Nanostructural Engineering of Vapor-Processed
Organic Photovoltaics for Efficient Solar Energy
Conversion trom Any Surface
MASS CHTSIstIr
MASSACHUSETT-S INSiUT
OF TECHNOLOGY
by
Jill Annette (Rowehl) Macko
JUN 10 2014
_______________
S.B., Massachusetts Institute of Technology (2008)
Submitted to the Department of Materials Science and Engineering
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Materials Science and Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2014
@ Massachusetts Institute of Technology 2014. All rights reserved.
Author
Signature redacted
Department of Materials Science and Engineering
April 16, 2014
Signature redacted
Certified by...
Vladimir Bulovic
Associate Dean, Professor
Thesis Supervisor
A ccepted by .......................
Signature redacted
Gerd Ceder
Chair of the Graduate Committee
2
Nanostructural Engineering of Vapor-Processed Organic
Photovoltaics for Efficient Solar Energy Conversion from
Any Surface
by
Jill Annette (Rowehli) Macko
Submitted to the Department of Materials Science and Engineering
on April 16, 2014, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy in Materials Science and Engineering
Abstract
More than two billion people in the world have little or no access to electricity. To
be empowered they need robust and lightweightrenewable energy conversion technologies that can be easily transported with high yield from our manufacturing centers
to their (often) rural homes. Few conventional photovoltaic technologies are robust
enough to fill this need, however organic photovoltaics (OPVs) are ideal candidates
due to their potential to be ultra-lightweight and flexible. However, this promising
technology is currently limited by its relatively low power conversion efficiencies.
This doctoral dissertation seeks to speed the eming of this promising technology.
As a proof of concept for the accessibility and ultra-lightweight of OPVs, we integrate
vapor-processed carbon-based electrodes and sub-30nm-thin encapsulations in organic
photovoltaics, leading to the demonstration of monolithic, robust solar cell arrays as
well as the first ever solar cells fabricated directly on paper. Furthermore, we have developed and advanced two unconventional approaches to enhancing power conversion
efficiency via conventional methods: (1) optimization of multijunction efficiency via
computational optical interference modeling and subcell photocurrent balance quantization and control, and (2) novel implementation of conventional vapor processing
methods in the formation of molecular semiconductor crystals. This work has confirmed the potential of carbon-based materials to enable robust, ultra-lightweight,
efficient solar arrays, thus advancing their capacity to empower our brothers and
sisters even at the ends of the earth.
Thesis Supervisor: Vladimir Bulovic
Title: Associate Dean. Professor
3
4
Acknowledgments
Anything that takes 10 years would surely result in significant indebtedness, as well
as (hopefully) significant gratitude. After ten years at MIT, I would need hundreds
(or thousands!) of pages to properly thank everyone whon I'n indebted too, but I
will do my best to keep it a little shorter than that.
First, of course, I thank Vladimir, who is technically my "research" advisor but his
advice has expanded to far beyond that. I cannot thank God enough for the past 7.5
years that I have been granted to study and work under your supervision. I have acquired an incomprehensible amount of knowledge on research organic optoelectronics,
but more than that. I've learned about communication, teaching, managing, interacting with others, etc. Thank you for your mentorship, and I look forward to seeing
how God's plan for you continues to unfold.
I believe you can tell a lot about a professor by looking at their group, and I
have had the honor to work with excellent and unforgettable group members over
the years.
John and Alexi, thank you for your mentorship and incredible patience
with me when I knew and understood nothing. It was a pleasure to be trained under
you.
PV sub-group (Johnny, Andrea, Patrick, Joel, Ko, Tim, Ni, Eletha, Melany,
Mengfei, Anna): Thanks for enabling my work through our many fruitful discussions
as well as commitment to up-keeping and trouble-shooting our lab. And the rest of
LOOE/ONE Lab: I can't believe how much I've learned outside of vapor-processed
organic photovoltaics. Thank you for expanding my knowledge, and thank you for
making ONE Lab such an enjoyable place to work and study.
I have also had the special privilege to work with a variety of amazing collaborators. Prof. Peko Hosoi and Tony; Prof. Karen Gleason and Miles, Dave, Rachel, Nan;
Prof. Jing Kong and Hyesung; Drs. Andrea Bernardi, Riccardo Po, Petra Scudo. I
thank you all, for our work and time together and for expanding imy viewpoints and
experiences to other fields and in greater depth. Keep up the great work!
Along that lines, I thank my many funding sources. Of course, this PhD wouldn't
have been possible without someone else's wallet to pay for it. My first year was gen-
erously supported by fellowships from Vasili and Danae Salapatas as well as Jerome
and Dorothy Lemelson. The following three years were supported by a NSF Graduate
Research Fellowship. And my final year was supported by an Energy Fellowship from
the MIT Energy Initiative. Above and far beyond all that was financial support from
eni, an energy-producing corporation based in Italy. I have never had to worry about
using obsolete equipment or not being able to afford more materials for my experiments due to their unequivocal support of solar research at MIT. My PhD work is
also indebted to them.
My life at MIT has been more than just research, however.
I'd like to thank
also Prof. Rajeev Ram and Prof. Craig Carter for their mentorship in my teaching
experiences. Thank you for your inspiration, your encouragement, your support, and
the practical experience that I will build upon in my career to come. Thank you also
to my coworkers in course 6.007: Prof. Marc Baldo and Bill, Lisa, David, Michael,
Goran.
To my family: thank you for your support and your training of me throughout my
life. My grandfather showed me the meaning of hard work and perseverance. Even
though he's no longer with us here to see the completion of my PhD, I am thinking
of him at this time and spurred on by his memory. My grandmother knew that
education would open doors to a better life, and when she pushed her children and
even refused to speak her native language to them for the sake of their education in
English, I know she was thinking of me, and I am indebted to her vision and passion.
My mother and father have pushed me to be the best I can be, both as a student and
as a person. They have loved me and sacrificed much for me, and I thank God for
them. My sisters, my aunts and uncles, my cousins, and the rest of the family: thank
you for your presence in my life. I couldn't be who I am and where I am without
you. My husband: thank you for suffering through my PhD and my defense prep
with me and for carrying me when I couldn't continue. I hope God-willing that we
will continue to point each other (and others!) to Christ for many years to come.
And finally I must turn to my spiritual family, at Antioch Baptist Church. My
life is truly living because of the work of Jesus Christ in me through you all. P Paul
6
Kim and Becky JDSN, thank you for founding this church, and taking the time and
heart to care for and train even me, a completely undeserving sinner who somehow
wandered into your flock. P Dave and Angela SMN, my life is forever changed because
of your true teaching of the beautiful, wonderful words of God. I can never repay my
debt to you. P Thomas and Peggy SMN, words cannot express my gratitude. Who
am I that you have invested so much in me and cared for me so much? Thank you
for shepherding me in every aspect of life. And what more can I say? Time would
fail me to tell of P Heechin and Jean SMN, P James and Donna SMN, P Donald and
JY SMN, P Roy and Vania SMN, P Sang and Emily SMN, Amy unni, Tiff, Sue, all
of YA, all of Antioch. I am not worthy of their presence in my life.
Ultimately, I must thank my lord and savior, Jesus Christ, through whom I have
true Life, which is far more important than the temporary things of this world. I
confess that the completion of this thesis, of my PhD, and of 10 years at MIT, is
only possible through his grace and power, which made me alive and carried me
throughout the years. My life and my entirety is wholly devoted to Him and to His
coming Kingdom. All that's left of my breath must praise the LORD God:
Now to Him who is able to do far more abundantly than all that we ask
or think, according to the power at work within us, to Him be glory in the
church and in Christ Jesus throughout all generations, forever and ever.
Amen. --Ephesians 3:20-21
7
8
Contents
1
Organic Photovoltaics: Motivations, Fundamentals and Objectives
23
The Needs at the Ends of the Earth . . . . . . . . . . . . . . . . . . .
24
The Potential of Organic Photovoltaics . . . . . . . . . . . . .
25
Fundam entals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
. . . . . . . . . . . . . . . .
28
. . . . . . . . . . . . . . . . . .
32
Objectives of this Thesis Work . . . . . . . . . . . . . . . . . . . . . .
38
1.3.1
Robust, Ultra-lightweight Solar Arrays . . . . . . . . . . . . .
38
1.3.2
Efficient Power Conversion . . . . . . . . . . . . . . . . . . . .
38
1.1
1.1.1
1.2
1.3
I
1.2.1
Physics of Organic Photovoltaics
1.2.2
Organic Photovoltaic Devices
Electrodes and Encapsulations for Robust, Ultra-lightweight
40
Solar Arrays
2
Robust, Ultra-lightweight Solar Arrays Enabled by Vapor-Processed,
41
Carbon-Based Electrodes
2.1
2.2
Doped Graphene Electrodes for Organic Solar Cells on Glass . . . . .
42
2.1.1
Graphene Electrodes
. . . . . . . . . . . . . . . . . . . . . . .
42
2.1.2
Graphene OPVs . . . . . . . . . . . . . . . . . . . . . . . . . .
43
oCVD-Printed Polymer Electrodes Enabling Direct Monolithic Integration of Organic Photovoltaic Circuits on Unmodified Paper
.
44
2.2.1
oCVD-printed polymer electrodes . . . . . . . . . . . . . . . .
46
2.2.2
oCVD-printed PVs . . . . . . . . . . . . . . . . . . . . . . . .
48
9
2.3
3
49
2.2.4
Large-area monolithic photovoltaic arrays
. . . . . . . . . . .
52
2.2.5
Integrated paper PV demonstrations
. . . . . . . . . . . . . .
55
C onclusions
56
Sub-30nm Thin Encapsulation for Enhanced Device Lifetimes
57
3.1
. . . . . . . . . . . .
57
3.1.1
Degradation of OPVs . . . . . . . . . . . . . . . . . . . . . . .
58
3.1.2
Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . .
58
Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . .
59
3.2.1
Device Fabrication
. . . . . . . . . . . . . . . . . . . . . . . .
59
3.2.2
ALD Process and Annealing . . . . . . . . . . . . . . . . . . .
59
3.2.3
Device Characterization
. . . . . . . . . . . . . . . . . . . . .
60
Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .
60
3.3.1
Initial Performance . . . . . . . . . . . . . . . . . . . . . . . .
60
3.3.2
Degradation and Failure Behavior . . . . . . . . . . . . . . . .
61
3.3.3
Encapsulation with ALD Nanothin Film . . . . . . . . . . . .
63
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
3.3
3.4
4
oCVD-printed PVs on flexible plastic and paper substrates
. . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
3.2
II
2.2.3
Introduction........................
Engineering Higher Efficiency Organic Photovoltaics
66
Introduction to Nanostructures and Architectures
67
4.1
Subcell Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . .
67
4.1.1
Bilayer/Planar (Tang) Cell . . . . . . . . . . . . . . . . . . . .
67
4.1.2
Bulk Heterojunction and Planar-Mixed Heterojunction
. . . .
70
4.1.3
Novel Nanostructures . . . . . . . . . . . . . . . . . . . . . . .
70
4.2
M ultijunction Architecture . . . . . . . . . . . . . . . . . . . . . . . .
71
4.3
Nanostructural Engineering via Processing . . . . . . . . . . . . . . .
72
5 Practical Efficiency Limits of Organic Photovoltaics
5.1
Practical Limits Derived from Literature . . . . . . . . . . . . . . . .
10
75
76
5.2
5.3
5.4
6
Computational Methods
. . . . . . . . . . . . . . . . . . . . . . . . .
77
5.2.1
M aterials Choice
. . . . . . . . . . . . . . . . . . . . . . . . .
77
5.2.2
Planar Architecture . . . . . . . . . . . . . . . . . . . . . . . .
78
5.2.3
Ideal Nanostructured Architecture . . . . . . . . . . . . . . . .
78
5.2.4
Multijunction Architecture . . . . . . . . . . . . . . . . . . . .
78
Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .
79
5.3.1
Single Junction: Planar Architecture
79
5.3.2
Single Junction: Ideal Nanostructured Architecture
. . . . . .
80
5.3.3
Tandem Cells (with the Same Subcells) . . . . . . . . . . . . .
81
5.3.4
Multijunction Cells (with Different Subcells) . . . . . . . . . .
82
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
82
C onclusions
. . . . . . . . . . . . . .
Materials and Architecture Design in Sub-100nm Multijunction Pho-
83
tovoltaics
7
6.1
Experimental Methods . . . . . . . . . . . .
84
6.2
Characteristics of Optimized Device . . . . .
89
6.3
Recombination Zone Development . . . . . .
91
6.4
Optical Optimization of Sub-100nin Subcells
93
6.5
Conclusions
. . . . . . . . . . . . . . . . . .
96
Subcell Photocurrent Balance in Multijunction Photovoltaics
97
7.1
Introduction . . . . . . . . . . . . . . . . . . . .
97
7.2
Experimental Methods . . . . . . . . . . . . . .
98
7.3
Calculation and Simulation
. . . . . . . . . . .
99
7.3.1
Subcell Photocurrent Fitting . . . . . . .
99
7.3.2
Subcell Photocurrent Balance . . . . . .
99
7.3.3
Circuit Simulations . . . . . . . . . . . .
100
R esults . . . . . . . . . . . . . . . . . . . . . . .
101
. .
101
. . . . . . . . . .
103
7.4
7.4.1
Subcells with Dissimilar Fill Factors
7.4.2
Subcell Photocurrents
11
7.4.3
7.5
Dependence of Multijunction Performance on Subcell Photocurrent B alance . . . . . . . . . . . . . . . . . . . . . . . . . . . .
104
D iscussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
106
7.5.1
PCE Optimization in Multijunctions
106
7.5.2
Dependence of Subcell Photocurrent Balance on Non-Standard
. . . . . . . . . . . . . .
C onditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.6
8
C onclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
Vapor-Processed Crystals and Aggregates of Organic Semiconductors
8.1
8.2
8.3
9
112
Organic Semiconductor Needles Formed via Solvent Annealing'
-
111
8.1.1
Experimental Results . . . . . . . . . . . . . . . . . . . . . .
112
8.1.2
Physical Picture . . . . . . . . . . . . . . . . . . . . . . . . .
114
8.1.3
Mathematical Model and Numerical Results . . . . . . . . .
116
8.1.4
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . .
117
Low Vacuum Thermal Evaporation of Organic Semiconductors . . .
117
8.2.1
T heory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
117
8.2.2
Results and Discussion . . . . . . . . . . . . . . . . . . . . .
119
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
120
Conclusions
ii [21
9.1
Sum m ary
9.2
Looking to the Future
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . .
121
123
A Photovoltaic Primer for U.S. Policy Makers
11
[27
B Fabrication of Vapor-Processed Organic Photovoltaics
133
B.1 Introduction . . . . . . . . . .
133
B.2 Dominance of DBP . . . . . .
133
B.3 Donor Layer . . . . . . . . . .
134
B.3.1
Manufacturer . . . . .
134
B.3.2
Purity . . . . . . . . .
137
12
B.3.3
B.4
. . . . . . . . . . . . . . . . . . . . . . . . . . .
137
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
138
Growth Rate
Acceptor Layer
B.4.1
M aterials Choice
. . . . . . . . . . . . . . . . . . . . . . . . .
138
B.4.2
Purity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
139
B.5 Anode Interlayer
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B.5.1
M aterials Choice
B.5.2
Thickness
. . . . . . . . . . . . . . . . . . . . . . . . .
139
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
139
B.6 Cathode Interlayer . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B.7
B.8
139
140
. . . . . . . . . . . . . . . . . . . . . . . . .
140
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
141
Substrates and Substrate Treatments . . . . . . . . . . . . . . . . . .
141
B.7.1
Substrate Choice . . . . . . . . . . . . . . . . . . . . . . . . .
141
B.7.2
Substrate Treatments . . . . . . . . . . . . . . . . . . . . . . .
142
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
143
B.6.1
M aterials Choice
B.6.2
Thickness
C Contributions Associated with This Thesis
13
145
14
List of Figures
1-1
Global distribution of population without access to electricity. ....
1-2
Photos comparing use of kerosene-fueled lamp and LED lamp (powered
24
by renewable energy). . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-3
Graph chartering the history of record efficiency cells, classified by
technology type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-4
25
26
(Top) Cartoon of a pentacene molecule and (Bottom) spatially-resolved
measurement of its electronic cloud. . . . . . . . . . . . . . . . . . . .
27
. . . . . . . . . .
28
1-5
Schematic of three steps in photovoltaic operation.
1-6
Absorption spectra of thin filns of the molecules C60, DBP and ClAlPc,
each showing multiple peaks of absorption at resonant frequencies. Inset: cartoons of molecular structures. . . . . . . . . . . . . . . . . . .
29
1-7
Schematic of energy states pertinent to organic photovoltaic operation.
30
1-8
Schematic of energy levels at an organic heterojunction. . . . . . . . .
31
1-9
Schematics of vertical and lateral device architectures.
. . . . . . . .
32
1-10 Schematic of the interior structure of a vacuum thermal evaporator. .
34
1-11 Cartoon of a current density-voltage characteristic, identifying the Voc,
Jsc, and M PP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
1-12 Cartoons illustrating the Voc, Jsc, and FF and their effects on the
maximum power point (and thus PCE) . . . . . . . . . . . . . . . . .
37
2-1
Transmittance of graphene sheets of one to three layers. . . . . . . . .
43
2-2
Current-Voltage characteristics of organic solar cells with different almodes under dark and simulated AM1.5G illumination at 100mW-cn15
2
44
2-3
Relationship between percent transmittance (550 nm) and sheet resistance in ohms per square ( Q/D) for the vapor-printed oCVD PEDOT used in the work. Upper inset: 200-nm thick PEDOT film vapor
printed on tissue paper in 15 pt. bold Verdana font. . . . . . . . . . .
2-4
45
Top: Deposition of oCVD PEDOT and solution-processed PEDOT
on a variety of surfaces (a-c). Multimeters demonstrate conductivity.
Bottom: Patterned, large-area deposition of PEDOT on a variety of
surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-5
47
Current density- voltage characteristics under illumination (AM1.5,
100 mW-cm
2
) for oCVD PEDOT PVs on glass differing only in
anode structure (the yellow and red curves are for reference and do
not include oCVD PEDOT). . . . . . . . . . . . . . . . . . . . . . . .
2-6
48
(a) Device characteristics and electrode conductivity for oCVD-printed
PVs on PET (5-mil thick) after repeated flexes to 5-mm radius. (b)
Current density-voltage characteristics for oCVD PVs vapor-printed
on as-purchased tracing paper, copy paper, and tissue paper. . . . . .
2-7
Internal and external quantum efficiency comparison for devices on
glass/ITO/PEDOT:PSS (black) and tracing paper/oCVD PEDOT. .
2-8
50
51
(a) Printing schematic for 250-cell, series-integrated monolithic arrays.
(b) Current-voltage performance curves for series-integrated photovoltaic arrays with vapor-patterned oCVD electrodes. (c) Spatial map
of individual cell open-circuit voltages across the arrays.
2-9
. . . . . . .
53
(a) Normalized efficiency of thin-film-packaged and unpackaged arrays
as a function of time. The right photograph shows the laminated paper
circuit powering an LCD display in air with ambient sunlight. (b) A
paper array is progressively folded in air while being tested. (c) The
iCVD-coated array (28 series-integrated cells) is submerged in water
during operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
54
3-1
Schematics of the device architectures studied in this work.
nesses are listed on right, however the image is not to scale.
3-2
Thick. . . . .
Comparison of representative current-voltage characteristics of solar
cells of varying architectures, as deposited. . . . . . . . . . . . . . . .
3-3
62
Representative current-voltage characteristics of devices initially and
after failure or after 2 weeks (whichever came first).
3-5
61
Comparison of device shelf lives without encapsulation in inert atno-
sphere or ambient atmosphere. . . . . . . . . . . . . . . . . . . . . . .
3-4
59
. . . . . . . . . .
63
Impact of ALD encapsulation process on device performance of polymer cells with Al electrode and molecular cells with Ag electrode.
Shown are representative current-voltage characteristics of: initial device, after 10 hour annealing, and after ALD process (includes 10 hour
annealing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-6
64
Effect of ALD encapsulation on lifetime of polymer cells and molecular
cells, both with Al electrodes, stored in ambient environment or in
inert environ nent.
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-1
Historic Advances in Nanlostructural Engineering of OPVs.
4-2
Calculation of the external quantum efficiency (EQE) for donor-acceptor
. . . . .
64
68
heterojunction as a function of CiLc (change in line style) and aLJ)
(change in color), where LD is exciton diffusion length, a is absorption
coefficient, and LC is carrier diffusion length, highlighting the exciton
diffusion bottleneck.
4-3
. . . . . . . . . . . . . . . . . . . . . . . . . . .
69
Left: Device structure of a planar heterojunction OPV. Right: Schematic
showing the cross-section of the rrP3HT:PCBM BIIJ device. Focused
(c) and defocused (e) cross-sectional TEM images; inset: the imagnified
image of the rrP3HT:PCBM BHJ layer .
17
. . . . . . . . . . . . . . . .
70
4-4
Effect of annealing on planar-mixed heterojunctions of CuPc-PTCBI.
Top (a-d) shows SEM images and bottom (e-h) shows simulations of
the morphology. Leftmost images are as deposited, images to right
show increased aggregation due to increasing annealing temperatures.
4-5
73
Effect of annealing on bulk heterojunction polymer solar cells. Performance increases with annealing.
SEM shows that this is due to
increased phase separation of the polymers and fullerene materials.
.
73
4-6
Effect of solvent annealing with different solvents on a polymer blend.
73
5-1
Practical Limit PCE for a range of optical bandgaps.
Black is for
single junction architecture and purple is for multijunction architecture
(plotted versus the top subcell bandgap). The bandgaps of DBP and
ClAlPc are indicated. . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-2
Modeled Jsc for DBP-C60 and ClAlPc-C60 cells with either planar and
ideal nanostructure. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-3
77
79
Calculated PCE of planar and ideal nanostructure single junctions
(lines) and planar and ideal nanostructure tandems (squares) for DBPC60 with current demonstrations of FF and Voc.
6-1
. . . . . . . . . . .
80
Absorption spectra of C60 (green), DBP (blue) and ClAlPc (red) thin
films, showing broad spectral response. Molecular structures are inset.
84
6-2
Energy levels of each component layer of the multijunction cells. . . .
85
6-3
Device cross-section of the multijunction cells. Thicknesses not to scale. 85
6-4
Schematic of the set up for external quantum efficiency measurement.
6-5
Representative current density-voltage characteristics of MJ (green),
SJ1 (red) and SJ2 (blue).
6-6
. . . . . . . . . . . . . . . . . . . . . . . .
88
89
Wavelength-resolved external quantum efficiency of SCI (red) and SC2
(blue) selected via optical bias (A=532nm laser, 50mW; and AM 1.5G
simulation, 100 mW/cm 2; respectively). . . . . . . . . . . . . . . . . .
6-7
90
Device Characteristics of MJ cels with recombination sonzes of various
architectures and thicknesses.
. . . . . . . . . . . . . . . . . . . . . .
18
92
6-8
Optical fields for wavelengths absorbed by SCI (A=785nm) and SC2
(A=530in) within the M\J cell modeled via T-matrix formalism.
6-9
93
Architectures of conventional and inverted subeell order. Thicknesses
not to scale. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
94
6-10 (Left) Modeled external quantum efficiency and (right) experimental current density-voltage characteristics of conventional and inverted
sub cell order.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
95
6-11 Simulated short-circuit currents for an array of thicknesses of subcell
1 and 2 with fixed donor-acceptor ratios.
