University of Southern California Dissertations and Theses

Squaraines and their applications to organic photovoltaics

(USC Thesis Other)

Squaraines and their applications to organic photovoltaics

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content
SQUARAINE DYES AND THEIR APPLICATIONS TO ORGANIC
PHOTOVOLTAICS

by

Siyi Wang

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)

August 2012

Copyright 2012 Siyi Wang

ii

Dedication

Dedicated to My Parents:
Mr. Jianguo Wang and Ms. Lili Zhang

iii

Acknowledgments
I would like to thank many people for their enormous assistance, contributions
and encouragement. Without their support, I will not come to point to finish this
dissertation.
Foremost, I would like to express my appreciation to my advisor Prof. Mark E.
Thompson for his long term mentoring, instruction and guidance on my way to purse
science. With his guidance on research, I was able to keep research on the right loop. I
would like to thank him for the freedom he provided that I was able to work on projects I
am really interested in. Besides my advisor, I would like to thank the rest of my
qualifying exam and dissertation committee members: Prof. Surya Prakash, Prof. Stephan
Haas, Prof. Hanna Reisler and Prof. Kyung W. Jung for their comments, insightful
suggestions and continuous encouragement.
I would like to thank Dr. Vyacheslav V. Diev for technical training, research
mentoring and helpful discussions. Also, same appreciation goes to Prof. Peter I.
Djurovich. All the thoughtful and meaningful discussions accompanied me all the way to
be a real scientist.
I would like to give my sincere appreciation to Prof. Stephen R. Forrest, Dr.
Guodan Wei, Xin Xiao, Brian Lassiter and Dr. Jeramy Zimmerman. With more than four
years’ collaboration, we have finished comprehensive and systematic study of squaraine
based OPVs, which make squaraines well recognized in organic photovoltaics research
community. I would like to extend my gratitude to Prof. Lincoln Hall, Dr. Alberto Bossi,
Dr. Wei Wei, Dr. Dolores Perez, Dr. Cody Schlenkler, Dr. Tissa Sajoto, Dr. Paulin

iv

Wahjudi, Dr. Kenneth Hanson, Yifei, Sarah and Francisco for their help and friendship in
MET group. In addition, I would like to thank Judy Hom for being very helpful in
organizations and administrations.
My thanks are also due to other staff members of Chemistry Department
especially Michele Dea, Katie Mckissick, Heather Connor and Marie de la Torre for their
tactical support. Allan Kershaw, Ross Lewis and Phil Sliwoski are also acknowledged for
their help with NMR, DSC, PotentioStat and Glass blowing.
Finally, I would like to take this opportunity to give special thanks to my parents
Mr. Jianguo Wang and Ms. Lili Zhang who gave me endless support through my life.
Especially, I would like to thank my fiancé Mr. Congxing Cai for spiritual support and
tolerance along the way.

v

Table of Contents

Dedication ........................................................................................................................... ii
Acknowledgments.............................................................................................................. iii
List of Tables .................................................................................................................... vii
List of Figures .................................................................................................................. viii
Abstract .............................................................................................................................. xv
Chapter 1. Introduction ........................................................................................................ 1
1.1 Energy from Sunlight ............................................................................................. 1
1.1.1 Abundance of Solar Energy .......................................................................... 1
1.1.2 Solar Spectrum ............................................................................................. 2
1.2 Comparing Organic and Inorganic Phovoltaics ...................................................... 4
1.2.1 Review of Inorganic Phovoltaics .................................................................. 4
1.2.2 Organic Photovoltaics Review ..................................................................... 6
1.2.3 Operation Principles ................................................................................... 10
1.2.4 Device Architectures .................................................................................. 11
1.2.5 Performance Evaluation of OPVs .............................................................. 15
1.3 Active Donor Materials for OPVs ........................................................................ 18
1.4 Seeking for New Materials Squaraines ................................................................. 25
1.5 Chapter 1: References ........................................................................................... 27
Chapter 2. Squaraines ........................................................................................................ 35
2.1 Introduction ........................................................................................................... 35
2.1.1 General ....................................................................................................... 35
2.1.2 Applications of Squaraines ......................................................................... 37
2.2 Review of Squaraine Synthesis ............................................................................ 43
2.2.1 General ....................................................................................................... 43
2.2.2 Product Contamination with 1,2 Substituted Derivatives .......................... 44
2.2.3 Ester route for squaraine synthesis ............................................................. 46
2.2.4 Unsymmetrical squaraine synthesis ........................................................... 47
2.3 Arylanilino Squaraine Synthesis ........................................................................... 49
2.3.1 Symmetrical Arylanilinosquaraine Synthesis ............................................. 49
2.3.2 Unsymmetrical Arylanilino Squaraines synthesis ...................................... 53
2.4 Summary ............................................................................................................... 56
2.5 Chapter 2: References ........................................................................................... 58
Chapter 3. Squaraine Photophysics.................................................................................... 61
3.1 Electronics Properties ........................................................................................... 61

vi

3.2 Thermal Stability .................................................................................................. 64
3.3 Single Crystal Data and Film Packing .................................................................. 66
3.4 Electrochemistry ................................................................................................... 71
3.5 UV-Vis of Solution and Films .............................................................................. 73
3.6 Photolumiescence Property .................................................................................. 74
3.7 Twist Insisted Charge Transfer State .................................................................... 76
3.8 Summary ............................................................................................................... 77
3.9 Chapter 3: References ........................................................................................... 79
Chapter 4. Squaraine Application in OPVs ....................................................................... 81
4.1 Vapor Deposited Squaraine OPVs ........................................................................ 81
4.1.1 Ultrathin Optimal Devices .......................................................................... 81
4.1.2 High V
oc
and Small Dark Current .............................................................. 82
4.1.3 Additional NPD Layer Covers Noncontinuous SQ 1 Film ........................ 84
4.2 Solution Processed Bilayer Squaraine OPVs ....................................................... 87
4.2.1 Solution Processed Alkylanilino Squaraine OPVs ..................................... 88
4.2.2 Solution Processed Arylanilino Squaraine OPVs ....................................... 90
4.2.3 High FF achieved by Compound Blocking Layer ...................................... 93
4.3 Solution Processed Bulk Heterojunction Squaraine OPVs .................................. 97
4.3.1 Low FF and Charge Carrier Mobility for SQ 1 Bulkheterojunction
OPVs ................................................................................................................ 97
4.3.2 Thermal Annealing Promotes Crystalline Squaraine Surface .................. 106
4.3.3 Solvent Annealed Squariane Bulk Heterojunction OPVs ........................ 113
4.4 Broad Solar Spectral Coverage ........................................................................... 120
4.5 Summary ............................................................................................................. 130
4.6 Chapter 4: References ......................................................................................... 132
Chapter 5. Large Scale Synthesis of Squaraines .............................................................. 136
5.1 Why Large Scale Synthesis? .............................................................................. 136
5.2 Difference between Academic Labs and Industry .............................................. 138
5.2.1 Safety ........................................................................................................ 138
5.2.2 Ideas to Real Products .............................................................................. 140
5.2.3 Detailed reaction plan and notebook record ............................................. 141
5.3 Large Scale Process of Squaraines ..................................................................... 142
5.3.1 An Air Stable Ligand ................................................................................ 143
5.3.2 The Purity of Precursor and Final Product ............................................... 144
5.3.3 Purity Characterization ............................................................................. 145
5.4 Summary ............................................................................................................. 147
5.5 Chapter 5: References ......................................................................................... 148
Bibliography .................................................................................................................. .. 149
Appendix A: Chapter 2 Supplemental information ......................................................... 164
Appendix B: Chapter 3 Supplemental information ......................................................... 175

vii

List of Tables

Table 3-1. Summary of crystal data, data collection and refinement parameters for
SQ 1 and SQ 3........................................................................................................ 68
Table 3-2. Electrochemical redox potentials of squaraines 1–10.
a
.................................... 72
Table 3-3. Photophysical data of SQ 1-10 in solution and as thin films. .......................... 75
Table 4-1: Photovoltaic device performance under 1 sun AM 1.5G simulated solar
illumination. The device structure is
ITO/Donor/C
60
(400Å)/BCP(100Å)/Al(1000Å). .................................................... 85
Table 4-2. Photovoltaic device performance under 1 sun AM 1.5G simulated solar
illumination. The device structure is ITO/SQ 3/C
60

(400Å)/BCP(100Å)/Al(1000Å). ............................................................................ 90
Table 4-3: Photovoltaic device performance under 1 sun AM 1.5G simulated solar
illumination. The device structure is
ITO/Donor/C
60
(400Å)/BCP(100Å)/Al(1000Å). .................................................... 93
Table 4-4. OPV performance for devices with different buffer layers under simulated
1 sun, AM1.5G illumination. ................................................................................. 97
Table 4-5. Summary of solar cell characteristics of different SQ:PC
70
BM ratios and
SQ/C
60
planar control cell. The active layers with a ratio of 3:1 and 1:1 were
casted with a concentration of 10 mg/ml; with a ratio of 1:2, 1:3 and 1:6, they
were cast with a concentration of 20 mg/ml. The SQ layer of the SQ/C
60

planar control cell was prepared from 1 mg/ml SQ solution in
dichloromethane solvent. ..................................................................................... 105
Table 4-6. Summary of SQ/C
60
solar cell characteristics under 1 sun, AM1.5G
simulated illumination (solar spectrally corrected) and in the dark. .................... 111
Table 4-7. Device performance of various 1-NPSQ, DPASQ and blended cells. .......... 129

viii

List of Figures

Figure 1-1. Global CO2 emission report since 1997 to 2010.
5
............................................ 2
Figure 1-2. US energy consumption in 2010 ( Source: U.S. Energy Information
Administration) ........................................................................................................ 2
Figure 1-3. Black body radiation and solar spectrum (source: www.newport.com) ........... 3
Figure 1-4. (a)Variable solar spectrum after sunlight diffusion (b) different air mass
depending on zenith angle (Source: www.newport.com) ........................................ 3
Figure 1-5. ASTM G173-03 AM1.5G spectrum from SMARTS v. 2.9.2 ........................... 4
Figure 1-6. The absorption coefficient α for the active region of organic D/A (solid
red trace), gallium arsenide (dashed black trace, GaAs), and crystalline
silicon (dotted blue trace, c-Si).
14
............................................................................ 7
Figure 1-7. Best research cell efficiency reported byNational Renewable Energy Lab
(Source: www.nrel.gov) ........................................................................................... 8
Figure 1-8. Photocurrent generation process of Organic Photovoltaics
29
.......................... 11
Figure 1-9. The cartoon of bilayer (left) and BHJ (right) PV structure ............................ 14
Figure 1-10. The current-potential plot of OPV device ..................................................... 16
Figure 1-11. The equivalent circcuit for a solar cell .......................................................... 18
Figure 2-1. (a) Squariane zwitterionic structure; (b) Squaraine resonance structure. ...... 36
Figure 2-2. Typical symmetrical squaraine structures: (a) Dimethylaniline
substituted; (b) Dimethylpyrrole substituted;(c) Benzothiazole substituted; (d)
Azulene substituted. ............................................................................................... 36
Figure 2-3. Representative structures of Cyanines, Merocyanines and Squaraines. ........ 37
Figure 2-4. The mechanism of photodynamic therapy. .................................................... 38
Figure 2-5. Structure modification of squaraine for photodynamic therapy
application. ............................................................................................................. 39
Figure 2-6. The hydrazone alkylpyrrole type squaraines. ................................................. 41
Figure 2-7. The squaraine achievied exceptional high short circuit current. .................... 41

ix

Figure 3-1.Comparision between the calculated and experiment bod length of
squaraine: calculated (DFT, R=Me), experimental (X-Ray, R=s-Bu). .................. 61
Figure 3-2. HOMO and LUMO picture of compound 1, 2 and 3 from DFT
calculation (B3LYP/6-31G*). ................................................................................ 62
Figure 3-3. HOMO and LUMO picture of compound 4 and 5 from DFT calculation
(B3LYP/6-31G*). .................................................................................................. 63
Figure 3-4. HOMO and LUMO picture of compound SQ 3 from DFT calculation
(B3LYP/6-31G*). .................................................................................................. 64
Figure 3-5. The TGA analysis of SQ 1, SQ 3 and SQ 4. ................................................... 65
Figure 3-6. DSC measurement of SQ 1 and SQ 3. ............................................................ 66
Figure 3-7. ORTEP diagrams for SQ 1 (top) and SQ 3 (bottom). ..................................... 69
Figure 3-8. Crystal packing diagrams for SQ 1 (top) and SQ 3 (bottom). View of SQ
1 showing herringbone structure (a) and molecular stacking arrangement (b).
Stacking arrangement of SQ 3 viewed down the short (c) and long (d)
molecular axes. Hydrogen atoms were removed for clarity. ................................ 70
Figure 3-9. Absorption spectra for SQ 1, SQ 3 and SQ 10 in CH
2
Cl
2
solution (open
symbols) and as neat films (solid lines). ................................................................ 74
Figure 3-10. Excitation (ex) and emission (em) spectra of SQ 1 and SQ 3 in toluene. ..... 74
Figure 4-1. Current density vs. voltage characteristics of ITO/donor/C
60
(400
Å)/BCP (100 Å)/Al(1000 Å): donor = SQ1- 65 Å (SQ-65), 110 Å (SQ 1-
110), NPD 50 Å/SQ 1 65 Å (NPD/SQ 1) and CuPc 400 Å (CuPc). Dark
current characteristics are shown as dashed lines. ................................................. 82
Figure 4-2.External quantum efficiency (EQE) characteristics of ITO/donor/C
60
(400
Å)/BCP (100 Å)/Al(1000 Å) devices: donor = SQ 1 65 Å (SQ-65), NPD 50
Å/SQ 1 65 Å (NPD/SQ 1) and CuPc 400 Å (CuPc). ............................................. 85
Figure 4-3. The solution processed SQ 1 device structure. ............................................... 88
Figure 4-4. The AFM images of vapor deposited and spin cast SQ 1 film. ...................... 89
Figure 4-5. The current-potential plot of SQ 1 devices and AFM image of SQ 1 spin
cast from chloroform. ............................................................................................. 89
Figure 4-6. AFM images of SQ 3 spin cast from (a) Chloroform, (b) Chlorobenzene,
and (c) Toluene. ..................................................................................................... 90

x

Figure 4-7. (a) The potential–current density plots and tabulated OPV performance
characteristics of squaraine donor devices with the structure ITO/donor C
60

(400 Å)/ BCP (100 Å)/Al. (b) The external quantum efficiency (EQE) plot of
squaraine donor devices ITO/donor /C
60
(400 Å)/ BCP (100 Å)/Al. .................... 92
Figure 4-8. (a) AFM images of films of SQ 4 (RMS = 0.51 nm) and (b) SQ 5 (RMS
= 1.61 nm) spin cast from chloroform. The scale bar in the top right corner is
1 μm. ...................................................................................................................... 93
Figure 4-9. Energy level diagrams of EBLs used in this work: a) PTCBI, which
transports via the LUMO; b) NTCDA, which transports via the LUMO but
has an electron extraction barrier; and c) compound NTCDA/PTCBI, which
transports via the LUMO. ...................................................................................... 94
Figure 4-10. Fill factor (FF) under spectrally corrected 1 sun, AM1.5G illumination
for devices with BCP buffer layers (black squares), PTCBI (red circles),
NTCDA (blue triangles), and compound NTCDA/PTCBI (green stars) as a
function of thickness. Lines are a guide to the eye. Inset: the molecular
structure of 1-NPSQ. .............................................................................................. 95
Figure 4-11. Spectrally corrected short-circuit current (Jsc) under 1 sun, AM1.5G
illumination for devices with BCP buffer layers (squares), PTCBI (circles),
NTCDA (triangles), and compound NTCDA/PTCBI (stars) as a function of
thickness. Solid lines are a guide to the eye. The dashed line is Jsc modeled
based on the optical intensity in the device for the case of the
NTCDA/PTCBI buffer........................................................................................... 96
Figure 4-12: X-ray-diffraction (XRD) spectra of as-prepared SQ thin film on indium
tin oxide (ITO) substrates spin-coated from chloroform solvent. XRD spectra
confirms that the SQ film annealed at 110
0
C and 130
0
C has the same cystal
structure as the starting powder and has (001) and (002) growth orientation. ...... 98
Figure 4-13. The absorption coefficient of pure squaraine, and PC
70
BM, and blends
on quartz substrate with different ratios of 3:1, 1:1, 1:2, 1:3 and 1:6. ................... 99
Figure 4-14: (a) The effect of blending ratios on the external quantum efficiency
(EQE) for the cells with a device structure of ITO/MoO3(80 Å) /SQ
1:PC
70
BM(x Å)/Al (1000 Å) at different blending ratios of 3:1, 1:1, 1:2 1:3
and 1:6 and the EQE of the SQ 1/C
60
planar control cell with a device
structure of ITO/MoO3(80 Å)/SQ 1 (62 Å)/C
60
(400 Å)/ BCP(100 Å)/Al(1000
Å); (b) the current density-voltage (J-V) characteristics for the five blend
bulk cells and one planar control cell illuminated at 1 sun. Here x represents
320 Å, 400 Å, 720 Å, 730 Å and 760 Å for the five blend cells. ........................ 101

xi

Figure 4-15: The power conversion efficiency (ηp), open circuit voltage (Voc) and
fill factor (FF) under 0.002, 0.02, 0.2, 0.6, 1.0 and 1.68 sun illumination at a
device structure of ITO/MoO
3
(80 Å) /SQ 1:PC
70
BM(760 Å)/Al (1000 Å) at
the (1:6) ratio of SQ 1:PC
70
BM; The zero field hole transport mobility (µ0)
and series resistance (RSA) versus the molecular weight ratio between
squaraine and PC
70
BM ......................................................................................... 102
Figure 4-16: (a) AFM topographic [(a),(c),(e),(g)] and TEM [(b),(d),(f),(h)] images
of SQ:PC70BM films spin-coated from different ratios: 3:1 ([(a) and (b)]),
1:1 [(c) and (d)], 1:2 [(e)and (f)], 1:3 [(g) and (h), and 1:6[(i) and (j)]. Images
of AFM are 5µmx5µm in size. ............................................................................ 104
Figure 4-17. (a) X-ray-diffraction patterns of squaraine (SQ 1) thin films spin-coated
from dichloromethane (DCM) solvent on indium tin oxide (ITO) coated glass
substrates. The patterns suggest that the neat SQ 1 film annealed at 110
0
C
and 130
0
C and DCM solvent annealed for 20 min, has the (011) and (022)
crystal axes oriented normal to the substrate plane. ............................................ 108
Figure 4-18. (a) External quantum efficiencies (EQE) of the control and five cells
annealed at temperatures shown in legend; (b) The power conversion
efficiency ( η
p
) versus annealing temperature at 1 sun, AM1.5G simulated
illumination for a device structure of ITO/MoO
3
(80Å)/SQ
1(62Å)/C
60
(400Å)/bathocuproine (100 Å)/Al(1000 Å). ...................................... 109
Figure 4-19. Atomic force microscope (AFM) images of: (a), (b) as-cast; (c), (d) 110
0
C; (e), (f) dichloromethane (DCM) solvent-annealed SQ 1 films deposited
on indium tin oxide (ITO) coated glass with a 80 Å thick layer of MoO
3
. ......... 110
Figure 4-20. The dark current saturation current density (Js) and opencircuit voltage
(Voc) measured at 1 sun, AM1.5G illumination vs annealing temperature. ....... 110
Figure 4-21: The x-ray diffraction patterns for squaraine (SQ 1):PC
70
BM (1:6) films
annealed in dichloromethane (DCM) solvent for 10min, 12 min and 30 min.
The inset shows the molecular structure of SQ 1. ............................................... 115
Figure 4-22: The effects of DCM solvent on film morphology. Transmission electron
microscopy and AFM of squaraine (SQ 1):PC
70
BM (1:6) films: (a) as-cast,
(b) annealed in DCM for 12 min, and (c) annealed in DCM for 30 min. The
inset shows the surface images measured by AFM. ............................................ 116
Figure 4-23. The effect of DCM solvent annealing as a function of time on squaraine
composite films a) UV-vis absorption, b) photoluminescence (PL), c) EQE,
and d) J-Vcharacteristics of the SQ 1:PC
70
BM (1:6) cells at 1 sun
illumination. ......................................................................................................... 117

xii

Figure 4-24: (a) The power conversion efficiency ( ηp) and (b) fill factor (FF) versus
power intensities as a function of dichloromethane (DCM) solvent annealing
time, for the device structure of ITO/MoO
3
(80 Å)/SQ 1:PC
70
BM (1:6 780
Å)/C
60
(40 Å)/BCP(10 Å)/LiF(8 Å)/Al(1000 Å). ................................................. 118
Figure 4-25: Absorption spectra of C
60
, 1-NPSQ, DPASQ and blended 1-NPSQ:
DPASQ (at weight ratio of 1:0.5) films. Inset: (upper) molecular structural
formula of 1-NPSQ; and (bottom) DPASQ. ........................................................ 121
Figure 4-26: Current density vs voltage characteristics under 1 sun, AM1.5G
simulated solar illumination for thermally-annealed, neat 1-NPSQ, DPASQ
and blended donor cells at various weight ratios of 1-NPSQ to DPASQ. Also
shown are characteristics for solvent-annealed (SA), blended cells at a 1:0.5
ratio (here, CB=compound buffer is used). ......................................................... 121
Figure 4-27: Ultraviolet photoelectron spectra of 10Å-thick 1-NPSQ and DPASQ
films on indium-tin-oxide-coated glass substrates. (a) Low energy cutoff; and
(b) high energy cutoff of the films.The dashed line crossings correspond to
intercepts with the energy axis. ............................................................................ 123
Figure 4-28: External quantum efficiency (EQE) spectra of devices in Figure 4-26.
Note that the clearly defined feature due to exciton generation in the DPASQ
film disappears for the CB and CB+SA films due to a significant increase in
the intensity of the C
60
spectra. The presence of DPASQ results in the
broadening of that feature. ................................................................................... 126
Figure 4-29: Atomic force microscope (AFM) images of a (a) neat 1-NPSQ film; (b)
1-NPSQ: DPASQ 1:0.5 blend; and (c) a 1:2 blend. Here, RMS indicates the
root mean square roughness of the films in the respective images. Small-size
surface clusters (possibly crystallites) were observed on neat 1-NPSQ film,
which leads to RMS = 17 Å. The surface of 1:0.5 blend is smoother, with
fewer clusters and RMS = 8 Å. The 1:2 blend has large clusters, with RMS =
12 Å. ..................................................................................................................... 128
Figure 5-1. The structures of squaraines and precursors made in Simga Aldrich. .......... 137
Figure 5-2. The process of converting air sensitive phosphines to air stable
phosphonium salts and how phosphine is released from phosphonium salts. ..... 143
Figure 5-3. The HPLC analysis of DPSQ synthesized in our lab. ................................... 145
Figure 5-4. The HPLC analysis of DPSQ synthesize in Sigma Aldrich. ......................... 146
Figure 5-5. The HPLC analysis of DPSQ recrystallized from hot chlorobenzene. ......... 147

