such as FeCl 3 or Br 2 , which react at the substrate via oxidative stepgrowth polymerization mechanism to form a conjugated polymer thin film; the oxidant
can subsequently dope the conjugated backbone to achieve conductive films [Figure 1-1]
[19-21, 30]. The entire process occurs in the vapor phase and under conditions of low
substrate temperature (20-100 C) and moderate vacuum (0. 1 Torr). The oCVD process
allows for direct control over thickness, dopant concentration, work function, and
conductivity by varying substrate temperature [23, 25] and can impart excellent substrate
adhesion through chemical grafting [28]. Since there are no solubility requirements,
oCVD polymers are not bound by the molecular weight and dopant restrictions that can
limit conductivity in conducting polymer solutions [31], and conductivities as high as
1000 S cm' have been reported for oCVD PEDOT [32]. Moreover, the conductive pdoped form of oCVD PEDOT is highly stable in ambient conditions, as has been proven
for the oxidized form of PEDOT thin films [33, 34].
34
arbitrary substrate (glass, plastic, paper, etc.)
idant
vpor
growing conjugated polymer film
monomer
PEDOT
rcEUA
luupau
Figure 1-1: Schematic representation of the oxidative chemical vapor deposition (oCVD)
process. The lower inset shows polymerization mechanism for PEDOT including the
following steps: (1) oxidation of EDOT to form cation radicals; (2) dimerization of cation
radicals; (3) deprotonation to form conjugation; (4) further polymerization from n-mer to
(n+1)-mer; (5) oxidative p-doping of PEDOT.
35
The laboratory-scale oCVD reactor setup that was built and used for the work in
this thesis is shown in Figure 1-2. The key system components are highlighted, including
oxidant and monomer delivery systems, the inverted temperature-controlled stage on
which the substrate is mounted, in situ thickness monitoring instrumentation, and
pumping exhaust with pressure control loop.
Resistively heated crucible delivers oxidants or other organics
Monomer flow through temperature-controlled line and MFC
Temperature-controlled stage controls reaction kinetics
in situ thickness measurement via laser Interferometry or QCM
Pressure set by butterfly-valve control loop in exhaust line
Figure 1-2: Oxidative Chemical Vapor Deposition (oCVD) reactor built for some of the
work in this thesis. Solid oxidizing agent is delivered through resistively heated crucible
(yellow). Heated liquid monomer vapor flow regulated by needle valve or mass flow
controller (0.1-10 sccm) (green). Substrate is inverted and the temperature is controlled
between 0-150'C. Thickness monitoring is possible by in situ laser interferometry or
quartz crystal microbalance (QCM) (red). Chamber is evacuated with a mechanical dry
pump and optional turbomolecular pump; the pressure is controlled between 1-500 mTorr
(white).
36
Many other synthetic routes have been created for processing of conjugated
polymers [35]. The main difficulty results from the rigid nature of the polymer backbone,
making these materials largely insoluble in common organic solvents [36]. Solvation
issues can be mitigated by the attachment of side chains, although conductivity usually
suffers as a result. Common techniques include oxidative polymerization in solution [3739], electropolymerization [40, 41], and solvation with flexible polymers which act as
dopants [42, 43]. Similarly, further engineering of electronic properties such as extension
to electron-transporting materials is often achieved by attachment of various functional
groups for donation or removal of electrons as well as to provide chemical stabilization
[44]. The addition of many groups in this way can result in bulky materials with low
solubility. Solvent processing can also be a concern in device fabrication where care must
be taken not to disturb underlying layers. Alternative vapor phase polymerization
techniques have also previously been explored for synthesizing and depositing
conjugated polymers. For example, plasma enhanced CVD (PECVD) was used to
polymerize monomers derivatives of thiophene [45-56], pyrrole [54, 57, 58], and anline
[59-61]; however, the resulting polymers lack conductivity and retention of the aromatic
ring structure due to the energetic and nonselective plasma polymerization process [20,
46, 48, 50]. Other vapor phase polymerization (VPP) techniques for oxidative
polymerization of conjugated polymers rely on solvent casting of the oxidants prior to
monomer exposure [62-70], which can damage underlying device layers and render
conformal, non-destructive deposition on many substrates inaccessible.
The absence of solvents in the oCVD process enables the deposition of
conjugated polymer films on virtually any substrate with no pre- or post-treatment. As an
37
a
SaranM wrap
b
Rice wrapper
(solublel
C 1-ply bathroom tissue
(porous, absorbent)
Figure 1-3: Conductive, 150-nm thick, oCVD PEDOT polymer electrodes (100-1000
S-cm-2) are uniformly deposited on ultra-delicate substrates with no pre- or post-treatment
and at identical conditions: -10-ptm thick SaranTM wrap (a, upper left), water-soluble rice
paper (b, upper middle), and a single ply of bathroom tissue that is porous and absorbent
(c, upper right). In contrast, the conventional drop-cast conducting polymer solution
(CleviosTM PH 750) does not wet the hydrophobic surface (a, lower left), easily dissolves
the soluble substrate (b, lower middle), and soaks through and easily damages the
delicate fiber matrix (c, lower right). Measurement in the lower half of the figure
indicates the 2-point film resistances of 130n, 12002, and 5.9 kK, respectively. From
supporting materials in [71].
example, Figure 1-3(a, left) shows a conductive oCVD PEDOT electrode film (~1000
S/cm) deposited on an ultrathin sheet of Saran Wrap@. This is compared to the highly
conductive commercially available PEDOT:PSS formulation ClevioSTM PH 750 (>650
S/cm as a dried film), which does not wet the hydrophobic surface, making processing
difficult Figure 1-3(a, right). Wetting problems can be mitigated by pretreatment with 02
plasma; however, this requires an extra processing step, and the high energy of the
plasma can easily destroy the properties of such a delicate substrate. oCVD can also be
38
used to deposit electrodes on substrates where it is difficult to imagine a controlled
deposition route via solution processes. For example, the oCVD PEDOT electrodes are
also compatible with delicate, ultraporous, fibrous substrates (bathroom tissue) and even
water-soluble substrates (rice paper) [Figure 1-3].
Newsprint
zou
Figure 1-4: Scanning electron microscope image of a 100 nm thick PEDOT electrode on
newsprint. The left side is bare newsprint and the right side is coated with the oCVD
PEDOT layer. The similarity between the uncoated and coated sides show that the
PEDOT layer conforms to the geometry of the underlying paper fibers at least to length
scale visible here. The interface between the two sides gives an idea of the resolution
achievable by the shadow masking step (-20 ptm). From supporting materials in [71].
Compatibility with substrates like papers and textiles is further enhanced by the
conformal nature of the oCVD process over non-planar substrates [27, 29, 30], which is
difficult to achieve by solution polymerization techniques due to solvent wetting effects,
or physical vapor deposition techniques due to line-of-site deposition [20]. For example,
Figure 1-4 shows a film of oCVD PEDOT on a fibrous newsprint substrate, in which the
bare paper looks nearly identical to that coated with the PEDOT electrode. No
39
pretreatment is necessary to fill in the spaces between the fibers, which retains the
breathability, deformability, and low weight of the underlying paper substrate.
Finally, the oCVD process can also be combined with in situ shadow masking to
"vapor-print" well-defined polymer patterns on the surface of choice [71], which is useful
for device manufacturing,
in which complementary patterning is required for
interconnection of successive device layers. The printed polymer patterns (down to 20
gm resolution) result from the presence of a shadow mask by maintaining the partial
pressure of the vapor-delivered oxidant species sufficiently close to its saturation pressure
at the substrate, which prevents significant mask undercutting (Figure 1-5).
Technology
Figure 1-5: 200-nm thick, oCVD PEDOT polymer electrodes are vapor-patterned in situ
directly on the substrate of choice. The figure shows examples of PEDOT printed in 15 pt.
bold Verdana font (line width: 0.5-1.0 mm) on 10-mil thick PET, SaranTM wrap, Tracing
Paper, and Tissue Paper. With the chemistry and conditions used here, the low vapor
pressure of the FeCl 3 oxidant at the substrate temperature (80*C) allows for pseudodirectional flow of this species through the mask to substrate. This, in combination with
its fast reaction rate with the more-volatile EDOT monomer, creates well defined oCVD
PEDOT anodes in the masked pattern and prevents significant mask undercutting. From
supporting materials in [71].
40
1.3
Scope of thesis
The objective of this thesis is to demonstrate CVD polymers for use in organic
photovoltaic devices, and we look to apply these polymer layers in a variety of roles and
device architectures. We primarily consider oCVD conjugated polymers, including the
electrically conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) for a variety
of electrode applications, but also consider semiconducting photoactive layers and
insulating materials for device encapsulation. Throughout, we look to highlight
applications that are enabled by the conformal, solvent-free, and low-temperature
characteristics of the CVD processing through device demonstrations. Each chapter is
formatted as a technical journal paper and can be read independently.
Chapter 2 explores oCVD PEDOT as transparent bottom electrode layer. We
demonstrate application of this layer as both a hole-transporting buffer layer on top of
conventional ITO transparent electrodes, as well as a standalone transparent electrode.
The
OPVs
utilize well-established,
vacuum-evaporated,
small-molecule
organic
heterojunctions based on copper phthalocyanine (CuPc) with fullerene C60 or 3,4,9,10perylene tetracarboxylic bisbenzimidazole (PTCBI). The oCVD PEDOT electrodes and
resulting OPVs are then applied to a variety of flexible lightweight substrates, including
plastics and common fiber-based papers, and the mechanical robustness under flexing,
folding, and stretching of these substrates is explored. We consider a variety of factors
influencing device performance, including tradeoffs between sheet resistance and
transparency for oCVD PEDOT, as well as substrate roughness and optical transmittance.
We also use in situ shadow masking to vapor-pattern the PEDOT electrodes over large
areas to monolithically fabricate OPV circuits on paper and glass, which we design and
41
demonstrate to power common electronic displays in ambient indoor lighting. Briefly, we
also demonstrate a variety of compatible encapsulation layers for the paper PVs,
including non-conjugated, hydrophobic, polymer thin films deposited by initiated CVD,
and show the effect on device lifetime and device functionality (e.g., water resistance).
Chapter 3 extends the results of from Chapter 2 to improve device compatibility
with non-transparent substrates (e.g. paper or metal foils) by positioning the oCVD
PEDOT transparent electrode as the top anode layer in optically inverted OPVs. This
structure necessitates exposing the full multilayered device stack to the oCVD process.
We find that device performance is improved by applying a molybdenum trioxide (MoO 3)
buffer layer prior to PEDOT deposition, which is shown to increase the device
photocurrent by preventing oxidation of the underlying photoactive semiconductor during
the CVD process. For the OPVs demonstrated in this chapter we also introduce a more
absorptive
active
layer
set
comprising
vacuum-evaporated
tetraphenyldibenzoperiflanthene (DBP) as the electron donor and fullerene C6 0 as the
electron acceptor.
In Chapter 4, PEDOT layers, including oCVD and solution processed versions,
are used as cathode interlayers in electrically inverted organic photovoltaic cells based on
DBP/C6 0 with an electron collecting transparent ITO bottom cathode. We use chemical
treatment of the as-deposited PEDOT layers with tetrakis(dimethylamino)ethylene
(TDAE) or cesium carbonate (Cs 2 CO 3 ) to reduce the work function for application on
the cathode side of the device. We find that the low-work function PEDOT layers prevent
the formation of an electron-limiting Schottky contact at the ITO cathode interface in
opposition to the DBP/C6 0 heterojunction. We also look to optimize the device
42
photocurrent devices, by modeling the optical interference patterns within the inverted
device, which we compare to experimental results.
In Chapter 5, we move away from the electrode layers of the OPV device and
explore oCVD for deposition of a photoactive semiconductor polymer for use in polymer
solar cells. Unsubstituted polythiophene (PT), which is insoluble and infusible and thus
typically difficult to process, is easily prepared by oCVD. The oCVD PT films are
characterized chemically and electrically, and a post-deposition methanol dedoping step
is considered. The resulting semiconducting PT is then applied as an electron donor in
bilayer heterojunction solar cells with a thermally evaporated C60 electron acceptor layer.
The absorption characteristics of the PT films are also compared with the external
quantum efficiency spectra of the OPVs to elucidate the origin of the photocurrent in the
devices.
Chapter 6 concludes the thesis by summarizing our findings and considering
future research directions. The remaining chapters, presented in the Appendix, represent
additional related works to which I contributed as a collaborator, some of which are not
directly related to the main points considered in Chapters 1-6.
Appendix A presents a simple one-step process to simultaneously create patterned
and functionalized three-dimensional PEDOT nanostructures, using grafting reactions
between oxidative chemical vapor deposition (oCVD) PEDOT conducting polymers and
functionalized polystyrene (PS) colloidal templates. The functionality of the colloidal
template is directly transferred to the surface of the grafted PEDOT, which is patterned as
nanobowls, while preserving the advantageous
electrical properties of the bulk
conducting polymer. Combined with the other results presented in the previous chapters,
43
these three-dimensional nanostructured PEDOT patterns may be useful for application as
high surface area electrodes or light-trapping structures in the OPVs.
In Appendix B, we develop a vapor-phase method using the complementary
techniques of oxidative and initiated chemical vapor deposition (oCVD and iCVD) to
modify substrate surfaces to achieve direct heating of temperature-responsive films. This
modification involves oCVD PEDOT for resistive heating, followed by an insulating
layer of iCVD poly(ethylene glycol dimethacrylate) (pEGDMA), and a subsequent iCVD
layer of poly(N-isopropylacryalmide-co-di(ethylene glycol) divinylether) [p(NIPAAmco-DEGDVE)]
that provides the temperature-responsive
surface properties.
We
characterize the heating behavior and temperature response of these structures and finally
apply them to temperature-responsive membranes.
Appendix C builds on the anode buffer layer results from Chapter 2, this time
incorporating the oCVD PEDOT hole-transporting buffer layers on top of transparent
graphene electrodes synthesized by LPCVD and APCVD. The solvent-free oCVD
process allows for facile uniform coverage of the underlying graphene layers, which is
challenging with PEDOT:PSS (hydrophilic) by spin-casting on the low surface free
energy graphene. The graphene/oCVD PEDOT electrodes are then applied in DBP/C6 0
heterojunction OPVs and compared to ITO reference devices.
44
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50
Chapter 2
Transparent PEDOT Bottom
Electrodes Deposited by oCVD
*
for Organic Photovoltaics
*Adapted from M.C. Barr, J.A. Rowehl, R.R. Lunt, J. Xu, A. Wang, C.M. Boyce, S.G. Im, V. Bulovid, and
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paper. Advanced Materials,23, 3499-3505, 2011.
51
2.1
Introduction
In this chapter, we examine the use of a substrate-independent vapor printing process to
deposit the conducting polymer poly(3,4-ethylenedioxythiophene)
in place of the
conventional transparent conductive electrode (e.g., indium-tin oxide (ITO)) in organic
solar cells on glass, plastic, and paper substrates. This process combines oxidative
chemical vapor deposition (oCVD) [1, 2] with in situ shadow masking to create welldefined polymer patterns on the surface of choice (Figure 2-1, inset and Figure 1-3). For
oCVD, the polymerized thin films form by simultaneous exposure to vapor-phase
monomer (EDOT) and oxidant (FeCl 3) reactants at low substrate temperatures (20100'C), moderate vacuum (~0.1 Torr). The printed polymer patterns (down to 20 pm
resolution) result from the presence of a shadow mask by maintaining the partial pressure
of the vapor-delivered oxidant species sufficiently close to its saturation pressure at the
substrate, which prevents significant mask undercutting. The vapor delivery of the
oxidant species makes this process unique from other techniques that rely on solvent
casting of oxidants prior to vapor delivery steps [3, 4]. Because the process is all dry,
there are no wettability or surface tension effects on rough substrates like paper, and
exactly the same process steps are used to fabricate devices on glass, plastics, and papers.
2.2
oCVD PEDOT transparent electrodes
For use as a transparent electrode, the conducting polymer layer must provide both low
sheet resistance (Rsh) and high optical transmittance (T) which are related by the
following equation [5, 6]:
52
-2
T= 1+
(2.1)
OP
2Rsh
dc )
where Zo is the impedance of free space (377 Q), and adc and aop are the dc and optical
conductivities, respectively. Figure 2-1 shows this relationship for oCVD PEDOT
deposited on glass at 80'C and 0.1 Torr. The data fits well to Equation 2.1 across the full
data range, giving acd,/op=9. This value is comparable to the best commercially available
conducting polymer solutions and is slightly lower than that reported for carbon nanotube
conductors
Gdc/Yop~15
and traditional metal oxide electrodes ad,/aopZ35 [5, 6]. The ability
to integrate these other electrode materials with paper has also not yet been demonstrated.
1001
_I
(U
_
_
_00
8
-104
E
C 60
2
1 o2
CJG =9.0
o
0
Op
40
- -
102
50
o
100
103
150
200
Thickness (nm)
--
- --
--
00,
104
l0s
Sheet Resitance (f/n)
Figure 2-1: Relationship between percent transmittance (550 nm) and sheet resistance in
ohms per square (Q/0) for the vapor-printed oCVD PEDOT used in the work. The red
line is a fit to equation (1) giving cdc/aop=9 (the dashed black line is for reference and
corresponds to adc/aop=35, representative for traditional metal oxide electrodes). Lower
inset: sheet resistance plotted versus film thickness. Upper inset: 200-nm thick PEDOT
film vapor printed on tissue paper in 15 pt. bold Verdana font.
53
2.3
Photovoltaic device integration
We first established the integration of oCVD PEDOT anodes into thin-film organic PV
devices
on
surfaces
glass
smooth
using the
well-documented
CuPc/C 6 0
(or
PTCBI)/BCP/Ag molecular organic heterojunction architecture [Figure 2-2, inset] [7-11].
Figure 2-2 compares representative current density-voltage characteristics for individual
devices on glass and differing only in anode structure: (i) oCVD PEDOT (50 nm); (ii)
indium
tin
(ITO);
oxide
(iii)
ITO/oCVD
PEDOT
(10
nm);
and
(iv)
ITO/PEDOT:poly(styrenesulfonate) (PSS).
10
light
Ag
-E
E
\//
|r-
BCP
C60
(PTCBI)
5 cufe
oCVD pomr
zr
suwbstat
a
0
1I. IMO
I. oCVD PEDOT (50 nm)
C
iv. ITO/PEDOT:PSS
Ii.
ITO/oCVD
EDOT (10 nm)
0
0.3
Voltage (V)
0.6i
Figure 2-2: Current density-voltage characteristics under illumination (AMI.5, 100
mW-cm ) for organic oCVD PEDOT PVs on glass differing only in anode structure (the
yellow and red curves are for reference and do not include oCVD PEDOT). The inset
shows the device structure used in the work; C60 was utilized as the acceptor unless
otherwise noted.
54
The series resistance (R,) and parallel (shunt) resistance (Rp) were determined
[Table 2-1] by fitting the dark current data (not shown) across the voltage range to the
generalized Shockley diode equation [12]:
J'dark
=
RP {Js expI q(V - JRS
R, +R,
nkT
+}
(2.2)
_ R,
where n is the diode ideality factor and Js is the reverse saturation current.
Table 2-1: Representative solar cell parameters for organic photovoltaics with different
anode layers. Summary of device performance for the devices shown in Figure 2-2 (AM
1.5, 100 mW-cm-2) differing only in anode composition.
Anode Material
Jsc
VOC
Fill
PCE
Rs
R,
(mA cm-2)
(V)
Factor
(%)
(a-cm2)
(a-cm)
0.44
0.41
0.48
0.47
0.54
0.57
0.60
0.56
0.90
1.03
1.30
1.18
4.4
0.5
1.2
4.2
1.1 x10 4
2.0 x10 4
1.5 x10 4
1.1 x10 4
3.8
oCVD PEDOT
4.4
ITO
ITO/oCVD PEDOT 4.5
4.5
ITO/PEDOT:PSS
The devices incorporating oCVD anodes (i and iii) perform comparably to the
devices with conventional ITO anode structures (ii and iv), and both oCVD PEDOT and
PEDOT:PSS devices exhibit improved open-circuit voltage (0.48V) relative to those on
bare ITO (0.41V) [7, 13, 14]. Both oCVD PEDOT and PEDOT:PSS have comparable
work functions (-5.2 eV) [1]; however, the conductance of oCVD PEDOT thin films is
several orders of magnitude more conductive than the PEDOT:PSS (CLEVIOSTM P VP
Al 4083) buffer layer (<10 S-cm- 1) [2], which contributes to the higher observed fill
factor (>0.6) in the devices with ITO/oCVD PEDOT electrodes (iii) due to the lower
device series resistance (1.2
2-cm2
vs. 4.2
g-cm 2 ).
For the ITO-free oCVD electrodes (i),
the trade-off between sheet resistance and transparency with thickness accounts for the
decrease in the fill factor (0.54) due to series resistance (4.4 Q-cm 2) and the short-circuit
55
current (3.8 mA-cm 2 ) relative to (ii) [Table 2-1]. Finally, we note that the power lost due
to the electrode sheet resistance for a cell of width w and length 1, where current is
collected along I can be estimated by the following equation [15]:
Rsh jSC 2W3
oss =
C
3
(2.3)
For the PEDOT electrodes used here, this corresponds to less than 1% fraction of
power lost (Press/generated total power) for these 0.5x2.0 mm 2 cells, and to prevent a
power loss of over 10% the cells should each be kept below 5 mm wide.
2.4
Mechanical durability
Uniquely, oCVD conducting polymers are in situ covalently bonded (grafted) to flexible
substrates possessing aromatic functional groups upon film formation (e.g. on common
polystyrenes (PS) and polyethylene terephthalates (PET)) [16]. The superior adhesion of
the grafted films is highly desirable for roll-to-roll processing and for flexible and
stretchable electronics, where mechanical stresses could otherwise result in catastrophic
cracks or delamination. Indeed, we observed the oCVD PEDOT electrodes and the full
OPV devices to be electrically robust to a variety of mechanical deformations.
2.4.1
Flexing
After 1000 flexing cycles at <5 mm radius, the electrical conductivity of oCVD PEDOT
on a PET substrate is minimally affected while that of commercial ITO-coated PET
decreases over 400 times due to formation of cracks visible under optical microscope
[Figure 2-3(a), inset], a direct result of the brittle nature of the ITO and relatively weak
substrate adhesion [17]. In contrast, the oCVD PEDOT exhibits no cracking and
56
maintains its electrical conductivity even through extreme flexes at a radius <1 mm.
Moreover, after over 100 flexing cycles, complete OPV devices with
a
E
104
10n
oCVD PEDOT on PET 11000 flexing cyclesi
W
0
'101
ITO on PET
1000 flexing cycles
100
0
b
200
600
400
Flexing Cycles
800
1000
6
4
E
2
E
U
ir
C
W) 0
-2
-4L_
-0.1
0
0.1
0.2
0.3
0.4
Voltage (V)
Figure 2-3: (a) Conductivity with respect to flexing cycle (radius <5 mm) for an asdeposited 50-nm thick oCVD PEDOT film covalently bonded to a flexible PET substrate
(blue) and commercial ITO-coated PET (red). Insets show surface of the films after 1000
flexing cycles; the scale bar is 200 pm. (b) J-V characteristics of an ITO-free OPV on
PET before and after flexing cycles (AM 1.5, 100 mW cm 2 ). The inset photograph
illustrates the orientation of flexing for an array of these OPVs.
57
oCVD PEDOT electrodes [device structure (i)] on PET display no noticeable change in
performance [Figure 2-3(b)]. We conclude that the flexibility of the oCVD electrode
allows the subsequent device layer to retain its integrity upon bending. In contrast, we
observed the equivalent ITO-based cells to become electrically shorted immediately upon
flexing. Further examination shows that upon flexing the cracking features observed for
the underlying ITO electrode become visible on top of the full device (cathode surface)
with feature heights on the order of the device thickness [Supporting Figure S2-1]. This
suggests that the electrode cracks compromise the integrity of the subsequent device
layers as has been reported elsewhere [18].
2.4.2
Stretching
The stretchability of oCVD PEDOT electrodes is exhibited on common SaranTM wrap
(~10-pm thick) substrates [Figure 2-4]. The oCVD PEDOT is covalently bonded to this
PET-based substrate, inhibiting slippage and ensuring that the polymer electrode is
stretched along with the substrate. Under moderate stretching (up to 25% extension) the
conductivity steadily increases, most likely due to molecular chain alignment, as has been
reported for other conducting polymer films [19, 20]. The conductivity then slowly
declines; however, significant electrical conductance is maintained until there is
mechanical failure of the underlying substrate at nearly 200% extension. This far exceeds
the stretchability of metals, metal oxides, graphene, and even "wavy" silicon ribbons
(electrical failure at ~5-30% extension) [17, 21-23] and is comparable to some of the best
reports for conducting polymer films and composites (electrical failure at -100-200%
extension) [20, 24, 25]. Microscopic cracks gradually form and become larger as the
oCVD PEDOT film is stretched, interrupting the current conduction along the stretch
58
direction; there are, however, complete conducting polymer pathways visible even at 150%
extension (Figure 2-4, lower).
150
.00
substrate
'U
I
0
0
0
50
100
150
%Extension (AL/L,
200
100U%
20%
150%
Stretch and Measurement DIrecton-
Figure 2-4: Conductivity of a 50-nm thick oCVD PEDOT electrode on SaranTM wrap
(10-prm thick) as it is anisotropically stretched until substrate failure. The images below
the plot show the surface of the electrode upon stretching; the scale bar is 10 pm. The
dashed line in the lower right image is to guide the eyes through a remaining continuous
film path in the view window.
2.4.3
Folding
The foldability of oCVD PEDOT electrodes on paper substrates is demonstrated by
repeatedly creasing the electrodes [Figure 2-5]. After an initial decrease over the first -10
creases, the conductivity levels off at 50 S cm-1 to 150 S cm~1, and significant
conductance is retained after over 100 folding iterations. This is in contrast to evaporated
59
metal films, which exhibit complete electrical failure after fewer than 10 folding
iterations [26]. Moreover, we measure conductivities of oCVD PEDOT films on paper of
over 100 S cm-1, which are comparable to conductivities of planar films grown on glass
substrates during the same deposition cycle. This is remarkable, considering that the
oCVD film thickness is much smaller than the RMS surface roughness of the paper (~2-4
gm). It is likely that the high conductivity of oCVD PEDOT on such a highly non-planar
surface and its endurance to repeated folding iterations is rooted in the ability of the
vapor-phase oCVD process to penetrate into the fibers of the paper substrate and create a
partially conformal coating throughout the fiber matrix, imparting good mechanical and
electrical connectivity.
300
250
200
Polymer
""'""'*
150
Pper
100
0
0
20
40
60
80
Folding iterations
100
Figure 2-5: Conductivity of a 100-nm thick oCVD PEDOT electrode on newsprint (~80
gm thick) after repeated folding. The inset shows the crease orientation: -180' (blue) or
+1800 (red). The paper was flattened back to 0* between each iteration.
60
2.5
Photovoltaics on paper substrates
There has been significant recent interest in integrating electronics into low-cost paper
substrates, including transistors, storage devices, displays, and circuitry [26-29]. Paperbased photovoltaics (PVs) could serve as an "on-chip" power source for these paper
electronics and also create attractive new paradigms for solar power distribution,
including seamless integration into ubiquitous formats such as window shades, wall
coverings, apparel, and documents. Module installation may be as simple as cutting paper
to size with scissors or tearing it by hand and then stapling it to roof structures or gluing it
onto walls. Moreover, paper is
-1000
times less expensive (~0.01 $-m-2 ) than traditional
glass substrates (-10 $-m- 2 ) [30, 31] and -100 times less expensive then common plastic
substrates (0.2-3 $-m2 ) [27], an important factor considering that the substrate represents
25-60% of total material costs in current solar modules [30, 31]. Additional cost savings
are anticipated to accrue from the combination of low weight and the ability to achieve a
compact form factor by rolling or folding for facile transport from the factory to the point
of use. Common tissue papers are also ultra-thin (1-10 mil thick) and ultra-light weight
(~10 g-m- 2) [27], making them highly desirable for mobile applications, where every inch
and gram counts. To date, however, the use of paper as a substrate for solar cells has been
relatively unsuccessful due to processing challenges including surface roughness and
poor wettability [27, 32, 33], and the majority of flexible solar cell demonstrations utilize
smooth plastic substrates, such as PET [18, 34].
