Development of Low-Temperature Solution-Processed
Colloidal Quantum Dot-Based Solar Cells
ARCHIVES
by
MASSACHUSETS 1N57
OF TECHNOLOGYy
Liang-Yi Chang
MAY 1 4 2014
B.S. Materials Science and Engineering
LIBRARIES
National Tsing Hua University, 2005
Submitted to the Department of Materials Science and Engineering
on August 5, 2013, in Partial Fulfillment of the
Requirements for the Degree of
Doctor of Philosophy in Materials Science and Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
September 2013
©Massachusetts Institute of Technology 2013. All rights reserved.
....................................
Author.....
Department of Materials Science and Engineering
August 5, 2013
Certified by.................
............................. .........................
Moungi G. Bawendi
Lester Wolfe Professor in Chemistry
Thesis Supervisor
............................
Certified by........
Lionel C. Kimerling
Thomas Lord Professor of Materials Science and Engineering
DepartmentalTesis Co-Supervisor
Accepted by.....................................
Gerbrand Ceder
Chair, Departmental Committee on Graduate Students
f
2
Development of Low-Temperature Solution-Processed
Colloidal Quantum Dot-Based Solar Cells
by
Liang-Yi Chang
Submitted to the Department of Materials Science and Engineering
on August 5, 2013, in Partial Fulfillment of the
Requirements for the Degree of
Doctor of Philosophy in Materials Science and Engineering
Abstract
Solution-processed solar cells incorporating organic semiconductors and inorganic colloidal
quantum dots (QDs) are potential alternatives to conventional solar cells fabricated via vacuum
or high-temperature
sintering
processes
for large-area,
high-throughput,
and
low-cost
manufacturing. In this work, we explored two types of solution-processed QD-based solar cells:
all-inorganic solar cells and organic/inorganic hybrid solar cells. In the all-inorganic device, three
QD deposition techniques (spin coating, dip coating, and spray coating) were first experimented
in order to prepare high-quality QD thin film for the photovoltaic application. The device was
based on the heterojunction formed between dip-coated PbS QD layers and CdS thin film that
was deposited via a solution process at 80'C. The resultant device, employing a 1,2-ethanedithiol
ligand exchange scheme, exhibits comparable power conversion efficiency (3.5%) to that of
high-temperature (260'C) sintered or sputtered ZnO/PbS (PbSe) QD devices. The initial device
yield issue associated with the pinhole formation was addressed, and a procedure for the
fabrication of reproducible devices was formulated. The demonstration of this device is a step
towards low-cost solar cell manufacturing. Through a combination of thickness-dependent
current density-voltage characteristics, optical modeling, and capacitance measurements, the
combined diffusion length and depletion width in the PbS QD layer is found to be approximately
170 nm. In the organic/inorganic hybrid device, poly(3-hexylthiophene-2,5-diyl)
(P3HT)
nanofibers and CdS QDs were employed as electron donors and acceptors, respectively.
Crystalline P3HT nanofibers, grown from amorphous P3HT solution, were blended with CdS
QDs to form bulk heterojunctions. By adding a large quantity of poor solvent to the blended
solution, we demonstrated preferential decoration of CdS QDs onto P3HT nanofibers and
stronger interaction between these two materials. The resultant device also showed improved
open-circuit voltage, short-circuit current density, and power conversion efficiency.
Thesis Supervisor: Moungi G. Bawendi
Title: Lester Wolfe Professor in Chemistry
3
4
Acknowledgements
This thesis is the result of inspiration, collaboration, encouragement, and support from
several professors, colleagues, friends, and families of mine. I am sincerely thankful for having
them before, during, and after my PhD study.
Prof. Tai-Bor Wu, an outstanding and esteemed professor in the Department of Materials
Science and Engineering at National Tsing Hua University in Taiwan and a kind father of one of
my best high school friends, introduced materials science to me and encouraged me to pursue my
study in the U.S. He unfortunately passed away from a heart attack in 2010, and I would like to
dedicate this thesis to him.
I am grateful to have Prof. Moungi Bawendi as my thesis advisor. He gave me plenty of
freedom when choosing research directions, support when equipment purchase and training was
needed, and guidance and patience when I was stuck in experimental failure. He has been
encouraging me to become an independent researcher.
Members in the Bawendi group have been very helpful throughout my study. In particular,
Dr. Scott Geyer instructed me QD synthesis and helped me to set up lab equipment for device
fabrication and characterization. I also would like to thank the device subgroup, including Darcy
Wanger, Gyu Weon Hwang, Jennifer Scherer, Chia-Hao Chuang, and Whitney Hess, for the
discussion
spanning
from material synthesis
and processing
to
device
fabrication
and
characterization. Most importantly, we worked together to synthesize batches and batches of PbS
QDs.
Prof. Vladimir Bulovic and members from his group have been very generous to share the
access to the Organic and Nanostructured Electronics Laboratory and to offer advise on device
physics and engineering. In particular, Dr. Richard Lunt wrote the majority of the MATLab code
5
for the optical modeling carried out in this study. Patrick Brown, Dr. Tim Osedach, and Geoffrey
Supran have been great resources to discuss device fabrication and characterization. Dr. Ni Zhao
has always cheered me up when my experiments failed in the early days.
My friends, especially Shih-Wei Chang, Chung-Hsiang Chang, Vivian Chuang, Hsu-Yi Lee,
Kevin Lee, Yi-Chun Lu, Amy Chi, Carl Yu, Tsao-Hsien Chen, Yu-Chung Hsiao, Elsie Li,
Chia-Hua Lee, Wei Yi, Kevin Chuang, and Kylie Yu, have undoubtedly completed my life
outside the lab. We explored cities, had meals, played sports, and had a lot of fun together.
The last but not least, I would like to thank my families in Taiwan for their endless love.
6
Preface
Parts of this thesis were reproduced from previously published manuscripts. The associated
copyright approval has been obtained as indicated below.
Chapter 5
Reprinted with permission from "Liang-Yi Chang, Richard R. Lunt, Patrick R. Brown,
Vladimir Bulovid, and Moungi G. Bawendi, Low-Temperature Solution-Processed Solar Cells
Based on PbS Colloidal Quantum Dot/CdS Heterojunctions, Nano Letters, 2013, 13 (3),
994-999." Copyright 2013 American Chemical Society.
Chapter 6
Reprinted with permission from "Shenqiang Ren, Liang-Yi Chang, Sung-Keun Lim, Jing
Zhao, Matthew Smith, Ni Zhao, Vladimir Bulovid, Moungi Bawendi, and Silvija Gradebak,
Inorganic-Organic Hybrid Solar Cell: Bridging Quantum Dots to Conjugated Polymer
Nanowires, Nano Letters, 2011, 11 (9), 3998-4002." Copyright 2011 American Chemical
Society.
7
8
Table of Contents
1 M otivation ............................................................................................................................
22
1.1 Solar Photovoltaic Background.....................................................................................
22
1.2 Low-Temperature Solution-Processed Colloidal Quantum Dot Solar Cells ...........
24
1.3 Thesis Outline..................................................................................................................28
1.4 Reference.........................................................................................................................29
2 Basics of Colloidal Quantum Dot Solar Cells......................................................................31
2.1 Colloidal Quantum Dot Synthesis ....................................................................................
31
2.2 Electrical Properties of Colloidal Quantum Dots ..............................................................
34
2.2.1 Carrier M obility ........................................................................................................
34
2.2.2 Carrier Type and Density ......................................................................................
36
2.3 Solar Cell Device Physics .............................................................................................
37
2.3.1 Photovoltaic Processes in Inorganic Solar Cells......................................................
37
2.3.2 Photovoltaic Processes in Organic Solar Cells .......................................................
41
2.3.3 Current-Voltage Characteristics of Solar Cells........................................................
43
2.4 Conclusion.......................................................................................................................47
2.5 Reference.........................................................................................................................48
3 Experim ental M ethods.........................................................................................................53
3.1 Fabrication of Pbs QD/CdS Bilayer Heterojunction Devices .........................................
53
3.1.1 PbS Colloidal QD Synthesis and Purification .......................................................
53
9
3.1.2 Substrate Cleaning ....................................................................................................
54
3.1.3 (3-mercaptopropyl)trim ethoxysilane Substrate Treatment.......................................
54
3.1.4 Deposition of PbS QD Film ...................................................................................
55
3.1.5 Deposition of CdS Thin Film .................................................................................
56
3.1.6 Deposition of Al Electrodes...................................................................................
59
3.2 Fabrication of P3HT Nanofiber/CdS QD Bulk Heterojunction devices..........................59
3.2.1 Growth of P3HT Nanofibers...................................................................................
59
3.2.2 CdS Colloidal QD Synthesis, Purification, and n-Butylamine Ligand Exchange ........ 60
3.2.3 Blending of P3HT N anofibers and CdS QDs .........................................................
61
3.2.4 Deposition of PEDOT:PSS.....................................................................................
61
3.2.5 Deposition and Treatment of P3HT Nanofiber/CdS QD Layer...............................61
3.2.6 Deposition of Bathocuproine and M g:Ag/Ag Layers .............................................
62
3.3 M aterial and Device Characterization ..........................................................................
62
3.3.1 M aterial Characterization .......................................................................................
62
3.3.2 Device Characterization ........................................................................................
63
3.4 Reference.........................................................................................................................63
4 PbS QD Schottky Photovoltaic Devices...........................................................................
65
4.1 Introduction .....................................................................................................................
65
4.2 Deposition of QD Film .................................................................................................
65
4.2.1 Spin Coating .............................................................................................................
65
10
4.2.2 Dip Coating...............................................................................................................66
4.2.3 Spray Coating............................................................................................................66
4.3 Film M orphology and Device Characteristics...................................................................67
4.4 Conclusion.......................................................................................................................70
4.5 Reference.........................................................................................................................70
5 PbS Q D/CdS Bilayer Heterojunction Photovoltaic Devices ...........................................
72
5.1 Introduction .....................................................................................................................
72
5.2 Device Structure and Fabrication ..................................................................................
73
5.3 Protocols for the Fabrication of Reproducible Heterojunction Devices ..........................
76
5.4 The Comparison between Schottky and PN Heterojunction Devices ............................
83
5.5 PbS QD Layer Thickness-Dependent J-V Characteristics.............................................
84
5.6 M odeling of PbS QD Layer Thickness-Dependent Jsc .................................................
86
5.7 Capacitance M easurement...........................................................................................
93
5.8 CdS Layer Thickness-Dependent Jsc.............................................................................
95
5.9 Conclusion.......................................................................................................................98
5.10 Reference.......................................................................................................................98
6 Efficiency Improvement in Inorganic-Organic Hybrid Photovoltaic Devices via
Solvent-Assisted Self Assem bly.............................................................................................102
6.1 Introduction ...................................................................................................................
102
6.2 Device Structure and Fabrication ...................................................................................
103
11
6.3 Material Characterization: Random Mix versus Self Assembly ...................
104
6.4 Device Characterization.................................................................................................109
6.5 Conclusion.....................................................................................................................
114
6.6 Reference.......................................................................................................................
115
7 O utlook ...............................................................................................................................
118
7.1 Conclusion.....................................................................................................................
118
7.2 Future Work ..................................................................................................................
119
12
List of Tables
1-1. Price of solar electricity in US cents per kilowatt-hour reported in 2012 for various
installation scales and climate conditions. The residential price is calculated based upon a
standard 2 kilowatt peak system; the commercial price, a 50 kilowatt peak system; the
industrial price, 500 kilowatt peak system ......................................................................
27
1-2. Theoretical power conversion efficiency of single- and multijunction solar cells and the
corresponding optimal band gap energies for the constituent junctions. Band gap energies
are expressed with equivalent photon wavelength. ........................................................
3-3. Parameters for PbS QD dip coating. A dip coating cycle consists of: (1)
30
dipping the
substrate into 10mg/ml PbS QDs in hexane for 1 second and drying the sample for 30
seconds, (2) dipping the film in 0.02% EDT in acetonitrile for 30 seconds, and (3) rinsing
by dipping in pure acetonitrile for 3 seconds, followed by drying for 60 seconds. The above
dip coating cycle was repeated to achieve desired PbS QD thicknesses. The dipping speed
for all the steps was 400 mm/min. Before and after dipping into QD solution, the samples
were dried naturally for 60 and 30 seconds respectively to avoid cross-contamination
between QD
solution
and acetonitrile
or EDT
solution, which
can produce
agglomerates in solution, resulting in poor film qualities. .............................................
13
QD
59
14
List of Figures
1-1. Schematic of a colloidal quantum dot with inorganic semiconductor core and organic ligand
sh e ll...................................................................................................................................2
8
1-2. The effect of quantum confinement on the energy levels of electrons (solid circle) and holes
(open circle) in semiconductor nanoparticles. ................................................................
29
1-3. Absorption spectra of PbS QDs with a series of particle diameters. The first exitonic
absorption peak of PbS QDs can be tuned readily from 600 to 2200 nm. .......................
30
2-1. A photograph of colloidal QD synthesis setup, including a Schlenk line, a three-neck flask, a
condenser, a stiffer, a temperature controlling system, an injection gun, and a mechanical
rotary pump (not shown in the picture). The black injection gun shown in this picture is
used for large-scale (gram-scale) synthesis, while for a conventional small-scale synthesis
(100 milligram-scale) synthesis, a syringe is used .........................................................
36
2-2. Cartoon depicting the precursor concentration evolution along different stages of colloidal
QD nucleation and growth under the framework of LaMer colloid growth model.......36
2-3. A Scanning electron microscopy image of a treated QD film. Cracks are usually developed
in thick QD film after post-deposition treatment. Cracks are usually developed in thick QD
film after post-deposition treatm ent ................................................................................
2-4. Energy band diagram of a pn homojunction at thermal equilibrium. ...............................
39
43
2-5. Energy band diagrams of Schottky junctions formed between (a) a metal and a p-type
semiconductor and (b) a metal and an n-type semiconductor at thermal equilibrium. ......... 44
2-6. A schematic diagram showing the mechanisms by which photons are converted to free
carriers in an organic solar cell. (a) Photon absorption in semiconductor layers generates
15
excitons, bound electron-hole pairs. (b) Excitons diffuse around in the material before
radiative or non-radiative recombination. (c) When excitons reach the interface between a
donor and an acceptor, charge transfer can occur. (d) Charges are spatially separated,
transported, and extracted at the electrodes. HOMO and LUMO is the abbreviation of
highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. 46
2-7. Ideal current-voltage characteristics of a solar cell in the dark (black curve) and under
AMi.5G illumination (red curve). An ideal solar cell I-V curve in the dark should exhibit
strong rectification.............................................................................................................48
2-8. An equivalent circuit diagram of the solar cell comprising a current source (IL), a diode
a shunt resistor
(Rsh),
(Id),
and a series resistor (R,). A load resistor (RL, not a part of the solar cell)
is shown in the diagram to represent the external load. ..................................................
48
3-1. Schematic of idealized MPTMS monolayer formation on hydroxylated oxide substrate
surface with the presence of a physically-absorbed water layer......................................58
3-2. A photograph of the multi-vessel dip coater used to deposit high-quality PbS QD film in this
thesis.................................................................................................................................5
9
3-3. Porous and rough film grown by a homogeneous reaction. ..............................................
61
3-4. A photograph of the custom-built Teflon substrate holder. ..............................................
62
3-5. Schematic illustration of overlaid patterned ITO, BCP, and Mg:Ag/Ag electrodes. The
overlap between ITO and metal electrodes defines the ten devices with an individual active
area of 1.24 mm 2 on a half-inch glass substrate. The same pattern was also employed for
PbS QD/CdS bilayer heterojunction devices. The pattern was designed by Dr. Alexi Arango.
..........................
................................................................................................................
16
65
4-1. A schematic representation of a spray coating setup composed of a handheld airbrush, a gas
regulator, and a nitrogen cylinder. Relevant processing parameters are also shown......70
4-2. Optical micrographs of PbS QD film deposited via (a) spin coating, (b) dip coating, and (c)
spray coating . ....................................................................................................................
71
4-3. AFM images of PbS QD film deposited via (a) spin coating and (b) dip coating........71
4-4. A three-dimensional profilometry image of spray-coated PbS QD film. .........................
72
4-5. J-V characteristics of PbS QD Schottky devices under simulated AMI.5G illumination. PbS
QD film in these devices was deposited via (a) spin coating, (b) dip coating, and (c) spray
co ating . .............................................................................................................................
72
5-1. (a) Schematic of the PbS QD/CdS heterojunction device fabricated in this study. (b) Energy
band diagram showing band edges of isolated 1.5eV PbS QDs and CdS along with electrode
w ork functions...................................................................................................................76
5-2. Absorbance spectrum of PbS QDs in solution used in this work. ....................................
77
5-3. SEM im age of the CdS thin film on glass. ........................................................................
78
5-4. Current-voltage characteristics of PbS QD and CdS thin films. The electrical conductivities
of PbS QDs and CdS were measured using a planar interdigitated configuration. For PbS
QDs, the channel length is 6 ptm, the channel width is 1.95 cm, and the film thickness is 100
nm. For CdS, the channel length is 120 ptm, the channel width is 0.75 cm, and the film
thickness is 120 nm. Consequently, the conductivities of PbS QDs and CdS are -2 x 10-8 and
~1 x 10-3 S/cm , respectively...........................................................................................
17
78
5-5. (a) Cross-sectional SEM image of the device. Platinum was deposited on top of the device
before focused ion beam milling to protect the underlying layers. (b) XPS depth profile of a
PbS QD/CdS device without Al electrodes. ..................................................................
79
5-6. J-V characteristics of nine PbS QD/CdS heterojunction devices fabricated on the same
substrate and measured under simulated AMi.5G solar illumination. The scattered J-V
characteristics manifest the device irreproducibility......................................................80
5-7. A representative top-view optical micrograph of a finished PbS QD/CdS device. Multiple
pinholes can be identified in the image. .........................................................................
80
5-8. (a) A representative top-view optical micrograph of finished PbS QD Schottky devices. No
pinhole is identified in this image. (b) J-V characteristics of nine PbS QD Schottky devices
fabricated on the same substrate and measured under simulated AMI.5G solar illumination
show good reproducibility. ...........................................................................................
81
5-9. Representative optical micrographs of (a) as-deposited PbS QD film, (b) PbS QD film with
a CdS overlayer and with no pinhole, and (c) PbS QD film with a CdS overlayer and with a
pinhole. The pervasive submicron dark spots in (b) and (c) are the CdS crystallites which
were initially formed in the bulk of the chemical bath solution and subsequently adsorbed
and em bedded in CdS film .............................................................................................
82
5-10. An optical micrograph of an ITO-coated glass substrate cleaned with the procedure
described in Chapter 3 still shows dust particles/contaminants (highlighted with the red
circles) adsorbed on the surface ......................................................................................
82
5-11. Optical micrographs of ITO-coated glass substrates (a) before and (b) after MPTMS
treatment. The red circles in (b) highlight the surface contaminants. ..............................
18
83
5-12. An optical micrograph of PbS QD film deposited from a solution of poor colloidal stability.
Multiple particle defects can be identified in the image.................................................
84
5-13. Photographs of centrifuged PbS QD solution added with (a) excess and (b) reduced amount
of ethanol. Complete purification process should be referred to Chapter 3. The supernatant
in the solution with excess amount of ethanol is much clearer than that with reduced amount
of ethanol, suggesting that most of the QDs was precipitated out during centrifugation in the
solution w ith excess amount of ethanol..........................................................................