xiii

Figure A-1.
1
H-NMR spectrum of (SQ) 2 in CDCl
3
at 60 ºC. ......................................... 164
Figure A-2.
13
C-NMR spectrum of (SQ) 2 in CDCl
3
at 60 ºC. ........................................ 164
Figure A-4.
13
C-NMR spectrum of (SQ) 3 in CDCl
3
at 25 ºC. ........................................ 165
Figure A-5.
1
H-NMR spectrum of (SQ) 4 in CDCl
3
at 60 ºC. ......................................... 166
Figure A-6.
1
H-NMR spectrum of aromatic region of (SQ) 4 in CDCl
3
at 60 ºC. .......... 166
Figure A-7.
13
C-NMR spectrum of (SQ) 4 in CDCl
3
at 60 ºC. ........................................ 167
Figure A-8.
1
H-NMR spectrum of (SQ) 5 in CDCl
3
at 25 ºC. ......................................... 167
Figure A-9.
1
H-NMR spectrum of aromatic region of (SQ) 5 in CDCl
3
at 25 ºC. .......... 168
Figure A-10.
13
C-NMR spectrum of (SQ) 5 in CDCl
3
at 25 ºC. ..................................... 168
Figure A-11.
1
H-NMR spectrum of (SQ) 6 in CDCl
3
at 60 ºC. ....................................... 169
Figure A-12.
1
H-NMR spectrum of (SQ) 7 in CDCl
3
at 60 ºC. ....................................... 169
Figure A-13.
1
H-NMR spectrum of aromatic region of (SQ) 7 in CDCl
3
at 60 ºC. ........ 170
Figure A-14.
13
C-NMR spectrum of (SQ) 7 in CDCl
3
at 60 ºC. ...................................... 170
Figure A-15.
1
H-NMR spectrum of (SQ) 8 in CDCl
3
at 60 ºC. ....................................... 171
Figure A-16.
1
H-NMR spectrum of aromatic region of (SQ) 8 in CDCl
3
at 60 ºC. ........ 171
Figure A-17.
13
C-NMR spectrum of (SQ) 8 in CDCl
3
at 60 ºC. ...................................... 172
Figure A-18.
1
H-NMR spectrum of (SQ) 9 in CDCl
3
at 25 ºC. ....................................... 172
Figure A-19.
13
C-NMR spectrum of (SQ) 9 in CDCl
3
at 25 ºC. ...................................... 173
Figure A-20.
1
H-NMR spectrum of (SQ) 10 in CDCl
3
at 25 ºC. ..................................... 173
Figure A-21.
1
H-NMR spectrum of (SQ) 10 in CDCl
3
at 25 ºC. ..................................... 174
Figure A-22.
13
C-NMR spectrum of (SQ) 10 in CDCl
3
at 25 ºC. .................................... 174
Figure B-1. Absorption spectra of squaraines (SQ) 1–10 in CH
2
Cl
2
solution. ................ 175
Figure B-2. Absorption spectra of films of squaraines (SQ) 1, 3–10 spin cast from
CH
2
Cl
2
.................................................................................................................. 175

xiv

Figure B-3. CV trace of squaraine (SQ) 1 in CH
2
Cl
2
. The large signal at 0 V is from
the ferrocene used as an internal reference. ......................................................... 176
Figure B-4. CV trace of squaraine (SQ) 2 in CH
2
Cl
2
. The large signal at 0 V is from
the ferrocene used as an internal reference. ......................................................... 176
Figure B-5. CV trace of squaraine (SQ) 3 in CH
2
Cl
2
. The large signal at 0 V is from
the ferrocene used as an internal reference. ......................................................... 177
Figure B-6. CV trace of squaraine (SQ) 4 in CH
2
Cl
2
. The large signal at 0 V is from
the ferrocene used as an internal reference. ......................................................... 177
Figure B-7. CV trace of squaraine (SQ) 5 in CH
2
Cl
2
. The large signal at 0 V is from
the ferrocene used as an internal reference. ......................................................... 178
Figure B-8. CV trace of squaraine (SQ) 6 in CH
2
Cl
2
.The large signal at 0 V is from
ferrocene used as an internal reference. ............................................................... 178
Figure B-9. CV trace of squaraine (SQ) 7 in CH
2
Cl
2
. The large signal at 0 V is from
the ferrocene used as an internal reference. ......................................................... 179
Figure B-10. CV trace of squaraine (SQ) 8 in CH
2
Cl
2
. The signal at 0 V is from the
ferrocene used as an internal reference. ............................................................... 179
Figure B-11. CV trace of squaraine (SQ) 9 in CH
2
Cl
2
. The large signal at 0 V is from
the ferrocene used an internal reference. ............................................................. 180
Figure B-12. CV trace of squaraine (SQ) 10 in CH
2
Cl
2
. The large signal at 0 V is
from the ferrocene used an internal reference. ..................................................... 180

xv

Abstract

As the worldwide demand for energy continues to rise, coupled with the problem
of the depletion of energy resources and concerns about the environmental impact of
fossil fuels, the search for cost-effective clean energy has become a growing global
priority. Chief among current clean energy sources, solar power has the biggest potential
to satisfy our energy demands. Currently, commercially available inorganic solar cells
have reasonable efficiency in the range of 15%-20%. Although their prices have been
dropping steadily, the high manufacturing cost still remains a limitation of inorganic
photovoltaics (IPV). Instead, organic photovoltaics (OPVs) have attracted tremendous
attention from both academia and industry due to the advantages of abundant materials,
along with inexpensive manufacturing and installation. Perceived as a viable alternative
to IPVs, the performance of OPVs (~10%) is quickly catching up with its inorganic
counterparts. However, to reach commercialization bar of 15%, some issues need to be
resolved for organic materials, such as high absorbance in broader region of solar
spectrum, low charge carrier mobility and short exciton diffusion length. New materials,
optimization of process conditions, and novel device architecture are key factors to
address these issues.
The aim of this dissertation is to explore a new type of functional material for
high performance OPVs. By introducing a new library of squaraines with varying
substituents, tunable absorption throughout visible and near-infrared, and energy levels
can be achieved. A range of photophysical properties have been studied to obtain more
useful information about this new type of material. The application of new squaraine

xvi

materials as active donors is covered in chapter 4. With good thermal stability and
solution processability, squaraine thin films can be fabricated by both vacuum deposition
and spin-casting methods. Squaraines have been utilized as donor materials in highly
efficient OPVs. A new approach of blending squaraines with complementary absorption
is proposed to achieve broadband absorption with strong spectral response from 500 nm
to 850 nm. In collaboration with Prof. Stephen Forrest’s group at University of Michigan,
squaraine-based OPVs have achieved marked increase in device performance. With post-
thermal and solvent annealing processes, squaraine-based nanocrystalline heterojunction
devices can lead to a significant increase in surface area and enhanced photocurrent.
Utilizing compound blocking layer can result in enhancing performance by over 25%.
The last chapter describes the synthesis of representative functional squaraine
materials on an industrial scale. These materials are commercially available at the time of
the preparation of this dissertation.

1

Chapter 1. Introduction
1.1 Energy from Sunlight
1.1.1 Abundance of Solar Energy
In 2005, gloabal consumption of energy was 13 terawatt (TW). Energy experts
estimate that this number will rise to 50 TW by 2050.
1
Currently energy sources fall
under three main categories: fossil fuels (petroleum, coal and natural gas), nuclear power
and renewable energy (solar, wind, biomass, hydropower and geothermal). As the
dominant energy source, fossil fuels have generated enormous amounts of carbon dioxide
CO
2
– a known green house gas, and its atmosphere level continues to be a growing
problem. Figure 1-1 illustrates CO
2
emission from 1971-2009. Emission level has been
increasing steadily with an exception during 2008-2009 created by widespread economic
crisis. Although fossil fuels offer dual advantages of reasonable prices and availability,
our dependence on these fuels comes at the price of risking environmental
destabilization. To address issue of climate change caused by increased energy demands,
the search for cost effective, efficient and carbon-free energy sources should become a
global priority.
Among all energy sources, renewable energy contributes only 8% to the total
energy demand (Figure 1-2) with solar, PV, and geothermal all together account only 1%
of all types of renewable energy. At the same time, solar energy is considered one of the
most promising sources, being extremely abundant as well as enviromental friendly.
2

The total solar energy hitting our planet is roughly 1.7x10
5
TW per year.
1, 3, 4
Counting

2

only the area available using practical means, the amount provided is still about 600
TW—greater than 10 times the estimated global demand of 50 TW in 2050.

Figure 1-1. Global CO2 emission report since 1997 to 2010.
5

Figure 1-2. US energy consumption in 2010 ( Source: U.S. Energy Information
Administration)
1.1.2 Solar Spectrum
Electromagnetic radiation emitted by the Sun ranges from ultraviolet to infrared,
including visible light. The Sun’s spectrum is similar to black-body radiation at a
temperature of 5800 K. The zenith angle at which sunlight strikes the Earth is likewise
important, varying from 0° to 90°. The light intensity is at the maximum when zenith
angle is 0°. Before reaching the Earth, sunlight is filtered as it passes through the

3

atmosphere, its intensity being reduced by such factors as absorption, scattering and
reflection. Thus, the amount of solar radiation that can be captured depends on such
factors as location, season and local landscape.

Figure 1-3. Black body radiation and solar spectrum (source: www.newport.com)

Figure 1-4. (a)Variable solar spectrum after sunlight diffusion (b) different air mass
depending on zenith angle (Source: www.newport.com)
Sunlight radiation is characterized by light intensity and the coverage of solar
spectrum. Airmass (AM) is the term to characterize how far sun light travels through
atmosphere and the spectrum of solar radiation. The number after AM is considered as

(a) (b)

4

the coefficient of air mass. For example, AM 0 refers to air mass before entering into
atmosphere. AM 1 is the solar radiation when sunlight travels through atmosphere with
Sun directly overhead. AM 1.5 is defined as the solar radiance after traveling through
atmosphere at zenith angle of 48.2°. In PV industry, AM 1.5 one sun (100 mW/cm
2
) is
used as a standard for solar cell performance evaluation. According to American Society
for Testing and Materials (ASTM), the AM 1.5 was chosen because it is representative of
average conditions in 48 contiguous states of the United States. It has peak photon flux in
the visible to red region with over 50% photon flux residing in the near infrared (NIR)
region.

Figure 1-5. ASTM G173-03 AM1.5G spectrum from SMARTS v. 2.9.2
1.2 Comparing Organic and Inorganic Phovoltaics
1.2.1 Review of Inorganic Phovoltaics
Photovoltaics historically began with Alexandre Edmond Becquerel’s discovery
of light-induced voltage in electrolytic cells in 1839. Further development of PV industry
1000 2000 3000 4000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6

Spectral Irradiance
(Wm
-2
nm
-1
)
Wavelength (nm)
Terrestrial Global 37 deg South Facing Tilt

5

followed with Bell labs’ invention of silicon solar cells in 1954. Significant progress and
cost-reduction were then driven by US space program. Besides PV’s NASA-based
application, more research is aimed at its use in daily life with further reduction on PV
cost.
Currently, the most advanced and mature PV cell is inorganic PV. There are three
groups of inorganic PVs: crystalline, polycrystalline and thin film ones. Its long dominant
history resulted in silicon PV having 83% share of the current PVmarket.
6, 7
Crystalline
silicon based PVs have achieved 25% power conversion efficiency (the ratio of energy
output to light input) by 2011. There are also III-V semiconductor materials based
inorganic PVs. One example of such materials is GaAs with certified efficiency of up to
28.3%.
8
For the state-of-art devices, efficiencies of 19.6% for thin film chalcogenide
CuInGaSe
2
cell (CIGS), 10.1% for amorphous and nanocrystalline Si, silicon crystal
25%, and 34.1% for multijunction cell GaInP/GaInAs/Ge have been reported.
9
A
multijunction GaInP/GaAs/GaInNAs with efficiency reaching 43.5% is reported under
concentrated light.
In silicon solar cells, free charge carriers are generated immediately upon photon
absorption due to strong covalent interactions between lattice sites and high dielectric
constants ( ε
r
=10-15). Generally, silicon has much higher hole mobility than organic
semiconductors. The highest reported hole mobility of small molecule single crystal is 15
cm
2
V
-1
s
-1 10
and

0.6 cm
2
V
-1
s
-1
for liquid crystalline polymers
11
in comparison with the
silicon hole mobility of 450 cm
2
V
-1
s
-1
. The single crystal mobility of organic

6

semiconductors is much higher than thin film mobility in real devices, which is usually at
the range of 10
-2
to 10
-6
cm
2
V
-1
s
-1
.
Although silicon is the dominant PV material, it is far from ideal. One major
disadvantage is that silicon is an indirect bandgap material which lacks ability to absorb
enough sun light. One simple example is that to absorb 90% of sunlight, only 1 μm GaAs
is needed while a 100 μm of Si is required to absorb same amount of sunlight.
12
With this
disadvantage, silicon materials with higher crystallinity are required to minimize charge
recombination. As a consequence, high temperature purification and crystal growth
directly leads to high cost of silicon PV cells.
7
Recently, there has been steady price drop
for inorganic PVs. However, high manufacturing fee still present a major hurdle for low
cost inorganic PVs.
1.2.2 Organic Photovoltaics Review
OPVs have made great progress over the past decade due to synergistic effort
from both academia and industry. OPV technology has the advantage of an abundant
supply of materials which are non-toxic, allow for mechanical flexibility, and is
potentially low in cost.
One of the main reasons that OPVs are actively pursued is the high absorption
coefficient of organic materials. The absorption coefficient is usually at the magnitude of
10
5
M
-1
cm
-1
, 1000 times higher than that of typical inorganic materials. Figure 1-6
compares absorption coefficient of conventional Copper Phthalocyanine (CuPc)/ C
60
pair
of organic materials, crystalline silicon (c-Si) and direct band gap gallium arsenide
(GaAs). Although GaAs is superior to c-Si in terms of light absorption, it still lags far

7

behind organic counterparts. With the same thickness, at the maximum photon flux of
630 nm, CuPc/C
60
pair can absorb up to 90% of solar spectrum, while the c-Si and GaAs
can absorb about 10% and 50% only.
13
With this feature, organic solar cells can be
fabricated with less than 100 nm thick layer. This is about 1000 times thinner than
crystalline silicon solar cells and also 10 times thinner than the current inorganic thin film
solar cells. Thus, OPVs offer the advantage of low materials consumption.

Figure 1-6. The absorption coefficient α for the active region of organic D/A (solid red
trace), gallium arsenide (dashed black trace, GaAs), and crystalline silicon
(dotted blue trace, c-Si).
14

Another advantage of organic materials is their low manufacturing cost. Organic
thin films can be prepared by spin casting method which does not consume as much
energy as inorganic PV’s fabrication, which also requires the use of clean room.
Additionally, the mechanical flexibility of OPVs creates the possibility of applying OPVs
on curved surfaces such as textiles or fabrics.
15

1
2
3
4
5
6

x10
-14
(cm
-2
s
-1
)
400 600 800 1000
0.2
0.4
0.6
0.8
1.0
1.2
1.4
GaAs
Organic D/A
c-Si

Wavelength (nm)

x10
-5

︵
c m
-1
︶
630 nm

8

Figure 1-7. Best research cell efficiency reported byNational Renewable Energy Lab
(Source: www.nrel.gov)
Figure 1-7 plots the photovoltaic cell development since 1970s including all types
of PVs. The inorganic PVs have already been commercialized and achieve much higher
efficiency than OPVs. However, the rapid growth of OPVs’ efficiency is not negligible.
Several high efficiency (~10%) OPVs have been reported since 2011. In Feb 2012, Yang
Yang group from UCLA reported a 10.6% tandem cell with 12 mm
2
area. By integrating
additional NIR absorber, it breaks earlier record of 8.62% in July 2011. This brings
promise for further improvement. Heliatek sets a new world record efficiency of 10.7 %
for its organic tandem cell in April 2012 with 1.1 cm
2
size based on small molecules.
16

This is the improvement four months after its 9.8% efficiency with 1.1 cm
2
size cell.

9

More importantly, the 10.7% cells confirmed their superior performance under low light
intensity and high temperature performance compared with traditional solar technologies.
Although no detailed information about exact materials or device structure was released,
new materials’ development and device optimization have been demonstrated to
stimulate higher OPVs’ performance.
OPVs can be fabricated from both small molecules and polymers materials. They
are quite different in aspects such as synthesis, purification and device fabrication. In
terms of materials synthesis, small molecules are easier to synthesize and purify with
defined structure. This significantly diminishes problems such as poor reproducibility of
batch-to-batch polymer synthesis as well as device fabrication.
However, polymers have the advantage of being solution-processed and having
film-forming properties. Polymers are mostly fabricated in a solution processed bulk
heterojunction (BHJ) structure. The BHJ structure refers to intimately mixed functional
layer blends which can overcome short exciton diffusion length of organic materials Most
often, a well performing BHJ cell needs correct solvent selection,
17
solvent mixture,
18

post annealing process
19-21
or processing additives
22, 23
to obtain optimal film morphology
with proper percolation pathway.
OPVs based on small molecules are more versatile in terms of device structure.
Small molecules can be processed by both thermal evaporation and solution process.
24, 25

With thermal stability, small molecules can be thermally evaporated to make either a
bilayer or BHJ device. Bilayer device has the advantage of a clean interface between
functional layers. It offers opportunity to probe device physics
26
and construct more

10

complex device architectures. For small molecules with reasonable solubility and film-
forming properties, a solution processed BHJ type cell can also be made.
Until now, there has been less research conducted on small molecule-based OPVs
than those based on polymers. However, small molecule based OPVs are catching up
with their polymer counterparts, with the highest reported power conversion efficiency of
6.7%.
25, 27, 28
This dissertation focuses exclusively on small molecules based OPVs.
1.2.3 Operation Principles
OPVs normally consist of an electron donor (D) and an electron acceptor (A)
layer. Light absorption in OPVs generates tightly bound excitons, which can either decay
to ground state or undergo charge transfer to form a hole and an electron. Efficient
exciton separation usually occurs at the D/A interface. Afterwards, a free hole and a free
electron transport through donor and acceptor layer respectively towards corresponding
electrodes. Then charge is extracted and collected by external circuit.
Thus, the photocurrent in OPVs are generated in the following four steps: 1)
Photon absorption ( η
A
) and subsequent generation of photon excited excitons; 2) Exiton
diffusion ( η
ED
); 3) Exciton dissociation to a free hole and a free electron ( η
CT
); 4) charge
transport and collection at electrodes ( η
CC
). The external quantum efficiency ( η
EQE
) is
defined as the ratio of free charge carriers collected at the electrode to the number of
incident photons. It characterizes the efficiency of converting one photon to one electron-
hole pair. The η
EQE
is the product of the above four steps’ efficiency.
η
EQE
= η
A*
η
ED*
η
CT*
η
CC (1) ,

11

Figure 1-8. Photocurrent generation process of Organic Photovoltaics
29

1.2.4 Device Architectures
The organics-based photovoltaic effect was first demonstrated in 1975
30
in a
Schottky diode cell, which is composed of one single layer sandwiched between two
electrodes. Such device achieved only 0.001% efficiency at that time. The poor
performance is due to low exciton dissociation rate in single layer, a result of large
exciton binding energy of 200-500 meV
31
in organic semiconductors. The available
thermal energy at RT is about 25 meV, which is insufficient for exciton separation at RT.
This issue was not resolved until C.W. Tang proposed the structure of bilayer
heterojunction in 1986.
32
The bilayer heterojunction is comprised of two functional donor

12

and acceptor layer with certain energy offset. Energy offset at donor/acceptor (D/A)
interface enables excited-state excitons to undergo charge dissociation.
OPVs’ are composed of donor, acceptor, buffer layer and electrodes. The donor
and acceptor layers are both photon absorbing materials. In a D/A pair, donor is easier to
be oxidized. It loses an electron upon photo excitation and donates the electron to the
acceptor material.
The cathodes are metals with different work function such as Al, Ca, Ag or Au.
The work function of cathodes is extremely important to avoid energy barrier to charge
collection. The work function and chemistry of metal are both important in solar cell
devices to avoid energy barrier to charge collection. For example, the oxidation of Al
creates barrier to charge collection and subsequent photocurrent loss. The interface
between metal/organics can be effectively modified by inserting a buffer layer as
discussed later.
The anode is transparent for sunlight absorption, normally indium tin oxide (ITO).
ITO has good transmittance (> 85%) in the visible region and performs well for most
OPVs. However, such drawbacks as high resistivity and high cost of ITO have prompted
a search for alternatives, including other metal oxides
33, 34
or carbon nanotube based
anodes.
35, 36
A research group from MIT has reported solar cells fabricated by graphene
anode having performance comparable with those made by ITO.
37
It was also found that
AuCl
3
doping on graphene can modify graphene surface properties, resulting in a higher
rate of device success. Our group has worked on replacing ITO with flexible graphene
electrode with comparable device performance as well.
38
By growing graphene film in

13

chemical vapor deposition (CVD) with continuous nature, graphene electrode has quite
low resistance of 230 Ω/sq at 72% transparency. More importantly, the CVD graphene
solar cell has superior bending capacity to ITO based solar cells.
The organic/electrode interface is ideally an ohmic contact to facilitate charge
collection and charge injection.
39, 40
The addition of a buffer layer has been demonstrated
to improve overall device performance.
41
The buffer layer between anode/organic
includes such benefits as reducing dark current leakage
42
and facilitating hole injection,
while cathode/organic buffer can block exciton or prevent damage of hot metal
deposition on top of acceptor. The common anode buffer layer is poly(3,4-
ethylenedioxythiophen)poly(styrenesulfonate) (PEDOT:PSS). This layer has the
advantage of high conductivity, modified abrupt surface contact between ITO and
organic materials
43
and forms ohmic contact with organic materials.
44
In addition, a
MoO
x
layer has been successfully applied in small molecule-based OPVs to prevent dark
current leakage and enhance open circuit voltage value.
42

The 2, 9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) is the most common
buffer layer for cathode/organic. In addition to acting as an exciton blocking and
prevention layer, BCP is transparent through the solar spectrum and can serve as an
optical spacer between photoactive region and metal electrode. It is thick enough to place
the region of the highest incident optical density at the D/A interface, enabling the
maximization of optical density.
45, 46

Unlike delocalized bands in inorganic materials, organic materials have weak
intermolecular interaction and short exciton diffusion length (L
D
) (2nm-20nm),
47
which

14

prevents thicker layer to be applied in bilayer OPVs. L
D
is defined as the length of
excitation can propagate prior to decay of the exciton population to 1/e (~35%) of its
initial value.
13
With such short L
D
, photo excited excitons are very likely to decay to
ground electronic state before reaching donor-acceptor interface.
Aside from the short L
D
of organic semiconductors, small interface area in bilayer
devices is also a problem for bilayer OPVs as well. The concept of BHJ structure can
overcome the above problems as illustrated in figure 1-8. In this case, the donor and
acceptor are intimately mixed at the scale of L
D
and a thicker optimized functional layer
can be incorporated into BHJ cells. But a high quality phase separation is required to
obtain high exciton dissociation rate.

Figure 1-9. The cartoon of bilayer (left) and BHJ (right) PV structure
In addition to bilayer and BHJ device structures, a more sophisticated tandem cell
structure was realized by Hiramoto et.al..
48
By connecting two or more single unit cells
together, the achieved solar cell can yield open circuit voltage as the sum of all single
cells while photocurrent matches with the minimum photocurrent of single unit cell.