61
2.5.1
Device performance
To demonstrate the versatility of oCVD-deposited electrodes, we fabricate organic
photovoltaics (OPVs) directly on various paper substrates. The transparent oCVD
polymer electrodes are conformally deposited on the paper fibers in a single step without
any paper pretreatment such as coatings with protecting layers. To our knowledge, this is
the first report of solar cells prepared on untreated paper substrates (see for example [32,
33] for earlier OPV demonstrations on pre-treated paper substrates). To minimize
occurrence of electrical shorting for these device structures we increase the thickness of
the CuPc thin films from 20 nm to 100 nm. Characteristic J-V performance curves
[Figure 2-6] show power generation for completed OPVs on tracing paper (b), tissue
paper (c), and printer paper (d). These structures are also easily patterned over large areas
and maintain photovoltaic power generation after folding into three-dimensional
structures. Figure 2-6(e) shows a thin-film photovoltaic cell integrated into the wings of a
paper airplane on a ~50 cm2 sheet of tracing paper that was first patterned with the
oCVD-electrodes (see Video S2-1). The oCVD fabrication process is also compatible
with other common papers, such as newsprint, without disturbing the underlying printed
ink [Figure 2-6(f)], and even wax paper [Figure 2-6(g)], which is typically resistant to or
damaged by common solvents.
62
a 10
ScP (12 nm)
S5
VIn.
0-
--0-- b) Tracing Paper
c) Tissue Paper
-+ d) Printer Paper
U-O-
-10
-0.2
-0.1
0
0.1
0.2
0.3
0.4
Voltage
Figure 2-6: oCVD-printed PVs on paper substrates. (a) J-V performance characteristics
(AM 1.5, 500 mW cm 2 ) of OPVs utilizing 100-nm thick oCVD PEDOT electrodes on
common as-purchased (b) tracing paper (~40-pim thick), (c) tissue paper (~40-pm thick),
and (d) printer paper (~1420-pm thick). Devices maintain power generation in air after
folding into three-dimensional structures; (e) shows a paper airplane with integrated
photovoltaic wings, folded from a ~50 cm 2 sheet of tracing paper that was first patterned
with the oCVD-enabled device structure (inset) (see Supplementary Video S2-1).
Functioning devices are also made on other common papers, including (f) printed
newspaper, without disturbing the underlying ink, and (g) wax paper, which is resistant to
many solvents, as shown by the droplet of conducting polymer solution (CLEVIOS
750) that beads up on the completed device.
63
PH
Paper substrates have a range of light transmission properties, typically
characterized by high light scattering (transmissive and reflective) due to surface
roughness
but low absorptive losses [Supporting Figure S2-2].
The measured
transmittance of these paper substrates at 630 nm is only 30%, 18% and 5%, for tracing
paper, tissue paper, and printer paper, respectively, significantly limiting the intensity of
light incident on the photoactive heterojunction. Thus, we illuminated the OPVs in Figure
2-6 with 500 mW cm- simulated solar illumination to enable a better comparison with
the OPVs on glass/ITO at 100 mW cm-2 shown in Figure 2-2a. The surface reflectivity is
evident in the performance curves for devices on various papers [Figure 2-6], in which
the short-circuit currents scale inversely with losses due to reflection.
30C60
PTCBI
--aperlo
I W------Paper/oCVD
cceptor
is
£20
Acceptor
nteral
interal
E
-
210
exenl0'
400
600
rd
800 400
Wavelength (nm)
external 4600
800
Figure 2-7: Internal and external quantum efficiency comparison for devices on
glass/ITO/PEDOT:PSS (black) and tracing paper/oCVD PEDOT. C60 and PTCBI were
used as the electron acceptor for the left and right graphs, respectively.
This is also evident by comparing the quantum efficiency and absorption spectra
for oCVD PEDOT devices on tracing paper with conventional glass/ITO/PEDOT:PSS
64
devices [Figure 2-7 and Supporting Figure S2-3]. While the external quantum efficiencies
(charges collected per incident photons) are lower on the paper substrates due to
substrate/electrode losses, the estimated internal quantum efficiencies (charges collected
per absorbed photons) are comparable across the visible spectrum, indicating similar
upper bounds on efficiency for both the paper and conventional ITO/glass structures if
effective light trapping schemes are incorporated. We note that such schemes may be
uniquely designable for paper substrates because of their good reflectance and scattering
(non-specular) properties, where >90% of the light absorbed by the active layer in the
paper-based devices stems from diffuse transmission (scattering) compared to <10% on
smooth ITO/glass [Supporting Figure S2-3].
We emphasize that these solar cells are integrated directly onto as-purchased
papers; no pretreatment is used to fill in the spaces between the fibers. Thus, we retain the
breathability, deformability, and low weight of the underlying paper, making the devices
truly wearable and portable. Because the surface of paper is quite rough, a key to this
breakthrough is the adherent formation of the first layer of the device conformal around
the individual fibers of the paper. This is evident under scanning electron microscope
(Figure 1-4) in which the bare paper looks nearly identical to that coated with the PEDOT
electrode. Because our methods do not expose the substrates to high temperature or
solvents, even delicate substrates like paper are not damaged.
2.5.2
Monolithic circuit integration
The ability to print patterned device layers on any substrate enables facile
monolithic integration of arrays of individual PV cells. We demonstrate this here by
2
fabricating 250-cell (0.1x.3 cm each) series-integrated monolithic arrays on both paper
65
and glass substrates. Figure 2-8(a) shows the pattern for each device layer (achieved by in
situ shadow masking), which ultimately create anode-to-cathode interconnections
between the individual cells. Ultimately, the specific circuit design should be explicitly
optimized to minimize efficiency losses due to parasitic resistances, absorption, and
fractional device coverage [35]. For this demonstration, we have chosen a relatively small
device area both to minimize the losses due to resistance through each device and
improve device yield at the expense of low fractional area coverage (~25%). The currentvoltage characteristics show high open-circuit voltages of 50V and 67V, respectively
[Figure 2-8(b)], representing a near summation of voltages from all working cells in each
series (minus losses from shorted devices).
Spatial distribution maps of individual solar cell performance in the seriesintegrated arrays were recorded to gain insight about overall cell statistics. The cell
voltage distributions on both glass and paper substrates [Figure 2-8(c)] show that devices
on each substrate achieve similar maxima (-0.4 V), with higher variance across the
rougher paper. Moreover, high (low) cell voltages are grouped spatially across the paper
substrate. More work is required to understand the exact failure mechanisms, but we
believe that the higher density of low open-circuit voltages on the paper substrate is likely
a result of small shunt resistances in local areas of high roughness across the lessconformal, evaporated photoactive layers. However, as was discussed recently for paperbased organic transistors [28], a specific correlation was not immediately apparent by
examining optical and SEM surface images of functional and non-functional cells.
66
j
-E
mmaF-:1=g
= ==
====
Anode
oCVD PEDOT
Cathode
Ag
Active Layers
CuPc/PTCBI/BCP
I
ff
Cmm
250 PV cells Inseries
I
M
%
I
C
< 10-6
u10-8
10-10
0
50
100
05
Fraction of Devices
Voltage (V)
Fraction of Devices
Figure 2-8: Large-area monolithic photovoltaic arrays. (a) Printing schematic for 250cell, series-integrated monolithic arrays. The photographs show the printed PEDOT (-50
nm) pattern (left) and a completed array (right) on tracing paper. (b) Current-voltage
performance curves for series-integrated photovoltaic arrays with vapor-patterned oCVD
electrodes on paper (red) and glass (black) under illumination (AM1.5, 80 mW-cm 2 )
(bold) and in the dark (thin). (c) Spatial map of individual cell open-circuit voltages
across the respective -50 cm2 arrays. The lower insets show the cumulative fraction of
devices producing at or below a given voltage.
67
2.5.3
Encapsulation and module demonstrations
As noted earlier, the light weight and foldability of these devices could provide an
advantage in reducing the cost of their installations and opening new venues for
application. However, for final deployment, these lightweight structures will require
flexible thin-film encapsulation to achieve sufficiently long lifetimes and provide other
environmental protections [36]. Thus, here we briefly show that simple, passive, flexible
thin-film encapsulation techniques can significantly improve cell lifetimes and can
provide other unique protective benefits while maintaining the foldability and papery
qualities of the unpackaged circuits [Figure 2-9]. 250-cell series-integrated arrays on
tracing paper were encapsulated on both sides with three encapsulants: (i) 5-mil thick
plastic laminate applied with an office laminating machine;
poly(monochloro-p-xylylene)
(parylene-C)
deposited
by
(ii) 750-nm thick
self-initiated
CVD
polymerization; and (iii) 750-nm thick hydrophobic and crosslinked poly(lH,1H,2H,2Hperfluorodecyl
acrylate)
(PPFDA)
film
deposited
by
initiated
CVD
(iCVD)
polymerization [1]. The packaged and unpackaged series-integrated arrays were then
aged in air, accelerated by exposure to constant illumination (80 mW-cm-2 halogen lamp)
and elevated temperature (42 0 C/108'F) at open-circuit [Figure 2-9(a) and Supporting
Figure S2-5].
The power-efficiency/time trajectory shows that each thin-film encapsulant
significantly improves lifetime over the unpackaged counterparts. The influence of
ultrathin films on circuit lifetime is also evident in the unpackaged cells in which
removing the 10-nm thick exciton blocking layer (BCP) [7] more than doubles the rate of
power decay. We emphasize that this lifetime test was performed on the full monolithic
68
series-integrated arrays and thus approaches a lower limit on power lifetime, since the
photocurrent of the full array is limited by that of the worst cell in the series. The longest
lifetime (half-life) observed here, >500 hrs, compares favorably with other reports of
shorter illuminated lifetime [37, 38], similar "shelf' lifetime of hybrid-encapsulated cells
[39], and similar illuminated lifetime for an encapsulated single tandem cell [40]. The
right hand inset, shows a laminated paper PV array powering an LCD display and related
circuitry in air using sunlight from the window, which has remained operational after
over 5000 hours of ambient-light shelf life in air. With specific engineering and more
sophisticated encapsulation techniques (active desiccants, multilayers, etc.) the lifetime
dynamics should be readily extendable.
The foldability of the full photovoltaic arrays on paper is shown in Figure 2-9(b),
in which high voltages are produced in each folded three-dimensional configuration and
are maintained during dynamic folding and unfolding [Supporting Video S2-2]. The
decrease in voltage with additional folds is partially a result of individual cell damage,
which we observed to manifest as localized short circuits to the next device in the series.
The voltage and current also decrease as a result of the geometry of illumination, as
evidenced by the increase in voltage when the accordion structure (~45' incidence) is
flattened out (normal incidence).
Finally, we note that the various encapsulating films can also add specific
functionalities to the integrated arrays of paper photovoltaics. The submicron iCVDcoated array is foldable and also hydrophobic, withstanding extended exposure to water
droplets and even complete water submersion without shorting or exhibiting significant
changes in performance [Figure 2-9(c), Supporting Video S2-3]. This paper photovoltaic
69
array was also resilient to subsequent laser-jet printing, where the whole cell was fed
through a roll-to-roll printer, while still maintaining efficiencies sufficient to power an
LCD clock [Supporting Video S2-4].
a
250 cell series-integated paper circuits
(air, 80 mWcm, 41*C (104 *F), Voc)
pm
1.6
1.4
1.2
C
laminateW
---
1
0.8
0.6
41 0.4
(PPFDA)
T*npaclaged
-unpakaged
0.2
0
250
(no
BCP)
5
1000 11
750
"Shelf"
Hours Illuminated
b
1.0
tonfodd
.0.
C
0.8
1.0-
0.6
0.
-0.4
.3
t
nsubmerge
,,,,,ct
t
0.4
0.2
0.0
a
submere
0.2
0
4
2
Number of Folds
0
6
10
20
Time (s)
30
Figure 2-9: Integrated paper PV demonstrations. (a) Normalized efficiency of thin-filmpackaged and unpackaged arrays (250 series-integrated cells) subjected to constant
illumination (80 mW-cm 2 , halogen lamp) in air at 42'C (108'F) as a function of time.
The right photograph shows the laminated paper circuit powering an LCD display in air
with ambient sunlight. The display's circuitry is powered by the photovoltaic alone and
regulates a constant voltage to the display for changes in light intensity. The demo has
maintained function after over 5000 hours of intermittent ("shelf') illumination. (b) A
2
paper array is progressively folded in air while being tested at AM 1.5, 80 mW-cm . The
final folded structure can be dynamically unfolded and refolded without loss of
performance in each three-dimensional configuration (See Video S2-2). (c) The iCVDcoated array (28-series integrated cells) is submerged in water during operation (See
Video S2-3). The right inset shows a nearly spherical droplet of water on the surface of
the paper photovoltaic array. The same device also withstood tortuous roll-to-roll
processing by an office laser-jet printer (See Video S2-4).
70
2.6
Conclusion
These demonstrations illustrate the near-term potential for implementation of paper-thin
photovoltaics in new venues and on nontraditional media. By using vapor-printed oCVD
polymer device layers, high-voltage, flexible, paper-thin integrated photovoltaic arrays
are monolithically fabricated directly on both conventional (glass and plastics) and
ubiquitous everyday substrates (papers) with identical fabrication steps. The paper PV
arrays produce >50V, power common electronic displays in ambient indoor lighting, and
can be tortuously flexed and folded without loss of function. The polymer vapor printing
process employs no solvents or rare elements (e.g., indium) and permits the substrate to
remain at low temperature. The vapor-printed electrodes conform to the geometry of
rough substrates, eliminating the need for more costly and heavier substrates such as
ultra-smooth plastics. Additionally, a thin-film vapor-deposited encapsulation layer
extends lifetime, even allowing for operation while submerged in water, but produces no
substantial change in weight, feel, or appearance of the paper circuits. This all-dry
fabrication and integration strategy should enable the design and implementation of new,
low-cost photovoltaic and optoelectronic systems without substrate limitations.
2.7
2.7.1
Experimental
Deposition of oCVD polymer electrodes
The oCVD process procedure and the reactor configuration are described in detail
elsewhere [1, 2, 41]. 3,4-ethylenedioxythiophene monomer (Aldrich, 97%) vapor was
metered at ~5 sccm and FeCl 3 oxidant (Aldrich) was controllably evaporated from a
71
resistively heated crucible at -250 0 C. The substrate temperature was maintained at ~80 0 C,
and the total pressure was maintained at -100 mTorr. The thickness of oCVD PEDOT
electrodes were determined by limiting the time of reaction (~5 min for a 50-nm thick
film). In situ patterning was achieved by positioning shadow masks cut from paper or
plastic in intimate contact with the substrate. PEDOT films were deposited for thin-film
and device characterization on pre-cleaned glass/ITO (described below) and various other
untreated substrates: PET (5-mil Melinex@ Q65FA, Dupont), copy paper (Office Depot,
20 lb, 92 brightness, #99964200), tissue paper (Office Depot, #48601-OD), tracing paper
(Canson, 25 lb, #702-321), glossy photo paper (Office Depot, #394-925), newsprint
(Pacon Papers, #3407), Reynolds® Cut-Rite® Wax Paper, and SaranTM Premium Wrap
(SC Johnson).
2.7.2
Device fabrication
The organic active layers (CuPc, C60 or PTCBI, and BCP) and Ag cathodes were
thermally evaporated at <10-6 Torr through shadow masks at rates of ~1 A s-1. The
overlap area between the anode and cathode defined the device areas which ranged from
0.005 to 0.02 cm2 , and were measured after testing with an optical microscope. The
copper phthalocyanine (CuPc, Acros Organics, ca. 95% purity), fullerene-C 60 (C60 ,
Luminescence
Technology
Corp.,
>99.5%),
3,4,9,1 0-perylene
tetracarboxylic
bisbenzimidazole (PTCBI, Sensient Imaging Technologies) and bathocuprine (BCP,
Sigma Aldrich, 99.99%) were each purified once via thermal gradient sublimation, and
the Ag (Alfa Aesar, 1-3 mm shot, 99.9999%) was used as received. All oCVD-based
devices were fabricated and tested in parallel with control devices with the same active
layer structure on pre-patterned and solvent-cleaned ITO-coated glass (Thin Film Devices,
72
sheet resistance = 20 K/0) with and without spin cast PEDOT:PSS [7-11, 42, 43]. For
the control devices with PEDOT:PSS, the PEDOT:PSS solution (CLEVIOS TM P VP Al
4083) was filtered (0.45 pm), spin-coated at 4000 rpm for 60 seconds, annealed at 210 'C
for 5 minutes in air, and exposed to oxygen plasma for <5 seconds (100 W, Plasma Preen,
Inc.). The following device structures were used in this study: Anode/CuPc (20 nm)/C 60
(40 nm)/BCP (10 nm)/Ag (1000 nm) for Figure 2-2 and Figure 2-3(b); Anode/CuPc (80
nm)/C6o (40 nm)/BCP (10 nm)/Ag (1500 nm) for Figure 2-6, Figure 2-7(left), and Figure
S2-3; Anode/CuPc (80 nm)/PTCBI (40 nm)/BCP (10 nm)/Ag (1500 nm) for Figure 27(right), Figure 2-8, and Figure 2-9. For integrated arrays, the individual layers were
shadow masked to create anode-to-cathode interconnections between cells. Three circuit
designs are shown in this chapter: 250 series-integrated cells, ~0.1xO.3 cm 2 each [Figure
2-8, Figure 2-9(a-b), Figure S2-5, and Video S2-2]; 128 series-integrated cells, ~0.2x0.3
cm 2 each [Figure 2-9(a, right)]; 4 parallel series of 28 series-integrated cells, ~0.1x0.3
cm 2 each [Figure 2-9(c), and Video S2-4].
2.7.3
Thin-film packaging
Before packaging the paper photovoltaics, wires were first connected to the electrical
contacts with conductive carbon tape and made to extend outside of the packaging area.
Laminated arrays were packaged in a nitrogen atmosphere using a multiuse office
laminating machine (Staples, Model 17466) with 5-mil thick plastic laminate pouches
(Staples, Model 17468); UV-curing epoxy was used to seal the resulting annular space
around the wires at the air-side connection. Arrays packaged with parylene-C were
loaded into a custom in-house CVD deposition system, and a 750-nm thick conformal
layer of poly (monochloro-p-xylylene) (parylene-C) was deposited at -2 A-s-' by self73
initiated CVD polymerization of thermally cracked dichloro-[2.2]paracyclophane dimer
(Daisan Kasei Co.) vapor using the Gorham process [44]. The total chamber pressure was
~25 mTorr and the substrate was at room temperature. Arrays packaged using initiated
chemical vapor depositions (iCVD) were first loaded into a pancake-style reactor [1, 2,
45], and a 750-nm thick poly(lH,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) layer
crosslinked with divinyl benzene was deposited conformally on the substrate, maintained
at 39'C, via free-radical polymerization of monomer 1H,1H,2H,2H-perfluorodecyl
acrylate (PFDA) (97%, Aldrich) and divinyl benzene (DVB) (Aldrich), initiated by hotfilament (260'C) decomposed tert-Butyl peroxide (TBPO) (Aldrich, 97%). PFDA, DVB,
and TBPO vapors were delivered at flow rates of 0.3 sccm, 0.88 sccm, and 1 sccm,
respectively, and the total chamber pressure was maintained at 300 mTorr.
2.7.4
Electrode and substrate characterization
Electrode thickness was measured with a Tencor P-16 profilometer and the sheet
resistance was measured using a Signatone S-302-4 four-point probe station with a
Keithley 4200-SCS semiconductor characterization system. The specular transmission
and reflection of the polymer electrodes and paper substrates was measured on glass
using a Varian Cary 6000i UV-Vis-NIR dual-beam spectrophotometer. The total (diffuse
plus specular) transmission and reflection of the paper substrates was measured using a
Cary 500i UV-Vis-NIR dual-beam spectrophotometer with a PTFE-coated integrating
sphere attachment. Flex testing for ITO/PET (Aldrich, 5-mil PET with 100 nm ITO, 60
Q/O) and oCVD PEDOT electrodes on PET was performed using an in-house flexingtest system and the sheet resistance was measured after each iteration. The flexing results
shown throughout this manuscript were obtained by flexing in the compressive
74
orientation (film on the inside of the bend); however, similar results were obtained in the
tensile orientation. Stretch testing was performed using a MTS Nano Instruments NanoUTM nanotensile tester. 5 mm x 1 mm oCVD PEDOT electrodes on Saran
wrap were
anisotropically stretched by 0.01 mm increments at a strain rate of 0.005 s-1 while the
resistance across the electrode (in the direction of extension) was simultaneously
measured using an Agilent U125A multimeter attached to the polymer electrode using
conductive tape and alligator clips. The stretching extension was defined as the change in
length of the electrode divided the initial length (AL/Lo), and the conductivity was
calculated assuming a constant film volume as described in the literature [20]. The
electrode adhesion on the PET and SaranTM wrap substrates was also confirmed by tape
testing (ASTM D3359-97) [16], and the surface after flexing and stretching was imaged
using an Olympus CX41 optical microscope with a maximum magnification of x100.
Electrode foldability was tested on ~100 nm oCVD PEDOT electrodes deposited on
untreated newsprint; the film thickness was based on films deposited on glass slides from
the same deposition. The folding tests (±180') were performed by creasing the
paper/electrode flat using gentle pressure applied with a pencil eraser; for folding at
+1800, the crease was made between two additional sheets of paper to prevent contact
between the eraser and the polymer electrode. After each iteration, the paper was
completely flattened back to 00 and the sheet resistance was measured across the crease
with a multimeter with probe spacing of 1 cm (0.5 cm on either side of the crease).
2.7.5
Device characterization
Current-voltage characteristics were measured for the individual cells in nitrogen
atmosphere with an Agilent 4156C Precision Semiconductor Parameter Analyzer and
75
simulated AM 1.5 solar illumination from a Newport 91191 1kW solar simulator at 100
mW-cm-2 and 500 mW-cm-2 and for the integrated arrays in air with an HP
4145B Semiconductor Parameter Analyzer using AM 1.5 solar illumination from a Oriel
94023A 450W solar simulator at 80 mW-cm-2 . Opaque masks were used to selectively
illuminate the active cell area, and optical density filters were used to achieve the desired
intensity as measured with a calibrated silicon photodiode. The external quantum
efficiency (EQE) spectra were measured with a Stanford Research Systems SR830 lockin
amplifier,
under
a
focused monochromatic
light generated by an Oriel
beam
of
variable
wavelength
1kW xenon arc lamp coupled to an Acton 300i
monochromator and chopped at 43 Hz. A Newport 818-UV calibrated silicon photodiode
was used to measure the incident monochromatic light intensity. Internal quantum
efficiency (IQE) spectra were estimated by dividing the EQE by the fraction of
incident light absorbed by the active layers (as estimated by the experiment described in
Supporting Figure S2-3 and Figure S2-4). The lifetime characteristics of the full
packaged and unpackaged arrays were measured by accelerated aging in air through
exposure to constant illumination (80 mW-cm-2, halogen lamp), elevated temperature
(42 0 C/108'F), and ambient relative humidity (10-20%) at open-circuit [Figure 2-9(a) and
Figure S2-5). The data for each circuit has been normalized by its initial performance
(t=0 hours).
76
2.8
Supporting Figures
C
200pm
200 m
oCVD PEDOT on PET
ITO on PET
~
200 pm
200 pm
r
oCVD PEDOT onl PET
Full Device
e
PET
uill on
Device
ITO
F
fMM
200-
E
4-J
01
0
106o
200
100
width (pm)
width (pm)
Figure S2-1: Optical microscope images after 100 compressive flexing cycles (film on
the inside of the bend) to radius <5 mm for (a) 100-nm thick oCVD PEDOT on 5-mil
PET and (b) commercial 100-nm thick ITO on 5-mil PET. The films correspond to those
shown in Figure 2-3(a). (c) and (d) show similar images for the cathode surface of the
entire devices [Anode/CuPc (20 nm)/C6o (40 nm)/BCP (10 nm)/Ag (100 nm)] grown on
the same starting surfaces and flexed in the same way as (a) and (b), respectively. (e) and
(f) show 2-dimensional profiles (left-to-right) across the surface of the devices in (c) and
(d), respectively. The device imaged in (c) and (e) correspond to the device tested in
Figure 2-3(b).
77
a
b
b
4
4oCa
1.
2C
2
I-.
~1~
so
400
600
wavelength (nm)
c
400
d
600
-- tracing paper
-- copy paper
newsprint
-- tissue paper
-- wax paper
-- photo paper
so
wavelength (nm)
100
'I.
so~
I-
0
4w
e
elnt
80(
wavelength (nm)
f
g
100
J50
0
400
soc
600
wavelength (nm)
Figure S2-2: Specular and diffuse contributions to UV-visible transmission, reflection,
and absorption spectra for various fiber-based papers (tracing paper, copy paper,
newsprint, tissue paper, wax paper, and photo paper). (a) and (b) show specular
transmission (%) and reflection (%), respectively; (c) and (d) show diffuse transmission
(%) and reflection (%), respectively; and (e) and (f) show total transmission (%) and
reflection (%), respectively. The absorption (%) losses (g) in the paper substrates were
calculated as 100-%T-%R using the total reflection and total transmission spectra.
78
a
100
401
C
b
Paper/oCVD PEDOT
400
01
--- - - 0
600
8m
wavelength (nm)
Glass/ITO/PEDOT:PSS
0.'
400
600
O00
%R
%A
--------
600
wavelength (nm)
8
0
01'
400
600
S00
0
wavelength (nm)
= (Substrate/Anode/Ag)
(Substrate/Anode/Active Layers/Ag)
===== %R=
-
400
GlassATO/PEDOT:PSS
wavelength (nm)
-
Paper/oCVD PEDOT
=
(Active Layers) =%R-%Rm
Figure S2-3: Total active layer absorption in CuPc/C 6o heterojunction devices on (a,b)
paper/oCVD PEDOT and (c,d) glass/ITO/PEDOT:PSS. The device structure is [CuPc (80
nm)/C60 (40 nm)/BCP (10 nm)/Ag (150 nm)] and the oCVD PEDOT thickness is 50 nm.
The active layer absorption (%AACT, red solid) was estimated as the difference between
the percentage of incident light reflected by the substrate/anode/Ag structure (%RsuBs,
black solid) and the percentage reflected by the complete device structure (%RTOT, black
dashed). The experiment is shown for both total reflection (as measured with an
integrating sphere) (a,c) and specular reflection (b,d) to illustrate the high contribution of
scattered light to active layer absorption in paper-based cells; however, the total
absorption spectra were used to calculate the internal quantum efficiency spectra [Figure
2-7].
79
a
a
40
100
b
wavelength (nm)
C
70
M.4
12
wavelength (nm)
G3lass/ITO/PEDOT:PSS
.0 g
d
wavelength (nm)
10-
Glass/ITO/PEDOT:PSS
02g
wavelength (nm)
----- %R = (Substrate/Anode/Ag)
----- %Ry = (Substrate/Anode/Active Layers/Ag)
....
%A = (Active Layers)=%R-R
Figure S2-4: Total active layer absorption in CuPc/PTCBI heterojunction devices on (a,b)
paper/oCVD PEDOT and (c,d) glass/ITO/PEDOT:PSS. The device structure is [CuPc (80
nm)/C60 (40 nm)/BCP (10 nm)/Ag (150 nm)] and the oCVD PEDOT thickness is 50 nm.
The active layer absorption (%AACT, red solid) was estimated as the difference between
the percentage of incident light reflected by the substrate/anode/Ag structure (%RSUBS,
black solid) and the percentage reflected by the complete device structure (%Rro, black
dashed). The experiment is shown for both total reflection (as measured with an
integrating sphere) (a,c) and specular reflection (b,d) to illustrate the high contribution of
scattered light to active layer absorption in paper-based cells; however, the total
absorption spectra were used to calculate the internal quantum efficiency spectra [Figure
2-7].