7-1
. . . . . . . . . . . . . . . .
95
Theoretical dependence of PCE loss on subeell balance for MJs with
similar FF subcells and dissimilar FF subcells. . . . . . . . . . . . . .
7-2
Circuit diagram for the MJ-OPV model.
. . . . . . . . . . . . . . . .
7-3
Experimental (solid lines) and simulated (dashed) current density-
98
100
voltage characteristics of single junction devices comprising SCI (red)
and SC 2 (blue). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-4
Current Density-Voltage Characteristics of single junction cells with
the SC I and SC 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-5
101
101
Representative reconstructed current density-voltage characteristics of
multijunction devices either balanced (left) or unbalanced (right). The
device on right is the maximal-PCE cell.
7-6
. . . . . . . . . . . . . . . .
103
Fitted photocurrent for an array of fabricated cells with the same SCI
thickness and varying SC2 thickness. Black lines are fits to the data.
Red and Blue lines are modeled subcell photocurrents.
7-7
. . . . . . . .
104
Dependence of FF, Jsc and PCE on SPB for an array of simulated
MJ-OPVs.
Left and Right show different ranges of SPB. Lines are
simulated values and points are experimental values.
19
. . . . . . . . .
105
7-8
(Left) Experimental Current Density-Voltage Characteristics of multijunction devices with increasing SC2 thickness and constant SCI thickness.
(Right) Simulated Current Density-Voltage Characteristics of
multijunction devices with balanced subeell photocurrents, large negative imbalance, or positive (optimal) imbalance . . . . . . . . . . . .
7-9
107
(Left) Experimental responsivities of max-PCE MJ and corresponding
SJ cells versus illumination intensity. (Right) Calculated subcell photocurrent balance and experimental power conversion efficiency for the
max-PCE MJ cell versus illumination intensity.
. . . . . . . . . . . .
108
7-10 Simulated MJ cell power output versus load resistance. . . . . . . . .
109
8-1
Process flow for growth of Alq3 needles.
112
8-2
(a) Optical micrographs of needles for different thicknesses of Alq3
. . . . . . . . . . . . . . . .
films after annealing for 1-5 hours. (b) SEM micrograph of rectangular
needles going from a common nucleation site after solvent annealing a
film with HAlq3=15nm. . . . . . . . . . . . . . . . . . . . . . . . . . .
8-3
113
(a) Successive snapshots of a cluster of needles growing during solvent
annealing. The top-left needle from the cluster is tracked over time
in (b). (b) Slice of micrograph pixels along the axis of a needle as a
function of tim e.
8-4
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
114
(a) Schematic of a needle growing into a fluid film. (b) Optical micrograph of the area surrounding an Alq3 needle after solvent annealing
for 3 hours.
8-5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
115
(a) Schematic of a needle growing into a fluid film. (b) Optical micrograph of the area surrounding an Alq3 needle after solvent annealing
for 3 hours.
8-6
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
Relation of "air" molecule's monolayer formation time and chamber
pressure. The typical length of an active layer deposition is identified
w ith the red line. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20
118
8-7
Relation of evaporate molecule's mean free path and chamber pressure.
. .
119
8-8
SEM image of CuPc "snow" formed by LVTE. . . . . . . . . . . . . .
120
A-1
Solar power per area separated by photon energy . . . . . . . . . . . .
128
A-2
Energy levels of photons and electrons in semiconductors. . . . . . . .
128
A-3
Excitement of electrons by photons in semiconductors . . . . . . . . .
129
B-1
Current Density-Voltage Characteristics of a variety of devices with
The throw distance of our chamber is identified with the red line.
nominally the same architecture and materials .
B-2
. . . . . . . . . . . .
135
Left: Schematic of device architecture highlighting the various layers
. . . . . . . . . . . . .
135
. . . . . . . . . . . . . . . .
136
. . . . .
136
. . . . . . . . . . . . . . . .
136
. . . .
137
. . . . . . . . . . . . . . . .
138
for optimization. Right: Device circuit model.
B-4
134
Current Density-Voltage Characteristics of a thickness optimization
run for two different batches of donor material.
B-3
. . . . . . . . . . . .
Thickness optimization of various layers.
B-5 Thickness optimization of DBP showing various parameters.
B-6
JVs with DBP from two manufacturers.
B-7
JVs with DBP as purchased or purified once or purified twice.
B-8
JVs with DBP grown at variety of rates.
B-9
JVs comparing C60 and C70 as acceptor, with the donor material ClAlPc. 138
B-10 JV and EQE for various C60 purities. . . . . . . . . . . . . . . . . . .
139
B-11 JV for various anode interlayers: MoOx and PEDOT. . . . . . . . . .
140
. . . . . . . . . . . . . . . . . . . .
140
B-12 JV for various MoOx thicknesses.
B-13 Left: JV comparing various cathode interlayers: BCP and Alq3. Right:
JVs of various thicknesses of BCP.
. . . . . . . . . . . . . . . . . . .
141
B-14 JVs of various substrates with either purified DBP or as-purchased DBP. 142
B-15 JVs of various substrates with various substrate treatments.
21
. . . . .
143
22
Chapter 1
Organic Photovoltaics:
Motivations, Fundamentals and
Objectives
The International Energy Agency estimates that one billion people worldwide have
zero access to electricity, and another one billion people have undependable access
to electricity. [1] However, the majority of these 2 billion people are at the ends of
the earth, far from the ends of the electric grid, thus filling their need is not as
simple as donating money to pay to flip a switch. Micro-generation of electricity is a
promising technology for these areas and is already demonstrating progress, however
gains are slow. We can speed the coming of omnipresent electricity by developing
photovoltaics that are robust and lightweight, thus easing utilization in areas of weak
to non-existent infrastructure.
Organic semiconductors have generated much interest due to their potential of enabling mechanically-robust, nano-thin photovoltaics, however their implementation is
still limited by their utilization of conventional substrate, electrode and encapsulation layers as well as their relatively low power conversion efficiencies. In this thesis,
we demonstrate application of nanomaterial-based electrodes and encapsulants fabricated via a variety of novel vapor-deposition processes for robust, lightweight photovoltaics on any surface/substrate. Furthermore, we present guidelines for fabrication
23
BANGLADESH 60
NEPAL $$
AFGHANISTAN N
F- 4AITI 2
KE4YA II
I
WANDA 6
CAMBODIA 71
SOUTH AFRICA 26
Porcentagp of populaion
without access to electricity
Source Uieid Nations
No date
0
25
50
75
.u
LI
Ouvelopmnenr Prograrn
a1
:. IIUS
Figure 1-1: Global distribution of population without access to electricity. [2]
and optimization of nano-thin multijunction photovoltaics, an architecture with large
potential for efficiency enhancement. Finally, we explore two novel approaches to
enhance subcell efficiency via enhancement of molecular order via vapor-processes.
1.1
The Needs at the Ends of the Earth
The two billion people in need of electricity are at the end of the earth, far from
the world's research institutions and industrial centers (Figure 1-1). [2] They are past
the end of the electric grid. They are past the end of the paved roads. They are
reached by camel and mule and foot. Mostly these people depend on burning carbon
for their basic energy needs: heat, light, cooking. However the burning of biofuels
leads to health and fire hazards, as well as consumes >30% of household incomes
in some areas. Furthermore, as these individuals seek to connect to the rest of the
world via radio and cell phones, they must seek out stores that sell electricity by the
minute, which are sometimes in neighboring villages tens of miles away.
One alternative is clean energy: electricity powered by natural sources. However,
24
Figure 1-2: Photos comparing use of kerosene-fueled lamp and LED lamp (powered
by solar energy). [3]
the availability of hydroenergy- the most prevalent form of clean energy today- is
highly localized and thus not capable of omnipresent micro-generation.
The same
is true for wind and geothermal energy. The sun, however, is available all over the
earth. Indeed. God "makes His sun rise on the evil and on the good." [Matthew 5:45]
Everyone has access to it. Therefore, if you want omnipresent clean energy: hydro,
wind and geothermal are good, but solar is a must.
1.1.1
The Potential of Organic Photovoltaics
Arguably, the best example of omnipresent clean energy today is plants. The
LORD God was the first engineer, and when He created the heavens arid the earth[Genesis
1:1] He designed arid implemented in plants a technology for converting solar energy
into chenmical energy (i.e. sugar) at efficiencies on the order of 3-6%. [4] This sounds
like aii unacceptably low efficiency to those of us in the solar research field, yet it
is evidently sufficient to power the earth due to the vastness of its implementation.
(Note also that plants are at the bottom of our food chain, so most of our bodily
energy is derived from them.)
Humankind has also invented and developed a variety of solar technologies. These
might be broadly classified under three types:
(1)
wafer-based technologies, (2)
thin-filn technologies, (3) nanomnaterials-based technologies. Wafer-based arid thin
filnm solar technologies are already common-place in the developed world, however
25
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nology type. [6,71
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nanomaterials-lases technologies are "emerging" from research laboratories to industry. Wafer-based technologies are characterized by their high efficiency and energyintensive fabrication processes. Thin film technologies have moderate efficiency and
moderate fabrication processes. Nanomaterials-based technologies are still in development but are currently characterized by low efficiency and potential for facile,
low-cost, high-throughput fabrication processes.
Nanomnaterial-based photovoltaics often possess the following characteristics: (1)
they consist of thin films (approx. 2 microns) of semiconductor material, (2) incorporate nanometer-scale optoelectronic materials that (3) have excitonic or quantumconfinement character and (4) can be fabricated entirely with low-temperature (<300C)
processing steps. [5] We limit the scope of this thesis to vapor-processed molecules.
This is not to suggest that other nanomaterials are not worth investigating, but rather
it indicates the expertise of this researcher as well as her limited time and cap~acities.
Note a fourth type labeled in Figure 1-3: "MJ-PV". This type has the highest
26
Figure 1-4: (Top) Cartoon of a pentacene molecule and (Bottom) spatially-resolved
measurement of its electronic cloud. [8]
efficiencies yet reported, up to 44.4%.
[7]
However the "MJ-PV" category is not a
specific materials set but rather a very special device architecture. The majority of
the MJ-PVs shown on this graph are comprised of wafer-based technologies aid are
thus difficult to process and commonly cost-prohibitive. In Part II of this thesis we
discuss the implementation of this architecture with nanoriaterials.
Conventional photovoltaics are already being significantly utilized in developing
nations (see figure 1-2). But due to their weight and fragility, they are limited to small
units with careful packaging. Can we instead fabricate a solar cell as lightweight arid
robust as a leaf, creation's original solar cell?' In this thesis we demonstrate a robust
and ultra-lightweight organic solar array as well as propose and explore multiple
approaches to enhancing its power conversion efficiency. But first, we describe the
fundamentals of organic photovoltaics: the physics and the device.
1.2
Fundamentals
Here we describe the key physical processes occurring within and the materials
arid device architectures of organic photovoltaics (OPVs). Note that Appendix A is
a primer that introduces physics anid devices of photovoltaics in elementary terms,
arid a newcomer to the field may find it beneficial to start there.
'Leaves may not seem very robust, but neither does glass and yet glass is used to stabilize and
protect conventional photovoltaics. It just goes to show that robustness is relative.
27
13
3
Figure 1-5: Schematic of three steps in photovoltaic operation.
1.2.1
Physics of Organic Photovoltaics
Fundamentally, the conversion of optical energy to electrical energy via a photovoltaic device requires 3 steps:
1. Interaction of photon and electron to form an exciton
2. Dissociation of exciton into charge carriers
3. Collection of charge carriers by an electrical circuit
Exciton Formation
Electrons on molecules 2 are like balls on springs, with an equilibrium position
but with some freedom to wiggle back and forth over the length of the molecule.
The spring constant of that wiggling (i.e. the attraction between positive nuclei and
electronic cloud) determines its resonant frequency. If you hit the electron with an
electric field wiggling at that resonant frequency (or close to it), the electron itself will
begin to wiggle back and forth at the same frequency and thence absorb the energy of
that electric field. [9] We know that the energy of a photon, EoPT, is proportional to
the frequency of its oscillating electro-magnetic field, v. Thus the electronic cloud's
2
Note that on the molecular scale (length scale of 1 nm=10- 9 m) electrons
should not be considered to exist at a single point, but rather to have a certain probability distribution, which looks
like a
cloud such as that shown in figure 1-4. However on the device scale (length scale of 1 micron=10- 6
m)
they are effectively point charges.
28
2.0
d
.
0
DBP
1.5
CIAIPc
a2)
0
o 1.0
C
0
0.5
0.0
C60
400
500 600 700 800
Wavelength ( nm )
900
Figure 1-6: Absorption spectra of thin films of the molecules C60, DBP and CIAlPc,
each showing multiple peaks of absorption at resonant frequencies. Inset: cartoons
of molecular structures.
resonant frequency will determine which Eop
0
it will absorb. The minimum value of
this is EGOPT, the molecule's optical bandgap energy.
If a ground-state electron absorbs a photon with an energy equal to the molecule's
optical bandgap, then it will be excited and form an exciton.
An exciton is the
3
bound pair of a positive polaron and negative polaron. The exciton diffuses freely
from molecule to molecule. However, note that it is neutrally-charged so it is neither
attracted nor repelled by electric fields.
Molecules have distributions of ground and/or excited state energy levels, and so
they will not have a sharp peak at resonance but rather a distribution. Furthermore,
molecules may have multiple resonant frequencies and therefore have multiple absorption peaks. An representative absorption spectrum is shown in figure 1-6. The
convolution of a device's absorption spectrum with the spectrum of the incident light
gives the number of excitons formed within a device. This is often identified as the
absorption efficiency, 'rA.
[10]
3 Polarons are the charge carriers in organics. They are simply slightly positive or slightly negative
electrical dipoles in the electrical cloud on each molecule. Positive polarons occupy the HOMO energy
band and negative polarons occupy the LUMO energy band. They move through an organic film
by hopping from one molecule to the next. They behave similarly to free electrons and holes, and
thus you will find many places in this thesis (as well as many, many publications elsewhere) where
polarons are mis-identified as electrons and holes. I apologize in advance but will continue to do so
for the sake of convenience and consistency.
29
EnergBd
EG,OPT
HOMO-LUMC
Photons
Ground
Exciton
Free
Carriers
AEDA
Free
at HJ
Figure 1-7: Schematic of energy states pertinent to organic photovoltaic operation.
Exciton Dissociation
Excitons are all well and good, but they do not give us electricity. We must also
convert the excitons into free charge carriers (which are typically polarons in organics,
as noted above), the flow of which is identified as current.
The energy needed to dissociate an exciton into a polaron pair is called the exciton
binding energy, EBinding.
In conventional semiconductors,
EBinding
is significantly
lower than kT at room temperature (=0.026 eV). [11] Such excitons can dissociate into
free charge carriers simply due to thermal variation. But in organic semiconductors,
EBinding
is >0-3eV.
[12-14] An energy step of this scale sometimes occurs at crystal
defects but such a low percentage of excitons were generated within a diffusion length
of these defects that the earliest organic photovoltaic efficiencies were unacceptably
low. [15,16]
In 1985 Tang introduced a radical new approach to exciton dissociation:
the
heterojunction. [17] By combining two organic semiconductors with different energy
levels, Tang introduced a photoactive interface where the molecular energy level difference was sufficient such that excitons dissociated with high probability. The next
step forward was the development of processing methods that distributed this photoactive interface throughout the active layer of the photovoltaic.
This work was
initially led by Heeger et al with the formation of the bulk heterojunction [18] and
has been pushed forward by a number of groups over the past two decades. A more indepth review of nanostructural engineering and its effect on photocharge generation
is presented in Chapter 4.
30
Donor
Energ
Acceptor
LUMO
AEDA
-HOMO
Position
Figure 1-8: Schematic of energy levels at an organic heterojunction.
The percentage of excitons converted to charge carriers is equal to the product of
the exciton diffusion efficiency,
'rEr
and the charge transfer efficiency, 'roT. Exciton
diffusion efficiency depends on materials properties and nanostructure. Charge transfer efficiency depends on voltage, light intensity, and temperature, [10] but is often
assumed to equal unity.
Charge Carrier Collection
Polarons will accumulate at the heterojunction interface as they are generated.
At this interface they are free to move in one direction (towards an electrode) but
not the other (towards the heterojunction) due to the energetic barrier. Thus, we
find that the distribution of polarons effectively spreads out towards the neighboring
electrode and is thus collected by the electrode.
Whether this spread towards to
the electrode is due to diffusion (simply the non-uniform concentration) and/or drift
(due to polaron repulsion or from an applied electric field) depends on the device
architecture and materials as well as operating voltage. The percentage of charge
carriers collected compared to charge carriers generated is often identified as the
carrier collection efficiency, 71cc.
Note that the electrical power collected is equal to current multiplied by voltage.
Care must be taken to maximize the number of charges collected (which leads to
current) as well as the potential energy of those charges (which leads to voltage).
This will be discussed in more detail below.
31
BCP
ACCEPTOR
DONOR
ITO
Glass
Figure 1-9: Left: Schematic of device architecture of a organic photovoltaic with
planar subcell, electrodes and interfacial layers. Note that the vertical length scale
is in the 100s of nanometers. Right: Schematic of lateral device architecture of a
organic photovoltaic with subcell (green), bottom electrode (pink) and top electrode
(blue). Note that the square is 0.5 inches long on each side.
1.2.2
Organic Photovoltaic Devices
Device Architecture
The central component of the OPV device is the photoactive layer (the subcell), on
which we have focused above. However, there are multiple other components. On the
most fundamental level, an electrode is required on either side of the device to collect
positive and negative charge carriers and channel them to a circuit for utilization. One
of the electrodes should be transparent to allow for the transmittance of photons to
the photoactive layer. Initial OPV demonstrations were simply thin films of molecules
deposited between thin films of two metals. [19] This is the simplest possible device
architecture.
However, significant enhancements in performance can be gained by adding interfacial layers (e.g. [20]), which assist in charge collection and minimize resistance
between the electrodes and the active layer. Furthermore, the device is usually prepared upon a substrate to provide stability. (Vapor-processed organic photovoltaics
have thicknesses <1 micron, thus they need additional support to prevent fracture.)
The sum of the above layers comprise our standard device architecture, which is
shown in figure 1-9a.
Additionally, devices benefit greatly from encapsulation to minimize exposure to
32
the environment. Most semiconducting materials have been shown to be significantly
affected by normal chemicals in the air such as water and oxygen. Thus solar panels
are almost always encapsulated to slow degradation. In chapter 3 of this thesis, we
introduce potential encapsulations for ultra-lightweight solar panels.
Please note,
however, that most of the devices reported in this thesis do not have encapsulation
and instead are fully fabricated and characterized in vacuum and/or inert atmosphere.
Note that this merely describes the vertical architecture of the device. It is also
necessary to design and control the lateral architecture of the device, including the
designation of positions where either of the electrodes can be electrically contacted
while ensuring that the electrodes are continually separated by the subcell to limit
shorting. The lateral architecture that we have primarily utilized in this thesis is
shown in figure 1-9b. [21]
Device Fabrication
Firstly, one must start out with the best possible materials, especially the organic
semiconductors. The impurity chemistry and concentration significantly impact the
optoelectronic behavior of semiconductors, [22] thus the source of materials matters and post-purchasing treatment must be considered. Organic molecules may be
purified via a three-zone furnace, which separates bulk materials by sublimation temperature. This process is described elsewhere. [21] The considerations of materials
manufacturers and purity for devices in this thesis are described in Appendix B.
The foundation of any thin film solar cell is a clean substrate, usually pre-coated
with commercially-deposited ITO (which is significantly higher quality than that
which is manufactured in house, see again Appendix B). Prior to any device fabrication, the ITO-coated glass substrates are solvent cleaned (dilute Micro-90, DI water,
Acetone and boiling Isopropanol), dried with a N2-gun, and stored in fluoroware in
inert atmosphere. In ONE Lab, our 'dry' glovebox is connected to a vacuum thermal
evaporation chamber (amongst others) via a ultra-high vacuum transfer line. This
minimizes exposure of the substrates and sequential layers to ambient atmosphere.
All materials discussed in this thesis are deposited via vacuum thermal evapora-
33
substrate
holder
thickness
monitor
gate
shutter
to
pump
stack
source
material
source
*boat*
power
supply
Figure 1-10: Schematic of the interior structure of a vacuum thermal evaporator. [23]
tion. A schematic of this chamber is shown in figure 1-10. Inside the chamber, a
crucible filled with the selected material is resistive-heated above sublimation temperature, thus vaporizing the material as individual molecules. Due to the ultra-high
vacuum pressure, the molecules travel line of sight out of the crucible and deposit
onto the substrate placed directly above. The deposition is non-conformal and may
lead to shadowing and non-uniform deposition, thus the substrate is rotated during
deposition. Thin film thickness is measured via a quartz crystal thickness monitor.
The thickness monitor is calibrated separately with a profiler.
The thermal evaporation process is repeated for each of the layers within the
device. Our chamber holds up to 6 crucibles of materials and thus usually does not
need to be reloaded at any point during the full device fabrication process. Electrode
thin films are patterned in situ via shadow masking to generate a controlled device
area.
Further details and tips for fabrication are included in Appendix B.
Device Characterization
The most fundamental method for characterizing the operation of a photovoltaic is
by measuring its electrical power output relative to the optical power input. This mea34
J
Power Conversion Efficiency,
PCE =
max. power point
input optical power
dark
Open Circuit Voltage, Voc
Vnt-
light
KV
Short Circuit Current, Jsc
Maximum
power
rectangle
Fill Factor,
FF =
max. power point
Voc - Jsc
Figure 1-11: Cartoon of a current density-voltage characteristic, identifying the Voc,
Jsc, and MPP .
surement gives us the power conversion efficiency, PCE. The conventional method for
doing so is to illuminate the PV with a lamp with known optical power (preferably a
solar simulator" with a color spectrum close to that of the sun) and measure current
output as you vary an applied voltage.
'
Electrical power is calculated by multi-
plying current and voltage, and the maximum power point (MPP)is determined by
investigating the full sweep of voltages in the fourth quadrant of the current-voltage
characteristic. A cartoon of a current density-voltage characteristic is illustrated in
figure 1-11.
5
There are 4 key parameters that can be derived from a current-voltage characteristic:
1. Open Circuit Voltage. V 00
Open circuit voltage, Voc; is the voltage that must be applied to the photovoltaic
such that zero current is flowing (i.e. the photovoltaic is in an open circuit). This
doesn't necessarily mean that there is zero motion of charges but rather that the
number of charges flowing out is equal to the number of charges flowing in. Voc
is related to the electronic energy levels, as shown in figure 1-7. Theoretically and
4
A terrific description of ONE Lab's opto-electrical characterization procedures is published in
the PhD thesis of Alexi Arango [21].
'Caution!: I usually intend to discuss current density, J, which is current per unit area. However
imany times I say simply "current" (which would be I).
practically the Voc is proportional to the optical band gap, [5, 241 however there is
much speculation within the OPV field that it is instead dependent on the donoracceptor interfacial energy difference. [25] In either case, there are multiple energy
losses within the device which leads to Voc significantly lower than both the donoracceptor interfacial energy (except in situations with recombination dominated by
the bulk rather than the interface [26]) and the optical band gap.
2. Short Circuit Current, Jsc
Short circuit current density, Jsc, is the current that is flowing out of the photovoltaic when there is zero voltage applied (i.e. the photovoltaic is shorted). Current
density is proportional to the number of positive charge carriers moving through the
device area over a given time, t. Thus Jsc is approximately equal to the flux of
photo-generated charges in the device due to the incident optical power. However
there are losses due to resistances and recombination within the device.