IT O
Al
Donor
A ccept or

e
-

h
+

ITO
Al

Donor
Accepto
r
Bulk

15

1.2.5 Performance Evaluation of OPVs
The plot of voltage and current is used to characterize OPV performance. In
addition, a plot of external quantum efficiency (EQE) identifies spectral response of
active components in OPVs. The OPVs’ performance is characterized by four
parameters: open circuit voltage V
oc
, short circuit current J
sc
, fill factor FF and power
conversion efficiency (PCE) η.
At open circuit voltage V
oc
, the photocurrent is zero. The V
oc
is not directly related
to built-in electrical potential, as in the Schottky barrier diode. The maximum photon flux
from solar radiance is at around 600 nm correlating with 2 eV. Most published V
oc
values
falls in the range of 0.5-1.0 eV. The resulting V
oc
is

less than half of available photon
energy. Moreover, there are a lot of photons left in NIR region that most materials do not
absorb. The NIR regionn corresponds to energy less than 1.65 eV. Thus, the resulting V
oc

of NIR materials can be even lower. This is why achieving high V
oc
and high J
sc

simultaneously remains a huge challenge.
V
oc
is significantly correlated with the energy difference ∆E
DA
(energy difference
between HOMO of donor and LUMO of acceptor). Aside from thermodynamic control,
kinetic factors such as the ratio of charge separation rate to the charge recombination rate
play a critical role as well. More specifically, the value of dark current and how fast the
recombination occurs both determined the value of V
oc
.
13

16

Figure 1-10. The current-potential plot of OPV device
q
E
J
J
q
nkT
V
DA
SO
SC
oc
2
ln












(2),

Perez et al., have derived equation (2) to estimate the value of V
oc
.
49
In equation
(2), the first part is contributed from kinetics factor while the second part accounts for
thermodynamic energy. If dark current J
s
is suppressed, V
oc
is mainly dependent on
thermodynamic energy which can be tuned by energy levels of functional materials. The
control of J
s
value can possibly be done by molecule design through proper interface;
Doing so may suppresses charge carrier recombination occurring in the interface.
Overall, the process suppresses the charge recombination, while maintaining the exciton
dissociation process, allowing it to then be optimized for minimum voltage loss.
13

Short circuit current J
sc
is the maximum current at no applied voltage. The value
of J
sc
depends on the overlap between functional materials’ absorption and the solar
spectrum, the absorptivity of functional materials, charge transfer rates, and charge

P max
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-6
-4
-2
0
2
4
Voltage (V)

Current density (mA/cm
2
)
Dark
Light
V
oc

J
sc

17

carrier mobility. During the charge transfer step, two processes might compete with photo
generations such as: 1. Geminate pair recombination of excited electron hole pair to
electronic ground state. 2. Bimolecular recombination of free charge carriers which
compete with charge transport to respective electrodes.
Many factors can impact the photocurrent of OPVs. One popular strategy of
enhancing J
sc
is by utilizing low band gap materials. While more than 50% sunlight
resides within the NIR region, current active materials are not effectively utilizing all the
sunlight. Ideally, the D/A blends can absorb from a wide range of sunlight while
maintaining efficient photoconductivity.
The FF is defined as the fraction of maximum power output over the product of
V
oc
and J
sc
. It evaluates how a solar cell performs compared with theoretical energy
output when V
oc
and J
sc
are both realized at the same time. The FF value is between 0.3-
0.8, depending on the charge carrier mobility and balance, series resistance (Rs) and
parallel resistance (R
p
) (Figure 1-11). The series resistance is derived from resistance of
current flows through the device, resistance of organic and metal contact and also the
resistance from two electrodes. The large series resistance can lead to a decrease in FF
and J
sc
. The series resistance should be ideally very small and close to 0 which would
require materials used in assembling solar cells have low resistivity. In comparison,
parallel resistance results from the defects inside the solar cell and can lead to another
pathway through which photocurrent can flow. It should be infinite to prevent any
“shorts” from allowing any current leakage. Overall, in a perfect solar cell, the R
s
is
expected to be 0 while R
p
is expected to be infinite.

18

Figure 1-11. The equivalent circcuit for a solar cell
η - Power conversion efficiency (PCE). The power conversion efficiency is equal
to the product of V
oc
, J
sc
and FF. Thus, to obtain a high performance solar cell,
resolutions to enhance all three parameters are necessary. In reality, three parameters
usually compromise each other. Thus, achieve high values for all three parameters at the
same time remains a major goal, and a major challenge.
1.3 Active Donor Materials for OPVs
As stated earlier, novel materials are the major driving force for understanding
OPVs’ mechanism and achieving higher PCE. This dissertation focuses on exploring new
donor materials while acceptor is fullerene or its derivatives. Before introducing new
squaraine materials, current small molecule based donors will be reviewed. There are
many types of chromospheres that have been identified as donors in OPVs such as
oligothiophenes, acenes and polycyclic arenes, push-pull chromospheres and
miscellaneous dyes.

J
sc

h ν
+
-

R
s

R
sh

19

Oligothiophenes – The earliest effort of applying oligothiophenes in OPVs starts
from 2006 when Roncali use three dimensional star shaped tetra(oligothienyl)silianes
pairing with PCBM.
50
However, due to absorption cutoff at 440 nm, OPVs based on this
type of oligothiophenes achieved only 0.3% efficiency under 80 mA/cm
2
light intensity.
Later, X- shaped oligothiophene substituted at phenyl core was synthesized. This X-
shaped oligothiophene has extended absorption into 520 nm and achieved 1.3% PCE in a
BHJ device.
51
However, the low FF of 0.4 and small domain size was attributed to the
poor phase separation inside device. It is also stated in this work that device performance
can be improved by increasing oligothiophene length with increased charge carrier
mobility.
The most successful oligothiophene based OPVs was achieved by a highly
branched thiophene dendrimer. Ma synthesized this dendrimer oligothiophene with
absorption extending to 600 nm.
52
Pairing 1:2 ratio of oligothiophene with PCBM, the
device achieved a performance of 1.72%. This is so far the record efficiency for
oligothiophene based BHJ OPVs.
Overall, oligothiophenes have not achieved performance comparable with
polymer P3HT. It is due to oligothiophenes’ low absorption in the visible region and poor
film properties in the solid state.
53
The branched oligothiophene has reduced
intermolecular interaction which does not form well-ordered films like P3HT in the solid
state.
Acenes and Polycyclic Arenes: Simple chains of fused benzene rings are active
donors for OPVs as well. They are known for excellent charge carrier mobility Among

20

acene derivatives, pentacene has a long exciton diffusion length, up to 70 nm, and high
hole mobility of 1~2 cm
2
V
-1
S
-1
.
54, 55
The first polycrystalline pentacene/C
60
based vapor
deposited bilayer device was published in 2004. Due to efficient light absorption and
ultrafast photoinduced electron transfer between pentacene and C
60
, high PCE of 2.7%
was obtained.
54

Diindenoperylene is a successful small molecule donor used in vapor deposited O
PVs. A planar-BHJ type cell (diindenoperylene/diindenoperylene:C
60
/ diindenoperylene )
achieved PCE up to 4.1%.
56
The high ionization potential of the diindenoperylene and su
itable energy level alignment with C
60
leads to a high V
oc
of 0.91 V. Although diindenope
rylene absorbs only in the range of 420-580 nm, its strong charge carrier collection still re
sults in reasonable photocurrent. Combined with favorable film morphology and high cry
stalline order, high performance diindenoperylene-based cells can be achieved in both pla
nar and planar-BHJ types.
The solution processed small molecule OPVs were initiated by pentacene
derivatives such as 6, 13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene).
57
The
TIPS side groups impart solubility and facile film formation to the insoluble pentacene
molecule. However, TIPS-pentacene is reactive with fullerene derivatives, leading to
inefficient photoinduced charge transfer. The optimized TIPS-pentacene has a PCE of
only 0.5%.
Some anthracene derivatives are solution processable for BHJ type cells. Marks
et.al have made a series of soluble anthracene derivatives bridged by olefinic versus
acetylenic π spacers.
58
In their work, anthracenes incorporated with acetylenic spacers

21

show better film crystallinity than the ones with olefinic spacers. This is likely due to
decreased steric repulsion in solid state films of anthracenes with acetlynic spacers. In
addition, anthrancenes with acetlynic spacers have broader absorption than the olefinic
ones. Overall, the anthrancene with acetlynic spacers achieved PCE up to 1.3%.
Push-pull Chromophores: Although there are a lot of chromospheres can be
considered as push-pull structure, the discussion here will be limited to structures with
obvious electron donating and electron accepting groups. A fashion of donor material
design is by Donor-Acceptor (D/A) approach.
59,60
The internal charge transfer
characteristics in D/A can lead to demonstrated low band gap, which show more double
bond characteristics within the molecule. The HOMO of final molecule largely resides on
the donor unit while the LUMO largely from the acceptor unit. With this design, it is easy
to tune molecular property from separate units. The most common and popular donor unit
is thiophene or arylamines which exhibit high charge carrier mobility.
61
The acceptor unit
can be electron withdrawing groups like dicyanovinylene,
62
2,1,3-benzothiadiazole,
63

thiadiazolo[3,4-c]pyridine,
61
diketopyrrolopyrrole,
64, 65
isoindigo,
66
tetracyanobutadiene
67

and borondipyrromethene.
68
Recently, Lin et.al published a small molecule with D-A-A
design achieving 5.8% efficiency by vacuum deposition pairing with C
70
acceptor.
69
This
molecule has a combination of donor ditolylaminothienyl with two electron accepting
groups 2,1,3-benzothiadiazole and dicyanovinylene. By employing two acceptor units,
the whole molecule is narrow bandgap with low lying HOMO. This can lead to relatively
high V
oc
while maintain high J
sc
. Meanwhile, the extended film absorption in NIR region

22

contributes to significantly enhanced photocurrent. The result is impressive and currently
carries the record efficiency for vacuum deposited small molecule OPVs.
The small molecule mimicking polymer design 5,5 ′-bis {(4-(7-hexylthiophen-2-
yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}-3,3 ′-di-2-ethylhexylsilylene-2,2 ′-
bithiophene, DTS(PTTh
2
)
2
have achieved a record efficiency of 6.7% with solvent
additive treatment.
25
This is a D-A-D-A-D type chromophore. The main advantage of this
molecule arises from its dithienosilole (DTS) core with better packing abilities and high
hole mobility compared to its carbon based homologues. The acceptor unit
pyridal[2,1,3]thiadiazole (PT) has high electron affinity and its asymmetry feature allows
for mono functionalization. The center core A-D-A offers strong intramolecular charge
transfer characteristics while the silane group with alkyl chain substitution yields better
solubility. The bithiophene with aliphatic substituents can allow better solution
processability and good charge carrier mobility. In addition, the whole molecule is planar
which can facilitate intermolecular π- π stacking and intermolecular charge transfer
characteristics.
Porphyrins: These donors resemble natural chlorophyll and have demonstrated
their capacity to be successful donors for OPVs applications. The high absorption
coefficient, synthetic versatility and photoconductivity mark them as extremely
promising donor candidates. The feasibility of porphyrin synthesis lies in the periphery
substitution and metal insertion which could tune energy levels. This feature is very
attractive for OPVs application. However, the absorption region of porphyrins is mainly
in the blue region thus poor spectral overlap with solar radiation. In addition, the short

23

exciton diffusion length of porphyrins sets a limit on high performance OPVs. The low
charge carrier mobility documented in literature is another factor contributing to low
performance of porphyrin based OPVs. A Porphyrin derivative 5,10,15,20- Tetraphenyl-
21H, 23H-porphine zinc (ZnTPP) dye is used in a BHJ device pairing with PCBM. Due
to quite low charge carrier mobility 10
-10
cm
2
/V.S of ZnTPP compared with PCBM
of10
-3
cm
2
/V.S, low PCE of only 0.21% was achieved.
70

Our group has worked on Pt and Pd based Tetraphenylbenzoporphyrin (TPBP) as
donors pairing with C
60
acceptor. Two porphyrin donors achieved comparable
performance as high as 1.9% but have optimal donor thickness with only 15 nm. This is
due to the short exciton diffusion length of porphyrin donor.
71
It bears mentioning that
both porphyrin donors are solution processable and have obtained comparable device
performance with vacuum deposited OPVs.
A porphyrin-fullerene dyad can serve as acceptor in BHJ device. By pairing this
acceptor with donor poly[(4,4’-bis(2-ethylhexyl)dithieno[3,2-b:2’,3’-d]silole)-2,6-diyl-
alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5’-diyl] (SiPCPDTBT), ambipolar
charge transport is demonstrated and high short circuit current of 16 mA/cm
2
is achieved.
By incorporating porphyrin unit, this dyad has much better absorbance than normal
fullerene derivative acceptor. In addition, the porphyrin and fullerene part can alternate
their arrangement in a way that enables both donor and acceptor to form a continuous
pathway, making bipolar transport possible.
72

Phthacyanines (PC): Phthacyanines are planar macrocycles. They are the
representatives of the subclass of azaporphyrin and can be considered to be a similar

24

family to Porphyrin dye. The PC derivatives usually have good thermal stability. Similar
to Porphyrins, PCs can have tunable optical and electronic property by structure
modification and metalation. Unlike Porphyrins, PCs have a broad and intense absorption
in the visible region. Normally, PCs have longer exciton diffusion length than Porphyrins
benefiting from structure and closer packing in the film.
45

The first breakthrough of bilayer heterojucntion OPVs was achieved by
CuPc/PTCBI.
32
At that time, with AM 2, 75 mA/cm
2
light intensity, nearly 1%
efficiency was achieved. Because of the chemical and thermal stability of CuPc, the
CuPc/C
60
has been a standard cell within OPV communities. A lot of research work has
followed up with this D/A combination to boost the power conversion efficiency. Later
on, using structure engineering, a three layer device of CuPc/CuPc:C
60
/C
60
could generate
a power conversion efficiency of 5.0% at 120 mA/cm
2
light intensity under AM 1.5
illumination.
73

Since CuPc does not absorb enough in the NIR region, PC derivatives such as
chloroaluminum phthalocyanine, ClAlPc
74
and oxotitanium phthalocyanine TiOPc
75
are
the ones with extended absorption and spectral response in the NIR region.
Another Pc derivative, boron subphthalocyanine chloride (SubPc) has C3
symmetry and bended structure. It has much better solubility due to its nonplanar
structure. The SubPc is composed of three N-fused diiminoisoindole rings centered on a
boron core. It has about 0.4 eV deeper HOMO than that of CuPc, which results in much
higher V
oc
of ~1.0 V.
76
The subnaphthalocyanine (SubNc), analogue of SubPc, was made
to enhance materials solubility and overcome the tendency to form aggregates. This work

25

demonstrates the possibility of low cost of solution processed SubNc based OPVs.
77
To
date, the highest SubPc based OPVs achieved high performance of 4.2% by graded
heterojunction donor-acceptor structure.
78

Merocyanines: As an intense absorbing dye, Merocyanines are featured with
tunable absorption. With suitable structure design, a merocyanine dye with good charge
transport ability and well separated percolation pathway with PCBM could achieve
power conversion efficiency of 2.59%.
79
Recently, Ojala et.al have manipulated the
orientation of merocyanine dye in a planar junction with C
60
by post thermal annealing.
The enhanced exciton dissociation in this structure leads to an improved performance of
3.9%.
80

Diketopyrolopyrroles (DPP) type: Electron donating unit DPP was first
proposed by Nguyen group and a lot of successful chromospheres have been derived
from it. The first DPP based chromophore from Nguyen group integrated DPP into
oligothiophene main chain. The achieved low band gap small molecule has broad solar
spectrum coverage and achieved a PEC of 3%.
81
Later, by replacing terminal thiophene
units with a fused benzofuran, the resulting (DPP(TBFu)
2
) has better intermolecular
interaction and stabilized HOMO energy. The record efficiency of 4.4% was obtained
after post thermal annealing of (DPP(TBFu)
2
) and PC
71
BM blends.
64

1.4 Seeking for New Materials Squaraines
The OPVs have advanced greatly over the past decade. High efficiency cells have
been achieved in both academia lab and industry. To achieve high PCE OPVs, the main
areas still needing to be addressed are: refinement of highly absorbing materials having

26

strong overlap with solar spectrum, enhancement of exciton diffusion length by new
materials or novel device architecture, better charge carrier mobility and minimizing open
circuit voltage losses. This dissertation is dedicated to a new family of squaraines as
active donor in OPVs research for being an efficient and broad absorber, and with
minimum open circuit voltage loss. The chapter 2 is focused on squaraines review in
terms of application and synthesis, concluding with a facile synthesis scheme for a new
family of arylanilino squaraines. Chapter 3 discusses squaraine photophysical property
characterizations that are necessary for OPVs application. Those data suggest that
squaraines are suitable donors to pair with fullerene and its derivatives in OPVs. Chapter
4 highlights all squaraines’ application in devices from vapor deposited OPVs, solution
process bilayer and BHJ devices. Ultimately, by incorporating new materials design and
process condition optimization, along with a new type of materials with minimal open
circuit voltage loss, decent photocurrent and high FF for OPVs are achieved.

27

1.5 Chapter 1: References
1. Service, R. F., Is It Time to Shoot for the Sun. Science 2005, 309, (5734), 548.

2. Herzog, A. V.; Lipman, T. E.; Kammen, D. M., Renewable Energy Sources. 2001

3. Potocnik, J., Renewable Energy Sources and the Realities of Setting an Energy
Agenda. Science 2007, 315, (5813), 810-811.

4. Schiermeier, Q.; Tollefson, J.; Scully, T.; Witze, A.; Morton, O., Energy
Alternatives: Electricity without Carbon. Nature 2008, 454, 816-823.

5. Friedlingstein, P.; Houghton, R. A.; Marland, G.; Hackler, J.; Boden, T. A.;
Conway, T. J.; Canadell, J. G.; Raupach, M. R.; Ciais, P.; Le Quéré, C., Update on Co
2

Emission. Nature Geosci 2010, 3, 811-812.

6. Service, R. F., Solar Energy: Can the Upstarts Top Silicon? Science 2008, 319,
718-720.

7. Goetzberger, A.; Hebling, C.; Schock, H. W., Photovoltaic Materials, History,
Status and Outlook. Mater. Sci. Eng., R 2003, 40, (1), 1-46.

8. Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D., Solar Cell
Efficiency Tables (Version 39). Progress in Photovoltaics 2012, 20, (1), 12-20.

9. Green, M.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E., Solar Cell
Efficiency Tables (Version 38) Progress in Photovoltaics 2011, 19, (5), 565-572.

10. Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.;
Gershenson, M. E.; Rogers, J. A., Elastomeric Transistor Stamps: Reversible Probing of
Charge Transport in Organic Crystals. Science 2004, 303, (5664), 1644-1646.

11. McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; Macdonald, I.; Shkunov,
M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W. M.; Chabinyc, M. L.; Kline, R. J.;
McGehee, M. D.; Toney, M. F., Liquid-Crystalline Semiconducting Polymers with High
Charge-Carrier Mobility. Nature Materials 2006, 5, (4), 328-333.

12. Schlenker, C. W.; Thompson, M. E., Current Challenges in Organic Photovoltaic
Solar Energy Conversion. Top Curr Chem 2012, 312, 175-212.

13. Schlenker, C. W.; Thompson, M. E., The Molecular Nature of Photovoltage
Losses in Organic Solar Cells. Chem. Commun. 2011, 47, (13), 3702-3716.

28

14. Augustine, G.; Rohatgi, A.; Jokerst, N. M.; Dhere, R., Contentration-Dependent
near and above Band-Edge Asorption in Doped Inp and Its Effect on Solar Cell
Modeling. J. Appl. Phys. 1995, 78, (4), 2666-2670.

15. Lipomi, D. J.; Tee, B. C. K.; Vosgueritchian, M.; Bao, Z., Stretchable Organic
Solar Cells. Adv. Mater. 2011, 23, (15), 1771-1775.

16. Heliatek GmbH, t., Heliatek Sets New World Record Efficiency of 10.7 % for its
Organic Tandem Cell 2012.

17. Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.;
Hummelen, J. C., 2.5% Efficient Organic Plastic Solar Cells. Appl. Phys. Lett. 2001, 78,
(6), 841-843.

18. Yao, Y.; Hou, J.; Xu, Z.; Li, G.; Yang, Y., Effect of Solvent Mixture on the
Nanoscale Phase Separation in Polymer Solar Cells. Adv. Funct. Mater. 2008, 18, (12),
1783-1789.

19. Li, G.; Yao, Y.; Yang, H.; Shrotriya, V.; Yang, G.; Yang, Y., "Solvent
Annealing" Effect in Polymer Solar Cells Based on Poly(3-Hexylthiophene) and
Methanofullerenes. Adv. Funct. Mater. 2007, 17, (10), 1636-1644.

20. Mihailetchi, V. D.; Xie, H. X.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M.,
Charge Transport and Photocurrent Generation in Poly (3-Hexylthiophene):
Methanofullerene Bulk-Heterojunction Solar Cells. Adv. Funct. Mater. 2006, 16, (5),
699-708.

21. Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J., Thermally Stable,
Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network
Morphology. Adv. Funct. Mater. 2005, 15, (10), 1617-1622.

22. Moule, A. J.; Meerholz, K., Morphology Control in Solution-Processed Bulk-
Heterojunction Solar Cell Mixtures. Adv. Funct. Mater. 2009, 19, (19), 3028-3036.

23. Groves, C.; Reid, O. G.; Ginger, D. S., Heterojeneity in Polymer Solar Cells:
Local Morphology and Performance in Organic Photovoltaics Studeid with Scanning
Probe Microscopy. Acc. Chem. Res 2010, 43, (5), 612-620.

24. Wei, G.; Wang, S.; Renshaw, K.; Thompson, M. E.; Forrest, S. R., Solution-
Processed Squaraine Bulk Heterojunction Photovoltaic Cells. Acs Nano 2010, 4, (4),
1927-1934.

29

25. Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J.,
Solution-Processed Small-Molecule Solar Cells with 6.7% Efficiency. Nature Materials
2011, 11, (1), 44-48.

26. Jadhav, P. J.; Mohanty, A.; Sussman, J.; Lee, J.; Baldo, M. A., Singlet Exciton
Fission in Nanostructured Organic Solar Cells. Nano Letters 11, (4), 1495-1498.

27. He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y.,
Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and
Fill Factor in Polymer Solar Cells. Adv. Mater. 2011, 23, (40), 4636-4643.

28. Chiu, S.-W.; Lin, L.-Y.; Lin, H.-W.; Chen, Y.-H.; Huang, Z.-Y.; Lin, Y.-T.; Lin,
F.; Liu, Y.-H.; Wong, K.-T., A Donor-Acceptor-Acceptor Molecule for Vacuum-
Processed Organic Solar Cells with a Power Conversion Efficiency of 6.4%. Chem.
Commun. 48, (13), 1857-1859.

29. Forrest, S. R., The Limits to Organic Photovoltaic Cell Efficiency. MRS Bulletin
2005, 30, 28-32.

30. Tang, C. W.; Albrecht, A. C., Photovoltaic Effects of Metal Cholorophyll-a-Metal
Sandwich Cells. J. Chem. Phys. 1975, 62, (6), 2139-2149.

31. Barth, S.; Bassler, H., Intrinsic Photoconduction in Ppv-Type Conjugated
Polymers. Phys. Rev. Lett. 1997, 79, (22), 4445-4448.

32. Tang, C. W., Two Layer Organic Photovoltaic Cell. Appl. Phys. Lett. 1986, 48,
(2), 183-185.

33. Banerjee, A. N.; Chattopadhyay, K. K., Recent Developments in the Emerging
Field of Crystalline P-Type Transparent Conducting Oxide Thin Films. Prog. Cryst.
Growth Charact. Mater. 2005, 50, (1-3), 52-105.

34. Kawazoe, H.; Yasukawa, M.; Hyodo, H.; Kurita, M.; Yanagi, H.; Hosono, H., P-
Type Electrical Conduction in Transparent Thin Films of Cualo2. Nature 1997, 389,
(6654), 939-942.

35. Tenent, R. C.; Barnes, T. M.; Bergeson, J. D.; Ferguson, A. J.; To, B.; Gedvilas,
L. M.; Heben, M. J.; Blackburn, J. L., Ultrasmooth, Large-Area, High-Uniformity,
Conductive Transparent Single-Walled-Carbon-Nanotube Films for Photovoltaics
Produced by Ultrasonic Spraying. Adv. Mater. 2009, 21, (31), 3210-3216.

36. Rowell, M. W.; Topinka, M. A.; McGehee, M. D.; Prall, H.-J.; Dennler, G.;
Sariciftci, N. S.; Hu, L.; Gruner, G., Organic Solar Cells with Carbon Nanotube Network
Electrodes. Appl. Phys. Lett. 2006, 88, (23).

30

37. Park, H.; Rowehl, J. A.; Kim, K. K.; Bulovic, V.; Kong, J., Doped Graphene
Electrodes for Organic Solar Cells. Nanotechnology 2010, 21, (50).

38. De Arco, L. G.; Zhang, Y.; Schlenker, C. W.; Ryu, K.; Thompson, M. E.; Zhou,
C., Continuous, Highly Flexible, and Transparent Graphene Films by Chemical Vapor
Deposition for Organic Photovoltaics. Acs Nano 2010, 4, (5), 2865-2873.

39. Hirose, Y.; Kahn, A.; Aristov, V.; Soukiassian, P.; Bulovic, V.; Forrest, S. R.,
Chemistry and Electronic Properties of Metal-Organic Semiconductor Interfaces: Al, Ti,
in, Sn, Ag, and Au on Ptcda. Phys. Rev. B 1996, 54, (19), 13748-13758.

40. Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K., Energy Level Alignment and Interfacial
Electronic Structures at Organic Metal and Organic Organic Interfaces. Adv. Mater.
1999, 11, (8), 605-625.