84
5-14. Optical micrographs of the PbS QD/CdS devices fabricated (a) without and (b) with
additional measures to prevent pinhole formation. The red circles in (a) highlight the
particle defects/pinholes. The blurred black spots in both images were the residual of the
QD film on the back side of the substrates. During a dip-coating process, both sides of the
substrates were coated, and the film on the back side was wiped off before the measurement.
Therefore, these blurred spots are not in the device and can be ignored..........................85
5-15. J-V characteristics of nine PbS QD/CdS heterojunction devices fabricated with the
newly-implemented experimental methods on the same substrate and measured under
simulated A M 1.5G solar illum ination............................................................................
85
5-16. J-V characteristics of typical PbS QD Schottky (red) and PbS QD/CdS heterojunction (blue)
devices measured in the dark and under AM1.5G simulated solar illumination..............86
5-17. J-V characteristics of an inverted CdS/PbS QD heterojunction solar cell measured in the
dark and under AMl.5G simulated solar illumination. To verify the origin of the device
rectifying behavior, symmetric top and bottom ITO contacts were employed. Due to some
spikes of chemical-bath-deposited CdS thin films, a thick (310 nm) PbS QD layer was used
19
in this inverted device to avoid shorting, which may result in a worse photovoltaic
perform ance than a normal device. ...............................................................................
87
5-18. (a) J-V characteristics of representative PbS QD/CdS heterojunction solar cells with
varying PbS QD layer thicknesses. (b) Performance characteristics of the above devices.
The error bars are evaluated from the standard deviation of 18 devices fabricated with the
same condition for each thickness of the PbS QD film...................................................89
5-19. Complex refractive indices of PbS QDs and CdS. ........................................................
90
5-20. AFM surface images of (a) the PbS QD thin film and (b) the CdS thin film...........91
5-21. Diffuse reflectance spectrum of a full PbS QD/CdS device (without Al electrode)......91
5-22. Measured EQE (red curves) and modeled active-layer absorption spectra (black curves) of
PbS QD/CdS devices with varying PbS QD thicknesses: (a) 40 nm, (b) 100 nm, (c) 160 nm,
(d) 220 nm , and (e) 280 nm . .........................................................................................
89
5-23. Modeled photon absorption rate (1/sec-cm 3 ) for PbS QD/CdS heterojunction devices with
varying PbS QD thicknesses: (a) 40 nm, (b) 100 nm, (c) 160 nm, (d) 220 nm, and (e) 280
nm. The x- axis (distance) is measured from the ITO surface where light is incident. The
horizontal stripes seen in the simulation plots result from the use of AM1.5G solar spectrum
as a light source for the calculated absorption/generation rate. There are multiple dips in the
solar spectrum due to light absorption by 03, 02, H 20, and CO 2 ................
. . . . . . . . .. . . . . . . . . . .
94
5-24. The modeled total generation rates in PbS QD layers of different thicknesses (offset for
clarity)...............................................................................................................................9
5
5-25. The integrated generation rate (1/sec-cm 2 ) as a function of the distance from the PbS
QD/CdS interface for the above PbS QD thicknesses.....................................................95
20
5-26. Jsc integrated from the modeled (black curve, assuming 100% IQE) and measured (red
squares) EQ E spectra....................................................................................................
96
5-27. Measured (black square) and modeled (red circle for the lower bounds; blue triangle for the
upper bounds) device capacitance at zero-bias as a function of PbS QD thicknesses. The
lower bounds were calculated, assuming that both the PbS QD and CdS layer are
fully-depleted; while the upper bounds were calculated, assuming that only the PbS QD
layer is fully-depleted and none of CdS layer is depleted. ..............................................
97
5-28. Modeled short-circuit current density of the PbS QD/CdS devices with various CdS layer
thicknesses. The thickness of PbS QD layer for this modeling is fixed at 170 nm, which is
effective carrier extraction range estimated in the previous sections of this thesis. .........
99
5-29. Simulated loptical electric field12 as a function of wavelength and position for various CdS
thicknesses: (a) 0 nm, (b) 50 nm, (c) 180 nm, (d) 280 nm, and (e) 400 nm. The thickness of
PbS Q D layer is fixed at 170 nm ......................................................................................
100
6-1. (a) The schematic device architecture and (b) the corresponding flat-band diagram..........107
6-2. TEM images of (a) P3HT nanofibers and (b) CdS QDs. ...................................................
108
6-3. UV-VIS absorbance spectra of (a) amorphous P3HT, P3HT nanofribers, and (b) CdS QDs.
..............................................................................................
..........................................
10 8
6-4. TEM images of P3HT nanofiber/CdS QD film synthesized (a) without self-assembly
(random-mix process) and (b) using a solvent-assisted self-assembly process. The inset
images show schematic representations of each; the random-mix and self-assembly method
is used to control the interface between CdS QDs (yellow spheres) and P3HT nanofibers
(purp le lin es). ..................................................................................................................
21
109
6-5. (a) An UV-VIS absorbance spectrum of self-assembled P3HT nanofiber/CdS QD film. (b)
Solution PL spectra of the random-mixed and self-assembled P3HT nanofiber/CdS QD
sam p les...........................................................................................................................1
10
6-6. XPS spectra of high resolution S2p in (a) random-mixed and (b) self-assembled P3HT
nanofiber/CdS QD film . ...............................................................................................
I 1
6-7. Time-resolved photoluminescence spectra of the random-mixed and self-assembled P3HT
nanofiber/CdS QD film. The film was excited by a pulsed diode laser (PicoQuant) at 414
nm, 2.5 MHz. Emission from the film was collected by an avalanche photodiode
(PicoQuant) with a 460nm band-pass filter. Photoluminescence decay of the sample was
measured with a time-correlated single photon counting module (PicoQuant)..................111
6-8. Tapping mode AFM images of P3HT nanofiber/CdS QD thin film (a) before and (b) after
EDT ligand exchange. (c) TEM image of curved P3HT NWs after the ligand exchange. . 112
6-9. Charge carrier recombination rate constants krec of pristine and EDT-treated devices
determined by transient open circuit voltage decay measurements at different illumination
lev els...............................................................................................................................
1 13
6-10. (a) J-V characteristics of an electron only P3HT nanofiber/CdS QD device before and after
EDT treatment. (b) J-V characteristics of a hole only P3HT nanofiber/CdS QD device
before and after EDT treatment. The effect of EDT treatment and P3HT nanofiber curving
on the transport properties of P3HT nanofiber/CdS QD film was demonstrated by the DC
J-V characteristics of the electron-only and hole-only devices. The electron-only device
consisted of Ag:self-assembled monolayer (SAM), P3HT nanofiber/CdS QD, and Mg:Ag
layers. The Ag:SAM layer blocks the hole injection [Error! Reference source not found.]. The
treated electron-only device shows a strong enhancement of electron transport over the
22
untreated
device.
The
hole
only
device
consisted
of ITO,
PEDOT:PSS,
P3HT
nanofiber/CdS QD, and Au layers, where the Au layer prevents electron injection. The
treated device showed a decrease in current density. This result suggests that the EDT
treatm ent reduces the hole transport.................................................................................114
6-11. (a) Cross-sectional TEM image of P3HT nanofiber/CdS QD hybrid solar cells. (b) Energy
dispersive X-ray spectroscopy elemental mapping (Red color: Cd; Green color: In; and Blue:
A g) of the im age show n in (a). ........................................................................................
6-12.
Cross-sectional
TEM
EDS
mapping
image
ITO/PEDOT:PSS/P3HT:CdS/BCP/Mg:Ag hybrid solar cell device. (a)-(d):
of
115
the
The Mg, Cd,
S, and In elem ent m apping im ages...................................................................................
115
6-13. J-V characteristics of random-mixed (blue) and self-assembled (red) P3HT nanofiber/CdS
QD so lar cells..................................................................................................................116
6-14. (a) J-V characteristics of pristine (blue) and EDT-treated (green) P3HT nanofiber/CdS QD
solar cells. (b) The photovoltaic performance summary (Jsc and Voc) of the EDT-treated
and self-assembled devices as a function of the CdS weight concentration.......................117
23
24
Chapter 1
Motivation
1.1 Solar Photovoltaic Background
Since the twentieth century, the living standard of most of the world population has
increased in a significant and unprecedented way. However, at the same time, human activity has
also casted a huge impact on the earth. Atmospheric carbon dioxide concentration and global
surface temperature have risen since 1960 [1, 2], and evidence suggests that the global warming
is a result of a rise in greenhouse gas emission caused by human activity [3]. Such change could
be unsustainable or even catastrophic, if it is continued in the current fashion. Sustainable
development,
the development
that meets the needs for economy and
society without
compromising the environment, is therefore one of the great challenges of the twenty-first
century.
Global greenhouse gas emissions by sector from 1990 to 2005, measured in carbon dioxide
equivalents, shows that the energy sector accounts for two thirds of total anthropogenic carbon
dioxide emissions [4]. As a result, in order to decrease global greenhouse gas emissions, people
cannot ignore the major contribution from the energy sector, including both energy production
and consumption. In addition, although hydraulic-fracturing technologies have recently opened
up considerable oil and gas resources, these reserves could last for only about sixty years (they
may be extended by new technologies and unconventional deposits) at current consumption rates.
Coal reserves will last for at least a century at current rates of consumption [5]. Nevertheless,
none of these energy resources are renewable and will be exhausted in the foreseeable future. To
25
summary, energy plays an important role for sustainable development. We need not only reliable
and affordable energy sources to continue economical and social development but also clean
energy
sources to mitigate the adverse effects produced during energy production and
consumption on the environment.
Among various energy sources, solar energy is particularly abundant [6], and solar
photovoltaics is relatively clean compared to coal-fired and natural gas power plants regarding
greenhouse gas emission [7]. For the reason given above, solar photovoltaics has been growing
rapidly in this past decade [8]. The global cumulative installed solar photovoltaic capacity has
surpassed 100 GW in 2012, achieving just over 101 GW [9]. However, photovoltaics still enjoys
marginal portions of the overall power generation. In 2010, solar photovoltaics accounted for
only 0.06% of total energy consumption [8].
To accelerate the deployment of solar photovoltaics, their cost must be reduced such that
solar electricity can be as cheap as the electricity generated from conventional sources such as
fossil and nuclear fuels. Table 1-1 lists solar electricity price in US cents per kilowatt-hour
reported in 2012 for various installation scales and climate conditions. Even the lowest price
(15.15
cents/kWh) of solar electricity generated in the industrial scale and sunny climate
condition is still well above that (~10 cents/kWh) of conventional electricity in most of the states
in the United States in 2012 [11]. The cost of solar electricity can be divided into a number of
components, including photovoltaic modules, electrical and structural balance of systems,
inverters, labors, and other soft costs. As of 2013, photovoltaic modules still account for the
largest portion (32%) of the solar electricity cost [12]. It is thus critical to cost down photovoltaic
modules to decrease overall solar electricity cost.
26
ResdenialSunny
Residential
Table 1-1.
28.91
Installed System
Cloudy
63.60
Commercial
Sunny
Installed System
19.42
Cloudy
42.73
Industrial
Sunny
15.15
Installed System
Cloudy
33.32
Price of solar electricity in US cents per kilowatt-hour reported
in 2012 for
various installation scales and climate conditions [10]. The
residential price is calculated based
upon a standard 2 kilowatt peak system; the commercial price,
a 50 kilowatt peak system; the
industrial price, 500 kilowatt peak system.
1.2 Low-Temperature Solution-Processed Colloidal Quantum
Dot
Solar Cells
Low-temperature
solution-processed
solar
cells
have
the
potential
to
decrease
manufacturing cost for photovoltaic modules. These solar
cells can be built on lightweight and
flexible substrates and are compatible with roll-to-roll manufacturing
process such as printing. In
addition, there is no vacuum process required to
manufacture these solar cells. These
characteristics give rise to not only low manufacturing
cost but also high manufacturing
throughput and short energy-payback time. Because these
solar cells can be built on lightweight
27
and flexible substrates, the transportation and installation cost for these photovoltaic panels will
be low as well.
Organic semiconductors and colloidal quantum dots are the two main material systems that
can be incorporated into low-temperature solution-processed solar cells. Colloidal quantum dots
are particularly interesting for photovoltaic applications because of their spectral tunability and
will be the focus of this thesis.
Colloidal quantum dots (QDs) are chemically synthesized semiconductor nanocrystals
coated with organic ligands (Figure 1-1) which passivate the surface of nanocrystals and provide
solubility to nanocrystals in various organic solvents. The particle size of QDs can be well
controlled via synthetic conditions which will be detailed in Chapter 2. When the particle size is
smaller than the exciton Bohr radius [13] of the constituent semiconductor, the allowed energy
levels for excitons (or electrons and holes) become quantized such that the band gap energy of
the semiconductor changes with the particle size, which is termed "quantum confinement effect."
The relationship between the band gap energy and particle size can be simplified and understood
with a particle in a spherical box quantum mechanical model (Eg
lI/d 2 , Eg = band gap energy,
and d = particle diameter) as illustrated in Figure 1-2. As a consequence, the band gap energy of
QDs can be tuned by controlling their particle size.
Inorganic semiconductor core
Organic ligand shell
Figure 1-1. Schematic of a colloidal quantum dot with inorganic semiconductor core and organic
ligand shell.
28
Particle in a box
Conduction Band
Valence Band
Decreasing particle sizes
Increasing band gaps
Figure 1-2. The effect of quantum confinement on the energy levels of electrons (solid circle)
and holes (open circle) in semiconductor nanoparticles.
Table 1-2 summarizes theoretical power conversion efficiency (Shockley-Queisser limit [14,
15]) of single- and multijunction solar cells and the corresponding optimal band gap energy for
the constituent junctions. Multijunction solar cells, consisting of multiple semiconductors of
different band gap energy, extract solar energy more efficiently and promise for higher efficiency.
For example, power conversion efficiency increases from 31% to 49% when triple junctions,
instead of single junction, are employed. However, since multiple semiconductors are involved
in the multijunction solar cells, considerable effort has to be exerted to optimize the properties of
constituent materials, which significantly increases the manufacturing complexity and cost of
these cells. Band gap tunability of QDs offers the opportunity to realize multijunction solar cells
29
with single semiconductor material, providing the potential to offset the manufacturing
disadvantages derived from multijunction devices. As shown in Figure 1-3, PbS QDs covers all
the optimal band gap energies for triple-junction solar cells.
Number of
junctions
23
Band gap
1100 nm
1320 nm
760nm
760 nm
1750
170nnm
1070 nm
680 nm
Efficiency
31%
44%
49%
Table 1-2. Theoretical power conversion efficiency of single- and multijunction solar cells and
the corresponding optimal band gap energies for the constituent junctions. Band gap energies are
expressed with equivalent photon wavelength.
6
5
7
UJ
C
0
4
i2
0400
800
1200
1600
2000
Wavelength (nm)
Figure 1-3. Absorption spectra of PbS QDs with a series of particle diameters. The first excitonic
absorption peak of PbS QDs can be tuned readily from 600 to 2200 nm [16-18]. On the other
hand, the band gap of bulk PbS is 0.41 eV, which is approximately 2958 mu.
It has to be noted that although the band gap of Cu(In,Ga)Se 2 , an extensive studied
semiconductor for a thin film solar cell, can be adjusted by substitution of Ga for In, its band gap
changes from 1.04 to 1.67 eV (or 1195 to 744 nm) [19] and does not cover most of the band gaps
30
calculated for multijunction solar cells (Table 1-2).
1.3 Thesis Outline
This thesis focuses on the electron-hole separation junction (or interface), one of the key
elements in solar cells. Specifically, it tackles inorganic-inorganic bilayer heterojunctions and
inorganic-organic bulk heterojunctions; both of which involve QDs.
Chapter 2 first gives an overview of QD synthesis then reviews electrical properties (carrier
mobility, carrier type, and carrier concentration) of QDs and solar cell device physics, including
photovoltaic mechanisms
in both inorganic
and organic
solar cells,
and current-voltage
characteristics of solar cells. The chapter provides the chemistry and physics background for
colloidal QD-based solar cells.
Chapter 3 elaborates the experimental procedures for the material synthesis and the
fabrication
and
characterization
of PbS
QD/CdS
bilayer
heterojunction
devices
and
poly(3-hexylthiophene-2,5-diyl) nanofiber/CdS QD bulk heterojunction devices.
Chapter 4 presents PbS QD Schottky devices fabricated by spin coating, dip coating, and
spray coating. Comparison of film morphology and device performance in the spin-, dip, and
spray-coated devices is reported. This chapter paves the way for the fabrication of PbS QD
photovoltaic devices with high yield and satisfying reproducibility.
Chapter 5 describes the realization, optimization and characterization of PbS QD/CdS
bilayer heterojunction devices, the first low-temperature solution-processed pn heterojunction
devices
employing
PbS
QDs.
Through
a
combination
of characterization
techniques,
fundamental material and device parameters are reported.
Chapter 6 presents a facile method to improve the power conversion efficiency of
31
poly(3-hexylthiophene-2,5-diyl) nanofiber/CdS QD bulk heterojunction solar cells through the
modification of the interfacial interaction between organic electron donors and inorganic electron
acceptors.
Chapter 7 summarizes the findings made from this thesis work and briefs the future work
that can be done to further improve the efficiency of QD-based photovoltaic devices.
1.4 Reference
Source: Met Office Hadley Centre; NOAA; Scripps Institute of Oceanography; Sydney
1
Levitus et al, GRL
2
Climate change: The measure of global warming, The Economist, 2013
3
Etheridge, D. M.; Steele, L. P.; Langenfelds, R. L.; Francey, R. J.; Barnola, J.-M. & Morgan,
V. I., Natural and anthropogenic changes in atmospheric C02 over the last 1000 years from
air in Antarctic ice and firn, Journal of Geophysical Research: Atmospheres, 1996, 101,
4115-4128
4
Climate Analysis Indicators Tool, World Resources Institute, 2009
5
Jeff Tollefson & Richard Monastersky, Nature, 2012, 491, 654-655
6
Graetzel, M.; Janssen, R. A. J.; Mitzi, D. B. & Sargent, E. H., Materials interface
engineering for solution-processed photovoltaics, Nature, 2012, 488, 304-312
7
U.S. Department of Energy, Report on the First Quadrennial Technology Review, 2011
8
REN2 1, Renewables Global Status Report, 2012
9
European Photovoltaic Industry Association, 2013.
10
Solarbuzz Retail Pricing Environment, 2012
11
U.S. Energy Information Administration, 2012
32
12
GTM Research, Solvida Energy Group, Inc., 2012
13
Excitons are bound electron-hole pairs.
The exciton Bohr radius is the avarage distance
between the electron and hole of an exciton in its ground state.
14
Shockley, W. & Queisser, H. J.Detailed Balance Limit of Efficiency of p-n Junction Solar
Cells, Journal of Applied Physics, 1961, 32, 510-519
15
Sargent, E. H., Infrared photovoltaics made by solution processing, Nat Photon, 2009, 3,
325-33
16
Hines, M. & Scholes, G., Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared
Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Size Distribution,
Adv. Mater., 2003, 15, 1844-1849
17
Liu, T.-Y.; Li, M.; Ouyang, J.; Zaman, M. B.; Wang, R.; Wu, X.; Yeh, C.-S.; Lin,
Q.;
Yang,
B. & Yu, K., Non-Injection and Low-Temperature Approach to Colloidal Photoluminescent
PbS Nanocrystals with Narrow Bandwidth, The Journal of Physical Chemistry C, 2009, 113,
2301-2308
18 Moreels, I.; Justo, Y.; De Geyter, B.; Haustraete, K.; Martins, J. C. & Hens, Z., Size-Tunable,
Bright, and Stable PbS Quantum Dots: A Surface Chemistry Study, ACS Nano, 2011, 5,
2004-2012
19
Wei, S.-H.; Zhang, S. B. & Zunger, A., Effects of Ga addition to CuInSe 2 on its electronic,
structural, and defect properties, Applied Physics Letters, 1998, 72, 3199-3201
33
Chapter 2
Basics of Colloidal Quantum Dot Solar Cells
This chapter is to provide introductory and theoretical background for colloidal QD solar
cells, including colloidal QD synthesis, optical and electrical properties of QDs and methods to
tune these properties, and solar cell device physics and electrical characteristics.