80
a
1.6
1.4
.2
0 1.2
parylene-C
laminated
iCvD
."I
(PPFDA)
0 0.8
unpackaged
0.6
unpackaged (no BCP)
C.
0.4 0
1000
500
Hours Illuminated
b1 6
-T 1.4
E 1.2
C
parylene-C
1
iCVD
(PPFDA)
*d0.8
$ .b0.6
laminated
npackage
0.4
unpackaged (no BCP)
0
0.2
0
500
1000
Hours Illuminated
Figure S2-5: Normalized open-circuit voltage (a) and short-circuit current (b) of thinfilm-packaged and unpackaged paper photovoltaic arrays (250 series-integrated cells) on
paper subjected to constant illumination (80 mW-cm 2 , halogen lamp) in air at 42'C
(108'F) as a function of time. These lifetime plots correspond to the arrays tested in
Figure 2-9.
81
Video S2-1: A paper airplane with thin-film photovoltaic cells integrated into the wings
is folded and tested in air. The final three-dimensional structure is folded from a flat ~50
cm2 sheet of tracing paper that was first patterned with an oCVD polymer electrode and
thermally evaporated active device layers and silver electrode. The device is illuminated
from above with a 20 W desk lamp. The video segments from 0:14-0:23 and 0:31-0:34
are played at 6 times the original frame rate.
(Available online at http://web.mit.edu/press/201 1/printable-solar-cells-0711 .html and
http://www.youtube.com/watch?v=gKbiXOkdpRA)
Video S2-2: The folded paper photovoltaic shown in Figure 2-9(c) is dynamically folded
and unfolded while simultaneously being tested at open circuit. The paper photovoltaic is
illuminated from below with simulated solar illumination (80 mW-cm-2 AM 1.5). The
video segment from 0:12-0:18 is played at 2 times the original frame rate.
Available online at http://onlinelibrary.wiley.com/doi/J0.1002/adma.201101263/suppinfo
and http://www.youtube.com/watch?v=21OtBe-Alk
82
Video S2-3: The iCVD-encapsulated circuit shown in Figure 2-9(d) is first folded into a
small ramp and exposed to copious water droplets while simultaneously being tested at
open circuit. A close-up view is shown in the segment from 0:30-0:44. The circuit is then
unfolded and completely submerged in a water bath while also being tested at open
circuit; the submersion of the hydrophobic paper photovoltaic is aided by physical
pressure applied with a glass rod. The paper photovoltaic array (and water bath) is
illuminated from below with simulated solar illumination (80 mW-cm- 2 AM 1.5). The
video segment from 0:12-0:28 is played at 3 times the original frame rate.
Available online at http://onlinelibrary.wiley.com/doi/0.1002/adma.201101263/suppinfo
Video S2-4: The same paper photovoltaic shown in Video S2-3 is shown powering the
LCD clock when illuminated. The entire integrated paper photovoltaic is then fed through
a roll-to-roll office laser-jet printer. The resulting ink spells MIT on the device side of the
paper array, which then continues to power the LCD clock. The clock circuitry is shown
at the end of the video, which is powered by the photovoltaic alone and regulates a
constant voltage to the display for variations in light intensity. The paper photovoltaic
array is illuminated from below with simulated solar illumination (80 mW-cm 2 AM 1.5).
The video segment from 0:04-0:05 and 0:24-0:26 are played at 6 times the original frame
rate.
Available online at http://onlinelibrary.wiley.com/doi/J0.1002/adma.201101263/suppinfo
and http://www.youtube.com/watch?v=5wBVktT9XO8
83
2.9
Acknowledgments
This work was supported by Eni SpA under the Eni-MIT Alliance Solar Frontiers
Program and by National Science Foundation Graduate Research Fellowships.
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S.G. Im and K.K. Gleason. Systematic control of the electrical conductivity of
poly(3,4-ethylenedioxythiophene) via oxidative chemical vapor deposition.
Macromolecules, 40, 6552-6556, 2007.
[42]
K.L. Mutolo, E.I. Mayo, B.P. Rand, S.R. Forrest and M.E. Thompson. Enhanced
open-circuit voltage in subphthalocyanine/C 6o organic photovoltaic cells. Journal
of the American Chemical Society, 128, 8108-8109, 2006.
[43]
M. Brumbach, D. Placencia and N.R. Armstrong. Titanyl phthalocyanine/C6o
heterojunctions: Band-edge offsets and photovoltaic device performance. Journal
ofPhysical Chemistry C, 112, 3142-3151, 2008.
[44]
W.F. Beach, C. Lee, D.R. Bassett, T.M. Austin and R. Olson. "Xylylene
polymers" in Encyclopedia of Polymer Science and Engineering. Wiley & Sons,
Inc., New York, 1989.
[45]
M. Gupta and K.K. Gleason. Initiated chemical vapor deposition of
poly(1H,1H,2H,2H-perfluorodecyl acrylate) thin films. Langmuir, 22, 1004710052, 2006.
87
88
Chapter 3
Top-illuminated Organic
Photovoltaics on a Variety of
Opaque Substrates with oCVD
PEDOT Top Electrodes*
*Originally prepared as M.C. Barrt, R.M. Howdent, R.R. Lunt, V. Bulovid, and K.K. Gleason Topilluminated organic photovoltaics on a variety of opaque substrates with vapor-printed poly(3,4ethylenedioxythiophene) top electrodes. Submitted to Advanced Energy Materials.
(tAuthors contributed equally.)
89
3.1
Abstract
Organic photovoltaics devices typically utilize illumination through a transparent
substrate, such as glass or an optically clear plastic. Utilization of opaque substrates,
including low cost foils, papers, and textiles, requires architectures that instead allow
illumination through the top of the device. Here, we demonstrate top-illuminated organic
photovoltaics, employing a dry vapor-printed poly(3,4-ethylenedioxythiophene) (PEDOT)
polymer anode deposited by oxidative chemical vapor deposition (oCVD) on top of a
small-molecule
organic
tetraphenyldibenzoperiflanthene
heterojunction
(DBP)
based
on
vacuum-evaporated
and C60 heterojunctions. Application of a
molybdenum trioxide (MoO 3) buffer layer prior to oCVD deposition increases the device
photocurrent nearly 10 times by preventing oxidation of the underlying photoactive DBP
electron donor layer during the oCVD PEDOT deposition, and resulting in power
conversion efficiencies of up to 2.8% for the top-illuminated, ITO-free devices,
approximately 75% that of the conventional cell architecture with indium tin oxide (ITO)
transparent anode (3.7%). Finally, we demonstrate the broad applicability of this
architecture by fabricating devices on a variety of opaque surfaces, including common
paper products, with over 2.0% power conversion efficiency, the highest to date on such
fiber-based substrates.
3.2
Introduction
Organic photovoltaics (OPVs) have gained significant momentum as a possible low-cost
energy source, and have recently reached record efficiencies of nearly 10% [1],
suggesting near-term commercial potential. One particularly promising direction is
90
deployment on the surface of everyday items, such as wall coverings, product packaging,
documents, and apparel, enabled by mechanically flexible device layers, low-temperature
manufacturing requirements, and low toxicity [2-4]. For example, there has been
significant recent interest in integrating electronics on paper substrates [5-10]. However,
for these applications, the OPV must be compatible with opaque substrates.
In the conventional orientation, the OPV is illuminated through a transparent
hole-collecting anode deposited on the substrate, typically indium tin oxide (ITO), and
electrons are collected by a low work function metal cathode top contact. This structure
necessitates that the substrate be transparent (e.g., glass or optically clear plastics).
Alternative top-illuminated OPV architectures that are compatible with opaque substrates
require deposition and patterning of a transparent electrode on top of the complete
organic device stack [11]. Such devices have previously been demonstrated with
sputtered ITO top anodes with a MoO 3 anode buffer layer, reaching efficiencies of 2.4%
with evaporated small molecule organic active layers [12], and 3% with a poly(3hexylthiophene) (P3HT): [6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk
heterojunction active layer [13]. Transparent ITO top cathodes have also been
demonstrated on top of a bathocuproine (BCP) exciton blocking layer in opaque [14] and
visible-light transparent [12] small molecule OPVs on glass substrates. However, in these
configurations, the ITO transparent electrode must be sputtered on top of the full device
which can damage underlying organic layers, and is prone to cracking on highly flexible
substrates [15-17]. As an alternative to sputtered metal-oxide transparent top electrodes,
ultrathin metal films deposited by vacuum thermal evaporation have also been
91
demonstrated as the top transparent cathode in top-illuminated, small molecule organic
OPVs, reaching efficiencies of 2.2% [16, 18-20].
Conducting polymer electrodes based on poly(3,4-ethylenedioxythiophene)
(PEDOT) are an attractive alternative transparent electrode due to their potential low cost,
ease of processing, and mechanical robustness on highly flexible substrates, such as
plastic, textiles, and paper [9, 10, 17]. Solution deposited PEDOT, doped with
poly(styrenesulfonate) (PEDOT:PSS), has been widely used as a hole-transporting anode
buffer layer on ITO [21, 22] and also demonstrated as a top anode in top-illuminated
polymer OPVs based on P3HT:PCBM with efficiencies up to 2.4% [9, 10, 23, 24].
However, PEDOT:PSS top-electrode configurations generally require that the underlying
device layers do not dissolve or intermix during the solvent deposition process, restricting
applicability in many multilayered and tandem device architectures, and limiting
demonstrations to single-junction P3HT:PCBM cells [9, 10, 15, 17, 23-25].
Oxidative chemical vapor deposition (oCVD), is a solvent-free, vacuum-based
technique, in which conjugated polymer films are formed directly on the substrate by
oxidative polymerization of vapor-phase monomer and oxidant precursors at low
temperature (25-150'C) [10, 26-28]. Well-defined polymer patterns can be "vaporprinted" on the substrate of choice when this process is combined with in situ shadow
masking. Thus, oCVD offers an attractive solvent-free route to transparent polymer top
electrodes, while maintaining the benefits of vacuum processing, including parallel and
sequential deposition, well-defined thickness control and uniformity, and inline
compatibility with standard vacuum process (e.g. thermal evaporation) [27].
92
Here, we demonstrate top-illuminated OPVs with an oCVD PEDOT transparent
anode on top of a small-molecule organic heterojunction based on vacuum-evaporated
tetraphenyldibenzoperiflanthene (DBP) and fullerene C60 planar heterojunctions [29].
Application of a molybdenum trioxide (MoO 3 ) buffer layer prior to oCVD deposition
increases the device photocurrent nearly 10 times by preventing oxidation of the
underlying photoactive DBP electron donor layer during the oCVD PEDOT deposition
and results in power conversion efficiencies of up to 2.8% for these top-illuminated, ITOfree devices, approximately 75% that of the conventional cell architecture with indium tin
oxide (ITO) transparent anode (3.7%). Finally, we demonstrate the broad applicability of
this architecture by fabricating devices on a variety of opaque fiber-based surfaces,
including common paper products with over 2.0% power conversion efficiency, the
highest to date on such substrates.
3.3
Results and discussion
3.3.1
Top-illuminated device structure
For the OPVs in this work, we used a simple single-junction bilayer structure based on
thermally
evaporated
small-molecule
organic
active
layers
tetraphenyldibenzoperiflanthene (DBP) as the electron donor and fullerene C6 0 as the
electron acceptor with a bathocuproine (BCP) exciton blocking layer at the cathode
interface [Figure 3-1(a)]. The conventional orientation device structure on glass/ITO has
been discussed previously for these materials, reaching efficiencies of up to 3.6% [29].
Here, we also optionally insert thermally evaporated molybdenum trioxide (MoO 3 ) layer
at the anode interface, which has previously been demonstrated as an electron-blocking
93
layer and physical buffer layer in polymer and small-molecule organic photovoltaics [3036].
a.
b-
Conventional
C- Top-Illuminated
Light
DBP
C60
Light
Substrate
NNN
S
N_
-N
H3C
BCP (7.Snm)
CH
3
Chmialstucurs
BCP
f
FeCI
BP CoBC, 3
On
CVD PEDOT
BCP (7.Snm)
CVD
plmrzdaddpdwt
PEDOT
(i
Figure 3-1: Schematics of the device structures and materials used in this report: (a)
Chemical structures
of~DBP, ~C60 , BCP,
and CVD PEDOT
polymerized (and doped
substrates
~
~O
wihteticnse2hwni0Fgr
3-1()] [MoO3
10m)B with
(2
FeCl 3 . (b) Conventional orientation PV device with transparent ITO anode (device is
illuminated from the substrate side). (c) Top-illuminated orientation PV device with
transparent CVD PEDOT anode (device is illuminated from the device side).
fwhilethe
deositin rder wa
revrsedstaring fom te
su
Strate:aBPDP
The conventional device structure was first optimized on ITO-coated glass
substrates with the thicknesses shown in [Figure 3-1(b)] [MoO 3 (20 nm)/DBP (25
nm)/C 6o
(40 nm)/BCP (7.5 nm)/Ag], and used subsequently as a point of comparison. For
the top-illuminated device, the same organic active layers and thicknesses were used,
while the order of deposition was reversed, starting from the substrate: Ag, BCP, DBP,
C6 0, (MoO 3 ).
For the transparent top electrode, instead of ITO, PEDOT layer was grown
by oCVD from the 3,4-ethylenedioxythiophene (EDOT) monomer with FeCl 3 oxidant
[Figure 3 -1(c)]. The thickness of the oCVD PEDOT electrodes used here were 60±1 0 nm,
which was controlled by the time of deposition, and resulted in films with a sheet
resistance of -200±50 Q2/E at the conditions used here (see Experimental Section). By
94
using the optimized DBP, C60 , and BCP thicknesses from the conventional device and
simply reversing the order of the stack, we expect a similar optical environment within
the device, since the reflective Ag node is maintained in the same position relative to the
DBP/C6o heterojunction interface, thus maintaining a similar positioning of the optical
electric field maxima within the respective device layers. We note, however, that in the
conventional orientation, defect states are created in the BCP layer when the metal
cathode is deposited on top it, which are proposed to provide efficient electron transport
through this layer [37]. In the reversed stack, the BCP organic layer is positioned on top
of the bottom Ag surface and thus likely absent of these states which may increase series
resistance through the device [38].
3.3.2
Device performance
Figure 3-2(a) compares the current density-voltage (J-V) characteristics of the optimized
conventional device on ITO with top-illuminated, oCVD PEDOT devices with and
without MoO 3 on glass substrates, and a summary of the performance parameters are
shown in Table 3-1. The optimized, conventional control device exhibits power
conversion efficiency (qp)= 3 .7 ±0.3 %, open-circuit voltage (Voc)= 0.93±0.03V, short-
circuit current (Jsc)= 6.72±(0.3) mA cm-2, and fill factor (FF)= 0.66±0.04, consistent
with previous reports [29]. Inverting the device and replacing the ITO with the oCVD
PEDOT top electrode, decreases the Jsc to 4.7t1.6 mA cm-2, Voc to 0.84±0.01 V, and FF
to 0.58±0.01, resulting in th, of 2.1±0.6%. The decrease in Jsc likely results from small
absorptive losses in the less transparent oCVD PEDOT electrode, as observed previously
with oCVD PEDOT bottom electrode devices [10]. The FF decreases due to increased
series resistance, observable in the J- V curve under forward bias, which is a result of the
95
more resistive oCVD PEDOT compared to ITO [10], and possibly detieriorated by the
lack of defect states in the BCP layer as discussed above. Removing the MoO 3 between
the oCVD PEDOT top electrode and DBP electron donor significantly decreases the Jsc
to 0.76+1.6 mA cm-2 , the Voc to 0.68±0.02 V, and FF to 0.44±0.02, resulting in a r/, of
only 0.21 ±0.1%.
a.
b.
16
(ITO)
.Conventional
-
S12
E
- -
onventional (ITO)
50
Top-Illuminated (CVD), 20 nm MoO
Top-Illuminated (CVD), No MoO
3
-
40
.
-
Top-Illuminated (CVD), 20 nm MoO
3
-
Top-Illuminated (CVD), No MoO 3
8
E
4
0
Cr
--
30
20
..
.
-
10
~4
-8
-0.50
LU
0.00
0.50
Voltage (V)
n
300
1.00
400
700
500
600
Wavelength (nm)
800
Figure 3-2: (a) Representative J-V performance curves measured under 1.1 sun
illumination and (b) external quantum efficiency spectra, for the conventional device with
ITO anode (dotted blue) and top-illuminated devices with CVD PEDOT anode, with
(solid red) and without (dashed black) MoO 3 as a buffer layer. All devices are on silvercoated glass substrates.
Table 3-1: Summary of performance parameters under 1.1 sun illumination for devices
on glass substrates [Figure 3-2].
Js
(mA/cm)
V0 c (V)
FF
PCE (%)
Conventional (ITO)
6.72±(0.3)
0.93±(0.03)
0.66+(0.04)
3.7±(0.3)
Top-illuminated (CVD),
4.7±(1.6)
0.84±(0.01)
0.58+(0.01)
2.1+(0.6)
0.76±(0.05)
0.68±(0.02)
0.44±(0.02)
0.21±(0.01)
Device
2
20 nm MoO 3
Top-Illuminated (CVD),
0 nm MoO 3
96
To understand the origin of the differences in device photocurrent, we measured
the external quantum efficiency spectrum for each of these device structures [Figure 32(b)]. As expected, the external quantum efficiency (EQE) of the top-illuminated CVD
device with MoO 3 is slighty lower than that of the conventional device across the
wavelength range, which is consistent with the lower observed Jsc due to absorptive
losses from the oCVD PEDOT electrode. The characterstic absorption peaks from DBP
(570 nm and 610 nm) are strongly evident in the EQE of the conventional device and topilluminated device with MoO 3 yet do not appear in the top-illuminated device without
MoO 3, which only shows photocurrent generation below 550 nm, in the region where C60
absorbs, indicating a loss of photocurrent originating from the DBP.
3.3.3
oCVD oxidant exposure to photoactive semiconductor layers
In the devices without MoO 3 , the surface of the DBP photoactive layer is exposed to
FeCl 3 oxidant precursor during the oCVD process. To better understand this effect,
absorption data was collected on blanket films of DBP and C60 both before and after
exposure to FeCl 3 , in the oCVD chamber, under the same pressure and temperature
conditions experienced during PEDOT polymerization. Figure 3-3 shows the UV-visible
absorbance data for the active layer films before and after FeCl 3 exposure both with and
without a MoO 3 buffer layer. Though the C60 absorption remains unchanged [Figure 33(b)], the bare DBP absorption peaks decrease significantly upon exposure to FeCl 3
[Figure 3-3(a)]. This is consistent with the EQE spectra for the top-illuminated device
without MoO 3 , which only shows photocurrent originating from C6 oand suggests that the
bare DBP layer is prone to oxidation by FeCl 3 while the more stable resonant structure of
C6 0
remains unaffected. With the addition of the MoO 3 buffer layer (20 nm) on top of the
97
DBP before exposing to FeCl 3, the absorption peaks of the DBP remain intact, indicating
that the thin MoO 3 layer physically protects the underlying DBP layer from chemically
interacting with the FeCl 3 during the oCVD process.
a-
b.
----
DBP
b
---- C60
-C60+FeC3
-DBP+FeCl3
0.6
o
- ---- DBP/MoO3
0.6
-- DBP/Mo3+FeCI3
U
6
C
---- C60/MoO3
-C60/MoO3+FeCl3
0.4
0.4
.0
0.2
0.2-
-
0
.0
300
400
500
600
700
Wavelength (nm)
800
300
400
500
600
700
Wavelength (nm)
800
Figure 3-3: UV-visible absorbance spectra for (a) glass/DBP [25 nm]/MoO 3 [0 nm
(black) and 20 nm (red)] and (b) glass/C 6 0 [40 nm]/MoO 3 [0 nm (black) and 20 nm (red)].
Films were measured before (dashed) and after (solid) exposure to FeCl 3 under CVD
polymerization conditions.
To further understand how this layer affects device performance, top-illuminated
devices with oCVD PEDOT electrodes were fabricated using varying thicknesses (0, 2
nm, 20 nm, 50 nm, and 100 nm) of the MoO 3 buffer layer. Figure 3-4 shows how the
main device characteristics (Jsc, Voc, FF,and r/,) vary with MoO 3 layer thickness. The
ultra-thin MoO 3 layer (2 nm) was found to be too thin to protect the underlying active
layers during PEDOT polymerization and these devices showed similarly low Jsc and
Voc characteristics as the devices with no MoO 3. Increasing the MoO 3 thickness further
from 20 to 100 nm resulted in a plateau in Jsc around 6.0 mA cm- 2 between 20 and 50 nm
[Figure 3-4(a)], demonstrating limited additional benefit in protecting the photocurrent
generation of the underlying semiconductors. Similarly, the Voc increases and eventually
98
plateaus with increasing MoO 3 thickness [Figure 3-4(b)]. The high Voc observed with the
MoO 3 layer present is supported by the high work function of MoO 3 (5.7±0.4 eV) [39],
compared with the work function of bare CVD PEDOT (5.2±0.1 eV) [40], resulting in
increased work function offset between
b.
a.
8.0
E
E
t
1
6.0
0.9
0
4.0
U
2.0
0.7
0.0
0
0.6
25
50
75 100
MoO3 Thickness (nm)
C.
0.8
'
'
0
25
50
75 100
MoO 3 Thickness (nm)
d.
4.0
0.7
'
0
3.0
0.6
a0 2.0
.- 0.5
1.0
0.4
0.0
0.3
0
25
50
75 100
0
25
50
75 100
MoO 3 Thickness (nm)
MoO 3 Thickness (nm)
Figure 3-4: Performance parameters for top-illuminated cells (solid symbols) with
different MoO 3 buffer layer thicknesses, measured under 1.1 sun illumination: (a) shortcircuit current density (blue diamonds), (b) open-circuit voltage (red circles), (c) fill
factor (green triangles), and (d) power conversion efficiency (black squares). The
conventional, bottom-illuminated cell with ITO anode is shown for reference at x=-15
(open symbols). Data points are the average of 3-5 devices measured across each
substrate, and error bars represent the maximum and minimum values recorded.
99
the anode and cathode. It has been also been reported that the MoO 3 work function and
band gap both increase with thickness, which can increase Voc by reducing electron
leakage current from the donor layer and enhancing hole extraction by increased band
bending at the MoO 3/donor interface [12, 39]. The FF gradually decreases with
increasing MoO 3 thickness due to increased series resistance through the device [Figure
3-4(c)]. The maximum observed rj of 2.8% was achieved for a device with a 100 nm
MoO 3 layer [Figure 3-4(d)]; however, the optimal MoO 3 thickness seems to be lower than
100 nm, where we see maximum values for Voc, Jsc, and FF. For example, a device
having the maximum Voc (0.91 V), Jsc (6.7 mA cm), and FF (0.61) observed with 50
nm MoO 3 (which were not all observed on the same individual device) would give an
efficiency of 3.4%.
3.3.4
Demonstrations on everyday opaque substrates
Finally, devices were fabricated and tested on a number of common opaque substrates to
demonstrate the wide applicability of this top-illuminated architecture [Figure 3-5].
Opaque substrates, made from everyday consumer products were selected, including
photo paper, magazine print, a U.S. First Class stamp, and plastic food packaging. The
OPV devices were fabricated and tested in the same way as the cells on glass substrates
with a 20 nm MoO 3 buffer layer, and all substrates were used as purchased. The ability to
seamlessly transition from fabrication on glass to paper substrates is made possible by
avoiding high temperatures and solvent wetting challenges in this all-dry process.
Notably, the J- V performance curves [Figure 3-5(a)] show that inverting the orientation
of illumination results in photocurrents (Jsc) that match closely with that of the cell on
the transparent glass substrate, despite the low substrate transparency.
100
a.
16
-
Photo Paper
Magazine Print
.
--- US First-Class Stamp
- -Plastic Food Packaging:.
-
-
.,12
E
8
E
Glass
-m4
.I
.1 i
I
0
. -4.
-8 '
-0.50
b.
0|
'"
. qOW
I e
..
0.00
0.50
1.00
Voltage (V)
Figure 3-5: (a) Representative J- V curves for top-illuminated OPVs fabricated on the top
side of some common opaque substrates under 1.1 sun illumination, including photo
paper, magazine print, a U.S. First Class stamp, plastic food packaging, and glass for
reference. (b) Photographs of completed 10 device arrays are also shown. All substrates
were used as purchased, so the original surface images are visible in the spaces below the
completed PV devices (i.e., printed text, Statue of Liberty image, and "Nutritional Facts"
text, respectively).
101
This represents a significant improvement over our previous demonstrations of
bottom-illuminated oCVD PEDOT OPVs on paper substrates, which suffered from low
photocurrents due to the low optical transmittance of paper substrates [10]. The low FF
for the device on magazine print is due to low shunt resistance observable in the J-V at
zero bias and likely due to shorting pathways across the thin semiconductor active layers
in areas of high surface roughness [10]. A summary of the performance parameters for
these devices are listed in Table 3-2. The efficiency observed here on paper substrates
(2.0%) has a performance just over half of the optimized conventional ITO device on
glass (3.7%) with the same active layer materials and is the best reported to date for an
OPV on a fibrous paper substrate [5, 9, 10, 41, 42].
Table 3-2: Summary of performance parameters under 1.1 sun illumination for devices
on common opaque substrates [Figure 3-5].
Device
J,c (mA/cm2)
Voc (V)
FF
PCE (%)
Photo Paper
5.7
0.61
0.47
1.5
Magazine Print
2.8
0.45
0.31
0.4
U.S. Stamp
4.8
0.81
0.57
2.0
Plastic Food Package
5.5
0.72
0.61
2.2
102
3.4
Conclusion
We have demonstrated top-illuminated organic photovoltaics, employing a vapor-printed
PEDOT polymer anode deposited by oCVD on top of a small-molecule organic
heterojunction based on vacuum-evaporated DBP as the electron donor and fullerene C60
as the electron acceptor. A MoO 3 buffer layer between the oCVD PEDOT top electrode
and DBP donor layer is shown to increase the device photocurrent nearly 10 times by
preventing oxidation of the underlying photoactive DBP electron donor layer during the
oCVD PEDOT deposition and results in power conversion efficiencies of up to 2.8% for
the top-illuminated, ITO-free devices, approximately 75% that of the conventional cell
architecture with indium tin oxide (ITO) transparent anode (3.7%). We also demonstrate
the broad applicability of this architecture by fabricating devices on a variety of opaque
substrates, including common paper products with ~2.0% power conversion efficiency.
By replacing the single-junction bilayer cell used here with tandem and bulk
heterojunction structures available today, we expect efficiencies in excess of 5% are
possible on opaque paper-based substrates without any pretreatment or special
manufacturing processes. This demonstrates the near-term potential for deploying organic
photovoltaics in the form of everyday opaque substrates, thus adding energy harvesting
functionality to otherwise passive products, such as wall and window coverings, product
packaging, documents, and apparel.
103
3.5
3.5.1
Experimental
Substrate preparation
Pre-cut glass substrates as well as pre-patterned ITO substrates (Thin Film Devices, 20
Q/o), for the conventional control devices, were cleaned by subsequent sonication in DI
water with detergent, DI water, acetone, and isopropyl alcohol, followed by 30 seconds
of 02 plasma (100 W, Plasma Preen, Inc.). Common opaque substrates [Figure 3-5] were
cut to size with scissors but used without any pretreatment or cleaning procedures: photo
paper (Office Depot, #394-925); magazine print (food network magazine, inner page);
U.S. First Class Stamp (2011 "Forever" stamp); and plastic food wrapper (Kellogg's@
Pop-TartsTM).
3.5.2
Device fabrication
The Ag cathode, organic active layers (BCP, C60 , and DBP), and MoO 3, were thermally
evaporated onto the substrate in sequence through shadow masks at a pressure of 1x 10-6
Torr at rates of -1.0 A/s. The C6 0 (Sigma Aldrich, 99.9%) and DBP (Luminescence
Technology Corp., >99.5%) were each purified once via thermal gradient sublimation
before use; the BCP (Luminescence Technology Corp., >99%), MoO 3 (Sigma Aldrich,
powder, 99.99%), and Ag (Alfa Aesar, 1-3 mm shot, 99.9999%) were all used as received.