41. Peumans, P.; Yakimov, A.; Forrest, S. R., Small Molecular Weight Organic Thin
Film Photodetectors and Solar Cells. J. Appl. Phys. 2003, 93, (7), 3693-3723.

42. Li, N.; Lassiter, B. E.; Lunt, R. R.; Wei, G.; Forrest, S. R., Open Circuit Voltage
Enhancement Due to Reduced Dark Current in Small Molecule Photovoltaic Cells. Appl.
Phys. Lett. 2009, 94, (2), 023307.

43. Peumans, P.; Forrest, S. R., Very High Efficiency Double Heterostructure Copper
Phthalocyanine/C
60
Photovoltaic Cells. Appl. Phys. Lett. 2001, 79, (1), 126-128.

44. Brabec, C. J.; Shaheen, S. E.; Winder, C.; Sariciftci, N. S.; Denk, P., Effect of
Lif/Metal Electrodes on the Performance of Plastic Solar Cells. Appl. Phys. Lett. 2002,
80, (7), 1288-1290.

45. Stubinger, T.; Brutting, W., Exciton Diffusion and Optical Interference in Organic
Donor-Acceptor Photovoltaic Cells. J. Appl. Phys. 2001, 90, (7), 3632-3641.

46. Lassiter, B. E.; Wei, G.; Wang, S.; Zimmerman, J. D.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R., Organic Photovoltaics Incorporating Electron Conducting Exciton
Blocking Layers. Appl. Phys. Lett. 2011, 98, (24), 243307.

47. Lunt, R. R.; Giebink, N. C.; Belak, A. A.; Benziger, J. B.; Forrest, S. R., Exciton
Diffusion Lengths of Organic Semiconductor Thin Films Measured by Spectrally
Resolved Photoluminescence Quenching. J. Appl. Phys. 2009, 105, (5), 053711.

48. Hiramoto, M.; Suezaki, M.; Yokoyama, M., Effect of Thin Gold Interstitial-Layer
on the Photovoltaic Properties of Tandem Organic Solar-Cell. Chem. Lett. 1990, (3), 327-
330.

31

49. Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E., Molecular and
Morphological Influences on the Open Circuit Voltages of Organic Photovoltaic Devices.
J. Am. Chem. Soc. 2009, 131, (26), 9281-9286.

50. Roquet, S.; de Bettignies, R.; Leriche, P.; Cravino, A.; Roncali, J., Three-
Dimensional Tetra(Oligothienyl)Silanes as Donor Material for Organic Solar Cells. J.
Mater. Chem. 2006, 16, (29), 3040-3045.

51. Kopidakis, N.; Mitchell, W. J.; van de Lagemaat, J.; Ginley, D. S.; Rumbles, G.;
Shaheen, S. E.; Rance, W. L., Bulk Heterojunction Organic Photovoltaic Devices Based
on Phenyl-Cored Thiophene Dendrimers. Appl. Phys. Lett. 2006, 89, (10), 103524.

52. Ma, C.-Q.; Fonrodona, M.; Schikora, M. C.; Wienk, M. M.; Janssen, R. A. J.;
Baeuerle, P., Solution-Processed Bulk-Heterojunction Solar Cells Based on
Monodisperse Dendritic Oligothiophenes. Adv. Funct. Mater. 2008, 18, (20), 3323-3331.

53. Dong, H.; Zhu, H.; Meng, Q.; Gong, X.; Hu, W., Organic Photoresponse
Materials and Devices. Chem. Soc. Rev. 2012, 41, (5), 1754-1808.

54. Yoo, S.; Domercq, B.; Kippelen, B., Efficient Thin-Film Organic Solar Cells
Based on Pentacene/C-60 Heterojunctions. Appl. Phys. Lett. 2004, 85, (22), 5427-5429.

55. Mayer, A. C.; Lloyd, M. T.; Herman, D. J.; Kasen, T. G.; Malliaras, G. G.,
Postfabrication Annealing of Pentacene-Based Photovoltaic Cells. Appl. Phys. Lett. 2004,
85, (25), 6272-6274.

56. Wagner, J.; Gruber, M.; Hinderhofer, A.; Wilke, A.; Broeker, B.; Frisch, J.;
Amsalem, P.; Vollmer, A.; Opitz, A.; Koch, N.; Schreiber, F.; Bruetting, W., High Fill
Factor and Open Circuit Voltage in Organic Photovoltaic Cells with Diindenoperylene as
Donor Material. Adv. Funct. Mater. 2010, 20, (24), 4295-4303.

57. Lloyd, M. T.; Anthony, J. E.; Malliaras, G. G., Photovoltaics from Soluble Small
Molecules. Materials Today 2007, 10, (11), 34-41.

58. Marrocchi, A.; Silvestri, F.; Seri, M.; Facchetti, A.; Taticchia, A.; Marks, T. J.,
Conjugated Anthracene Derivatives as Donor Materials for Bulk Heterojunction Solar
Cells: Olefinic Versus Acetylenic Spacers. Chem. Commun. 2009, (11), 1380-1382.

59. Zhang, Q. T.; Tour, J. M., Alternating Donor/Acceptor Repeat Units in
Polythiophenes. Intramolecular Charge Transfer for Reducing Band Gaps in Fully
Substituted Conjugated Polymers. J. Am. Chem. Soc. 1998, 120, (22), 5355-5362.

32

60. Havinga, E. E.; Tenhoeve, W.; Wynberg, H., Alternate Donor-Acceptor Small-
Band-Gap Semiconducting Polymers - Polysquaraines and Polycroconaines. Synth. Met.
1993, 55, (1), 299-306.

61. Steinberger, S.; Mishra, A.; Reinold, E.; Levichkov, J.; Uhrich, C.; Pfeiffer, M.;
Baeuerle, P., Vacuum-Processed Small Molecule Solar Cells Based on Terminal
Acceptor-Substituted Low-Band Gap Oligothiophenes. Chem. Commun. 47, (7), 1982.

62. Fitzner, R.; Reinold, E.; Mishra, A.; Mena-Osteritz, E.; Ziehlke, H.; Koerner, C.;
Leo, K.; Riede, M.; Weil, M.; Tsaryova, O.; Weiss, A.; Uhrich, C.; Pfeiffer, M.;
Baeuerle, P., Dicyanovinyl-Substituted Oligothiophenes: Structure-Property
Relationships and Application in Vacuum-Processed Small-Molecule Organic Solar
Cells. Adv. Funct. Mater. 21, (5), 897-910.

63. Li, W.; Du, C.; Li, F.; Zhou, Y.; Fahlman, M.; Bo, Z.; Zhang, F.,
Benzothiadiazole-Based Linear and Star Molecules: Design, Synthesis, and Their
Application in Bulk Heterojunction Organic Solar Cells. Chem. Mater. 2009, 21, (21),
5327-5334.

64. Walker, B.; Tomayo, A. B.; Dang, X.-D.; Zalar, P.; Seo, J. H.; Garcia, A.;
Tantiwiwat, M.; Nguyen, T.-Q., Nanoscale Phase Separation and High Photovoltaic
Efficiency in Solution-Processed, Small-Molecule Bulk Heterojunction Solar Cells. Adv.
Funct. Mater. 2009, 19, (19), 3063-3069.

65. Loser, S.; Bruns, C. J.; Miyauchi, H.; Ortiz, R. P.; Facchetti, A.; Stupp, S. I.;
Marks, T. J., A Naphthodithiophene-Diketopyrrolopyrrole Donor Molecule for Efficient
Solution-Processed Solar Cells. J. Am. Chem. Soc. 133, (21), 8142-8145.

66. Mei, J.; Graham, K. R.; Stalder, R.; Reynolds, J. R., Synthesis of Isoindigo-Based
Oligothiophenes for Molecular Bulk Heterojunction Solar Cells. Org. Lett. 12, (4), 660.

67. Leliege, A.; Blanchard, P.; Rousseau, T.; Roncali, J., Triphenylamine/Tetracyano
butadiene-Based D-a-D Pi-Conjugated Systems as Molecular Donors for Organic Solar C
ells. Org. Lett. 13, (12), 3098-3101.

68. Rousseau, T.; Cravino, A.; Bura, T.; Ulrich, G.; Ziessel, R.; Roncali, J., Bodipy
Derivatives as Donor Materials for Bulk Heterojunction Solar Cells. Chem. Commun.
2009, (13), 1673-1675.

69. Lin, L.-Y.; Chen, Y.-H.; Huang, Z.-Y.; Lin, H.-W.; Chou, S.-H.; Lin, F.; Chen,
C.-W.; Liu, Y.-H.; Wong, K.-T., A Low-Energy-Gap Organic Dye for High-Performance
Small-Molecule Organic Solar Cells. J. Am. Chem. Soc. 133, (40), 15822-15825.

33

70. Khan, S. M.; Kaur, M.; Heflin, J. R.; Sayyad, M. H., Fabrication and
Characterization of Zntpp:Pcbm Bulk Heterojunction (Bhj) Solar Cells. J. Phys. Chem.
Solids 72, (12), 1430-1435.

71. Perez, M. D.; Borek, C.; Djurovich, P. I.; Mayo, E. I.; Lunt, R. R.; Forrest, S. R.;
Thompson, M. E., Organic Photovoltaics Using Tetraphenylbenzoporphyrin Complexes
as Donor Layers. Adv. Mater. 2009, 21, (14-15), 1517-1520.

72. Wang, C.-L.; Zhang, W.-B.; Van Horn, R. M.; Tu, Y.; Gong, X.; Cheng, S. Z. D.;
Sun, Y.; Tong, M.; Seo, J.; Hsu, B. B. Y.; Heeger, A. J., A Porphyrin-Fullerene Dyad
with a Supramolecular "Double-Cable" Structure as a Novel Electron Acceptor for Bulk
Heterojunction Polymer Solar Cells. Adv. Mater. 2011, 23, (26), 2951-2956.

73. Xue, J. G.; Rand, B. P.; Uchida, S.; Forrest, S. R., A Hybrid Planar-Mixed
Molecular Heterojunction Photovoltaic Cell. Adv. Mater. 2005, 17, (1), 66-71.

74. Bailey-Salzman, R. F.; Rand, B. P.; Forrest, S. R., Near-Infrared Sensitive Small
Molecule Organic Photovoltaic Cells Based on Chloroaluminum Phthalocyanine. Appl.
Phys. Lett. 2007, 91, (1), 013508.

75. Brumbach, M.; Placencia, D.; Armstrong, N. R., Titanyl Phthalocyanine/C-60
Heterojunctions: Band-Edge Offsets and Photovoltaic Device Performance. J. Phys.
Chem. C 2008, 112, (8), 3142-3151.

76. Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.; Thompson, M. E.,
Enhanced Open-Circuit Voltage in Subphthalocyanine/C
60
Organic Photovoltaic Cells. J.
Am. Chem. Soc. 2006, 128, (25), 8108-8109.

77. Ma, B.; Woo, C. H.; Miyamoto, Y.; Frechet, J. M. J., Solution Processing of a
Small Molecule, Subnaphthalocyanine, for Efficient Organic Photovoltaic Cells. Chem.
Mater. 2009, 21, (8), 1413-1417.

78. Pandey, R.; Holmes, R. J., Graded Donor-Acceptor Heterojunctions for Efficient
Organic Photovoltaic Cells. Adv. Mater. 22, (46), 5301-5305.

79. Buerckstuemmer, H.; Kronenberg, N. M.; Gsaenger, M.; Stolte, M.; Meerholz,
K.; Wuerthner, F., Tailored Merocyanine Dyes for Solution-Processed Bhj Solar Cells. J.
Mater. Chem. 20, (2), 240-243.

34

80. Ojala, A.; Petersen, A.; Fuchs, A.; Lovrincic, R.; Poelking, C.; Trollmann, J.;
Hwang, J.; Lennartz, C.; Reichelt, H.; Hoeffken, H. W.; Pucci, A.; Erk, P.; Kirchartz, T.;
Wuerthner, F., Merocyanine/C60 Planar Heterojunction Solar Cells: Effect of Dye
Orientation on Exciton Dissociation and Solar Cell Performance. Adv. Funct. Mater. 22,
(1), 86-96.

81. Tamayo, A. B.; Dang, X.-D.; Walker, B.; Seo, J.; Kent, T.; Nguyen, T.-Q., A Low
Band Gap, Solution Processable Oligothiophene with a Dialkylated Diketopyrrolopyrrole
Chromophore for Use in Bulk Heterojunction Solar Cells. Appl. Phys. Lett. 2009, 94,
(10), 103301.

35

Chapter 2. Squaraines
2.1 Introduction
2.1.1 General
Squaraine dyes are 1,3 di-substituted derivatives of squaric acid (3,4-
dihydroxycyclobut-3-en3-1,2-dione) with resonance stabilized zwitterionic structures
(Figure 2-1). They contain electron donating groups attached to a central electron
accepting core resulting in donor-acceptor-donor type molecules. The electron donating
groups can be electron rich arenes, aromatics with substantial π-bonding or heterocycles.
Squaraines represent a versatile group of molecules because of the wide variety of donor
molecules that can be used in their synthesis. This is a very important feature of
squaraines which make them amenable for use in several applications. Figure 2-2 shows
some typical symmetrical squaraines with aniline, pyrrole, benzothiazole and azulene
substituents.
Theoretical calculations suggest π electrons are extensively delocalized over the
squaraine molecule.
1
Bigelow and Freund have studied the electronic properties of
dimethylanilino squariane (Figure 2-2. (a))
2
using MNDO and CNDO semiempirical
molecular orbital approximations. The charge transfer (CT) during ground S
0
state to S
1
electronic excitation was shown. However, such CT is mainly confined in the central
C
4
O
2
unit with 0.094 e CT from each oxygen atom to C
4
O
2
. A small amount of CT (0.019
e) from aniline moiety to the C
4
O
2
unit is also present.
3
Such intramolecular charge
transfer and the tendency of squaraines to aggregate give squaraine films with
panchromatic absorption (400-800 nm). Due to the localization of electronic transitions,

36

modifications in donor groups have little impact on the maximum absorption wavelength
( λ
max
) of squaraines.

Figure 2-1. (a) Squariane zwitterionic structure; (b) Squaraine resonance structure.

Figure 2-2. Typical symmetrical squaraine structures: (a) Dimethylaniline substituted;
(b) Dimethylpyrrole substituted;(c) Benzothiazole substituted; (d) Azulene
substituted.
N N
O
O
2+
O
O
2+
N
N
O
O
2+
S
N
N
S
O
O
2+
(a) (b)
(c) (d)

O
O
R
2
R
1
2+
O
O
R
2
R
1
O
O
R
2
R
1
(a)
(b)
O
O
R
2
R
1
O
O
R
2
R
1
(b)

37

Squaraines share structure similarity with merocyanine and cyanine dyes. Cyanine
dyes are planar, conjugated, open chained system of sp
2
-hybridizied carbon atoms with
an odd number of methine groups and an even number of π electrons according to the
general formula:
X-(CR)
n
-X’
The cyanine dyes consist of two nitrogen centers, one of which is positively
charged and linked by a conjugated chain of carbon atoms to the other nitrogen.
4
In
contrast, merocyanines are polymethines capped on one end with a nitrogen atom and
with an oxygen-containing group on the other. The whole molecule is neutral. The
common characteristics of these three types of dyes are their highly intense absorptions,
strong emissions and the tendency to form aggregates. The structural differences of three
types of dyes are shown in figure 2-3.

Figure 2-3. Representative structures of Cyanines, Merocyanines and Squaraines.
2.1.2 Applications of Squaraines
The desirable properties of squaraines, for example, their intense absorption in the
red/NIR region, photostability and photoconductivity are attractive for applications in
different fields such as photodynamic therapy,
5
two photon absorption,
6
organic solar
cells,
7
fluorescent probes
8
and field effect transistors.
9

38

Photodynamic therapy (PDT): Squaraines have been applied as photosensitizers
in PDT, which is known for non-invasive treatment for cancer and acne.
10
PDT utilizes a
photosensitizer, light and singlet oxygen. The mechanism involved is the intersystem
crossing of the singlet state of the photosensitizer to the triplet state, which undergoes
electron/energy transfer to generate highly reactive singlet oxygen (Figure 2-4). A
suitable photosensitizer should have strong absorption from 600 nm to 900 nm.
Squaraines render themselves as good candidates for use in PDT because of their intense
absorption and fluorescence in the NIR region. However, only water soluble squaraines
with high intersystem crossing yield are preferred for PDT application. One of the main
challenges in development of squaraines in PDT relies on the improvement of their
intersystem crossing yield.

Figure 2-4. The mechanism of photodynamic therapy.
Some efforts were made at synthesizing halogenated squaraines with different
donor groups such as benzothiazole, quinoline, pyrrole and qunaldine in order to enhance
intersystem crossing yield.
5,11,12
Wang, et.al.
13
have shown that iodinated aniline
squaraine (Figures 2-5) can enhance singlet oxygen (
1
O
2
) quantum yield from 0.02 to
0.54. Since squaraines are not generally water soluble, there are also efforts to enhance

39

water solubility by incorporating polar functional groups such as sulfonate, carbohydrate,
hydroxyl and phosphonate to squaraines.
14

Figure 2-5. Structure modification of squaraine for photodynamic therapy application.
Two Photon Absorption: Two photon absorption (2PA) is a non linear process in
which a molecule absorbs two photons at the same or different frequency and becomes
promoted to one of its excited states. The energy difference of excited state and ground
state is equal to two photon energy. 2PA is different from linear absorption in that its
intensity is dependent on the square of light intensity. 2PA can potentially be used in
fluorescence microscopy,
15
3D microfabrication
16
and optical pulse suppression.
17
D-A-D
type molecules are of interest because of their large 2PA cross sections δ. Large δ values
is obtainable by extended conjugation in symmetrical chromophores.
The pioneering 2PA work with squaraines was conducted by Scherer et.al. using
analogs with dimethylindoline anhydrobase donors.
6
A high δ value of 5000 GM was
reported with 2PA peaks in 820-890 nm range. A possible route to enhance 2PA
performance is the extension of conjugated π-system of squaraines. Record high values of
33000 and 14000 GM were reported by Marder et al.
18

A prerequisite for strong 2PA is a large transition dipole moment, which is
available in both porphyrin and squaraine molecules. A coupled product, Bis(Porphyrin)-

40

substituted Squaraine, showed substantial electronic coupling between porphyrin and
squaraine moieties. The overall performance is more than an additive result of the
porphyrin and squaraine subunits. This results in a large δ over a 750 nm wide
wavelength and can be applied in broadband NIR pulse suppression.
19

Organic solar cells: The exceptional absorption coefficient, reasonable charge
carrier mobility
20
and photoconductive characteristics of squaraines made them
promising candidates for use in organic solar cells. However, there are only few
examples of application of squaraines in this regard. More importantly, the properties of
squaraines are easily tunable via structure modification.
The first squaraine based OPV was fabricated in a single layer Schottky diode cell
in 1978.
21
It was vacuum deposited dye sandwiched between two metal electrodes with
V
oc
=1.2 V, Jsc =1.8 mA/cm
2
, FF =0.25, resulting in a PCE of 0.7%. However, it was not
until 2009 that the first squaraine-based BHJ was reported by Tobin Marks’ group.
7
This
work was based on blended films of hydrazone alkylpyrrole substituted squaraine and
PC
60
BM (Figures 2-6). The use of different alkyl substituent groups allows solubility
control and offers a tool to control solid state properties. For example, the squaraine with
alkyl chain R
1
(2-ethylhexyl) has a better performance with V
oc
of 0.62 V, Jsc of 5.7
mA/cm
2
, FF of 0.35 and PCE of 1.24%. The noticeable poor FF is likely due to
unbalanced hole and electron mobility. The same group later reported new squaraines
substituted with n-hexyl and hexenyl chains.
22
The N-alkenyl substituent affords a more
compact solid state structure with thin film transistor hole mobility five times enhanced,
and OPV performance of 2.05% was achieved.

41

Figure 2-6. The hydrazone alkylpyrrole type squaraines.
Later, U. Mayerhoffer et. al reported squaraine based BHJ OPVs with a
significantly higher J
sc
value of 12.6 mA/cm
2
.
23
The authors claimed that the
approximately 10 fold increase in Jsc was mainly due to the introduction of dicyanovinyl
functional groups (figure 2-7). Studies are underway to better define and understand
structure/photovoltaic response relationship. Overall, there are not many squaraine-based
OPVs reported in literature. It is the goal of this dissertation to understand the properties
of a new family of squaraines. By analyzing their performance in OPVs, we are able to
understand structure-correlated properties of OPVs and design squaraines with molecular
structures that are better for OPVs’ applications.

Figure 2-7. The squaraine achievied exceptional high short circuit current.
Fluorescent probes: Fluorescent molecules can be used in biological research for
detecting protein location, activated complex formation and quantification. The

42

mechanism involves enhanced fluorescent response due to non covalent bonding between
fluorescent molecules and proteins such as human serum albumin (HAS) and bovine
serum albumin. The quantitative determination of HAS level is of significant importance
in clinical diagnosis. Fluorescent probes have the advantage over traditional methods due
to their high selectivity, sensitivity and convenience. However, fluorescent dyes with
water solubility, photostability and low toxicity are required. Some squaraines can be
used as fluorescent probes because they show intense absorption in between 600-900 nm
and have strong fluorescence emission. Based on the findings from the previous
application of squaraines as fluorescent probes, Volkova, K et. al
8
have shown that a
series of benzothiazole and benzoselenazole squaraine derivatives can be used for HAS
and bovine serum albumin detection. It was found that the binding affinity of squaraine
dyes is related to their pendent alkyl group length. The squaraines with a long N-hexyl
pendent group exhibit much stronger emission in the presence of proteins than the ones
with a short N-ethyl chain. The N-hexyl pendent squaraines show about 100-540 fold
fluorescence intensity increase with protein addition. More importantly, the
benzothiazole squaraines have selectivity to albumin and a linear correlation of
fluorescence intensity with HAS concentration. This offers opportunity for HAS
quantification by benzothiazole squaraines. Research focusing on squaraines with better
water solubility was conducted using analogs with sulfo group terminated alkyl
substituents.
24

Organic field effect transistors (OFETs): Acenes and thiophene derivatives are
often used in fabrication of organic field effect transistors (OFETs) due to their high

43

charge carrier motilities. However, some squaraines with enhanced charge carrier
mobility can be used in OFETs as well. Wöbkenberg. et.al
9
have used a solution-
processed guaiazulenyl-derived squaraine dye in the fabrication of a bottom gate contact
type OFET. From the transfer curves of transistor, both electron and hole mobility of
approximately 10
-4
cm
2
Vs
-1
for guaiazulenyl squaraine were identified, which also
decreased with increasing channel length. This is probably due to the grain boundaries
present in the channels. This work demonstrates that single squaraine can act as
ambipolar organic semiconductors, which could significantly reduce fabrication cost.
Further enhancement of squaraine OFETs can be achieved by growing thin films with
larger crystalline domains and fewer grain boundaries.

2.2 Review of Squaraine Synthesis
2.2.1 General
The first symmetrical pyrrole and phloroglucinol based squaraines were
synthesized by Treibs and Jacob in 1965.
25,26
Later on, aniline based squaraines were
synthesized by Sprenger and Ziegenbein in 1966.
27
Traditional squaraine synthesis is
carried out by the reaction of one equivalent of squaric acid and two equivalents of
electron rich donors in azeotropic solvents mixtures containing toluene or benzene.
Higher reaction yields are often achieved with more electron rich donors. The scope of
squaraine compounds possible is mainly dependent on the available electron-rich donors.

44

In the early years, squaraine compounds made by this synthetic route were often
contaminated with impurities from squaric acid. However, this is no longer an issue since
high quality squaric acid is commercially available nowadays.