2.1 Colloidal Quantum Dot Synthesis
The investigation of colloidal quantum dots began in early 1980s, when CdS nanocrystals
with broad size distribution were precipitated and grown via Ostwald ripening in aqueous
solution [1]. A breakthrough in synthesizing monodisperse colloidal QDs was made by Chris
Murray, David Norris, and Moungi Bawendi in 1993, when hot injection method [2] was first
introduced. Since then, there have been numerous studies attempting to improve size distribution
and optical property of colloidal QDs and also change the morphology, composition, and
chemistry of colloidal QDs. However, hot injection method remains the mainstream colloidal
QD synthesis protocol because of the narrow size distribution it can provide. Therefore, colloidal
QDs in this thesis are synthesized via hot injection method.
Detailed description of synthesis procedure for colloidal QDs of different compositions will
be provided later in the corresponding chapters. Here, a generalized colloidal QD synthesis is
presented. Colloidal QD synthesis is performed with a Schlenk line which provides air- and
moisture-free environment. Figure 2-1 shows the photograph of the setup used for colloidal QD
synthesis. One precursor compound is first dissolved in a heated high-boiling point solvent
contained and degassed in a three-neck flask. The precursor solution is further heated to the
reaction temperature. Then, another degassed precursor solution is swiftly injected to the flask,
34
which raises the precursor concentration above the nucleation threshold and leads to a nucleation
burst. Figure 2-2 illustrates precursor concentration as the reaction evolves according to LaMer
colloid growth model [3]. Rapid injection ensures that most of nuclei formed at the same time. In
an ideal case, the lowering of precursor concentration due to the nucleation event and the
reducing of overall solution temperature due to the addition of a room-temperature precursor
solution terminates nucleus formation. Precursors are then consumed by the nanocrystal growth.
The quenching of nucleus formation prior to or at the early stage of nanocrystal growth is a key
for narrow particle size distribution achieved with hot injection colloidal QD synthesis because it
allows all nanocrystals to grow at the same time. Eventually, the reaction will reach equilibrium
between material in nanocrystals and precursor monomers, and the size distribution will start to
increase because big particles grow at the expense of small particles' shrinkage known as
Ostwald ripening.
Particle size can be controlled via various synthesis conditions, such as precursor reactivity,
coordinating/non-coordinating solvent ratio, growth temperature, and growth period. When more
reactive precursor is used, more nuclei are formed, resulting in smaller particle size and larger
size distribution [4]. Coordinating solvent promotes precursor consumption by nanocrystal
growth, which leads to larger particle size [5]. Higher growth temperature and longer growth
period also increase particle size.
35
Figure 2-1. A photograph of colloidal QD synthesis setup, including a Schlenk line, a three-neck
flask, a condenser, a stirrer, a temperature controlling system, an injection gun, and a mechanical
rotary pump (not shown in the picture). The black injection gun shown in this picture is used for
large-scale (gram-scale) synthesis, while for a conventional small-scale synthesis (100
milligram-scale) synthesis, a syringe is used.
C
0
C
U
o
Z
.0
C
4.
Nucleation threshold
=
U
~
(U
+
Ostwald ripening
Time
Figure 2-2. Cartoon depicting the precursor concentration evolution along different stages of
colloidal QD nucleation and growth under the framework of LaMer colloid growth model.
36
2.2 Electrical Properties of Colloidal Quantum Dots
After synthesis, colloidal QDs are capped with long-chain organic ligands, such as oleic
acid, oleylamine, trioctylphosphine, and trioctylphosphine oxide, which provide solubility to
QDs in a range of organic solvents. The capping ligands also play an important role in the
electrical properties of colloidal QDs, such as carrier mobility, carrier type and concentration,
and carrier traps.
2.2.1 CarrierMobility
In a thin film composed of colloidal QDs, carriers are transported via either tunneling or
hopping mechanisms which are largely affected by the distances between adjacent QDs.
Therefore, a lot of effort has been made to shrink the inter-QD distances to improve carrier
mobility. This is generally achieved via solution- or solid-phase chemical treatments by which
long-chain organic ligands are replaced by short organic or inorganic ligands. Although carrier
mobility can also be increased by a high-temperature annealing process which sinters QD thin
film [6], the quantum confinement effect is lost and thus is not within the focus of this thesis.
Solution-phase chemical treatment here is a pre-deposition ligand exchange process
occuring in a QD solution and requires the preservation of colloidal stability of QDs after ligand
exchange for the following thin film deposition with small roughness. Common organic
molecules for solution-phase ligand exchange are pyridine [7] and n-butylamine [8]. Recently,
Dmitri Talapin's group developed molecular metal chalcogenide complex ligands and achieved
electron mobility as high as 16 cm 2/Vs for CdSe colloidal QDs with In 2 Se2 ligands [9]. Cherie
Kagan's group also demonstrated thiocyanate-capped CdSe QD thin film with high electron
mobility of 1.5 ± 0.7 cm 2/Vs [10]. Both of these values are acquired by field effect transistor
37
measurement and are considered very high for non-sintered colloidal QD thin film. Although the
physical size of the ligands used in these two cases are tiny, these ligands charge QD surfaces
and provide electrostatic repulsion between QDs, which prevents aggregation and offers
solubility in polar solvents.
Solid-phase chemical treatment, on the contrary, is a post-deposition ligand exchange
process carried out after QD thin film is deposited. Small ligands, such as 1,2-ethanedithiol [11],
1,4-benzenedithiol [12], 3-mercaptopropionic acid [13], and hydrazine [14], used in this process
drastically shorten inter-QD distances and insolubilize QDs. Untreated oleic acid capped-PbS
QDs with hole field-effect mobility of 3 x 10- cm 2/Vs is reported [15]. After treated with
1,2-ethanedithiol, 1,4-benzenedithiol, and 3-mercaptopropionic acid, hole field-effect mobility is
increased to 1.7 x 10-3 3.8 x 10-, and 2.7 x
1 0 -3 cm
2
/Vs, respectively [16]. In addition, Yao Liu
et al. have also shown that electron and hole field-effect mobility decrease exponentially with
increasing ligand length [17]. Recently, Edward Sargent's group has also demonstrated that with
halide treatments, such as cetyltrimethylammonium bromide, the electron field-effect mobility of
PbS QDs can be improved to 4.4 x 102 cm 2/Vs [18]. Because of the large volume shrinkage,
cracks usually form in the treated QD film as shown in Figure 2-3. Therefore, layer-by-layer
coating techniques are employed to fill the cracks generated in the underlying layers to prevent
shorts.
38
Figure 2-3. A Scanning electron microscopy image of a treated QD film. Cracks are usually
developed in thick QD film after post-deposition treatment.
In summary, carrier mobility in QD film is strongly correlated with the capping ligands: the
shorter the ligand, the higher the mobility. Various solution- and solid-phase chemical treatments
are implemented to replace bulky native ligands with short ligands, providing a pathway toward
solution-processed high-performance electronic and optoelectronic devices.
2.2.2 Carrier Type and Density
Carrier type and density are important factors for the operation of solar cells as will be
described later in this chatper. It has been reported in several QD material systems that carrier
type and density can be controlled via chemical treatment and are also sensitive to the
environment to which QDs exposed.
Conduction of PbSe QDs can be switched from p- to n-type by hydrazine post-deposition
treatment and reversed to p-type as the hydrazine desorbed [14]. By increasing the doping level,
silver can be changed from n- to p-type dopants for CdSe QDs [19]. N-type doping of CdSe QDs
can also be realized by thermal diffusion of indium [9], and electron transfer from sodium
39
biphenyl [20]. Metal chalcogenide complexes allow switching of CdTe QD conduction from
p-type to ambipolar
to n-type
[21].
Pre-deposition incorporation
of cadmium switches
conduction of InAs QDs from n- to p-type [22]. Chemical treatments, including organic and
halide ligands, are employed to tailor doping type and concentration of PbS QDs [23, 24, 25].
Oxidation is found to increase p-type doping of PbS QDs [15]. In addition, carrier type and
density in PbSe QDs are shown to be connected to QD's stoichiometry: Pb-rich QDs are n-type;
Se-rich, p-type [26].
2.3 Solar Cell Device Physics
A solar cell is a semiconductor diode which directly converts sun light into electricity. In a
conventional inorganic solar cell, this conversion process primarily consists of photon absorption
(carrier generation) and carrier separation. While in an excitonic solar cell, such as an organic
solar cell [27], very similar but not exactly identical processes are involved, namely photon
absorption (exciton generation), exciton diffusion, charge transfer, charge separation, carrier
transport and carrier extraction [28]. Since this thesis work involves both inorganic and organic
solar cells, in the following sections I will first describe the details of these processes in
conventional inorganic solar cells, followed by the explanation of similar conversion process in
organic solar cells.
2.3.1 Photovoltaic Processes in Inorganic Solar Cells
Photon Absorption and CarrierGeneration:
When light shines on a solar cell, photons, if not reflected from the solar cell, with energy
equal to or greater than the band gap of semiconductors can be absorbed. Such photon absorption
40
event excites electrons from valence bands to conduction bands and generates free carriers
(electrons and holes). However, not all the photons with energy equal to or greater than the band
gap of semiconductors are absorbed. In materials with low absorption coefficient or small
thickness, photons can transmit without being absorbed, even though their energy is sufficient to
excite electron from valence bands to conduction bands. Therefore, reflection and transmission
are two of the loss mechanisms that need to be eliminated or minimized to achieve high
photocurrent density and high device performance.
The number of free carriers generated by photon absorption is limited by the flux of photons
with enough energy to excite electrons from valence bands to conduction bands. The smaller the
band gap of semiconductors, the more free carriers can be generated by photon absorption.
However, because carriers generated by high-energy photons (with energy greater than the band
gap of semiconductors) relax rapidly to the band edge of semiconductors and also because of
energy conservation, the usable energy is smaller when semiconductors of smaller band gaps are
employed. Due to this trade-off, to realize high device efficiency, there is an optimal band gap
for the semiconductor used in a solar cell as mentioned in chapter one.
Although there are approaches proposed to exploit or better use photons with energy smaller
or greater than the band gap of semiconductors, such as an intermediate-band solar cell [29], a
multiple exciton generation solar cell [30], and a hot carrier solar cell [31], these types of device
are still in their very early stage of research and rather immature and thus are beyond the scope
of this thesis.
Carrier Separation:
After photogeneration, carriers need to diffuse and drift to the corresponding electrodes to
perform electrical work to the loads in an external circuit. The relevant parameters for diffusion
41
and drift processes are minority carrier diffusion length and depletion width. For the carriers
generated outside of the depletion region, they need to diffuse to the depletion region, but the
distance they can diffuse before annihilated via recombination is limited by the minority carrier
diffusion length. For the carriers generated in or successfully diffusing into the depletion region,
they can be swept by the built-in electric field to the corresponding electrodes and contribute to
electric current. Built-in electric field is typically formed in a semiconductor-semiconductor pn
junction or a metal-semiconductor Schottky junction which forms the basis of a solar cell and is
elaborated in the following paragraphs.
When p-type and n-type semiconductors are joined together, due to the electron and hole
concentration gradient across the p-type and n-type semiconductor junction, holes in the p-type
semiconductor diffuse to the n-type semiconductor, and electrons in the n-type semiconductor
diffuse to the p-type semiconductor to establish chemical potential equilibrium. The diffusion of
free carriers leaves behind uncompensated positively charged donor atoms in the n-type
semiconductor and negatively charged acceptor atoms in the p-type semiconductor. The charged
atoms created the so-called built-in electric field (or built-in potential, Vbi, or energy band
bending) which sweeps away free carriers in the junction region, and the junction region where
free carriers are depleted is named depletion region whose width is denoted as Wd and equal to:
q2
W4/c ==[2-cs-o
where es,
co,
NA+ND
(NA+ND)
Vbi
bi]1/2
21
21
q, NA, and ND are the dielectric constant of the semiconductor, vacuum permittivity,
elementary charge, acceptor concentration and donor concentration, respectively. Figure 2-4
shows the energy band diagram of a pn homojunction at thermal equilibrium. The depletion
width, WP, in the p-type semiconductor and the depletion width, W,, in the n-type semiconductor
are given by:
42
WP
= [Iq
Wn =
ND
NA(NA +ND)
Vbi
ND (NA+ND)
bi
1/2
1/2
(2-2)
(2-3)
It can be seen from equation 2-2 and 2-3 that the depletion region is not evenly distributed in the
p-type and n-type semiconductors; instead, it spreads mostly in the less-doped side of the
semiconductor (either in the p-type or n-type semiconductor). As a result, in heterojunction solar
cells such as CdTe/CdS and CIGS/CdS solar cells [32, 33], it is preferred to have the absorber
layer less doped than the window or buffer layer to extend most of the depletion width in the
absorber layer and facilitate carrier separation.
depletion region
EC
electrons
EC
EF
holes
EF
w
W
p-type
n-type
Figure 2-4. Energy band diagram of a pn homojunction at thermal equilibrium.
When a metal and semiconductor are brought into contact, energy band bending and
depletion regions can also be established as in the case of pn semiconductor junctions. Such
metal-semiconductor junction is known as a Schottky junction in which the metal can be
conceived as a very heavily doped semiconductor and essentially all the depletion region resides
in the semiconductor side as depicted in Figure 2-5.
43
(a)
EF----
-
-
- -
- --- -
EF
W
metal
p-type
(b)
EC
EF
- - - - -
- - - - - - - - - - - -EF
Ev
W
n-type
metal
Figure 2-5. Energy band diagrams of Schottky junctions formed between (a) a metal and a p-type
semiconductor and (b) a metal and an n-type semiconductor at thermal equilibrium.
2.3.2 Photovoltaic Processes in Organic Solar Cells
Conversion of light to electricity in an organic solar cell takes on the mechanistic steps very
similar to the ones in an inorganic solar cell but with some differences due to the material
properties of organic semiconductors.
Exciting organic semiconductors with light does not generate appreciable free electron-hole
44
pairs immediately. In conjugated polymers, only 10% of photoexcitations with energy above the
band gap directly create free charges [34]. The majority of the photoexcitations in organic
semiconductors forms coulombically bound electron-hole pairs, called excitons, due to small
dielectric constants and resultant large exciton binding energy (estimated to be 0.4-0.5 eV [35])
in these materials [27]. These excitons have to diffuse to dissociation sites to be broken apart into
free carriers. The dissociation process can be achieved by blending two organic semiconductors
of different energy levels together such that it is energetically favorable to transfer electrons from
a donor material to an acceptor material and holes from an acceptor material to a donor material.
Although the transferred electrons and holes (charge-transfer exitons) can still attract each other,
these excitons have decreased binding energy because of increased separation between electrons
and holes and thus more susceptible to dissociation under electric fields or thermal activation.
The spatially separated charges then need to transport to and be extracted at an anode and
cathode in order to contribute to photocurrents. The sequential photovoltaic process starting from
photon absorption to carrier extraction in an organic solar cell is schematically illustrated in
Figure 2-6.
45
(b)
(a)
Energy
Donor
LUMO
Acceptor
I
I
I
LUMO |
I
I
I
LUMO
HOMO
HOMO
HOMO
(d)
(c)
LUMO
Donor
LUMO
Acceptor
LM
LUMO
HOMO
HOMO__
HOMO
IHOMO
Figure 2-6. A schematic diagram showing the mechanisms by which photons are converted to
free carriers in an organic solar cell. (a) Photon absorption in semiconductor layers generates
excitons, bound electron-hole pairs. (b) Excitons diffuse around in the material before radiative
or non-radiative recombination. (c) When excitons reach the interface between a donor and an
acceptor, charge transfer can occur. (d) Charges are spatially separated, transported, and
extracted at the electrodes. HOMO and LUMO is the abbreviation of highest occupied molecular
orbital and lowest unoccupied molecular orbital, respectively.
2.3.3 Current-Voltage Characteristics of Solar Cells
Although the photovoltaic processes involved in inorganic and organic solar cells are
different, current-voltage characteristics of these devices are identical. Figure 2-7 presents ideal
46
current-voltage (I-V) curves of a solar cell in the dark and under AM1.5G illumination. When a
forward bias is applied on a solar cell in the dark, carriers are injected across the potential
barriers at a semiconductor-semiconductor or a metal-semiconductor interface such that the
current flowing through the device increases exponentially with a positive bias as described in
the Shockley diode equation:
Id= Io
SqV
(en'
- 1)
(2-4)
where Id is the current flowing through the diode, 1o is the reverse saturation current, n is the
ideality factor, k is the Boltzmann constant, and T is the absolute temperature. Under
illumination, a photocurrent (IL) is generated and can be superimposed to the diode current,
shifting the diode current downward in a current-voltage plot:
I
= JO (en
qV
- 1) - 1,
(2-5)
Current, voltage, and thus output power is available from the device with the power conversion
efficiency (PCE) defined as the ratio of maximum output power to input power and is a critical
figure of merit for a solar cell. Input power is the product of device area and the incident light
intensity, and the standard incident solar spectrum at the Earth's surface is AM1.5G with
integrated power density equal to 100 mW/cm 2. Maximum output power (Po
0 t) is the product of
optimal current (Im.) and voltage (Vm,) and can be expressed as:
Pout = Isc X Voc X FF
(2-6)
where ISc is the short-circuit current, VOC is the open-circuit voltage, and FF is the fill factor. Fill
factor is the ratio of the optimal current and voltage product to the short-circuit current and
open-circuit voltage product, defining the "squareness" of the current-voltage curve in the fourth
quadrant of a solar cell under illumination.
47
--
AM1.5G
Dark
41-J
Voltage
Figure 2-7. Ideal current-voltage characteristics of a solar cell in the dark (black curve) and under
AM1.5G illumination (red curve). An ideal solar cell I-V curve in the dark should exhibit strong
rectification.
However, a solar cell in reality is not merely composed of an ideal diode, and thus the I-V
curve can deviate appreciably from the one depicted in the Figure 2-6. To account for the
non-ideality, shunt resistance (Rsh) and series resistance (R,) are included in the equivalent circuit
diagram (Figure 2-8) used to describe the electrical behavior of the device. The components in
the equivalent circuit diagram are expatiated in the following paragraph.
RS
ld
IshI
L
sh
V
RL
-0-
Figure 2-8. An equivalent circuit diagram of the solar cell comprising a current source (IL), a
diode (Id), a shunt resistor (Rh), and a series resistor (R,). A load resistor (RL, not a part of the
solar cell) is shown in the diagram to represent the external load.