3. Fill Factor, FF
Fill factor simply describes the shape of the J-V characteristic. As illustrated in
figure 1-12, the shape of the curve significantly affects the maximum power output.
The effect of shape is quantified as the fill factor,
FF
=
VOC * Jsc
(1.1)
4. Power Conversion Efficiency, PCE
Power Conversion Efficiency, PCE is the ratio of electrical power out to optical
power in:
o=*Jsc*FF
PCE =
PIN
(1.2)
PIN
where MPP is maximal electrical power outputted and PIN is optical power. Power
conversion efficiency is the most commonly cited parameter of a photovoltaic and
arguably the most important, however the other parameters listed above allow us to
identify limitations and potentials of photovoltaics and guide us in the enhancement
of power conversion efficiency.
36
J
dark
Voc
light
V
dark
J
V
light
dark
V
light
Jsc
Figure 1-12: Cartoons illustrating the Voc, Jsc, and FF and their effects on the
maximum power point (and thus PCE).
Spectral Response
There is another perspective of characterizing a photovoltaic: spectral response. The
spectral response is necessary for understanding how the device responds to sunlight
as well as identifying which layer is behaving how. This is quantified by measuring
the external quantum efficiency, EQE. External quantum efficiency is equal to the
number of positive charge carriers moving through the device area during time t
divided by the number of photons incident on that device area during that same time
t, i.e.:
Holes Collected
Photons Incident
This parameter is also sometimes called Incident Photon to Current Efficiency, IPCE.
Internal quantum efficiency, IQE is a measure of the device performance regardless
of optical effects. It is equal to the number of positive charge carriers moving through
a device area during time t divided by the number of photons absorbed by that device
area during that same time t, i.e.:
Holes Collected
Photons Absorbed
37
(1.4)
1.3
1.3.1
Objectives of this Thesis Work
Robust, Ultra-lightweight Solar Arrays
Organic photovoltaics (OPVs) have generated much excitement in part due to
their potential as ultra-lightweight, flexible solar modules [27-30]. This potential is
primarily due to the favorable mechanical properties of the photoactive layer (namely,
their minimally ordered nanostructure), however a complete solar module is limited
by the electrodes, substrates, and encapsulates. Herein, we report on development of
carbon-based electrodes enabled by novel vapor-processes, which can be deposited on
any surface/substrate (Chapter 2). Furthermore, we discuss development of a nanothin, conformally deposited encapsulant, for encapsulation of ultra-lightweight solar
cells (Chapter 3).
1.3.2
Efficient Power Conversion
Every percentage increase in power conversion efficiency equates to a decrease
in area (and weight) required of solar cells for generation of a specified amount of
electrical power. Photovoltaics generally available in the US have PCE in the range
of 15-25%. However, current record organic photovoltaics are only 11.1%.
[7]
Can
organic photovoltaics match the efficiency levels of conventional solar technology?
This is where I have directed the remainder of this thesis.
Firstly, we survey the current OPV nano-structures and architectures in literature
(Chapter 4). Then we explore the trends of best OPVs in literature and apply it to
two standard OPV structures to understand the practical efficiency limits of these
standard sets if we were to increase the cells to previously demonstrated bests in
every aspect. Chapters 6-8 reports our work advancing OPV power efficiency via
multijunction architectures and novel vapor processes for organic semiconductors.
38
Multijunction Photovoltaics
The theriodynamic limit for converting solar power to electricity on Earth is
89%, but the limit for a single-junction photovoltaic at 33%. [24] The tried method
for simultaneously reducing thermalization and absorption losses is to stack multiple
cells with varying bandgaps. Indeed, this approach has led to record PV efficiencies
of 44.4% with triple junction cells. [7,31] While stacking of high efficiency inorganic
cells is limited by epitaxial growth considerations (i.e., lattice matching) and can
add substantial cost to PV fabrication, nanostructured materials do not have such
restrictions. This makes stacked growth of nano-PVs feasible on arbitrary substrates
and with arbitrary combinations of subcells, as already demonstrated for molecular
PV structures. [32]
Optimization of multijunction cells, however, requires cognizant choice of complementary subcells as well as balancing of the photocurrents (for series connection)
or photovoltages (for parallel connection) of the subcells. Chapters 6 and 7 of this
thesis discusses the design and optimization of nano-thin mnultijunction organic photovoltaics.
Nano-Structural Engineering
Ultimately, however, multijunction OPVs are still limited by the subcells within.
One common approach towards enhanced subcell efficiency is generating greater mixing of the donor and acceptor materials, however this can impede photocharge collection and detrimentally increase series resistance. An alternative approach is to enhance molecular ordering to extend the exciton and charge carrier diffusion lengths,
thus potentially increasing Jsc concurrently with VOC and FF. In Chapter 8, we
present two novel approaches to nano-structural engineering of vapor-processed organic semiconductors for enhancement of power conversion efficiency.
39
Part I
Electrodes and Encapsulations for
Robust, Ultra-lightweight Solar
Arrays
40
Chapter 2
Robust, Ultra-lightweight Solar
Arrays Enabled by
Vapor-Processed, Carbon-Based
Electrodes
Electrodes connect the photoactive materials with the real world; they collect
photo charges from the active layer for utilization as electricity. The most commonly
used electrode materials in OPVs are tin-doped indium oxide (ITO) and aluminum.
ITO utilized in a full 95% of published devices [33]. However ITO is an intrinsically
critical material due to the scarcity of indium. [34-37] Furthermore, the inechanical properties of ITO thin films are incompatible with lightweight and/or flexible
substrates. [38 -42]
Therefore the OPV field has been collectively seeking a substitute for ITO which
is transparent, conductive, flexible, easily processed, and earth abundant. [34, 4345] Current materials under research are:
conductive polymers, graphene/carbon
nanotubes, thin metal films or grids, and metal oxides. In spite of demonstrations
and progress, the opto-electrical properties need much further improvement before
reaching the "gold standard" of ITO.
41
Here I describe two approaches that we have taken in collaboration with other
groups here at MIT. Both are vapor-deposited, carbon-based electrodes.
One is
graphene (planar, crystalline carbon), [46] and the other is PEDOT (amorphous polymerized carbon). [38]
2.1
Doped Graphene Electrodes for Organic Solar
Cells on Glass
2.1.1
Graphene Electrodes
Graphene is a hexagonal arrangement of carbon atoms forming a one-atom thick
planar sheet. This layer is the building block of graphite and carbon nanotubes and
it has been studied widely by theorists since the middle of the last century [47, 48].
The successful isolation of single- and few-layer graphene by the mechanical cleaving
of highly ordered pyrolytic graphite (HOPG) [49] has led to an explosion of research
activities, and significant attention has been focused on their high electron and hole
mobility (up to 200,000 cm 2 y-is') [50, 51], high current carrying capability (up to
3x108 A-cm- 2 ) [52], and high mechanical robustness [53]. Graphene has also been
shown to have a uniformly high transparency in the visible and near infrared region
and thus can be utilized to form ultra-thin transparent electrodes [54].
Recently, the Kong group at MIT has developed a procedure to produce large,
area, continuous graphene sheets on copper foil via low pressure chemical vapor deposition (LPCVD). These sheets may then be transferred via a PMMA stamp to an
arbitrary substrate. Since the as-grown graphene is mostly single layered, in principle, one can control the number of graphene layers through multiple transfers which
results in overall improvements in the conductivity of the graphene electrode: for
the OPVs in this report, three-layered graphene sheets were used. The sheet resistance (Rsh) of graphene on quartz substrates can be varied from 500 to 300 Q/sq
and transmittance from 97.1% to 91.2% for 13-layered graphene sheets (figure 2-1).
As shown in figure 1(b), the optical transparency of our graphene sheet agrees quite
42
100
95
iF
90
C 96
so
70
4
Soo0
600
700
800
Wavolength (nm)
Figure 2-1: Transmittance of graphene sheets of onethree layers. As-grown LPCVDsynthesized graphene films are mostly single layered and each additional layer contributes approximately 2.3%O opacity over the range of wavelength. The inset indicates
the transmittance at 550 nm as a function of the number of graphene layers.
well with the rnasuremient performed by Nair et al [55] where each graphene layer
was reported to have approximately 2.3% opacity. This result confirms that our Cu
grown graphene layers are monolayer and the multiple transfer steps are successful to
maintain the integrity of the graphene layers. Furthermore, Kim [56] reported that
AuCl 3 (loping on graphene films resulted in up to 77% decrease in Rah with only 2%X
decrease in transmittance.
2.1.2
Graphene OPVs
Here we report the implementation of this large area, continuous, highly conductive and transparent graphene sheet, with controllable number of layers, as an anode
material in organic photovoltaic cells. The rest of the device architecture followed our
typical structure (electrode/PEDOT: PSS/active layer/BCP/Ag). The active layer of
the photovoltaics was comprised of copper phthalocyanine (CuPc) and fullerene (C 60 ).
Optimization of the device included adjustment of electrode surface energy via
fabrication and cleaning procedures as well as enhancement of graphene electrical
(i.e., withproperties. Typical power conversion efficiency (PCE) of pristine graphene
out doping) based solar cells using copper plithalocyanine (CuPc) and fullerene (Co)
is 1.37%, which is about 77.3% of the PCE of the equivalent indium tin oxide (ITO)
43
(d)
M~A ITO (0, 0lama)-daft
0 IIA. ITO (0,P"naLMurm0Wd1
* tC. Grapheme (DopdWdark
7t1C. Graphieme (DOped)Uumrnne
0
I-Jo.
E2
0
*-2
vs. Graphene
STO
-0.4
-0,2
0.0
0.2
0.4
0.6
0.A
Voltage (V)
Figure 2-2: Current Density-Voltage characteristics of organic solar cells with different
anodes under dark and simulated AM1.5G illumination at 100mW-cm- 2 .Shown here
is the comparison of performances of ITO with modified PEDOT:PSS by 02 plasma
(IIIA) and graphene doped with AuCl3 (10 mM) (1IC).
based cells (1.77%).
By chemically doping (p-type) the graphene with AuCl3 , we
found the PCE of graphene solar cells to be further improved (PCE: 1.63%), which
is comparable to (92.1% of) the device performance of ITO based solar cells.
Continuing work from the Kong group has further improved power conversion
efficiency
strates
2.2
[57] as well as demonstrated graphene electrodes deposited on flexible sub-
[58].
oCVD-Printed Polymer Electrodes Enabling
Direct Monolithic Integration of Organic Photovoltaic Circuits on Unmodified Paper
Here 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) [59, 60] with in situ shadow masking to create
44
a
100
800
so
0-00
13100'"0
0
op
'Thickness
50
100
150 20
(n)
Sheet Resitance (fl/o)
Figure 2-3: Relationship between percent transmittance (550 nm) and sheet resistance
in ohms per square ( Q/D) for the vapor-printed oCVD PEDOT used in thre work.
The red line is a fit to Equation (1) giving udc/u 0 p=9 (the dashedl black line is for
reference and corresponds to udc/ug),= 3 5, representative for traditional metal oxidle
electrodes). Lower inset: sheet resistance plottedl versus film thickness. Upper inset:
200-nm thick PEDOT film vapor printed on tissue paper in 15 pt. bold Verdana font.
well-defined polymer patterns on the surface of choice (Fig. 2-3, inset). For oCVD,
the polymnerized thin films form by simultaneous exposure to vap~or-p~hase monomer
(EDOT) and oxidant (FeCl3 ) reactants at low slubstrate temperatures (20-100~ C),
moderate vacuum (~-0. 1 Torr). The printed polymer patterns (down to 20 ipm resolution) result from the presence of a shadow mask by maintaining the partial pressure of
the vapor-dlelivered oxidanit species sufficiently (lose 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. [61, 621 Because the process is all
dry, there arc no wettability or surface tension effects on rough substrates like paper,
and exactly the same process steps are used to fabricate devices oni glass, plastics,
andl papers.
45
2.2.1
oCVD-printed polymer electrodes
For use as a transparent electrode, the conducting polymer layer must provide
both low sheet resistance (Reh) and high optical transmittance (T) which are related
by the following equation: [63,64]
T=
1+
(2.1
ZOop
2Rsh
(2.1
where ZO is the impedance of free space (377 Q), and Ordc and oo
optical conductivities, respectively.
PEDOT deposited on glass at 80
are the dc and
Figure 2-3 shows this relationship for oCVD
and 0.1 Torr.
The data fits well to equation 1
across the full data range, giving fcfd/JOO =9. This value is comparable to the best
commercially available conducting polymer solutions and is slightly lower than that
reported for carbon nanotube conductors
rdc/aOP=15 and traditional metal oxide
electrodes adc/cop=35. [63,64] The ability to integrate these other electrode materials
with paper has also not yet been demonstrated.
As shown in Figure 2-4, conductive, 150-nm thick, oCVD PEDOT polymer electrodes (100-1000 S-cm- 1 ) are uniformly deposited on ultra-delicate substrates with no
pre- or post-treatment and at identical conditions:
~10-pni 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
130Q, 1200Q, and 5.9 kQ respectively. (D) 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 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 FeCl3 oxidant at the substrate allows for
pseudo-directional flow of this species through the mask to substrate. This, in com-
46
A
Saran"m wrap
(hydrophobic)
B
Rice wraper
(solub)
C 1-ply bathroom tissue
(porous, absorbent)
5.9kl
pDchusetts
Fu2:
v ts
E
Institute of Technolo
achusetts Institute
of Technoto
m
lgaring daper
Meassachusetts Institute of Tech
TissuP per
W
assachus
Figure 2-4: Top: Deposition of oCVD PEDOT and solution-processed PEDOT oil a
variety of surfaces (a-c). Multimieters demionstrate conductivity. Bottomn: Patterned,
large-area deposition of PEDOT on a variety of surfaces.
47
b10
a f
iv. ITO/PEDOT:PSS
0
0.3
0.6
Vo1tage MV
Figure 2-5: Current density- voltage characteristics under illumination (AM1.5, 100
mW-cm-2 ) for oCVD PEDOT PVs on glass differing only in anode structure (the
yellow and red curves are for reference and do not include oCVD PEDOT).
bination 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.
2.2.2
oCVD-printed PVs
We first established the integration of oCVD PEDOT anodes into thin-film or-
ganic PV devices on smooth glass surfaces using the well-documented CuPc/C6
0
(or PTCBI)/BCP/Ag molecular organic heterojunction architecture (Fig. 2-5, inset).
[20,65-68] Fig. 2-5 compares representative current density-voltage characteris-
tics for individual devices on glass and differing only in anode structure:
(i) oCVD
PEDOT (50 nm), (ii) indium tin oxide (ITO), (iii) ITO/oCVD PEDOT (10 nm),
(iv) ITO/PEDOT:poly(styrenesulfonate (PSS). The series resistance (R)
allel
(shunt) resistance (R)
were determined by fitting the dark current data
shown) across the voltage range to the generalized Shockley diode equation:
Jdark
--
and par-
Ruas
(gnp (--VojRn
)
where n is the diode ideality factor and J is the reverse saturation current.
48
(not
[10]
(2 .2)
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). [20,69,70] Both oCVD PEDOT and PEDOT:PSS have
comparable work functions (~5.2 eV); [59] however, the conductance of oCVD PEDOT thin films is several orders of magnitude more conductive than the PEDOT:PSS
(CLEVIOST P VP Al 4083) buffer layer (<10 S-cm-1) [60], 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 Q-cm 2 vs. 4.2 Q-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-ci
2 )and
the short-circuit current (3.8 mA/cm 2 ) relative to
(ii). Moreover, we note that the power lost due to the electrode sheet resistance for
a cell of width a) and length 1, where current is collected along I can be estimated by
the following equation: [71]
PIOSS
=
sI 3sc)3
(2.3)
For the PEDOT electrodes used here, this corresponds to less than 1% fraction of
power lost (Pi0 s./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 mmni wide.
2.2.3
oCVD-printed PVs on flexible plastic and paper substrates
Next, we demonstrate the processing versatility of oCVD printed electrodes, by
fabricating these organic photovoltaics-device structure (i)-directly on various flexible
substrates, including common fiber-based papers (Fig. 2-6). We observed the oCVD
PEDOT electrodes and the full device to be electrically robust to mechanical deformations, which is highly desirable for low-cost roll-to-roll processing and potential
applications (Fig. 2-6a). After 1000 compressive flexing cycles at <5 mm radius, the
electrical conductivity of oCVD PEDOT on PET is minimally affected. In contrast,
49
a
b
niwe.
PMust (PET)
Substrate
10Tssue
0.6.cg
V.t
Paper
8
0Jsc
Cp
Paer
10
1
-
-
Paper
%R
10S
10'
PEW
101
1061 0
.7s000mwea
25
50
75
0
100
Flexing Cycles (R=S mm)
0.1
0.2
0.3
Voltage (V)
Figure 2-6: (a) Device characteristics under illumination (AM1.5, 100 mW -cm-
2
)
and electrode conductivity for oCVD-printed PVs on PET (5-mil thick) after repeated
flexes to 5-mm radius. The dashed black line shows the conductivity of ITO/PET
for reference. (b) Current density-voltage characteristics under illumination (AM1.5,
500 mW - cm- 2 ) for oCVD PVs vapor-printed on as-purchased tracing paper (~40-
pm thick), copy paper ( ~120-pm thick), and tissue paper ( ~40- pm thick).
flexing of commercially fabricated ITO-coated PET substrates decreases the film conductance over 400-fold due to formation of cracks visible under optical microscope.
Moreover, after over 100 compressive flexing cycles, devices with oCVD PEDOT electrodes on PET, display no significant change in performance. 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. This suggests that
the electrode cracks compromise the integrity of the subsequent device layers as has
been reported elsewhere, [72] in contrast to the flexible oCVD PEDOT devices, which
maintain function and are visibly unchanged after repeated flexing.
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. The surface reflectivity is evident in the performance curves for
devices on various papers (Fig 2-6b), in which the short-circuit currents scale inversely
with losses due to reflection. This is also evident by comparing the quantum efficiency
and absorption spectra for oCVD PEDOT devices on tracing paper with conventional
50
C 30 c6
PTCBI
PAcceptor
Acceptor
a,
10
4W0
Figure 2-7:
60W
NO 4W0
IWAWFIGmOR Wh (n)
6W0
SW
Internal and external quantum efficiency comparison for devices on
glass/ITO/PEDOT:PSS (black) and tracing paper/oCVD PEDOT.
glass/ITO/PEDOT:PSS devices (Fig. 2-7). 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.
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 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.
51
2.2.4
Large-area monolithic photovoltaic arrays
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 fabricating 250-cell (0.1xO.3 cm 2 each) series-integrated monolithic arrays on both paper
and glass substrates. Figure 2-8 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. [73] 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 current-voltage characteristics show high open-circuit voltages of 50V
and 67V, respectively (Fig. 2-8b), 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 series-integrated
arrays were recorded to gain insight about overall cell statistics. The cell voltage distributions on both glass and paper substrates (Fig. 2-8c) 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
less-conformal, evaporated photoactive layers. However, as was discussed recently for
paper-based organic transistors, [74] a specific correlation was not immediately apparent by examining optical and SEM surface images of functional and non-functional
cells.
52
now
y
Anod
AnofAc~
CuPf/PTC8l/BCP
oCVD PEDOT
Ag
-
c
b..
Series-lntegrated oCVDOIPV
250 PV cells In serles
C66010
7 cm
i.
-
(MV)
3501
0
Paper
110.
Glass
rCIUft
Voc-SOV
0
50
rCIult
j400
Voc-67V
10 0
0
MM
O
Figure 2-8: (a) Printing schematic for 250-cell, series-integrated monolithic arrays.
The photographs show the printed PEDOT (~50-nm thick) 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
2
paper (red) and glass (black) under illumination (AM1.5, 80 mW - cm- ) (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 given voltage.
53
a
IAL
4
somWem .IddevWca
0.6
0.4
-N
7"n
s
L"o
Nomr Innwtd
0
b 0.2
1.0
p
640*
90
-uww
~
LO
KEWM@nsftpqpwdftA
y\
0.4
0.2
0.0 .
0.2
0
2
0
4 3 6
Nunber of Fokds
10
20
TWO (s)
A
Figure 2-9: (a) Normalized efficiency of thin-film-packaged and unpackaged arrays
2
(250 series-integrated cells) subjected to constant illumination (80 mW - cm- , halogen lamp) in air at 42 C (108 0 F) as a function of time. The right photograph shows
the laminated paper circuit powering an LCD display in air with ambient sunlight.
(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. (c) The iCVD-coated
array (28 series-integrated cells) is submerged in water during operation. 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.
54
2.2.5
Integrated paper PV 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. [75] 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 (Fig. 2-9).
250-cell series-integrated ar-
rays on tracing paper were encapsulated on both sides with three encapsulants 1)
5-mil thick plastic laminate applied with an office laminating machine, 2) 750-un
thick poly (monochloro- p-xylylene) ("parylene-C") deposited by self-initiated CVD
polymerization, and 3) 750-urn thick hydrophobic and crosslinked poly (1H,1H,2H,2Hperfluorodecyl acrylate) ("PPFDA") film deposited by initiated CVD (iCVD) polymierization [59].
The packaged and unpackaged series-integrated arrays were then
aged in air,, accelerated by exposure to constant illumination (80 mW-cim-2, halogen
lamp) and elevated temperature (42' C/108' F) at open-circuit (Fig. 2-9a).
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) [20] more than doubles the rate of
power decay. We emphasize that this lifetime test was performed on the full monolithic 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 illumninated lifetime, [76, 77] similar " shelf' lifetime of
hybrid-encapsulated cells, [78] and similar illuminated lifetime for an encapsulated
single tandem cell. [79] The right hand inset, shows a laminated paper PV array
powering an LCD display and related circuitry in air using sunlight from the window,
55
which has remained operational after over 6000 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 paper photovoltaic arrays is shown in Fig. 2-9b, in which
high voltages are produced in each folded three-dimensional configuration and are
maintained during dynamic folding and unfolding.
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 is evident by the
increase in voltage when the accordion structure (- 450 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 iCVD-
coated 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 (Fig. 2-9c).
This paper photovoltaic 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 the LCD
clock.
2.3
Conclusions
Herein we report our investigations of novel carbon-based electrode materials from
the Kong and Gleason groups and demonstrated their capacity for use as transparent electrodes. Further, we develop and optimized surface treatments and fabrication methods for integrating these electrodes with organic semiconductors. Finally,
we demonstrated the first ever solar cell fabricated directly on paper, and further
demonstrated a monolithic solar cell array on paper. This advances the development
of robust nano-thin electrodes for use with ultra-lightweight, low cost substrates.
56
Chapter 3
Sub-30nm Thin Encapsulation for
Enhanced Device Lifetimes
3.1
Introduction
Organic photovoltaics (OPVs) have generated much excitement in part due to
their potential as ultra-lightweight, flexible solar modules [28 30], however the current range of device lifetimes in literature (0-5 years) [80-82] do not yet achieve the
lifetimes necessary for commercialization (5-10 years). [5] Device lifetime is a key figure of merit for solar cells because it impacts the levelized energy cost ( $/(kW-hr) )
of a photovoltaic module as well as its carbon release factor. [5] Its importance necessitates the solar cell encapsulation, however the prevalent commercial encapsulation
(polymer thin film plus glass sheet [81,83,84]) is inflexible and significantly increases
the weight of the module, thereby increasing the installation and balance-of-systems
costs.