Figure 2-9. Pyrrole (a) and phloroglucinol (b) based squaraines.
2.2.2 Product Contamination with 1,2 Substituted Derivatives
In this synthetic route described in section 2.2.1, the 1,2 substitued isomer is
identified as a side product which behaves differently from 1,3 substituted squaraines.
Recently, Pagani., et.al studied the regioselective synthesis of 1,2 and 1,3-squaraines
specifically with benzothiazole substituted squaraine.
28
Understanding the conditions
under which the 1,2 substituted squaraine derivatives are formed can provide more
insight in synthesizing regio-regular 1,3 substituted squaraines. As shown in scheme 2,
depending on whether compound a (3-hexyl-2-methylene-2,3-dihydrobenzo[d]thiazole)
react with, either the 1,2 substituted compound c is formed or both 1,3 substituted
compound e (main product) and 1,2 substituted compound c are generated. The
difference depends on the relative position of carbonyl to benzothiazole methylene that
compound a attacks. In the first case, the carbonyl α to the benzothiazole methylene is
more electrophilic while in the latter case, the nucleophilic attack occurs at the carbonyl β
to the methylene benzothiazole because the carbonyl α is conjugated with enolate which

45

makes it electron rich. Overall, more electron rich donor is required to form 1,2-
squaraine. From this work, regioregular synthesis of squaraine is possible by selecting
suitable electron rich precursor.
In the follow up study to the regioregular benzothiazole squaraine synthesis,
28
the
author also characterized UV-Vis absorption and electrochemical properties of 1,2
substituted squaraines. The 1,3 substituted squaraines are known for their excellent
optical properties since they form intensely colored solutions. It is surprising that the less
conjugated 1,2 substituted squaraine c also has an intense color but with blue shifted
absorption. Additionally, compound c is stable with completely reversible redox
properties. Such properties are very important for possible future application of these
compounds as photosensitizers in photoconducting devices.

Scheme 1. Regioselective synthesis of benzothiazole squariane.
28

46

2.2.3 Ester route for squaraine synthesis

Scheme 2. The ester route of HSq synthesis.
In addition to traditional methods, squaraine synthesis can also be achieved using
mono as well as dialkyl esters of squaric acid. Lee et al have described an alternative
route using squarate esters to form squaraines with satisfactory yield and purity.
29
In
Lee’s work, a mechanism was proposed based on the synthesis of molecular Bis(4-
dimethylaminophenyl)squaraine. The typical route using squaric acid was considered as
an acid route. In the “acid route” with squaric acid and N,N-dimethylaniline, no reaction
occurs in non-hydroxylic solvents, secondary alcohols or tertiary alchohols.
29
Unlike the
acid route, the ester route (illustrated in Scheme 2) is catalyzed by sulfuric acid and water.
In this example, di-n-butyl squarate h is hydrolyzed to n-butyl squarate g. In this process,
the concentrations of sulfuric acid and water are important. However, too much sulfuric

47

acid can result in the protonation of the N,N-dimethylaniline and consequently a lower
reaction yield. In the ester route synthesis, a steric effect is apparent. For example, the
hydrolysis step f and arylation step g are both sensitive to substituent size. Studies show
that the reaction yield decreases with longer substituent chain.
2.2.4 Unsymmetrical squaraine synthesis

Scheme 2. The ester route of HSq synthesis.
In the ester synthesis route, the main precursor is semisquaraine, which is also the
building block for unsymmetrical squaraines. By coupling with other aniline reactants,
unsymmetrical squaraines can be formed through this route. Thus, the independent
synthesis of compound m is quite important because it provides a versatile route to the
synthesis of unsymmetrical squaraines. In route (a), the compound m is formed after
hydrolysis of mono-acid chloride from the reaction of acid dichloride of squaric acid and

48

N,N-dimethylaniline. Once the semisquaraine is obtained, the unsymmeterical squaraine
is formed by reacting semisquaraine with other electron rich donors like anilines or
anhydrobases. A simple example is shown in scheme 4.

Scheme 3. Synthesis routes for semisquaraines.
30

Scheme 4. Typical nonsymmetrical squaraine synthesis.
31

49

2.3 Arylanilino Squaraine Synthesis
2.3.1 Symmetrical Arylanilinosquaraine Synthesis
In this dissertation, new derivatives of symmetric squaraine dyes with N,N-
diarylanilino substituents that have high solubility and high absorptivity (ε = 0.71–4.1 x
105 M
-1
cm
-1
, α = 3.5–4.6 x 10
5
cm
-1
) in the red solar spectral region ( λ
max
= 645–694 nm)
which make them promising candidates for application in organic photovoltaics (OPVs)
are reported. Compounds SQ 2–8 were all made by a similar synthetic procedure.

1. R1 = R2 = isobutyl 5. R1 = 2-naphthyl, R2 = phenyl
2. R1 = methyl, R2 = phenyl 6. R1 = R2 = 4-biphenyl
3. R1 = R2 = phenyl 7. R1 = R2 = 2-biphenyl
4. R1 = 1-naphthyl, R2 = phenyl 8. R1 = 1-pyrenyl, R2 = phenyl
Scheme 5. Synthesis of symmetrical arylanilino squaraines
The synthesis of SQ 3 is detailed here and was followed for the preparation of the
other symmetric arylanilino-squaraines. The first procedure given is for the synthesis of
N (3,5 dihydroxyphenyl)diphenylamine, while the second is for the reaction of this
material with squaric acid to form the N,N diphenylanilino squaraine dye. The
characterization data for SQ 2–8 is provided in the following.

50

N-(3,5 dihydroxyphenyl)diphenylamine: Diphenylamine (3 g, 17.7 mmol),1-brom
o-3,5-dimethoxybenzene(4.6g,21mmol),Pd2(dba)3[tris(dibenzylidineacetone)dipalladium
(0)] (0.78 g, 0.8 mmol), sodium tert-butoxide (2.6 g, 25 mmol), and tri-tert-butylphosphin
e (0.52 g, 2.5 mmol) were dissolved in 100 ml of toluene and refluxed under N
2
for 5 hou
rs. The reaction mixture was cooled down and passed through a plug of silica gel with tol
uene to remove insoluble material. The crude product was recrystallized from toluene/Me
OH to obtain 4.5 g (83%) of N-(3,5-dimethoxyphenyl)diphenylamine. Next, this triaryla
mine (2.0 g, 6.5 mmol) was added to 70 ml anhydrous CH
2
Cl
2
in a 250 ml round bottom f
lask equipped with a stir bar. Boron tribromide (65 ml of 1 M solution in CH
2
Cl
2
, 65 mm
ol) was added and the solution was stirred under room temperature for 2 days. The solutio
n was then decanted to 30 ml of ice water and after 30 minutes of hydrolysis the organic l
ayer was extracted twice with 100 ml of CH
2
Cl
2
. The organic layer was washed twice wit
h 100 ml of cold water to neutralize any excess of BBr3. The solvent was removed under
reduced pressure and the resulting solid was further purified on silica gel column chromat
ography using hexane and ethyl acetate (6:1) to obtain 1.6 g (88%) of N (3,5 dihydroxyph
enyl)diphenylamine.
SQ 3: 2-[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N,N-diphenyliminio)-2,6-
dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate,. A mixture of N (3,5
dihydroxyphenyl)diphenylamine (1 g, 3.6 mmol) and excess squaric acid (0.25 g, 2.2
mmol) in 1-butanol (40 ml) and toluene (120 ml) were refluxed under N2 overnight.
After reaction was cooled down, the solvents were removed under reduced pressure using
a rotary evaporator leaving behind a green solid. The crude solid was recrystallized from

51

a 1:1 volume ratio of dichloromethane methanol mixture to afford metallic green crystals
of 3 (0.70 g, 61%).
1
H-NMR (CDCl
3
, 500 MHz): 11.00 (s, 4H), 7.41 (t, 8H, J = 7.5 Hz),
7.29 (t, 4H, J = 7 Hz), 7.23 (d, 8H, J = 8 Hz), 5.87 (s, 4H).
13
C-NMR (CDCl
3
, 125 MHz):
181.36, 163.50, 163.06, 159.51, 144.08, 129.81, 127.57, 127.05, 104.96, 98.75. MS
(ESI+): m/z 633.3 (MH
+
); calcd: 632.66 amu. Elemental analysis for
C
40
H
28
N
2
O
6
•CH
2
Cl
2
•MeOH: calcd: C 74.42, H 4.42, N 4.23; found: C 74.58, H 4.31, N
4.35.
SQ 2: 2-[4-(N-phenyl-N-methylamino)-2,6-dihydroxyphenyl]-4-[(4-(N-phenyl-N-methyli
minio)-2,6-dihydroxy¬phenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate. Yield =
17%.
1
H-NMR (CDCl
3
, 500 MHz, 60 °C): 10.95 (s, 4H), 7.44 (t, 4H, J = 8 Hz), 7.33 (t, 2
H, J = 7.5 Hz), 7.19 (d, 4H, J = 8.5 Hz), 5.77 (s, 4H), 3.41 (s, 6H).
13
C-NMR (CDCl
3
, 125
MHz, 60 °C): 184.45, 163.25, 158.38, 144.08, 130.09, 127.68, 127.03, 104.96, 95.49, 86
.48, 53.29. MS (ESI
+
): m/z 508.25 (M
+
); calcd: 508.52 amu. Elemental analysis for C
30
H
2
4
N
2
O
6
•MeOH: calcd: C 68.88, H 5.22, N 5.18; found: C 69.44, H 5.48, N 5.18.
SQ 4: 2-[4-(N-phenyl-N-1-naphthylamino)-2,6-dihydroxyphenyl]-4-[(4-(N-phenyl-N-1-
naphthyliminio)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate.
Yield = 51%.
1
H-NMR (CDCl
3
, 500 MHz, 60 °C): 10.91 (s, 4H), 7.91 (d, 2H, J = 7.5
Hz), 7.88 (t, 4H, J = 7.5 Hz), 7.47-7.54 (m, 8H), 7.39 (d, 2H, J = 7 Hz), 7.30-7.37 (m,
8H), 7.22 (m, 4H), 5.78 (s, 4H).
13
C-NMR (CDCl
3
, 125 MHz, 60 °C): 181.34, 163.78,
163.24, 160.31, 144.15, 140.10, 134.99, 130.48, 129.71, 128.83, 128.69, 127.57, 127.31,
126.77, 126.74, 126.40, 126.03, 123.08, 104.87, 98.27. MS (ESI+): m/z 733.2 (MH
+
);

52

calcd: 732.78 amu. Elemental analysis for C
48
H
32
N
2
O
6
: calcd: C 78.68, H 4.40, N 3.82;
found: C 78.07, H 4.28, N 3.85.
SQ 5: 2-[4-(N-phenyl-N-2-naphthylamino)-2,6-dihydroxyphenyl]-4-[(4-(N-phenyl-N-2-
naphthyliminio)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate.
Yield = 38%.
1
H-NMR (CDCl3, 500 MHz, 60 °C): 10.99 (s, 4H), 7.86 (d, 2H, J = 8.8
Hz), 7.82-7.86 (m, 2H), 7.73-7.77 (m, 2H), 7.66 (d, 2H, J = 2 Hz), 7.48-7.52 (m, 4H),
7.40 (t, 4H, J = 7.5 Hz), 7.32 (dd, 2H, J = 8.8, 2.1 Hz), 7.28 (t, 2H, J = 7.5 Hz), 7.27 (t,
4H, J = 7.5 Hz), 5.93 (s, 4H).
13
C-NMR (CDCl3, 125 MHz, 60 °C): 181.59, 164.10,
163.46, 159.84, 144.44, 141.78, 134.01, 132.14, 129.90, 129.86, 127.86, 127.81, 127.70,
127.12, 126.93, 126.59, 125.86, 125.58, 105.45, 99.33. MS (ESI
+
): m/z 733.3 (MH
+
);
calcd: 732.78 amu. Elemental analysis for C
48
H
32
N
2
O
6
: calcd: C 78.68, H 4.40, N 3.82;
found: C 78.74, H 4.33, N 3.84.
SQ 6: 2-[4-(N,N-4,4'-biphenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N,N-4,4'-biphenylimi
nio)-2,6-dihydroxy¬phenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate. Yield=28%
.
1
H-NMR (CDCl
3
, 500 MHz, 60 °C): 11.00 (s, 4H), 7.63 (d, 8H, J = 8.5 Hz), 7.60 (d, 8H,
J = 7.8 Hz), 7.45 (t, 8H, J = 7.4 Hz), 7.36 (t, 4H, J = 7.4 Hz), 7.31 (d, 8H, J = 8.5 Hz), 6.
00 (s, 4H). The solubility of 6 was too poor to obtain an acceptable
13
C-NMR spectrum.
Elemental analysis for C
64
H
44
N
2
O
6
: calcd: C 82.03, H 4.73, N 2.99; found: C 83.9, H 4.3
1, N 1.75.
SQ 7: 2-[4-(N,N-2,2'-biphenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N,N-2,2'-biphenylimi
nio)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate. Yield = 19%.

1
H-NMR (CDCl
3
, 500 MHz, 60 °C): 11.00 (s, 4H), 7.24 (m, 4H), 7.22 (t, 8H, J = 7.5 Hz)

53

, 7.08-7.14 (m, 8H), 6.97 (d, 8H, J = 7.1 Hz), 6.82 (t, 4H, J = 7.1 Hz), 6.27 (d, 4H, J = 7.8
Hz), 5.89 (s, 4H).
13
C-NMR (CDCl
3
, 125 MHz, 60 °C): 181.44, 163.11, 163.06, 140.26,
140.15, 139.62, 131.43, 129.91, 128.55, 128.25, 127.38, 127.27, 127.02, 104.76, 98.00.
MS (ESI
+
): m/z 937.4 (MH
+
); calcd: 937.04 amu. Elemental analysis for C
64
H
44
N
2
O
6
: cal
cd: C 82.03, H 4.73, N 2.99; found: C 81.98, H 4.70, N 3.02.
SQ 8: 2-[4-(N-phenyl-N-1-pyrenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N-phenyl-N-1-
pyrenyliminio)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate.
Yield = 60%.
1
H-NMR (CDCl
3
, 500 MHz, 60 °C): 10.89 (s, 4H), 8.24 (d, 2H, J = 7.5
Hz), 8.20 (d, 2H, J = 7.4 Hz), 8.19 (d, 2H, J = 8.2 Hz), 8.02-8.15 (m, 10H), 7.85 (d, 2H, J
= 8.1 Hz), 7.32-7.39 (m, 4H), 7.20-7.24 (m, 2H), 5.81 (s, 4H).
13
C-NMR (CDCl
3
, 125
MHz, 60 °C): 181.52, 164.17, 163.63, 160.78, 144.70, 137.32, 131.29, 131.01, 129.79,
129.44, 128.39, 127.07, 126.99, 126.71, 126.62, 126.09, 125.88, 125.84, 124.73, 122.07,
105.28, 98.79. MS (ESI
+
): m/z 881.5 (M
+
); calcd: 884.97 amu. Elemental analysis for
C
60
H
40
N
2
O
6
•CH
2
Cl
2
•MeOH: calcd: C 80.16, H 4.52, N 3.06; found: C 80.10, H 3.81, N
3.11.
2.3.2 Unsymmetrical Arylanilino Squaraines synthesis
The synthesis of SQ 9 and SQ 10 were carried out by similar procedures. The first
step involved the synthesis of diphenylaminosquarate, followed its reaction with either N-
(3,5-dihydroxyphenyl)diisobutylamine or N-(3,5-dihydroxyphenyl)diphenylamine to give
SQ 9 and SQ 10.

54

9. R = isobutyl 10. R = phenyl
Scheme 6. Synthesis of unsymmetrical arylanilino squaraines
3-Diphenylamino-4-hydroxycyclobut-3-ene-1,2-dione
(diphenylaminosquarate).30 A solution of diphenylamine (5.0 g, 30 mmol) in 100 mL of
propan-2-ol was added to a solution of 3,4-diisopropoxycyclobut-3-ene-1,2-dione (4.7 g,
24 mmol) in 50 mL of the same solvent. Concentrated HCl (1 mL) was then added and
the mixture was refluxed for approximately 3 h. The solvent was removed on a rotary
evaporator and the residue dissolved in CHCl
3
and filtered to remove any squaric acid.
The filtrate was then pumped down and the residue was dissolved in 150 mL of acetone
and followed by 150 mL of 6 M HCl. This solution was refluxed for 4 hrs with vigorous
stirring followed by evaporation under reduced pressure to give a residue that was
subsequently dissolved in CHCl
3
and filtered to remove any further quantities of residual
squaric acid. The solvent was removed from the filtrate under reduced pressure and the
residue repeatedly extracted with hot water to give green crystals of the product. This
material was recrystallized once more from hot water to obtain 3.7 g (59%) of

55

diphenylaminosquarate.
1
H-NMR (CDCl
3
, 500 MHz, 60 °C): 7.41 (t, 2H, J = 7.5 Hz),
7.35 (t, 1H, J = 7.5 Hz), 7.16 (d, 2H, J = 7.5 Hz), 5.61 (s, 1H), 3.41(s, 3H).
13
C-NMR
(CDCl3, 125 MHz, 60 °C): 181.36, 163.06, 159.51, 144.08, 129.81, 127.57, 104.96,
98.75.
SQ 9: 2-[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-(4-diphenyliminio)-2,5-dien-
1-ylidene}-3-oxocyclobut-1-en-1-olate. A solution of diphenylaminosquarate (1.5 g, 5.7
mmol) and N-(3,5-dihydroxyphenyl)diisobutylamine (1.35 g, 5.7 mmol) in 45 ml of
toluene and 15 ml of 1-butanol was refluxed under N2 overnight. The resulting reaction
mixture was dried over rotary evaporator to obtain a crude red solid. The final product
was purified by elution with CH
2
Cl
2
/hexane on a silica gel column to obtain 0.76 g (28%)
of 9.
1
H-NMR (CDCl3, 500 MHz, 60 °C): 12.00 (s, 2H), 7.43-7.47 (m, 4H), 7.36-7.39
(m, 2H), 7.21-7.23 (m, 4H), 5.76 (s, 2H), 3.21 (d, 4H, J = 7.5 Hz), 2.12 (sep, 2H, J = 6.7
Hz), 0.90 (d, 12H, J = 6.7 Hz).
13
C-NMR (CDCl3, 125 MHz, 60 °C): 175.59, 163.59,
139.98, 129.17, 128.07, 125.31, 93.82, 60.25, 27.56, 20.16. MS (ESI+): m/z 485.4
(MH+); calcd: 484.58 amu. Elemental analysis for C
30
H
32
N
2
O
4
: calcd: C 74.36, H 6.66,
N 7.78; found: C 74.33, H 6.75, N 5.80.
SQ 10: 2-[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]-4-(4-diphenyliminio)-2,5-dien-
1-ylidene}-3-oxocyclobut-1-en-1-olate. A solution of diphenylaminosquarate (0.5 g, 1.9
mmol) and N-(3, 5 dihydroxyphenyl)diphenylamine (0.56 g, 2.0 mmol) in 45 ml toluene
and 15 ml of 1-butanol was refluxed under N
2
overnight. The resulting reaction mixture
was dried under reduced pressure in a rotary evaporator to obtain a crude red solid. The
final product was purified by elution with CH
2
Cl
2
/hexane on a silica gel column to obtain

56

0.12 g (11%) of SQ 10.
1
H-NMR (CDCl
3
, 500 MHz, 60 °C): 11.88 (s, 2H), 7.48-7.51 (m,
4H), 7.42-7.46 (m, 2H), 7.35-7.39 (m, 4H), 7.26 (m, 2H), 7.24-7.26 (m, 4H), 7.21-7.23
(m, 4H), 5.84 (s, 2H).
13
C-NMR (CDCl
3
, 125 MHz, 60 °C): 175.34, 163.41, 144.36,
139.50, 129.56, 129.12, 128.51, 127.50, 126.42, 125.28, 99.14. MS (ESI
+
): m/z 525.35
(MH
+
); calcd: 524.56 amu. Elemental analysis for C
34
H
24
N
2
O
4
•CH
2
Cl
2
: calcd: C 76.27, H
5.19, N 4.81; found: C 76.35, H 4.49, N 5.28.
Both SQ 9 and SQ 10 were formed with low reaction yields together with
symmetrical squaraines. A hydrolysis of diphenylamino squarate leads to the formation
of free squaric acid. Reaction of an aniline precursor can compete with both squarate and
squaric acid, thus, lowering the yield of unsymmetrical squaraine. The following
procedure is described for compound SQ 10 with side product of SQ 3. The crude
reaction mixture should be suspended in ca. 50 ml of DCM per 1g of the reaction mixture
and crystals 90% enriched with SQ 3 filtered off after 3 hours. The remaining solution is
approximately 85-90% SQ 10 which can be separated on SiO2 column (hexanes -
dichloromethane). Scalability of up to 1g of pure unsymmetrical SQ 10 has been
demonstrated.
2.4 Summary
In this chapter, a general introduction of squaraines is presented. Squaraines are
very attractive dyes because of their excellent absorption properties, their versatility with
respect to structure modification and photoconductivities. These properties give them
advantages as promising candidates in a wide range of applications. Normally,
squaraines-both symmetrical and unsymmeterical-can be synthesized via two

57

nucleophilic type reaction routes, which could be both symmetrical and unsymmetrical
squaraines. Such D-A-D type dyes could be modified by varying their donor substituents.
In the synthesis section, possible contamination with 1,2 substituted squaraine side
products is discussed. In this chapter, the synthesis of a total of ten squaraines, mainly
symmetrical arylanilino and a few unsymmetrical ones is described. By controlling
conjugation length, squaraine absorption can possiblely be tuned to extend to cover
green, red and NIR region of the solar spectrum. The detailed characterization data are
presented in chapter 3. Due to the versatility of the synthetic scheme described, the
synthesis of more squaraines with specific property demands is anticipated.

58

2.5 Chapter 2: References
1. Meyers, F.; Chen, C.-T.; Marder, S. R.; Brédas, J.-L.; Meyers, F., Electronic
Structure and Linear and Nonlinear Optical Properties of Symmetrical and
Unsymmetrical Squaraine Dyes. Chem. Eur. J. 1997, 3, (4), 537.

2. Bigelow, R. W.; Freund, H. J., An Mndo and Cndo/S(S + Des Ci) Study on the
Structural and Electronic-Properties of a Model Squaraine Dye and Related Cyanine.
Chem. Phys. 1986, 107, (2-3), 159-174.

3. Law, K. Y., Organic Photoconductive Materials: Recent Trends and
Developments. Chem. Rev. 1993, 93, (1), 449-486.

4. Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.; Behera, G. B., Cyanines
During the 1990s: A Review. Chem. Rev. 2000, 100, (6), 1973-2011.

5. Reis, L. V.; Serrano, J. P.; Almeida, P.; Santos, P. F., The Synthesis and
Characterization of Novel, Aza-Substituted Squarylium Cyanine Dyes. Dyes and
Pigments 2009, 81, (3), 197-202.

6. Scherer, D.; Dorfler, R.; Feldner, A.; Vogtmann, T.; Schwoerer, M.; Lawrentz,
U.; Grahn, W.; Lambert, C., Two-Photon States in Squaraine Monomers and Oligomers.
Chem. Phys. 2002, 279, (2-3), 179-207.

7. Silvestri, F.; Irwin, M. D.; Beverina, L.; Facchetti, A.; Pagani, G. A.; Marks, T. J.,
Efficient Squaraine-Based Solution Processable Bulk-Heterojunction Solar Cells. J. Am.
Chem. Soc. 2008, 130, (52), 17640-17641.

8. Volkova, K. D.; Kovalska, V. B.; Losytskyy, M. Y.; Bento, A.; Reis, L. V.;
Santos, P. F.; Almeida, P.; Yarmoluk, S. M., Studies of Benzothiazole and
Benzoselenazole Squaraines as Fluorescent Probes for Albumins Detection. J Fluoresc
2008, 18, 877-882.

9. Wöbkenberg, P. H.; Labram, J. G.; Swiecicki, J.-M.; Parkhomenko, K.;
Sredojevic, D.; Gisselbrecht, J.-P.; de Leeuw, D. M.; Bradley, D. D. C.; Djukic, J.-P.;
Anthopoulos, T. D., Ambipolar Organic Transistors and near-Infrared Phototransistors
Based on a Solution-Processable Squarilium Dye. J. Mater. Chem. 2010, 20, (18), 3673-
3680.