48
The current source generates current (IL) from the photogenerated carriers before any
recombination loss. The diode, which is the basis of a solar cell as mentioned in the previous
section, gives rise to the rectifying current-voltage characteristic. To maximize the current
flowing to the external load (RL), shunt resistance must be maximized, whereas series resistance
has to be minimized. Thin film defects, such as pinholes and cracks, and recombination in the
device result in small shunt resistance, while series resistance is determined by device electrical
conductance that is affected by carrier mobility and film thickness. Because the diode is not
conductive at zero or small bias (V), and because shunt resistance is much greater than series
resistance, shunt resistance can be approximated by the inverse slope of the I-V curve at zero
bias. On the other hand, because the diode becomes very conductive at a bias greater than the
turn-on voltage of the diode, series resistance can be approximated by the inverse slope of the
I-V curve at this bias. Since the slopes of the I-V curve at zero and a turn-on bias determine the
"squareness" of the curve,
Rsh
and R, influence the fill factor of a solar cell.
From the viewpoint of energy conservation, the upper limit of open-circuit voltage is the
band gap or the effective band gap (conduction band-valence band offset of n- and p-type
semiconductors or LUMO-HOMO offset of acceptors and donors) of the semiconductors
employed in a homojunction or heterojunction solar cell, respectively. However, in reality V,, in
a homojunction solar cell is limited by the built-in potential because it is the driving force for
charge separation. From the equation derived below, it can be shown that V.
in either a
homojunction or a heterojunction solar cell is further reduced by recombination and shorts.
Using Kirchhoff's circuit laws, the current (I) and voltage (V) in Figure 2-8 can be related
as:
(IL - Ia - I)RSh = V +
IRS
(2-7)
Replacing Id in equation (2-7) with the Shockley diode equation (2-4) in which V = V - IRs and
49
rearranging the equation, the current (I) can be written as:
V
L
1
=
Rsh
V-IRs
(
-
enkT /
-)
(2-8)
Rsh
With the assumption that R, << Rsh in a solar cell and I = 0 at open circuit condition, the
dependence of open-circuit voltage on the reverse saturation current (Io) and shunt resistance (Rsh)
can be obtained:
V
c=
In(L-Voc/Rsh
Therefore, higher recombination, meaning larger lo and smaller Rsh, decreases the VOc. With more
shorts, meaning smaller Rsh, the V., will be lower as well.
The effect of recombination and shorts on V.c can also be understood by considering the
competition between dark current (Id + Ish) and photocurrent (IL). Since at open circuit condition,
the total current (I) equals to zero, dark current can be regarded as a counterbalance to
photocurrent: the higher the dark current, the lower the Vc. Consequently, recombination and
shorts, which can increase dark current, will reduce Vc.
Short-circuit current (Isc), the current (I) at short circuit condition (V = 0), is the
photocurrent (IL) subtracted by the diode current (Id) and the shunt current (Ish) as can be seen
from Figure 2-8. As mentioned before, the diode current is negligible at zero bias, so it is
favorable to keep the shunt current small in order to extract photocurrent as much as possible.
Photocurrent is determined by the number of photogenerated carriers or split excitons as
discussed in previous section 2.2.1 and 2.2.2 and thus is also proportional to light intensity.
2.4 Conclusion
Colloidal QDs are the building blocks of the photovoltaic devices studied in this thesis. The
50
property of these QDs can significantly influence solar cell performance and hence needs to be
controlled during or after synthesis. This chapter begins by reviewing the synthesis of colloidal
QDs with the emphasis on the control of particle size, the parameter directly determining the
band gap of QDs. Then, strong dependence of QD electrical properties, namely carrier mobility,
carrier type, and carrier density, on postsynthesis chemical treatments is enumerated to manifest
the diversity of colloidal QDs. Finally, relevant device physics and electrical characteristics of
inorganic and organic solar cells are introduced in preparation for better understanding the
behaviors of the QD-based photovoltaic devices fabricated in the following studies.
2.5 Reference
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Colloidal Quantum Dot Solar Cells,
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3374-3380
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Talapin, D. V. & Murray, C. B., PbSe Nanocrystal Solids for n- and p-Channel Thin Film
Field-Effect Transistors, Science, 2005, 310, 86-89
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Klem, E. J. D.; Shukla, H.; Hinds, S.; MacNeil, D. D.; Levina, L. & Sargent, E. H., Impact
of dithiol treatment and air annealing on the conductivity, mobility, and hole density in PbS
colloidal quantum dot solids, Applied Physics Letters, 2008, 92, 212105
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Bisri, S. Z.; Piliego, C.; Yarema, M.; Heiss, W. & Loi, M. A., Low Driving Voltage and
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Advanced Materials, 2013
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Liu, Y.; Gibbs, M.; Puthussery, J.; Gaik, S.; Ihly, R.; Hillhouse, H. W. & Law, M.,
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Mater, 2011, 10, 765-771
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Shim, M. & Guyot-Sionnest, P., n-type colloidal semiconductor nanocrystals, Nature, 2000,
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Geyer, S. M.; Allen, P. M.; Chang, L.-Y.; Wong, C. R.; Osedach, T. P.; Zhao, N.; Bulovic, V.
& Bawendi, M. G., Control of the Carrier Type in InAs Nanocrystal Films by Predeposition
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Charge-Orbital Balance Picture of Doping in Colloidal Quantum Dot Solids, ACS Nano,
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E. H., N-Type
Colloidal-Quantum-Dot
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Oh, S. J.; Berry, N. E.; Choi, J.-H.; Gaulding, E. A.; Paik, T.; Hong, S.-H.; Murray, C. B. &
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Their Electronic and Optoelectronic Device Performance, ACS Nano, 2013, 7, 2413-2421
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55
Chapter 3
Experimental Methods
In Chapter 1 and 2, motivational and theoretical background for QD-based photovoltaic
devices is presented. In this chapter, detailed description of the main synthesis, fabrication, and
characterization techniques is given, while a number of techniques also adopted but less common
to this thesis will be introduced thereafter when needed.
3.1 Fabrication of Pbs QD/CdS Bilayer Heterojunction Devices
3.1.1 PbS Colloidal QD Synthesis and Purification
Materials: lead acetate trihydrate, oleic acid, 1-octadecene, hexamethyldisilathiane, anhydrous
hexane, methanol, ethanol, isopropyl alcohol, butanol, and acetone are purchased and used
without purification.
Due to the need for large quantity of materials, scale-up synthesis of PbS colloidal QDs,
initially developed by a former graduate student, Dr. Scott Geyer, in the group, is adopted in this
study.
Synthesis was carried out with a Schlenk line. 11.380 g of lead acetate trihydrate, 21 mL
of oleic acid, and 300 mL of 1-octadecence was loaded into a IL four-neck flask equipped with a
thermal couple, condenser, mechanical stirrer, and heating mantle. The solution was stirred and
underwent rough degassing until no vigorous bubbling observed.
Then, mild heating at 100
degree Celsius is used to dissolve lead acetate trihydrate and form the lead precursor, lead oleate.
Degassing process was continued overnight until 300 millitorr or lower pressure is achieved.
3.15 mL of hexamethyldisilathiane and 150 mL of degassed 1-octadecence were loaded into an
injection barrel in a nitrogen glovebox, which was used as the sulfur precursor. The degassed
lead precursor was further heated to 120 degree Celsius under nitrogen, followed by swift
injection of the sulfur precursor. The solution changed from colorless to black, indicating the
formation of PbS QDs. To implement the injection process, an injection gun pressurized at 70 psi
was used. It should be noted that the heating apparatus was switched off right before the
injection to avoid abrupt heating which could cause non-uniform solution temperature rise and
56
undesired particle size distribution. After injection, the heating mantle was removed to allow
cooling of QD solution.
After synthesis, the solution was transferred to a vacuumed and sealed flask via a cannula
and was pumped into a nitrogen glovebox for initial purification. Appropriate amount of acetone
was added to the solution that was then centrifuged at 3900 rpm for 3 minutes to precipitate QDs.
The supernatant was decanted and the precipitated QDs were redispersed in hexane at high
concentration for the convenience of further purification. The resultant QD hexane solution was
stored as a stock solution in a nitrogen glovebox.
Before applying to devices, QDs were purified (centrifuged) for two more times with the
following recipe. Appropriate amount of ethanol was added to QD solution, accompanied by the
addition of butanol with volume equal to one tenth of QD solution to prevent phase separation
between ethanol and hexane. The solution mixture was centrifuged at 3900 rpm for 3 minutes to
precipitate QDs that were redispersed in hexane and run through 0. 1-micron PTFE syringe filters.
The resultant QD solution added with acetone was centrifuged again at 3900 rpm for 3 minutes
to precipitate QDs which were redispersed in hexane or octane for dip coating, spin coating, and
spray coating as will be discussed in the following Chapter.
3.1.2 Substrate Cleaning
Patterned ITO (tin-doped indium oxide)-coated glass substrates purchased from Thin
Film Devices Inc. were cleaned by ultrasonication in 2% Micro-90, deionized water, acetone,
and boiling isopropyl alcohol for 20 minutes each, followed by five-minute oxygen plasma
which removes hydrocarbon contaminants. (The plasma treatment also improves wetting of
aqueous solution of PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate))
used in P3HT (Poly(3-hexylthiophene-2,5-diyl)) nanofiber/CdS QD bulk heterojunction devices
as will be introduced later in this chapter.)
3.1.3 (3-mercaptopropyl)trimethoxysilane Substrate Treatment
(3-mercaptopropyl)trimethoxysilane
(MPTMS) treatment was performed to improve
adhesion between QDs and ITO-coated glass substrates. Without MPTMS treatment, PbS QD
film was often peeled off during the subsequent postdeposition ligand exchange process. Cleaned
substrates were immersed in a jar containing 20 miroliter of MPTMS and 8 mL of toluene in a
57
nitrogen glovebox overnight. Treated substrates were rinsed with isopropyl alcohol in a nitrogen
was
glovebox and immersed into another jar filled with isopropyl alcohol. The jar with substrates
sealed and brought out to the air for 10-minute ultrasonication, after which it was pumped back
into a nitrogen glovebox. The substrates were rinsed again with isopropyl alcohol and blew dry
with nitrogen. Formation of idealized MPTMS monolayer is illustrated in Figure 3-1 as proposed
oxide
by other group [1]. In a nutshell, the oxysilane group covalently binds the hydroxylated
surface, and the thiol group binds the metal atoms (such as lead atoms, not shown here) in QDs
through metal-thiolate bonds.
SH
I
(CH 2)3
I.
SH
I
(CH 2)3
MeO
I
OMe
OMe
MeO
SH
CH 30H
Sii
Ome
OMe
OH OH OH OH OH
H20
I
layer I
(CH 2)3
I
(CH
SH
SH
SH
I
I
)
23
H2 0
111
t
I OH HO I OH
OH O OHOH O
I I1
I
HO
(CH 2 )3
HOI
OH O
(CH 2 )3
0 I OH
OH O OH
Figure 3-1. Schematic of idealized MPTMS monolayer formation on hydroxylated oxide
substrate surface with the presence of a physically-absorbed water layer.
3.1.4 Deposition of PbS QD Film
PbS QD film in PbS QD/CdS bilayer heterojunction devices was deposited via dip
coating due to the superior film quality this coating technique can achieve. Purified PbS QDs
were redispersed in hexane at a concentration of 10 mg/ml with the addition of 3 droplets of
oleic acid. The QD solution was filtered through 0.02-miron AnotopTM syringe filters and
contained in a 1 OmL beaker. 0.02% (by volume) 1,2-ethanedithiol (EDT) in acetonitrile and pure
acetonitrile was contained in another two beakers separately. MPTMS-treated ITO substrates,
held by a steel clamp, were sequentially dipped into the beakers containing QD solution, EDT
solution, and acetonitrile, using a multi-vessel dip coater (Nima DC Multi 8, Nima Technology)
as shown in Figure 3-2. Specific dip coating parameters are provided in Table 3-1. Because
hexane evaporates readily, it was added to QD solution every hour to keep total solution volume
constant. After dip coating, QD film on the glass side of ITO substrates were wiped off with a
cotton swab moistened with hexane.
58
Figure 3-2. A photograph of the multi-vessel dip coater used to deposit high-quality PbS QD film
in this thesis.
Beaker
Dipping speed
(mm/min)
immersion
time(s
QD solution
400
1
30
EDT solution
400
30
0
Acetonitrile
400
3
60
Dyngtm(s
Table 3-1. Parameters for PbS QD dip coating. A dip coating cycle consists of: (1) dipping the
substrate into 10mg/ml PbS QDs in hexane for 1 second and drying the sample for 30 seconds, (2)
dipping the film in 0.02% EDT in acetonitrile for 30 seconds, and (3) rinsing by dipping in pure
acetonitrile for 3 seconds, followed by drying for 60 seconds. The above dip coating cycle was
repeated to achieve desired PbS QD thicknesses. The dipping speed for all the steps was 400
mm/min. Before and after dipping into QD solution, the samples were dried naturally for 60 and
30 seconds respectively to avoid cross-contamination between QD solution and acetonitrile or
EDT solution, which can produce QD agglomerates in solution, resulting in poor film qualities.
3.1.5 Deposition of CdS Thin Film
CdS thin film was deposited onto PbS QD-coated ITO substrates via chemical bath
deposition in a chemical fume hood. Chemical bath deposition can be regarded as a liquid-phase
59
analogy of chemical vapor deposition. In this section, the mechanism for chemical bath
deposition of CdS thin film is first reviewed, followed by the detailed description of the
experimental method.
Two types of reaction (heterogeneous and homogeneous) occur in chemical bath
deposition. The heterogeneous reaction is characterized by the atom-by-atom growth of film on
substrates [2], while the homogeneous reaction competes with the heterogeneous reaction and
consumes the reactants to form particles in the bulk of the solution [3]. It also has been observed
that the predominant homogeneous reaction terminates the heterogeneous reaction, and the film
grown by the absorption of massive suspending particles results in porous morphology. Figure 33 shows a rough film grown predominantly by a homogenous reaction. Since a rough film with
peaks on the order of micrometer will cause the shorting especially when the overlaying PbS thin
film is just hundred-nanometer thick, it is desired to suppress the homogeneous reaction and
promote the heterogeneous reaction to obtain smooth CdS film.
The following equations list the proposed reactions that occur in the CdS chemical bath
deposition [2, 3]:
(3-1)
NH 3 + H2 0 - NH+ + OH-
Cd2+ + 4NH 3 - Cd(NH 3 ) 4 2 + (3-2)
Cd2+ + 20H
SC(NH 2) 2 + 20H -
-3 Cd(OH) 2 (s)
(3-3)
S2 - + CN 2 H2 + 2H 2 0 (3-4)
Cd2+ + S2- -> CdS (s)
(3-5)
Cd(NH 3) 4 2 + + 20H- + site # [Cd(OH) 2 ]ads + 4NH 3 (3-6)
[Cd(OH) 2]ads + SC(NH 2 ) 2 -+ [Cd(SC(NH 2) 2)(OH) 2 ]ads (3-7)
[Cd(SC(NH 2 ) 2 )(OH) 2]ads -* CdS + CN 2 H2 + 2H 2 0 + site (3-8)
Equations 3-(3-4 and 3-(3-5 are responsible for the homogeneous reaction which generates a
large number of CdS particles in the bulk solution and cause the roughness in the resultant thin
film. Ammonium hydroxide is added to form cadmium tetraammine complex ions (equation 3(3-2) and prevent the reaction in equation 3-(3-5. Moreover, the reaction in euqation 3- (3-3 also
needs to be suppressed because it forms Cd(OH) 2 grains in the bulk solution. By the addition of
ammonium hydroxide, cadmium ions are predominantly used up in equation 3-(3-2, and the
60
product of [Cd2+] and [OH] is ensured to be less than the solubility product (1.2 X 10-4) of
Cd(OH) 2. Since ammonium hydroxide introduces both NH' and OH (OH~ promotes the
unwanted reaction in equation 3- (3-3), ammonium acetate is further added to control the
concentration of OH- and stabilize cadmium tetraammine complex ions.
On the other hand, equation 3-(3-6 through 3-(3-8 manifest the mechanism of
heterogeneous reaction through which smooth and compact films can grow on the substrate
surface. This involves reversible adsorption of cadmium hydroxide species on the susbstrate
(equation 3-(3-6), formation of surface complex with thiourea (equation 3-(3-7), and formation
of CdS with site regeneration (equation 3-(3-8).
Figure 3-3. Porous and rough film grown by a homogeneous reaction.
In the following, the detailed deposition recipe is presented. 138 mg of cadmium acetate
and 308 mg of ammonium acetate were dissolved in 85.9 mL of deionized water in a 150mL
beaker at 80 degree Celsius and stirred at 300 rpm. It is noteworthy that because of the slow
temperature equilibrium between the reaction solution and the oil bath and because of the
sensitivity of the reaction to the temperature, the actual solution temperature was measured. 4.1
mL of ammonium hydroxide (28% NH 3) and 91 mg of thiourea in 10 mL of deionized water
were then sequentially added to the 150mL beaker, followed by the immersion of PbS QDcoated ITO substrates which were vertically held by custom-built substrate holders (Figure 3-4)
made of Teflon. The reaction was timed since the immersion of substrates. The solution began to
61
turn yellow after 3 minutes, indicating the formation of CdS. The substrates were withdrawn
from the solution after 15 minutes and rinsed with deionized water, resulting in 50nm thick CdS
film on PbS QD-coated ITO substrates. After naturally drying in the air, the CdS/PbS QD-coated
substrates were pumped into an evaporator-integrated glovebox.
Figure 3-4. A photograph of the custom-built Teflon substrate holder.
3.1.6 Deposition of Al Electrodes
Al electrodes were deposited through a shadow mask via thermal evaporation under a
base pressure of 2 x 10-6 torr at a rate of -1 A/s. The deposition thickness was monitored with a
crystal monitor, and a typical thickness of 100 nm is used for electrodes.
3.2 Fabrication of P3HT Nanofiber/CdS QD Bulk Heterojunction
devices
3.2.1 Growth of P3HT Nanofibers
P3HT
nanofibers were
grown from a solvent-assisted
self-assembly
method.
Regioregular P3HT (Sepiolid P200, Rieke Metals, Inc) was dissolved in 1,2-dichlorobenzene at
10 mg/ml. 5% (by volume) cyclohexanone was added to the P3HT solution, which was then
allowed to sit on the shelf without stirring overnight. The appearance of the solution, changing
62
from clear and orange to opaque and purple, indicates the formation of crystalline P3HT
nanofibers.
3.2.2 CdS Colloidal QD Synthesis, Purification, and n-Butylamine Ligand
Exchange
Materials: cadmium oxide, oleic acid,
1-octadecene,
sulfur powder, anhydrous hexane,
methanol, isopropyl alcohol, butanol, and n-butylamine were purchased and used without
purification.
Synthesis was carried out with a Schlenk line. 514 mg of cadmium oxide, 20 mL of oleic
acid, and 150 mL of 1-octadecence was loaded into a 500mL three-neck flask equipped with a
thermal couple, condenser, magnetic stirrer, and heating mantle. The solution was stirred and
heated to 250 degree Celsius under nitrogen flow to dissolve cadmium oxide and form the
cadmium precursor, cadmium oleate. The appearance of the solution, changing from turbid and
brown to clear and colorless, indicated the formation of cadmium oleate. The solution was
cooled to 100 degree Celsius for degassing. 64 mg of sulfur powers and 20 mL of 1 -octadecene
was loaded into a 40mL septa-capped vial stirred and degassed at 100 degree Celsius, followed
by heating to 160 degree Celsius to dissolve sulfur powders and form sulfur precursor. The
degassed sulfur precursor was swiftly injected to the degassed cadmium precursor solution at
300 degree Celsius under nitrogen flow. After injection, the solution temperature was plummeted
to and maintained at 280 degree Celsius for 30 seconds. Then, the solution was cooled down to
room temperature naturally with the heating mantle removed.