PEDOT top electrodes were then deposited directly on top of the partially completed
inverted OPVs in a vacuum chamber using oCVD, which is described in detail elsewhere
[28, 40, 43]. Here, the oCVD PEDOT top electrodes were all synthesized during the same
deposition at a reactor pressure of -le-4 Torr and a substrate temperature of 150'C, via
simultaneous exposure to vapors of 3,4-ethylenedioxythiophene (EDOT) monomer
104
(Aldrich 97%) metered at -5 scem and FeCl 3 oxidant (Sigma Aldrich, 99.99%)
controllably evaporated from a resistively heated crucible at ~170 'C for ~20 min. No
post-treatment or solvent rinsing steps were used, as has been described previously [43].
The PEDOT electrodes were patterned in situ during the oCVD process by positioning
pre-cut metal shadow masks in intimate contact with the substrate, which were aligned by
hand with the pattern of the bottom device layers. The overlap area between the PEDOT
top anode and the Ag bottom cathode defined the device area (-0.012 cm2 ), which was
measured after testing with an optical microscope. The resulting device structures were
either Glass/ITO/MoO
3
(20 nm)/DBP (25 nm)/C6 0 (40 nm)/BCP (7.5
nm)/Ag
(conventional control orientation) and Substrate/Ag/BCP (7.5 nm)/C 6o (40 nm)/DBP (XX
nm)/MoO 3 (20 nm)/oCVD PEDOT (top-illuminated orientation).
3.5.3
Characterization
Current density-voltage (J-V) characteristics were measured for the completed OPV
devices in nitrogen atmosphere using a Keithley 6487 picoammeter. Devices were tested
using 110± 10 mW cm-2 illumination provided by a 1kW xenon arc-lamp (Newport
91191) with an AM 1.5G filter, and the solar simulator intensity was measured with a
calibrated silicon photodiode. The EQE spectra were measured with a Stanford Research
Systems SR830 lock-in amplifier, under a focused monochromatic beam of variable
wavelength light generated by an Oriel 1kW xenon arc lamp coupled to an Acton 300i
monochromator and chopped at 43 Hz. A Newport 818-UV calibrated silicon photodiode
was used to measure the incident monochromatic light intensity. Optical transmittance
measurements were made on the DBP and C60 films before and after FeCl 3 exposure
using a Varian Cary 6000i UV-Vis-NIR dual-beam spectrophotometer. PEDOT electrode
105
thicknesses were measured on bare glass slides (positioned next to the OPV devices
during oCVD deposition) with a Tencor P-16 profilometer, and the sheet resistance was
measured using a Signatone S-302-4 four-point probe station with a Keithley 4200-SCS
semiconductor characterization system.
3.6
Acknowledgments
This work was supported by Eni SpA under the Eni-MIT Alliance Solar Frontiers Center
and by a National Science Foundation Graduate Research Fellowship. This research was
also supported in part by the Department of Energy Office of Science Graduate
Fellowship Program (DOE SCGF), made possible in part by the American Recovery and
Reinvestment Act of 2009, administered by ORISE-ORAU under contract no. DE-AC05060R23 100.
3.7
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110
Chapter 4
Cathode Buffer Layers Based on
Vacuum and Solution Deposited
PEDOT for Efficient Inverted
Organic Photovoltaics*
*Originally prepared as M.C. Barr, C. Carbonera, R. Po, V. Bulovid, and K.K. Gleason. Cathode buffer
layers based on vacuum and solution deposited PEDOT for efficient inverted organic solar cells. Submitted
to Applied Physics Letters.
111
4.1
Abstract
Vacuum and solution processed versions of poly(3,4-ethylenedioxythiophene) (PEDOT)
are used as cathode interlayers in inverted organic photovoltaic cells comprising
tetraphenyldibenzoperiflanthene (DBP) as the electron donor and C60 as the electron
acceptor.
Chemical
treatment
of
tetrakis(dimethylamino)ethylene (TDAE)
the
as-deposited
PEDOT
layers
with
or cesium carbonate (Cs 2 CO 3 ) reduces the
work function by up to 0.8 eV. Inserting these PEDOT layers at the ITO cathode results
in improved electron collection and efficiencies of up to 2.3±0.2%, approaching the
3.2±0.3% of the conventional device. This illustrates the potential for efficient polymer
cathode materials and inverted device architectures compatible with either solution or
vacuum processing.
4.2
Introduction
Organic photovoltaic cells (OPVs) have received significant interest for their potential as
a low-cost energy source, and efficiencies have recently reached records nearing 10%
using both solution and vacuum deposition methods [1]. In the conventional orientation,
the OPV is illuminated through a transparent hole-collecting anode deposited on the
substrate, [e.g. indium tin oxide (ITO)], and electrons are collected by a low work
function metal cathode top contact. Development of efficient inverted device structures
(i.e. electrons collected by a transparent cathode) would: (i) provide more degrees of
freedom in designing OPV fabrication schemes, including tandem and semitransparent
devices [2-4]; (ii) allow for protection of the delicate organic semiconductor layers below
metal oxide anodic buffer layers (e.g. MoO 3 ) prior to subsequent top layer deposition
112
steps [5, 6]; and (iii) are proposed to be more stable by allowing use of higher work
function metal top contacts (e.g. Au or Ag) [7] and protecting the air-sensitive electron
acceptor layer (e.g. C60 ) [8]. However, due to the high work function of common
transparent conductors (e.g. ITO and conductive polymers) such structures require a
readily processable low work function (LWF) interfacial cathode buffer layer to provide
sufficient electric field through the device and allowing for ohmic contact with the
adjacent electron acceptor [4, 7, 9].
We explore the use of poly(3,4-ethylenedioxythiophene) (PEDOT) deposited by
both solution and vacuum deposition methods as a cathode buffer layer in
organic
solar
cells
on
ITO.
The
solution
deposited
version
inverted
doped
with
poly(styrenesulfonate) (PEDOT:PSS) has been widely used as a hole-transporting anode
buffer layer on ITO [9, 10], and high-conductivity analogues have been proposed for use
as the OPV anode [3, 11, 12] or cathode [3, 13]. Similarly, vacuum deposited PEDOT, in
which the polymer film is formed directly on the substrate by oxidative polymerization of
vapor phase precursors, has been explored for use as an anodic buffer layer [14, 15],
anode [14, 16, 17], and cathode [15]. However, there have been few reports of PEDOT as
a cathode buffer layer, due to its high work function [18]. It has previously been shown
that the organic reductant tetrakis(dimethylamino)ethylene (TDAE) lowers the work
function of PEDOT:PSS [19] and vapor-deposited PEDOT [20]; however, these layers
were not incorporated in an OPV device. Similarly, Cs 2 CO 3 has been demonstrated to
lower the work function of ITO electrodes by both vacuum thermal evaporation and
solution casting [5, 9, 21]. Here we incorporate PEDOT:PSS and vacuum deposited
PEDOT treated with TDAE or Cs 2 CO 3 as cathode interlayers in inverted OPVs on ITO,
113
which we demonstrate improves electron collection efficiency compared to devices with
no cathode buffer layer.
4.3
Experimental
PEDOT films were deposited onto ITO-coated glass substrates (Kintec Co., sheet
resistance = 15 a/sq) that were first solvent-cleaned and treated with 02-plasma. The
vapor deposited PEDOT was deposited in a vacuum chamber using the oxidative
chemical vapor deposition process (oCVD) [14, 22] at 0.1 Torr and a substrate
temperature of 80'C via simultaneous exposure to vapors of 3,4-ethylenedioxythiophene
monomer (Aldrich 97%, ~5 sccm) and sublimed FeCl 3 oxidant (Aldrich) for 2 minutes.
The PEDOT:PSS (CLEVIOS TM P VP Al 4083) was filtered (0.45 pm), spin-coated at
4000 rpm for 60 seconds, and annealed at 200 'C for 5 minutes in air. Resulting film
thicknesses were ~20 nm for oCVD PEDOT and -50 nm PEDOT:PSS, measured by
profilometer (Tencor P-16). Sets of both PEDOT films where then either (i) treated with
liquid TDAE in nitrogen atmosphere for 60 seconds before spinning dry at 5000 rpm and
transferring into vacuum or (ii) coated with -1 nm of Cs 2 CO 3 via vacuum thermal
evaporation. Work function measurements were performed in nitrogen atmosphere with a
Scanning Kelvin Probe (SKP5050, KP Technology Ltd.) using a 2 mm gold tip and were
calibrated relative to a gold reference surface assumed to be 5.1 eV, which was measured
before and after each sample and was stable over the course of the experiment. Average
and standard deviation values were calculated from a scan of 220 points recorded evenly
across a 1 cm2 surface. OPVs were then fabricated using a bilayer heterojunction
structure
of
C60
(Aldrich,
sublimed)
114
as
the
electron
acceptor
and
tetraphenyldibenzoperiflanthene (DBP, Lumtec) as the electron donor [23], with an
MoO 3 anode buffer layer and Ag top electrode. The DBP and C60 were first purified once
by thermal gradient sublimation, and then the OPV layers were sequentially deposited
onto the ITO substrates (with and without the various PEDOT buffer layers) by thermal
evaporation at <10-6 Torr and rates of 0.1 nm s-1. Shadow masks were used to define a 1
mm by 1.2 mm active device area. The resulting device structures were ITO, MoO 3 (20
nm), DBP (25 nm), C6 0 (40 nm), Ag (conventional orientation) and ITO, (PEDOT), C6 0
(40 nm), DBP (XX nm), MoO 3 (20 nm), Ag (inverted orientation). Current densityvoltage (J-V) characteristics were measured in nitrogen atmosphere under simulated
AMI.5G illumination from a 1kW xenon arc-lamp (Newport 91191). Optical interference
modeling was performed using transfer matrix formalism [24], with exciton diffusion
lengths LDC6 0=20 nm and
LDDBP=5-20
nm and optical constants measured by variable
angle spectroscopic ellipsometry (J. A. Woollam M-2000S).
4.4
4.4.1
Results and discussion
Low work function PEDOT
Figure 4-1(a) shows the change in work function measured for PEDOT:PSS and oCVD
PEDOT films on ITO before and after treatment with TDAE and Cs 2CO 3. The decrease
in work function upon treatment with liquid TDAE is ~0.6 eV with PEDOT:PSS (5.52±
0.05 eV to 4.97± 0.03 eV) and ~0.8 eV for the oCVD PEDOT (5.35± 0.04 eV to 4.53±
0.05 eV). The observed decreases in work function are consistent with the trends
observed previously for TDAE with PEDOT and ITO [19, 20, 25]. Previous reports for
oCVD PEDOT showed that lowering the substrate temperature used during the
115
deposition resulted in a 0.3 eV higher work function; thus, the change here suggests that
the oCVD PEDOT can be modified by over 1 eV through a combination of deposition
conditions and chemical treatment. The change in surface work function observed upon
evaporation of Cs 2 CO 3 was -0.3 eV for both the PEDOT:PSS (5.52± 0.05 eV to 5.16±
0.04 eV) and oCVD PEDOT (5.35± 0.04 eV to 5.03± 0.03 eV) films. We note that the
values observed for the as-deposited PEDOT:PSS and CVD PEDOT as much as -0.2 eV
higher than previously reports [22, 26], which is most likely explained by different
conditions and techniques used, as the kelvin probe and ultraviolet photoelectron
spectroscopy measurements are highly sensitive to environmental conditions [27].
(a)
(b)
4.4
*
*
(c)
TDAE
Metal Anode
4.8
.0
c
TOAE
5.
tron Donor
--
C32Co0
3-
Electron Acceptor
J 5.2
PEDOT:PSS
5.6IVPEDOT
Treatment
A
LM4.
Lm
+-Cathode Buffer
_ Transparent Cathod
LW PEDOT
5.4
5-6
3.5
ode Buffer
4.6
Glass
1.WF ITO
5.7
5.5
6.2
Figure 4-1: (a) Work function of both PEDOT:PSS and CVD PEDOT treated with
TDAE or Cs 2 CO 3 . (b) Schematic of the inverted device architecture with transparent ITO
cathode and low work function PEDOT buffer layer inserted between ITO/C6 0 interface.
(c) Flat band energy level diagram for the inverted device architecture. The top-right inset
shows the proposed electron-limiting Schottky-contact formed at an unbuffered ITO/C60
interface. Band energies are taken from the literature.
116
4.4.2
Inverted organic photovoltaics
The ability to decrease the work function of the various PEDOT surfaces motivates their
incorporation as cathode layers for electron collection in OPVs. Figure 4-1(b) shows the
schematic of inverted OPVs used here, in which the LWF PEDOT layer is inserted
between the transparent ITO electrode and the adjacent C60 electron acceptor. In the
inverted orientation, the ITO collects electrons and the Ag collects holes. Figure 4-1(c)
shows the flat band energy level diagram for the inverted device, with the proposed LWF
PEDOT layer raising the ITO cathode work function closer to the lowest unoccupied
energy level (LUMO) of the C60 layer and the MoO 3 layer raising the work function of
the Ag anode towards the highest occupied energy level (HOMO) of the DBP layer.
The J-V characteristic of devices with and without the LWF PEDOT buffer layers
are shown in Figure 4-2. Devices utilizing an unmodified ITO cathode and as-deposited
PEDOT cathode layers show a distinct "S" shape at reverse bias, suggesting the presence
of a Schottky junction at the cathode contact in opposition to the diode formed at the
DBP/C6o heterojunction interface, which would create a barrier to electron collection
[Figure 4-1(c), inset], which is evident in the J-V curve by the poor fill factor (FF) (<0.4)
and reduced open-circuit voltage (Voc) relative to the conventional device orientation
with identical layer thicknesses. Replacing the as-deposited PEDOT layers with the LWF
versions [Figure 4-2(b)] results in improved FF (>0.6) and Voc comparable to the
conventional device (0.9 V). This high Voc is supported by sufficient work function offset
between the high work function MoO 3 layer at the anode [28] and the LWF PEDOT layer
at the cathode.
117
(a)
E0
Conventional: -0Inverted: -0-
S-10 M-r-
ITO
ITO
ITO/CVD PEDOT
- ITO/PEDOT:PSS
0
) -15
64
E
5 -
E 0
C
Inverted w/ LWF
Cathode Buffer Layer:
-5-
CVD PEDOT/TDAE
-r-
,vr-CVD PEDOT/Cs CO
PEDOT:PSS/TDAE
L4)-10 -
-0- PEDOT:PSS/Cs 2CO,
-15
-1.0
-0.5
0.0
0.5
1.0
Voltage (V)
Figure 4-2: J-V curves under AMI.5G simulated solar illumination. (a) Comparison of
the conventional device orientation (ITO/MoO 3 20 nm/DBP 25 nm/C60 40 nm/Ag) (open
diamonds) with the same device stack inverted (ITO cathode) with no buffer layer on the
ITO (open squares), and with untreated CVD PEDOT (filled triangles) and PEDOT:PSS
(filled circles) on the ITO. (b) The same inverted device structure incorporating a LWF
PEDOT buffer layer on the ITO cathode: CVD PEDOT/TDAE (filled triangles), CVD
PEDOT/Cs 2 CO 3 (open triangles),
PEDOT:PSS/TDAE (filled circles), and
PEDOT:PSS/Cs 2 CO 3 (open circles).
118
4.4.3
Optimization and optical modeling
We then optimize this inverted device structure using the PEDOT:PSS/TDAE cathode
buffer layer by varying the DBP thickness [Figure 4-3], subsequently moving the position
of the reflective Ag interface relative to the DBP/C 60 interface. The structure is optimized
I
I
-o
I
I
I
1.0
0.8 8
4
0.6
-n
3
0.4
0
a-
2
1
0
I
-
I
I
.
I
.
I
.
I
.
I
5
4
20
"E
E
_,
3
10 Or
2
S t7
1
0
0
10
20
30
40
DBP Thickness (nm)
Figure 4-3: Power conversion efficiency, r, (filled triangles), open-circuit voltage (filled
circles), fill-factor (filled diamonds), and. short-circuit current (filled squares) as a
function of the DBP electron donor layer thickness for the inverted device with
PEDOT:PSS/TDAE buffer layer. The dashed lines show short-circuit current vs. DBP
thickness calculated from optical interference simulations using DBP exciton diffusion
lengths from 5-20 nm (as noted). The control cell in the conventional orientation
[ITO/MoO3 (20 nm)/DBP (25 nm)/C60 (40 nm)/Ag] is included as x=0 for reference
(open symbols). Solid lines are to guide the eyes.
119
at a DBP thickness of -10 nm, corresponding to short-circuit current (Jsc)=4.6 mA/cm 2
and maxima in Voc=0.89 V, FF=0.6, and power efficiency (%p)=
2 .5%.
The observed
optimum Jsc is consistent with simulations modeling the optical interference of light
inside the device with light reflected by the Ag anode [24] and correspond to a DBP
exciton diffusion length (LDDBP ) between 10 and 15 nm (Figure 4-3, dashed lines). For
this donor/acceptor pair, the optimal photocurrent is lower for the inverted structure than
the conventional structure, because the short wavelength absorber (C60 , peak <400 nm) is
positioned farther from the reflective node, while the long wavelength absorber (DBP,
peak -610 nm) is closer to the reflective node, creating a mismatch in the positioning of
the peak-wavelength optical electric field maxima within the respective layers. With bulk
heterojunction structures and donor/acceptor pairs with different peak absorption
characteristics, higher efficiencies should be possible with such inverted OPVs.
4.5
Conclusion
In conclusion, we have demonstrated the use of vacuum and solution deposited low work
function PEDOT cathode buffer layers. Incorporation of these materials at the cathode in
inverted DBP/C 60 heterojunction OPVs prevents the formation of an electron-limiting
Schottky junction at the cathode in opposition to the diode formed by the DBP/C6 0
heterojunction, thereby increasing FF and Voc relative to cells with an improperly or unbuffered cathode interface. We find an optimal efficiency of 2.5% using an inverted
DBP/C6o heterojunction structure. This illustrates the potential for efficient polymer
cathode materials and inverted device architectures compatible with solution and vacuum
processing.
120
4.6
Acknowledgments
This work was supported by Eni SpA under the Eni-MIT Alliance Solar Frontiers
Program and by a National Science Foundation Graduate Research Fellowship.
4.7
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S.-I. Na, S.-S. Kim, J. Jo and D.-Y. Kim. Efficient and flexible ITO-free organic
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Y.H. Kim, C. Sachse, M.L. Machala, C. May, L. Mueller-Meskamp and K. Leo.
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F. Nickel, A. Puetz, M. Reinhard, H. Do, C. Kayser, A. Colsmann and U. Lemmer.
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M.C. Barr, J.A. Rowehl, R.R. Lunt, J. Xu, A. Wang, C.M. Boyce, S.G. Im, V.
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123
124
Chapter 5
Unsubstituted Polythiophene
Photoactive Layers via oCVD
for Polymer Photovoltaics*
*Originally published as D.C. Borrellit, M.C. Barr', V. Bulovid, and K.K. Gleason. Bilayer heterojunction
polymer solar cell using unsubstituted polythiophene via oxidative chemical vapor deposition. Solar
Energy Materialsand Solar Cells, 99, 190-196, 2012.
(tAuthors contributed equally.)
125
5.1
Abstract
We demonstrate the use of a vacuum-based, vapor phase technique for the deposition of a
donor polymer for use in polymer solar cells. Unsubstituted polythiophene (PT), which is
insoluble and infusible and thus typically difficult to process, is easily prepared by
oxidative chemical vapor deposition (oCVD). The oCVD process results in a conductive
PT film that is heavily doped with FeCl 3 , which is used as the oxidizing agent. A postdeposition methanol rinse sufficiently dedopes the film and removes spent oxidant,
leaving semiconducting PT with an optical bandgap close to 2 eV. Drastic changes in the
film color, absorption spectra, and film composition confirm the dedoping process. The
resulting semiconducting PT is then applied as an electron donor in bilayer heterojunction
solar cells with a thermally evaporated C60 electron acceptor layer, resulting in power
conversion efficiencies up to 0.8%. The absorption edge of the PT at ~620 nm closely
matches the edge present in the external quantum efficiency spectra, indicating that the
oCVD PT contributes to the photocurrent of the devices. This demonstrates that the
oCVD technique can be used in the processing and design of polymer active layers for
polymer solar cells and hybrid small molecular organic solar cells without solubility,
temperature, or substrate considerations.
5.2
Introduction
Semiconducting polymers and low molecular weight organic molecules have received
significant attention for their application as active layers in organic solar cells, due to
their potential low cost, high mechanical flexibility, wide array of functionalities, and
well-understood structure-composition-property
126
relationships [1-4].
In fact, both
solution-printed polymer solar cells and vacuum-deposited small molecule organic solar
cells have independently reached record certified efficiencies of 8.3% [5] through careful
materials selection and device architecture engineering.
Vapor-deposited polymer solar cells would enhance the ability to integrate
attractive materials into organic solar cells. For example, unsubstituted polymers, which
are reported to be more stable because their highly compact structures prevent oxygen
permeation into the polymer bulk [6, 7], are also insoluble and infusible because of their
compact structures. Furthermore, the vacuum fabrication of multilayered devices is not
constrained by the requirement of finding solvents that will not dissolve the underlying
layers to prevent mixing between layers during deposition. However, the high
temperatures necessary to physically deposit polymers by vacuum thermal evaporation
leads to polymer degradation [8], limiting materials to low-molecular-weight organics.
The few reports in the literature of the use of a vapor deposition technique (such as
physical deposition [9], plasma polymerization [10], and thermal chemical vapor
deposition [11]) to deposit a polymer photoactive layer resulted in low corresponding
device efficiencies (<0.3%). Thus a soluble derivative (e.g. poly(3-hexylthiophene)) or an
oligomeric version are typically used to facilitate processing by standard solution printing
or vacuum thermal evaporation. This leaves a largely unexplored domain of materials for
use in organic solar cells [Figure 5-1]. This work demonstrates an alternative vacuum
fabrication method for the utilization of this region of polymers that is independent of
solubility properties.
Oxidative chemical vapor deposition (oCVD) offers a facile route to processing
conjugated polymers (including insoluble polymers) via vacuum deposition, offering a
127
potential link between many of the above considerations. In oCVD, conjugated polymers
are simultaneously synthesized from vapor phase precursors (monomer and oxidant) and
deposited on the substrate at low temperature (25-100'C) and moderate vacuum (~0.1
Torr) [12, 13]. Thus, oCVD offers the well-cited processing benefits of vacuum
processing, including parallel and sequential deposition, well-defined thickness control
and uniformity, and inline integration with other standard vacuum processes (e.g.,
vacuum thermal
100
E
-cM
104
103
0
102
101
0.001
0.01
0.1
1
10
100
Solubility (mg/mL)
Figure 5-1: Properties of materials deposited by various techniques for use in organic
solar cells. Traditionally, the use of polymers is limited to those that can be dissolved and
therefore can be deposited by a solution-based technique. Vapor deposition is usually
limited to molecules with low enough molecular weight to be thermally evaporated. This
leaves a region of vapor-deposited polymers that is difficult to access by traditional
methods.
128
evaporation). Moreover, oCVD is conformal over nonplanar substrates, enabling
compatibility with substrates such as paper and textiles [13]. In contrast, vacuum thermal
evaporation is generally subject to line-of-sight deposition, while conformal deposition of
liquid-phase systems is complicated by surface tension effects around micro- and nanoscale features [14]. oCVD has previously been used to conformally deposit thin films of
doped conducting polymers, such as poly(3,4-ethylenedioxythiophene) (PEDOT) [12],
which have recently been incorporated as transparent electrodes in small molecule
organic solar cells on a variety of substrates, including unmodified paper [13].
Unsubstituted polythiophene (PT) has been prepared by several techniques in the
literature, including electropolymerization [15, 16], chemical polymerization [17, 18],
thermo-cleavage of solubilizing side chains [7, 19], and various types of vapor deposition
techniques such as plasma polymerization [20, 21] and others [22-25]. However, largely
because of difficulties in processing PT due to its insolubility or harsh deposition
conditions, PT has only been reported in polymer solar cells via electropolymerization
[26, 27] and thermo-cleavage of a solution-processable alkylated polymer precursor [7,
28-30].
Here we report the preparation, characterization, and application of unsubstituted
PT by oCVD for use as a photoactive semiconductor in organic solar cells. We
characterize as-deposited and methanol-rinsed oCVD PT films to confirm polymer
dedoping upon post-processing with methanol. The resulting semiconducting PT is then
applied as an electron donor in bilayer heterojunction solar cells with a thermally
evaporated C60 electron acceptor layer, resulting in power conversion efficiencies up to
0.8%. This demonstrates that the oCVD technique can be used in the processing and
129
design of polymer active layers for polymer solar cells and hybrid small molecule organic
solar cells without solubility, temperature, or substrate considerations.
5.3
5.3.1
Experimental
Polythiophene depositions
The polymer deposition procedure using the oCVD process and the custom-built reactor
configuration are described in detail elsewhere [31, 32]. Briefly, the oCVD reactor
consists of a vacuum chamber with monomer inlet ports and an exhaust to a pump. A
heated crucible holding the oxidizing agent is in the bottom of the chamber, and directly
above it is an inverted stage for the substrate. The stage and reactor body were
maintained at 30'C and 45'C, respectively. The chamber pressure was held constant at
150 mTorr using a butterfly valve. Iron(III) chloride (FeCl 3, 97%, Sigma-Aldrich) and
thiophene (>99%, Sigma-Aldrich) were used as purchased. Quartz slides, silicon wafers,
and ITO-coated glass were used as substrates. FeCl 3 was used at the oxidizing agent and
it was sublimed at 340'C. Polymer film thickness was controlled by varying the amount
of FeCl 3 loaded in the crucible. Vapor phase thiophene monomer was introduced into the
reactor from a side port on the reactor. The thiophene monomer jar was maintained at a
temperature of 25'C and a needle valve was used to limit the flow rate to about 1 sccm. A
deposition time of 20 minutes was used for all films. After deposition, the films were
rinsed in methanol (>99.9%, Sigma-Aldrich) for 2 minutes to remove reacted oxidant.
130
5.3.2
Polymer characterization
UV-vis spectra of the studied films on quartz substrates were measured with a Varian
Cary 5000 UV-vis spectrophotometer. Transmission and reflection spectra were
measured. The reflection spectra were obtained using a specular reflectance accessory
and an Al standard reference mirror (ThorLabs). Fourier transform infrared (FTIR)
measurements of PT films on silicon wafers were performed on a Nexus 870, Thermo
Electron Corp. spectrometer. Film compositions were estimated by XPS using a Surface
Science Instruments (SSI) model SSX-100 with operating pressure <2x10~9 Torr utilizing
monochromatic AlKa X-rays at 1486.6 eV. Photoelectrons were collected at an angle of
55-degrees from the surface normal. Film thicknesses were measured using a Veeco
Dektak
150 surface profilometer. PT film thicknesses used for devices were
approximated by measuring the thickness of PT on glass slides placed close to the ITO
substrates from the same deposition. The sheet resistance of the as-deposited PT films
were measured with a Jandel four-point probe in air. Conductivity values were calculated
using the measured sheet resistivity and thickness measured with the profilometer. Cyclic
voltammetry measurements were conducted using a 660D potentiostat (CH Insturments)
with a standard three-electrode configuration under a nitrogen atmosphere. The oCVD PT
film on ITO/glass was the working electrode, Ag/AgNO 3 (0.01 M in acetonitrile) was the
reference electrode, and a platinum mesh attached to a platinum wire was used as the
counter
electrode.
The
measurements
were
performed
in
acetonitrile
with
tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte at a scan
rate of 100 mV s-. The Fc/Fc+ redox couple was used to calibrate the Ag/Ag+ reference
electrode.