In this work we characterize the device behavior of OPVs of various archi-
tectures over time, and further we implement a 27nm-thin film of alternating layers
of HfOx and AlOx deposited by atomic layer deposition (ALD) as an encapsulation
layer [85] and characterize its impact on shelflife.
57
3.1.1
Degradation of OPVs
Literature contains many investigations of degradation in OPVs, most pointing to oxygen, water, and light exposure as dominant sources of performance decay. [86-89] However, device degradation behavior depends greatly on the architecture of the OPV: on its active layer, interlayers, electrodes, substrate, and encapsulation. [75,80,84,87,90-96] In this letter, we compare the impact of degradation
on device characteristics for two different active materials: the archetypal polymer
system P3HT:PCBM (solution-deposited) [97, 98] and the common small molecular
system ClAlPc:C60 (vapor-deposited) [99, 100], as well as electrodes of either Al or
Ag composition (both vapor-deposited).
3.1.2
Encapsulation
A variety of thin film encapsulations have been proposed and explored, including
many that are based on organic and/or inorganic thin films that are potentially flexible and light-weight [80,101-108]. Atomic Layer Deposition (ALD) is promising for
controllably depositing nanothin inorganic films, however this process is often done
at relatively high temperatures and with exposure of the device to reactive reagents
(e.g.
water). [109, 110] In this letter we implement an ALD nano-thin composite
proposed by Chang et al. [85, 111] Even with thickness <30nm, the WVTR of the
nano-multilayer film is <5 x 10- 4 g-m
2
-day- 1 , [111] at or below the proposed thresh-
old necessary foreffective encapsulation of organic electronics [75].
Furthermore, it
is highly transparent, potentially flexible, and can be conformally deposited over
3-D large areas on any surface (beneficial for PVs deposited on highly structured
substrates such as fibrous paper or for nanostructured PVs for enhanced optical behavior).
58
Molecular PMHJ
Polymer BHJ
25 nm
Encapsulant
100 nm
Al or Ag
25 nm
110nm
BCP
ACCEPTOR
125 nm
MIXED
-
40 nm
DONOR
110 nm
|
ITO
S
|
ITO
i
90 nm
Glass
Glass
Figure 3-1: Schematics of the device architectures studied in this work. Thicknesses
are listed on right, however the image is not to scale.
3.2
3.2.1
Experimental Methods
Device Fabrication
The device architectures studied in this work are shown in Figure 3-1. The photovoltaic devices are fabricated on solvent-cleaned, pre-patterned ITO-coated glass
(Thin Film Devices, 20 Q/D). The polymer solar cells consisted of an anodic buffer
layer of PEDOT:PSS and an active layer of a blend of P3HT and PCBM (Plexcore
PV1000). The molecular solar cells consisted of 20nm of MoOx as the anodic interlayer, 10nm of ClAlPc, 20nm of a 1:1 blend of ClAlPc:C60, 20 nm of C60, and
7.5nm of BCP as cathodic interlayer. Stripes of either 100nm of aluminum or 100nm
silver were deposited via vacuum thermal evaporation orthogonal to the ITO layer,
generating a 0.0121 cm 2 device area.
3.2.2
ALD Process and Annealing
An ALD barrier multilayer film comprised of AlOx and HfOx was deposited on
top of completed devices using a Savannah 200 system by Cambridge NanoTech.
Trimethylaluminium (TMA) and water were used as the precursors for the aluminum
59
oxide layers. Tetrakis(dimethylami-do)hafnium (TDMAH) and water were used as
precursors for hafnium oxide layers. The growth rates for the two oxides were respectively 1.2 A/cycle for AlOx and 0.9A/cycle for HfOx. In this study we used four
cycles of deposition of AlOx and 4 cycles of HfOx, and we repeated the deposition
of the two materials 30 times for a final thickness of 27nm. The ALD process took
approximately 10 hours, and the substrates were held at a substrate temperature of
100'C during the entire deposition time. In order to make more comparable the fabrication processes for the cells with and without the barrier film, all completed devices
that were not encapsulated were annealed at 100'C for 10 hours in inert atmosphere.
3.2.3
Device Characterization
OPV shelf life was tested according to the recommended protocol ISOS-D-1 Shelf
[112]. To describe briefly, the devices were stored in dark at open circuit, at ambient
temperature and humidity (approx. 25'C, 55% RH). All opto-electrical characterization was completed in a glovebox with inert atmosphere. Current density-voltage
characteristics are measured under dark conditions and under 1 sun illumination
(100 mW/cm 2 , AM 1.5G), using a 150 W solar simulator (Newport, 6255) illuminated through a AM1.5G filter (Newport, 81094) calibrated with an NREL-certified
monocrystalline-Si photodiode (Newport, 91150V). Results are not corrected for spectral mismatch. Electrical characteristics are measured using a picoammeter (Keithley,
6487) employing a switch mainframe (Keithley, 7001) for switching between cells.
3.3
3.3.1
Results and Discussion
Initial Performance
The performance of the cells as fabricated are shown in Figure 3-2. The polymer system P3HT:PCBM gives a higher power conversion efficiency (3.1%) than the
molecular system ClAlPc:C60 (2.0%). This is primarily due to the higher Jsc (8.84 vs
5.45 mA/cm 2 ), which is due to higher absorption efficiency of the polymer cell under
60
4-
0~
-2-
-
Polymer, Al
Polymer, Ag
-
Molecular, Al
Molecular, Ag
1.0
-
-0.5
0.5
0.0
Voltage [V ]
Figure 3-2: Comparison of representative current-voltage characteristics of solar cells
of varying architectures, as deposited.
solar illumination.
The polymer cells have highest initial performance with aluimnun electrodes, and
the molecular cells have highest initial performance with silver electrodes. However,
we fabricated both active layers with both electrodes to study the electrode effects
on lifetime.
Utilizing an Ag electrode with the polyier cell decreases the overall
efficiency (3.1% to 0.5%), primarily by decreasing the shunt resistance, likely due to
shorting of the Ag atons through the polymer blend layer. The molecular cell, oii the
other hand, has a BCP thin film between the active layer and cathode, that significantly decreases the issues with Ag shorting through the device. [20] Anm alunminun
electrode decreases the short circuit current of the molecular cell in comparison with a
silver electrode (5.45 mnA/cmn2 to 3.45 mA/cm'). This gives a significant but smaller
decrease in efficiency (2.0% to 1.2%).
3.3.2
Degradation and Failure Behavior
Figure 3-3 shows the lifetime of each of the solar cell architectures after storage in
inert atmosphere or in ambient atmosphere without encapsulation. The polymer solar
cells "died" within 20 hours of storage in ambient atmnosphere. However the molecular
cells continued to generate power after 450 hours in ambient atmosphere, regardless
of electrode composition. The significant difference in shelflife between polymeric and
61
Inert Atmosphere
I
3.0
- -
40
II . I II . I
e
h
Ambient Atmos
Atmnounhuru
13.5
Inert
AtosphereAmbient
.
3.0
- -
Polymer, A]
2.5
2.5
2.0M
2.0-
U 1.5-
1.5-
1.0.
0.5 -
S
Molecular, Ag
1.0t
y?
r
0.0
0
200
400
600
800
1000
0.5
a
Molecular Ar
0.
P lymer. Ag
1200
0
Time (hours)
100
-
200
-
300
400
500
Time (hours)
Figure 3-3: Comparison of device shelf lives without encapsulation in inert atmosphere
or ambient atmosphere.
molecular cells with either electrode rules out electrode oxidation as the dominating
factor in polymeric cell degradation. Therefore the dominate degradation mechanism
can be atributed to the effect of moisture on either P3HT or PEDOT:PSS. [88,89]
For cells with an Ag electrode, the fraction of devices shorted increased over time,
far faster in comparison with the Al electrode cells. This suggests that the silver
atoms continue to migrate even after fabrication to cause shorts, and the BCP isn't
completely protective. [20]
All solar cells show substantial decreases in performance over 500 hours in ambient
atmosphere (75-100% of initial PCE), however they have significantly lower PCE
decay when stored in inert atmosphere over the same length of time (8-11% of PCE)
(Figure 3-3), thus they have the potential for substantial enhancement in lifetime due
to encapsulation from atmosphere.
Figure 3-4 shows the J-Vs of devices initially and after failure or after two weeks,
whichever was first. The molecular cells primarily degrade in conductivity and fill
factor by various amounts (possibly due to introduction of an energetic barrier). Most
polymer cells also exhibit significant conductivity degradation and lose their power
conversion efficiency relatively rapidly. Further, all polymer cells with Ag electrodes
shorted within 1 week, either before or after substantial conductivity decay (see IV
curve of Polymer, Ag, Ambient in Figure 3-4).
This indicates that Ag electrodes
lead to device degradation that is not caused by environmental exposure and thus -if
62
4-
4--
2-
2-
C,"
E
E
-2-
-2 Z0
-6---
1 ___
-0.5
_
0.0
0.5
Molecular
Al, Initial
At, Inert
Al, Ambient
Ag, Initial
Ag, Inert
Ag, Ambient
Ambien
1.0
2
d
Polymer
Al, Initial
Inert
6-Al,
-8
Al, Ambient
Ag, Initial
Ag, Inert
-
1__Ag,_Amb__
-6.5
nt
0.0
Voltage[ V ]
Voltage [V ]
0.5
1.0
Figure 3-4: Representative current-voltage characteristics of devices initially and after
failure or after 2 weeks (whichever cane first).
utilized- necessitate strategies in addition to atmospheric encapsulation. [113]
3.3.3
Encapsulation with ALD Nanothin Film
Effect of ALD Process on Device Performance
ALD requires exposure of the device to elevated temperatures for an extended
period of tinie, and additionally the device is exposed to precursors (e.g. water) during
thin filn deposition. [109, 110] Chang et al optiniized their ALD process conditions for
optimal annealing of a specific device architecture and conposition. [85] To investigate
the impact of temperature vs precursor exposure, we separately annealed a batch
of devices with no ALD film for the same length of time and temperature as the
devices which had ALD fihn deposited on then (10 hours at 100 C). Figure 3-5 shows
perforimance of polymer cells with aluminum electrode and small niolecule cells with
silver electrode (1) initial perforniance, (2) after 10hours of annealing at 100'C, and
(3) after encapsulant deposition (which includes 10 hours of annealing at 100 0 C).
For the polymer cells, annealing at 100 0 C for 10 hours increases the Jsc by 12.5%
but has no effect on overall PCE due to small decreases in FF (5%) and Voc (7%).
For the nolecular cells, annealing at 100'C for 10 hours increases the fill factor by
6%, the Jsc by 4% and the PCE by 11%. However the ALD process had negligible
effect on the performance parameters of either the polyner cells or the molecular
63
4-
_______I
2-
0-2 -
Polymer with At
Initial
--Annealed
--ALD (inci annealing)
Molecular with Ag
C
- 000_0_
-6-
'7
S
-.
S- - - --
-
Initial
--
-
-0.50
-0.25
0.00
0.25
0.50
Annealed
ALD (inc annealing)
0.75
1,00
Voltage [ V I
Figure 3-5: Impact of ALD encapsulation process on device performance of polymer
cells with Al electrode and molecular cells with Ag electrode. Shown are representative current-voltage characteristics of: initial device, after 10 hour annealing, and
after ALD process (includes 10 hour annealing).
Polymer
1.0.
-
Molecular
1.0-
ALD Inert
0.8-
0.8-
*To 0.6.
noALDlnert
...-
M 0. -
Z 0.4 -
LD Ambient
w
a. 0.2-
0.0-
.....-II..
SAmbe
Z
noALD, Ambient
0
200 400
0.4-
a2. 0.2-
600 800 1000 1200
noALD, Ambient
0
Time (hours)
200
400
600
800
1000
1200
Time (hours)
Figure 3-6: Effect of ALD encapsulation on lifetime of polymer cells and molecular
cells, both with Al electrodes, stored in ambient environment or in inert environment.
cells. Therefore, the annealing at 100'C for 10 hours slightly improves morphology
of the organic active layers for device performance, [114,115] but either the effect of
precursors during ALD counteracts whatever gains are caused by annealing, or the
temperature of the devices during ALD or during annealing in inert atmosphere is
not equal to the specified temperatures. In either case, the ALD process implemented
here does not significantly affect the performance of the devices.
64
Effect of ALD on Device Shelflife
Figure 3-6 shows the effect on shelflife of presence of encapsulation and exposure
to ambient environment vs an inert environment of polymer cells and molecular cells
with Al electrodes. Polymer cells with ALD encapsulation and stored in inert atmosphere have the same lifetime as the cells with no encapsulation and stored in inert
atmosphere (11% decrease over 1200 hours), confirming that the ALD nanothin film
is not a source of degradation. However, encapsulation does have a beneficial effect
on shelflife of cells exposed to ambient atmosphere. Whereas polymer cells stored in
ambient atmosphere with no encapsulation degrade to zero efficiency in less than 20
hours, ALD-encapsulated polymer cells retain >50% of its efficiency after 450 hours
and continues to generate electrical power after 1200 hours. The molecular cell with
ALD encapsulation shows negligible degradation due to ambient atmosphere exposure
after 1150 hours. There is a small decrease in efficiency for molecular cells stored in inert atmosphere (whether encapsulated or unencapsulated) and for ALD-encapsulated
molecular cells stored in ambient atmosphere after 1150 hours (8% vs. 13%). This
demonstrates the effective encapsulation of ALD films, especially for molecular cells.
3.4
Conclusions
III conclusion, our study indicates that Ag electrodes lead to shorting and significant decrease of device performance, regardless of atmospheric exposure, whereas
Al electrodes lead to greater stability, for both polymeric and molecular cells. The
ALD deposition process itself has negligible effect on device performance, although
simply annealing at the same specifications slightly increases efficiency. Furthermore,
we have shown that although device lifetime depends dramatically on the active materials choice as well as cathode choice, ALD nano-thin encapsulation is effective for
organic solar cells of a variety of device architectures. Herein we demonstrated significant increases in shelf life (>50x) for polymer cells and molecular cells, and >1200
hours shelf life for molecular PVs stored in air due to a ianothin encapsulation film.
65
Part II
Engineering Higher Efficiency
Organic Photovoltaics
66
Chapter 4
Introduction to Nanostructures
and Architectures
Some of the greatest advances in nano-PV technology have been due to nanostructural engineering: Tang designed a cell with a exciton-dissociating interface within
nanometers of the photoactive region. [17] Heeger et al designed a cell that further increased the surface area of this exciton-dissociating interface through a bulkheterojunction.
[18]
Matsuo et al designed a cell that kept a large exciton-dissociating
interface while maintaining molecular order through nanopillars. [116] All of these
nanostructural advances have brought the OPV field closer to the ideal 33%, as well
as up to and beyond the supposed limit of commercial viability, 10%. Thus we can
hope that further advances in nanostructuring will further push this technology to
a status of serious impact on the world.
This chapter provides a background on
nanostructural engineering in OPVs.
4.1
Subcell Nanostructures
4.1.1
Bilayer/Planar (Tang) Cell
One of the distinguishing features of small-molecule and polymer semiconductors
is the presence of strongly bound excited states known as Frenkel excitons. These
67
b
a
C
Gold Wire Conboct
-BP
--
n2O3
Cuft
Gkos
(Aor
"'a
Ligt
ITO
-PEDOT
-
-
ITO
MunWnOM
wstate
----
glass
100 nm
Figure 4-1: Historic Advances in Nanostructural Engineering of OPVs. a) The Tang
cell (1985) introduced the photoactive interface on the nanoscale to achieve 1% PCE.
[17] b) Heeger et al (1995) introduced the bulk heterojunction to achieve 1.5% PCE.
[18] c) Matsuo et al (2009) fabricated a nanopillar structure that achieved 10.1%
PCE. [116]
excitons have significant impact on many of the processes in photovoltaic devices
from photocurrent generation to characteristic absorption features. [117] For example,
organic semiconductors typically exhibit static dielectric constants in the range of 25, [118] and accordingly, exhibit large exciton binding energies 0.2-0.8eV.
To overcome this exciton binding energy, bilayer donor-acceptor architectures
which have now become common to promote exciton separation to free carriers
[17,119,120] as shown in Figure 4-2a. However, there still remains an inherent tradeoff
between the absorption efficiency (a) and exciton diffusion efficiency (LD) to dissociating interfaces that results in thickness optima. The external quantum efficiency
(EQE) can be described by constituent efficiencies as EQE =
/AT/EDT/cTrlcc where TiA
is the absorption efficiency, r/ED is the exciton diffusion efficiency,
TICT
is the charge-
transfer or exciton dissociation efficiency, and r/cc is the charge collection efficiency.
In the absence of optical interference, the absorption efficiency is,
rjA =
T(1 - exp(-ad))
(4.1)
where T is the total transmission through the substrate and TCO to the active layer
(typically limited to 75%), a is the absorption coefficient, and d is the active layer
68
In
C
0.8
----- 0.5
- 2
-100
0.6
3CP
NCCEPTOR
Uj
wi
A
0.41
=
100
-2
0.2
--00
-
DONOR
ITO
Glass
0.0 0.5 1.0
1.5 2.0 2.5 3.0 3.5 4 .0
dot
Figure 4-2: Calculation of the external quantum efficiency (EQE) for donor-acceptor
heterojunction as a function of aLo (change in line style) and aLD (change ini color),
where LD is exciton diffusion length, a is absorption coefficient, and LC is carrier
diffusion length, highlighting the exciton diffusion bottleneck. [5]
thickness; the exciton diffusion efficiency is,
d
rlhD =CXP(-
LD
)
(4.2)
offset;
the charge transfer efficiency, qI-, is approximately 1 for a large donor-acceptor
and the charge collection efficiency is,
_
Lc
d(1 - exp(-
))
(4.3)
where LC is the charge collection length. [5, 121, 122] This model for the EQE is
plotted in Figure 4-2b as a function of ad for various values of aLj and aLc. Typical
values for aLD are <<1 so that for planar structures, exciton diffusion is the limiting
process. [117]
69
Figure 4-3: Left: Device structure of a planar heterojunction OPV. Right: Schematic
showing the cross-section of the rrP3HT:PCBM BHJ device. Focused (c) and
defocused (e) cross-sectional TEM images; inset: the magnified image of the
rrP3HT:PCBM BHJ layer. [124]
4.1.2
Bulk Heterojunction and Planar-Mixed Heterojunction
Bulk-heterojunction (BHJ) and planar-mixed heterojunction (PM-HJ) structures
aim to decouple these two limiting efficiencies through the use of interpenetrating
networks of donor and acceptor (shown in Figure 4-3), which enable the surface area of
the dissociating interface to be dramatically increased. This manifests as an effective
enhancement of the exciton diffusion length which, in turn, enables the optimal device
thickness to be increased as shown in Figure ??. However, as the interdigitation of
the donor-acceptor network increases the pathways for charge collection becomes
hindered, which is also a common limitation even in planar structures where aGLc
can be <<1. Indeed the mobility (p) of organic semiconductors is generally quite low
10- 7 101cm 2 /V
-
s. [123] Thus, there is a sensitive interplay between high photocharge
generation and charge collection which requires careful morphology optimization.
4.1.3
Novel Nanostructures
While solid state BHJ structures typically suffer from reduced carrier transport,
[125 they nonetheless remain the highest performance solid-state solar cells to date.
70
Alternative approaches to simultaneously increase r/ED and T/cc include (1) long-range
ordering which can increase both LD [126--128] and p [116,123,129] or (2) molecular
design of highly absorptive materials reducing required thicknesses for high absorption
efficiency. Most notable is the nanopillar architecture (Figure 4-1c) which in 2011
was the first OPV to surpass the 10% PCE mark and has recently been reported to
produce a 11.1% PCE cell (the current world-record OPV cell). [7,116,129]
4.2
Multijunction Architecture
Single junction solar cells are caught in a catch-22 of sorts: the larger the bandgap,
the larger the Voc but the smaller the Jsc. The smaller the bandgap, the smaller the
Voc but the larger the Jsc. This means that we can never quite utilize all of the
energy of all the photons.
One way around this is the multijunction architecture.
(This architecture is also sometimes called "tandem" or "stacked".) A multifunction
photovoltaic is comprised of two subcells in series (or in parallel). In this way, higher
energy photons can be absorbed by a subcell with higher Voc, and lower energy
photons are absorbed by a subcell with lower Voc.
The first MJ-OPV was reported only a few years after the first SJ-OPV. [130]
In this report they fabricated a lual-junction device with two of the same subeells
stacked upon each other. The intention of this structure is simply to increase the
amount of excitons dissociated, as the subcells were planar cells.
The MJ device,
however, exhibited significantly lower PCE than the SJ device (0.3% vs 1.0%) due
to (1)
an ineffective recombination zone and (2) unoptimized subcell thicknesses.
With time the field moved to using complementary subcells [131] and empirical and
computational optimization of subcell thicknesses. [132]. The current record holder
MJ cell has a PCE of 10.6%. [7,133]
71
4.3
Nanostructural Engineering via Processing
Thermal annealing of organic photovoltaics has been thoroughly studied in solutiondeposited organic films, to the effect of significant enhancements in OPV performance,
however this technique has yet to be significantly implemented in vapor deposited materials systems. Herein we study the effect of thermal annealing on molecular thin
films, covering multiple donor materials and nanostructures and devices comprising
such.
The nanostructure of OPVs is highly dependent on processing conditions. Through
annealing the films, we enable the molecules to move towards and perhaps reach their
most stable state. For most materials utilized in OPVs thus far, aggregation (longrange order) is the preferred state, which would thus lead to complete phase segregation. However the annealing is stopped before the material has reached equilibrium,
trapping the photoactive film in a blend of aggregation leading to higher charge mobility but sufficiently close phase interfaces leading to effective exciton dissociation
(see Figure 4-5). [114,115,134] This has been demonstrated in small-molecule mixed
OPVs (see Figure 4-4) [65,135] and in polymer blends [115,136] to lead to significant
enhancements in power conversion efficiency. However much research must be performed for each materials set to determine its ideal temperature and length of time
for optimized PCE.
Often the term "annealing" refers to thermal annealing, however that is not necessarily the case. Solvent annealing is also applicable to this field, and is used regularly in polymer blends through the use of low-boiling point solvents during deposition. [137] It has also been applied to fabrication of crystal needles of small molecules
(see the next chapter), [138,139] and has only been sparsely experimented with in
molecular OPVs. [140,141] Solvent annealing has the benefit of enabling the organic
materials to move through unstable states into more stable states at low temperatures. Many substrates that are of interest to OPVs (particularly plastics) degrade
quickly at high temperatures but are unaffected by solvent annealing.
72
100
0Tn
80
70
60
50
40
30
20
10
0
40
40
40
40
0
0 0
0
0
40
0
00
Figure 4-4: Effect of annealing on planar-mixed heterojunctions of CuPc-PTCBI.
Top (a-d) shows SEM images and bottom (e-h) shows simulations of the morphology.
Leftmost images are as deposited, images to right show increased aggregation due to
increasing annealing temperatures. [65]
0
As Dep
7Q*
'04
-6.
-3
0 01
150.0:
-10
0* 0.J
nm
0.510
V
Figure 4-5: Effect of annealing on bulk heterojunction polymer solar cells. Performance increases with annealing. SEM shows that this is due to increased phase
separation of the polymers and fullerene materials. [115]
1.68110
Toluene
Chlorobenzene
Figure 4-6: Effect of solvent annealing with different solvents oil a polymer blend. [137]
73
74
Chapter 5
Practical Efficiency Limits of
Organic Photovoltaics
Five decades since Shockley and Queisser derived the power conversion efficiency
limit of single-junction photovoltaic cells, researchers have still not demonstrated such
high performance for any photovoltaic device system.