10. Reidy, K.; Campanile, C.; Muff, R.; Born, W.; Fuchs, B., Mthpc-Mediated
Photodynamic Therapy Is Effective in the Metastatic Human 143b Osteosarcoma Cells.
Photochem. Photobiol. 2012, 88, (3), 721-727.

59

11. Jyothish, K.; Arun, K. T.; Ramaiah, D., Synthesis of Novel Quinaldine-Based
Squaraine Dyes: Effect of Substituents and Role of Electronic Factors. Org. Lett. 2004, 6,
(22), 3965-3968.

12. Beverina, L.; Abbotto, A.; Landenna, M.; Cerminara, M.; Tubino, R.; Meinardi,
F.; Bradamante, S.; Pagani, G. A., New Pi-Extended Water-Soluble Squaraines as Singlet
Oxygen Generators. Org. Lett. 2005, 7, (19), 4257-4260.

13. Luo, C.; Zhou, Q.; Jiang, G.; He, L.; Zhang, B.; Wang, X., The Synthesis and
1
o
2

Photosensitization of Halogenated Asymmetric Aniline-Based Squaraines. New J. Chem.
2011, 35, (5), 1128-1132.

14. Beverina, L.; Salice, P., Squaraine Compounds: Tailored Design and Synthesis
Towards a Variety of Material Science Applications. Eur. J. Org. Chem. 2010, (7), 1207-
1225.

15. Denk, W.; Strickler, J. H.; Webb, W. W., 2-Photon Laser Scanning Fluorescence
Microscopy. Science 1990, 248, (4951), 73-76.

16. Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.; Dyer, D. L.; Ehrlich, J. E.;
Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Lee, I. Y. S.; McCord-Maughon, D.; Qin, J.
Q.; Rockel, H.; Rumi, M.; Wu, X. L.; Marder, S. R.; Perry, J. W., Two-Photon
Polymerization Initiators for Three-Dimensional Optical Data Storage and
Microfabrication. Nature 1999, 398, (6722), 51-54.

17. Ehrlich, J. E.; Wu, X. L.; Lee, I. Y. S.; Hu, Z. Y.; Rockel, H.; Marder, S. R.;
Perry, J. W., Two-Photon Absorption and Broadband Optical Limiting with Bis-Donor
Stilbenes. Opt. Lett. 1997, 22, (24), 1843-1845.

18. Chung, S.-J.; Zheng, S.; Odani, T.; Beverina, L.; Fu, J.; Padilha, L. A.; Biesso, A.;
Hales, J. M.; Zhan, X.; Schmidt, K.; Ye, A.; Zojer, E.; Barlow, S.; Hagan, D. J.; Van
Stryland, E. W.; Yi, Y.; Shuai, Z.; Pagani, G. A.; Bredas, J.-L.; Perry, J. W.; Marder, S.
R., Extended Squaraine Dyes with Large Two-Photon Absorption Cross-Sections. J. Am.
Chem. Soc. 2006, 128, (45), 14444-14445.

19. Odom, S. A.; Webster, S.; Padilha, L. A.; Peceli, D.; Hu, H.; Nootz, G.; Chung,
S.-J.; Ohira, S.; Matichak, J. D.; Przhonska, O. V.; Kachkovski, A. D.; Barlow, S.;
Bredas, J.-L.; Anderson, H. L.; Hagan, D. J.; Van Stryland, E. W.; Marder, S. R.,
Synthesis and Two-Photon Spectrum of a Bis(Porphyrin)-Substituted Squaraine. J. Am.
Chem. Soc. 2009, 131, (22), 7510-7511.

20. Smits, E. C. P.; Setayesh, S.; Anthopoulos, T. D.; Buechel, M.; Nijssen, W.;
Coehoorn, R.; Blom, P. W. M.; de Boer, B.; de Leeuw, D. M., Near-Infrared Light-

60

Emitting Ambipolar Organic Field-Effect Transistors. Adv. Mater. 2007, 19, (5), 734-
738.

21. Morel, D. L.; Ghosh, A. K.; Feng, T.; Stogryn, E. L.; Purwin, P. E.; Shaw, R. F.;
Fishman, C., High Efficiency Orgaic Solar Cells. Appl. Phys. Lett. 1978, 32, 495.

22. Bagnis, D.; Beverina, L.; Huang, H.; Silvestri, F.; Yao, Y.; Yan, H.; Pagani, G.
A.; Marks, T. J.; Facchetti, A., Marked Alkyl- Vs Alkenyl-Substitutent Effects on
Squaraine Dye Solid-State Structure, Carrier Mobility, and Bulk-Heterojunction Solar
Cell Efficiency. J. Am. Chem. Soc. 2010, 132, (12), 4074-4075.

23. Mayerhoeffer, U.; Deing, K.; Gruss, K.; Braunschweig, H.; Meerholz, K.;
Wuerthner, F., Outstanding Short-Circuit Currents in Bhj Solar Cells Based on Nir-
Absorbing Acceptor-Substituted Squaraines. Angew. Chem. Int. Ed. Engl. 2009, 48, (46),
8776-8779.

24. Umezawa, K.; Cittierio, D.; Suzuki, K., Water-Soluble Nir Fluorescent Probes
Based on Squaraine and Their Application for Protein Labeling. Anal. Sci. 2008, 24, (2),
213-217.

25. Treibs, A.; Jacob, K., Cyclotrimethine Dyes Derived from Squaric Acid. Angew
Chem., Int. Ed. Engl. 1965, 4, 694.

26. Treibs, A., Pyrrole Dyes. Chimia 1966, 20, (9), 329.

27. Sprenger, H.; Ziegenbein, W., Condensation Products of Squaric Acid and
Tertiary
Aromatic Amines. Angew. Chem. Int. Ed. 1966, 5, 894.

28. Ronchi, E.; Ruffo, R.; Rizzato, S.; Albinati, A.; Beverina, L.; Pagani, G. A.,
Regioselective Synthesis of 1,2- Vs 1,3-Squaraines. Org. Lett. 2011, 13, (12), 3166-3169.

29. Law, K. Y.; Bailey, F. C., Squaraine Chemistry. Synthesis of Bis(4-
Dimethylaminophenyl)Squaraine from Dialkyl Squarates. Mechanism and Scope of the
Synthesis. Can. J. Chem. 1986, 64, (12), 2267-2273.

30. Law, K. Y.; Bailey, F. C., Squaraine Chemistry-a New Approach to Symmetrical
and Unsymmetrical Photoconductive Squaraines-Characterization and Solid State
Properties of These Materials Can. J. Chem. 1993, 71, (4), 494-505.

31. Beverina, L.; Ruffo, R.; Patriarca, G.; Angelis, F. D.; Roberto, D.; Righetto, S.;
Ugo, R.; Pagani, G. A., Second Harmonic Generation in Nonsymmetrical Squaraines :
Tuning of the Directional Charge Transfer Character in Highly Delocalized Dyes. J.
Mater. Chem. 2009, 19, 8190-8197.

61

Chapter 3. Squaraine Photophysics
3.1 Electronics Properties
To better understand squaraine electronic properties, density functional theory
(DFT) calculation is conducted by B3LYP/6-31G* method. DFT calculation provides
useful insights for structure design of squaraines. It guides possible ways to tune HOMO
and LUMO as well. It bears mentioning that DFT calculation has fairly good correlation
with experimental X-ray data. Figure 3-1 compares the calculated and experimental
results of bond length in squaraines. The results are quite close which support validity of
DFT calculation.

Figure 3-1.Comparision between the calculated and experiment bod length of squaraine:
calculated (DFT, R=Me), experimental (X-Ray, R=s-Bu).
DFT calculation starts with a basic squaraine compound 1. This compound has an
estimated HOMO and LUMO of 5.94 eV and 3.44 eV respectively. Interestingly, the OH
substituents can either act as a donor or an accepter to squaraine 1 depending on the OH
position. The para-OH acts as a donor in squaraine 2 which destabilize both HOMO and
LUMO energy. In comparison, the ortho-OH serves as an acceptor in squaraine 3 which
largely stabilize both HOMO and LUMO energy to 6.19 eV and 3.6 eV. The ortho-OH

62

groups stabilize the anionic charge on adjacent oxygen of C
4
O
2
unit, making it less
electron donating for squaraine 3 (Figure 3-2). More importantly, the ortho-OH groups
can form hydrogen bonding with CO groupsof four-membered ring. Such structure offers
planar squarate core and is beneficial for squaraine thermal stability.
1

Figure 3-2. HOMO and LUMO picture of compound 1, 2 and 3 from DFT calculation
(B3LYP/6-31G*).

LUMO (mesh) 3.02 eV 3.6 eV

HOMO (transparent) 5.47 eV 6.19 eV

LUMO (mesh)

3.44 eV

HOMO (transparent) 5.94 eV

63

By introducing electron donating dimethyl amino substituent in compound 1,
HOMO and LUMO of compound 4 is destabilized with upshift over 1 eV. As
demonstrated in compound 3, the compound 5 can be stabilized by incorporating ortho-
OHs into compound 4.

Figure 3-3. HOMO and LUMO picture of compound 4 and 5 from DFT calculation
(B3LYP/6-31G*).
While dimethyl amino substituent acts as strong donor, the diaryl amino
substituent functions more as a destabilizing unit. With the replacement of end alkyl
substituent with aryl one, stabilizing effect arise more from LUMO while HOMO is
nearly unaffected. This is confirmed by DFT calculation of compound SQ 3 (Figure 3-4).
From DFT calculations, the most valuable finding is that OH substituent position
can effectively tune HOMO level, which can be incorporated in squaraines to lower
HOMO energy and likely to result in a larger V
oc
in OPVs.

LUMO (mesh)

2.38 eV

2.61 eV

HOMO (transparent) 4.67 eV 5.06 eV

64

Figure 3-4. HOMO and LUMO picture of compound SQ 3 from DFT calculation
(B3LYP/6-31G*).
3.2 Thermal Stability
The thermal stability of squarianes was measured by thermal gravimetric analysis
(TGA) and differential scanning calorimetry (DSC). Both TGA and DSC characterize
thermal properties with change of temperatures, in which TGA determines weight change
with change in temperature. Unlike TGA, DSC measures the heat flow difference at
different temperature which characterizes the phase transition of compounds.
The replacement of N-aryl groups on squaraines was expected to enhance the
thermal stability of squaraine compounds. TGA analysis was also performed on squaraine
SQ 1, SQ 3 and SQ 4 to assess if thermal stability was improved by N-aryl substituents,
which are SQ, DPSQ and 1NPSQ respectively. The squaraines were found to undergo
28% weight loss at 308 °C (SQ), 58% weight loss at 318 °C (DPSQ) and 73% weight
loss at 326 °C (1NPSQ). The modest increase in decomposition temperature for N-aryl
derivatives is in accordance with DSC measurements, which also suggests that the

LUMO (mesh) 2.84 eV

HOMO (transparent) 5.08 eV

65

thermal breakdown occurs at the squarate moiety. Consequently, attempts to sublime
samples of DPSQ and 1NPSQ led to extensive decomposition.

Figure 3-5. The TGA analysis of SQ 1, SQ 3 and SQ 4.
In DSC measurement, the compound SQ 1 and SQ 3 were selected because they
represent the alkylanilino and arylanilino squaraine types respectively. DSC was
measured with a scanning range from 35 °C to 350 °C. The samples were first scanned at
a heating rate of 10 °C min
−1
and were cooled down to 35 °C rapidly using liquid N
2
. The
second and third scans were performed at a heating rate of 6 °C min
−1
. As shown in
figure 3-7, SQ 1 is melted at 308 °C undergoing decomposition at temperature of 316 °C.
The second cycle is featureless because the sample is already decomposed at that
moment. Similarly, compound SQ 3 is likely to melt and undergo decomposition process
as expected from TGA analysis. However, the melting temperature is likely out of the
measurement range but decomposition occurs at 322 °C. No other phase transition was
identified. A plain curve of SQ 3 was also observed for second and third cycle.
0 100 200 300 400 500
20
40
60
80
100
SQ 1
SQ 3
SQ 4

Weight (%)
Temperature (
o
C)

66

Figure 3-6. DSC measurement of SQ 1 and SQ 3.
3.3 Single Crystal Data and Film Packing
A metallic green prism-like crystal of SQ 1 obtained from CH
2
Cl
2
/MeOH was
used for the X-ray crystallographic analysis. The X-ray intensity data were measured on a
Bruker SMART APEX diffractometer equipped with an APEX CCD system and a Mo
sealed-tube fine-focus source ( λ = 0.71073 Å). A total of 1850 frames were collected
50 100 150 200 250 300 350
-10
0
10
20
30
40
50

SQ cycle 1
SQ cycle 2
Decomposition
Heat flow
Temperature
Melting

50 100 150 200 250 300 350
-20
-10
0
10
20
30

DPSQ cycle 1
DPSQ cycle 2
DPSQ cycle 3
Decomposition
Heat flow
Temperature

67

over a total exposure time of 10.3 hours. The frames were integrated with the Bruker
SAINT V6.02A software package using a narrow-frame algorithm. The calculated
minimum and maximum transmission coefficients (based on crystal size) are 0.782 and
0.983. The X-ray crystallographic analysis for SQ 3 was performed on a metallic green-
blue plate-like specimen obtained from toluene. The X-ray intensity data were measured
on a Bruker SMART APEX DUO diffractometer equipped with a APEX II CCD system
and a Cu ImuS microfocus source ( λ = 1.54178 Å). A total of 2764 frames were collected
over a total exposure time of 69.10 hours. The frames were integrated with the Bruker
SAINT V7.68A software package using a narrow-frame algorithm. The calculated
minimum and maximum transmission coefficients (based on crystal size) are 0.6087 and
0.7516. Both structures were solved and refined using the Bruker SHELXTL V6.12
software package. Additional refinement details and the resulting factors for SQ 1 and SQ
3 are given in Table 3-1.
Single crystal x-ray structures were obtained for crystals of SQ 1 obtained from
CH
2
Cl
2
/MeOH and SQ 3 obtained from toluene (Figure 3-7). The metrical parameters of
both derivatives are similar to values reported for related bis[N,N-di(n-
butyl)anilino]squaraines.
2,3
A relatively short aryl–squarate bond distance (C2–C3 =
1.402(2) Å in SQ 1, C16–C19 = 1.409(7) Å in SQ 3) is consistent with strong
conjugation between the anilino moieties and squarate core. Likewise, the short nitrogen–
anilino bond distances (N1–C6 = 1.359(2) Å in SQ 1, N1–C13 = 1.385(6) Å in SQ 3)
demonstrate strong conjugative stabilization imparted by the amino substituent to the

68

squaraine, particularly when compared to the considerably longer nitrogen–phenyl bonds
in SQ 3 (average = 1.435(6) Å).
Table 3-1. Summary of crystal data, data collection and refinement parameters for SQ 1
and SQ 3.
compound 1 3
empirical formula C
32
H
44
N
2
O
6
C
40
H
28
N
2
O
6

formula weight 552.69 632.64
temperature 143(2) K 100(2) K
radiation wavelength 0.71073 Å 1.54178 Å
crystal system monoclinic triclinic
space group P2(1)/n P-1
unit cell dimensions a = 6.2034(16) Å
= 90°
a = 11.1895(9) Å
= 96.371(8)°
b = 16.478(4) Å
= 92.406(4)°
b = 11.7874(11) Å
= 91.176(7)°
c = 14.518(4) Å
= 90°
c = 11.9792(9) Å
= 92.485(7)°
volume 1482.7(6) Å
3
1568.3(2) Å
3

Z 2 2
density (calculated) 1.238 Mg/m
3
1.340 Mg/m
3

absorption coefficient 0.085 mm
-1
0.738 mm
-1

F(000) 596 660
crystal size 0.33 x 0.09 x 0.05 mm
3
0.10 x 0.09 x 0.02 mm
3
theta range for data collection 1.87 to 27.52° 3.71 to 59.29°
index ranges -8 ≤ h ≤ 8
-21 ≤ k ≤ 20
-18 ≤ l ≤ 18
-12 ≤ h ≤ 12
-13 ≤ k ≤ 11
-13 ≤ l ≤ 13
reflections collected 12527 14554
independent reflections 3349 [R(int) = 0.0508] 4202 [R(int) = 0.0753]
completeness to theta = 27.52° 98.4 % 99.0 %
absorption correction semi-empirical multi-scan
transmission factors min/max 0.796 min/max: 0.810
refinement method full-matrix least-squares on F
2

69

Table 3-1 (Continued)
data / restraints / parameters 3349 / 0 / 193 4202 / 6 / 437
goodness-of-fit on F
2
1.014 1.084
final R indices [I > 2 (I)] R1 = 0.0477
wR2 = 0.1021
R1 = 0.0819
wR2 = 0.2240
R indices (all data) R1 = 0.0890
wR2 = 0.1147
R1 = 0.1154
wR2 = 0.2434
largest diff. peak and hole 0.218 and -0.184 e.Å
-3
0.619 and -0.322 e.Å
-3

The π-system of SQ 1 is nearly planar as there is only a small dihedral twist (<4
degrees) between the dihydroxy-phenyl ring and squarate core, whereas SQ 3 displays a
slightly twisted conformation with dihedral angles of ca. 10º and 13º between the
dihydroxy-phenyl rings and squarate core. The amine groups of SQ 1 and SQ 3 are
planar; the sum of the three C–N–C bond angles for the amine groups of both SQ 1 and
SQ 3 are 360°. The amine groups of SQ 3 show larger dihedral twists with respect to the
dihydroxyphenyl group (12.2º and 13.2º) as compared to the corresponding dihedral
angles for the amine groups of SQ 1 (0.95º).

Figure 3-7. ORTEP diagrams for SQ 1 (top) and SQ 3 (bottom).

70

Figure 3-8. Crystal packing diagrams for SQ 1 (top) and SQ 3 (bottom). View of SQ 1
showing herringbone structure (a) and molecular stacking arrangement (b).
Stacking arrangement of SQ 3 viewed down the short (c) and long (d)
molecular axes. Hydrogen atoms were removed for clarity.
Packing diagrams of SQ 1 and SQ 3 show significant differences in the
organization of squaraine molecules in the crystal lattice (Figure 3-8). Squaraine
molecules in crystals of SQ1 are organized in staircase slip-stacks arranged in
herringbone-like arrays that alternate at skewed 90º angles with respect to the molecular
long axis (Figure 3-8. (a)). To describe the angular inclinations away from the molecular
plane within the stacks, it is instructive to use “pitch” and “roll” angles, terms coined by
Curtis, et. al.
4
The pitch angle defines the displacement with respect to the long molecular
axis while the roll angle identifies the displacement relative to the short molecular axis.
The squaraine planes are separated by ca 3.2 Å and have pitch and roll angles of 45º and
(a)

(b)

(c)

(d)

71

36º, respectively. The small roll angle in particular leads to a displacement along the
short axis by more than 3 Å such that there is no overlap between aromatic rings in
adjacent molecules (Figure 3-8. (b)). In contrast, crystals of SQ 3 are composed of
parallel stacks of dimers separated by ca 3.4–3.5 Å between the molecular planes. The
dimers units of SQ 3 are also organized in a slip-stack staircase arrangement with the
long molecular axis having a pitch angle of ca. 38º (Figure 3-8. (c)). However, unlike SQ
1, a large roll angle of 80º within the stack leads to a small displacement along the short
molecular axis (Figure 3-8. (d)). While there are no significant interactions between the
pendant phenyl rings of adjacent squaraine molecules, the small lateral displacement
leads to considerable overlap between the anilinosquaraine ring systems within the stack.
The large pitch angle also leads to a slip offset in the long molecular axis such that the
squarate core is situated above the aromatic anilino ring in adjacent molecules. Molecular
solids with large roll angles and offsets, and consequent cofacial stacking as found in
crystals of SQ 3, have been proposed to have better charge and exciton transport
properties in ordered films than materials with small roll angles and poor overlap, as
observed in SQ 1.
4

3.4 Electrochemistry
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were
performed using an EG&G potentiostat/galvanostat model 263 under N
2
atmosphere.
Anhydrous DCM was used as solvent for the scan from -2 V to 2 V and 1.0 M
tetrabutylammonium hexafluorophosphate (TBAH) was used as the supporting
electrolyte. A glassy carbon rod was used as a working electrode, and a silver wire was

72

used as a pseudoreference electrode. Electrochemical reversibility and redox potentials
were determined using CV and DPV, respectively. The redox potentials were calculated
relative to an internal ferrocenium/ferrocene (Fc
+
/Fc) reference.
Table 3-2. Electrochemical redox potentials of squaraines 1–10.
a

squaraines E
ox
(V)

E
red
(V) HOMO(eV)
b
LUMO(eV)
c
1 0.49, r -1.23, i 5.29 (5.1)
d
3.31
2 0.54, q -1.25, i 5.36 3.29
3 0.62, r -1.08, i 5.47 (5.3)
d
3.49
4 0.59, q -1.24, i 5.43(5.3)
d
3.30
5 0.58, r -1.11, i 5.41 (5.3)
d
3.45
6 0.59, q -1.12, i 5.43 3.45
7 0.60, r -1.10, r 5.44 3.47
8 0.65, q -0.98, q 5.51 3.61
9 0.59, r -1.40, r 5.43 3.11
10 0.70, r -1.45, i 5.58 (5.4)
d
3.05
(a) Recorded in 0.1 M Bu
4
N
+
PF
6
-
in CH
2
Cl
2
, referenced to internal Fc
+
/Fc; r =
reversible, q = quasi-reversible and i = irreversible. Values for irreversible waves
are cathodic peak potentials. (b) HOMO values are calculated by literature.
6
(c)
LUMO values are calculate by literature.
7
(d) HOMO values are reported from
UPS measurement.
Electrochemical analysis of SQ 1–10 using both cyclic and differential pulse
voltammetry were performed in CH
2
Cl
2
and referenced to Fc
+
/Fc as an internal standard,
redox data is listed in Table 3-2. Squaraines SQ 1–8 display reversible or quasi-reversible
oxidation waves in the range 0.49–0.65 V and irreversible reduction waves between -0.98
V and -1.25 V. Compared to SQ 1 (E
ox
= 0.49 V), the oxidation potentials of SQ 2–8 are
0.05 to 0.16 V lower. Unsymmetrical squaraines SQ 9 and SQ 10 show reversible

73

oxidation at comparable potentials (E
ox
= 0.59 V and 0.70 V, respectively), but display
reversible reduction at higher potentials (E
red
= -1.40 V and -1.45 V, respectively), which
indicate that the blue shift in absorption is primarily due to an increase in the energy of
the LUMO. The energy of the HOMO for films of SQ 1, SQ 3, SQ 4, SQ 5 and SQ 10
has also been determined using ultraviolet photoelectron spectroscopy (UPS) (see
appendix). The first ionization potential of SQ 3, SQ 4 and SQ 5 are the same (IP
1
= 5.3
eV) and 0.2 V higher than SQ 1 (IP
1
= 5.1 eV),
5
while SQ 10 has an ionization energy of
5.4 eV.
3.5 UV-Vis of Solution and Films
The absorption properties of squaraines SQ 1–10 in CH
2
Cl
2
solution and in neat
thin film are listed in Table 3-3 and representative spectra are shown for SQ 1, SQ 3 and
SQ 10 in Figure 3-9. In solution, squaraines 1–8 display an intense ( ε≈ 10
5
M
-1
cm
-1
)
absorption band with λ
max
= 650–700 nm. Relative to the absorption maximum of SQ 1
( λ
max
= 645 nm) and other N, N-dialkyl derivatives,
8
N-aryl squaraines SQ 2–8 show red-
shifted peak maxima. In contrast, the unsymmetrical squaraines SQ 9 and SQ 10 have
absorption peaks that are blue-shifted, λ
max
 530 nm, relative to both N-alkylanilino and
N-arylanilino based symmetric squaraines. The absorption bands of SQ 1 and SQ 2 in
solution display narrow full-width half maxima (FWHM) of 630 cm
-1
and 890 cm
-1
,
respectively, while the absorption bands of SQ 3–10 are markedly broader, with FWHM
between 1280 and 1980 cm
-1
. The absorption spectra for all the squaraines red-shift and
undergo significant broadening in thin films (Figure 3-9). The broadening is important
for OPVs since it leads to improved spectral overlap with the solar irradiance spectrum.