After synthesis, appropriate amount of isopropyl alcohol was added to the QD solution
which was then centrifuged at 6000 rpm for 6 minutes in the air to precipitate QDs. The
supernatant was decanted and the precipitated
QDs were redispersed in hexane at high
concentration for the convenience of further purification. The resultant QD solution in hexane
was stored as a stock solution in the air.
Before n-butylamine ligand exchange, QDs in hexane were precipitated for two more
times following the same procedure as described in the preceding paragraph. The precipitated
QD powders were pumped into a nitrogen glovebox, redissolved in n-butylamine, and stirred at
room temperature for three days. Small amount of butanol and appropriate amount of methanol
were then added to the solution, followed by centrifuging at 3900 rpm for 3 minutes to
63
precipitate QDs, which were redissolved in either octane or 1,2-dicholorobenzene at a
concentration of 40 mg/ml.
3.2.3 Blending of P3HT Nanofibers and CdS QDs
1:1 (by volume) of 10mg/ml P3HT nanofiber solution and 40mg/ml CdS QD solution
were mixed and stirred at 500 rpm and room temperature for three days. Because of the viscosity
of the solution, care should be taken during stirring to minimize the splash of solution onto the
vial wall.
3.2.4 Deposition of PEDOT:PSS
PEDOT:PSS (Clevios TM P VP CH 8000) was filtered through 0.2-micron PES filters and
spun cast on cleaned ITO-coated substrates (using the same cleaning procedure as has been
described in section 3.1.2), resulting in 40nm thick film. The spin speed was 5000 rpm with
acceleration speed of 6000 rpm/s, and the spin time was 60 seconds. After spin coating, a
cleanroom swab moistened with deionized water was used to remove PEDOT:PSS film on the
ITO pad close to the substrate edge for electrical contact. Otherwise, the PEDOT:PSS film
between the ITO pad and the electrical probe used during the electrical characterization increases
resistance, leading to poor photovoltaic performance. Finally, the substrates were baked at 140
degree Celsius for 10 minutes on a hot plate in the air to eliminate residual water in the film and
pumped into a nitrogen glovebox where the rest of fabrication process was carried out.
3.2.5 Deposition and Treatment of P3HT Nanofiber/CdS QD Layer
30
microliter
of P3HT
nanofiber/CdS
QD
solution was
spun coated
onto
PEDOT:PSS/ITO-coated substrates at 1000 rpm and acceleration of 1000 rpm/s for 60 seconds.
After drying naturally, the substrates were dipped into an acetonitrile solution with 0.02% (by
volume) EDT for 30 seconds and pure acetonitrile for another 30 seconds. The substrates were
then placed on a hot plate with the ramping rate of 450 degree Celsius/minute and baked at 175
degree Celsius for 10 minutes.
64
3.2.6 Deposition of Bathocuproine and Mg:Ag/Ag Layers
Deposition of bathocuproine (BCP) and Mg:Ag/Ag layers was performed in the Organic
and Nanostructured Electronics Laboratory. Mg was degassed in the thermal evaporator
at a base
pressure of I x 10~ torr and at the roughly same power as would be used to evaporate Mg.
After
Mg degassing, a 1 Onm thick BCP layer was deposited at the rate of 0.1 nm/s. Then,
50nm thick
of Mg and Ag alloy layer was co-evaporated through a shadow mask at the rate of 0.3
and 0.03
nm/s respectively, followed by the evaporation of 70nm thick Ag layer at the rate of
1.5 nm/s.
Figure 3-5 illustrates the overlaid patterns of ITO, BCP, and Mg:Ag/Ag electrodes.
ITO
BCP
Mg:Ag/Ag
Probe contact
Figure 3-5. Schematic illustration of overlaid patterned ITO, BCP, and Mg:Ag/Ag
electrodes.
The overlap between ITO and metal electrodes defines the ten devices with an individual
active
2
area of 1.24 mm on a half-inch glass substrate. The same pattern was also employed
for PbS
QD/CdS bilayer heterojunction devices. The pattern was designed by Dr. Alexi Arango.
3.3 Material and Device Characterization
3.3.1 Material Characterization
Various microscopy and spectroscopy characterization techniques were performed
to
analyze the optical property, morphology, chemical composition of the materials
synthesized for
this study.
Optical
absorption
spectra
were
obtained
using
a
Cary
5000
UV-vis-NIR
spectrophotometer to study the absorption profiles of QD and P3HT in solution
and thin film, as
65
well as CdS thin film. Diffuse reflectance of the PbS QD/CdS device was measured with an
Ocean Optics ISP-REF Integrating Sphere. Complex refractive indices of PbS QD and CdS thin
film were determined using a spectroscopic ellipsometer (J. A. Woollam Co. XLS- 100).
Film thicknesses of each layer in PbS QD/CdS and P3HT nanofiber/CdS QD devices
were determined using a Veeco Dektak 6M stylus profilometer.
The Cross-sectional scanning electron microscopy (SEM) image was taken with a Helios
Nanolab 600 dual beam focused ion beam milling system to verify the construction of multilayer
thin film devices. The acceleration voltage for electrons and Ga ions were 10 and 30 KV,
respectively. The top-view SEM image was acquired with a JEOL 6320FV Field-Emission Highresolution SEM under acceleration voltage of 5 kV to characterize the morphology of CdS thin
film. Transmission electron microscopy (TEM) images of drop-cast QDs and P3HT from the
sample solution on copper grid (400 mesh) with carbon/formvar support film were obtained with
a JEOL 200CX General Purpose TEM to investigate the morphology of QDs and P3HT. Height
and phase images of P3HT, PbS QD, and CdS thin film was acquired using a Digital Instruments
Dimension 3100 atomic force microscope (AFM) operating in tapping mode to study
morphology as well.
X-ray photoelectron spectroscopy (XPS) was carried out using a PHI VersaProbe II
spectrometer equipped with a monochromated Al Ka X-ray source (1486.6 eV) operating at
50W power and 200ptm spot size to examine local bonding of atom in P3HT/CdS QD blend and
elemental composition of PbS QD/ CdS bilayer thin film as a function of depth.
3.3.2 Device Characterization
Current-voltage and external quantum efficiency measurement provides simple and
decisive characterization to photovoltaic devices and was routinely performed to evaluate and
study the device performance.
Current-voltage characteristics were acquired using a Keithley 2636 system SourceMeter.
During the measurement, samples were placed in a custom-built probe fixture in a nitrogen
glovebox and connected to the SourceMeter through a 10-lead cable and a Keithley 7001 switch
system, which enabled automatic and individual test of nine devices on the samples. AM 1.5G
solar illumination was provided by a Newport 96000 solar simulator and calibrated by a Newport
91150V reference solar cell.
66
The measurement of external quantum efficiency was carried out in the Organic and
Nanostructured Electronics Laboratory. The incident light was generated by a quartz tungsten
halogen lamp (Oriel Instruments), chopped at 45 Hz by a Stanford Research Systems SR540
Optical Chopper, dispersed by an Acton monochromator, transmitted through an optical fiber,
and focused onto sample surfaces with beam size smaller than the device active area. The
second-order diffraction allowed by the monochromater was removed, using a series of low-pass
filters. The light intensity was calibrated using a Newport silicon photodetector (818-UV) and
the photocurrent at a short-circuit condition was measured with a Stanford Research Systems
SR830 lock-in amplifier.
3.4 Reference
1
Singh, J. & Whitten, J. E., Adsorption of 3-Mercaptopropyltrimethoxysilane on Silicon
Oxide Surfaces and Adsorbate Interaction with Thermally Deposited Gold, The Journal
of Physical Chemistry C, 2008, 112, 19088-19096
2
Lincot, D. & Borges, R. 0., Chemical Bath Deposition of Cadmium Sulfide Thin Films.
In Situ Growth and Structural Studies by Combined Quartz Crystal Microbalance and
Electrochemical Impedance Techniques, Journal of The Electrochemical Society, 1992,
139, 1880-1889
3
Ortega-Borges, R. & Lincot, D., Mechanism of Chemical Bath Deposition of Cadmium
Sulfide Thin Films in the Ammonia-Thiourea
System: In Situ Kinetic Study and
Modelization, Journal of The Electrochemical Society, 1993, 140, 3464-3473
4
Oladeji, I. 0. & Chow, L., Optimization of Chemical Bath Deposited Cadmium Sulfide
Thin Films, Journal of The Electrochemical Society, 1997, 144, 2342-2346
67
Chapter 4
PbS QD Schottky Photovoltaic Devices
4.1 Introduction
As mentioned in section 1.2, by virtue of their solution processability, near-infrared
sensitivity, and band gap tunability, PbS QDs are promising material for the absorber layer in
photovoltaic devices. (It is worth noting that although PbSe QDs possess same processing and
optical advantages, they have been shown to be less stable than PbS QDs. [1]) It has been
previously reported by Jason P. Clifford et al. that Schottky barriers can form at junctions
between p-type PbS QDs and shallow-work function metals. Because of the structural simplicity,
this thesis began with the fabrication and characterization of PbS QD Schottky photovoltaic
devices.
4.2 Deposition of QD Film
Due to short minority carrier diffusion length and narrow depletion width, the thickness
of PbS QD film is typically limited to few hundreds. However, the need for a thin film imposes
challenges on the fabrication of devices with high yield and reproducibility because shorts and
shunting paths are easily formed. As a consequence, this thesis began with experimentation and
evaluation of a couple of deposition methods, namely spin coating, dip coating, and spray
coating, in search of a reliable thin-film coating technique. For each deposition method tested
herein, PbS QDs were synthesized and purified per section 3.1.1, and the substrates were cleaned
and treated with MPTMS per section 3.1.2 and 3.1.3, respectively.
4.2.1 Spin Coating
PbS QD solution (25 mg/ml) in octane was filtered through 0.1-micron PTFE filters and
spun cast on ITO-coated glass substrates. The spin speed was 1500 rpm with an acceleration
speed of 1500 rpm/s, and the spin time was 60 seconds. EDT treatment was performed by
dipping the substrates into an acetonitrile solution with 0.02% (by volume) EDT for 30 seconds
68
PbS QD spin coating and EDT treatment was
and pure acetonitrile for another 30 seconds.
repeated alternately for 8 times.
4.2.2 Dip Coating
PbS QDs were redispersed in hexane at a concentration of 10 mg/ml, filtered through
0.02-miron AnotopTM syringe filters and contained in a IOmL beaker. 0.02% (by volume) EDT
solution in acetonitrile and pure acetonitrile solution were contained in another two beakers
separately. ITO-coated substrates were sequentially dipped into the beakers containing QD
solution for 1 second, EDT solution for 30 seconds, and acetonitrile for 3 seconds. The dip
coating and EDT treatment was repeated alternately for 8 times. After dip coating, QD film on
the glass side of ITO substrates were wiped off with a cotton swab moistened with hexane.
4.2.3 Spray Coating
A commercially available handheld airbrush (Badger Air-Brush Co 200-1 Siphon Feed
200NH Airbrush) powered by compressed nitrogen at 8 psi was used to spray QD thin film.
Figure 4-1 shows the schematic representation of the spray coating setup employed in this study.
PbS QDs were dissolved in octane at a concentration of 5 mg/ml and contained in a jar
connected to the airbrush. The flow of compressed nitrogen gas through a venturi creates a local
reduction in gas pressure, sucking QD solution from the jar. The high velocity of nitrogen gas
atomizes QD solution into fine droplets and carries the droplets onto the substrate surface. The
spray of QD solution was raster scanned on the substrate surface to produce a thin layer of QD
film, followed by EDT treatment and acetonitrile rinse as done with spin or dip coating. The
spray coating and EDT treatment was repeated alternately for 8 times with the substrate rotated
90 degrees between each iteration.
69
Nitrogen gas pressure
Solvent vapor pressure
Solution feed rate
Tip-to-substrate distance
Figure 4-1. A schematic representation of a spray coating setup composed of a handheld airbrush,
a gas regulator, and a nitrogen cylinder. Relevant processing parameters are also shown.
To achieve a good film quality, several spray coating parameters have to be diligently
controlled. Major solvent evaporated before the droplet arriving at the substrate surface would
lead to poor QD adsorption on the substrate surface and therefore should be avoided. For this
concern, the relevant parameters are solvent vapor pressure (or the solvent evaporation rate) and
the distance over which the droplets need to travel before arriving at the substrate surface (called
tip-to-substrate distance). On the contrast, once the droplets arrive at the substrate surface, rapid
solvent evaporation is preferred to minimize the migration of QDs to the circumference of the
droplets, the so-called coffee stain effect, which would cause non-uniform distribution of QDs on
the substrate surface. For this concern, the relevant parameters are the solvent evaporation rate
and the solution feeding rate. Additionally, over high speed of carrier gas (nitrogen gas) would
smash the droplets onto the substrate surface, further spreading out QDs, and hence should be
avoided as well. For this concern, the relevant parameters are the pressure of nitrogen gas and the
tip-to-substrate distance.
4.3 Film Morphology and Device Characteristics
Optical micrographs of PbS QD film deposited via spin coating, dip coating, and spray
coating are shown in Figure 4-2. QD film deposited via spin coating and dip coating were
70
generally very smooth, although spin-coated film did exhibit more circular or comet-like defects.
AFM images (Figure 4-3) of spin-coated and dip-coated film confirm that the sub-micron
morphology of this film indeed is very similar to each other. Root means squared roughness of
the spin coated and the dip coated film is 3.9 nm and 5.1 nm, respectively. On the contrast, spray
coated film is much rougher as can been seen in Figure 4-4, resulting in root mean squared
roughness of 72.8 nm. The thicknesses of spin-coated, dip-coated, and spray-coated QD film are
150 nm, 170 nm, and 180 nm, respectively.
100 m
Figure 4-2. Optical micrographs of PbS QD film deposited via (a) spin coating, (b) dip coating,
and (c) spray coating.
35.0 nm
5pVm
r5Pm
Figure 4-3. AFM images of PbS QD film deposited via (a) spin coating and (b) dip coating.
71
A
6000
6888.5 A
5000
4000
3000
399
pm
2000
1000
400 PM
0
Figure 4-4. A three-dimensional profilometry image of spray-coated PbS QD film.
Figure 4-5 presents the J-V characteristics of AM 1.5G illuminated Schottky devices with
PbS QD film deposited via spin coating, dip coating, and spray coating. All of these devices
demonstrated similar J-V characteristics. The smaller shunt resistance and lower fill factor of the
spray-coated device may be attributed to its higher film roughness and thus higher device
leakage current.
2
U
E
0-
2.
3
4.
Spin Coating
-0.1
0.0
0.1
0.2
0.3
-4
-
0.4 -0.1
0.0
'
0.1
Dip Coating
'I
0.2
0.3
~4
~
0.4 -0.1
Spray Coating
0.0
0.1
0.2
0.3
0.4
Voltage (V)
Figure 4-5. J-V characteristics of PbS QD Schottky devices under simulated AM1.5G
illumination. PbS QD film in these devices was deposited via (a) spin coating, (b) dip coating,
and (c) spray coating.
Consequently, film morphology and J-V characteristics suggest that spray coating would
be a more reliable deposition technique when a thick film, rather than a thin film, is required. It
should be also noted that spray coating wastes less QD solution than spin coating [3] and spray
coating [4] do and thus is a promising technique when thick and large-area deposition is
72
necessary in terms of material saving. However, spin coating and dip coating are highly
recommended when a smooth film is needed. In a film-quality-demanding case, such as the
fabrication of PbS QD/CdS bilayer devices, dip coating has to be adopted to deposit a nearly-
defect-free film.
4.4 Conclusion
In summary, while spray coating can be a viable and cost-effective technology for the
deposition of large-area and thick QD film, dip coating provides the best film quality (in terms of
smoothness and reduced number of defects) among the three coating technique examined in this
thesis and hence is preferred when film quality is the top priority. Overall, the Schottky devices
exhibit poor PCE. It has been known that a Schottky diode (a majority-carrier device) typically
shows a higher dark current and thus a lower turn-on voltage and Vc than a pn diode (a
minority-carrier
device)
does
[5].
Additionally,
Fermi-level pinning
semiconductor interface is also responsible for low V,
at the
metal and
in a Schottky device [6]. Therefore, the
bulk of this thesis work was to develop a PbS QD solar cell based on a pn junction which will be
detailed in Chapter 5.
4.5 Reference
I
Luther, J. M.; Law, M.; Beard, M. C.; Song,
Q.;
Reese, M. 0.; Ellingson, R. J. & Nozik,
A. J., Schottky Solar Cells Based on Colloidal Nanocrystal Films, Nano Letters, 2008, 8,
3488-3492
2
Clifford, J. P.; Johnston, K. W.; Levina, L. & Sargent, E. H., Schottky barriers to
colloidal quantum dot films, Applied Physics Letters, 2007, 91, 253117
3
The majority of the solution is spun out of substrates and wasted during spin coating.
4
QD solution is contaminated due to repeated substrate immersion and necessitates routine
filtration which sacrifices some of the solution. In addition, QDs coated on the back side
of the substrate need to be removed and are wasted, too.
5
Green, M. A., Solar Cells, 1998, 176
73
6
Luther, J. M.; Law, M.; Beard, M. C.; Song, Q.; Reese, M. 0.; Ellingson, R. J. & Nozik,
A. J., Schottky Solar Cells Based on Colloidal Nanocrystal Films, Nano Letters, 2008, 8,
3488-3492
74
Chapter 5
PbS QD/CdS Bilayer Heterojunction Photovoltaic Devices
5.1 Introduction
Low-temperature solution-processed solar cells incorporating organic semiconductors [1,
2] and colloidal quantum dots (QDs) [3] are a potential alternative to conventional solar cells
fabricated via vacuum or high-temperature sintering processes for large-area, high-throughput,
and low-cost manufacturing. While there has been significant effort to synthesize low-band gap
organic semiconducting molecules for photovoltaic applications [4], PbS and PbSe QDs can
already be tuned across the near-infrared spectrum from k=2200nm to k=600nm wavelength [5,
6, 7], encompassing the range of ideal band gaps for optimum efficiency in both single- and
multi-junction solar cells [8, 9]. For example, tunable band structures facilitate implementation
of cascaded band gaps in the multi-junction stack geometry, which can optimize the light capture
of the solar cell stack. Since the report of the first PbS QD Schottky junction solar cell in 2008
[10] there have
photovoltaics.
been steady improvements in the power conversion
The highest
currently-reported
efficiency
efficiency of QD
of 7.0% was achieved through
passivation of the PbS QDs with halide anions and organic thiol, incorporating TiO 2 annealed at
520"C [11]. Recently, the low-temperature fabrication of PbS/Bi 2 S 3 QD bulk heterojunction and
PbS QD pn junction solar cells exhibiting efficiencies of 4.9% [12] and 5.4% [13], respectively,
were reported. Here, we explore an alternative heterojunction solar cell based on PbS QDs and a
low-temperature solution-processed CdS n-type inorganic semiconductor window layer adapted
from traditional CdTe photovoltaic technology.
Our fabrication process utilizes the chemical bath deposition technique, which has been
shown to be a low-temperature, solution-phase, and scalable method for the fabrication of
numerous inorganic semiconductors [14, 15]. For example, chemical bath deposition is used to
fabricate CdS thin films as n-type window layers for conventional CdTe and CIGS solar cells
with efficiencies as high as 15.8% [16, 17]. It has been shown that the superior performance of
CdTe solar cells coated with CdS is due to field quenching, attributed to the excitation of trapped
holes and recombination of electron-hole pairs in the CdS [18]. Analogous to this work, in the
75
present study we chose chemical-bath-deposited CdS to form a rectifying heterojunction with
PbS QDs, enabling us to assess the compatibility of this deposition process and PbS QDs.