131
5.3.3
Device fabrication and characterization
The OPVs were fabricated on glass substrates that were precoated with a 150 nm thick,
patterned indium tin oxide (ITO) transparent anode with 15
/o sheet resistance (Kintec
Co.). Prior to use, the substrates were successively cleaned by ultrasonic treatment in
detergent solution (Micro 90), 2 x de-ionized water, 2 x acetone and 2 x isopropanol for 5
minutes each. The substrates were then treated with 02 plasma for 30 seconds. A PT film
of varying thickness was deposited onto the cleaned ITO via oCVD as described above.
Samples were exposed to air for approximately 10 minutes in transferring them to a
glovebox. C60 (99.9%, sublimed, Sigma-Aldrich) was purified once by vacuum train
sublimation prior to loading, while bathocuproine (BCP, from Luminescence Technology
Corp.) and Ag (Alfa Aesar, 1-3 mm shot, 99.9999%) were used as purchased. C60 , BCP
(8 nm), and a 100 nm thick Ag cathode were sequentially deposited via thermal
evaporation at a rate of 0.1 nm/s. The cathode films were deposited through a shadow
mask for single devices, defining a 1 mm x 1.2 mm active device area, and there were 10
devices per substrate. The current-density-voltage (J-V) measurements were recorded by
a Keithley 6487 picoammeter and 100 ± 10 mW cm-2 illumination was provided by a
nitrogen-glovebox-integrated 1kW xenon arc-lamp (Newport 91191) equipped with an
AM1.5G filter. The solar simulator intensity was measured with a calibrated silicon
photodiode. The external quantum efficiency (EQE) spectra were measured with a
Stanford Research Systems SR830 lock-in amplifier, under a focused monochromatic
beam of variable wavelength light generated by an Oriel 1kW xenon arc lamp coupled to
an Acton 300i monochromator and chopped at 43 Hz. A Newport 818-UV calibrated
silicon photodiode was used to measure the incident monochromatic light intensity.
132
5.4
Results and discussion
5.4.1
PT synthesis
Following
a
procedure
similar
to
that
for
the
deposition
of
poly(3,4-
ethylenedioxythiophene) (PEDOT) by oCVD, thiophene monomer vapor was fed into a
reaction chamber [31]. Thiophene reacted via oxidative polymerization with sublimated
iron(III) chloride on a substrate to result in the deposition of a solid, polymer film [Figure
5-2(1)]. The generally accepted mechanism for the oxidative polymerization of thiophene
involves the formation of radical cations [1]. Further oxidation results in the formation of
polarons and bipolarons, making the polymer conductive [33]. The cations are charge
balanced by counter anions [Figure 5-2(2)]. The resulting polymer film from the oCVD
process is a conductive, blue film [Figure 5-3]. This suggests that FeCl3 is present in
large enough concentrations during the oCVD process to over-oxidize the PT, as has been
seen during chemical polymerization in solution. Based on conductivity measurements in
air, the doped PT film conductivities ranged from 10 to 20 S cm-1.
A post-deposition rinse treatment of the films with methanol caused them to
become nonconductive and undergo an abrupt color change to red [Figure 5-3]. The
conductivities of the methanol-rinsed films were below the detection limit of the
equipment used (<10-4 S cm-1). The observed changes in conductivity and color of the
films suggest that the methanol rinse dedoped the PT [Figure 5-2(3)].
133
S
(1)
FeCI 3
(2) FeCl 3
CH3 0H
A
(3)
A
Figure 5-2: Processes (1) and (2) occur during the oCVD deposition process, while
process (3) is a post-deposition step. (1) Oxidative polymerization of thiophene to
polythiophene. (2) Oxidation of the polymer chain leads to the formation of polarons and
bipolarons (shown), which are charge balanced by counteranion dopants. (3) Rinsing the
deposited film with methanol reduces it back to neutral PT.
Figure 5-3: As-deposited oCVD PT film (left) and methanol-rinsed film (right)
uniformly deposited on 25 x 75 mm glass slides. The blue PT film is doped with FeCl 3
and has a conductivity between 10 and 20 S cm~1, whereas the red film is neutral PT and
nonconductive.
134
Composition measurements from X-ray photoelectron spectroscopy (XPS) survey
scans show that the methanol rinse significantly reduces the amount of iron and chlorine
in the PT film [Table 5-1]. It is possible that the dedoping mechanism follows a similar
mechanism as that for other reactions involving the oxidation of primary alcohols with
strong electrophiles [34]. Additionally, the high solubility of iron in methanol results in
rapid removal of most of the reacted oxidant.
Table 5-1: Atomic ratios in PT films from XPS survey scans after various methanol
rinsing times.
5.4.2
Rinsing time (min)
C:S
Fe:S
Cl:S
0
16.0
1.6
2.8
2
6.1
0.05
0.07
60
5.8
0.05
0.07
240
5.7
005
0.07
UV-vis and fourier transform infrared (FTIR) spectroscopy
The UV-vis absorption spectra of the as-deposited and methanol-rinsed PT films are
shown in Figure 5-4. The presence of midgap energy states in the as-deposited PT film is
indicative of the presence of polarons or bipolarons [35]. The energy levels of the peaks
(0.8 and 1.6 eV) suggest that the conductive PT film is heavily doped and contains
bipolarons [35, 36]. The maximum of the absorption coefficient of the methanol-rinsed
PT occurs at 495 nm (-2.5 eV). The optical band gap, taken as the intersection of the line
tangent to the band edge with the x-axis, is 1.96 eV.
These values match those of
electrochemically and chemically polymerized neutral PT [36, 37]. This further supports
the hypothesis that the methanol rinse reduces the PT film.
135
1.80
-
1.60
-
1.40
-
1.20 -
P.I
100
-
0.80
-
e
0.60
-
0.40
-
0.20
-
0.00
0
1
2
3
4
Energy (eV)
Figure 5-4: Absorption coefficient of doped (- -) and dedoped (-) oCVD PT films on
quartz. The energy levels of midgap peaks in the doped film suggest that it is heavily
doped, resulting in bipolarons in the film.
The FTIR spectra of the as-deposited and methanol-rinsed films are shown in
Figure 5-5 along with a reference spectra for neutral polythiophene [38]. The MeOHrinsed spectrum matches the neutral reference spectrum closely, suggesting that neutral
(dedoped) PT is indeed formed. All spectra show peaks at around 790 cm-1, attributed to
C-H out-of-plane vibration for 2,5-substituted thiophene, along with peaks at 1450 and
1490 cm- due to 2,5-substituted thiophene ring stretching, and a C-H stretching peak at
3060 cm- [39, 40]. To elucidate the relative strength of doping-induced absorption peaks
in the as-deposited PT sample, all spectra were normalized by the C-H vibrational peak at
790 cm-1, as its intensity has previously been reported to be independent of doping effects
[39]. The as-deposited PT film shows strong peaks throughout the spectral range that are
not present in the spectra for the neutral reference or MeOH-treated oCVD PT film. It has
previously been reported for oCVD PEDOT films that the presence of broad and strong
136
PTDoping
PT-Cl
FeC13 Oxidant
7*
C
oCVD PT: as-deposited
oCVD PT. MeOH-rinsed
PT Reference. neutral
3600 3400 3200 3000
1700 1500
1100
1300
Wavenumber (cm
900
700
500
1)
Figure 5-5: FTIR spectra of oCVD PT film before and after MeOH rinse. The bottom
spectrum is a reference spectrum for neutral PT [38]. All spectra are normalized by the CH vibrational peak at 790 cm-1, as indicated by the asterisk.
absorption peaks in the 1400-700 cm- 1 range films are indicative of doping of the
conjugated polymer chain [31], as are evident in the as-deposited PT film. The peaks
indicated with arrows at 1320, 1200, 1190, and 1020 cm' match closely with the dopinginduced peaks observed in electrochemically prepared PT, independent of the dopant
species [39]. The broad peak below 700 cm-1 is ascribed to Cl-specific dopant
interactions with the thiophene ring, as observed for plasma-polymerized thiophene
doped with Cl [41]. Finally, the sharp peak at 1600 cm-1 and the characteristic -OH peak
at 3500-3300 cm
1
are indicative of atmospheric water interactions with residual iron
chloride oxidant in the PT film, which is known to be strongly hydroscopic. Both of these
peaks have been observed in FeCl 3-doped poly(phenylacetylene) [42], and are also
strongly evident in the FeCl 3 and FeCl 2 spectra themselves [43]. These doping- and
oxidant-related peaks are removed for the MeOH-rinsed film, which agrees with the
137
neutral PT reference spectrum, supporting the claim that methanol post-treatment
removes the oxidant residue and dedopes the as-deposited oCVD PT film.
5.4.3
Electrochemical properties
Cyclic voltammetry was used to study the electrochemical properties of oCVD PT. PT
films were deposited onto ITO-coated glass and rinsed in methanol to use as the working
electrode. Ferrocene/ferrocenium (Fc/Fc) was used as an external standard. The halfwave potential (Eu2) of the Fc/Fc
couple was measured under the same testing
conditions to be 0.096 V to the Ag/Ag* electrode. The cyclic voltammogram of the PT
film is shown in Figure 5-6.
2.0
S
1.5
-
1.0
-
0.5
-
0.0
-
a -0.5
-1.0
-1.5
-2.0
-2
-1
0
1
Potential (V vs Fc/Fc*)
Figure 5-6: Cyclic voltammogram of the oCVD PT film deposited onto ITO-coated glass
in an acetonitrile solution of Bu 4NPF 6 (0.1 M) at a scan rate of 100 mV s-1. The Ag/Age
reference electrode was calibrated using the Fc/Fc redox couple.
138
The pre-peaks that appear before the peaks for both the n-doping and p-doping processes
are due to charge trapping [41, 42]. This phenomenon is often seen during consecutive pand n-doping cycles of conducting polymers [43, 44]. The onset of the oxidation and
reduction peaks were estimated as being 0.36 V and -2.09 V vs Fc/Fc, respectively. The
energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) were calculated according to the following equations [45]:
EHOMO
= -
E LUMO =
-
E
onset,ox vs Fc/Fc+
+ 5.1) (eV)
(5.1)
E
onset,red vs Fc/Fc*
+ 5. 1) (eV)
(5.2)
These equations assume that the redox potential of Fc/Fc has an absolute energy
level of -5.1 eV relative to vacuum, although several other values have been used in the
literature [45]. The calculated HOMO and LUMO levels are -5.46 eV and -3.01 eV,
respectively. The electrochemical band gap is 2.45 eV.
5.4.4
Photovoltaic device performance
Dedoped PT was then prepared on patterned ITO-coated glass substrates for
incorporation as the electron donor layer in bilayer heterojunction photovoltaic cells. The
PV devices were completed by vacuum thermal evaporation of fullerene C60 as the
electron acceptor, bathocuproine (BCP) as an exciton blocking layer, and silver (Ag) as
the cathode. The resulting device structures were: ITO/PT/C 60 /BCP (8 nm)/Ag (100 nm).
First, the thickness of the C60 layer was optimized by varying its value and using a
PT layer thickness of ~30 nm. Representative current-density-voltage (J-V) curves
obtained under one sun of air mass 1.5G (AM 1.5G) irradiation (100 mW cm 2) are
139
shown in Figure 5-7(a). The fill factor (FF) remained relatively constant with variation in
C 60
thickness, whereas the open circuit voltage (Voc), short-circuit current (Jsc), and power
conversion efficiency (PCE) achieve a maximum at around 30 nm of C60 [Figure 5-7(b)].
3I
(2)
1 1
1'
..- I
1
0
-1
_3
'
-0.1
-
0.1
-
1
-
I
0.0
0.2
11 -0-3
0.4
0.5
0.6
Voltage (V)
(b)
1i2.5
0.6
0.5
2
0
-.4
0.4
1.5
0.3
El
0.2
1
0.7
0.8
0.6
0.6
0
4
0.5
0.4
0.2
0.3
0
0
10
20
30
40
*
0.4
50
I
0
10
20
I
30
40
50
C60 Thickness (nm)
Figure 5-7: (a) J- V characteristics of devices with structure ITO/PT (-3Onm)/C 6 0 /BCP (8
nm)/Ag under 100 mW cm-2 AMI.5G simulated solar illumination. (b) Performance
characteristics of the above devices. Markers and error bars correspond to the average
and maximum and minimum values obtained. An efficiency maximum is achieved for a
30 nm-thick C60 layer.
140
The change in short-circuit current with increasing C60 thickness is expected due
to the changes in optical interference patterns within the thin multilayer device stack, as
the position of the reflective Ag interface is moved farther from the PT/C60 interface [46].
The optical electric field is expected to be maximized for shorter wavelengths (e.g., C60
absorption peak) closer to the reflective node and for longer wavelengths (e.g., PT
absorption peak) farther from the reflective node, which should vary the relative amount
of photocurrent originating from excitons generated in the C60 and PT layers, respectively.
This effect is evident in Figure 5-8, which shows the variation in the external quantum
efficiency (EQE) as the thickness of the C60 is changed.
35
6
- -- 40nm
30
30
~_30n
0
-
0o
----
-25
--
0
300
00
10
00
0
00
-
300
400
500O
0
80
-0
600
700
800
Wavelength (nm)
Figure 5-8: The thin lines show the EQE spectra (left axis) of the devices in Figure 6 in
which the C60 thickness is varied. The bold lines show the absorption coefficients of C60
(- -) and oCVD PT (-) (right axis). The absorption edge past 600 nm in the EQE
suggests that the oCVD PT is functioning as a photoactive layer.
141
The bold lines show the absorption coefficients of C6 0 and oCVD PT. Any EQE
past about 550 nm should be due to excitons generated in the PT layer and EQE below
400 nm primarily due to C60 excitons. As the C60 becomes thicker, the shoulder in the
EQE curve around 600 nm becomes larger, likely due to additional excitons generated in
the PT as the optical field maxima for longer wavelengths are positioned within an
exciton diffusion length of the heterojunction interface. In contrast, the EQE at short
wavelengths near the C60 absorption peaks decrease as the C60 thickness is increased.
This is likely due to loss of excitons that are generated too far from the heterojunction
interface to diffuse and separate before recombining, as the optical field maxima for the
shorter wavelengths are positioned deeper into the C60 layer. These observations suggest
that both PT and C6 o are contributing to the device photocurrent, which is balanced at
around 30 nm of C6 0 . Lastly, the Jsc values calculated by integrating the product of the
EQE and the AM1.5G solar spectrum are 1.7, 2.0, 2.5, and 2.4 mA/cm 2 for 10, 20, 30,
and 40 nm of C60 , respectively. These values are in close agreement with the Jsc values
shown in Figure 5-7.
Devices were then fabricated with a fixed C60 thickness of 30 nm and a varying
PT thickness. Representative J- V curves for these devices under AM 1.5G (100 mW cm-2)
are shown in Figure 5-9 (a). The Js for the devices remained constant, but the FF
decreased with increasing PT thickness [Figure 5-9(b)]. The decrease in fill factor is most
likely explained by an increase in series resistance through the device with thicker PT
layers, which was generally observed to manifest as a lower slope in the J-V curves at
positive bias above Vc. Additionally, for devices with a PT layer thicker than about 35
nm, there is much more variability in the values of Vc. A maximum PCE of 0.8% was
142
3
(a)
E
1
0
-1
-3 '-0.1
0.5
0.3
0.1
Voltage (V)
(b)
2.4
0.6
4
0.5
2.2
+
0.4
I
0.3
'
I
2
1.8
1
0.8
0.6
f
0.5
0.4
0.6
'4
0.4
4
I
0
'
'
I
I
i
S10
0.2
I
10 20 30 40 50 60
0
0
10 20 30 40 50 60
PT thickness (nm)
Figure 5-9: (a) J-V characteristics of devices with structure with structure ITO/PT/C 60
(30 nm)/BCP (8 nm)/Ag. (b) Performance characteristics of the above devices. Markers
and error bars correspond to the average and maximum and minimum values obtained. A
maximum efficiency of 0.8% was obtained for a 25 nm PT layer with 30 nm of C60 .
achieved using about 25 nm of PT and 30 nm of C60 . This is the highest efficiency
achieved to date for the use of a vapor-phase deposition of the donor polymer for a
polymer solar cell. Furthermore, despite using a bilayer structure, this efficiency is also
comparable to bulk heterojunction devices made with PT and similar acceptor materials
143
[Table 5-2]. Higher efficiencies should be possible using bulk heterojunction device
structures instead of bilayer structures and with the use of different acceptor materials.
Table 5-2: Summary of device structures and performance that use PT as the donor
material. The devices using oCVD PT provide comparable or better performance
compared to other PT deposition methods.
Deposition Method
Device Structure
Acceptor
PCE (%)
Source
oCVD
Bilayer
C60
0.8
This work
Solution processing/
thermocleavage
Bulk heterojunction
(60)PCBM
0.6, 0.84
[7], [28]
Solution processing/
thermocleavage
Bulk heterojunction
(70)PCBM
1.5
[7]
Electropolymerization
Bilayer
(60)PCBM
0.1
[27]
Electropolymerization
Single layer
(Schottky device)
-
0.02
[26]
5.5
Conclusion
In conclusion, we have used oCVD to obtain unsubstituted polythiophene. A doped,
conductive form of the polymer is deposited during the oCVD process. Rinsing the film
with methanol is sufficient to dedope the PT to obtain the semiconducting form, as
confirmed by UV-vis, FTIR, and XPS. By directly depositing onto ITO substrates, the
neutral
PT was
successfully incorporated
into efficient bilayer
heterojunction
photovoltaic devices with C60 . The external quantum efficiency spectra demonstrate that
the oCVD PT contributes to the photocurrent generation of the devices, which is
successfully balanced with photocurrent from C6 0 through variation in the layer
thicknesses.
144
It is expected that device fabrication using oCVD active layers will be directly
compatible with other substrates, including those that are rough, lack the ability to
withstand high temperature, and/or degrade upon exposure to solvents. This technique
can easily be extended to the deposition of other semiconducting polymers by changing
the monomer used. By utilizing oCVD, the selection of the monomer is no longer
constrained by the requirement that the resulting conjugated polymer must be soluble or
stable at high temperatures for thermal evaporation. Thus, this opens up a range of
materials with potentially desirable properties that can be considered for an active layer
material with the goals of improving device efficiency and stability. Additionally, with
the use of different monomers to deposit polymers with different bandgaps, oCVD can
provide another route for the fabrication of tandem polymer solar cells capable of energy
conversion across the solar spectrum. Thus, oCVD is a viable technique that can combine
the benefits of vacuum processing and the use of semiconducting polymers for
fabricating organic photovoltaics.
5.6
Acknowledgments
This work was supported by Eni SpA under the Eni-MIT Solar Frontiers Center and by
National Science Foundation Graduate Research Fellowships. The authors thank Jill A.
Rowehl for insightful discussions. This work made use of the X-ray photoelectron
spectrometer of the Cornell Center for Materials Research (CCMR) with support from the
National Science Foundation Materials Research Science and Engineering Centers
(MRSEC) program (DMR 1120296).
145
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149
prepared
150
Chapter 6
Conclusion
151
6.1
Summary
The broad goal of this thesis was to explore CVD for deposition of polymer thin films for
application in organic photovoltaics. In Chapter 2, we first found oCVD to be suitable for
depositing transparent bottom electrodes and charge-transport buffer layers based on
poly(3,4-ethylenedioxythiopene).
The
oCVD
PEDOT
performed
superiorly
to
PEDOT:PSS hole-transport buffer layers on ITO transparent electrodes by minimizing
series resistance, which we also applied to transparent graphene electrodes in Appendix C.
We then used the oCVD PEDOT to replace the conventional ITO anode in CuPc/C 6o and
CuPc/PTCBI bilayer heterojunction OPVs. The OPVs based on oCVD PEDOT
electrodes devices performed around 90% that of the same device structure on ITO
electrode, and were found to be limited by tradeoffs between sheet resistance and
transparency for the oCVD PEDOT films. We then showed the oCVD-based OPVs to be
applicable to a variety of substrates, including common plastics and papers. The vaporprinted electrodes conform to the geometry of rough substrates, and were found be
mechanically robust under repeated flexing, folding, and stretching, while traditional ITO
electrodes are brittle and show significant cracking.
In the second part of Chapter 2, we used in situ shadow masking to vapor-print
and pattern the PEDOT electrodes over large areas to monolithically fabricate OPV
circuits directly on paper substrates. We demonstrated module integration of these
circuits by designing them to produce high-voltages (>50V) under 1 sun solar
illumination, and thus power common electronic displays in ambient indoor lighting. The
performance of these bottom-illuminated paper PVs were shown to be limited by the
optical transmittance of the highly reflective paper substrate as well as electrical shunting
152
through the non-conformally evaporated active layer materials in areas of high substrate
roughness. Additionally, thin film encapsulation layers based on CVD Parylene-C, iCVD
poly(1H,1H,2H,2H-perfluorodecyl acrylate) were found to extend device lifetime, even
allowing for operation while submerged in water, but produce no substantial change in
weight, feel, or appearance of the paper circuits. Video demonstrations were provided to
demonstrate that the paper PV arrays can be tortuously flexed and folded without loss of
function.
In Chapter 3, we found the oCVD PEDOT electrode to be applicable as a topelectrode in optically inverted devices, which are illuminated from above and thus
compatible with non-transparent substrates (e.g., paper or metal foils). We also
introduced
a more
absorptive
active
layer
set comprising vacuum-evaporated
tetraphenyldibenzoperiflanthene (DBP) as the electron donor and fullerene C60 as the
electron acceptor. Application of a molybdenum trioxide (MoO 3 ) buffer layer prior to
oCVD deposition increased the device photocurrent nearly 10 times by preventing
oxidation of the underlying photoactive DBP electron donor layer during the oCVD
PEDOT deposition. The resulting power conversion efficiencies were up to 2.8% for the
top-illuminated, ITO-free devices, approximately 75% that of the conventional, bottomilluminated, cell architecture with ITO transparent anode (3.7%). Finally, we fabricated
the top-illuminated oCVD devices on a variety of opaque surfaces, including common
paper products with over 2.0% power conversion efficiency, the highest to date on such
substrates.
In Chapter 4, we then explored the use of CVD PEDOT and PEDOT:PSS as
cathode interlayers in electrically inverted DBP/C6 0 OPVs. Chemical treatment of the as-
153
deposited PEDOT layers with tetrakis(dimethylamino)ethylene (TDAE)
or cesium
carbonate (Cs 2 CO 3 ) reduced the work function by up to 0.8 eV. Inserting these PEDOT
layers at the ITO cathode resulted in improved electron collection and efficiencies of up
to 2.3%, approaching the 3.2% of the conventional device. We found that the low-work
function PEDOT layers prevented the formation of an electron-limiting Schottky contact
at the transparent ITO cathode interface in opposition to the DBP/C 60 heterojunction. The
optimized photocurrent was found to be lower in the inverted structure due the reversed
positioning of the short and long wavelength absorbers (C6 0 and DBP, respectively)
relative to the reflective Ag anode, thus creating a mismatch in peak-wavelength optical
electric field maxima within the respective layers, which we observed experimentally and
by modeling the optical interference patterns within the device.
Finally, in Chapter 5 we extended the oCVD process to a photoactive
semiconductor layer, unsubstituted polythiophene (PT), which is insoluble and infusible
and thus typically difficult to process. We found that oCVD polymerization of the
thiophene monomer resulted in a conductive PT film heavily doped with FeCl 3, which
was used as the oxidizing agent. A post-deposition methanol rinse sufficiently dedoped
the film, leaving semiconducting PT with an optical band gap close to 2 eV. The resulting
semiconducting PT was applied as an electron donor in bilayer heterojunction solar cells
with a thermally evaporated C60 electron acceptor layer, resulting in power conversion
efficiencies up to 0.8%. The absorption edge of the PT at ~620 nm closely matched the
edge present in the external quantum efficiency spectra, indicating that the oCVD PT
indeed contributes to the photocurrent of the devices.
154
6.2
Implications
We have demonstrated CVD to be a simple and versatile technique to deposit a variety
polymer device layers for organic photovoltaics, including transparent electrodes, chargetransport layers, photoactive semiconductors, and encapsulating films. This all-dry
fabrication strategy should enable the design and implementation of polymer thin films
without solvent, temperature, or substrate limitations. Moreover, the CVD process is
amendable to low-cost, roll-to-roll processing with flexible substrates and inline
compatibility with other well-established physical vapor deposition steps (e.g., vacuum
thermal evaporation, and sputter coating).
By changing the monomer used, the oCVD technique can easily be extended to
the deposition of other conjugated polymers than those considered herein. Thus, selection
of monomers is no longer constrained by the requirement that the resulting conjugated
polymer must be soluble or stable at high temperatures for thermal evaporation. Thus,
this opens up a range of materials with potentially desirable properties that can be
considered as OPV device layers with the goals of improving device efficiency and
stability.
Finally, as we have demonstrated here, CVD fabrication enables design of OPVs
for nearly any surface, including those that are flexible, lightweight, fibrous, and opaque,
which could enable attractive new paradigms for solar deployment. For example, the
vapor-printed solar cells are integrated directly onto as-purchased fiber-based papers, in
which the vapor-printed device layers conform to the geometry of the rough substrate,
eliminating the need for more costly and heavier substrates such as ultra-smooth plastics.
Module installation may be as simple as cutting paper to size with scissors or tearing it by
155
hand and then stapling it to roof structures or gluing it onto walls. Additional cost savings
are anticipated to accrue from the combination of low weight and the ability to achieve a
compact form factor by rolling or folding for facile transport from the factory to the point
of use. Moreover, by adding energy harvesting functionality to surfaces that are already
commonly used in society, consumer adoption is simplified and electronic functionalities
can be added to otherwise passive products, such as wall coverings, product packaging,
documents, and apparel.
6.3
Future directions
There is not enough time in a Ph.D. to pursue every path of interest, much less those that
are opened along the way. Below are some possible future research directions that have
piqued my interest. It is left to the reader to assess their merit and determine whether they
should be pursued.
6.3.1
Multilayered CVD device structures
In this thesis, we have demonstrated CVD device layers in a variety of roles within
typical organic photovoltaic devices; however, these demonstrations were primarily
performed one layer at a time for the sake of systematic performance comparisons, and
the devices were completed using well-established physical vapor deposition techniques
(e.g., vacuum thermal evaporation). For example, in Chapter 2, we replaced the
conventional ITO bottom electrode with oCVD PEDOT, while maintaining the rest of the
OPV device constant (i.e., active layers and cathode). In combination, however, the
results and demonstrations presented herein hypothetically represent a full OPV using
only CVD processing (i.e., bottom electrode, active layer, top electrode, and encapsulant).
156
Thus, multiple CVD layers could be implemented in a single OPV, which would
streamline manufacturing and potentially enable improved performance and functionality.
For example, a straightforward extension of the results presented in Chapters 2-4, would
be to use oCVD PEDOT for the top and bottom electrodes in the same device, resulting
in a semi-transparent OPV that could be applied to windows for energy harvesting.
A particularly interesting direction is to utilize CVD multilayers to improve
device performance on non-planar substrates. For example, one of the biggest challenges
in employing paper substrates for thin-film electronic devices is the high multi-scale
roughness associated with these substrates. RMS roughnesses are typically on the order
of 1-5 [tm, however, depending on the paper, there is also local, nanometer-scale
roughness. This is complicated by the fact that each the device deposition techniques vary
significantly in their conformality. Specifically, the oCVD PEDOT electrodes have been
shown to be quite conformal over rough features, whereas, vacuum evaporation of smallmolecule organic semiconductors and metal cathodes is typically more directional (nonconformal). This inevitably creates the potential for shorting pathways between the
oCVD PEDOT anode and the metal cathode through the 50-100 nm active layers in areas
of high local roughness [Figure 6-1]. This is evident in the current-voltage characteristics
of our PVs on rough paper substrates, in which high dark currents and generally poor
diode behavior (fill factors) are observed to severely limit efficiency [Chapter 2]. This
performance can be modeled as parallel connections of structures that are shorted,
partially shorted, or non-shorted; the shorted structures increase the dark current and
mask the characteristic diode behavior of the non-shorted structures. This motivates a
more thorough study of the effect of multi-scale roughness on failure mechanisms and the
157
performance of the paper-based PVs, ultimately enabling better engineering of device
structures for various paper substrates with different roughness profiles.