74

Figure 3-9. Absorption spectra for SQ 1, SQ 3 and SQ 10 in CH
2
Cl
2
solution (open
symbols) and as neat films (solid lines).
3.6 Photolumiescence Property

Figure 3-10. Excitation (ex) and emission (em) spectra of SQ 1 and SQ 3 in toluene.
Squaraines SQ 1–10 fluoresce in the wavelength range of λ
max
= 658–758 nm
(Table 3-3), representative excitation and emission spectra for SQ 1 and SQ 3 are shown
400 500 600 700 800 900
0.0
0.2
0.4
0.6
0.8
1.0
ex, SQ 1
em, SQ 1
ex, SQ 3
em, SQ 3

Intensity (au)
Wavelength (nm)

400 500 600 700 800 900
0.0
0.2
0.4
0.6
0.8
1.0
SQ 1
SQ 3
SQ 10

Absorbance (au)
Wavelength (nm)

75

in Figure 3-10. The photoluminescent quantum efficiency ( Ф) of SQ 1 in toluene is 0.80,
whereas SQ 2–10 are lower, ranging from 0.01 to 0.66. The Stokes shift of the N-
alkylanilino-squaraine SQ 1 is 9 nm in toluene and is unaffected by solvents of higher
polarity. The small, solvent-insensitive Stokes shift is comparable to behavior observed
for other related N,N-dialkylanilino squaraines
8, 9
indicating that only minor structural
reorganization occurs in the excited state.
10

Table 3-3. Photophysical data of SQ 1-10 in solution and as thin films.
absorption emission
solution in CH
2
Cl
2
or (toluene) thin film toluene solution

ε

(10
5
M
-1
cm
-1
)
λ
max
(nm)
FWHM
(cm
-1
)
λ
max
(nm)
FWHM
(cm
-1
)
λ
em
(nm)
Ф
1 4.1 652(649) 630 692 3140 658 0.80
2 1.7 645 (645) 890
*

*
672 0.14
3 1.9 674 (677) 1470 717 3240 750 0.29
4 2.0 666(669) 1280 691 3280 726 0.36
5 1.9 687 (689) 1590 717 2770 768 0.07
6 0.71 694 (693) 1790 697 2910 786 0.02
7 2.1 678 (673) 1440 686 2490 721 0.66
8 1.4 679 (682) 1530 712 2520 738 0.02
9 1.3 529 (529) 1570 552 2840 582 0.01
10 1.0 535 (539) 1980 560 3160 625 0.02
* Solubility is too poor to form a uniform film.
On the other hand, the N-arylanilinosquaraines display much larger Stokes shifts
that range between 27 nm for SQ 2 to 79 nm for SQ 5. The large Stokes shifts observed
for SQ 2–10 suggests that excited state formation in the N-arylanilino derivatives is

76

accompanied by a greater degree of intramolecular distortion. In addition, the Stokes shift
increases with increasing solvent polarity (positive solvatochromism). For example, the
Stokes shift of SQ 3 is 52 nm in cyclohexane, 73 nm in toluene and 112 nm in 2-methyl
tetrahydrofuran (2MeTHF). The solvatochromic response indicates that the excited state
of SQ 3 is both more polarizable than that of SQ 1 and strongly stabilized by polar media.
3.7 Twist Insisted Charge Transfer State
The solvent-dependent luminescence properties of squaraines SQ 1 and SQ 3
were examined in more detail. The quantum yield of SQ 1 is largely independent of
solvent polarity, ranging from 0.80 in toluene to 0.73 in acetonitrile. This behavior differs
from the trend reported for bis[4-(N,N-dimethyl)anilino]squaraine (SQ-DMA),
11
an
analogue of SQ 1 without the meta-hydroxyl groups. The quantum yield of SQ-DMA is
also high in toluene (Ф= 0.85) but decreases dramatically in acetonitrile ( Ф= 0.08). The
lower quantum yield of SQ-DMA in polar media was attributed to the formation of a
twisted intramolecular charge transfer (TICT) state that acts as a nonradiative decay
channel.
12
Rotation around the N, N-dimethylanilino–squarate bond axis was proposed to
accompany formation of the TICT state. The weak solvent dependence in the quantum
yield of SQ 1 suggests that an analogous TICT state is not accessible in this compound.
The difference between SQ 1 and SQ-DMA likely arises due to hydrogen bonds formed
between the four hydroxyl groups to the squarate oxygens in SQ 1 that suppress twisting
along the phenyl–squarate bond. Like SQ-DMA, the quantum yield of SQ 3 is strongly
affected by solvent polarity, being highest in cyclohexane ( Ф= 0.55) and decreasing
sharply in 2-MeTHF ( Ф = 0.05). This suggests that an alternate TICT state may be

77

accessible in SQ 3. Since both squaraine SQ 1 and SQ 3 have equivalent hydroxyl
groups, twisting of the diphenylamino moiety with respect to the molecular core might
account for the dependence of quantum yield on the solvent polarity. Considering that the
dipolar structure of SQ 3 should lead to a nominally nonpolar ground state, the
luminescent behavior implies that charge separation, along with consequent breaking of
the molecular symmetry, occurs during excited state formation.
3.8 Summary
In this chapter, discussion is focused on the photophysics of squaraines. First, the
DFT calculation of squaraines is presented. The fair correlation between experimental
and calculation results demonstrates the effectiveness of DFT calculation. The squaraine
structure with 2,6-OH substituted is preferred for enhanced thermal stability and deeper
HOMO level, which can possibly result in higher V
oc
value as well. The thermal stability
of squaraines is evaluated by both TGA and DSC based on two representative molecules
SQ 1 and SQ 3. With the goal to improve thermal stability by introducing aryl
substituents, SQ 3 only exhibits slightly enhancement in thermal stability. The possible
reason is that squaraine decomposition occurs at squarate moiety. From single crystal X-
ray data, stronger π- π stacking is observed in arylanilino squaraines SQ 3 which is
proposed to have better charge and exciton transport properties in SQ 3 films. In terms of
optical properties, variable squaraines with absorption from green, red to NIR are
obtained with excellent extinction coefficient. Importantly, all of these squaraines show
broadened absorption in films which is beneficial for solar cell application. From

78

electrochemistry estimation, deeper HOMO values are measured for arylanilino
squaraines which can result in higher Voc in OPVs’ application.

79

3.9 Chapter 3: References

1. Tian, M. Q.; Furuki, M.; Iwasa, I.; Sato, Y.; Pu, L. S.; Tatsuura, S., Search for
Squaraine Derivatives That Can Be Sublimed without Thermal Decomposition. J. Phys.
Chem. B 2002, 106, (17), 4370-4376.

2. Dirk, C. W.; Herndon, W. C.; Cervanteslee, F.; Selnau, H.; Martinez, S.;
Kalamegham, P.; Tan, A.; Campos, G.; Velez, M.; Zyss, J.; Ledoux, I.; Cheng, L. T.,
Squarylium Dyes - Structural Factors Pertaining to the Negative 3rd-Order Nonlinear-
Optical Response. J. Am. Chem. Soc. 1995, 117, (8), 2214-2225.

3. Ashwell, G. J.; Bahra, G. S.; Brown, C. R.; Hamilton, D. G.; Kennard, C. H. L.;
Lynch, D. E., 2,4-Bis[4-(N,N-Dibutylamino)Phenyl] Squaraine: X-Ray Crystal Structure
of a Centrosymmetric Dye and the Second-Order Non-Linear Optical Properties of Its
Non-Centrosymmetric Langmuir-Blodgett Films. J. Mater. Chem. 1996, 6, (1), 23-26.

4. Curtis, M. D.; Cao, J.; Kampf, J. W., Solid-State Packing of Conjugated
Oligomers: From Pi-Stacks to the Herringbone Structure. J. Am. Chem. Soc. 2004, 126,
(13), 4318-4328.

5. Wei, G. D.; Wang, S. Y.; Renshaw, K.; Thompson, M. E.; Forrest, S. R.,
Solution-Processed Squaraine Bulk Heterojunction Photovoltaic Cells. ACS Nano 2010,
4, (4), 1927-1934.

6. D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P. I.; Polikarpov, E.;
Thompson, M. E., Relationship between the Ionization and Oxidation Potentials of
Molecular Organic Semiconductors. Org. Electron. 2005, 6, 11-20.

7. Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E., Measurement of
the Lowest Unoccupied Molecular Orbital Energies of Molecular Organic
Semiconductors. Org. Electron. 2009, 10, (3), 515-520.

8. Law, K. Y., Squaraine Chemistry - Effects of Structural-Changes on the
Absorption and Multiple Fluorescence Emission of Bis[4-
(Dimethylamino)Phenyl]Squaraine and Its Derivatives. J. Phys. Chem. 1987, 91, (20),
5184-5193.

9. Law, K. Y., Squaraine Chemistry - Effects of Solvent and Temperature on the
Fluorescence Emission of Squaraines. J. Photochem. Photobiol. A-Chem. 1994, 84, (2),
123-132.

80

10. Wolf, J.; Law, K. Y.; Myers, A. B., Hole-Burning Subtracted Fluorescence Line-
Narrowing Spectroscopy of Squaraines in Polymer Matrices. J. Phys. Chem. 1996, 100,
(29), 11870-11882.

11. CornelissenGude, C.; Rettig, W.; Lapouyade, R., Photophysical Properties of
Squaraine Derivatives: Evidence for Charge Separation. J. Phys. Chem. A 1997, 101,
(50), 9673-9677.

12. Rettig, W., Charge Separation in Excited-States of Decoupled Systems - TICT
Compounds and Implications Regarding the Development of New Laser-Dyes and the
Primary Processes of Vision and Photosynthesis. Angew. Chem. Int. Ed. Engl. 1986, 25,
(11), 971-988.

81

Chapter 4. Squaraine Application in OPVS
4.1 Vapor Deposited Squaraine OPVs
4.1.1 Ultrathin Optimal Devices
To prevent molecular decomposition on sublimation for most squaraine
compounds, squaraines with hydroxyl substituent groups at the 2’, 6’-positions of two
phenyl rings are used in bilayer heterojunction devices.
1
The squaraine used is 2,4-Bis[4-
(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ 1). This is one of few
squaraines studied in this dissertation that have sublimation ability. This compound was
synthesized according to literature procedures
1
and purified by multiple cycles of thermal
gradient sublimation in vacuum.
2
The sublimation yield is as low as ~30%. The
absorption spectrum of SQ 1 in CH
2
Cl
2
solution shows a narrow, intense band at 652 nm
( ~ 4.5x10
5
M
-1
cm
-1
), which broadens and red-shifts upon film formation.
3,4
The
broadening of the absorption band in the solid state results from a strong excitonic
interaction between neighboring molecules. Electrochemistry of SQ 1 in

CH
2
Cl
2
shows
reversible oxidation waves at 0.49 V, and a quasi-reversible reduction wave at -1.23 V
(both versus ferrocene), respectively, corresponding to a thin film HOMO energy of 5.3
eV
5, 6
and a LUMO energy of 3.4 eV.
7
These values suggest SQ 1 can serve as a donor in
a heterojunction structure when used with the acceptor C
60
, whose HOMO and LUMO
energies are 6.2 eV and 3.7 eV, respectively.
8, 9

Devices with the structure: ITO/SQ 1(x)/ C
60
(400 Å)/BCP(100 Å)/Al(1000 Å), x
= 65, 110, 150 and 200 Å, exhibited performances shown in Figure.4-1 and Table. 4-1.
For comparison, the data are provided for devices prepared in a single fabrication run, to

82

eliminate run-to-run variations. The highest power conversion efficiencies are observed
for devices with x = 65 Å. Devices with doubly sublimed SQ 1 fabricated on five
different runs give efficiencies in the range of 3.2±0.3%. In contrast, a device using only
singly sublimed source material results in a reduced power conversion efficiency of
2.2%, while OPVs with triply sublimed SQ 1 gave similar efficiencies to those with
doubly sublimed SQ 1, indicating that continued exposure to the sublimation purification
step has a diminishing influence on device performance.

Figure 4-1. Current density vs. voltage characteristics of ITO/donor/C
60
(400 Å)/BCP
(100 Å)/Al(1000 Å): donor = SQ1- 65 Å (SQ-65), 110 Å (SQ 1-110), NPD 50 Å/SQ 1 65
Å (NPD/SQ 1) and CuPc 400 Å (CuPc). Dark current characteristics are shown as
dashed lines.
4.1.2 High V
oc
and Small Dark Current
The measured open circuit voltage for the devices employing SQ 1 donor are V
oc
=
0.75-0.84, or 300-400 mV greater than that observed for a comparable CuPc device. The
higher observed V
oc
is consistent with an increased donor-acceptor interface energy gap
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-8
-6
-4
-2
0
2
4
SQ 1-65
SQ 1-110
NPD/SQ
CuPc

Current density (mA/cm
2
)
Voltage (V)

83

ΔE
DA
.
7, 10, 11
Here, ΔE
DA
= 1.8 eV for the SQ 1/C
60
heterojunction is 100 mV larger than
that of CuPc/C
60
heterojunction. However, the increase in V
oc
is larger than the observed
differences in ΔE
DA
. The open circuit voltage of OPVs is approximated by V
oc
=
(nk
B
T/q)ln(J
sc
/J
s
), where n is diode ideality factor, k
B
is Boltzmann’s constant, T is the
temperature, q is the fundamental charge, J
sc
is the short circuit current density, and J
s
is
the saturation dark current density, which is given by J
s
= J
SO
exp(- ∆E
DA
/2nkT).
11, 12
For
the SQ 1 based devices, J
s
= 10
-2
-10
-3
μA/cm
2
, while analogous CuPc based devices give
J
s
 1 μA/cm
2
.
12, 13
The ideality factor, n, for devices with CuPc and SQ 1(x = 110 Å) are
2.1±0.1 and 2.5±0.1 respectively. For the x = 110 Å device, J
s
= 0.02 μA/cm
2
. These
values result in a calculated V
oc
of 0.40 and 0.79 V (assuming ∆E
DA
= 1.7 eV and 1.8 eV)
for CuPc and SQ 1 devices, respectively. These are approximately equal to the observed
values of 0.42±0.01 and 0.83±0.01 indicating that differences in dark current largely
account for V
oc
differences between CuPc and SQ 1 based devices. The differences in J
s

for SQ 1 and CuPc based devices is, therefore, largely due to their pre-exponential
currents, J
SO
, which are 16 mA/cm
2
and 2.1x10
4
mA/cm
2
, respectively. Such difference
in J
SO
is likely due to different electronic coupling between donor and acceptor in
devices. CuPc is flat molecule and packs in a way that it has strong intermolecular
interaction with acceptor C
60.
In comparison
,
squaraine molecules in crystals of SQ 1 are
organized in staircase slip-stacks arranged in herringbone-like arrays that alternate at
skewed 90º angles with respect to the molecular long axis. Annealed SQ 1 films exhibit
peaks at (011) and (022). By viewing from 0hl plane, two different SQ 1 orientations are
observed. One type of orientations is with SQ 1 standing out of ITO plane with dihedral

84

angle of 60°, which leads to diisobutyl amine group facing towards acceptor C
60
. The
other orientation has SQ1 lying down with dihedral angle of about 30° to ITO plane.
Both SQ 1’s orientations hinder direct π- π interaction with acceptor C
60
, effectively
suppressing charge recombination process. Overall, the reduced π accessibility and weak
intermolecular interaction between SQ 1 and C
60
result in corresponding low J
SO
and
unexpectedly large photovoltage.
4.1.3 Additional NPD Layer Covers Noncontinuous SQ 1 Film
EQE measurements in Figure 4.2 show that a significant fraction of the
photocurrent is contributed by SQ 1 layer. The highest J
sc
is observed for OPV with x =
65 Å, and monotonically decreases as the SQ thickness is increased. The fill factor (FF)
also decreases at x > 65 Å, although FF > 0.5 for the range of x  200 Å studied. The
trends observed for J
sc
, and FF as functions of SQ 1 thickness are consistent with
increased resistance to hole transport in the thicker SQ films. For the thinnest layers, the
SQ 1 may not form a continuous film on the ITO surface whose root mean square
roughness is approximately 1 nm. Discontinuous films can lead to direct contact between
the C
60
layer and the anode. The C
60
/ITO contact is expected to form a Schottky
junction,
14
which can act as a competing interface for exciton dissociation, and thereby
introducing current shunts. To reduce the effects of a discontinuous SQ 1 layer, a 50 Å
thick layer of the wide energy gap, hole transporting material, N,N'-di-1-naphthyl-N,N'-
diphenyl-benzidine, NPD, was inserted between the ITO anode and the donor film. Here,
we expect NPD to form a continuous film on ITO, while maintain high hole
conductivity.
15, 16

85

Figure 4-2.External quantum efficiency (EQE) characteristics of ITO/donor/C
60
(400
Å)/BCP (100 Å)/Al(1000 Å) devices: donor = SQ 1 65 Å (SQ-65), NPD 50 Å/SQ 1 65 Å
(NPD/SQ 1) and CuPc 400 Å (CuPc).
Table 4-1: Photovoltaic device performance under 1 sun AM 1.5G simulated solar
illumination. The device structure is
ITO/Donor/C
60
(400Å)/BCP(100Å)/Al(1000Å).
Donor-Thickness
(Donor-x, Å)
V
oc
(± 0.01, V)
J
sc

(± 0.05, mA/cm
2
)
FF
(±0.05)
η
(±0.1, %)
SQ 1-65, ave
a
0.76(0.03) 7.01(0.38) 0.56(0.05) 3.1(0.3)
SQ 1-65
b
0.75 7.13 0.60 3.2
SQ 1-110
b
0.83 6.89 0.55 3.2
SQ 1-150
b
0.84 6.44 0.53 2.9
SQ 1-200
b
0.84 5.98 0.50 2.6
NPD-50/SQ 1-65 0.82 6.67 0.55 3.0
CuPc-400 0.42 4.99 0.58 1.2
NPD-50/CuPc-
400
0.43 4.74 0.58 1.2

a.
The values are averages for each prameter measured on five different substrates
prepared in different fabrication runs. The values in parentheses are the standard
deviations.
400 500 600 700 800
0
10
20
30
SQ 1-65
NPD/SQ
CuPc

EQE (%)
Wavelength (nm)

86

b.
The SQ OPV data listed for ITO/SQ/C
60
/BCP/Al devices (x = 65-200 Å) prepared in
parallel in a single fabrication run. The data are averages for several devices on each
substrate.
The addition of the NPD layer into the structure: ITO/NPD (50 Å)/SQ 1 (65
Å)/C
60
/BCP/Al, gives V
oc
comparable to that observed for thicker (x  100 Å) SQ 1
films, and J
sc
comparable to a device with x = 65 Å as shown in Table 4-1. This suggests
that the lower V
oc
for x = 65 Å may indeed be due to partial contact of C
60
with the ITO
anode. Introduction of a 50 Å thick NPD layer in CuPc(400 Å)/C
60
control cells shows
no change in device performance, which is consistent with previous observations that
such thick CuPc films are continuous.
Device Fabrication: Photovoltaic cells were grown on ITO-coated glass substrates
that were solvent cleaned and treated in UV-ozone for 10 minutes immediately prior to
loading into a high vacuum (base pressure ~ 3x 10
-6
Torr) chamber. The organic
materials, SQ 1, copper phthalocyanine (CuPc, Aldrich), C
60
(MTR limited) and 2,9-
dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (Aldrich) were purified by thermal
gradient sublimation in vacuum prior to use. Squaraine samples require multiple
sublimation steps to achieve high device efficiency, possibly due to improved purity
attained via this process as shown previously for CuPc.
17
Metal cathode materials, Al
(99.999% pure, Alfa Aesar) were used as received. Materials were sequentially grown by
vacuum thermal evaporation at the following rates: SQ (0.3-0.6 Å/sec) or CuPc (4 Å/sec),
C
60
(2 Å/sec) and metals: 1000 Å thick Al (3 Å/sec). The cathode was evaporated through
a shadow mask with an array of 1 mm diameter openings. Current–voltage measurements
were performed in ambient using a Keithley 2420 SourceMeter® in the dark and under
corrected 1 kWm
-2
white light illumination from a 300W Xe arc lamp (Newport® Oriel

87

Product Line) equipped with an AM 1.5G filter. Incident power was adjusted using a
NREL-calibrated Si photodiode to match one sun intensity (100 mW/cm
2
), and spectral
response was measured using a monochromated light source. Spectral mismatch
correction was performed as described in the literature,
18
using a Si photodiode
(Hamamatsu S1787-04), calibrated at the National Renewable Energy Laboratory
(NREL), and frequency modulated illumination (250 Hz) from a Xe source coupled to a
Cornerstone
TM
260 ¼ M monochromator (Newport® 74125) in conjunction with an
EG&G 7220 DSP Lock-In amplifier. This monochromatic system was also used to
collect all external quantum efficiency data.
4.2 Solution Processed Bilayer Squaraine OPVs
As discussed in chapter 3, most squaraines are not thermally stable under
sublimation. The replacement of alkyl aniline substituents with arylanilino substituents
only slightly enhances thermal stability without solving decomposition issues. Thus, an
alternate squaraine device processing method is necessary for further exploration of
squaraines in OPVs. In contrast to vapor deposition method, solution processed OPVs are
made with spin casted functional layers without sublimation requirement. High quality
spin cast film is the prerequisite for achieving high performance device. In this section,
the importance of solvent selection and film morphology for solution processed SQ 1
based OPVs will be discussed.
The device structure is ITO/spin cast SQ 1(80 Å)/C
60
(400 Å)/BCP (100 Å)/Al
(1000 Å) (Figure 4-3). The thickness of spin casted SQ 1 film can be controlled by
concentration of SQ 1 solution. The 80 Å spin casted SQ 1 thin film is prepared by spin

88

coating SQ 1 solution on precleaned ITO with spin rate of 3000 rpm for 40s in air. Then
C
60
, BCP and AL cathode are deposited on spin cast film accordingly under high vacuum
10
-6
torr.

Figure 4-3. The solution processed SQ 1 device structure.
4.2.1 Solution Processed Alkylanilino Squaraine OPVs
Many factors have impact on spin casted film quality such as solvent, humidity
or spin cast condition. The following discussion will show the solvent importance in a
spin cast film. CH
2
Cl
2
is first selected for SQ1 solution process device. Figure 4-4
compares AFM film morphology of SQ 1 prepared from vapor deposition and spin casted
films from CH
2
Cl
2.
It

is noticeable that the vapor deposited film is amorphous while spin
casted SQ 1 is quite rough.
The corresponding films were fabricated in a bilayer device with structure as
shown in figure 4-3. The resulting V
oc
(~0.24 V) of solution processed SQ1 device is
significantly lower than that of vapor deposited SQ 1 device (0.73 V) fabricated with the
same device structure. This is likely due to two competing junctions formed in real
device in which high leakage current results in low V
oc
. One junction is formed by
ITO/SQ 1/C
60
, while the other is from ITO/C
60
junction.

Spin cast
Vapor deposition
ITO
80 Å SQ
400 Å C
60

100 Å BCP
1000 Å Al
Glass

89

Figure 4-4. The AFM images of vapor deposited and spin cast SQ 1 film.

Figure 4-5. The current-potential plot of SQ 1 devices and AFM image of SQ 1 spin cast
from chloroform.
When solvent CHCl
3
with higher boiling point is selected, smooth film
morphology is formed. Such difference in film morphology directly impacts SQ 1 OPVs
performance. The high V
oc
is instantly recovered from good film morphology. The
optimal SQ 1 device generates high J
sc
of 6.46 mA/cm
2
, and V
oc
of 0.69 V , resulting in
overall power conversion efficiency of 2.8 %.This is comparable to a vapor deposited
SQ 1 bilayer devices and offers opportunity to study more squaraine OPVs by solution
process.
-0.4 0.0 0.4 0.8 1.2
-8
-6
-4
-2
0
2
4
6

SQ1 solution process from chloroform
SQ1 vapor deposite
Current density (mA/cm
2
)
Potential (V)

Vapor deposited SQ 1 film Spin cast SQ 1 film from CH
2
CL
2

90

4.2.2 Solution Processed Arylanilino Squaraine OPVs
Solvent plays a significant role in arylanilino squaraines based devices as well.
SQ 3 is the one with diphenyl amino substituent and has reasonable solubility in most
solvents. Three solvents are used for device fabrication. They are chloroform,
chlorobenzene and toluene. From AFM images, dramatic difference in film morphology
can be observed especially from the RMS roughness.