[7]
For example, the power con-
version efficiency of the best single-crystal-Si single-junction PVs is q= 25% (without
solar concentration), [7,142] which is only 76% of the SQ limit. Hence, in evaluating
the achievable performance of a comparatively new PV technology, such as nano-PVs,
it is prudent and necessary to inake an estimate of the upper limit of the achievable
Herein we discuss the trends of the best demonstrations found in literature, and
utilize this to determine the practical limits of nanomaterial-PVs. Further, we apply
these trends to two standard material sets, to understand our expectations of their
efficiency limits in single junction architecture as well as multijunction architecture.
Using an optical model based on transfer-imatrix formalism [143], we can model the
maximum Jsc accessible with the given materials set by varying the subcell thicknesses. We utilize this model to explore different nanostructures of these materials
sets, namely: planar cells, ideal nanostructure cells, and tandem and mnultijunction
cells.
75
5.1
Practical Limits Derived from Literature
Here we report a numerical evaluation of trends of the best technical demonstrations. [5] Our literature survey of nano-PV technologies collects device data of the
best experimentally reported performance in terms of EQE , FF , and VOC over the
last two decades. From this data a semi-empirical estimate for the practical upper
limit to the power conversion efficiency of nm-PVs is derived.
The maximum demonstrated EQE across the spectrum is
75% with internal
quantum efficiencies approaching 100%; the losses in EQE are due primarily to combined optical losses (~10% loss from transparent electrode absorption, ~5% reflection
from substrate, and ~10% from internal reflections), which likely will be difficult to
eliminate in a practical device structure. Assuming this maximum EQE of 75% at
wavelengths extending to the band edge, upper limits of JSC are calculated as function of optical bandgap.
Upper limits for the FF as a function of optical gap were similarly assumed to be
that of the highest reported, i.e. FF = 0.75 across the spectrum. Deviations in FF
from the SQ limit generally stem from a combination of large series resistance and
low shunt resistance.
While many empirical forms have been proposed for the functional form of VOC
as a function of EG, [144, 145] it is found that maximum photovoltages follow a
trend of reaching 80% of the value predicted by the SQ limit; the difference can be
attributed to a number of recombination phenomena including internal conversion,
interface recombination, and exciton binding losses as discussed below. Nonetheless,
this threshold is consistent with the framework of the SQ limit.
We combine the photo voltage trend with the limiting photocurrents and fillfactors to estimate the ultimate nanostructured efficiency limits as shown in Figure 51. We find that a semi-empirically derived maximum efficiency of 17% is achievable
for a single-junction cell with a bandgap of approximately 1.1-1.4 eV. This maximum efficiency is lower than that reported in recent semi-theoretical studies in which
practical considerations described above are not taken into account and which led to
76
CIAIPc DBP
30
-
,
- .
- ,
-
.-
,
-
25MJ
20
SJ
W 15
10.
5-
0_
0.75
-
.
1.00
1.25
.
1.50
. . .
1.75
2.00
-.
2.25
2.50
Optical Bandgap (eV)
Figure 5-1: Practical Limit PCE for a range of optical bandgaps. Black is for single
junction architecture and purple is for multijunction architecture (plotted versus the
top subcell bandgap). The bandgaps of DBP and CIAlPc are indicated.
maximum exciton-limited efficiencies ranging from 2227%. [146] Further, Figure 5-1
also shows practical limits for dual-junction cells. We calculate a maximum efficiency
of 24% for stacked, 2-cell, current-matched nanostructured tandems (compared to
43.4% predicted in the SQ limit [147,148]).
5.2
5.2.1
Computational Methods
Materials Choice
We study the DBP-C60 (DBC6) and ClAlPc-C60 (CAC6) subcells that are utilized
throughout this thesis work. The single junction architecture is ITO/MoOx/subcell/BCP/Ag
and the multijunction architecture is ITO/MoOx/subcell1/BCP/Ag*/MoOx/subcell2/BCP/Ag.
These are standard materials sets and device architectures.
Optical parameters have been derived from ellipsometry measurements of each
of these materials.
Exciton diffusion lengths (utilized in the modeling of the pla-
nar architecture) are derived from EQE measurements of an array of active layer
thicknesses. [149]
77
5.2.2
Planar Architecture
Simply, we first computationally model the wavelength-resolved and spatiallyresolved optical intensity throughout the photoactive region of the device, via transfermatrix formalism. [143] Then we convolute this with the absorption spectra of the
photoactive materials.
For planar cells, any photons absorbed within an exciton diffusion length (which
is experimentally determined) of the photoactive interface is assumed to contribute
to the generated photocurrent. Here, we model the generated photocurrent for an
array of total active layer thicknesses, assuming the ratio between donor and acceptor
remains constant.
5.2.3
Ideal Nanostructured Architecture
Herein we do not make any assumptions of what the ideal nanostructure would be
except what is necessary for the model: (1) that both the donor and acceptor will be
uniformly distributed throughout the thickness of the active layer, and (2) that every
photon absorbed by the active layer will generate a photo charge that is collected by'
the electrode. With these assumptions and the same materials optical properties and
other device layers as selected above, we model the maximum Jsc.
5.2.4
Multijunction Architecture
Building upon above, we simply add more layers to the device that is optically
modeled as well as a second exciton dissociation interface.
Note that, within our
optical model, two "Jsc"s are generated: one for each subcell. For the sake of determining practical limits, we take the lowest subcell Jsc as the MJ Jsc, however we
note that this is simply to acquire a general value. In chapters below, we will discuss
the relationship between subcell Jsc and actual PCE.
78
20
18
160
1 CIAIPc-C60
- -'
S
10.
E
.
IQE =100%
- --
-.
DBP-C60
/-
S6.
4 0 5;
2
50
10
10
20
Planar
25
30
160
1 50
200
250
300
Total Active Layer Thickness [nm ]
Figure 5-2: Modeled Jsc for DBP-C60 and C1AlPc-C60 cells with either planar and
ideal nanostructure.
5.3
5.3.1
Results and Discussion
Single Junction: Planar Architecture
We find DBC6 Planar to generate Jsc =6mA/cm2 and CAC6 Planar to generate
Jsc= 10mA/cm2. Even though DBC6 has a longer exciton diffusion length and higher
peak absorption coefficient, CAC6 has a lower optical bandgap (1.7eV vs 2.0eV) and
thus is able to absorb a larger proportion of the solar spectrum.
If we combine these Jsc's with best demonstrated FF and Voc of each materials
set (FF= 0.55 and 0.75, and Voc= 0.74 and 0.92, respectively), we get a practical
limit PCE of 4.1% for planar architectures of both CAC6 and DBC6. Interestingly,
this has been experimentally attained for DBC6 but not for CAC6. [149]
However it is clear that planar devices are limited by their exciton diffusion
lengths. The active materials could absorb more light, but these extra excitons would
be wasted because they are too distant from the photoactive interface to be converted
to free photocharges. This limitation can be obviated through nano-structuring of
the active layer.
79
7-
MJ Planar
SMJ BHJ
6- -Planar
2
1
-- BHJ
5-
w
4-
5
BHJ: 70% inc
Planar:
DJ: 8% inc
TJ: 10% inc
PJ: 19% inc
3"
1 "2
321
0
500
1000
1500
2000
2500
3000
Total D+A thickness
Figure 5-3: Calculated PCE of planar and ideal nanostructure single junctions (lines)
and planar and ideal nanostructure tandems (squares) for DBP-C60 with current
demonstrations of FF and Voc.
5.3.2
Single Junction: Ideal Nanostructured Architecture
As discussed above, the "Ideal" nanostructure simply assumes that all excitons
generated collected carriers. DBC6 Ideal generates Jsc = 11mA/cm2 and CAC6
ideal generates Jsc=19 mA/cm2. We find that Jsc is increased by 70%
and 90%,
respectively, over the maximal modeled planar cells of DBC6 and CAC6, respectively.
If somehow the "ideal" nanostructure exhibited the empirical best FF and Voc
rather than decaying (which has never been reported before, but we are imagining
an ideal device), we get PCE of 7.6% and 7.3% for DBC6 and CAC6, respectively.
These are on par with the highest bulk heterojunction device efficiencies currently
demonstrated, but still fall short of the goal of 10% which is highly lauded for commercialization.
Further, we apply the practical limit trends derived from literature: setting FF=0.75
(the highest reported, which is equal to that demonstrated for DBP-C60) and Voc
equal to 80% of the SQ limit. The parameters derived from this are shown in Table 5.1. This leads to practical limit PCE values of 11% and 16% for DBC6 and
CAC6 materials sets, respectively. The bandgap of ClAlPe, 1.7 eV, is closer to the
ideal bandgap of 1.1eV to 1.4eV, leading to a significantly higher practical limit. [5]
80
Table 5.1: Current and Practical Limit Performance Parameters for SJ-OPVs of
DBP-C60 and ClAlPc-C60
Jsc
[mA/cm']
Voc
[V]
FF
[frac]
PCE
[%]
DBC6 Current
DBC6 Practical Limit
DBC6 Potential Gain
6.2
11
77%
0.70
0.75
6%
0.92
1.35
47%
4.0%
11.1%
178%
CAC6 Current
CAC6 Practical Limit
CAC6 Potential Gain
7.1
19
167
0.51
0.75
47%
0.78
1.13
44%
2.8%
16.1%
475%
In conclusion, realistic PCE increases of 70-90% can be achieved with these specific
niaterials sets if the exciton diffusion length can be overcome via nano-structuring,
and further PCE increases of up to 500% can be imagined if all PV parameters are
enhanced to the practical limit.
5.3.3
Tandem Cells (with the Same Subcells)
We further examine another approach to overcone the exciton diffusion limit:
tandem cells. Tandem cells are a series of optically thin cells on top of each other.
This increases the photoactive interface by adding one on top of the other, but this
doesn't increase Jsc since the current of every subeell niust equal. Rather, the voltages
add, so you get an increase in Voc. This can be useful in applications where high
voltage is needed, such as water-splitting. [150]
We model the maximal Jsc of tandemns for 2-, 3- and 5-junctions comprised of
planar subcells, and we further calculate the maximal PCE by taking this Jsc and
multiplying it by the empirical best FF and Voc and the number of junctions. We see
that this tandem approach can lead to moderate improvements in PCE of approxiiately 20% over the planar single-junction architecture.
Finally, we examine the approach of tandems comprised of "ideal rianostructure"
subcells. This only serves to decrease PCE, according to our model, because of need of
balancing the currents. If we were more thorough in the thickness intervals chosen for
modeling, we should find that the PCE will niatch exactly that of the single junction,
81
but for this study we have chosen fairly big jumps in thickness (10nm) for the sake
of computational time efficiency.
5.3.4
Multijunction Cells (with Different Subcells)
In the following chapters we discuss the engineering and optimization of MJ cells,
which achieve PCE of 5.5% utilizing these two materials sets. Ultimately, however we
find that this structure is limited by the comprising subcells. If we calculate the MJOPV performance utilizing the practical limit subcells discussed above, we see that
PCE could rise to 17.7%. This assumes current matching as well as implementation
of the conventional subcell order. Note that the inverted subcell order discussed in
the following chapters is beneficial only in MJ cells where the subcells are sub-100nm
thickness.
Table 5.2: Current and Practical Limit Performance
DBP-C60 and ClAlPc-C60
Voc
Jsc
2
[mA/cm ]
[V]
Current
4.9
0.68
Practical Limit
9.5
0.75
Potential Gain
94%
10%
5.4
Parameters for MJ-OPVs of
FF
PCE
[frac]
[%]
1.65
2.48
50%
5.5%
17.7%
222%
Conclusions
In conclusion: nanostructuring of these standard donor-acceptor materials sets
could potentially double PCE, if only we can increase exciton dissociation efficiency
while not decreasing the carrier collection efficiency nor increasing the series resistance. This would lead to PCE of 7-8% with the materials sets utilized in this thesis.
If we could also achieve enhanced FF and Voc up to the practical limit for OPVs, we
could see efficiencies up to 16% with materials sets utilized in this thesis. Furthermore, if combined in the multijunction architecture (as discussed below) PCE of up
to 17.7% could be achieved.
82
Chapter 6
Materials and Architecture Design
in Sub-100nm Multijunction
Photovoltaics
Organic photovoltaics (OPVs) have generated much interest due to their potential
as flexible, semi-transparent solar cells. However low power conversion efficiency has
A major limiting factor is the trade-off
thus far limited their connercialization.
between photon absorption and conversion to current and photon energy conversion to
voltage, which can be manipulated by the choice of donor and acceptor energy levels.
This trade-off can be minimized through implementation of a mnultijunction (MJ)
architecture, [5, 147] in which multiple subcells of differing optical band gaps lower
thermal losses. Organic semiconductors uniquely demonstrate structured absorption
spectra, which enables MJ-OPVs in which donor materials are transparent to each
other.
[?,
151] This minimizes competitive absorption between subcells and enables
facile optical device optimization. [152,153] Power conversion efficiencies up to 10.6%
have now been reported for MJ-OPVs comprising complementary donor materials.
[133]
Herein [149] we present optical and electrical optimization of an MJ-OPV incorporating a near-infrared-absorbing
visibly-transparent phthalocyaine-based donor
(ClAlPc) and a complementary visible-absorbing perylene-based donor (DBP) (Fig-
83
2.0
*2
DBP
CIAIPc
IV 1.5
0
o 1.0
.: 0.51C
0.0
0
400
500
600
700
800
900
Wavelength ( nm )
Figure 6-1: Absorption spectra of C60 (green), DBP (blue) and ClAlPc (red) thin
films, showing broad spectral response. Molecular structures are inset.
ure 1). [100, 122,154] The two donor materials are transparent to one another, enabling the positioning of the narrower band-gap cell in front of the wider band-gap
subcell in order to optimize optical absorption within the weak optical microcavity of
the nanoscale PV. [152,153] The subcell thicknesses were computationally optimized
for maximum generated photocurrent using T-matrix formalism. These subcells were
connected with a variety of recombination zone (RZ) architectures to optimize subcell
current recombination and thus minimize charge build-up and voltage loss. Optimization of the structure of the MJ cells leads to open-circuit voltage equal to near perfect
summation of the subcell open-circuit voltages (1.65±0.02V), near maintenance of the
high fill factor of the perylene-based subcell (0.685±0.002), broad spectral response
(in the wavelength range of 350nm to 850nm), and OPV power conversion efficiencies
of 5.5±0.2%.
6.1
Experimental Methods
The MJ cells are fabricated monolithically entirely by vapor deposition and incorporate chloroaluminum phthalocyanine (ClAlPc) as a near-infrared-absorbing visiblytransparent donor and tetraphenyldibenzoperiflanthene (DBP) as a visible-absorbing
84
SC
ITO MoOx CIAIPc-C60
3.2
RZ
BCP Ag MoOx
SC2
DBP-C60
3.5
4.0
BCP Ag
3.5
4.3
4.3
g4.8 .4m-
5.4 4.9
5.4
IRO...
lease
6.5
6.5
Figure 6-2: Energy levels of each component layer of the multijunction cells.
RZ
BCP
ITO|
Glass
Figure 6-3: Device cross-section of the mnultij unction cells. Thicknesses not to scale.
85
donor, with buckminsterfullerene (C6 0 ) as a wide bandgap acceptor in both subcells.
The photovoltaic devices are fabricated via vacuum thermal evaporation on solvent
cleaned, pre-patterned ITO-coated glass (Thin Film Devices, 20 Q/E). Prior to device growth, ClAlPc (TCI, C1167) and C
60
(Aldrich, sublimed 99.9%) are purified
once by vacuum train sublimation, while DBP (Lumtec, LT-N4003), BCP (Lumtec,
LT-E304), MoOx (Alfa, 99.9995%), and Ag (Alfa-Aesar, Premion 11357 99.9999%)
are used as purchased.
The subcell and RZ layers are blanket deposited, while the top Ag electrode is
patterned via shadow masking in stripes orthogonal to the bottom ITO electrode
to generate a device area of 0.0121 cm 2 . Each subcell and each RZ are fabricated
without breaking high-vacuum (<10-- Torr), and all fabrication transfer and characterization is performed in the controlled environment of N2 -filled gloveboxes. The
multijunction (MJ) cells structure consists of glass/ITO 150nm/MoOx 20nm/ (subcelli)
/
(RZ)
/
(subcell2) /BCP 6.5nm/Ag 1100nm. Single junction (SJ) cells with
structure glass/ITO 150nm/MoOx 20nm/ (subcell) /BCP 6.5nm/Ag 1100nm are fabricated in parallel with MJ cells in order to characterize the opto-electrical response
of individual subcells. A planar-mixed heterojunction of ClAlPc and C6 ocomprises
subcell 1 (SCI) and a planar heterojunction of DBP and C60 comprises subcell 2
(SC2).
The optimal SJ configuration (highest rqp in a SJ architecture) of subcelli (SC1) is
ClAlPc 10nm/ClAlPc:C60 (1:1 ratio) 10nm/C 6 o20nm, and the SC1 in an optimal MJ
cell consists of ClAlPc 11.4nm/ClAlPc:C 60 (1:1 ratio) 7.2nm/C 60 9.6nm. The optimal
SJ configuration of subcell2 (SC2) is DBP 20nm/C 6 o40nm, and the SC2 in an optimal
MJ cell is DBP 20nm/C
60 23.2nm.
Film thicknesses are measured during film growth
using a quartz-crystal monitor pre-calibrated by thin film measurement with a profiler
(KLA Tencor, P-16+), leading to a film thickness error of <1nm.
Current density-voltage characteristics are measured under dark conditions and
under 1 sun illumination (100mW/cm 2. AM 1.5G), using a 150W solar simulator
(Newport, 6255) illuminated through a AM1.5G filter (Newport, 81094) calibrated
with an NREL-certified monocrystalline-Si photodiode (Newport, 91150V). Results
86
are not corrected for spectral mismatch. Electrical characteristics are measured with
a picoammeter (Keithley, 6487) employing a switch mainframe (Keithley, 7001) for
switching between cells.
The external quantum efficiency (EQE) is measured with AC monochromatic
light and DC light bias (Figure 6-4).
For MJ cells, the measured photocurrent is
equivalent to the response of the current-limiting subcell, thus the DC illumination
wavelengths and intensities are selected such that the designated subcell is under
isun conditions and the other subcell is over-illuminated so that its photocurrent
is limited by the designated sub-cell. [155,156] The two subcells are complementary
light absorbers (i.e. with limited absorption spectrum overlap) resulting in a distinct
span of the excitation wavelengths for each subcell. The EQE shown here was taken
while illuminating with AC monochromatic light signals scanned from wavelength
A=350nm to 900nm at 2mW and 353Hz (using Oriel, 100W Xe arc lamp, 66921
and Princeton Instruments, SpectraPro 300i with a light chopper) and DC solar
simulation light bias at 100 mW/cm 2 (Newport, 6255) and DC monochromatic light
bias of A=532nm (absorbed predominately by DBP) at 50mW, CW (Extreme Lasers,
GHQ-50 x1). The AC light intensity is calibrated with NREL-certified Si photodiode
(Newport, 818-UV). Electrical characteristics are measured with a lock-in amplifier
(Stanford Research Systems, SR830) employing a switch mainframe (Keithley, 7001)
for switching between devices.
We apply T-mnatrix formalism to model the wavelength-resolved optical field intensities within the full MJ cell structure and from these solve the ID exciton diffusion
equations to calculate short-circuit currents as a function of layer configurations. [143]
The optical properties of each material are measured via spectroscopic ellipsomnetry
(J. A. Woollam, M-2000S).
Ideal dissociation and charge collection efficiencies are
assumed and exciton diffusion lengths are extracted from fitting single junction EQE
thickness dependencies.
Numerical optimization of tandem structures is performed
by calculating the MJ photoresponse for cells over a series of SCI and SC2 donor and
acceptor thicknesses (4 variables).
87
W Lamp
Cho per Monochromator
La sers
78 n 532nm
Solar Si
4
ND Filter
I-
I
I
Figure 6-4: Schematic of the set up for external quantum efficiency measurement.
88
5.0
E 2.5
SJ 1SJ2
E
--
MJ
+--
110.0
-
-5.0
-7
-0.5
0.0
0.5
1.0
1.5
2.0
Voltage ( V )
Figure 6-5: Representative current density-voltage characteristics of MJ (green), SJ1
(red) and SJ2 (blue). Solid curves are taken under isun illumination (AM1.5G), and
dashed curves are taken in (lark. Shown SJ1 and SJ2 were fabricated in parallel with
the subcells of shown MJ.
6.2
Characteristics of Optimized Device
Figure 6-5 shows the current density-voltage characteristics of the optimal MJ cell
and SJ cells fabricated in parallel with the subcells of the shown MJ under dark conditions and under illumination of simulated solar spectrum (100 mW/cm 2 , AM1.5G,
not corrected for spectral mismatch).
The (Voc of the MJ cell (1.65±0.02 V) is
equal to the sum of those of the SJ cells (1.64±0.01 V), and the FF (0.685±0.002)
is within error of the average of those of the SJ cells (0.65±0.02). The Jsc 4.9±0.3
mA/cm 2 ) demonstrates the minimal loss of photocurrent, consistent with the coinplementary absorption of the subcells. The highest qp of SCI and SC2 are 2.8±0.1%
and 4.0±0.1%, respectively, and the highest qp of the MJ cell is 5.5±0.2%.
Figure 6-6 shows wavelength-resolved external quantum and efficiency (EQE) of a
MJ cell with similar subcell thicknesses thicker RZ as the empirically optimized structure. The measured photocurrent is due to the photoresponse of the current-linmiting
subcell, thus a subcell can be selected for EQE measurement via properly chosen
optical bias. SCI (red) and SC2 (blue) were selected via optical bias of A=532nm
89
3
35-8
SCI
530
SC2
S25
E
**
+:15e
CV
h
10
X 0
W
400
500
600
700
800
900
Wavelength ( nm )
Figure 6-6: Wavelength-resolved external quantum efficiency of SC1 (red) and SC2
(blue) selected via optical bias (A=532nm laser, 50mW; and AM 1.5G simulation,
100 mW/cm 2 ; respectively).
2
laser, 50mW; and simulated AM 1.5G, 100 mW/cm ; respectively; such that the selected subcell generates a photocurrent within an order of magnitude of the measured
photocurJsc under 1sun and the deselected subcell generates a significantly larger
rent. With application of the optical model described above, we calculate that under
A=532nm laser illumination SC2 generates 412% more photocharges than SCI and
under simulated AM1.5G illumination SC1 generates 28% more photocharges than
SC2. Through varying optical bias intensity and spectra, we find that this level of
photocharge generation unbalance is sufficient to clearly select SC2 for EQE measurement (data not shown). The EQE spectrum of SC1 corresponds to the optical
absorption spectrum of ClAlPc thin film (over the spectral range A=600nm-850nm)
with a small response corresponding to C60 film (A <500nm), and the EQE spectrum of SC2 corresponds to a summation of the optical absorption spectrums of C60
(A <500nm) and DBP (A=500nm-650nm).