5.2 Device Structure and Fabrication
Figure 5-1 (a, b) shows the device architecture and the band diagram of our PbS QD/CdS&
bilayer heterojunction solar cells. PbS QDs of a wide band gap (-1.5 eV, as determined from the
first absorption excitonic peak at X=850 nm shown in Figure 5-2) are used to ensure
energetically-favorable electron transfer from PbS QDs to CdS and to reduce the interfacial
recombination current [19]. Energy levels of the constituent materials are taken from the
literature [20, 21]. Light is illuminated on the heterojunction from the side of the PbS where
excited states are mainly generated in the smaller-energy-band-gap PbS QDs, and the larger gap
CdS can act as an optical spacer. Aspects of optical design are subsequently explored with the
use of optical simulations.
(a)
(b)
CdS
4.2 eV
Al
ITO
PbS QDs
ITO-Glass-
Al
4.2 eV
PbS
4.8 eVI
CdS
6.6 6V
Figure 5-1. (a) Schematic of the PbS QD/CdS heterojunction device fabricated in this study. (b)
Energy band diagram showing band edges of isolated 1.5eV PbS QDs and CdS along with
electrode work functions.
76
,-2.0
. ,
PbS QDs
M1.0 U
C
20.50
<0.
300
400
500
600
700
800
900 1000
Wavelength (nm)
Figure 5-2. Absorbance spectrum of PbS QDs in solution used in this work.
Material synthesis and device fabrication have been detailed in Chapter 3 and is briefly
summarized here. PbS QDs were prepared by a modified recipe reported by Hines et al. [5], then
deposited onto ITO-coated glass substrates using a layer-by-layer dip-coating method [22]
employing EDT ligand exchange to increase carrier mobility and insolubilize the QDs. The
thickness of the PbS QD layer was determined by the number of dip-coating cycles and ranged
from 40 nm to 280 nm as measured with a profilometer. CdS thin films were deposited onto PbS
QDs at 80 0C in an aqueous solution. The SEM image in Figure 5-3 displays the polycrystalline
nature of these chemical-bath-deposited CdS thin films with grain sizes around 200 nm. The
electrical conductivity measurement provided in Figure 5-4 reveals that CdS thin films are five
orders of magnitude more conductive than PbS QD thin films of similar thickness. The relatively
high conductivity of as-synthesized CdS thin films guarantees that the series resistance added to
the PbS QD devices is negligible, thus eliminating the need for further vacuum or annealing
processes. The cross-sectional SEM image and depth-profile XPS presented in Figure 5-5
confirm the layer structure of the PbS QD/CdS heterojunction devices.
77
Figure 5-3. SEM image of the CdS thin film on glass.
1E-5
1E-6
1E-7
1E-8
C
1E-9
1E-10
-+-PbS
+ CdS
1E-11
1E-12
-1.0
-0.5
I
0.0
0.5
1.0
Voltage (V)
Figure 5-4. Current-voltage characteristics of PbS QD and CdS thin films. The electrical
conductivities of PbS QDs and CdS were measured using a planar interdigitated configuration.
For PbS QDs, the channel length is 6 ptm, the channel width is 1.95 cm, and the film thickness is
100 nm. For CdS, the channel length is 120 pm, the channel width is 0.75 cm, and the film
thickness is 120 nm. Consequently, the conductivities of PbS QDs and CdS are ~-2 x10-8 and
-1
x
10-3 S/cm, respectively.
78
(a)
(b)
500
450
-Cd3p3/2
01s
-- Pb4f
-- S2s
400
350
Pt
300
Al2s
*~250
200
CdS
S150-
PbS QDs100
50
ITO0
.l..-
0
10
20
30
40
50
60
70
Sputter Time (min)
Figure 5-5. (a) Cross-sectional SEM image of the device. Platinum was deposited on top of the
device before focused ion beam milling to protect the underlying layers. (b) XPS depth profile
of
a PbS QD/CdS device without Al electrodes.
5.3 Protocols for the Fabrication of Reproducible Heterojunction
Devices
During the course of study, device yield and reproducibility had once been an issue such
that drawing a convincing and reliable conclusion had been proven difficult. J-V characteristics
of nine PbS QD/CdS devices made on the same substrate were plotted in Figure 5-6
to illustrate
this problem. The representative optical microscopy image (Figure 5-7) reveals that the
film in
these devices was characterized by pinholes. Pinholes can create shunting paths and
shorts in the
devices, leading to a leaking diode with I-V characteristics akin to those shown in
Figure 5-6.
Therefore, we attempted to find out the origin of pinhole formation.
79
20NJ'
E
15
-
Dark
-- AM1.5 G
10-
E
VLb
E
0
U -15
-1
-0.8 -0.6 -0.4 -0.2
0
0.2
0.4 0.6 0.8
1
Voltage (V)
Figure 5-6. J-V characteristics of nine PbS QD/CdS heterojunction devices fabricated on the
same substrate and measured under simulated AMI.5G solar illumination. The scattered J-V
characteristics manifest the device irreproducibility.
Figure 5-7. A representative top-view optical micrograph of a finished PbS QD/CdS device.
Multiple pinholes can be identified in the image.
Because this was a through-thickness pinhole, we were able to concentrate our attention
on its formation in either ITO or PbS QD layer. The fact that ITO is mechanically and
chemically robust during PbS QD and CdS deposition processes allow us further exclude the
possibility of pinhole formation in the ITO layer. Therefore, we could focus solely on the PbS
QD layer.
Intuitively, the question was whether the pinhole developed during or after PbS QD
deposition (i.e. during subsequent fabrication processes). From the optical micrographs shown in
Figure 4-2 and Figure 5-8(a), we know that the dip-coated QD film was nearly defect-free: there
80
particles but no pinhole observed. In addition, the Schottky devices fabricated
with the same recipe in the same batch exhibits reproducible J-V characteristics as shown in
Figure 5-8(b). This observation infers that the pinhole did not develop during PbS QD
were only few
deposition.
(a)
(b)
20
' 15
E
10
-- Dark
- AM1.5 G
E 54-J
o
0
-5-
4-J
-10
- 15
-'
91 -Ov8
0 0.2 0.4 0.6 0.8
Voltage (V)
-O.6 -0.4 -0.2
1
Figure 5-8. (a) A representative top-view optical micrograph of finished PbS QD Schottky
devices. No pinhole is identified in this image. (b) J-V characteristics of nine PbS QD Schottky
devices fabricated on the same substrate and measured under simulated AM1.5G
solar
illumination show good reproducibility.
After carefully comparing the PbS film before (Figure 5-9(a)) and after CdS deposition
(Figure 5-9(b) and (c)), we found that although some particles stayed in the film,
some were
peeled off from the film, forming pinholes. Therefore, we attempted to tackle the pinholes
by
minimizing their predecessors-particles.
81
Figure 5-9. Representative optical micrographs of (a) as-deposited PbS QD film, (b) PbS QD
film with a CdS overlayer and with no pinhole, and (c) PbS QD film with a CdS overlayer and
with a pinhole. The pervasive submicron dark spots in (b) and (c) are the CdS crystallites which
were initially formed in the bulk of the chemical bath solution and subsequently adsorbed and
embedded in CdS film.
After examining the fabrication process, we identified three possible routes for the
formation of particle defects in the PbS QD film and took the according measures to prevent.
First, the dust particles or other surface contaminants, if not eliminated during the substrate
cleaning procedure, on the ITO-coated glass substrates would be covered by PbS film and appear
as particle defects in the film. The optical micrograph (Figure 5-10) of the substrate undergoing
one cycle of the substrate cleaning process detailed in Chapter 3 shows the existence of dust
particles/contaminants. Therefore, to ensure particle- and contaminant-free surfaces, the
substrates underwent multiple cleaning cycles and were inspected under an optical microscope
prior to subsequent fabrication processes.
Figure 5-10. An optical micrograph of an ITO-coated glass substrate cleaned with the procedure
described in Chapter 3 still shows dust particles/contaminants (highlighted with the red circles)
adsorbed on the surface.
82
Second, we found that thoroughly-cleaned substrates were contaminated again during the
MPTMS treatment as shown in Figure 5-11. It was reported by Minghui Hu et al. that excess
water in solvents used for the treatment accelerates self-polymerization of MPTMPS molecules,
which not only become the sources of particle defects but also reduce the binding capability of
the adhesion layer formed by these molecules [23]. To avoid the self-polymerization, the
MPTMS treatment, including subsequent rinsing steps, was performed in a nitrogen dry
glovebox.
500
_____M
gim
Figure 5-11. Optical micrographs of ITO-coated glass substrates (a) before and (b) after MPTMS
treatment. The red circles in (b) highlight the surface contaminants.
Last, QDs with poor colloidal stability aggregates and grows into big particles. Figure 512 presents an optical micrograph of the PbS QD film deposited from a solution with poor
colloidal stability. It can be seen that the film is scattered with small spots. Figure 5-13
demonstrates that essentially all of the QDs in the solution added with excess amount of ethanol
were precipitated at the bottom of the tube after centrifugation. Excess alcohol, such as ethanol,
added in the purification process is linked to loss of surface ligands [24], which can cause the
aggregation of QDs. Therefore, to preserve the native ligands on QD surfaces and thus maintain
good colloidal stability, the amount of ethanol and acetone added to the QD solution during
purification process described in Chapter 3 was reduced, and two drops of native ligands (oleic
acid) were added to the final purified QD solution (~10 mL). Additionally, before and during the
film deposition processes, the QD solution was filtered with 0.02-miron AnotopTM syringe filters
to remove aggregated QDs.
83
500 pm
Figure 5-12. An optical micrograph of PbS QD film deposited from a solution of poor colloidal
stability. Multiple particle defects can be identified in the image.
Figure 5-13. Photographs of centrifuged PbS QD solution added with (a) excess and (b) reduced
amount of ethanol. Complete purification process should be referred to Chapter 3. The
supernatant in the solution with excess amount of ethanol is much clearer than that with reduced
amount of ethanol, suggesting that most of the QDs was precipitated out during centrifugation in
the solution with excess amount of ethanol.
After the implementation of the aforementioned measures, nearly-particle-free and
pinhole-free devices as shown in Figure 5-14(b) have been routinely made. The current densityvoltage characteristics (Figure 5-15) of the nine PbS QD/CdS devices fabricated according to the
newly-implemented measures on the same substrates also exhibit good reproducibility.
84
1
500 4m
1500
Figure 5-14. Optical micrographs of the PbS QD/CdS devices fabricated
(a) without and (b) with
additional measures to prevent pinhole formation. The red circles
in (a) highlight the particle
defects/pinholes. The blurred black spots in both images were the
residual of the QD film on the
back side of the substrates. During a dip-coating process, both
sides of the substrates were
coated, and the film on the back side was wiped off before the measurement.
Therefore, these
blurred spots are not in the device and can be ignored.
30
r4
E
20
U
-Dark
-AM1.5
G
E 10
U
-20
-30
-1
-0.8 -0.6 -0.4 -0.2
0
0.2 0.4 0.6 0.8
Voltage (V)
1
Figure 5-15. J-V characteristics of nine PbS QD/CdS heterojunction
devices fabricated with the
newly-implemented experimental methods on the same substrate
and measured under simulated
AMI.5G solar illumination.
85
5.4 The Comparison between Schottky and PN Heterojunction
Devices
The advantages of utilizing pn heterojunctions over Schottky junctions for photovoltaics
have been discussed by Pattantyus-Abraham et al. [25] In particular, heterojunctions can
minimize the Fermi-level pinning that frequently occurs at metal-semiconductor interfaces,
which limits the Schottky barrier height and V0 c. In addition, wide band discontinuities at pn
heterojunctions can suppress the contribution of electron or hole injection to the dark current and
benefit both the FF and the Vc. Figure 5-16 displays the representative dark and light J-V
characteristics of PbS QD Schottky and PbS QD/CdS bilayer heterojunction devices under
AM1.5G illumination. Both of these devices consist of a 200nm thick layer of PbS QDs,
prepared using the same EDT ligand exchange procedure, and sandwiched between an ITO
bottom anode and an Al top cathode. The Vc, J, and PCE of heterojunction devices
unambiguously outperform those of Schottky devices, indicating the effectiveness of using
chemical-bath-deposited CdS as the n-type semiconductor in heterojunction devices.
10
r4i
E
5-
E0
-5-
--
-15-
.2 -0.1
Schottky, dark
Schottky, AM1.5G
Heterojun., dark
Heterojun., AM1.5G
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Voltage (V)
Figure 5-16. J-V characteristics of typical PbS QD Schottky (red) and PbS QD/CdS
heterojunction (blue) devices measured in the dark and under AMi.5G simulated solar
illumination.
The applicability of CdS as an n-type material in pn heterojunctions is further
demonstrated by the inverted solar cell shown in Figure 5-17. The inverted CdS/PbS QD solar
cell with symmetric ITO top and bottom contacts displays inverted polarity compared with the
86
normal PbS QD/CdS, PbS QD Schottky, or ITO/PbS/ITO solar cells and an apparent V, of -0.32
V, indicating that the built-in potential of the CdS/PbS QD heterojunction is greater than the top
Schottky barrier that has been reported in several paper [22, 26, 27].
-N 2.0
E
E
-
1.5
1.0
0.5
0.0
In
-0.5
0 -1.0
-AM1.5G
-1.5
-dark
U -2n
-0.5-0.4-0.3-0.2-0.1 0.0 0.1 0.2 0.3 0.4 0.5
Voltage (V)
Figure 5-17. J-V characteristics of an inverted CdS/PbS QD heterojunction solar cell measured in
the dark and under AM1.5G simulated solar illumination. To verify the origin of the device
rectifying behavior, symmetric top and bottom ITO contacts were employed. Due to some spikes
of chemical-bath-deposited CdS thin films, a thick (310 nm) PbS QD layer was used in this
inverted device to avoid shorting, which may result in a worse photovoltaic performance than a
normal device.
5.5 PbS QD Layer Thickness-Dependent J-V Characteristics
Because the performance of thin film solar cells is highly dependent on the active layer
thickness due to a tradeoff between light absorption and carrier extraction, we studied the effect
of the PbS QD layer thickness on the J-V characteristics to optimize device performance.
Representative J-V curves for PbS QD/CdS heterojunction solar cells with various PbS QD
thicknesses (40, 100, 160, 220, and 280 nm) are plotted in Figure 5-18(a), and the photovoltaic
parameters are summarized in Figure 5-18(b). A decrease of V" with increasing thickness of
PbS QD film is observed, which is attributed to a reverse Schottky contact (non-ohmic contact)
that emerges at the anode, impeding electrical conduction and acting as an opposing diode in
series, thus reducing the voltage [26, 27]. When the thickness is small, the built-in potential from
the depletion region is sufficient to assist carriers tunneling through this non-ohmic contact.
While when the thickness is large, this non-ohmic contact becomes important. Nevertheless, the
87
PCE initially increases with PbS QD layer thickness to a maximum at 160 nm, then decreases
with greater PbS QD layer thicknesses. With a 160 nm thick PbS QD layer, we achieved an
average J,, of 15.3 ± 1.1 mA/cm2, Voc of 0.54 ± 0.02 V, FF of 0.42 ± 0.01, and PCE of 3.5 ±
0.3%. The trend in PCE is primarily correlated with that of the Js, as illustrated in Figure 5-18.
We therefore investigated the origin of the dependence of Js, on PbS QD thickness.
88
(a)
I
10
_
E 01
-10
-15
-20
-0.2 -0.1
0
0. 1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Voltage (V)
(b)
E
E
18
0.5
15-
0.4
g
2-0.3
L
6
0.1
S3.
00.70.6
0.0
4.0
3.5
3.0
*
0.5-
0.4.
2.5
u-3 2.0
1.5.
0.3
> 0.20.1
0.0
0
0.2
1.0.
50
100 150 200 250 300
PbS Thickness (nm)
0.5
0.0
0
50 100 150 200 250 300
PbS Thickness (nm)
Figure 5-18. (a) J-V characteristics of representative PbS QD/CdS heterojunction solar
cells with
varying PbS QD layer thicknesses. (b) Performance characteristics of the above devices.
The
error bars are evaluated from the standard deviation of 18 devices fabricated with
the same
condition for each thickness of the PbS QD film.
5.6 Modeling of PbS QD Layer Thickness-Dependent Jc
The generation of photocurrent in a pn junction solar cell can be described in terms
of
two fundamental processes [28]. The first process involves the absorption of incident
photons
89
and the creation of electron-hole pairs. Since the constituent materials and fabrication processes
are identical for the devices of different thickness, any variation in the generation efficiency of
electron-hole pairs following photon absorption can be neglected. Consequently, the first process
is determined only by the absorption of incident photons where it is well-known that the optical
electric field (and correspondingly the generation rate) inside multi-layered thin films with
reflective metal contacts can be modulated dramatically as a result of optical interference
between incident and reflected light [29, 30, 31]. In order to understand the spatial distribution of
generation rates, we employed an optical model as previously described by Pettersson et al. [32]
where the complex indices of refraction (reported in Figure 5-19) and layer thicknesses of the
materials were determined by spectroscopic ellipsometry and profilometry.
2.5
2
1.5
- CdS n
--- CdSk
1
.5
0
-- PbS n
PbS k
0--
400
500
600
700
800
900
1000
Wavelength (nm)
Figure 5-19. Complex refractive indices of PbS QDs and CdS.
The model assumes smooth interfaces between each layer, which is justified in this work.
Figure 5-20 shows the AFM surface images of PbS QD and CdS thin film. Although there are
some spikes observed on the CdS surface, the root mean squared roughness of CdS is ~5 nm,
which suggests most of the interface in the device is smooth. The reflectance measurement of the
full PbS QD/CdS device (without Al electrode) carried out with an integrating sphere also
suggests only a small diffuse reflectance across the visible region (Figure 5-21) where most of
photons are absorbed in the device. In addition, the relatively good agreement between the EQE
and modeled absorption spectra at low PbS thicknesses shown in Figure 5-22 indicates interface
scattering is not a significant effect. Therefore, the optical modeling was simplified without the
inclusion of interface roughness.
90
(a)
(b)
15.0 nm
50.0 nm
I
I
0.0
Height
1.5 um
0.0
Height
1.5 uM
Figure 5-20. AFM surface images of (a) the PbS QD thin film and (b) the CdS thin film.
100
. . . . . . . . . . . . .
90
U
a
In
80
70
60
50
40
30
20
10
0
300 400
500 600
700
800 900 1000
Wavelength (nm)
Figure 5-21. Diffuse reflectance spectrum of a full PbS QD/CdS device (without Al electrode).
91
(a)
.100
90
$
., 80
-r- 70
. 60
0 50
c 40
.2 30
t 20
L10
0
(b)
absorptionEQE
-
--
(e)
100
90
80
70
oc
4.OR60
0 50
C 40
0 30
U
20
Cu
10
U0
4-1
100
90
80
70
60
O..
-c
40 50
C 40
30
20
U
10
absorption
EQE
C 40
.0 30
S20
IL
0
400 500 600 700 800 900 1000
400 500 600 700 800 900 1000
Wavelength (nm)
(c)
.100
#
90
- 80
70
60
(d)
absorption-
-
-- EQE
400 500 600 700 800 900 1000
--6
7
'100
*90
+j 80
-r- 70
60
-O
o50
c 40
. 30
U 20
10
0
absorption
400 500 600 700 800 900 1000
Wavelength (nm)
Wavelength (nm)
-
Wavelength (nm)
absorption
EQE
400 500 600 700 800 900 1000
Wavelength (nm)
Figure 5-22. Measured EQE (red curves) and modeled active-layer absorption spectra (black
curves) of PbS QD/CdS devices with varying PbS QD thicknesses: (a) 40 nm, (b) 100 nm, (c)
160 nm, (d) 220 nm, and (e) 280 nm.