CVD
Active
Shorting
Layers
Pathway
PEDOT
Figure 6-1: Cross-sectional representation of an OPV device on a highly non-planar
substrate (e.g., paper), in which the electrode layer (CVD PEDOT) is conformal over the
surface topography, while the active layers and metal electrode are not.
The ideal device structure on a rough substrate would employ all perfectly
conformal layers, such that the cross-sectional layer profile is identical at each point on
the substrate. Such a device would be hypothetically possible using the conformal CVD
process for each layer. As an initial step towards this type of structure a second conformal
CVD semiconducting polymer layer could be used in addition to the conductive oCVD
PEDOT layer. For example, semiconducting oCVD polythiophene (PT) could be
implemented as a hole-transport layer or photoactive layer directly on top of (or
underneath) the oCVD PEDOT layer, combining the results presented in Chapters 2-3
and Chapter 5. This would replace any parallel shorting pathway with a resistive metalsemiconductor-metal pathway, thereby reducing shunting pathways through the device.
We also note that in such devices, charge transport may be improved by forming covalent
bonds between adjacent device layers during the CVD polymerization reaction.
158
The ability to conformally deposit complete devices on non-planer surfaces using
CVD multilayers could also be coupled with more structured three-dimensional
substrates to optimize optical absorption, through light trapping, increased surface area,
and more uniform performance at off-angle illumination. One possible route for this
would be the three dimensionally patterned oCVD PEDOT electrodes presented in
Appendix A. In this work, we used colloidal lithography to create well-ordered,
hexagonal-close-packed, inverse hemispheres of PEDOT with tunable dimensions (radii
of 200-1000 tm), which could serve as an interesting substrate/electrode on which to
build conformal CVD OPVs.
6.3.2
Application to other organic electronics
Although we have focused our initial efforts on applying CVD polymers in photovoltaic
devices, the demonstrations represent broader applicability to organic electronic devices,
including light emitting diodes (OLED) [1-4] and thin film transistors (OTFTs) [5-7].
Such devices could also benefit from characteristics afforded by CVD processing, such as
applicability with substrates like paper due to conformal and mechanically flexible device
layers. Indeed, there has been much recent interest in integrating various electronic
components with low-cost paper substrates, including transistors, storage devices,
displays, and circuitry [8-11]. One motivation for pursuing such devices is to enhance the
utility of paper OPVs as an "on-chip" power source for other paper-based electronics;
since many of the benefits of the paper form factor will be lost if the OPV must be
connected to non-paper electrical components (e.g., rigid silicon circuits).
As one example, oCVD polymers offer a promising set of materials for charge
injection, charge transport, emissive, and electrode layers in OLEDs due to their
159
flexibility, low cost, ease of processing, and molecular tunability. Application of oCVD
PEDOT transparent electrodes could be extended to OLED heterojunction structures
[Figure 6-2] on glass or paper substrates. This should be a straightforward extension of
the PV heterojunction structures demonstrated in Chapters 2 and 3 by using well-known
evaporated
molecular
organic
layers,
such
as
N,N'-bis(3-methylphenyl)-N,N'-
diphenylbenzidine (TPD) and tris-(8-hydroxyquinoline) aluminum (Alq3 ), which can be
evaporated at similar thicknesses as in the photovoltaic structures described in Chapters 2
and 3.
Electron Transport Layer
Hole Injection Layer
Transparent Substrate
Figure 6-2: Cross-sectional schematic of a generic organic light emitting diode.
Also of interest for OLED devices are oCVD PEDOT interfacial electrode layers
for charge injection and transport (a potential extension of the hole-transporting PV
buffer layer examples in Chapter 2 and Appendix C and the PV cathode buffer layers
presented in Chapter 4). PEDOT doped with polystyrene sulfonic acid (PEDOT:PSS) is
commonly used for hole-injection and layers in OLEDs [12]. By doping with the flexible
PSS system, PEDOT is made water-soluble for solution processing. Thus, the
PEDOT:PSS solution can be spin coated on anode material to lower the hole-injection
160
barrier between the anode and hole-transporting layer. Subsequent device processing
must be water free to avoid solvation of the PEDOT:PSS layer. oCVD PEDOT offers an
attractive, solvent-free, alternative to PEDOT:PSS hole-injection layers. The ability to
modify the work function and conductivity of oCVD PEDOT ([13] and Chapter 4) should
allow for fine-tuned matching of work function with hole-transporting layers to reduce
device resistances and improve performance.
6.4
Concluding remarks
I hope that the studies and demonstrations presented here have not only been interesting
to the reader but also contribute practically to the engineer's tool box for organic
electronic device design and fabrication. While this thesis concludes my time as a Ph.D.
researcher at MIT, the exploration of CVD polymers and their applications to organic
electronics continues.
6.5
References
[1]
M. Gross, D.C. Muller, H.G. Nothofer, U. Scherf, D. Neher, C. Brauchle and K.
Meerholz. Improving the performance of doped pi-conjugated polymers for use in
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[2]
W.H. Kim, A.J. Makinen, N. Nikolov, R. Shashidhar, H. Kim and Z.H. Kafafi.
Molecular organic light-emitting diodes using highly conducting polymers as
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[3]
J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H.
Friend, P.L. Bums and A.B. Holmes. Light-emitting diodes based on conjugated
polymers. Nature, 347, 539-541, 1990.
[4]
G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri and A.J. Heeger.
Flexible light-emitting diodes made from soluble conducting polymers. Nature,
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A. Dodabalapur. Organic and polymer transistors for electronics. Materials Today,
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[6]
M. Halik, H. Klauk, U. Zschieschang, G. Schmid, W. Radlik and W. Weber.
Polymer gate dielectrics and conducting-polymer contacts for high-performance
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[7]
B.S. Ong, Y. Wu, Y. Li, P. Liu and H. Pan. Thiophene polymer semiconductors
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[8]
D. TobjOrk and R. Osterbacka. Paper electronics. Advanced Materials,23, 19351961, 2011.
[9]
U. Zschieschang, T. Yamamoto, K. Takimiya, H. Kuwabara, M. Ikeda, T.
Sekitani, T. Someya and H. Klauk. Organic electronics on banknotes. Advanced
Materials, 23, 654-658, 2010.
[10]
L. Nyholm, G. Nyhstr6m, A. Mihranyan and M. Stroomme. Toward flexible
polymer and paper-based energy storage devices. Advanced Materials,23, 37513769, 2011.
[11]
A.C. Siegel, S.T. Phillips, M.D. Dickey, N.S. Lu, Z.G. Suo and G.M. Whitesides.
Foldable printed circuit boards on paper substrates. Advanced Functional
Materials,20, 28-35, 2010
[12]
N. Koch, A. Kahn, J. Ghijsen, J.J. Pireaux, J. Schwartz, R.L. Johnson and A.
Elschner. Conjugated organic molecules on metal versus polymer electrodes:
Demonstration of a key energy level alignment mechanism. Applied Physics
Letters, 82, 70-72, 2003.
[13]
S.G. Im, K.K. Gleason and E.A. Olivetti. Doping level and work function control
in oxidative chemical vapor deposited poly(3,4-ethylenedioxythiophene). Applied
PhysicsLetters, 90, 152112, 2007.
162
Appendix A
Patterned and Functional
Grafted Conducting Polymer
Nanostructures via oCVD*
Originally published as N.J. Trujillot, M.C. Barrt, S.G. Lmt, and K.K. Gleason. Oxidative chemical vapor
deposition (oCVD) of patterned and functional grafted conducting polymer nanostructures, Journal of
Materials Chemistry, 20, 3968-3972, 2010.
(tAuthors contributed equally.)
163
A.1
Abstract
We present a simple one-step process to simultaneously create patterned and amine
functionalized biocompatible conducting polymer nanostructures, using grafting reactions
between oxidative chemical vapor deposition (oCVD) poly(3,4-ethylenedioxythiophene)
(PEDOT) conducting polymers and amine functionalized polystyrene (PS) colloidal
templates. The functionality of the colloidal template is directly transferred to the surface
of the grafted PEDOT, which is patterned as nanobowls, while preserving the
advantageous electrical properties of the bulk conducting polymer. This surface
functionality affords the ability to couple bioactive molecules or sensing elements for
various applications, which we demonstrate by immobilizing fluorescent ligands onto the
PEDOT nanopatterns. Nanoscale substructure is introduced into the patterned oCVD
layer by replacing the FeCl 3 oxidizing agent with CuCl 2.
A.2
Introduction
High-resolution functional polymer patterns have recently gained exposure from their
widespread applications in tissue engineering and biosensors [1-4], anti-biofouling [5],
microelectronics [6], optics [7], and MEMS [8]. Micro- and nanopatterning of functional
polymeric materials enables precise definition of reactive chemical moieties that can be
used to selectively immobilize target molecules for a desired application. For example,
patterned bioactive molecules such as cell growth factors and matrix proteins induce
anisotropy in cardiac myocytes [9], which can increase their viability. Furthermore,
selectively patterned antibodies can facilitate spatially directed deposition of mammalian
cells for biosensor applications [10].
164
It is no surprise that micro- and nanopatterned conducting polymers have been an
integral component in state-of-the art biosensors and various biomedical applications [11,
12]. Conducting polymers, such as poly(3,4-ethylenedioxythiophene) (PEDOT), can
provide a biocompatible platform for interfacing biological systems with electronic
devices [13]. These devices can convert biological signals into electronic signals, and
vice versa. Furthermore, the softness and flexibility of these organic materials closely
mimic the natural environment for tissue growth and provide an innocuous alternative to
inorganic conductors [13]. For example, it has been shown that PEDOT does not induce a
cytotoxic response in heart tissue [14] and demonstrates high cell viability when
polymerized around living neural cells [15]. Moreover, a variety of counter ions can be
incorporated into PEDOT, providing avenues for ionically modulating cell-substrate
interactions [14].
It is known that functionalized conducting polymers with the attachment of
biomotifs, such as growth factors, enable even greater ability to control the adhesion and
growth of cells on surfaces [16-18]. However, complex synthesis routes are commonly
required for imparting functional groups onto commercially available conducting
polymer precursors [18-20]. Alternatively, "grafting to" techniques have been used to
impart amine functionality to conducting polymer films [16, 21, 22]. In addition to
complex synthesis routes, these techniques require surface activation with UV light or
plasma discharge, which can damage the underlying conducting polymer. Carboxylate
functionalized oCVD copolymers have directly been deposited [23], but this approach
has not yet been extended to other functional groups, often because required monomers
are not readily available.
165
Chemical vapor deposition (CVD) is compatible with complementary metal oxide
semiconductor (CMOS) processing and is the most promising method to accommodate in
vivo measurements using systems-on-chip [24]. Within this framework, oxidative
chemical vapor deposition (oCVD) can conformally deposit thin, adherent coatings of
PEDOT and other conducting polymers on practically any substrate via a step-growth
polymerization process [25]. This process allows for direct control over dopant
concentration, work function, and conductivity by varying substrate temperature [26]; we
have reported conductivities as high as 1000 S cm 1 [27].
Here, we present a novel process in which well-ordered conducting polymer
nanostructures are simultaneously patterned and functionalized by combining oCVD with
colloidal lithography. Colloidal lithography can reduce the costs and complexity when
compared to conventional lithographic techniques, and cost-effective biosensors that
utilize patterned structures from colloidal lithography have been demonstrated [28-30].
Moreover, it has been shown that the spatial confinement and regularity presented by
colloidal patterns can be useful for aligning cells for studying their interactions [31].
Similar to our recently reported technique that uses colloidal lithography and
initiated chemical vapor deposition (iCVD) to pattern grafted non-conductive polymer
nanostructures [32], we employ commercially available polystyrene (PS) beads to create
arrays of grafted conducting polymer nanobowls with large surface area. In both
techniques, substrate grafting is essential to successful patterning. However, in contrast to
the iCVD patterning technique, here we exploit the unique polymerization mechanism of
the oCVD process to directly transfer the functionality of the colloidal template to the
surface of the bulk conducting polymer patterns. Thus, a well-ordered interface with
166
specific functionality or bioactivity can be created by using surface-functionalized PS
beads, such as those containing amine, carboxylate, hydroxylate, sulfate, or protein
conjugation, while maintaining the advantageous properties of the underlying bulk
conducting polymer.
A.3
Experimental
A.3.1
Preparation of functionalized PEDOT nanobowls
The procedure used to cast a monolayer of colloidal particles has been detailed elsewhere
[32]. Briefly, monodisperse 1 ptm polystyrene spheres, both non-functionalized or
primary amine functionalized (Polysciences), were diluted in a 1:1 ratio with a solution of
methanol and Triton X-100 and cast onto plasma cleaned silicon wafers in discrete 2 ptL
droplets. To remove any residual water, the samples were then placed into a nitrogen
purged vacuum oven (VWR, 1400E) that was maintained at 60 'C and -15 in. Hg gauge
pressure. After 3 min the nitrogen flow was stopped, and the samples were exposed to
tricholorvinylsilane (Aldrich, 98%) vapor at -30 in. Hg gaugepressure for 6 min.
Conducting polymer PEDOT films were deposited using a custom-built reactor
that has been previously detailed [33]. Low roughness PEDOT films were deposited
using FeCl 3 (Aldrich) as the oxidizing agent and nanoporous PEDOT films were
deposited using CuCl 2 (Aldrich) as the oxidizing agent using previously detailed reaction
conditions [26, 34]. The substrate temperature was maintained elevated at 85"C to help
enhance the film conductivity.
167
The PEDOT patterns were ultrasonicated with a 70 W ultrasonic cleaner in 30 mL
of THF (Aldrich, >99%) for 2 min to completely remove the colloidal template. The
samples were subsequently dried under nitrogen for 10 seconds.
After the template was removed, amine-functionalized PEDOT patterns were
immersed for 4 hours in a 1 wt% solution of NHS-Fluorescent dye (NHS-Fluorescein,
494 nm excitation wavelength, 521 nm emission wavelength, Pierce Scientific) in DMSO.
The samples were then triple rinsed with copious quantities of DMSO to remove any
unbound fluorescent ligands.
A.3.2
Characterization
The oCVD film composition was elucidated by transmission mode FT-IR spectroscopy
(Nicolet Nexus 870 ESP) using a DTGS-KBr detector. Spectra were obtained through the
patterned samples over the range of 400-4000 cm-1 at 4 cm-1 resolution averaged over 64
scans and baseline corrected. Three types of spectra were obtained: as-deposited oCVD
PEDOT on a silicon wafer, non-functionalized PS colloidal template cast onto a silicon
substrate, and oCVD-pattemed conducting polymer nanostructures on silicon in which
any unbound PS template was removed by rigorous rinsing and ultrasonication (2 min) in
THF.
The patterned samples were sputter coated with 5 nm of gold, and SEM images were
obtained using a JEOL JSM 6060 with 5 kV acceleration voltage. High-resolution
patterns were imaged using a FEI/Philips XL30 FEG ESEM with a 5 kV acceleration
voltage. Fluorescence microscope images were obtained at 50x magnification using a
Zeiss Axiovert 200 inverted microscope with FITC illumination. The film conductivity
was determined using a two-point probe measurement (Agilent) with probe spacing of 3
168
mm on unpatterned PEDOT films; bulk resistivity values were obtained across patterned
regions.
A.4
Results and discussion
A.4.1
3D patterning via colloidal lithography
This process takes advantage of the unique properties of oCVD, which creates PEDOT
films that are covalently grafted to PS and other aromatic-containing surfaces by the
Kovacic mechanism [35, 36]. In this process, a vaporized oxidant, such as FeCl 3, forms
radical cations in the EDOT monomers and the benzene moieties on the PS chains. With
the Friedel-Crafts catalyst as the stabilizing agent, these radical cations combine to form
covalent bonds between the growing PEDOT film and the PS substrate. This process
generates a robust interface that prevents delamination during patterning applications [35].
Similarly, oCVD PEDOT films can be covalently grafted to Si wafers by pre-treating the
wafers with a linker material such as trichlorovinyl silane (TCVS). During the oCVD
process, the oxidant converts these surface vinyl groups to radical cations, which
combine with the EDOT to covalently tether the PEDOT film onto the Si wafer.
Figure A-1 illustrates how this enhanced surface adhesion to both Si and PS is
utilized here for simultaneous patterning and functionalization of conducting PEDOT
films. Monodisperse PS beads are cast onto oxygen-plasma treated Si wafers and allowed
to dry under ambient conditions, forming a 2-D self-assembled hexagonal close-packed
(HCP) colloidal template. This patterned surface is then exposed to TCVS vapor, which
modifies the exposed area of the Si wafer by forming a monolayer of covalently bound
vinyl groups on the surface [Figure A-1(a)]. This occurs only in the exposed regions of
169
the substrate through the colloidal template and provides an anchor point for subsequent
grafting of oCVD conducting polymer. A blanket PEDOT conducting polymer film is
then deposited via oCVD, in which the substrate is exposed to EDOT monomer vapor
while oxidizing agent (FeCl 3 or CuCl 2 ) is simultaneously sublimed to the surface. The
substrate is maintained at elevated an elevated temperature (ca. 85'C) to maximize
PEDOT conductivity [26] without destroying the underlying PS template.
PS Beads ==-=
(a) Vinyl-Teated
Silicon -
(b)
Grafted
PEDOT
Grafted
(C)
PEDOT=mm*Pattern With
/
IL
-L-L-A
+
Fdedui-Crlt Catalyst
0
\L /
0
S
Q31, \S/*
0
o
PS Surface
Layer
Figure A-1: PS beads are self-assembled on a vinyl-treated Si wafer to form a 2-D HCP
colloidal template (a). PEDOT is deposited via oCVD, conformally coating the void
space of the colloidal template with conducting polymer, which grafts to the surfaces of
the PS beads and the vinyl-treated wafer (b). The non-grafted cores of the PS template are
removed via dissolution and rigorous ultrasonication in THF, revealing well-ordered
PEDOT nanostructures with a grafted PS surface layer.
When the oCVD process is applied to the colloidally patterned substrate, the
resulting PEDOT film conformally covers the PS beads and the coating infiltrates
through the interstices to the bottom of the HCP pattern. The oCVD PEDOT coating
becomes covalently bound to the surfaces of both the PS beads and the Si wafer [Figure
A-1(b)].
The non-grafted cores of the PS beads are then dissolved out with
170
tetrahydrofuran (THF) [Figure A-1(c)]. Since the PEDOT coating is grafted to the Si
wafer, the solvent rinsing step does not delaminate the patterned PEDOT film from the Si
wafer. After the THF rinsing, the covalently bound PS polymer chains remain on the
PEDOT coating as a grafted capping shell.
A.4.2
oCVD PEDOT nanobowls
Figure A-2 contains SEM images of the grafted PEDOT conducting-polymer
patterns obtained using the oCVD colloidal patterning scheme described above.
Figure A-2: SEM images show a PEDOT shroud in which the internal PS template has
been dissolved away by brief ultrasonication in THF (a). Rigorous ultrasonication reveals
the hemispherical HCP geometry of the patterned PEDOT nanostructure below (b). Using
FeCl 3 as the oxidant yields a well-defined PEDOT/PS nanostructure with a very smooth
texture (c). When CuCl 2 is used as the oxidant, hierarchically patterned PEDOT
conducting polymer films are formed with tunable nanoporosity (d). The inset captures
the morphology of the porous PEDOT on an unpatterned substrate.
171
The conformal nature of the oCVD process coupled with the growth mechanism
of the oCVD polymerization allows the colloidal assembly to act as a template in which
the grafted PEDOT pattern preserves shape of the PS colloidal template. This is evident
upon dissolution of the PS by ultrasonication in THF. Due to PEDOT grafting and the
solubility contrast between PS and PEDOT in THF, the internal PS template is dissolved
away after brief ultrasonication (<30 sec) while a PEDOT shroud remains [Figure A-2(a)].
It is suspected that the retention of the complete hollowed-out, 3-D, HCP geometry is due
to the unique ability of the oCVD process to graft PEDOT to the exterior polymer chains
of the PS beads. This results in enhanced mechanical stability, especially at the tangency
points between adjacent beads, which helps to preserve the top shroud. More rigorous
ultrasonication (>2 min) causes this shroud to break away, revealing the hemispherical
HCP geometry below [Figure A-2(b) and (c)]. These patterns can typically form over
areas as large as 0.3 cm 2 for a 1 pm diameter template. The typical feature heights and
separation widths of the nanobowls range from 300 to 500 nm and 50 to 150 nm,
respectively, and will vary depending on the template diameter used. Using FeCl3 as the
oxidant for the oCVD polymerization yields a well-defined PEDOT/PS nanostructure
with a very smooth texture. Alternatively, it has been shown that CuCl 2 can be used to
produce PEDOT conducting polymer films with tunable nanoporosity [34]. This was
coupled with the templating technique described here to produce unique hierarchical
nanostructures [Figure A-2(d)]; internal basalt-like nanoporosity is achieved while
maintaining the large-scale HCP pattern. The high conductivity of oCVD PEDOT films
was maintained in these patterned structures. The bulk resistance laterally across the
patterned PEDOT nanostructures ranged from 3-15 k92, measured by two-point probe
172
with 3 mm probe spacing. Due to the wide variation in film thickness inherent in the
patterned structures, the bulk conductivity of the conducting polymer patterns was not
calculated; however, the resistance of the unpatterned PEDOT from the same depositions
ranged from 1-10 k, which corresponded to conductivities of 2-100 S/cm.
The FTIR spectra of the bulk oCVD PEDOT [Figure A-3(a)], analyzed in detail
elsewhere [26] and the self-assembled PS colloidal template [Figure A-3(b)] are
compared to that of the oCVD-patterned conducting polymer nanostructures [Figure A3(c)] in which the PS template was removed by rigorous rinsing and ultrasonication in
THF [Figure A-2(b)].
(c)
3200
3100
3000
2900
1650
1450
Wavenumber
1250
1050
850
650
(cm1)
Figure A-3: FT-IR spectra of a blanket oCVD PEDOT conducting polymer film (a), a 2D monolayer of 1 pm diameter polystyrene colloidal template (b), and oCVD-pattemed
conducting polymer nanostructures (c) in which the PS template was removed by
rigorous rinsing and ultrasonication in THF. The observation of PS peaks in (c) is
evidence of grafting between the PEDOT nanobowls and the surface of the PS beads.
173
The characteristic peaks of the PS colloidal template (blue) are retained in
addition to those of the bulk oCVD PEDOT (yellow), indicating chemical adhesion of a
PS shell to the surface of the bulk conducting polymer. This retention of the chemical
characteristics of the PS colloidal template is unique to the oCVD process and is not
observed when other CVD polymerization mechanisms are used [32].
A.4.3
Surface functionalization
The grafted PS capping shell on the PEDOT colloidal pattern can be further utilized for
applications by imparting specific functionality via appropriate chemical modification of
the colloidal template. To demonstrate this, we employed commercially available aminofunctional PS beads for the PEDOT colloidal patterning process. Upon patterning
PEDOT with these beads, the surface of the PS capping shell contains primary amine
groups. These surface amine moieties can be coupled to a variety of ligands such as
proteins, carbon nanotubes, or nanoparticles [Figure A-4(a)]. Since the bulk PEDOT
under-layers demonstrate high electrical conductivity and are wired to the functional PS
capping shell via conjugated molecular linkages, these high surface area PEDOT patterns
are attractive for various applications, including biosensors [11], electrical drug release
[37], and plasmonic devices [38]. Figure A-4(b) shows SEM images of the oCVD
PEDOT patterned using amino-functional PS beads as the colloidal template. After the
non-grafted portion of the amino-functional colloidal template was removed by rigorous
rinsing and ultrasonication in THF, the presence of amino-functional groups on the
surface of the patterned PEDOT nanostructures was easily confirmed by applying an
NHS-tethered fluorescent dye (NHS-Fluorescein, 494 nm excitation wavelength, 521 nm
emission wavelength). The fluorescence from the immobilized dye was captured using a
174
(a)
Grafted aminofunctional PS
surface
dler
(NHS)
Amino Functional PS Beads
L
Grafted
PEDOT-
Bulk
=
H
Patterned
PEDOT
Nanostructures
0-
I
Unpatterned
Bulk
PEDOT
"
Figure A-4: Surface amine moieties provide a linkage point between the bulk conducting
polymer nanostructure and various ligands, demonstrated here by attachment of NHStethered fluorescein (a). SEM image of oCVD PEDOT patterned using 1 jtm aminofunctional PS beads (b); the inset captures the nanobowl shape, which becomes evident at
the interface with the unpatterned bulk PEDOT. Fluorescence microscopy shows
fluorescence areas patterned using amino-functional PS beads (c). The focal plane of the
fluorescence microscope causes the fluorescence signal to appear greatest along the
circumference of the bowls. The inset shows the interface (highlighted by the dotted
white line) between the fluorescent patterned regions and the unpatterned bulk PEDOT,
which does not exhibit fluorescence after the identical functionalization treatment.
fluorescence microscope, as shown in Figure A-4(c). The green fluorescence image
mirrors the oCVD PEDOT pattern shown in Figure A-4(b). No fluorescence was detected
on the unpatterned bulk PEDOT [inset Figure A-4(c)], and the pattern fluorescence was
only observed after functionalization with the fluorescent dye. This confirms that the PS
capping shell on the oCVD PEDOT pattern contains functionalizable surface amine
groups. The ability to tailor the surface functionality enables the coupling of bioactive
molecules or sensing elements to the bulk PEDOT patterns for various applications.
175
A.5
Conclusion
We simultaneously patterned and functionalized grafted PEDOT conducting polymer
nanostructures using a simple process combining oxidative chemical vapor deposition
and colloidal lithography. The robust polymer/template interface is established by
covalent coupling of oCVD PEDOT to the aromatic surface groups of the PS template.
Functionalized PS beads can be used to impart specific functional groups to the surface of
the patterned PEDOT layers while maintaining the high conductivity of the bulk
conducting polymer. Here, we used amino-functional PS beads to tether amine
functionality to the conducting polymer nanostructures as demonstrated by the
immobilization of fluorescent ligands. The ability to tether biomolecules or sensing
elements to well-defined arrays of biocompatible conducting polymer nanostructures
makes this technique of great interest for biosensor and cell growth applications;
furthermore, the excellent substrate adhesion makes it practical for device applications.
A.6
Acknowledgments
This research was supported in part by the U.S. Army through the Institute for Soldier
Nanotechnologies, under Contract DAAD-19-02-0002 with the U.S. Army Research
Office. This work made use of the shared electron microscopy facility in the MIT Center
for Materials Science and Engineering (CMSE).
176
A.7
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180
Appendix B
Facile Vapor Phase Method to
Prepare Dynamically Switchable
Membranes*
* Originally prepared as M.E. Alf, M.C. Barr, and K.K. Gleason. Facile vapor phase method to prepare
dynamically switchable membranes. Submitted to MacromolecularRapid Communications.
181
B.1
Abstract
We developed a vapor-phase method using the complimentary techniques of oxidative
and initiated chemical vapor deposition (oCVD and iCVD) to modify substrate surfaces
to achieve direct heating of temperature-responsive films. This modification involves
oCVD of poly(3,4-ethylenedioxythiophene) (pEDOT) for resistive heating, followed by
an insulating layer of iCVD poly(ethylene glycol dimethacrylate) (pEGDMA), and a
subsequent
iCVD
layer
of
poly(N-isopropylacryalmide-co-di(ethylene
glycol)
divinylether) (p(NIPAAm-co-DEGDVE)) that provides the temperature-responsive
surface properties. We achieved heating of flat surfaces in air up to temperatures of 70'C
at an applied voltage of 20 V AC and successfully model this heating behavior using a
simple energy balance. Flat surfaces show a static contact angle shift of 30' when heated
above the responsive film's lower critical solution temperature (LCST). X-ray
photoelectron spectroscopy (XPS) survey scans confirm successful functionalization
steps on 2 gm pore diameter polycarbonate (PC) track-etched membranes. These
membranes show temperature changes of 10'C heating in water, sufficient for
temperature control on both sides of the LCST transition and a corresponding change in
flux of 50 - 150% when the temperature is controlled below and above the LCST.