Comparison of the SJ and MJ cell current density-voltage characteristics demonstrates effective opto-electrical optimization of the MJ cell structure. The MJ cell
90
,t/p is 5.5±0.2%, a nearly 40% improvement over the most efficient SJ cell made with
these materials. The minimal loss of V oc from the two subeells confirms the efficient
operation of the RZ. The similarity of the Jsc of the SJ cell and MJ cell as well as
the spectral response of the MJ subcells shown in Figure 2b demonstrate the complementary absorption of the subcells and the broad utilization of the solar spectrum
from A=350nm to A=850nm. Integrating the EQE spectra of each subeell with the
AM1.5G spectrum gives a generated Jsc for SCI of 3.6+0.4 mA/cm
[5]
and for SC2
of 3.2±0.3 mA/cm2 , as compared to the measured MJ cell Jsc of 3.9+0.2 mA/cm2 .
Both integrated subcell Jsc are low by 10-20%, possibly due to spectral aberration
of the calibration spectrum and/or of the solar simulated illumination. It is expected
that if each subcell peak EQE was increased from the present 25±5% to 70%, a
2
practical limit already achieved in the best OPVs, then the Jsc and rpwould be
additionally enhanced up to three-fold, suggesting a practically-achievable potential
r/p of >10% with these organic materials and the MJ device architecture.
6.3
Recombination Zone Development
A key component in advancing from fabrication of single junction cells to multijunction cells is the selection and deposition of an efficient recombination zone.
Figure 6-7 shows the dependence of MJ cell performance on the recombination
zone (RZ) composition and layer thicknesses. Figure 6-7a compares the
i-V
charac-
teristics under 1 sun of MJ cells with either no RZ (i.e. SCI in direct contact with
SC2) or RZ compositions of: Ag 0.5mn/BCP 5nn/MoOx 5nm, or BCP 5mn/Ag
0.5nrn/MoOx 5nm. Comparing the MJ cell with no RZ and the MJ cell with the full
RZ, the FF increases by 40% (from 0.40±0.002 to 0.56±0.002) and the
T/p
increases
by 160% (from 1.9±0.2% to 5.0±0.2%).
MJ cells fabricated without aiy recombination zone or with just BCP/MoOx or
just Ag nanoclusters exhibit S-shaped current density characteristics, suggesting a
build-up of space-charge [157] due to inefficient recombination of subcell currents
of opposing charge.
However the complete 3-layer RZ (BCP/Ag/MoOx) has clear
91
Ag thickness ( nm)
0.6
f"---no RZ
4- ...Ag nanoclusters
BCP/MoOx
-Full RZ: BCP/Ag/MoOx /
2 --
2.0
1.5
1.0
0.5
0.64
3.2
0.62
3.0
0.60
C
0.
0.58-
-2-
LL0.56
0.54
.
C-)
C
2.8 o
0--.
2.6
0
-2.40
0.52
-6
-1.0
a
0.50
-0.5
0.0
0.5
1.0
1.5
2.0
2
3
4
5
6
7
8
9
2.2
1lul
BCP, MoOx Thickness ( nm)
Voltage ( V )
Figure 6-7: (Left) Current Density-Voltage characteristics of MJ cells with either
no RZ (dash-dot) or RZ of: Ag 0.5nm (short dash), BCP 5nm/MoOx 5nm (long
dash), BCP 5nm/Ag 0.5nm/MoOx 5nm (solid). (Right) Fill factor of MJ cells with
BCP x nm/Ag 0.5nm/MoOx 5nm (solid circles) or BCP 5nm/Ag 0.5nm/MoOx x nm
(open circles) as the RZ. And short-circuit current of MJ cells with BCP x nm/Ag
0.5nm/MoOx 5nm (squares) as the R.Z . Measurements were performed under 100
mW/cm2, AM1.5G.
rectification, resulting in high FF and Voc.
The RZ can be further optimized by tuning each layer thickness. In Figure 67b, the thickness of each layer of the RZ was varied while keeping all other layers
constant. Solid circles correspond to BCP x nm/Ag 0.5nm/MoOx 5nm, open circles
correspond to BCP 5nm/Ag 0.5nm/MoOx x nm, and squares correspond to BCP
5nm/Ag x
nm/MoOx 5nm. By increasing the BCP and MoOx thicknesses from
5nm to 7.5nm and 5nm to 10nm, respectively, the FF decreases by 5% and 11%,
respectively. By increasing the Ag thickness, the J sc decreases by 22%.
Other
photovoltaic parameters were negligibly affected.
For BCP and MoOx film thicknesses >5nm, the FF is sub-optimal. Since BCP
and MoOx are semiconducting thin films, the series resistance of the cell increases
with the thickness of either layer, consequently decreasing FF while maintaining a
high Voc. Furthermore, Jsc decreases with increasing Ag thickness. This is due to
the non-negligible absorption of Ag nanoclusters, decreasing the optical intensity in
the subcells. Due to these effects, the optimized RZ is BCP 2.5nm/Ag 0.5nm/MoOx
2.5nm and is used in devices throughout this work unless otherwise indicated.
92
1.6-
TO . . .
Aq
1.4~
1.2
1.0- 1.
85nm
530nm
L.8
-0.6-
0.4-
0
50
100
150
200
250
300
Position (nm )
Figure 6-8: Optical fields for wavelengths absorbed by SCI (A=785nm) and SC2
(A=530nm) within the MJ cell modeled via T-matrix formalism.
6.4
Optical Optimization of Sub-100nm Subcells
Figure 6-8 shows the calculated optical field distribution within the empiricallyoptimized MJ cell for two wavelengths (A=530nm, absorbed primarily by DBP, and
A=785nm, absorbed only by ClAlPc). The optical interference due to the reflective
top electrode and the nanoscale-thickness of the MJ cell is evident in the spatially-
oscillating optical intensity. The optical intensity of A=785nm light (which is primarily absorbed by the ClAlPc film) peaks farther from the reflective electrode than
A=530nm light (which is primarily absorbed by the DBP film). This suggests that
a greater photocurrent would be generated if the NIR-absorbing subcell (SC1) is
placed further from the reflective electrode than the visible-absorbing subcell (SC2)
(see modeled EQE shown in Figure 6-10.).
This is contrary to tandem stacking of typical inorganic or organic solar cells where
the largest bandgap sub-cell must be positioned closest to the incident light [143,147]
for best MJ performance. However the structured absorption spectrum of our NIR
cell (SCI) which has minimal absorption of visible light, enables us to place the SC1
in front of SC2 and optimize our MJ design for the peaks in the optical fields of
different colors of light.
Modeled EQE of devices either conventional or inverted structure is shown below.
93
Inverted MJ structure:
Small bandgap in front
Conventional MJ structure:
large bandgap in front
BCP
RZ
BCP
ITO
Glass
ITO
Glass
Enabled by
complementary absorption
Figure 6-9: Architectures of conventional and inverted subcell order. Thicknesses not
to scale.
We calculate that a Jsc enhancement of 20% can be achieved by inverting the subcell
order. Indeed, experimental data confirms this enhancement, as shown in figure 6-10.
For series-connected MJ cells, an equal current density must flow through both
of the subcells, which leads to a dependence of the overall photovoltaic performance
on the relative photocurrent generated in each subcell. Therefore the photocurrent
generation of each subcell was modeled for a range of subcell donor:acceptor thickness
ratios and total thicknesses. By numerically varying the four free variables, we find the
modeled-maximum-Jsc for MJ structure with SC1: ClAlPc 9.5nm/ClAlPc:C6 o6nm/C 6o
8nm and SC2: DBP 25nm/C 6o29nm, which closely matches the empirically optimized structure, SCI: ClAlPc 11.4nm/ClAlPc:C 6o 7.2nm/C 60 9.6nm and SC2: DBP
20nm/C 60 23.2nm. Figure 4b shows the calculated Jsc for an array of SCI and SC2 total thicknesses varying from 19nm to 28nm and from 43nm to 65nm, respectively, and
with donor:acceptor thickness ratios restrained to those of the modeled-maximum-Jsc
MJ cell.
The overlaid text is the empirical data of MJ cells fabricated with the designated
pair of subcell thicknesses. Over this range, the simulated Jsc are within error of
the experimental Jsc. Additionally, the trend of the simulated Jsc closely follows
94
1.0
L) 0.8
04
E
E 0.6
Ej
Inverted MJ
-
- Conventional MJ
ai)
w
E
0
2-
0
SC1
SC2
0.4
C
5
II
C
/
-2
-3
--
Conventional MJ l,=2.9%
0 0.2
Inverted MJ
U
(D 0.0
wr
400
500
600
700
800
-5-0.5
900
0.0
0.5
3,=3.3%
1.0
1.5
2.0
Voltage ( V)
Wavelength (nm)
Figure 6-10: (Left) Modeled external quantum efficiency and (right) experimental
current density-voltage characteristics of conventional and inverted subcell order.
64
62 48*0.2
5.3±
E 60
C
58
O56
42 5.6±0.2
5.5*0.2
5.4d
.r 52
F-
C%4 50
( 48
~46
44
±
4 4.9±0.2
4
20
22
18
a
--
.9:
24
_T
I
26
28
Subcell1 Thickness ( nm )
Figure 6-11: Simulated short-circuit currents for an array of thicknesses of subcell 1
and 2 with fixed donor-acceptor ratios. Overlaid is text of experimental JSC for nine
samples corresponding to the thicknesses specified by their position. Experimental
Jsc error is ±0.3 mA/cm 2 and thickness error is ±1nm.
95
that of the experimental Jsc, confirming the model's validity when used for optical
optimization.
6.5
Conclusions
In conclusion, we demonstrated a multijunction cell with Tp of 5.5±0.2% by incorporating near-infrared-absorbing visibly-transparent ClAlPc and visible-absorbing
DBP, with all layers fabricated via vapor deposition. These devices are enabled by an
efficient hybrid tunnel junction RZ and optimized optically for maximum photocurrent generation. The MJ cells shown here are ultimately limited by the low EQEs of
the component subcells (of between 20% and 30%), thus further enhancement of subcell EQE to practical limits [5] (e.g. EQE between 70% and 80%) could lead to OPVs
with Tp of > 10%. Multijunction architectures are an important approach to reduce
thermal losses and improve overall efficiencies of PVs composed of nanostructured
and organic materials and will likely be important to their commercial viability.
96
Chapter 7
Subcell Photocurrent Balance in
Multijunction Photovoltaics
7.1
Introduction
Multijunction (MJ) cells are a key approach to enhancing photovoltaic power
conversion efficiency, surpassing the Shockley-Queisser limit and potentially advanc-
ing OPVs into the range of commercial viability. [5, 24, 147, 158] There has been
much work in the field regarding this architecture, primarily focusing on rmaterials selection and architecture optimization to maximize short circuit current (Jsc).
[133,143,149, 151-153, 159] However, there has been little discussion on utilizing the
imbalance of subcell photocurrents for efficiency optimization. Recently, Forrest et al
reported analytical calculations of MJ organic photovoltaic (OPV) device parameters
and their dependence on the ratio of subcell photocurrents. [160] They conclude that
for MJ cells made with subcells with very similar fill factors (FF) power generation
is maximized when subcell photocurrents are balanced, confirming popular thought.
But for MJ cells with subcells with very dissimilar FFs, power generation can be max-
imized when the higher-FF subcell is slightly current-limiting. Most MJ-OPV reports
thus far have utilized subcells with similar FF, and thus where subcell balance has
been explored, the most balanced device architecture was found to be optimized. [133]
However there has not yet been a report of an MJ-OPV architecture that exhibits
97
(a) 7
(a) 8
7
6
6
..
- -Sub-cell
...
.....
3
2
(b) 2
(b) 7
6
5
0 0
Balanced
Z3
I 0
5.
......
4.........
..............
(C)
2
5.
3
iL oss
5
Least
Loss
53
2
Least
A
.....
FF
.. -Tandem
Dissimilar...
--- sub-cell I
--
8
7.
Tandem B
Sub-coll I
Sub-cell 2
%.
(c)
46
I SCI
0
:10
gre
-An=9.0%
Z3
.
-0.5
0.0
52
0.5
1.0
1.5
-1.0
2.0
Applied bas (V)
-0.5
0:0
0.5
1.0
Applied bias (V)
1.5
2.0
Figure 7-1: Theoretical dependence of PCE loss on subcell balance for MJs with
similar FF subcells and dissimilar FF subcells. [160]
maximum efficiency when the subcells are imbalanced.
Herein we present an MJ-OPV architecture comprising subcells with dissimilar
FFs, for which the maximum reported PCE cell is comprised of imbalanced subcells.
For this materials set, we report that FF gain from subcell imbalance dominates
over Jsc loss, and the highest Jsc cell is not the highest PCE cell.
Further, we
introduce a highly accessible technique to determine subcell photocurrent balance
simply from the reconstruction of 3-V curves of the MJ device from representative
SJ devices. Finally we simulate this specific MJ architecture under a broad range of
subcell photocurrent balances to further explore the dependence of Jsc, FF and PCE
on subcell photocurrent balance.
7.2
Experimental Methods
The subcells in this report are: (subcell 1) CIAlPc/ClAlPc:C60/C60, and (subcell
2) DBP/C60. The bottom contact is ITO/MoOx, the top contact is BCP/Ag, and
the recombination zone is BCP/Ag*/MoOx. The contacts and recombination zone
98
are kept constant throughout this work. Single junction (SJ) architectures are also
fabricated in parallel with the MJ subcells, with the same bottom and top contacts
as the MJ cell. Current-voltage characteristics are performed under AM1.5G 1 sun,
not corrected for spectral mismatch. A more in depth description of the materials
and methods are described elsewhere. [149]
7.3
7.3.1
Calculation and Simulation
Subcell Photocurrent Fitting
Subcell photocurrents for each MJ cell were determined by utilizing two fitting
parameters while re-constructing the MJ current-voltage (JV) curve from the two SJ
curves. The method of constructing an MJ JV curve from two SJ curves is proposed
and described elsewhere. [156,161] Briefly: for two subeells in series, the current going
through each of the subcells must be equal, i.e.,
JAI = JcI = Jsc2
(7.1)
Furthermore, the voltage across the entire MJ cell equals the sui of the voltage across
each subeell, i.e.
VAIJ = Vsc1 + VsC 2
(7.2)
Therefore, at any given current magnitude, we add the voltages of each subcell, and
that voltage sum is equivalent to the voltage of the MJ cell at that current magnitude.
The photocurrents (i.e., effective short circuit currents) for the two subcells are used
as fitting parameters for matching the constructed MJ curve to the empirical MJ
curve.
7.3.2
Subcell Photocurrent Balance
After identifying each of the subcell photocurrents within the MJ cell, we then
quantify their balance within the MJ cell with a parameter identified as subcell pho99
RsI
lPr2
R=10 Ohm
SC2
D2
12
ls=8e-16A
N=1.2
VI=1.2 V
Rshl
R=2000 Ohm
Rs
R=11 Ohm
V1
P
11
i=z
4'
Ash
R=550 Ohm
U=Uce
DI
Is=6e-1OA
ZSN=1.8
vj=o.e v
Figure 7-2: Circuit diagram for the MJ-OPV model.
tocurrent balance, SPB, which equals the difference between the two subcell photocurrents divided by the sum of the two subeell photocurrents:
SPB =
7.3.3
± SF2
SP1 + SP2
(7.3)
Circuit Simulations
We simulate our MJ-OPV circuit via the Quite-Universal Circuit Simulator. [162,
163] The MJ-OPV is modeled as two SJ-OPVs in series. Each SJ-OPV has a series
resistance in parallel with a diode, a DC current source, and a shunt resistance. The
diode characteristics and shunt resistance are varied to match each of the fabricated
subcells in single junction architecture. The recombination zone is not considered as
a separate component, since it has been confirmed as a lossless interconnection. [149]
The circuit structure and its parameters are reported in Figure 7-2. Current-Voltage
characteristics of fabricated SJ cells and simulated SJ cells are compared in Figure 7-3.
100
Experimental & Simulated
0
E..
-1
L
-
E
-2(D)
-3E3
-
--S '
-4-
-
(0
-5SC2-
-6-7-0.5
1.0
0.5
0.0
Voltage [ V ]
Figure 7-3: Experimental (solid lines) and simulated (dashed) current density-voltage
characteristics of single junction devices comprising SCI (red) and SC2 (blue).
N
42-I
E
0
/
-
-2.
SJ1
U)
-4-3%
SJ2
-6-8
-10-
-0.5
0.0
0.5
1.0
1.5
Voltage (V)
Figure 7-4: Current Density-Voltage Characteristics of single junction cells with the
SC1 and SC2.
7.4
7.4.1
Results
Subcells with Dissimilar Fill Factors
We utilize two subcells with complementary absorption and dissimilar FF. [149]
SCI has FF of approximately 0.55 and SC2 has FF of approximately 0.73. Over a
thickness range of ± 20% of the optically modeled-optimal MJ cell, the FF of SC1
and SC2 in the SJ architecture vary by only 3±1% and 5±3%, respectively. This
data is reported in Table 7.1.
101
Table 7.1: Pe rformance parameters for SJ-OPVs of varying SCI and SC 2 thicknesses.
SC T SC2 T
Jsc
Voc FF
PCE
2
[%]
[V] [frac]
[mA/cm ]
[nm]
[nm]
18.9
-
3.4
0.66
0.54
1.2
23.5
28.2
-
-
4.1
4.9
5.8
6.4
6.4
0.71
0.74
0.91
0.92
0.92
0.55
0.55
0.75
0.73
0.71
1.6
2.0
3.9
4.3
4.2
43.2
54.0
64.8
Table 7.2: Performance parameters for MJ-OPVs of varying SCI and SC2 thicknesses.
Fitted photocurrents and calculated SPB are included.
SC1 T SC2 T
SPi
SP2
SPB
Jsc
Voc
FF
PCE
2
2
2
[%]
[V]
[frac]
]
[mA/cm
[mA/cm ] [mA/cm ]
[nm]
[nm]
18.9
43.2
5.3
5.3
0.00
5.3
1.56
0.63
5.2
18.9
54.0
4.9
6.3
-0.13
5.8
1.61
0.56
5.2
18.9
64.8
4.4
5.4
-0.10
5.1
1.59
0.57
4.6
23.5
43.2
5.6
5.0
+0.06
5.1
1.63
0.67
5.6
23.5
54.0
5.55
5.7
-0.01
5.7
1.65
0.60
5.7
23.5
64.8
4.9
6.1
-0.11
5.7
1.65
0.55
5.2
28.2
43.2
6.0
5.0
+0.09
5.1
1.66
0.69
5.8
28.2
54.0
5.3
5.7
-0.04
5.7
1.66
0.59
5.6
28.2
64.8
5.1
6.2
-0.10
5.9
1.66
0.54
5.3
102
Reconstructed & Experimental
Reconstructed & Experimental
E
0
-1E
0
E -2-
E
0-4
0
-2-3--
-5
6-
-0.5
1
50O
0.0
0.5
1.0
Voltage [ V
1.5
-0.5
2.0
0.0
0.5
1.0
1.5
2.0
Voltage [ V ]
]
Figure 7-5: Representative reconstructed current density-voltage characteristics of
multijunction devices either balanced (left) or unbalanced (right). The device on
right is the maximal-PCE cell.
7.4.2
Subcell Photocurrents
First, we determine the photocurrent generated by each of the subcells within
each fabricated MJ cell. Two methods previously proposed for determining the subcell photocurrent balance are (1) utilizing optical models of the subcells [156] or (2)
integrating the subcell EQEs under solar simulation [133]. Optical models can exhibit
significant variation from experimental results (see Figure 7-6). [149] Furthermore, accurate EQE measurements of subcells are difficult to perfect and may also lead to
significant error [133].Therefore here we utilize a simple method to quantify each
subcell photocurrent and calculate the subcell photocurrent balance, SPB of each
fabricated MJ-OPV, based on the re-construction of the MJ cell JV curve from SJ
JV curves.
Subcell photocurrents for each MJ cell were determined by utilizing two fitting
parameters while re-constructing the MJ JV curve from the two SJ curves. Two
examples are shown in Figure 7-5. The fitted subcell photocurrents and calculated
SPB for each MJ-OPV are listed in Table 7.2.
The array of devices formed by
fabricating subcells equal to, 20% thicker or 20% thinner than those of the modeledoptimal structure (SCI T
=
23.5nim, SC2 T = 54.Onm) leads to SPB between -0.13
and +0.09.
To simulate the MJ-OPV circuit, the subeell current sources must be programmed
103
8 -
SIP1 Modeled
C~4
U7 -
E
E
SP1 Exp
6p
SP2 Exp
U
1W
0
5
SP2 Modeled
4 11
40
60
50
SC2 Thickness [nm]
70
Figure 7-6: Fitted photocurrent for an array of fabricated cells with the same SCi
thickness and varying SC2 thickness. Black lines are fits to the data. Red and Blue
lines are modeled subcell photocurrents.
with an array of realistic subcell photocurrents. The tradeoff between the photocurrent of one subcell and that of the other is complex and depends both on absorption
spectra as well as the weak micro-cavity effect formed by the silver electrode. The
optical model utilized in our previous publication is highly useful for initial selection
of subcell order and thickness, but contains inherent sources of error, such as assumptions of unity carrier collection efficiency and potential discrepancies in thickness and
in optical behavior (especially of the silver nanoparticle layer). Therefore, we fit our
fitted subcell photocurrent data to trend lines to estimate subcell photocurrents outside of the range of our fabricated devices. Even though this may not lead to accurate
models of photocurrent values for real devices, it enables us to visualize PCE dependence on SPB to a larger range. The data and their fits alongside the optical model
results are shown in Figure 7-6.
7.4.3
Dependence of Multijunction Performance on Subcell
Photocurrent Balance
Table 7.2 reports the performance parameters for the array of 9 MJ cells as well
as their fitted subcell photocurrents (SP) and calculated SPB. The dependence of
104
6.5-
0.75070-
0
6,5-
6.5-
0.70
.
5
C
4.5
5
PC
5.5
5.5-
LL
' T 0.65 A.
4 5
.5
0.50
-0.15
6
-
JSC
0.00 0.05 0.10
Subcell Photocurrent Balance
-0.10
-0.05
40
4.5
0.15
L-
0
4
0C.
0.0 0.1 0.2 0.3 0.4 0.5
Subcell Photocurrent Balance
-0.5 -0.4 -0.3 -0.2 -0.1
Figure 7-7: Dependence of FF, Jsc arid PCE on SPB for an array of simulated MJOPVs. Left arid Right show different ranges of SPB. Lines are simulated values and
points are experimental values.
FF, Jsc, and PCE on SPB are shown in Figure 7-7. Within this range of data, there
is a clear monotonic trend of increasing FF with increasing SPB, with a change of
28% of the maximnal-FF. Jsc decreases slightly with SPB, with a change of 14% of the
maximal-Jsc. Voc (not shown) has no dependence on SPB, and varies over a range
of 6% of the maxinmal-Voc. PCE increases with SPB, with a change of 21% of the
maximal-PCE. The maximal PCE cell shown here is the highest PCE cell we have
reported for this materials set.
To have a more detailed look at a larger range of SPB, we simulate our MJ-
OPV circuit via the quite-universal circuit simulator, [162, 163] and set the subeell
photocurrents to follow the fits shown in Figure 7-6. Figure 7-7 shows the dependence
of FF and PCE on SPB for an array of sirmulated MJ-OPVs. For 0 <SPB <0.2, FF
increases while Jsc decreases, leading to an inflection point of maximum PCE. For
SPB >0.2, FF stabilizes as Jsc continues to decrease rapidly, leading to rapid decrease
in PCE. For SPB <0.2, both FF arid Jsc decrease, leading to decrease of PCE. At
SPB =±0.06, FF is increased by 8% arid Jsc has decreased by 5% versus the balanced
device. This has the maximum PCE which is 3% greater than the balanced device
(SPB=0)
and 16% greater than the maximal-Jsc device (SPB= -0.15).