In Figure 5-22, the EQE spectra roughly match the absorption spectra for (a), (b), and (c).
The discrepancy seen in (a), (b), and (c) may be due to weak diffusive scattering [33] resulting
from slight interface roughness but more likely stems from slight inaccuracies in the thickness of
each layer and variations in refractive indices which can sensitively impact the optical
92
interference and cause shifts in the optical spectra; this conclusion is supported by the
observation that all the absorption spectra simply appear to be slightly shifted from the measured
EQE, but have the same spectral features (peaks, valleys, and tails). The large discrepancy in the
short-wavelength range seen in (d) and (e) is attributed to poor carrier extraction for those
carriers generated by short-wavelength photons since these photons are absorbed further away
from the PbS QD/CdS junction (generating a position dependent IQE). The reason this
wavelength (position) dependent IQE is not seen in (a)-(c) is because the thicknesses are smaller
than the depletion width.
The simulated wavelength- and distance-dependent photon absorption rates under
AMl.5G solar photon flux for the complete device with various PbS QD thicknesses are
illustrated in Figure 5-23. For ease of visualization, we also integrate the absorption rate with
respect to wavelength and plot the total distance-dependent exciton generation rate in the PbS
QD layers in Figure 5-24. It is found that the generation rate (or, correspondingly, the rate of
photon absorption) does not follow a simple exponential decay with thickness; instead, a local
maximum occurs near the interface between PbS QDs and CdS regardless of PbS QD thickness.
To elucidate whether such an generation pattern would favor a particular thickness of PbS QDs
and because carrier separation is supposed to take place at the heterojunction interface, we
integrate the full-spectrum generation rate displayed in Figure 5-24 with respect to the distance
from the PbS QD/CdS interface to obtain the integrated exciton generation rate, as depicted in
Figure 5-25. Integrated to a given distance from the interface, the generation rates are smaller for
thicker PbS QD films since more of the light is absorbed near the incident interface close to the
ITO. With this simulation result, we find that even though there is an optical cavity effect in our
devices, the thinner devices still have more excited states generated close to the PbS QD/CdS
interface depletion region than in the thicker devices.
93
(a)
Al
PbS CdS
ITO
x 1019
(b)
x
109
6
6
5
E
E
4
4
3
C
(U
CU
0M
2
2
Cu
1
1000M
100
200
Distance (nm)
0
(C)
PbS
ITO
CdS
300
0
0
100
200
0
300
Distance (nm)
Al
X1o 9
(d)
x 101
5
4
E
E
4
3
4-J
to
a
0j
W)
2
2
1
0
100
200
300
400
1
0
0
Distance (nm)
(e)
100
200
300
400
500
-0
Distance (nm)
,119
x 101,
5
E
4
-C
C
2
0M
1
0
100
200
300
400
500
0
Distance (nm)
Figure 5-23. Modeled photon absorption rate (1/sec-cm 3 ) for PbS QD/CdS heterojunction
devices with varying PbS QD thicknesses: (a) 40 nm, (b) 100 nm, (c) 160 nm, (d) 220 nm, and
(e) 280 nm. The x- axis (distance) is measured from the ITO surface where light is incident. The
horizontal stripes seen in the simulation plots result from the use of AM1.5G solar spectrum as a
light source for the calculated absorption/generation rate. There are multiple dips in the solar
spectrum due to light absorption by 03, 02, H2 0, and CO 2 .
94
E 2
40 nm
in
0
1
100 nm
0 ------------------------------------ --CDi 0-16
lnm
C 1220
01
nm
L1
280 nm
Co0
CU
0
50
100
150
200
250
300
Distance (nm)
Figure 5-24. The modeled total generation rates in PbS QD layers of different thicknesses (offset
for clarity).
E
U
14
Xx10
-
40nm
12
100nm
-- 160nm
10 220nm
a8
-280nm
i8,
u
4
24-0
0
50
100
150
200
250
300
Distance from the PbS/CdS Interface (nm)
Figure 5-25. The integrated generation rate (1/sec-cm 2 ) as a function of the distance from the
PbS QD/CdS interface for the above PbS QD thicknesses.
The second
process involved
in photocurrent
production is the extraction of
photogenerated carriers. As mentioned above, the carrier extraction efficiency is described by the
sum of excition/carrier diffusion length and depletion width relative to PbS QD thicknesses and
can be estimated by comparing the device data in Figure 5-18 with the optical modeling results
95
in Figure 5-25. The devices with 40nm and 220nm thick PbS QD layers exhibit similar Jc as
shown in Figure 5-18. Consequently, the amount of carriers extracted must be the same for the
two devices. Since 40 nm is rather thin compared to the convoluted length of typical depletion
width and diffusion length reported for PbS QDs [19, 27], we can assume that all of the carriers
generated in the device are extracted. This assumption is supported by the high internal quantum
efficiency (IQE) of 93% calculated from the ratio of measured to modeled Jsc (from the modeled
total generation rate and assuming 100% IQE) and the good agreement between the measured
and modeled J,, (assuming 100% IQE) integrated from the external quantum efficiency (EQE)
spectra at low PbS QD thicknesses as illustrated in Figure 5-26. From Figure 5-25, the integrated
rate of carrier collection of the 220 nm thick device is seen to match that of the 40 nm thick
device at a distance roughly 170 nm into the PbS film from the PbS/CdS interface; this distance
constitutes an estimate of the convolution of the diffusion length and the depletion width within
the 220 nm thick PbS QD layer.
20 . . . .
4-'15
E
E
10.
modeled
--
measured
150 200 250 300
*
0
50
100
PbS QD Thickness (nm)
Figure 5-26. J,, integrated from the modeled (black curve, assuming 100% IQE) and measured
(red squares) EQE spectra.
5.7 Capacitance Measurement
The depletion width can also be determined from a measurement of the device
capacitance. In order to obtain the depletion width in the PbS QD layer in our full device, we
measured the capacitance of PbS QD/CdS devices with various PbS QD thicknesses under zerobias conditions [34, 35] rather than implementing Mott-Schottky analysis in which a Schottky
junction or a pn junction with n or p much more heavily-doped than p or n must be employed
[36]. The capacitance is expected to decrease with increasing thickness as long as the thickness is
96
smaller than the depletion width, in which case the fully-depleted diode resembles a parallelplate capacitor with a dielectric thickness equal to the depletion width. However, when
the
thickness exceeds the depletion width, the capacitance should be insensitive to the change
of
thickness. The measured and modeled capacitances are plotted in Figure 5-27.
5 .
m Measured
A
4
LLA
o Modeled lower bound
A Modeled upper bound
A
AU
U
r2
4-A
3 .-
A
CU'
U0~
0
50
100 150 200 250 300
PbS Thickness (nm)
Figure 5-27. Measured (black square) and modeled (red circle for the lower bounds; blue
triangle
for the upper bounds) device capacitance at zero-bias as a function of PbS QD thicknesses.
The
lower bounds were calculated, assuming that both the PbS QD and CdS layer are fully-depleted;
while the upper bounds were calculated, assuming that only the PbS QD layer is
fully-depleted
and none of CdS layer is depleted.
The capacitances of PbS QD/CdS heterojunction devices were modeled based on simple
parallel-plate capacitors in series, in which case the capacitance C is given by
C=
-CPbS
=
CCdS
(PbS-E'
dPbS)
+
ECdS Eo
dCdS)
(5-1)
where Cpbs is the capacitance contributed from the PbS QD layer, Ccds
is the capacitance
contributed from the CdS layer, EPbs is the dielectric constant of PbS QDs (=
18) [34], Ccds is the
dielectric constant of CdS (= 8.9) [37], so is the permittivity of free space,
A is the device area (
= 1.24 mm 2 ), dPbs is the depletion width in the PbS QD layer, and dcds
is the depletion width in
the CdS layer ( = 50 nm for the lower bound; = 0 nm for the upper bound).
The modeled lower bound is calculated with the assumption that both the PbS
QD and
CdS layers are fully depleted, while the modeled upper bound assumes that the
PbS QD layer is
fully depleted but that the CdS layer is not depleted. However, the measured capacitance
deviates
from the modeled capacitance bounds and becomes relatively constant for PbS
QD thicknesses
97
greater than -150 nm, suggesting that the PbS QD layer is no longer fully depleted. The ~150 nm
depletion width implied from the capacitance measurement is consistent with the -170 nm
combined diffusion length and depletion width estimated from the optical model. The fact that
the highest J,, was obtained with a 160 nm thick layer of PbS QDs also correlates with this
characteristic length. As the PbS QD thickness is smaller than this length, the J,, increases with
the thickness owing to improved light absorption. However, when the PbS QD thickness exceeds
this length, more photons are absorbed in the quasi-neutral region and fewer photons are
absorbed in the combined diffusion length and depletion width region, causing inefficient carrier
extraction and a lower Jsc.
5.8 CdS Layer Thickness-Dependent Jsc
The optical model can be used to guide the device design as well. As mentioned earlier in
this chapter, CdS layer can also act as an optical spacer to modulate the optical electric field and
concentrate the light absorption in the PbS QD layer. Based on the optical model developed in
previous sections, we simulated the Jsc of the PbS QD/CdS devices as a function of CdS layer
thicknesses and plotted the results in Figure 5-28. It can be seen that the optimal CdS thickness
for maximum Jsc occurs at -50 nm, which is actually the thickness of CdS layer that have been
adopted in this study. However, it is also found that the change of the Jsc with respect to CdS
layer thicknesses is trivial, which can be rationalized with the distribution of optical electric
fields as shown in Figure 5-29. The distances between adjacent Jelectric field12 peaks are roughly
equal to the effective carrier extraction range (-170 nm) in the PbS QD layer, suggesting that the
concentration of optical electric field and accordingly the variation of Jsc due to the optical
spacer (CdS layer) is marginal.
98
16.2
16.0
E
E
15.815.6
15.4
15.2
-
15.0
0
14.814.6
0
50
100 150 200 250 300 350 400
CdS Thickness (nm)
Figure 5-28. Modeled short-circuit current density of the PbS QD/CdS devices with various CdS
layer thicknesses. The thickness of PbS QD layer for this modeling is fixed at 170 nm, which is
effective carrier extraction range estimated in the previous sections of this thesis.
99
(a)
301
(b)
Al
PbS
ITO
2
2
401
E 504
C
1.5C
1.5
J+--601
ba
C
1
-
701
~801
0.5
0.5
901
100
lu
200
Distance (nm)
2011
400
3UU
Distance (nm)
(d)
(c)
30,
3
40
1.5E
E so
2.5
C
2
~60
1
00
C
1.5
0
-~70
1
~80 0
0.5
0.5
90
100
(e)
3C
0
200
100
300
400
0
500
Al
CdS
ITO PbS
100
200
300
400
500
600
Distance (nm)
Distance (nm)
2.5
2
E
C
1.5
m 8
1
0 .5
ZOO
400
600
800
Distance (nm)
2
Figure 5-29. Simulated loptical electric field as a function of wavelength and position for
various CdS thicknesses: (a) 0 nm, (b) 50 nm, (c) 180 nm, (d) 280 nm, and (e) 400 nm. The
thickness of PbS QD layer is fixed at 170 nm.
100
5.9 Conclusion
In summary, we demonstrated the fabrication of PbS QD/CdS heterojunction devices
based on a low-temperature solution process with good device reproducibility and an average
PCE of 3.5%. Thickness-dependent J-V characteristics, optical modeling, and capacitance
measurements suggest that the combined diffusion length (~20 nm) and depletion width (150
nm) in the PbS QD layer is ~170 nm. The integration of colloidal QDs and low temperaturedeposited inorganic semiconductors in this work demonstrates the applicability of QDs in a
potentially scalable device fabrication process and also opens up the possibility of pairing PbS
QDs with other chemical-bath-deposited semiconductors, which may lead to further performance
improvement.
5.10 Reference
I
Brabec, C. J.; Gowrisanker, S.; Halls, J. J. M.; Laird, D.; Jia, S. & Williams, S. P.,
Polymer-Fullerene Bulk-Heterojunction Solar Cells, Advanced Materials, 2010, 22,
3839-3856
2
Walker, B.; Kim, C. & Nguyen, T.-Q., Small Molecule Solution-Processed Bulk
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104
Chapter 6
Efficiency
Improvement
in
Inorganic-Organic
Hybrid
Photovoltaic Devices via Solvent-Assisted Self Assembly
6.1 Introduction
Solution-processed nanostructured hybrid solar cells consisting of organic and inorganic
components have recently attracted considerable interest due to the combined advantages of both
material classes [1, 2]. In hybrid organic-inorganic bulk heterojunction (BHJ) solar cells [3, 4],
electron-donating conijugated polymers are blended with inorganic semiconductor nanomaterials,
taking advantage of their solution processability [5, 6]. So far, a variety of nanomaterials such as
GaAs [7], CdSe [8], PbS [9], TiO 2 , and ZnO [10, 11], nanowires or nanocrystals have been
investigated for applications in hybrid solar cells. To improve BHJ solar cell device efficiency,
several design criteria have to be taken into consideration. The domain size of the organic donor
and inorganic acceptor materials should be comparable or smaller than the exciton diffusion
length to increase the probability of exciton dissociation across the heterojunction interface. For
many conjugated polymers, the exciton diffusion length is less than 10 nm [12], but in standard
BHJ nanostructured solar cells processed from solution, device morphology cannot be precisely
controlled on such a small scale. In addition, efficient transport of charge carriers to their
respective electrodes before recombination is the key for high device current and voltage.
Therefore, high electron and hole mobilities, controllable nanomorphology, and a well-structured
interface are critical factors for developing efficient hybrid solar cells [13].
QDs are particularly well-suited for applications in hybrid BHJ solar cells because their
absorption can be tuned to cover a broad spectral range. QDs can have relatively high electron
mobility [14], and good photo and chemical stability. However, achieving a controllable
bicontinuous percolation network and a well-controlled interface between QDs and the polymer
matrix remain challenging due to the chemical difference between inorganic and organic
components. In addition, the inorganic QDs tend to vertically phase separate from non-polar
conjugated polymers at higher loading concentrations, decreasing the interfacial area [15], and
this phase separation of the photoactive films can be only partly mitigated by selection of solvent
105
and spin-coating conditions. Therefore, the next generation of hybrid QD photovoltaics will
require a strategy for controlling phase separation, increasing the interfacial area, and improving
the optoelectronic interactions of inorganic QDs and organic polymers.
Here, we demonstrate a facile method to synthesize and control the nanoscale
morphology of P3HT nanofiber and CdS QD hybrids using solvent-assisted self-assembly [16]
and ligand exchange methods [17]. Solvent-assisted self assembly and ligand exchange were
used to control the interface of P3HT nanofiber/CdS QD nano-hybrids and the CdS QD
interparticle distance, respectively. The effects of controlling morphology on the optical and
electrical properties are discussed. We find that the formation of an interpenetrating and
percolating P3HT nanofiber/CdS QD BHJ network gives rise to efficient charge transfer and
charge transport, significantly improving photovoltaic performance.
6.2 Device Structure and Fabrication
The synthesis of CdS QDs followed procedures reported by Peng et al. [18] and is
described in Chapter 3. After the synthesis, the CdS QDs were centrifuged, precipitated with
isopropanol, and redispersed in hexane. P3HT nanofibers were prepared using solution-phase
self assembly. Nonpolar P3HT was initially completely solvated in a nonpolar solvent, 1,2dichlorobenzene (1,2-DCB). To weaken the interaction between P3HT and the solvent, the
solvent polarity was increased by adding a marginal solvent, cyclohexanone. As a consequence,
the interaction among P3HT polymer chains becomes dominant, puffing P3HT into an
aggregated state. In this stage, the P3HT solution forms a dark purple gel whose viscosity can be
strongly decreased under manual agitation, indicating the formation of P3HT nanofibers.
To form the photoactive layer of the hybrid solar cell whose structure is shown in Figure
6-1(a), P3HT nanofibers were blended with CdS QDs. We selected CdS QDs due to the wide
offset between the HOMO level of P3HT and the conduction band edge of CdS QDs, which can
potentially provide large Voc (Figure 6-1(b)) [19]. Hybrid solar cells consisting of P3HT
nanofiber/CdS QD random-mixed and self-assembled structures were prepared by a solventassisted method. For the random-mix process, the same solvent, 1,2-DCB, was used for CdS
QDs and P3HT nanofibers. On the contrast, in the self-assembly process, the 1-D coaxial
nanofiber/QD structures were formed by first dissolving the P3HT nanofibers and CdS QDs into
1,2-DCB and octane, respectively, and then mixing these two solutions together. The P3HT
106
nanofiber/CdS QD blended solution was spun onto ITO-coated glass substrates to form a 140-nm
thick film. The coated devices were heat-treated at 175 *C for 10 min and quickly cooled to
room
temperature. Finally, the 10-nm thick bathocuproine (BCP) hole blocking layer
and the top
Mg/Ag electrode were evaporated and the final device area was defined as the overlap
between
the top and bottom electrodes.
(a)
(b)
Mg:Ag
ITO
- - - -PEDO
4.8 eV
5.0 eV
ITO
Glass
3.7 eV
P3HT
CdS
BCP
6.6eV
Figure 6-1. (a) The schematic device architecture and (b) the corresponding flat-band
energy
diagram.
6.3 Material Characterization: Random Mix versus Self Assembly
The photoactive material of our hybrid solar cells consists of the electron-donor P3HT
nanofibers and electron-acceptor CdS QDs. Figure 6-2(a), (b) shows representative
TEM images
of P3HT nanofibers and colloidal CdS QDs (-4 nm in diameter) capped with oleic
acid ligands,
respectively. As prepared P3HT nanofibers have an aspect ratio of 100 with an
average length of
1000±150 nm and diameter of 10±2 nm. Figure 6-3 shows the UV-VIS absorbance
spectra of
P3HT and CdS QD components. In comparison with amorphous P3HT phase,
a significant redshift and an additional vibronic feature (600 nm) are observed in the absorbance
spectrum of
P3HT nanofibers (Figure 6-3(a)). These results indicate the formation of
I-D self-organized
P3HT nanofibers with improved structural ordering. It has been reported that
the n-n interactions
between polymer chains lead to superior electronic properties with a two-order-of-magnitude
increase of the hole mobility in P3HT nanofibers [20].
107
(a)
(b)
Figure 6-2. TEM images of (a) P3HT nanofibers and (b) CdS QDs.
1.0
(b)
(a)
1.0 -
nanofiber
amorphous
0.8-
0.8-
0.6 -
(
0.6 C
.2.
2
~~0.
CU
0
0
0.2-6
~0.2
0.0
300
400
5
6
700
Wavelength (nm)
800
0.0
350
400
450
500
550
600
Wavelength (nm)
Figure 6-3. UV-VIS absorbance spectra of (a) amorphous P3HT, P3HT nanofribers, and (b) CdS
QDs.
Because the diameters of P3HT nanofibers are below the exciton diffusion length of
P3HT phase, we studied the effect of the self-assembly process on exciton dissociation between
P3HT nanofibers and CdS QDs by comparing a randomly blended system (random-mix process)
with 1-D coaxial nanostructures formed by decorating P3HT nanofibers with CdS QDs (selfassembly process). A key to the self-assembly process is to introduce a poor solvent, allowing
P3HT nanofibers and CdS QDs to form aggregates to minimize their surface energy. In the
present study, the P3HT nanofibers and CdS QDs were dissolved in 1,2-DCB and in octane,
respectively. Octane is known to be a good solvent for CdS QDs but a poor solvent for P3HT.