B.1.1
Introduction
Poly(N-isopropylacrylamide) (pNIPAAm) and its copolymers are thermally responsive
materials exhibiting a lower critical solution temperature (LCST) [1, 2]. When
submerged in water below the LCST, free polymer chains exhibit a hydrated coil
conformation and cross-linked gels are swollen. A sharp change in hydrophilicity occurs
182
above the LCST, resulting in a hydrophobic, collapsed globule conformation for free
chains and a dehydrated, collapsed gel for cross-linked polymers. Polymers containing
NIPAAm have been studied and explored for applications in widely varying arenas
including drug delivery, sensors, actuators, pumps, and separations [3-5]. For all of these
applications, a means of changing temperature must be designed in order to induce the
polymer's response. For drug delivery systems, switching occurs as the polymer is moved
between ambient conditions and body-temperature. For polymers utilized in single
location, such as membrane immersed in water, one can choose to heat the entire
enviroment surrounding the polymer or, alternatively, to directly heat only the polymer.
Of the two approaches, direct heating of polymer is expected to provide a faster
switching response. Several direct heating methods have been explored including heating
of pNIPAAm-modified gold nanocages via near infrared radiation [6], attachment of
pNIPAAm-modified PDMS to a black substrate which can subsequently be heated by a
strong light beam [7], as well as metal deposition on substrate backing for resistive
heating used mainly for MEMS devices or micro-reactors [3].
Here, we propose, demonstrate, and analyze a three-step fabrication method for
the direct heating of thermally responsive polymer layers. Rapid switching is achieved on
both flat substrates (e.g. glass) and micro-porous surfaces (e.g. membranes). A schematic
of the surface functionalization is depicted in Figure B-I along with an illustration of how
our design can provide flow control to a membrane. The functionalization utilizes both
oxidative and initiated chemical vapor deposition (oCVD and iCVD) as surface
modification methods. First, oCVD is used to deposit a conductive polymer poly(3,4ethylenedioxythiophene) (pEDOT) film, which provides resistive heating capabilities. A
183
thin (-200 nm) layer of poly(ethylene glycol dimethacryalte) (pEGDMA) follows to
provide an electrically insulating layer between the conductive pEDOT and the
environment. This layer is thick enough to provide electrical insulation, but thin enough
to minimize thermal conduction resistance to the subsequent temperature responsive
poly(N-isopropylacrylamide-co-di(ethylene
glycol)
divinyl
ether)
(p(NIPAAm-co-
DEGDVE)) layer. Silver paint was added along the length on opposite sides of the
substrates as electrodes to facilitate conduction across the full surface area.
(a)
Glass Substrate
p(NIPAAm-co-DEGDVE)
PC Track Etched Membrane
pEGDMA
Buffer Layer
Silver Paint
pEDOT
Substrate
(b)
T <LCST
-
--
TLCST
Figure B-1: (a) Cross-sectional schematic of samples developed with dynamically
switchable properties for both flat glass substrates and a track-etched polycarbonate (PC)
membrane. The deposition steps for pEDOT and pEGDMA were performed from the top
side, while the p(NIPAAm-co-DEGDVE) layer was deposited from the top side for the
glass substrate and the bottom side for the PC track etched membrane. (b) Schematic of
how the temperature-responsive polymer film will control the water flux through the
membrane. Below the LCST, the polymer swells, narrowing the membrane pores and
causing a decrease in flux, while above the LCST, the polymer deswells against the pore
walls, creating a larger effective pore size and higher flux.
184
Using these structures we are successfully able to prepare substrates that show
responsive surface and membrane permeation properties when AC voltage is applied.
Importantly, the polymers and deposition methods used here are both conformal and
substrate independent, making this fabrication scheme directly applicable to a wide range
of surfaces for a variety of applications.
B.1.2
Deposition methods
Many of the varied applications for NIPAAm-based polymers require surface
modification, which gives the polymer's responsive characteristics to a base substrate.
Several methods have been used to provide this functionalization including solutionphase methods such as "grafting-to"[8-10] and "grafting-from," [11-13] as well as vaporphase methods such as plasma-enhanced chemical vapor deposition (PECVD) [14-16]
and initiated CVD (iCVD) [17]. We have chosen the iCVD deposition method because it
has been shown to have several distinct advantages over other surface functionalization
methods. Between the substrate-independent nature of the process and its ability to
conformally coat structured surfaces, iCVD is a technique with applicability that spans a
broad range of arenas. Also from a processing standpoint, iCVD provides dynamic
control over film composition and thickness, with achievable deposition rates upwards of
100 nm min-' and film thicknesses that can range from the nanometer to micrometer
range [18]. Additionally, surface modification with responsive polymers depends heavily
on functional retention to preserve desired polymer properties. iCVD follows the
traditional free-radical reaction pathway, which is equivalent to the solution-phase
process and therefore provides complete functional retention, which is not the case with
other higher energy vapor-phase processes (e.g. PECVD). For these reasons, we have
185
chosen iCVD to form both the insulating (pEGDMA) and responsive (p(NIPAAm-coDEGDVE)) layers of the structure in Figure B-1.
While
iCVD is very useful
for monomers
that react via free-radical
polymerization, oCVD is a complimentary technique that capitalizes on monomers that
react via step-growth polymerization, including a variety of conductive polymers. Here
we utilize oCVD to deposit the conductive polymer pEDOT to provide "on-chip"
resistive heating of our substrates. Like iCVD, oCVD is also conformal over textured
surfaces and is compatible with a wide range of substrates [18]. The conformality of
oCVD is one characteristic that differentiates it from evaporated metals that have
previously been used to resistively heat substrates to induce temperature-responsive
behavior [3]. The line-of-sight nature of evaporation leads to non-conformal coverage,
with thicker coatings at the pore mouth and thin, if any coating, down the length of the
poor. Thus, oCVD, due to the conformal nature of the polymer films it deposits [19, 20],
has the ability to provide targeted heating even in micro- or nano-structured geometries,
including the internal surfaces of membranes and foams [21]. Therefore, instead of
needing to provide enough energy to heat the entire substrate, it is possible to merely
provide enough energy to heat the responsive polymer it is close thermal contact with.
Here, we utilize its conformality to provide targeted heating across a membrane, while
preserving the structure of the underlying membrane. Additionally delamination can be
avoided, as oCVD films can exhibit excellent adhesion due to the conducting polymer
chains can inherently graft to substrates containing aromatic functionality (e.g.
poly(styrene), PC) [22].
186
B.2
Experimental
B.2.1
Deposition conditions
Conductive
poly(3,4-ethylenedioxythiophene)
(pEDOT)
films
were
conformally
deposited directly onto all substrates via oxidative chemical vapor deposition (oCVD).
The oCVD process procedure and the reactor configuration are described in detail
elsewhere [19, 23, 24]. 3,4-ethylenedioxythiophene monomer (Aldrich, 97%) vapor was
metered at ~5 sccm and FeCl3 oxidant (Aldrich) was controllably evaporated from a
resistively heated crucible at ~25 0 'C. In order to enhance the conductivity of the resultin
film, resistive heating was used to maintain the substrate temperature at ~80'C. The total
pressure was maintained at ~100 mTorr. The thicknesses of oCVD PEDOT films were
controlled by limiting the time of reaction (~30 min for a 1- tm thick film) and measured
later via profilometry (Tencor-P16).
Following pEDOT depositions, films of both pEGDMA and p(NIPAAm-coDEGDVE) were subsequently deposited on all substrates using iCVD. All iCVD
depositions took place in a custom-built reactor as described previously [18]. NIPAAm
(Acros, 99%), DEGDVE (Aldrich, 99%), EGDMA (Aldrich, 98%) and tert-butyl
peroxide (TBPO) initiator (Aldrich, 98%) were purchased and used without further
purification. To deposit the iCVD polymer films, TBPO vapors (room temperature) were
fed into the reactor through a mass flow controller (MKS 1490A) at a flow rate of 1.0
sccm. The monomers EGDMA, NIPAAm and DEGDVE were kept at 85'C, 75'C and
50'C respectively and fed into the reactor via fully open diaphragm valves at respective
flow rates of 0.3 sccm, 0.15 sccm and 1.0 sccm. For the pEGDMA films, only EGDMA
187
and TBPO were fed into the reactor kept at a pressure of 140 mTorr. For the cross-linked
p(NIPAAm-co-DEGDVE) films, NIPAAm, DEGDVE and TBPO vapors were fed
concurrently, while keeping the reactor pressure at 200 mTorr. For all depositions, the
substrate was kept at a temperature of 45 'C to promote monomer adsorption and the
filaments were held at 200'C throughout the course of the deposition.
For each
deposition, the sample substrates (coated glass slides or membranes) were included along
with a reference Si wafer (WaferWorld) to be used for subsequent variable angle
spectroscopic ellipsometry (VASE) thickness measurements, and which could also be
used for in situ interferometry with a 633 nm HeNe laser source (JDC Uniphase) to
monitor to achieve approximate desired film thicknesses.
B.2.2
Sample preparation
For the flat surface samples, glass slides (2.5 cm x 25 cm) were used as substrates for the
depositions. First pEDOT was deposited in varying thicknesses (~50 - 200 nm) in order
to achieve different resistances (400 - 3,000 Q). For the preliminary resistive heating
tests, silver paint was then applied along the length of the sample on opposite sides to act
as electrodes, leaving an area of approximately 2 cm x 2.5 cm between the electrodes for
electrical conduction through the pEDOT for resistive heating. To prepare samples for
contact angle measurements, the silver electrodes were masked and followed by
subsequent pEGDMA and p(NIPAAm-co-DEGDVE) depositions.
To prepare the membrane samples, similar processing was used. The substrates
were 4.7 cm diameter track-etched PC membranes with 2 pm pores (Millipore) for the
pEDOT depositions (-200 nm). Prior to pEGDMA depositions, masking of opposite
membrane edges was performed, leaving approximately 4 cm between the masked edges
188
open for deposition. pEGDMA was then deposited from the same direction as the
pEDOT. After the deposition was performed, the mask was removed and the membrane
flipped. A new mask was placed on the edges, masking the same membrane area prior to
the p(NIPAAm-co-DEGDVE) deposition (150 - 500 nm).
After this deposition, the
membrane was removed and silver paint was painted on the side where the pEDOT
deposition was performed to assure contact with the pEDOT and therefore conduction
and heating across the membrane.
B.2.3
Sample characterization
Film thicknesses of pEGDMA and p(NIPAAm-co-DEGDVE) were measured
using variable angle spectroscopic ellipsometry (VASE) (JA Woollam M-2000). An
incident angle of 65' was used and the data were fit to a Cauchy-Urbach isotropic model.
Contact angle measurements on the fully-functionalized flat substrates were performed
on a goniometer equipped with an automatic dispenser (Model 500, Rame-Hart) using a 4
pL DI water droplet. Measurements were taken both at room temperature (~23 'C) and
after heating the substrate to -35'C through application of 15 V AC. X-ray photoelectron
spectroscopy (XPS) (SSX-100, Surface Science Instruments) survey scans measured the
surface composition of the membrane samples after each deposition step. Scans were
carried out at 150 eV pass energy with an electron emission angle of 55'.
B.2.4
Membrane permeation
The membrane permeation experiments were performed by mounting a
membrane between two pieces of glass tubing with o-rings and keeping a constant water
2
head to provide back-pressure for permeation over an area of 4.5 cm . Water height was
189
maintained at 28 cm, providing a backpressure of 2.7 kPa. Water was allowed to
permeate through the membrane for 1 hour prior to taking the first measurement, but was
allowed to equilibrate for only 20 minutes after each change of condition (e.g. heating or
cooling cycle). An AC voltage was applied across the membrane using a variac
transformer (model SC-20M) to provide heating. For cooling, samples were simply
allowed to dissipate heat to the atmosphere. For data analysis, a baseline shift was applied
to account for a small amount of drift in the base permeation values.
B.3
Results and discussion
B.3.1
Proof of concept on flat surfaces
To first investigate the heating properties of conductive pEDOT, we began by heating
various resistances of pEDOT film on glass slides with AC current [Figure B-2 (a)]. Data
points were taken after allowing the temperature to come to equilibrium for
approximately 10 minutes after each voltage step. We were able to model this heating
behavior by performing a basic energy balance on the glass slide system. By making the
assumptions that the glass temperature is the same as the pEDOT temperature, that there
is only convective heat loss to the air, and that data points are taken at equilibrium, the
energy balance of temperature as a function of applied voltage reduces to,
T=
v 2
2RhA
+ TIm,
(B.1)
where T and Tatm are the temperatures of the surface and atmosphere respectively, V is the
applied voltage, R is the resistance across the pEDOT film, h is the convective heat
transfer coefficient to stagnant air, and A is the area of the glass surface.
190
(b)
(a)
*970 Ohm
:5WOh
L.60
140
,,4r
20
0
10
Applied DC Voltage V
(c) T < LS
(d) T > LCST
20
c
8
Figure B-2: (a) Temperature curves for pEDOT-coated glass substrates with varying
resistances over a range of applied AC voltages. The data points were taken after
allowing 10 min. of temperature equilibration at each voltage step. The dashed lines
represent Equation 1 as fitted to the data using only one fitting parameter (h = 13 ± 1 W
m-2 K-). (b) Setup used to obtain static contact angle measurements of the responsive
surface using pEDOT coating to provide resistive heating. Static contact angles for glass
slides coated with pEDOT followed by pEGDMA and p(NIPAAm-co-DEGDVE) both (c)
below the film's LCST (~22'C) and (d) above the film's LCST (~ 35'C) as achieved via
application of 15 V AC across the substrate (RpEDOT =,200 D).
Equation (B.1) shows good agreement with the experimental data, as does the
value obtained for the fit parameter h for the four samples of different resistances
spanning an order of magnitude. The convective heat transfer coefficient determined by
these fits was 13 1 1 W m-2 K , which lies within the range expected for stagnant air [25].
Using this value, we were able to verify the validity of the assumption that the glass and
191
pEDOT temperatures were approximately equivalent by calculating the dimensionless
Biot number (Bi) for the system, which compares heat transfer resistances inside of a
body to those at the surface. Using the fitted value for h and 1.05 W m-1 K- as the
thermal conductivity of glass [26], Bi was calculated to be 0.006. Because Bi is
significantly less than one, it implies the temperature profile within the glass slide is not
significant and therefore our assumption is valid. Also, although generally good
agreement is seen between the fitted equation and the data, there are some deviations at
higher temperatures. This can be attributed to the fact that at higher temperatures heat
loss mechanisms other than convection, such as conduction, become significant. This
would then cause greater total heat loss than estimated by Equation
and, therefore,
temperatures below the model prediction as observed. Therefore, the model would need
to be modified to include these factors if we wish to go to high temperature values, but
because these temperatures are well past the responsive temperature for our system, it is
unnecessary for this application.
After understanding the heating properties of pEDOT, the buffer and temperature
responsive layers were added to the surface. The LCST of the temperature responsive
polymer film, p(NIPAAm-co-DEGDVE) is estimated to be -28 'C [17]. To test the
combination of voltage-responsive heating and temperature-responsive hydrophilicity
changes, static contact angle measurements were performed [Figure B-2 (a) and (b)]. At
ambient temperatures of approximately 22'C, below the LCST of the p(NIPAAm-coDEGDVE) film, the static contact angle was 50'. Subsequently, an AC voltage of 15 V
was applied across the silver electrodes on the substrate
(RpEDOT =
1,200 9) leading to a
temperature increase to 35 C, taking the temperature-responsive polymer layer well
192
above its LCST. At these temperatures, a contact angle of 800 was observed, which
illustrates the hydrophobic switch of the surface layer. The 300 contact angle swing was
previous observed flat surfaces functionalized with a NIPAAm-based polymer [17, 27].
B.3.2
Membrane application
Material Characterization.Several membrane samples were prepared via the deposition
steps outlined previously to use for further characterization and analysis. The thickness of
the different layers as measured on reference Si wafers using VASE are given in Table B1. The primary difference between the two samples shown is the difference in thickness
of the p(NIPAAm-co-DEGDVE) responsive layer, where sample 2 had approximately
half the thickness of this polymer film compared to sample 1.
Table B-1: Details of deposition thickness as measured from profilometry for pEDOT
and VASE for pEGDMA and p(NIPAAm-co-DEGDVE) on reference Si wafers for the
diameter track-etched PC membrane samples used in
two different 2 pm pore
subsequent permeation studies.
Film Thickness*
[nm]
p(NIPAAmpEGDMA
co-DEGDVE)
Sample
Resistance
[Tp]
pEDOT
1
200
620
320
530
2
650
450
360
240
*Thicknesses are as measured on a flat reference Si wafer.
After each deposition step, a piece of the membrane was removed for elemental
analysis via XPS. Figure B-3 and Table B-2 confirms the elements we would expect to be
present after each deposition step. After the pEDOT deposition, sulfur can be seen, which
is not present in any of the other polymers or scans. After the pEGDMA deposition,
193
peaks are only seen for carbon and oxygen and the sulfur is no longer present, indicating
that the pEDOT layer has been fully covered by pEGDMA. In the final scan, nitrogen is
present along with carbon and oxygen, indicating that NIPAAm is present at the surface.
These scans are all taken from the top side of the membrane (top is defined as per Figure
B-1). Scans were also taken of the bottom of the membranes with qualitatively similar
results, showing the functionalization steps occurred similarly on the bottom side of the
membrane and that the monomers were able to pass through the membrane pores to
polymerize on the bottom side of the membrane. This indicates that the pore structure of
the membrane was retained, because if the pores became blocked, we would not observe
deposition on the bottom side of the membranes.
An analysis of elemental percentages was used to further investigate the
properties of the polymer films. For the pEDOT layer, the doping level is 25% as
calculated by comparison of the elemental percentages of chlorine and sulfur. This is very
close to the maximum possible doping level, theoretically calculated to be 33% [28],
therefore we expect the resistances achieved are close to the lowest possible for the given
polymer structure and film thicknesses. Also, although 4.4% sulfur was observed on the
top side of the membrane, only 1.1% was present on the back side, indicating that
although the pEDOT did penetrate the membrane fully, less reached through to the back
side of the membrane. The proportion of doped sulfur remained at approximately 25%,
indicating that the polymer properties were consistent between the top and bottom side of
the membrane.
194
Table B-2: Expected and actual elemental compositions as calculated from XPS survey
scans on the top side of 2 ptm pore size membranes after pEDOT, subsequent pEGDMA
and subsequent p(NIPAAm-co-DEGDVE) depositions.
Elemental Percentages
Deposition
0
N
C
Cl
S
Expected
21.4
-
64.3
3.6
10.7
Actual
26.1
-
68.4
1.1
4.4
Expected
28.6
-
71.4
-
-
Actual
28.5
-
71.5
-
-
Expected*
15.7
9.8
74.5
-
-
Actual
14.6
9.6
75.7
-
-
pEDOT
pEGDMA
p(NIPAAm- co-DEGDVE)
Expected percentage calculated using 83% NIPAAm 17% EGDMA composition
for copolymer as determined from amount of N as compared to 0 and C in survey
scan.
*
o
N
CS
(a)
(b)
(c)
1000
800
600
400
200
0
Binding Energy [eV]
Figure B-3: XPS survey scan taken of the top side (as in
etched membrane after (a) pEDOT deposition followed
p(NIPAAm-co-DEGDVE). The important elemental peaks
present at the surface only after the pEDOT deposition,
following the subsequent pEGDMA deposition, and N
subsequent p(NIPAAm-co-DEGDVE) deposition.
195
Figure B-1) of a PC trackby (b) pEGDMA and (c)
are highlighted where S is
only C and 0 are present
is present only after the
The pEGDMA expected ratio of carbon to oxygen is very similar to that observed
experimentally; indicating pEGDMA is in fact what is present on the surface. From the
elemental percentage obtained for the final p(NIPAAm-co-DEGDVE) surface layer, the
nitrogen composition was used to estimate the percent NIPAAm and DEGDVE
monomers present in the film. A calculated percentage of 83% NIPAAm to 17%
DEGDVE gives very good agreement to the experimental data. This ratio of NIPAAm to
DEGDVE is slightly higher than that observed previously [17], and can be attributed to
the higher flow rate of DEGDVE during the deposition, which allowed for more
DEGDVE to be incorporated into the films.
Although the XPS analysis showed successful functionalization of the membranes,
the other important characteristic of the membranes is their uniform pore structure. SEM
images were obtained before and after functionalization [Figure B-4 (a) and (b)] which
show full retention of the uniform pore shape. By visual inspection, the pore size appears
to have decreased significantly post-functionalization as would be expected based on the
conformal nature of the oCVD and iCVD depositions. Through analysis of approximately
100 pores for each membrane, the pore diameter decreased from 1.7 ± 0.2 pm for the bare
member to 1.17 ± 0.07 pm after functionalization. Note that this pore diameter change is
somewhat less than what would be expected based on the coating thicknesses as
measured by VASE on reference wafers, but, importantly, the XPS analysis confirms full
functionalization of the membranes.
Permeation Study. The responsive characteristics of the membrane systems were
studied by monitoring the heating properties and water permeation rate through the
membrane maintaining a backpressure supplied by a water head above the membrane.
196
The volumetric flow rate of the permeate was measured at various temperatures (applied
AC voltages) around the LCST. The results of the permeation study are depicted in
Figure B-5(a).
(a)
(b)
I
ZA
0
0
0.
U.
(c)
C
0
0
E.
)Ih
0
C')
Figure B-4: SEM images of the top side [as depicted in Figure B-1 (b)] of (a) the blank,
2 pm pore diameter PC track etched membrane used for Sample 1, (b) Sample 1 after the
three deposition steps, (c) Sample 1 after the membrane permeation study. The scale bars
represent 5 pm.
197
From these data, we see a distinctive trend that at high temperatures, the
membrane flux is 50% - 150% higher than the flux at low temperatures, which is the
general trend expected. At low temperatures, the p(NIPAAm-co-DEGDVE) film swells
in the pores, effectively making them smaller, which decreases flow rates. When the
temperature is increased above the LCST, the film collapses onto the pore walls, making
the pores larger and allowing for higher flow rates. This percent change in permeability is
within the range expected for similar NIPAAm-grafted membranes where permeability
changes from 50% up to 600% have been seen [29-31]. This also indicates with some
additional tuning of functionalization parameters such as film thicknesses and crosslinking, it should be possible to achieve larger changes in flux, but that the current system
has successfully achieved the desired results with this novel functionalization method.
These data also show the dual functionality of the membrane. In addition to
permeation control, we were successfully able to achieve membrane heating via
application of an AC voltage across the functionalized membrane (Figure B-5).
Temperature changes of up to 100 C were observed. We believe even greater temperature
changes could be induced based on the relatively small amount of voltage applied to
achieve this heating, especially for Sample 1, which had significantly lower resistance
than Sample 2 (Table B-1), allowing for more current to provide greater heat generation.
But, for this application, it was best to keep the temperature swing as low as possible to
minimize work input into the system. This does show the versatility of our design, which
could be applied to other systems with different temperature requirements.
198
(a)
~
3
A
30
ALCST
20
1
10
12
HeaUng Cycle
(b)
-%12
3
(c)
-
- 10 8
4
**
6
42
22
Iqq
0
6
0
30
20
10
AC VItage (V
0
2
4 6 8
Thme [mini
10
Figure B-5: (a) Data representing the normalized (to the first low temperature flux,
Sample 1: 2.7 ± 0.1 mL-m-2*- 1, Sample 2: 3.8 ± 0.1 mL m-2 -s-1) DI water permeation rate
of membrane Sample 1 (*) and 2 (0). The temperature at which the measurements were
taken are given in red open points on the second axis and the LCST of the polymer film is
represented by the dashed line at 28'C, illustrating that low temperature measurements
were taken below this value and high temperature measurements above it. (b)
Temperature changes as measured by a thermocouple affixed to the membrane in DI
water from the base system temperature with applied AC voltage for the two different
membrane samples. The dashed (sample 1) and solid (sample 2) lines are included merely
as guides. (c) Transient temperature change profiles for a AC voltage jump of 0 to 12 V
on two different runs (open vs. solid points) for sample 2.
The heating and permeability switching were both shown to be repeatable over
multiple cycles. [Figure B-5(a)]. Sample 1 was subjected to three heating cycles, and
199
repeatable permeability and heating was seen for all three. This indicates that both the
temperature-responsive and conductive polymer did not degrade significantly throughout
several swelling/deswelling cycles or after carrying a current. The retention of the
membrane structure after the permeation study was verified through SEM images as
given in Figure B-4(a). Repeatable membrane cycles have been shown in literature for up
to three cycles [32], but for many studies permeability was not investigated [29-31],
indicating that these membranes have as good or better stability than others of similar
composition and structure.
From an application standpoint, it is also important to understand the response
time of the system. We observed the transient temperature profile for a voltage jump of 0
to 12 V AC for Sample 1. The system heating response behavior has a time constant of
approximately 42 ± 9 seconds (Figure B-6). This response time is relatively fast because
based on the membrane design, it is not necessary to heat all the water in the system. In
fact only the temperature-responsive film needs to be heated to above the LCST of the
polymer. Although the system has fast temperature-switching characteristics, due to the
swelling constraints of the temperature-responsive layer, we would expect the time
constant for this part of the system to be slower. Data points in Figure B-5(a) were taken
after allowing for equilibration, and after 20 minutes the permeation had reached a steady
value.
The time constant for the system response was determined by fitting data for the
transient temperature profile to an equation for the response (temperature) of a first order
system to a step change input (voltage)
AT=A
1- exrVZ ,
200
(B.2)
where AT is the temperature change, A is the system gain, k is the magnitude of the step
input, t is the time and r is the process time constant. Figure B-6 shows the fit equation
with the data.
8
o
aa
o
246
&
6"Data
-Fit
2
0q0
0
2
4
6
8
10
Time (min]
Figure B-6: Transient temperature change profiles for a AC voltage jump of 0 to 12 V on
two different runs (open vs. solid points) for sample 2 with corresponding fit to Equation
S1 to determine the time constant of the response (A = 0.60 ± 0.02C-V-, k = 12 V, =
42 ± 9 seconds).
B.4
Conclusion
We successfully achieved functionalization of both flat and porous surfaces using the
vapor-phase techniques of oCVD and iCVD. The heating of flat surfaces was
successfully modeled using a simple energy balance and only one fit parameter over a
voltage range of 0 - 20 V AC, achieving temperatures up to 70 0C and for surfaces with
resistances spanning an order of magnitude. These fully-functionalized flat surfaces
showed a static contact angle change of 500 to 80' as the temperature is increased via
application of 15 V AC to temperatures above the responsive polymer, p(NIPAAm-coDEGDVE)'s LCST. XPS data confirms functionalizaton of membranes with the same
201
responsive polymer layers. Membranes were then successfully heated in water through
application of an AC voltage, resulting in temperature changes of up to 10'C, sufficient
to achieve temperature switching above and below the LCST. Control over water
permeation rate was achieved, in which fluxes increased by 50%-150% above the LCST.
These results illustrate the general applicability of this functionalization technique in
providing dynamically switchable surface properties to a wide variety of substrates for a
range of applications.
B.5
Acknowledgments
The authors thank Jonathan Shu at the Cornell Center for Materials research for
performing XPS measurements. The authors thank Rachel Howden of the Gleason Group
at MIT for performing the profilometry measurements to determine pEDOT thicknesses.
This research was supported by, or supported in part by, the US Army through the
Institute for Soldier Nanotechnologies, under Contract DAAD-19-02-0002 with the US
Army Research Office.
B.6
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206
Appendix C
Graphene Organic Solar Cells
with oCVD PEDOT
Hole-Transporting Layers*
* Originally prepared as H. Park, R.M. Howden, M.C. Barr, V. Bulovid, K.K. Gleason, and J. Kong..
Graphene organic solar cells with vapor-printed poly(3,4-ethylenedioxythiophene) hole transporting layers.
Submitted to Nano Letters.