105
7.5
7.5.1
Discussion
PCE Optimization in Multijunctions
The increase in FF with SPB in this array of devices experimentally confirms
empirical observations stated elsewhere that the FF of a MJ-OPV depends on the
balance of subeell photocurrents, particularly noting that FF seems to be dominated
by the current-limiting subcell [132, 156,164]. Further, we note that this relatively
small change in FF in the single-junctions indicates that the significantly larger change
in FF in the multijunctions is not simply due to any increase in series resistance of
the subcells. Jsc varies without a clear dependence on PCB due to its dependence
primarily on absorption efficiency (and thus optical environment) rather than electrical characteristics of MJ architecture. Voc varies minimally over this thickness range
because with the optimized hybrid tunnel junction RZ the Voc is equivalent to the
sum of the SJ cells, and the Voc of the SJ cells are essentially independent of subcell
thickness. PCE shows variation due to Jsc, but the trend of increasing PCE with
increasing PCB (due to increasing FF) is evident.
Figure 7-8a shows current-voltage characteristics for MJ-OPV devices with constant SCI thickness (28.2 nm) but three different SC2 thicknesses: 43.2, 54.0, 64.8
nm. Simulated current-voltage characteristics for multijunctions with SPB = 0 (balanced), -. 15, and +.06 (optimal) are shown in Figure 7-8b. The dominance of FF
over Jsc in maximizing PCE is evident. Thus, even though optical models are useful
for initial optimization of subcell order and thickness, [143,149] further PCE gains
are made by utilizing imbalanced subcell photocurrents. We note, however, that the
tradeoff of Jsc and FF will vary for each materials set. [158,160] Ultimately, the gains
(and losses) in FF can be probed via adding any two SJ JVs with varying PCB, as
explained above.
106
Simulated
Experimental
E
_-2
E -2
-3-
-3
CD-4-
a) -4-
5
Max-PCE
-5
--
07
-0.5
-Matched
Max-Jsc
-
70.0
0.5
1.0
1.5
,
-0.5
2.0
0.0
0.5
Voltage [ V
Voltage [ V
,
,
1.5
1.0
2.0
]
Figure 7-8: (Left) Experimental Current Density-Voltage Characteristics of multijunction devices with increasing SC2 thickness and constant SCI thickness. (Right)
Simulated Current Density-Voltage Characteristics of multijunction devices with balanced subcell photocurrents, large negative imbalance, or positive (optimal) imbalance.
7.5.2
Dependence of Subcell Photocurrent Balance on NonStandard Conditions
Illumination
We further note that herein we vary SPB with subcell thickness, but SPB also
depends on spectrum of illumination and on light intensity. Therefore, effects of
SPB on PCE should be considered for all solar cells that will be under natural solar
illumination, which indeed exhibits significant intensity and spectral shifts throughout
the day and year. [165]
Power Output Dependence on Load Resistance
Often within the research lab we focus solely on power conversion efficiency, however out in the field power generation and utilization is not so simple. In Figure 7-10
we plot the power output of three simulated MJ cells (max-Jse, balanced, and maxPCE cells) versus the load resistance. The maximal-PCE cell (with slight positive
SPB) produces the highest power output at a load resistance of approximately 300
, 3% higher than the balanced cell and 16% greater than the max-Jsc cell. However
this advantage in power output decreases as the load resistance is varied from the
107
0.07 -
. .. ,
. . ...
. ...
0.06
,0.06
0.8 0.75
. . ...
T
-
0.05'
::,0.04
0
!
J2
't-
S J1
Qf 0.01
0.00
.. .!.7
---.
6
. .
4
'45
0.5-
0.03
r.0
, ..
S.
0.1
ioU1
0
3
0.2 .
0.1
100
0
1
]*-
0.
Intensity [mW/cm2]
0c
0.3
0.4-
0
0.1
10
1
Intensity [mW/cm2]
100
D
3:
0
Figure 7-9: (Left) Experimental responsivities of max-PCE MJ and corresponding
SJ cells versus illumination intensity. (Right) Calculated subcell photocurrent balance and experimental power conversion efficiency for the max-PCE MJ cell versus
illumination intensity.
maximum power point. The balanced subcell produces greater power for load resishas
tances less than 200 Ohms (towards short circuit). Furthermore, the max-Jsc cell
the advantage of having a significantly smaller variance in power output in the region
of maximum power point (a change of 1% in power output for a change in 10% of load
resistance versus a change of 3% for the maximal-PCE cell). This is an unexpected
advantage of the lower FF and higher shunt resistance of the max-Jsc cell. Therefore,
for systems without sophisticated MPP tracking systems or seeking higher flexibility
in load resistance may choose to utilize the max-Jsc cell with lower FF and lower
PCE. Likewise, for applications seeking lower load resistance, the balanced cell may
be preferable to the maximal-PCE cell.
7.6
Conclusions
We first demonstrate the importance of fill factor in optimizing MJ-OPVs. We
then show a simple method for calculating subcell photocurrent balance, SPB, from
current-voltage characteristics and find that the fill factor for multijunctions comprised of this materials set show a linear dependence on SPB within a limited thickness range. Finally, we simulate a broad array of subcell thicknesses and confirm that
is a
MJ FF can be adjusted via subcell photocurrent imbalance, and that when there
108
7
6
E
S5
Ea
I-4
0~
'53
-.15
-SPB=
-SPB= 0
-SPB= +.06
.
0
--------
0
200
600
400
Load Resistance [Ohm]
800
1000
Figure 7-10: Simulated MJ cell power output versus load resistance.
large difference in FF of the subcells then the gain in MJ FF can dominate losses in
Jsc. These methods are broadly applicable to multijunction photovoltaics and should
be utilized to fully optimize power conversion efficiency.
109
110
Chapter 8
Vapor-Processed Crystals and
Aggregates of Organic
Semiconductors
As discussed above, high long-range order can lead to increased short circuit current within OPVs. The highest possible ordered nano-structure is the crystal. Herein
we explore two novel processes for fabricating organic micro-crystals:
via solvent
annealing (first demonstrated by Mascaro et al [166] and here analyzed and modeled [167]) and via low vacuum thernial evaporation.
Both of these processes are
compatible with conventional device fabrication techniques.
8.1
Organic Semiconductor Needles Formed via
Solvent Annealing
Mascaro et al [166] produced extremely high-aspect-ratio Alq3 needles up to a
centimeter long, with characteristic cross-sectional dimensions of less than a micron.
After evaporating thin (10-20 nim) filis of amorphous Alq3 onto silicon and glass substrates, Mascaro et al annealed these filns in a solvent vapor (chloroform or methanol)
at room temperature and atmospheric pressure to promote the growth of single-crystal
111
Glass
substrate
i
Amorphous
Alq 3 film
Alq 3 deposition
.
I
Alq3 crystals
and droplets
Methanol annealing
Figure 8-1: Process flow for growth of Alq3 needles. [167]
Alq3 needles. This process is shown in Fig. 8-1. They demonstrate that the rate of
Alq3-needle growth and its morphology depended on substrate topography, substrate
properties, and solvent properties.
Motivated by Mascaro et al we examine the growth of needle-like crystals from a
binary fluid mixture. Through experiments, numerical modeling, and analytic scaling
laws, we characterize the general growth process on planar substrates.
8.1.1
Experimental Results
A few minutes into the solvent annealing process, images of the substrate as
viewed through a microscope darken, and later the substrate develops a speckled
texture. This visual transformation corresponds to an initially-uniform film breaking
up into small drops driven by either spinodal dewetting or hole nucleation [168]. The
first Alq3 crystals begin to appear during this transformation. These crystal needles
tend to grow in clusters surrounding a common nucleation point, as shown in figure 82. For thicker films, these points of nucleation appeared as large splotches (see the
micrograph for HAlq3=60nm in figure 8-2) that were tens of microns in diameter;
the periphery of those splotches acted as nucleation sites for crystal needles. These
nucleation sites were not seeded; instead it appears the came form imperfections
during deposition, or, possible, form particulate contamination during handling. It is
interesting to note that the morphology and thickness dependence of these splotches
112
(a)
(b)
HAtq, = 10 nm
HAlq3 = 15 nm
H Alq = 30 nm
H441q
= 60
nm
Figure 8-2: (a) Optical micrographs of needles for different thicknesses of Alq3 films
after annealing for 1-5 hours. (b) SEM micrograph of rectangular needles going from
a common nucleation site after solvent annealing a fihn with HAIq3=15nm. [167]
resembles hole formation in nucleation-driven dewetting (e.g. [169]). The thick rims of
such holes could serve as nucleation sites, which is in agreement with our observations.
In experiments, Alq3 tends to solidify into single-crystal, high-asp ect-ratio needles.
Note that Alq3-needle formation is not specific to these experiments; needle formation
is also observed in crystals grown from physical-vapor deposition [170] and liquid
solutions of Alq3 and solvent [171]. Thus, it is likely that Alq3 has an anisotropy in
the growth due to an anisotropic surface energy, which favors needle-like morphologies
[172]. Nevertheless, the rectangular geometry breaks down for large Alq3 thicknesses.
The needles for HAq3=60 nmni are tapered and have a sharp (as opposed to flat) tip.
In addition, these tapered needles grow along slightly curved paths, in contrast to the
straight paths observed in thinner filns.
For rectangular needles, we track their lengths in experiments by taking a onedimensional slice of pixels along the needle's long axis. When sequential pixel slices
are placed side-by-side, they form a simple visualization of the evolution of the needle
length, as shown in figure 8-3. The dark curved line in figure 8-3b gives the position
of the needle tip as a function of time and corresponds to the length of the needle,
since the pixel slices start at the base of the needle.
113
(a)
5 min
20 min
60 min
120 min
(b)
5
30
5 ptm
120
90
60
Time (min)
150
180
4 pm
Figure 8-3: (a) Successive snapshots of a cluster of needles growing during solvent
annealing. The top-left needle from the cluster is tracked over time in (b). (b) Slice
of micrograph pixels along the axis of a needle as a function of time. [167]
Needle growth appeared to exhibit a power-law behavior over one decade, such
that the needle length grows like
Lneedle
~ T'
(8.1)
where T is time and y is the growth exponent. In our experiments, To was taken
to be the start of the experiment. Needle growth slows down over time, such that
0 < -y < 1. Simulations, discussed below, suggest that this slowing of growth is due
to the depletion of liquid Alq3: at late times, growth saturates as nearby liquid Alq3
solidifies, thus reducing the availability of mobile material.
8.1.2
Physical Picture
In these experiments, methanol vapor interacts with solid Alq3 film to produce
a liquid mixture. Thus, the evolution of the Alq3-methanol film is governed by the
dynamics of thin liquid films. Furthermore, we neglect condensation and evaporation
of solvent during needle growth. In what follows, we consider a time after a needle
has nucleated from the binary mixture.
114
Alq 3 needle
Wetting layer
\
|
Droplets
/
Figure 8-4: (a) Schematic of a needle growing into a fluid fih. (b) Optical micrograph
of the area surrounding an Alq3 needle after solvent annealing for 3 hours. [167]
The mixture of Alq3 and methanol forms a thin, liquid film with characteristic
film thickness H and characteristic length L in the plane of the fihn. For filns that
are approximately 100nn thick or less, intermolecular interaction between liquid,
substrate, and surrounding vapor become important. For van der Waals interactions,
the interaction strength is given by the combined Hamaker constant for the solidliquid-vapor system, ASLV.
In addition to the intermolecular pressure, surface tension produces a pressure
juimp across a curved liquid/vapor interface. Thus the total fluid pressure is a sum
of the surface tension as well as van der Waals interactions between liquid, substrate,
and surrounding vapor. The competition between surface tension and intermolecular
forces causes the initially-uniform film to break up into drops through either spinodal
or nucleation-driven deleting. Note that the short-range repulsive force in the molecular interactions prevents formation of dry spots.
Although the term 'dewetting'
suggests complete removal of fluid, here and in the literature it describes progression toward an ultra-thin filn rather than film rupture. After dewetting, the drops
coarsen, during which capillary pressure denominates in drops, while internolecular pressure dominates in the ultra-thin film forming a slowly-evolving quasi steady
configuration.
In our system, the fluid film is composed of a binary mixture of Alq3 and solvent
molecules, lience a second equation is required to track motion of solvent relative to
mixture. Solvent is advocated by fluid flow and driven down concentration gradients
115
([-
1=0
(b)L
r=256
(c)
IA
A
A
A
~
~
A
A
t= 1000
(d)
-ineedle
A
t= 1000000
A
5
0
Figure 8-5: Evolving Thin Film. A solid needle grows from the left. [167]
by diffusion. Furthermore, at the solid/liquid interface of a growing crystal needle,
we assume local equilibrium at the needle tip, such that the solvent concentrations
at the tip are fixed.These fixed values provide the relevant boundary condition at the
needle tip if solidification is limited by transport of Alq3, not by interface kinetics.
8.1.3
Mathematical Model and Numerical Results
Based upon this physical picture of Alq3 crystal growth, my collaborators in the
Hosoi group developed a mathematical model [167]. Then, these governing equations
were solved numerically using centered finite differences and fully implicit time steps.
At each time step, the system was solved using a two-step procedure: first they solved
for film hight and concentration while holding the needle tip position fixed; next they
advanced the needle tip.
The result of this simulation is shown in figure 8-5. At time t=0, the fluid film
has a uniform, but rough, thickness. At a characteristic time (here at t=256), the
uniform film becomes unstable and breaks up into small drops (t=1000).
At late
times, drops collapse and coalesce to form a larger, more coarsely-spaced drops. The
growth exponent could be manipulated by varying the thickness of the Alq3 film. The
measured growth exponents were consistent with the predicted range, and increased
with increasing film thickness, as predicted by the model.
116
8.1.4
Conclusions
Although these experiments were conducted on a single system, Alq3-methanolglass, the results should be applicable to many molecule-solvent-substrate systems.
In particular, the behavior of the system is governed by a set of dimensionless parameters, which could be tuned using different molecules, solvents, and substrates.
8.2
Low Vacuum Thermal Evaporation of Organic
Semiconductors
8.2.1
Theory
It is well established that the vacuum pressure of a deposition chamber has significant effects on purity of a thin film. The "air" molecules (or whatever else might be
floating around in a dirty chamber) will land on a substrate at a uniform rate. The
time for a monolayer of "air" to form on a surface is given by:
tmoflo =
3.61
P
(8.2)
where tmOO is the monolayer formation time in seconds, and P is the pressure in pascals (1 pascal = .0075Torr). (Note: the constant value will depend on the chemistry
of the "air" molecules, but the possible variance is negligible for our consideration.)
Since we typically deposit organic thin films at 1 Angstrom/second and at a pressure
of 10-' Torr, an typical active layer of 60nm (roughly 60 monolayers of molecules) will
include approximately 200 monolayers of "air".
(Air is made primarily of Nitrogen,
which has a covalent atomic radius of 71 pmn, thus we can roughly approximate 200
monolayers of air as 14nm thick, or >20% of the thickness of the active layer.) This,
apparently, is not too bad. If we lower the pressure to 10'
Torr, we can get down
to only 2 monoloayers of air, but if we increase the pressure to 1 mTorr (10'
Torr),
the thin film will contain 200,000 monolayers of air.1 We see here that pressure has
'Note that our quartz thickness monitor stops working at approximately 10-4 Torr.
117
1E+04
G1E+03
IE+02 -
E
1E+02
1E+03 0
E
1E+01 -
0 IE+OO
U-
1E-02
o
-
1E-02
0
S1E-03
1E-09
____
1E-06
1E-07
1E-08
1E-05
1E-04
1E-03
Chamber Pressure [Torr]
Figure 8-6: Relation of "air" molecule's monolayer formation time and chamber pres-
sure. The typical length of an active layer deposition is identified with the red line.
a significant influence on thin film purity.
However, vacuum pressure also affects the mean free path (i.e. line-of-sight flight
distance) of an evaporant. At higher pressures, there are more "air" molecules floating
around in the chamber, and thus, as an evaporant shoots off towards the substrate,
it may be subject to collisions with these "air" molecules.
The mean free path, 1
between collisions for a molecule is given by:
I =
kBT
BT
(8.3)
23
J/K), T is the temwhere kB is the Boltzmann constant in J/K (1.3806488x10-
perature in K (here, 300K), P is pressure in pascals , and d is the diameter of the
molecules in meters (typically 10'
m).
For the situation described here, we can
simplify this equation to:
1-
0.00125
P
(8.4)
The number of collisions depends both on the vacuum pressure as swell as the
throw distance of the chamber.
Both of ONE Lab's vacuum thermal evaporation
chambers have a throw distance of approximately 50cm. The comparison of mean
free path and ONE Lab's throw distance is shown in figure 8-7.
We see that for
pressures lower than 1 mTorr there is very low probability of an evaporant colliding
118
1E+03 -
E
1E+02
) 1E+01
C 1E+00
1lE-01
1E-02
.L 1E-03
1E-04
1E-05
1E-06
-
- -
-
1E-06 1E-05 1E-04 1E-03 1E-02 1E-01 1E+00 1E+01 1E+02 1E+03
Chamber Pressure [Torr]
Figure 8-7: Relation of evaporate molecule's mean free path and chamber pressure.
The throw distance of our chamber is identified with the red line.
before landing on the substrate. However, for pressures higher than 1 mTorr there
is high probability of scattering events. At atmospheric pressure (760 Torr) we can
anticipate > 105 collisions before deposition on the substrate surface. This is definitely
no longer line-of-sight and we anticipate will result in a conformal coating.
8.2.2
Results and Discussion
However, what we anticipate is usually not what actually happens. In experiment,
we find that pressures of 1 mTorr result in aggregation of the evaporant (in this case,
CuPc) in mid-"air", and deposition of huge (>1 micron) clusters of crystalline CuPc.
In the field, this is called "snowing".
An image of a snowy thin film is shown in
Figure 8-8.
In an attempt to utilize our CuPc snow, we soak the snowed-on substrate in
methanol to dissolve the CuPc. (CuPc is nearly insoluble, but methanol is the best
attempt.) We then fabricate a solution-processed solar cell with dissolved CuPc snow
(a donor) blended with PCBM (an acceptor), capped with conventional electrodes
and interlayers. In current density-voltage characteristics, the snowy PV exhibits an
extremely low shunt resistance, such that diode behavior is non-existent. We propose
that the 'gigantic' CuPc nanocrystals (micron-scale versus the sub-100nm-scale of the
device) allowed shorting from the bottom electrode to the top electrode, bypassing
119
Figure 8-8: SEM image of CuPc "snow" formed by LVTE.
any potential heterojunction interface.
Further work should include intentionally varying the size of the CuPc snow-likely
through vacuum pressure control- as well as development of more suitable device
architectures.
8.3
Conclusions
The novel processes reported here for generation of long range order in molecular systems are highly accessible due to their utilization of conventional capital
equipment and materials in the organic optoelectronics field. We propose that these
processes will be compliant with current OPV fabrication, however demonstration
of efficient solar cells with the organic crystals demonstrated in this thesis is not
yet completed. Furthermore, the micron- and centimeter-scale crystals reported here
would also be of interest in pairing with a vapor-processed, conformally deposited
semiconductor material, such as oCVD semiconducting polymers (e.g. [173]). The
macro-scale molecular ordering presented here holds potential to produce solar cells
with enhanced Jsc, Voc, and FF due to extended exciton and polar diffusion lengths
in molecular semiconductors.
120
Chapter 9
Conclusions
9.1
Summary
With the development of the first "efficient" organic photovoltaic, Tang kicked off
an era of rapid development in nanostructured solar energy. Significant improvements
have been made to the point of near-connercialization in niche markets today. However continued improvements in power conversion efficiency, enhancements to device
lifetime, and better processability and mechanical properties are still needed to realize
broad market presence in both developing and developed nations.
In this thesis we tackle the problems of processability and module mechanical
properties via implementation and development of Mapor-processed carbon-based electrodes and nano-thin encapsulant. We further tackle the problem of power conversion
efficiency with two approaches: first through development of optimization guildelines
for sub-100nm multijunction photovoltaics and secondly through novel methods for
formation of molecular semiconductor crystals.
Firstly, we investigated novel robust electrode materials with the Kong and Gleason groups and have reported:
" development of vapor-processed organic conductors and integration with organic
semiconductors
* demonstration of monolithic organic solar cell arrays with carbon-based elec-
121
trodes
e first ever solar cell fabricated directly on paper
In addition we investigated the device lifetime of our array of standard OPV structures
and materials and explore the implementation of nano-thin ALD encapsulants. We
report:
* distinguishment between shelflife decay due to atmospheric exposure and to
materials aging
* >50 times improvement in device shelflife via <30nm-thin encapsulation
The combination of this nano-thin encapsulant and carbon-based electrodes, enabled
by their ease of processing and earth abundance, will lead to solar cells empowered
for vast impact and distribution throughout developing nations.
Furthermore, we advanced power conversion efficiency through development of
sub-100nm multijunction photovoltaics. Specifically, we report:
" Demonstration of simple recombination zone with negligible electrical loss
* Investigation of optical interference in nano-thin photovoltaics, leading to photocharge generation enhancement via inversion of subcell order
We also reported on the rarely explored impact of subcell photocurrent balance on
multijunction photovoltaic optimization, including,
" empirical demonstration of fill factor dominating over short-circuit current in
efficiency maximization of multijunctions comprising subcells with dissimilar fill
factor
* development of a highly accessible technique for determining subcell photocurrent balance
Multijunction photovoltaics enable surpassing of the SQ limit, and these reports will
enable significant steps towards practical devices.
Finally, we report on nano-structuring of organic semiconductors for utilization
in photovoltaics, including,
122
"
modeling of potential enhancement via nanostructuring for two standard materials sets. These results suggest that an ideal nanostructure of either DBP-C60
or ClAlPc-C60 would lead to approximately doubling of efficiency compared to
the planar nano-architecture.
" demonstration of low-temperature thermal annealing of completed devices for
enhancement of power conversion efficiency
" formation of organic semiconductor crystals via (1)
atmospheric thermal evap-
oration and (2) solvent annealing
These unconventional applications of conventional processing methods suggest a path
to development of efficient OPVs.
9.2
Looking to the Future
While considerable progress towards efficient solar cells on every surface is reported
in this thesis, that goal is by no means yet achieved.
The wide-ranging and practical demonstrations in Part 1 illustrate the near-termn
potential for the design and implementation of low-cost solar energy conversion on
non-traditional substrates, without mechanical or processing limitations.
However
the tradeoff of conductivity and transparency in these carbon-based electrodes continues to be a limitation for their utilization as transparent electrodes of large area
devices. Further improvements in transparency and/or conductivity or implementation of metallic grids (such as that found in conventional solar cells) could expand
the practicality of these novel electrode materials. Furthermore, the enhancement in
lifetime via a sub-30nm thin film shown here is significant, but is still impractical
for long term (>2 months) applications. Further work exploring new chemistries of
encapsulation layers as well as more stable active layers and electrode materials will
enable robust, ultra-lightweight solar arrays for long term deployment to the ends of
the earth.