108
Therefore, we expect CdS QDs preferentially attach to the surface of P3HT nanofibers
to reduce
surface energy. Figure 6-4(a) shows a TEM image of the randomly blended
P3HT/CdS system;
the CdS QDs are randomly distributed and there is no obvious ordering interaction
between
P3HT nanofibers and CdS QDs. To increase the organic-inorganic interaction and
maximize the
interfacial area, we self-assembled CdS QDs onto one-dimensional
(1-D) P3HT nanofibers,
producing P3HT nanofiber/CdS QD coaxial hybrids (Figure 6-4(b)). This architecture
carries
advantages over a randomly distributed system because phase segregation
is minimized and the
P3HT nanofiber/QD interface is maximized, which can potentially improve
the dissociation of
photo-generated excitons in P3HT nanofibers due to the conduction band offset
at the interface.
Figure 6-5 shows the film absorbance and solution photoluminescence (PL)
spectra produced
from random-mixed and self-assembled P3HT nanofibers with 80 wt% CdS
QDs. The vibronic
absorption feature at ~600 nm in the self-assembled film is clearly resolved,
indicating that the
crystalline P3HT nanofibers are preserved after CdS QD self assembly. The
PL intensity of the
P3HT nanofiber/CdS QD self-assembled film is significantly quenched, presumably
due to the
efficient nonradiative channel for charge transfer from P3HT nanofibers to
CdS QDs.
(a)
(b)-'1 '
Figure 6-4. TEM images of P3HT nanofiber/CdS QD film synthesized
(a) without self-assembly
(random-mix process) and (b) using a solvent-assisted self-assembly
process. The inset images
show schematic representations of each; the random-mix and self-assembly
method is used to
control the interface between CdS QDs (yellow spheres) and
P3HT nanofibers (purple lines).
109
(a)
(b)
0.8
7
Random m ix
6
0.
0.6
0
e (au)
C
C
3.2
5
4
32-
400
300 350 400 450 500 550 600 650 700
Wavelength (nm)
j
Self-assem bly
Soo
600
700
800
900
Wavelength (nm)
Figure 6-5. (a) An UV-VIS absorbance spectrum of self-assembled P3HT nanofiber/CdS QD
film. (b) Solution PL spectra of the random-mixed and self-assembled P3HT nanofiber/CdS QD
samples.
In order to understand the bonding structure of the self-assembled P3HT nanofiber/CdS
QD hybrids, XPS spectra of the samples without and with self-assembly were obtained (Figure
6-6(a,b), respectively). We focus on the XPS spectra of S 2p peaks to more precisely identify the
bond formed during self assembly. Figure 6-6(a) shows the S 2p peaks for random-mixed
samples where the S bonding energies is well matched through peak deconvolution with the
independent P3HT and CdS components. In comparison, XPS spectrum of the self-assembled
P3HT nanofiber/CdS QD hybrids (Figure 6-6(b)) reveals a new intermediate bonding energy
which can be assigned to the C-S-Cd bond, connecting the P3HT polymer backbone and CdS
QDs [21]. These findings are further supported by transient PL measurements of P3HT
nanofiber/CdS QD hybrids with and without self-assembly (Figure 6-7), which show that selfassembled samples have a shorter exciton lifetime and thus faster exciton dissociation. Taken
together, these results can be understood as follows: the self assembly of CdS QDs onto the
surface of highly ordered P3HT nanofibers increases the interaction between these two materials
and creates a large number of interfaces for charge transfer [22], which results in a shorter PL
lifetime, quenched PL intensities, and increased charge transfer rates.
110
Random-mixed
(b)
Self-
CsCd
o 5. assembled
Mo4
3P3HT
3
A
C
162
A
2
-
162
156
158
160
Binding Energy (eV)
156
160
158
Binding Energy (eV)
Figure 6-6. XPS spectra of high resolution S 2p in (a) random-mixed and (b) self-assembled
P3HT nanofiber/CdS QD film.
10
-- Self-assembled
Random-mixed
10
10
10n
4-J
10
10
U
-D
N
1024
-
E
0
104
-10
0
10
20
30
40
50
Time (ns)
Figure 6-7. Time-resolved photoluminescence spectra of the random-mixed and self-assembled
P3HT nanofiber/CdS QD film. The film was excited by a pulsed diode laser (PicoQuant) at 414
nm, 2.5 MfHz. Emission from the film was collected by an avalanche photodiode (PicoQuant)
with a 460nm band-pass filter. Photoluminescence decay of the sample was measured with a
time-correlated single photon counting module (PicoQuant).
It was recently demonstrated that the performance of solar cells based on PbS QDs can be
improved by ligand-exchange treatments involving thiols [23]. The beneficial effect of thiols has
111
been attributed to a shortened QD spacing, improved cross-linking of QDs, and improved surface
passivation. The photoconductivity of different QDs has been shown to be largely enhanced with
a consequence of the ligand exchange treatment [17]. We performed the ligand exchange in our
organic P3HT nanofiber-inorganic CdS QD hybrid system to shorten the interdistance between
CdS QDs and P3HT nanofibers and among CdS QDs themselves. The CdS QDs were originally
capped with oleic acid and n-butylamine surfactants, which were removed through an
ethanedithiol (EDT) treatment in acetonitrile solvent. Acetonitrile is a relatively good solvent for
ligands, but a poor solvent for P3HT nanofibers. The solvent quality induces ordering in the
polymer phase and changes the degree of phase separation. The surface morphology before and
after the ligand exchange was investigated using AFM and TEM (Figure 6-8(a)-(c), respectively)
and it was found that the P3HT nanofiber/CdS QD film forms small agglomerates after the
ligand exchange, which indicates that CdS QDs get closer to one another.
Figure 6-8. Tapping mode AFM images of P3HT nanofiber/CdS QD thin film (a) before and (b)
after EDT ligand exchange. (c) TEM image of curved P3HT NWs after the ligand exchange.
6.4 Device Characterization
Next, we examined the effect of the controlled nanoscale morphology on electrical
properties. To directly measure charge recombination rates we fabricated a device after self
assembly but with and without ligand-exchange treatment, and performed small perturbation
transient V,, decay measurements [24, 25]. In this measurement, a low-intensity pulsed laser
light at 520 nm wavelength is used to induce a small perturbation to the Vc by transiently
generating additional electrons and holes. The resulting additional transient photovoltage (V)
then decays with a lifetime that is determined by the recombination rate constant (kre) of the
electrons and holes. In Figure 6-9, kec is shown as a function of the solar simulator light intensity.
112
For all devices, we observe that the EDT-treated device exhibits k,,c approximately
two times
higher than that of the non-treated device, which is likely due to the fact that
photogenerated
charge carrier concentrations of EDT-treated devices are significantly higher than
that of the
pristine device. The ligand exchange process reduces the distance among
CdS QDs and P3HT
nanofibers and thus increases the concentration of free charge carriers
due to more efficient
exciton dissociation, consequently increasing the charge recombination rate
constant. The charge
carrier concentration and charge transport is highly dependent on the carrier
mobility of the
materials, which also strongly influences the photovoltaic performance. Therefore,
both electron
and hole mobility in CdS and P3HT components should be matched and high
enough to extract
charges efficiently and prevent build-up of space charge. We have tested
the electron-only and
hole-only devices with self-assembled hybrid film (Figure 6-10), which
indicates the wellmatched electron and hole mobility after the EDT treatment process.
We note that the
conductivity of P3HT nanofibers is reduced by the EDT treatment, which
may result from the
decreased P3HT molecular ordering as can be manifested as the curving
features shown in the
TEM image (Figure 6-8(c)).
6x1 03
5x 10
4x103
3x103
S2x1 0
Pristine
EQ
103
10
100
Power Dens ity
(mnW/cm)
Figure 6-9. Charge carrier recombination rate constants krec of pristine
and EDT-treated devices
determined by transient open circuit voltage decay measurements at different
illumination levels.
113
(a) 0.004
(b) 0.'014
electron W EDT
electron w/b EDT
0.003
0. 03
0.002
0. 02
E 0.001
E 0. 01
0.000
0. 00'
-0.001
-0. 01
0 -0.002
-hoe/EDT
hole w/o E DT
-
0 -0. 02
-0.003
-0. 03-
-_n flA
5
-4 -3
-2 -1
0 1
Voltage (V)
2
3
4
5
-4
-3
-2
-1
0 1
Voltage (V)
2
3
4
5
Figure 6-10. (a) J-V characteristics of an electron-only P3HT nanofiber/CdS QD device before
and after EDT treatment. (b) J-V characteristics of a hole-only P3HT nanofiber/CdS QD device
before and after EDT treatment. The effect of EDT treatment and P3HT nanofiber curving on the
transport properties of P3HT nanofiber/CdS QD film was demonstrated by the DC J-V
characteristics of the electron-only and hole-only devices. The electron-only device consisted of
Ag:self-assembled monolayer (SAM), P3HT nanofiber/CdS QD, and Mg:Ag layers. The
Ag:SAM layer blocks the hole injection [26]. The treated electron-only device shows a strong
enhancement of electron transport over the untreated device. The hole-only device consisted of
ITO, PEDOT:PSS, P3HT nanofiber/CdS QD, and Au layers, where the Au layer prevents
electron injection. The treated device showed a decrease in current density. This result suggests
that the EDT treatment reduces the hole transport.
Following these promising measurements, devices based on self-assembled and EDTtreated films were prepared with the structure described in Figure 6-1(a). The bright-field TEM
image of a cross-section of a typical complete device (Figure 6-11(a)) shows that CdS QDs were
uniformly distributed across the 150 nm thick photoactive layer. The self assembly of CdS QDs
onto P3HT nanofibers is confirmed using energy dispersive x-ray spectroscopy elemental
mapping image (Figure 6-11(b) and Figure 6-12). In comparison with the QD vertical phase
separation that often occurs in randomly blended inorganic-organic hybrid devices [15], the
image suggests that the P3HT nanofiber structure and self-assembly process plays a crucial role
in templating CdS QDs across the vertical direction and improving the interface between P3HT
nanofibers and CdS QDs.
114
CP
Cd$.-P3HT
Figure 6-11. (a) Cross-sectional TEM image of P3HT nanofiber/CdS
QD hybrid solar cells. (b)
Energy dispersive X-ray spectroscopy elemental mapping (Red color:
Cd; Green color: In; and
Blue: Ag) of the image shown in (a).
Figure
6-12.
Cross-sectional
TEM
EDS
mapping
image
of
the
ITO/PEDOT:PSS/P3HT:CdS/BCP/Mg:Ag hybrid solar cell device.
(a)-(d): The Mg, Cd, S, and
In element mapping images.
Figure 6-13 shows the current density-voltage characteristics
of the P3HT nanofiber/CdS
QD hybrid photovoltaic devices using random-mixed film
and self-assembled film as a
photoactive layer. The device at 80 wt% CdS QDs without
self assembly shows Jsc of 1.92
115
mA/cm 2 , Voc of 0.78 V, and a FF of 0.41 under simulated AM 1.5 illumination (100 mW/cm 2 ),
and gives power conversion efficiency of 0.61%. In contrast, the device with the self-assembly
process exhibits higher Jsc of 5.75 mA/cm 2, Voc of 1.00 V, a FF of 0.39, and power conversion
efficiency of 2.24%. The enhancement of the photovoltaic performance in the device with the
self-assembly process was attributed to the increased charge transfer efficiency in the
photoactive film, enabled by self-assembling CdS QDs onto highly ordered P3HT nanofibers as
evidenced in previous sections.
2
Eo
E>
n
-2
4-
40
-6
Random-mixed
Self-assembled
-8
-10
--
-
-
-
-
-I
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Voltage (V)
Figure 6-13. J-V characteristics of random-mixed (blue) and self-assembled (red) P3HT
nanofiber/CdS QD solar cells.
Figure 6-14 (a) shows the current density-voltage characteristics of the pristine and EDTtreated P3HT nanofiber/CdS QD hybrid photovoltaic devices, and Figure 6-14(b) shows a
summary of the Jsc and Voc as a function of CdS wt% for the samples after the self assembly
and EDT treatment. Our results show that a significant improvement of the photovoltaic
performance is achieved in hybrid devices fabricated using self assembly and EDT treatment
processes, giving an improved Jsc of 10.96 mA/cm 2, Voc of 1.06 V, FF of 0.35 and the
maximum efficiency of 4.06% (average power conversion efficiency of 3.2% measured over 100
devices). The improvement of the performance in the device after EDT treatment originated from
the enhanced charge transfer efficiency and charge collection efficiency provided by shorter
116
P3HT nanofiber/CdS QD distances and QD/QD
distances, which has been supported by the
microscopy images (Figure 6-8) and conductivity
measurement (Figure 6-10) presented in
previous sections.
(a)
4
. ,
. , . ,
,
, .
(b) 12
.
2
E
E
4-J
LI
(U
1+~*Voc
0
-2
-4t
-6
-8-
0.7 8
04
-10-12
-
Pristine
-14
-
EDT-treated
-16
- ' - ' - ' - ' - ' . '
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
1.1
-T 1.0
0.9
0.8E
2.
0.6>
0.5
0.4
1.4
Voltage (V)
50
60
70
80
90
CdS Weight Concentration (W/o)
Figure 6-14. (a) J-V characteristics of pristine (red)
and EDT-treated (green) P3HT
nanofiber/CdS QD solar cells. (b) The photovoltaic performance
summary (Jsc and Voc) of the
EDT-treated and self-assembled devices as a function
of the CdS weight concentration.
6.5 Conclusion
In summary, we have presented a facile and simple method
to synthesize and control the
nanomorphology of P3HT/CdS coaxial nanofiber BHJ
hybrid solar cells. The self assembly of
CdS QDs onto crystalline P3HT nanofibers results in
not only the controllable dispersion of CdS
QDs within the P3HT matrix, but also stronger electronic
interaction between CdS QDs and
P3HT nanofibers. The formation of bicontinuous
donor/acceptor phases and a well-defined
interface in the hybrid photoactive film through self
assembly and ligand exchange can largely
enhance charge transfer and transport efficiency, which
are essential for hybrid inorganic-organic
solar cells. The results shown here represent a direction
for increasing the efficiency of hybrid
solar cells via enhancing the interfacial interactions
and controlling nanomorphology between
the organic and inorganic materials.
117
Author Contributions
This chapter is an equal contribution between Dr. Shenqiang Ren and' Liang-Yi Chang.
Dr. Shengqiang Ren is responsible for the device fabrication and electrical measurements. LiangYi Chang is responsible for the material synthesis and material characterization of the work.
6.6 Reference
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120
Chapter 7
Outlook
7.1 Conclusion
Colloidal QDs and organic semiconductors are promising building blocks for lowtemperature, solution-processed, and hence low-cost solar cells. This work developed the solar
cell employing these two material systems with the focus on the junction interface-pn
heterojunction in an all-inorganic solar cell and donor/acceptor interface in an organic/inorganic
solar cell. The findings and contributions of this thesis are summarized below:
a) Dip coating provides the best film quality (in terms of surface morphology) and is preferred
in the application where a defect-free film is a must, such as PbS QD/CdS bilayer
heterojunction devices. However, in most cases, such as QD Schottky devices, ZnO/PbS QD
devices, and inorganic/organic hybrid QD devices, spin coating is good enough and results in
device performance on a bar with that of dip-coated devices. On the other hand, the film
deposited by spray coating is considerably rougher and can cause leakage or shunting issues
in thin devices, but we also noticed that spray coating is an economic deposition technique in
view of its solution/material consumption. Therefore, spray coating may be an attractive
technique when large-area and thick coating is essential.
b) Prior to this work, PbS or PbSe QD heterojunction photovoltaic devices relied on sintered or
sputtered n-type transition metal oxides such as ZnO and TiO 2 to form rectifying junctions
with p-type PbS or PbSe QDs. This thesis demonstrated a new junction, namely PbS QD/CdS,
with a low-temperature solution process that has not been employed in this field before. The
resulted PCE was comparable to that of devices based on sputtered or sintered ZnO when
using similar dithiol treatments on PbS or PbSe QDs.
c) The measured thickness-dependent Jsc and device optical modeling suggested a combined
exciton/carrier diffusion length and depletion width of -170 nm, consistent with the
depletion width (~150 nm) measured by capacitance measurement, and implied a short
exciton/carrier diffusion length of~20 nm.
121
d) The PbS QD/CdS device yield and reproducibility issue stemming from pinholes was
successfully addressed, and a procedure for the fabrication of highly-reproducible devices
was created.
e) P3HT nanofibers were pre-formed in a good solvent (1,2-DCB) by the addition of a marginal
solvent (cyclohexanone). The resultant thin film spin-cast from the nanofiber solution created
a three-dimensional percolation network for carrier collection.
f)
P3HT nanofiber/CdS QD blended film prepared with mixed solvents (octane and 1,2-DCB)
revealed preferential decoration of CdS QDs on P3HT nanofibers and stronger interaction
between CdS QDs and P3HT nanofibers as opposed to the film prepared with single solvent
(1,2-DCB). The J-V characteristic of the device fabricated with the mixed-solvent scheme
also exhibited better photovoltaic performance than that of the device fabricated with single
solvent.
7.2 Future Work
Although reasonable PCE has been demonstrated using a low-temperature solution
process, further investigations are required for closing the gap between the current performance
and the threshold performance for commercialization. Possible improvements can be achieved
through both the device structure and material aspects which are outlined below:
a) Carrier concentration in PbS QD layer was shown to increase by five-fold after exposing to
the aqueous bath during CdS deposition, leading to a narrower depletion width and less
efficient carrier separation in the layer. Therefore, to avoid exposing PbS QDs to the aqueous
bath, the development of an inverted CdS/PbS QD device in which the CdS layer will be
deposited prior to the PbS QD layer is proposed.
b) As mentioned in Chapter 2, chemical treatment can modulate doping concentration, carrier
mobility, and trap states in QDs. Consequently, a treatment that can realize graded doping to
increase built-in potential and simultaneously maintain low doping level in the absorber layer,
that can improve carrier mobility, and that can diminish the density of trap states, especially
those deep in the band gap, should be optimized and integrated into the device processing
steps.
c) On the other hand, carrier concentration in the window layer (CdS) should be raised to
increase built-in potential and extend the depletion region in the absorber layer. Other
122
chemical-bath-deposited semiconductors can also be explored to form rectifying junctions
with PbS QDs.
d) The crossover between photo and dark J-V characteristics may originate from the previouslyreported Schottky barrier between ITO and PbS QD layers, opposing to the main diode
between PbS QD and CdS layers in the PbS QD/CdS device or between PbS QD and Al
layers in the PbS QD Schottky device. Therefore, the interface between ITO and PbS QDs
should be further engineered (e.g. insertion of a buffer layer, MoO 3 ) to reduce contact
barriers.
e) In the P3HT nanofiber/CdS QD hybrid device, the morphology of CdS QDs can be
experimented (such as the employment of nanorods) to improve carrier transport through the
QD phase.
f)
Lower-band-gap acceptor QDs can also replace CdS QDs to increase light absorption in the
red and near-infrared regions.
g) Once the efficiency of the QD photovoltaic device approaches the threshold (~10 % under
favorite policies and economics) for commercialization, the lifetime of the device has to be
shown to be at least 10 years, therefore requiring substantial efforts to achieve the stability
under ambient conditions.
123