207
C.1
Abstract
For the successful integration of graphene as a transparent conducting electrode in
organic solar cells, proper energy level alignment at the interface between the graphene
and the adjacent organic layer is critical. The role of a hole transporting layer (HTL) thus
becomes more significant due to the generally lower work function of graphene
compared
to ITO.
A
commonly used HTL material with ITO
anodes
is
poly(ethylenedioxythiophene) with poly(styrenesulfone) (PSS) as the solid state dopant.
However, graphene's low surface free energy renders uniform coverage of PEDOT:PSS
(hydrophilic) by spin-casting very challenging. Here, we introduce a novel, yet simple,
vapor printing method for creating patterned HTL PEDOT layers directly onto the
graphene surface. Vapor printing represents the implementation of shadow masking in
combination with oxidative chemical vapor deposition (oCVD). The oCVD method was
developed for the formation of blanket layers of pure PEDOT (e.g. no PSS) with
systematically variable work function. In the unmasked regions, vapor printing produces
complete, uniform, smooth layers, of pure PEDOT (e.g. no PSS) over graphene.
Graphene electrodes were synthesized under both LPCVD (~300 Q/sq at ~92%T) and
APCVD (~450 Q/sq at -92%T) conditions using a copper catalyst. We demonstrate that
using the donor material tetraphenyldibenzoperiflanthene (DBP) with highly efficient
graphene electrodes (without any doping) yields organic solar cells with performances
comparable to those of the ITO reference devices (17p,
and r/,
ITO
= 3.20%).
208
LPCVD=
3.01%,
qp, APCVD
2.49%,
C.2
Introduction
Graphene is hexagonal arrangement of carbon atoms forming a one-atom thick planar
sheet. The successful isolation of single- and few- layer graphene by the mechanical
cleaving of highly ordered pyrolytic graphite (HOPG) [1] has led to a significant increase
in studies in numerous research areas. Of the many interesting properties of graphene,
such as superior electron and hole mobility (up to 200,000 cm2 V-I s-1) [2, 3], high current
carrying capability (up to 3 x 108 A cm 2 ) [4], and mechanical robustness [5], its
uniformly high transparency in the visible and near infrared region, with moderate
conductivity, place graphene as a promising candidate for an alternative to indium tin
oxide (ITO) [6] as a transparent conducting electrode (TCE).
Several criteria, such as electrical conductivity, optical transmittance, and work
function (WF), need to be optimized for the integration of graphene sheets as TCEs in
organic photovoltaics (OPV).
Recent reports [7, 8] have already demonstrated that
graphene has high transmittance with moderate conductivity. The most critical factor,
however, is the energy level alignment between the work function of graphene and the
highest occupied molecular orbital (HOMO) of the donor material.
Bae et. al. [8]
reported a WF value of 4.12 eV (electron volts) for a monolayer of graphene synthesized
from LPCVD, which is considerably lower than for ITO (-4.5 eV and can be increased
up to -5.0 eV after oxygen (02) plasma treatment) [9, 10]. This low value for graphene
is obviously not a good match for the donor material considered in this work
(tetraphenyldibenzoperiflanthene (DBP), HOMO = 5.5 eV) [11], as well as other
common donor materials such as copper phthalocyanine (CuPc) (HOMO = 5.2 eV) [11]
209
or poly(3-hexylthiophene) (P3HT) (HOMO = 5.2 eV) [12], which can induce a large
energy barrier at the interface between the graphene and the organic layer.
For
ITO
anodes,
a
thin
layer
ethylenedioxythiophene):poly(styrenesulfonate)
of
conducting
polymer,
poly(3,4-
(PEDOT:PSS), is commonly inserted
before the deposition of the donor material in order to favor an ohmic-contact at the
junction. The PEDOT:PSS hole transporting layer (HTL) with a WF of 5.2 eV not only
facilitates the injection/extraction of holes but is also known to help planarize the rough
surface of the ITO, which often becomes a possible source of local shorting through the
ultra-thin active layers, thus improving the overall device performance [13, 14].
Therefore smooth and complete coverage of the PEDOT:PSS layer on the underlying
electrode surface plays a crucial role in the general OPV device performance.
Application of PEDOT:PSS onto the graphene surface is problematic since the two
surfaces are not compatible with each other, i.e. graphene is hydrophobic and
PEDOT:PSS is hydrophilic.
The sputtered ITO surface is also hydrophobic but it is
almost always pretreated with 02 plasma, which renders the hydrophobic surface into an
hydrophilic one by introducing hydroxyl (OH) and carbonyl (C=O) groups [15] that
enables conformal coverage of PEDOT:PSS. Active oxygen species from plasma disrupt
the aromatic rings of the graphene and greatly reduces the conductivity. In the case of
single layer graphene electrodes, a graphene film can completely lose conductivity after
such plasma treatments.
Recently, Park et. al. [16] reported that the wettability of PEDOT:PSS on the
graphene surface can be significantly improved by doping it with gold (III) chloride
(AuCl 3 ). However, the doping process introduces large Au particles (up to 100 nm in
210
diameter), which can create shorting pathways through the device. This method also
suffers from the high cost of AuCl 3 dopant. Wang et. al. [17] later reported using a
molybdenum oxide (MoO 3 ) HTL with acid-doped graphene electrodes, which is a
common HTL material used with ITO electrodes [18]. However, the device performance
was not as efficient as the ITO control device with a MoO 3 layer alone and still required
the use of PEDOT:PSS on top of the MoO 3 interfacial layer, which allowed better
wetting of PEDOT:PSS on the MoO 3-coated graphene.
In this work, we introduce a novel, yet simple, HTL fabricated by vapor printing
of PEDOT [19] directly onto the unmodified graphene surface. In a single step, vapor
printing combines (i) the synthesis of conducting polymer chains from vapor-phase
EDOT monomer, (ii) thin film formation of the PEDOT HTL, and (iii) patterning by in
situ shadow masking.
Vapor printing is derived from the oxidative chemical vapor
deposition (oCVD) (steps (i) and (ii) only). The oCVD blanket PEDOT layers readily
integrate with a wide range of substrates, since difficulties with film dewetting and
substrate degradation by solvents or high temperatures are completely avoided [19] and
the WF can be tuned by controlling the doping level of Cl- ions [20]. In this work, oCVD
will also be shown to be compatible with graphene substrates. The oCVD polymer layer
is
formed
by
directly
exposing
the
substrate
to
vaporized
monomer
(3,4
ethylenedioxythiophene) and an oxidizing agent (in this case, FeCl 3) under controlled
reactor conditions. oCVD is a dry process, which eliminates the wettability issues that
arise when using solution-processed PEDOT:PSS, and allows for complete coverage of
the graphene substrate with a smooth PEDOT layer.
The relatively mild deposition
conditions (low temperature 120'C, moderate pressure ~10 mTorr, and no use of solvents)
211
allow for PEDOT to be deposited without damaging or delaminating the graphene
electrode. Vapor printing is substrate independent: no substrate-specific optimization of
the oCVD process or substrate pretreatment was required to achieve vapor printing of
PEDOT onto graphene.
We then demonstrate that graphene based solar cells, fabricated with vapor
printed PEDOT HTLs, achieve ~94% of the performance of their ITO counterparts
without any additional treatment to the graphene sheets such as chemical doping. We
also show, for the first time, that graphene films synthesized under atmospheric pressure
chemical vapor deposition (APCVD) conditions, using a common copper foil (25 gm in
thickness and 99.8% purity, ALFA AESAR), can be well adopted as TCEs. The APCVD
films demonstrate comparable performance to graphene films synthesized using low
pressure chemical vapor deposition (LPCVD) conditions and we propose possible
benefits over the LPCVD process.
C.3
Results and discussion
C.3.1
Graphene electrodes
Graphene films considered in this work were synthesized under 2 different
conditions: LPCVD and APCVD.
Under the LP condition, graphene anodes were
prepared through layer-by-layer transfers by stacking 3 mono-layers of graphene sheets.
Under AP conditions, the anodes were constructed from only one layer of graphene sheet
which consists of a few layers.
Graphene electrodes (3-layer) using LPCVD have
average sheet resistance (Rsh) and transmittance values of -300 a/sq and -92% (at 550
nm), respectively, and the electrodes prepared using APCVD have average values of
212
~450 Q/sq and -92% (at 550 nm), respectively. From the transmittance values, it can be
inferred that a few layers of graphene can be grown under the AP condition whereas
under the LP condition the graphene growth process is self-limited and yields mostly
mono-layers [7]. As shown in Figure C-1, the graphene growth under the AP condition
does not seem to be uniform. It appears that there are discontinuous regions of a few
layers of graphene on the mono-layer background [optical images in Figure C-i (a-b) and
atomic force microscopic (AFM) image in Figure C-i (d)]. A mono-layer graphene sheet
from LPCVD is also shown in Figure C-1(c, e) for comparison. This non-uniformity
might be problematic in other applications but, for use in OPVs as TCEs, no significant
Figure C-1: Optical (a-c, scale bar: 10 pm) and AFM (d-e, scale bar: 1tm) images of
graphene transferred on the SiO 2 (300 nm) substrates synthesized under different pressure
conditions: (a,b,d) APCVD. (c,e) LPCVD images are shown here as a reference for
APCVD. (a-b) APCVD graphene consists of non-uniformly distributed multilayer
regions on top of the mono-layer background, which can be clearly identified from the
image taken at the edge of the graphene (b). (b) The dotted area indicates a region of
graphene film broken due to the transfer process, which further confirms the existence of
the mono-layer background. (d) AFM image which illustrates the non-uniformity of
APCVD graphene. The rms roughness of APCVD graphene is 1.66 nm compared to 1.25
nm for LPCVD graphene in (e).
213
problems were demonstrated in terms of device functionality. After patterning the
graphene electrodes, PEDOT (PEDOT:PSS or vapor printed PEDOT) and organic layers
were subsequently deposited followed by the top capping electrode via thermal
evaporation. The final device structure is: anode (ITO or graphene)/HTL (PEDOT:PSS
or vapor printed PEDOT)/DBP/C 60 (fullerene)/BCP (bathocuproine)/Al (aluminum). The
entire device fabrication process is schematically illustrated in Figure C-2 and further
details are explained in the experimental section.
C.3.2
oCVD PEDOT layers on graphene electrodes
We first demonstrate a few characteristics of vapor printed PEDOT, its morphology on
the graphene surface, and its effect on the electronic properties of graphene by analyzing
WF shift (evaluated using a Kelvin probe).
We then characterized the OPV device
performance under various experimental conditions such as vapor printed PEDOT
thickness, different types of graphene, and active layer thickness.
Shown in Figure C-3 are sheet resistance (Rsh) and transmittance values of vapor printed
PEDOT with varying thicknesses. The thinnest PEDOT layers (2, 7, and 14 nm) have
high transmittance values (generally >90%) that are ideal for HTL layers, where losses in
optical absorption through the transparent electrode can significantly decrease device
performance. The sheet resistance also decreases with increasing thickness, with a large
shift moving from 2 nm to the thicker layers (7, 14, and 40 nm), as the amount of charge
pathways increases. Having a lower sheet resistance HTL results in better charge transfer
to the graphene electrode, however, the thicker the HTL, the further the charge must
travel through the layer to reach the graphene electrode and the greater the transmission
losses due to absorption. The data points are summarized in Table C-1.
214
(a)
Synthesis
Transfer
MOCue~w
(b)
spin-casting
Graphene
VaporprintedDOTvia oCVD
\ Shadow-masked
graphene
Growing
PEDOTHTL
FeCl3
(c)
o ddjant
___
o
---------------------------
WN
BCP
Anode (Graphene/ITO)
Figure C-2: Schematics outlining the fabrication of graphene electrodes, PEDOT HTLs,
and OPV devices. (a) Graphene synthesis and transfer. The last part of the transfer
procedure is repeated to prepare 3-layer graphene stacks for LPCVD graphene. Detailed
growth parameters are graphically illustrated in Figure C-7, below. (b) PEDOT:PSS
spin-coating vs. vapor printing of PEDOT deposition. The spin-casting layer covers the
graphene and the surrounding quartz substrate while the vapor printed patterns align to
produce PEDOT only on the graphene electrodes. (c) Graphene anode OPV structure:
Graphene/PEDOT/DBP/C 60/BCP/Al.
215
100
90
--
80 ---
C
I-
2 nm, Rsh ~ 100,000 ohms/sq
7 nm, Rsh - 3000 ohms/sq
15 nm, Rsh - 900 ohms/sq
40 nm, Rsh ~ 500 ohms/sq
70-
60
300
400
500
600
700
800
Wavelength [nm]
Figure C-3: Transmittance data for the oCVD PEDOT HTL layers, measured using
ultraviolet-visible spectroscopy (UV-Vis) over wavelengths from 350-800 nm. The
oCVD PEDOT layers decrease in transmittance and sheet resistance with increasing
thickness. The three thinnest PEDOT layers (2, 7, and 15 nm) have high transmittance
values (>90% over a majority of the range), which are preferred for HTL layers. The
thickest (40 nm) PEDOT layer has a transmittance <70% over the wavelength range,
which causes performance losses in the devices due to optical absorption. The oCVD
PEDOT sheet resistance was measured using a 4-point probe (taking the average of 10
measurements). With increasing oCVD PEDOT thickness, there are more pathways for
charge transfer, so the sheet resistance (Rsh) decreases. Rsh decreases dramatically from
the thinnest (2 nm) to thicker PEDOT layers (7, 14, and 40 nm).
Table C-1: Optical transmittance (%T) and sheet resistance (Rsh) of oCVD PEDOT with
varying thicknesses described in Figure 3.
Transmittance
at 550 nm (%T)
Rsh
(LI/sq)
oCVD PEODT (2 nm)
98.0
100,000
oCVD PEODT (7 nm)
96.4
3,000
oCVD PEODT (15 nm)
94.1
900
oCVD PEODT (40 nm)
67.4
500
216
Figure C-4 shows optical images of graphene after the vapor printing of PEDOT
(15 nm). The optical image illustrates that most of the spin-casted PEDOT:PSS dewetts
over the graphene electrode as well as the adjacent bare quartz [Figure C-4(a,c,d)]. The
dewetting of the PEDOT:PSS is clearly observed over the entire substrate. In contrast,
vapor printing provides a well-defined PEDOT region (light blue) aligned only with the
graphene electrode [Figure C-4(b)]. As oCVD is a dry process, the PEDOT forms a
uniform film on the graphene without any visible defects or any problems with dewetting.
Figure C-4: Macroscopic images comparing HTL coverage of actual graphene devices:
(a) A blanket (i.e. unpatterned) PEDOT:PSS spun-coated on the entire 0.5" square
substrate. The coating is irregular and most of the PEDOT:PSS layer is dewetted from
the substrate with dark macroscopic defects visible to the naked eye. (b) In contrast, pure
PEDOT, containing no PSS, can readily be confined left side of the substrate if the area
on the right side is shadow masked. In the unmasked region on the left side, the oCVD
PEDOT appears as a uniform light blue coating. (c-d) Optical micrographs of the spincast PEDOT:PSS on the graphene surface illustrating the poor wettability of PEDOT:PSS
on the graphene.
217
The relative change in WF values between the graphene and vapor printed
PEDOT (7 nm)/graphene was evaluated by the Kelvin probe method. The WF of vapor
printed PEDOT on the graphene (3-layers) was measured to be -5.1 eV, similar to that of
PEDOT:PSS (-5.2 eV), where the WF of 3-layers graphene films was previously
reported as -4.3 eV from the ultraviolet photoelectron spectroscopy (UPS) [8].
This
increase in the WF (-0.8 eV) indicates the injection/extraction of holes from the HOMO
of donor now becomes energetically favorable compared to the interface of graphene
only.
C.3.3
Graphene/oCVD PEDOT anodes in OPV devices
Small molecule organic solar cells with graphene anodes (prepared from both APCVD
and LPCVD) were fabricated with device structures mentioned earlier. Figure C-5(a)
displays the current density-voltage (J-V) measurements of devices with various
configurations using 3-layer graphene anodes (LPCVD): graphene with PEDOT:PSS and
vapor printed PEDOT (15 nm) HTLs along with ITO reference (Graphene device with
PEDOT:PSS (i.e. graphene with poor wetting of PEDOT:PSS) is shown to emphasize the
dramatic improvement achieved using PEDOT deposited from the oCVD process). J-V
responses using vapor printed PEDOT HTLs clearly show significantly improved photoresponse compared to the graphene with PEDOT:PSS, which demonstrates that vapor
printed PEDOT is a desirable HTL for integration with graphene-based OPV devices.
Moreover, the performance (Jsc (short-circuit current density) = 5.69 ± 0.17 mA cm-2,
Voc (open-circuit voltage) = 0.88 ± 0.01 V, FF (fill factor) = 0.60 ± 0.01, and qp (power
conversion efficiency, PCE) = 3.01 ± 0.05%) is quite comparable to the ITO reference
218
device with PEDOT:PSS (Jsc=5.14± 0.12 mA cm 2 , Voc = 0.92 ± 0.01 V, FF = 0.68
0.01, and qp = 3.20 ± 0.05%).
(a)
-q"
2
E
E
0
-2
C
-4
-8
-0.5
0.5
0.0
1.0
Voltage [vM
(b)
3.5
4.5
5.5 -
6.5 [eV]Graphene I PEDOTI DBP / C6 0 /
IA
GrapheneI PEDOT I CuPc I C60 1
IAl
Figure C-5: J-V characteristics of representative graphene (3-layer, LPCVD) OPV
devices (Graphene/PEDOT:PSS (40nm), vapor printed PEDOT/DBP, 25nm/C60 ,
40nm/BCP, 7.5nm/Al, 100nm) under simulated AM 1.5G illumination at 100 mW/cm 2 .
(a) Graphene devices with PEDOT:PSS and vapor printed PEDOT (15nm) HTLs,
compared with ITO/PEDOT:PSS reference device. (b) Energy level diagram of complete
OPV device structure.
For a device with a non-optimized structure and undoped graphene with only 3
layers, our value of /p = 3.01% is already the highest value reported for a graphene anode
and small molecule based OPV (-94 of PCE of ITO based device). This number is even
219
higher than the recently reported value of i, = 2.5% by Wang et. al. [17]
However,
considering the fact that they used 4-layer acid-doped graphene stacks and, more
importantly, polymer based organic materials (P3HT:PCBM ([6,6]-phenyl-C 61-butyric
acid methyl ester)), which generally yield higher PCEs than small molecule based
devices, our results (higher PCE and comparable performance to the ITO reference
device) suggest that graphene is now at a more prominent position as an ITO replacement
in transparent conducting films (TCFs) applications. The generally high efficiency can
be attributed to the increased Voc observed in the cells compared to devices fabricated
using CuPc as donor materials, one of the most widely used donor materials in the small
molecules based OPV structure. As shown from the energy level diagram in Figure C5(b), the maximum Voc achievable from DBP/C 60 pair is ~1.0 V,
~0.3 V higher than
CuPc/C 60. Therefore, improvements in Voc mostly originate from the deep-lying HOMO
level of DBP compared to that of CuPc.
Graphene synthesized from the Cu catalyst under LP condition succeeded in
producing high quality mono-layer sheets, which could not be achieved using nickel (Ni)
catalyst [7, 21]. However, due to the self-limiting process of the LPCVD, achieving
multi-layers of graphene from the Cu is not feasible [7]. In practice, a mono-layer of
graphene sheet can be hardly used as an electrode due to defects induced from the
processing issues such as transfer or patterning, as well as the generally lower
conductivity compared to stacked multi-layers
(Rsh
decreases as a function of additional
layers [16]). In this work, we were able to synthesize a few layers of graphene under AP
condition with comparable optical and electrical properties to LPCVD counterparts,
although the uniformity was not as good as the films from the LPCVD process.
220
Nevertheless, for the particular application in TCFs, we verified that non-uniformity of
the graphene surface does not impede the device functionality as long as the surface is
fully coated with PEDOT which can be achievable via oCVD processing. Moreover,
since the APCVD graphene requires only one-time transfer, a significant amount of time
can be saved compared to the multiple transfer processes necessary for the LPCVD
graphene during the electrode preparation.
In Figure C-6, we demonstrate that solar cells fabricated with graphene anodes
prepared under APCVD condition perform close to the devices made via LPCVD
condition (/p,
2.49% and
APCVD
3.01%).
1/p, LPCVD
Figure C-6(a) shows the J-V
characteristics of graphene device with vapor printed PEDOT (Jsc=5.89 ± 0.03 mA cm-2,
Voc = 0.89 ± 0.03 V, FF = 0.48 + 0.01, and r/p = 2.49 ± 0.06%) and ITO reference device
(b)
(a)
2
E
-ITO/PEDOT:PSS
Graphen (AP)/vapor printed PEDOT
2
///
'
.-
E
---
LP Graphene
AP Graph ene
E
E
-2
.-
2
-4
-e4 -~
0
*
- 6-
-0.5
0.0
0.5
1.0
-0.6
0.0
0.6
1.0
Voltage [
Voltage [
Figure C-6: (a) J-V characteristics of representative graphene (APCVD) OPV devices
(Graphene/vapor printed PEDOT, 15nm/DBP, 25nm/C60 , 40nm/BCP, 7.5nm/Al, 100nm)
along with ITO/PEDOT:PSS reference device under simulated AM 1.5G illumination at
100 mW/cm 2 . (b) Comparison of graphene-based device performances, where graphene
electrodes are prepared under different conditions, i.e. LPCVD vs. APCVD.
221
with PEDOT:PSS (Jsc=5.14± 0.12 mA cnf, Voc = 0.92 ± 0.01 V, FF = 0.68 ± 0.01, and
r/, = 3.20 ± 0.05%).
Figure C-6(b) compares the device performance using graphene
electrodes from different synthesis conditions (LPCVD vs. APCVD), illustrating
comparable performances (APCVD graphene performs ~83% of LPCVD graphene).
C.4
Conclusion
In summary, we introduce a novel, yet simple, method for vapor printing PEDOT onto
the graphene surface, which yields well defined patterns using in situ shadow masking.
The oCVD process, which is the foundation for vapor printing results in smooth,
complete coverage of PEDOT on the graphene electrode.
In contrast, spin-casting
PEDOT:PSS from an aqueous solution does not coat the graphene surface as well due to
graphene's low surface free energy [12]. We also showed that a few layers of graphene,
although not uniform, can be grown under AP condition with commonly used Cu foil and
used as TCFs.
Organic solar cells with graphene anodes under different growth
conditions (APCVD: one time transfer, LPCVD: 3-layered stack through layer-by-layer
transfers) were fabricated and tested. We demonstrated that using the small molecule
donor material DBP combined with a vapor printed PEDOT HTL, yields highly efficient
graphene based devices with performances comparable to those of ITO reference devices.
Our results confirm that graphene is indeed a suitable candidate as an alternative TCF for
the replacement of ITO and open up opportunities in other applications as well, such as
organic light-emitting diodes (OLEDs).
222
C.5
Experimental
C.5.1
Graphene synthesis (LPCVD, APCVD)
Copper foil (25 prm in thickness, ALFA AESAR) was used as a metal catalyst for both
conditions. LPCVD. The CVD chamber was evacuated to a base pressure of 30-50
mTorr. The system was then heated to a growth temperature of 1000 'C under hydrogen
(H2 ) gas (~320 mTorr) and annealed for 30 minutes. Subsequently, methane (CH 4) gas
was introduced (total pressure: -810 mTorr) and graphene growth was carried out for 30
minutes.
The chamber was then cooled down at ~45 'C/min to room temperature.
APCVD. The chamber was heated to 1000 'C under H2 gas (170 sccm) and annealed for
30 minutes. After annealing, H2 was reduced to 30 sccm and CH 4 (1 sccm) and Ar (1000
sccm) were additionally introduced followed by 30 minutes of growth. After growth, the
chamber was cooled at -100
'C/min to room temperature.
Both processes are
schematically illustrated in the Figure C-7.
C.5.2
Graphene transfer and anode preparation.
Transfer was carried out using poly(methyl methacrylate) (PMMA, 950 A9, Microchem).
Graphene on one side of the foil was removed via reactive ion etching (RIE) with 02 gas
(Plasma-Therm, 100 Watt at 7x 10-5 Torr). Cu was etched by a commercial etchant (CE100, Transene). Graphene films were then thoroughly rinsed with diluted hydrochloric
acid (10 %) and de-ionized (DI) water to remove residual iron ions from the Cu etchant.
The PMMA layer was removed by annealing at 500 'C for 2 hours under H2 (700 sccm)
and Ar (400 sccm). Repeated transfers were performed for 3-layer graphene films. The
transferred graphene films were patterned into the desired shape through RIE.
223
(a)
1000 *C
Hl
H21
cm H2 10 seem
CH4 20 sccm
100 *C
20 min
30 min
30 min
(b)
45 *CImin
Time
1000 *C
H17som H230 sccm
CH. 1 sccm
4,
Ar 1000Osccm
100 * C
20 min
30 min
30 min
100 *CumIn
Time
Figure C-7: Illustration of graphene growth process at different stages: (a) LPCVD and
(b) APCVD.
Vapor Printed PEDOT. The oCVD reactor configuration and general process
procedure are described elsewhere [22]. The oCVD PEDOT HTLs were all deposited
under the same reaction conditions. The reactor pressure was held at ~10 mTorr and the
substrate
temperature
was
maintained
at
120'C.
The
monomer
3,4-
ethylenedioxythiphene (Sigma Aldrich, 97%), EDOT, was used as purchased.
The
EDOT was heated to 140'C and introduced into the reactor at a flow rate of ~5 sccm.
Iron (III) chloride (Sigma Aldrich, 99.99%) was evaporated from a heated crucible
between 130-160'C. Different thicknesses were achieved by varying the time of reaction
(1, 2, 4, and 8 minutes respectively to get HTL thicknesses of 2, 7, 15, and 40 nm).
(PEDOT:PSS (CleviosTM P VP Al 4083) was filtered (0.45 pm), spin-coated at 4000 rpm
224
for 60 seconds, and annealed at 210'C for 5 minutes in air). The PEDOT was patterned
using a pre-cut metal shadow mask with the same dimensions as the graphene electrode.
The mask was visually aligned such that the PEDOT was deposited directly on top of the
graphene.
C.5.3
OPV device fabrication process
Organic layers (DBP (Luminescence Technology Corp., >99%), C60 (Sigma Aldrich,
99.9%), BCP (Luminescence Technology Corp., >99%)) and top cathode (Al (Alfa Aesar,
3.175 mm slug, 99.999%)) were thermally evaporated through shadow masks at a base
pressure of 1 x 10-6 Torr at rates of 1.0 A/s and 1.5 A/s, respectively. C60 was purified
once via thermal gradient sublimation before use. DBP, BCP, and Al were used as
received. Pre-patterned ITO (Thin Film Devices, 20 Q/sq) substrates were cleaned by
solvents followed by 30 seconds of 02 plasma (100 W, Plasma Preen, Inc.). Patterned
graphene substrates were cleaned by annealing at 500'C for 30 minutes under H2 (700
sccm) and Ar (400 sccm). The device area defined by the opening of the shadow mask
was 1.21 mm.2
C.5.4
Characterization
The surface morphology of graphene sheet was characterized by AFM (Dimension 3100,
Veeco) and the transmittance was measured from the UV-VisNIR spectrometer (Cary
5000, Varian). Work function measurements were performed using a SKP5050 Kelvin
probe system from Kelvin Technology Inc. with analysis taken at various locations on
each sample with 50 measurements collected per location (using 30 point averaging).
Current-voltage measurements were recorded by a Keithley 6487 picoammeter in
225
nitrogen atmosphere. 100 mW cm- 2 illumination was provided by 150 W xenon arc-lamp
(Newport 96000) filtered by an AM 1.5G filter.
C.6
Acknowledgments
This work was supported by Eni S.p.A. under the Eni-MIT Alliance Solar Frontiers
Center and by a National Science Foundation Graduate Research Fellowship.
This
research was also supported in part by the Department of Energy Office of Science
Graduate Fellowship Program (DOE SCGF), made possible in part by the American
Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under contract
no. DE-AC05-060R23 100.
C.7
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