123
The demonstration of organic photovoltaics with mechanical properties that can
be utilized in developing countries are highly motivating, however their current status
of low efficiency continues to limit their actual utility. However, the tools and guidelines for enhanced power conversion efficiency reported here present significant steps
towards useful operation of organic photovoltaics. Multijunction architectures are an
important approach to reduce thermal losses and can significantly increase the ultimate performance of these relatively low efficiency organic materials. Further work
should be performed on implementing the multijunction guidelines reported in this
work with higher efficiency active layer materials. One should note, however, that
some of the methods presented here will be less beneficial with better performance
subcells, namely: (1) as OPV nano-structuring enables more absorbant and/or thicker
subcells, conventional subcell order will likely give higher efficiencies than the inverted
subeell order presented here. And (2) optimization via subcell photocurrent balance
is most beneficial with subcell sets wherein at least one of the subeells has very low
FF. As subcells are all brought to similarly high FF (>0.75), the optimal SPB will
likely draw closer to zero (i.e. current matched). However these optimization methods will ocntinue to be useful as we work towards "practical limit" efficiency subeells.
In addition, the novel processes reported here for formation of organic semiconductor
crystals should be compatible with current solar cell fabrication processes, however
demonstration of efficient solar cells with the organic crystals demonstrated in this
thesis is yet be completed. Thus, our hypothesis that molecular ordering will concurrently produce greater Jsc, Voc, and FF is left for confirmation via experimental
work and/or simulation.
Ultimately, incorporation of these high efficiency approaches with the flexible,
processable electrodes and encapsulants demonstrated in this work may lead to functional solar technologies for both developing and developed nations.
The field of
organic photovoltaics is rapidly advancing, yet these very real limitations are inhibiting their broad distribution. The impact of developing countries' lack of electricity as
well as developed countries' dependence on polluting electricity generation schemes
should motivate our field towards advancing the production and utilization of robust,
124
ultra-lightweight, efficient solar arrays. I hope and pray that this work has sped the
coming of clean electricity generation to those in need around the world.
125
126
Appendix A
Photovoltaic Primer for U.S.
Policy Makers
The word photovoltaic comes from the Latin words for light and electricity. The
function of a photovoltaic (PV) device is to convert optical power into electrical power.
Light is comprised of photons (i.e. chunks of light), each with a specified energy.
Our eyes identify the different photon energies as different colors.
Sunlight looks
white because it is comprised of photons with energies ranging from 0.5 to 4 electronVolts (e-V). When light is incident on a material, it can be reflected, absorbed, or
transmitted.
Only light that is absorbed by the PV device can be used to make
electricity.
Semiconductors are the essential building blocks of modern day electronics (e.g.
transistors, flash memory, LED displays). In semiconductor materials, electrons reside
in energy states that are grouped in bands, and each of the bands is either completely
filled or completely empty of electrons. The highest energy band that is completely
filled when the material has received no optical or other energy is called the valence
band (VB), and the lowest energy band that is completely empty is the conduction
band (CB). If an electron in the VB is given enough energy, it can be excited to al
empty state in the CB, where it is mobile and may thus conduct electricity.
The
energy difference between the VB and CB is called the bandgap (EG). Bandgap
energies of semiconductors range from 1 to 5 eV. Coincidentally, this energy range
127
Wavelength [nm]
1.75
1.50
C 1.25
C14 1.00
9
0.75
0 50
S0.25
0.00
0
0.5
1
15
2
2.5
3
3.5
4
4.5
5
Energy [eV]
Figure A-1: Solar power per area separated by photon energy.
Energy
CB
Energy
EG
VB
Position
Color
Figure A-2: Energy levels of photons and electrons in semiconductors.
overlaps with the energy range of sunlight, and thus situates semiconductors to be
efficiently utilized as the primary component of PV devices.
Excitation energy for an electron can be supplied by photons. If a photon with
energy of exactly EG is incident on a VB electron, then the photon is absorbed and
the energy excites the electron to the CB. An excited positive charge (i.e. a hole)
is formed simultaneously and also becomes mobile. If the photon has energy greater
than EG, then the electron will still be excited to the CB but the extra light energy
is wasted as heat. If the incident photon has energy lower than EG, it will not be
absorbed and the electron will not be excited. (Note that more complex behavior
can occur in some unconventional semiconductors.) Excited electrons must then be
expelled from the semiconductor by energetic asymmetry within the device. This
asymmetry can be generated either by introducing a p-n junction (a boundary or
128
Excited Excited
+Heat
Not
Excited
CB
Energy
VB
Figure A-3: Excitement of electrons by photons in semiconductors
interface between two types of semiconductor material p-type and n-type inside a
single crystal of semiconductor, achieved by incorporating impurities/dopants) or a
junction between the semiconductor and another material. Excited electrons are then
removed via electrical contacts.
A single PV device is called a solar cell. It is comprised of semiconductor(s),
electrical contacts and, potentially, optical and/or encapsulation material(s). Semiconductors can be comprised either of a single element (e.g. Silicon (Si)) or of an alloy
of two or more elements (e.g. Cadmium Telluride (CdTe) or Copper Zinc Tin Sulfide
(CZTS)). The semiconductor component can either be fabricated as a wafer (i.e. a
slab of crystalline semiconductor that is hundreds of microns thick) and then cut to
size with the other layers deposited upon it (wafer based); or it can be fabricated as a
micron-scale thin film upon a substrate with other layers also deposited inunediately
preceding or subsequent (thin filn based). Electrical contacts are utilized to output
the generated electricity from the semiconductor. Optical materials may be added to
enhance light absorption in the semiconductor (most commonly by reducing reflection). Encapsulation materials may be added to minimize environmental exposure of
the semiconductor and contacts, therefore increasing operational lifetime of the solar
cell.
A solar cell generates electricity only when light is shining on it. The amount of
electricity generated is characterized by the number of electrons flowing as well as
129
the potential energy of those electrons. Each photon absorbed in the semiconductor
can excite one electron: this translates to current. Each electron volt (1eV) of energy
of the bandgap converts to IV of electrostatic potential energy of the electrons: this
translates to voltage. However, there are losses for both current and voltage, so generated electricity is significantly lower than potential generation. The actual electrical
output of the solar cell is characterized by the power conversion efficiency (PCE),
which is the ratio of the electric power outputted to the optical power inputted. PCE
is dominated not only by the opto-electrical properties of the semiconductor but also
incorporates gain and loss mechanisms within the optical and/or conductive materials.
PCE is typically reported under standard test conditions, e.g. light with an intensity
of 1000 W/m2 and a spectrum of AM1.5G (the lighting condition representing the
yearly average solar light intensity and spectrum at sea level in global mid-latitudes
(e.g. the United States, Europe, China, and Australia)). The total power output of
a solar cell is calculated by multiplying the PCE by the total incident optical power.
The theoretical maximal PCE for the conversion of sunlight to electricity on Earth
(calculated from fundamental thermodynamic principles) is 89% (the Carnot limit).
However, if we account for the physics of single semiconductor junction PV devices,
the theoretical maximal PCE is 33% (the Shockley-Queisser (SQ) Limit). This is
due to (1) the non-utilization of photons with energy less than the semiconductor
bandgap, (2) the loss of photonic energy greater than the bandgap, and (3) fundamental recombination losses within semiconductors. Currently, the maximal reported
PCE for a single junction solar cell is 29%, nearly the theoretical limit. However, most
solar cells commercially available today are single junctions with PCEs ranging from
12-23%.
However there are multiple methods for exceeding the SQ Limit. First is the
implementation of multi-junction solar cells, in which multiple solar cells with semiconductors of differing bandgaps are fabricated on top of each other to better utilize
the solar spectrum. A triple-junction solar cell could theoretically achieve 49% PCE.
This approach has produced the highest reported solar cell thus far, with a PCE of
44.4%. However, fabrication of such a complex cell is cost prohibitive for most appli130
cations. The second method is usage of optics and/or imechanical tracking systems
to concentrate the intensity of light incident on the solar cell.
For most (but not
all) semiconductors, this increases efficiency while decreasing overall cost. The third
method is utilization of semiconductors with unconventional opto-electronic physics,
however such PV technologies are still in the R&D stage.
Solar cells are connected mechanically and electrically to form PV modules (also
known as solar panels).
The typical mechanical connection is adherence to a glass
pane and mounting on an aluminum frame. The cells are connected either in series
and/or in parallel with thin wires to provide the desired voltage and current output.
Typical PV modules output voltages of 5V to 30V.
A full PV system further includes electrical components.
PV modules gener-
ate DC electric power, which is the unidirectional flow of electrons.
DC power is
useful primarily for charging batteries and powering personal electronics, ideal for
residential-scale applications.
However, the electric grid runs on AC power, where
the flow of electrons periodically switches direction (in the US, at a frequency of 60
Hz). Household AC power in the US is at 120V, and transmission line power is at
110,000 V or above (to reduce the energy lost when transmitting over long distances).
Therefore, for grid-scale applications, an inverter is necessary to convert between DC
power and AC power in addition to other electronics for modification of voltage output.
131
132
Appendix B
Fabrication of Vapor-Processed
Organic Photovoltaics
B.1
Introduction
There are many variables that impact on device performance. Figure B-1 are all
the same photoactive materials and device structure, and all are optimal performances
for an experimental run, yet PCE range from 0.6%-4.0%.
Note: typical standard
deviation 10%.
Thickness of the photoactive layers is a key factor in planar OPVs. Too thin and
the electrodes/interlayers may short through the device. Too thick and they will add
too much series resistance and optical absorption losses. (Figure B-2 shows devices
with two different batches of DBP.)
B.2
Dominance of DBP
Why do we mostly focus on DBP optimization?
materials, we get a significantly smaller effect.
Because when we vary other
(Note: this doesnt hold for every
material set and device structure. But it does here, and is the foundation of the
research presented in this chapter.)
From the circuit model we note that lower series resistance, Rs, is better for fill
133
4-
2
i
-
-
20110309
20120328
20120918
20120925
20121003
20121121
20130124
4.0%
2.7%
1.2%
1.6%
0.6%
1.8%
2.7%
1,
0
-2
-
--
-4
-6
-1.0
-0.5
0.0
0.5
1.0
Voltage [V]
Figure B-1: Current Density-Voltage Characteristics of a variety of devices with nominally the same architecture and materials.
factor as well as short circuit current density. Whereas lower shunt resistance, RSH,
is worse for fill factor as well as short circuit current density.
B.3
Donor Layer
B.3.1
Manufacturer
DBP material purchased from Sigma Aldrich shows a 40-50% improvement in
PCE over material purchased from Lumtec. The SA batch shown here is purified
once whereas the Lumtec batch shown here is purified two times. These two JVs are
from two separate device runs.
The improvement in PCE is due to increases in both Jsc and FF. This may be
due to differences in impurity concentration or impurity chemistry.
Note that we should further experiment with comparing different purities of the
different manufactures.
However also note that our evaporator is only capable of
134
4-
----
3.-
;:
-
2-
-
1-
Onm
DBP
DBP
DBP
DBP
DBP
4-
2.5nm
5nm
10nm
20nm
2-
~/i'1I'I
E
-2
-1
20nm
2-3
-5
-050
-0.25
0.00
0,25
0.50
0.75
10nm
-4t
4 DBP
1.00
DBP
00 oa5
-1V0
Voltage [ V I
05
1.0
1.5
Voltage (V)
2.5
I
I*I*I
*
I
-U-- 20130130
-U- 20121003
2.0U
1.5-
IR
0L.
0
U
1.0-
U
0.5U
0.00
5
10
15
20
25
30
35
40
Donor Thickness (nm)
Figure B-2: Current Density-Voltage Characteristics of a thickness optimization run
for two different batches of donor material.
TOR
RS
-
Glass
Figure B-3: Left: Schematic of device architecture highlighting the various layers for
optimization. Right: Device circuit model.
135
U
U
U
IvPCE
-, 2 0
R81
tVPCE - 2
0
-
ivPCF
8
500-
s50-
500
U
I
E
I
200-
15
20
.
5
10
25
30
40
35
-
05
I
W
I
100-
Ii
i,., o
45
U
U
100,
-
0
0
300
200-
U
I
I10W
2 200
0-.
.
20
400U
E 300-
a
E300
- 1,5
400-
1 5
400
20
10
50
30
40
50
60
70
U!
80
IW
20
40
C60
DBP [nm
60
MoOx
Figure B-4: Thickness optimization of various layers.
10000,
jINI
1000
E
*
Rs @1.25V
Rsh @-1.25V
N
I
*
iv PCE
iv Jsc
Siv FF
RshU
Jsc
1
-
IL
IL
0
100-
LU
-Ci
It
FF
INe
Rs
PCE
10U
0
5
I 0.1
15
10
20
25
35
30
40
45
50
DBP [nm]
Figure B-5: Thickness optimization of DBP showing various parameters.
2-
2-
1 - I
E
t
0
0
f I
-
Lumtec 2x
SA 1x
I
2.8
3.2
0.89
0.90
0.52
0.67
1.3
2.0
-1.1
-3
(D2
-3
-
-0.50
-0.25
0.00
20121121 3.1
0.90
0.65
1.8%
20130124 3.9
0,90
0.70
2.5%
0.25
0.50
0.75
1.00
-0.5
0 5
0.0
Voltage [V I
1.25
Voltage [ V ]
Figure B-6: JVs with DBP from two manufacturers.
136
1.0
80
100
05
OU
120
--
2-
as purch DBP 20120925 3.4
1x pur DBP 20120925
3.4
2x pur DBP 20130107
3.2
0.89
0.90
0.88
0.45
0.53
0.64
2-
1.4%
1.6%
18%
---
0-
E
----
E
1
I
unpur DBP
1x pur DBP
-1 -
U
-2-
-2
.3 -
:3
-3
-0.5
-0.50
-025
0.00
0.25
0.50
Voltage [ V ]
05
1.00
0.0
0.5
1.0
Voltage [ V I
1.25
Figure B-7: JVs with DBP as purchased or purified once or purified twice.
holding two donor materials at once. (So, to experiment with more batches of donor
material, the evaporator would have to be re-loaded midst a device run.)
B.3.2
Purity
For the specific manufacturer Lumtec, we explored the effect of purity via comparison of different purification runs on device performance. We note that higher purity
led to higher fill factors, resulting in a 30% increase in PCE. We speculate that the
higher FF is due to lower concentration of traps and thus higher mobility within the
donor layer.
We also note that higher purity led to higher yield and stability of devices on a
given substrate (each substrate has 10 devices).
This is evident in the graph with
JV curves from every device pad on a substrate from either unpurified DBP or once-
purified DBP.
B.3.3
Growth Rate
Optimizing the growth rate of the DBP layer results in PCE enhancements of up
to 20%. Growth rate is well known to cause changes in nanostructure, which of course
effects performance. We hypothesize that the higher growth rate either results in a
more disordered film which thus is missing grain boundaries that are sources of traps,
137
22--
E
-1
01
-2-
0
7
-
-.
-I
-1
-3
0
-4
e
-1.0
,
-0,5
, ---
,
0.0
.5
DBP 0.2N/s -- 1.7%,
0.5
-1.0
1.0
DBP A/s - 1.1%
-DBP 1A/s - 1.4%
DBPO0.5A/s - 1.5%
__________
-0.5
0.0
0.5
1.0
Voltage [ V
Voltage LVI
Figure B-8: JVs with DBP grown at variety of rates.
-
C60
C70
35
40
075
0.75
0.37
0.36
1.1%
1 .2%
5.02.5E0. 0 -2.5
-5.0
-7.5
-0.5
0.0
Voltage ( V )
05
1.0
Figure B-9: JVs comparing C60 and C70 as acceptor, with the donor material ClAlPc.
or it results in a more ordered film, the bulk of which has less traps and thus higher
mobility. Further experiments must be performed to confirm either option.
B.4
Acceptor Layer
B.4.1
Materials Choice
Note that this data is for a different donor material: CIAlPe. We have not yet successfully performed this experiment with DBP. (Each attempt coincided with issues
with the evaporator.)
Substituting C70 for C70 led to a 30% increase in short circuit current, and a 9%
increase in PCE. We note that C70 has stronger and broader absorption than C60
yet very similar in other characteristics, thus explaining the focused improvement in
Jsc.
138
2010-11-16
20-
10
_
MoOx+C60
--
MoOx+C60 Oxpur
1xpur
87
8-
>10
(
0 -2-
201011 16
-oOxoC6O
Ipur
MoOxvC6O Oxpur
060 1ixpur
-10
s
-05
_
00
060
Oxpur
0
05
300
400
500
600
700
800
Wavelength [nm]
Voltage [VI
Figure B-10: JV and EQE for various C60 purities.
B.4.2
Purity
Purer C60 leads to an increase in incident photon to electron efficiency for C60
absorption range, and additionally increases FF. This is likely due to decrease in trap
states which could lead to recombination.
B.5
Anode Interlayer
B.5.1
Materials Choice
PEDOT shows a 10-40% PCE improvement over MoOs as an anode interlayer.
We speculate that this is due to a smnoother/more favorable contact between DBP and
PEDOT vs DBP and MoOx. However it may also be due to PEDOT leading DBP to
grow in a more power conversion efficient morphology. Note that PEDOT is also more
sensitive to age etc. The double-diode JV in blue is due to an old batch of PEDOT.
The rest of these experiments are performed with a more recently purchased bottle
of PEDOT. (The manufacturer tells us that PEDOT expiration is at approximately
6 months.)
B.5.2
Thickness
Thicker MoOx decreases PCE but increases yield. Thus we utilize 50nmn as our
standard thickness, which is a blanace between shorting (yield) and resistance (PCE).
139
1.0-
4-
MoOx 20130124
MoOx 20130130
PEDOT 20130124
PEDOT 20130130
-
3-
-
2-
-
0.5
E
--
0C--
-- - -+
-+
E
'-
-
E
-'
C
-2-
-0.5 -MoOx
-
-
-1.0
-1.5
PEDOT
PEDOT/MoOx
-
-2.0 -
-3-
-
-4-
-2.5
-3.0
-0.50
-5
-0.50
-0.25
0.00
0.25
0.50
Voltage [ V
1.00
0.75
-0.25
]
0.00
0.25
0.50
0.75
1.00
Voltage [V
Figure B-11: JV for various anode interlayers: MoOx and PEDOT.
1.0- MoOx
--
0.5- -
Yield
4/10
8/10
S00nm 9/10
20nm
50nm
E
E
-0.5
C
0D
-1.0
-1.5-2.0
-2.5
-0.50
-
-0.25
0.00
0.25
0.50
0.75
1.00
Voltage [ V I
Figure B-12: JV for various MoOx thicknesses.
B.6
Cathode Interlayer
B.6.1
Materials Choice
BCP is the conventional cathode interlayer for vapor-processed molecular PVs,
however we experiment with Alq3, a conventional electrode transport layer. We find
a 21% increase in PCE, due to the lower resistivity of the Alq3 film. Note that the
thicknesses shown here have each been optimized for this particular donor-acceptor
device structure.
140
4.
1-
BCP
.-
0-
-
I
2.
E
E
Onm
-
3.
E
-
1-
2.5nm
6.5nm
10nm
18nm
18nm
0-
-1
C
-2-
-2 -
-
none
BCP 6.5nm
-3-
Alq 10nm
-0.5
0.5
0.0
1.0
1.0
-0,5
0.0
0.5
10
1.5
Voltage [V ]
Voltage [ V]
Figure B-13: Left: JV comparing various cathode interlayers: BCP and Alq3. Right:
JVs of various thicknesses of BCP.
Due to the significant enhancement in Alq3, we recounend that further experiments be employed to explore the change in "ONE Lab conventional OPV" from
BCP to Alq3.
B.6.2
Thickness
In optinzing the thickness of BCP layer, we find that there is a narrow optninum
point. We speculate that this is due to the combination of Ag traps (thus necessitating
a thicker BCP layer) and high inherent resistance (necessitating a thinner BCP layer).
B.7
Substrates and Substrate Treatments
B.7.1
Substrate Choice
We find that although substrate choice has significant effect on as-purchased
Lumtec DBP, there is negligible difference in devices grown with purified DBP. This
suggests the growth morphology of as-purchased DBP is highly sensitive to surface
conditions.
Note that Jill's devices are most commonly grown on "current large ITO" and
Andrea's devices are most commonly grown on "current small ITO", simply as a
matter of preference.
141
as purch DBP
1x pur DBP
. -
2-
-
. -
1 -
-
-
---
current large ITO
old large ITO
old small ITO
0E
-1 .
C
-2
-
-3 -
II
-0.5
e
a
I
0.5
0.0
1.0
Voltage [ V ]
Figure B-14: JVs of various substrates with either purified DBP or as-purchased
DBP.
However, due to the clear dependence of substrate effects on the DBP batch, this
experiment should be repeated with every new batch of DBP.
B.7.2
Substrate Treatments
We first compare depositing devices on ITO substrates just solvent cleaned vs ITO
substrates that are solvent cleaned plus exposed to oxygen plasma. We find that 30
seconds of 02 plasma has no effect on device performance. Thus the plasma either
does not effect the ITO, or the MoOx interlayer minimizes any effects.
We further explore the effect of 5 minutes 02 plasma or 6 minutes of UV-Ozone
(a slightly gentler technique). We see a slight decline ( 10%) in PCE in both devices
exposed to long 02 plasma and long UV-Ozone.
For less conventional techniques of influencing the ITO, we also rinse the ITO with
dhilte HCl (in an attempt to slightly etch it and remove rough spots), which results in
a slight decrease in Jsc. Further, we fabricated devices utilizing ITO sputter deposited
in ONE Lab (rather than purchased pre-deposited), and find that all devices utilizing
our ITO are shorted, suggesting production of a very rough ITO film.
142
1.0-
1-
no plasma
30s 02 plasma
0.5-
CN.
E
E
E
E
-0.5-1.0-
-1
0D -1.5
--------
-
-02
-
-2.0
-2.5-
-
0.0
0.5
Voltage [V
1
,U I
0
-0.50
I
-0.25
0.00
0.25
0.50
plasma 30"
2 plasma 5'
UV-Ozone 6'
dilute HCI
sputtered ITO
0.75
1.00
Voltage [ V
]
Figure B-15: JVs of various substrates with various substrate treatments.
B.8
Conclusions
" More pure -+ higher FF -+ 30%X larger PCE
" Lunitec -+ SA
--
40-50% larger PCE
" DBP higher Growth Rate
-4
6-20% larger PCE
" MoOx -+ PEDOT -+ 10-40%X larger PCE
" BCP -+ A 1 3
-a
20% larger PCE
" Substrates have no effect on PVs with stable DBP
" Plasma or UV- Ozone
-*
largest change is 8% decrease in PCE
143
144
Appendix C
Contributions Associated with
This Thesis
1. Macko, J. A., Lunt, R. R., Osedach, T. P., Brown, P. R., Barr, M. C., Gleason,
K. K., & Bulovic, V. (2012). Multijunction organic photovoltaics with a broad
spectral response. Physical Chmistry Chemical Physics. doi: 10. 1039/c2cp43000b
2. Lunt, R. R., Osedach, T. P., Brown, P. R., Rowehl, J. A., & Bulovic, V. (2011).
Practical Roadmap and Limits to Nanostructured Photovoltaics. Advanced Materials, 23(48), 57125727. doi:10.1002/adnia.201103404
3. Barr, M. C., Rowehl, J. A., Lunt, R. R., Xu, J., Wang, A., Boyce, C. M., In,
S. G., Bulovic, V., & Gleason, K. K. (2011). Direct monolithic integration of
organic photovoltaic circuits on unmodified paper. Advanced Materials, 23(31),
34993505. doi:10.1002/adma.201101263
4. Park, H., Rowehli, J. A., Kim, K. K., Bulovic, V., & Kong, J. (2010). Doped
graphene electrodes for organic solar cells. Nanotcchnology, 21, 505204. doi:10.1088/0957-
4484/21/50/505204
145
146
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