Figure 4-6. AFM images of SQ 3 spin cast from (a) Chloroform, (b) Chlorobenzene, and
(c) Toluene.
Table 4-2. Photovoltaic device performance under 1 sun AM 1.5G simulated solar
illumination. The device structure is ITO/SQ 3/C
60

(400Å)/BCP(100Å)/Al(1000Å).
Solvent V
oc
(V) Jsc (mA/cm
2
) FF η (%)
Chloroform 0.84 6.68 0.59 3.29
Chlorobenzene 0.65 2.33 0.27 0.41
Toluene 0.56 0.61 0.37 0.13
The performance of OPVs fabricated from the corresponding films is shown in
table 4-2. The V
oc
of devices spun cast from chloroform solvent is about 200 mV higher
than those from solvent chlorobenzene and toluene. This is consistent with thin film

(a) Chloroform RMS 1.1 nm; (b) Chlorobenzene RMS 11 nm; (c) Toluene RMS 11
nm.

91

morphology data. The rough films from chlorobenzene and toluene solvent are likely to
form islands which can cause non continuous films covering ITO substrate. Moreover, a
significant lower FF is also observed in devices from chlorobenzene and toluene solvents.
This is possibly due to poor charge transport inside rough films.
Solar cells with the structure ITO/squaraine (100 Å)/C
60
(400 Å)/BCP(100
Å)/Al(1000 Å) were fabricated using SQ 3, SQ 4 and SQ 5 to study the effects of N-aryl
substitution on the optoelectronic characteristics of OPVs. The performance of these
devices using layers of the squaraines spun cast from chloroform is shown in Figure 4-7.
The power conversion efficiencies of squaraines are SQ 3 (  = 3.2 ± 0.1%) and 4 (  =
2.5 ± 0.1%). The V
oc
for devices made using SQ 3 and SQ 4 are also 0.23 V higher than
that of SQ 1, consistent with the decrease of the HOMO energy by 0.2 V. Further,
devices made using SQ 3 have an increased short circuit current–density (J
sc
= 6.71 ± 0.1
mA/cm
2
). An increase in fill factor (FF) from 0.51 ± 0.3 for SQ 1 to near 0.60 for SQ 3
and SQ 4 is consistent with improved charge carrier transport for the latter materials. The
role of the N-aryl substituent position is illustrated by comparing the performance of
solar cells fabricated using isomeric squaraines SQ 4 and SQ 5. The device made using
SQ 5 has a lower V
oc
and J
sc
that cannot be explained solely on the basis of differing
photophysical and electrochemical characteristics of the two donor molecules. AFM
images show that amorphous films of SQ 4 (Figure 4-8 (a)) are smooth (RMS = 0.51
nm), whereas films cast from the less soluble SQ 5 (Figure 4-8 (b)) are rough (RMS =
1.61 nm). The AFM data suggest that the aromatic substituents in isomeric squaraines SQ
4 and SQ 5 strongly influence the molecular packing and aggregation in the films. Thus,

92

the performance of OPVs made using SQ 4 and SQ 5 show a strong correlation with the
differences in squaraine solubility.

Figure 4-7. (a) The potential–current density plots and tabulated OPV performance
characteristics of squaraine donor devices with the structure ITO/donor C
60
(400 Å)/ BCP
(100 Å)/Al. (b) The external quantum efficiency (EQE) plot of squaraine donor devices
ITO/donor /C
60
(400 Å)/ BCP (100 Å)/Al.

-0.5 0.0 0.5 1.0
-8
-4
0
4
8
12
1
3
4
5

Current density (mA/cm
2
)
Potential (V)
(a)
400 500 600 700 800 900
0
5
10
15
20
25
30
35
(b)

1
3
4
5
EQE (%)
Wavelength (nm)

93

Figure 4-8. (a) AFM images of films of SQ 4 (RMS = 0.51 nm) and (b) SQ 5 (RMS =
1.61 nm) spin cast from chloroform. The scale bar in the top right corner is 1 μm.
Table 4-3: Photovoltaic device performance under 1 sun AM 1.5G simulated solar
illumination. The device structure is
ITO/Donor/C
60
(400Å)/BCP(100Å)/Al(1000Å).
Donor Thickness
(Å)
V
oc
(V) Jsc
(mA/cm2)
FF η
p

(%)
SQ 1 85 0.59±0.05 5.58±0.16 0.51±0.03 1.8 ± 0.1
SQ 3 85 0.82±0.02 6.71±0.1 0.59±0.01 3.2 ± 0.1
SQ 4 85 0.86±0.01 5.11±0.2 0.57±0.02 2.5 ± 0.1
SQ 5 66 0.63±0.01 4.2±0.2 0.54±0.02 1.4 ± 0.1

4.2.3 High FF achieved by Compound Blocking Layer
Our collaborators from University of Michigan have achieved high FF in
squaraine OPVs by integrating compound blocking layer. It is demonstrated that 3,4,9,10
perylenetetracarboxylic bisbenzimidazole (PTCBI) and 1,4,5,8-napthalene-
tetracarboxylic-dianhydride (NTCDA) can function as electron conducting and exciton
blocking layers when interposed between the acceptor layer and cathode. A low
resistance contact is provided by PTCBI, while NTCDA acts as an exciton blocking layer
and optical spacer as shown in figure 4-9.

(a) (b)

94

Here, a third type of EBL is utilized where the lowest occupied molecular orbital
(LUMO) is aligned with that of the acceptor, allowing for low-resistance transport of
electrons directly from acceptor to cathode.
19
It is shown that 3,4,9,10
perylenetetracarboxylic bisbenzimidazole (PTCBI) serves as a trap-free electron
conductor and forms a low energy barrier contact with the Ag cathode.

Figure 4-9. Energy level diagrams of EBLs used in this work: a) PTCBI, which transports
via the LUMO; b) NTCDA, which transports via the LUMO but has an electron
extraction barrier; and c) compound NTCDA/PTCBI, which transports via the LUMO.
Devices were fabricated with the following structure: glass/150 nm ITO/8 nm
MoO
3
/15 nm 1-NPSQ/40 nm C
60
/buffer(s)/100 nm Ag. Figure 4-10 shows FF as a
function of buffer layer thickness x for BCP, PTCBI, NTCDA, and compound buffers
consisting of (x-5) nm NTCDA/5 nm PTCBI. Optimal performance for devices with BCP
occurs at a thickness of 5 nm, with FF = 0.60 ± 0.01, beyond which there is sharp drop in
efficiency due to the limited depth of damage-induced transport states extending into the
film from the surface.
20
In contrast, devices with PTCBI exhibit FF = 0.70 ± 0.01, with
only a small reduction as x 50 nm, confirming the low resistance transport in this

95

material. The optimum thickness for PTCBI is 10 nm, where η
p
decreases for thicker
films due to a decrease in J
sc
, since PTCBI absorption overlaps with that of the active
acceptor and donor layers. Devices with NTCDA buffer layers show FF = 0.62 ± 0.01. In
contrast, devices with a compound 15 nm NTCDA/5 nm PTCBI buffer have FF = 0.68 ±
0.01, which is similar to that of PTCBI alone.

Figure 4-10. Fill factor (FF) under spectrally corrected 1 sun, AM1.5G illumination for
devices with BCP buffer layers (black squares), PTCBI (red circles), NTCDA (blue
triangles), and compound NTCDA/PTCBI (green stars) as a function of thickness. Lines
are a guide to the eye. Inset: the molecular structure of 1-NPSQ.
The compound NTCDA/PTCBI buffer layer leads to increases in J
sc
compared to
PTCBI alone. Unlike PTCBI, the wide energy gap NTCDA is transparent across the
visible spectrum. Hence, the PTCBI is kept sufficiently thin (5 nm) to provide a low-
barrier cathode contact without introducing excessive optical absorption. At the same
time, the NTCDA thickness is adjusted to maximize the optical field at the donor-
acceptor junction without increasing series resistance, contrary to the case with BCP. The
trend in J
sc
as a function buffer layer agrees with optical modeling using the transfer-
0 10 203040 50
0.0
0.2
0.4
0.6
Buffer Thickness (nm)

Fill Factor
BCP
PTCBI
NTCDA
Compound

96

matrix approach,
21
shown by the dashed line in Figure 4-11. Optimized devices
employing compound buffers achieve J
sc
= 8.0 ± 0.1 mA/cm
2
compared to 7.2 ± 0.1
mA/cm
2
for BCP and 7.1 ± 0.1mA/cm
2
for PTCBI, as seen in Figure 4-9. For devices
without a buffer and with BCP, PTCBI, NTCDA, and PTCBI/NTCDA buffers, we
measure η
p
= 2.8 ± 0.1, 4.0 ± 0.1, 4.6 ± 0.1, 3.2 ± 0.1, and 5.1 ± 0.1 % respectively. These
results are summarized in Table 4-4.

Figure 4-11. Spectrally corrected short-circuit current (Jsc) under 1 sun, AM1.5G
illumination for devices with BCP buffer layers (squares), PTCBI (circles), NTCDA
(triangles), and compound NTCDA/PTCBI (stars) as a function of thickness. Solid lines
are a guide to the eye. The dashed line is Jsc modeled based on the optical intensity in the
device for the case of the NTCDA/PTCBI buffer.
In summary, the use of electron conducting EBLs in OPVs is demonstrated. Here,
electrons are transported via the LUMO states directly from that of the acceptor to the
cathode. By using PTCBI as a buffer layer, we find FF = 0.70 ± 0.01, compared to FF =
0.60 ± 0.01 for conventional BCP-based devices. Adding an NTCDA electron-
conducting EBL in combination with PTCBI allows for optimized optical spacing and
efficient exciton blocking, leading to an increase in η
p
that is >25 % for an analogous

97

squaraine/C
60
/BCP OPV. The increased stability of PTCBI compared to BCP may also
potentially extend the operational lifetime of OPVs employing blocking layers.
Table 4-4. OPV performance for devices with different buffer layers under simulated 1
sun, AM1.5G illumination.
Buffer
Layer
Thickness
(nm)
V
oc
(V)
FF R
(mA/W)
η
p

(%)
R
s

( Ωcm
2
)
none 0 0.90 0.59 53 2.8 ± 0.1 18 ± 6
BCP 5 0.93 0.60 72 4.0 ± 0.1 0.44 ± 0.02
PTCBI 10 0.94 0.70 71 4.6 ± 0.1 2.2 ± 1.4
NTCDA 10 0.94 0.62 56 3.2 ± 0.1 165 ± 14
Combination 15/5 0.94 0.68 79 5.1 ± 0.1 1.5 ± 0.6

4.3 Solution Processed Bulk Heterojunction Squaraine OPVs
The solution processed squaraine OPVs is a collaboration work between me and
Dr. Guodan Wei at University of Michigan. By incorporating BHJ and nanocrystalline
device with post thermal and solvent annealing process, high performance squaraine
OPVs are achieved.
4.3.1 Low FF and Charge Carrier Mobility for SQ 1 Bulkheterojunction OPVs
Neat SQ 1 thin films is measured with a hole mobility of approximately10
-5

cm
2
/V-s, which is comparable with CuPc. The combination of the optical, electrical and
chemical properties of SQ 1 blended with [6, 6]-phenyl C
70
butyric acid methyl ester
(PC
70
BM), results in cells with power conversion efficiencies η
p
=2.75 ± 0.09%, with J
sc
=
9.0 ± 0.67 mA/cm
2
, V
oc
=0.90 ± 0.02 V at 1 sun intensity, AM1.5G simulated solar
emission, and a peak external quantum efficiency EQE=42% at wavelengths of λ=685
nm.
Experimental

98

Samples for AFM operated in the tapping mode were prepared by first casting
SQ 1 :PC
70
BM blend thin films on ITO coated glass substrate with an 80 Å thick MoO
3

film deposited on its surface by thermal evaporation. The films were annealed at 70
0
C
for 10 minutes in N
2
glovebox.
Results
SQ 1 powder characterized by XRD (Figure 4-12) indicates crystallization in a
monoclinic phase [space group P2/m with a=18.36, b=5.87, c=14.93 Å, β=91.24
0
].
22
Thin
films annealed at 110
0
C and 130
0
C for 10 minutes exhibit (011) and (022) peaks at
2θ=7.58
0
and 2 θ=15.29
0
(inset, Figure 4-12) corresponding to an intermolecular spacing
distance of d=14.92 Å and d=7.50 Å. There are no peaks for SQ 1, PC
70
BM and SQ
1:PC
70
BM blends

with five different ratios of 3:1,1:1, 1:2, 1:3 and 1:6 annealed at 70
0
C,
indicating that they are amorphous.

Figure 4-12: X-ray-diffraction (XRD) spectra of as-prepared SQ thin film on indium tin
oxide (ITO) substrates spin-coated from chloroform solvent. XRD spectra confirms that
the SQ film annealed at 110
0
C and 130
0
C has the same cystal structure as the starting
powder and has (001) and (002) growth orientation.
510 15 20
200
400
600
800
1000
1200
1400
1600
1800
90
o
C
110
o
C
130
o
C
70
o
C
Intensity (a.u.)
2 
(011)
(022)
ITO (211)
N o annealin g

99

The absorption spectra of pure SQ 1 and blends of SQ 1: PC
70
BM films on quartz
substrates are shown in Figure 4-13. The absorption of the SQ 1 films extends to λ=800
nm and exhibits one absorption peak at λ=700 nm. When mixed with PC
70
BM with
absorption centered at λ=373 nm and λ=463 nm, the resulting film has a good visible
solar spectral coverage. As the fraction of PC
70
BM increases, the absorption peak at
λ=700 nm is suppressed with a concomitant increase in absorption from λ=300 nm to
λ=500 nm characteristic of PC
70
BM. The peak at λ=700 nm for pure SQ 1 films shifts to
λ=685 nm for 1:6 blend films.

Figure 4-13. The absorption coefficient of pure squaraine, and PC
70
BM, and blends on
quartz substrate with different ratios of 3:1, 1:1, 1:2, 1:3 and 1:6.
The EQE (Figure 4-14 (a)) and J-V (Figure 4-14 (b)) characteristics of the
blended and vacuum deposited control solar cells are summarized in Table 4-5. The
addition of PC
70
BM broadens the photocurrent spectral range. With the increase of the
PC
70
BM fraction, the spectral response from 350 nm to 550 nm regions is increased from
400 500 600 700 800
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
2.5x10
5
3.0x10
5
Absorption coefficient (cm
-1
)
Wavelength(nm)
SQ 1
SQ 1:PC
70
BM=3:1
SQ 1:PC
70
BM=1:1
SQ 1:PC
70
BM=1:2
SQ 1:PC
70
BM=1:3
SQ 1:PC
70
BM=1:6
PC
70
BM

100

3:1, 1:1, 1:2, 1:3 to 1:6. The EQE spectra of SQ 1 materials in the range of λ=550-800 nm
increases accordingly. The highest EQE (48% at λ=350 nm) was obtained for a ratio of
1:6. The optimized BHJ 1:6 SQ 1:PC
70
BM has EQE = 42% at λ=685 nm, and
photoresponse extending to λ=730 nm. Interestingly, this BHJ 1:6 SQ 1:PC
70
BM has
higher EQE response than the SQ 1/C
60
planar control cells. The integrated J
sc
of 1:6 SQ
1:PC
70
BM cell from EQE data is 11.70 mA/cm
2
at 1 sun, which is higher than the
measured J
sc
of 9.07 ± 0.67 mA/cm
2
(Table 4-5). The observed V
oc
values range between
0.60 V and 0.90 V, consistently higher than previously reported SQ 1: [6, 6]-phenyl C
60

butyric acid methyl ester (PCBM) bulk cells. The incorporation of MoO
3
buffer layer
leads to reduced dark current.
12
The SQ 1/C
60
planar cells have the most squared J-V
curve as shown in Figure 4-12 (b) resulting the FF as high as 0.57. There is no
progressively evolution of the FF when the ratio increases from 3:1 to 1:6 (Table 4-5).
They are about 40% as low as compared with the SQ 1/C60 planar control cell. The
device efficiency based on 1:6 ratio of SQ 1:PC
70
BM under different power intensity
irradiation drops from 3.5 ± 0.32%with FF=0.41 ± 0.02 at 0.2 sun (20 mW/cm
2
) AM1.5G
illumination, to η
p
=2.75 ± 0.09% with FF=0.34 ± 0.016 at 1 sun, as shown in Figure 4-
13.
As shown in Figure 4-15, the specific series resistance, R
SA
, which is composed of
the contact resistances (electrode resistivity and metal-material interfaces) and the ohmic
loss (due to the bulk resistivity of the materials),
23
is fitted from the slope of the J-V
characteristic curves of the SQ 1 only diode and five bulk cells under dark condition

101

using an ideal diode equation.
24
It gradually decreases from 21.10 ± 0.72 Ω·cm
2
(pure SQ
films) to 0.734 ± 0.02 Ω·cm
2
as the blend ratios vary from 3:1 to 1:6.

Figure 4-14: (a) The effect of blending ratios on the external quantum efficiency (EQE)
for the cells with a device structure of ITO/MoO3(80 Å) /SQ 1:PC
70
BM(x Å)/Al (1000
Å) at different blending ratios of 3:1, 1:1, 1:2 1:3 and 1:6 and the EQE of the SQ 1/C
60

planar control cell with a device structure of ITO/MoO3(80 Å)/SQ 1 (62 Å)/C
60
(400 Å)/
BCP(100 Å)/Al(1000 Å); (b) the current density-voltage (J-V) characteristics for the five
blend bulk cells and one planar control cell illuminated at 1 sun. Here x represents 320 Å,
400 Å, 720 Å, 730 Å and 760 Å for the five blend cells.
The hole transport mobility in the pristine SQ 1 films and these SQ 1: PC
70
BM
components is obtained with space charge limited current (SCLC) method. It decreases
from 1 × 10
-5
cm
2
/V-S to 2.61 × 10
-7
cm
2
/V-s when the molecular weight percentage of
SQ decreases from 100% (pure SQ 1 thin films) to 42% (corresponding to weight
ratio1:3). Then it increases slightly to 3.6×10
-7
cm
2
/V-s at the molecular weight
percentage of 26% (weight ratio 1:6).
400 500 600 700 800
0
10
20
30
40
50
EQE(%)
W avelength (nm )
3:1
1:1
1:2
1:3
1:6
C ontrol
(a)
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8
-10
-8
-6
-4
-2
0
2
4
Current density (mA/cm
2
)
Voltage (V)
3:1
1:1
1:2
1:3
1:6
(b)
Planar control

102

Figure 4-15: The power conversion efficiency ( ηp), open circuit voltage (Voc) and fill
factor (FF) under 0.002, 0.02, 0.2, 0.6, 1.0 and 1.68 sun illumination at a device structure
of ITO/MoO
3
(80 Å) /SQ 1:PC
70
BM(760 Å)/Al (1000 Å) at the (1:6) ratio of SQ
1:PC
70
BM; The zero field hole transport mobility (µ0) and series resistance (RSA) versus
the molecular weight ratio between squaraine and PC
70
BM
Discussions
The greatly reduced R
SA
with the increase of the molecular weight of PC
70
BM in
Figure 4-13 indicates the incorporation of more conductive PC
70
BM acceptors in the
composites create more donor and acceptor interfaces for exciton dissociation and
10
1.6
2.0
2.4
2.8
3.2
3.6
4.0
0.0
0.2
0.4
0.6
0.8
1.0

p
(%)
Power intensity (mW/c m
2
)

p
V
oc
FF
FF, V
oc
(V)

20 30 40 50 60 70 80 90 100110
0
20
40
60
80
100
0
5
10
15
20

0
(10
-7
cm
2
/V-S)
molecular weight (% ) of Squaraines

0
PC
70
BM
Squaraines
(3:1)
(1:1)
(1:2) (1:3)
(1:6)
R
SA

103

facilitate the extraction of photocarriers, resulting greatly increased J
sc
from 2.04 ± 0.04
mA/cm
2
to 9.07 ± 0.67 mA/cm
2
(Table 4-5).
The µ
0
decreases by a factor of 30 in the 1:6 BHJ cells compared with pure SQ 1
thin film. As less SQ 1 molecules exist in the composites, the average distance between
the SQ 1 donor molecules becomes significantly larger and there is lack of continuous
path for the hole transport. As a result, the hole hopping and tunneling in the small
molecules is inhibited. Therefore, appropriate control of the nanoscale morphology in the
SQ 1:PC
70
BM composite is necessary to form a continuous pathway for both electron and
hole carriers extracted to their respective electrodes.
The correlation of active layer morphology to the device performance is
investigated by AFM and TEM images of the five different blends (Figure 4-14) on
MoO
3
(80Å) buffer layers deposited on ITO substrates. AFM images show that the active
layer of a 5µm x 5µm surface area sample has an average root-mean-square (RMS)
roughness of 0.82 nm, 0.70 nm, 1.01 nm, 1.60 nm, and 0.90 nm for the ratios of 3:1, 1:1,
1:2 and 1:3 and 1:6, indicating the presence of a smooth surface. In TEM images, the
dark area is attributed to PC
70
BM clusters because the electron scattering of PC
70
BM is
higher. The main morphological feature of the SQ 1:PC
70
BM mixture is that a
homogeneous distribution of the donor and acceptor materials within the nanoscale,
resulting large areas of interface between SQ 1 and PC
70
BM materials which allows
better free charges and desirable photocurrent. In the AFM of 1:3, the interconnected
network of PC
70
BM can be formed which will help collect the free electron carriers.
Further increase of SQ 1 and PC
70
BM ratios to 1:8 and 1:9 results in short condition of

104

the devices, indicating that fully continuous connection of PC
70
BM between anode and
cathode electrodes happens when enough amount of PC
70
BM is added. But there is no
obvious interpenetrating network of SQ 1-rich regions has been observed here when the
ratio of the blend is increased to 1:3 and 1:6, the extracted hole carriers through SQ 1
materials will be limited. Thus, refiner mixture of SQ 1 and PC
70
BM is beneficial to form
interfacial areas for efficient charge separation, but at a cost of the discontinuous channel
for the hole transport. The lack of the interpenetrating network of SQ 1 molecules inhibit
the further increase of the performance.
25

Figure 4-16: (a) AFM topographic [(a),(c),(e),(g)] and TEM [(b),(d),(f),(h)] images of
SQ:PC70BM films spin-coated from different ratios: 3:1 ([(a) and (b)]), 1:1 [(c) and (d)],
1:2 [(e)and (f)], 1:3 [(g) and (h), and 1:6[(i) and (j)]. Images of AFM are 5µmx5µm in
size.
The pure SQ 1 films strongly absorb red light and µ
0
at the order of 10
-5
cm
2
/V-s.
Blended with different ratios of the PC
70
BM, SQ 1 proves to an effective solution

105

processable donor material for BHJ cells with efficiencies up to 2.75 ± 0.09%and V
oc
up
to 0.90 ± 0.02 V. As more PC
70
BM component is uniformly mixed with SQ 1, it creates
morphological order for exciton dissociation and facilitates the extraction of the free
carriers. However, the increasing PC
70
BM content disrupts the aggregation of the SQ 1
and discontinuous SQ 1 phase is formed. Therefore, there is no continuous hole transport
path which limits the low FF.
Table 4-5. Summary of solar cell characteristics of different SQ:PC
70
BM ratios and
SQ/C
60
planar control cell. The active layers with a ratio of 3:1 and 1:1 were
casted with a concentration of 10 mg/ml; with a ratio of 1:2, 1:3 and 1:6, they
were cast with a concentration of 20 mg/ml. The SQ layer of the SQ/C
60

planar control cell was prepared from 1 mg/ml SQ solution in
dichloromethane solvent.
SQ:PC
70
BM
ratio
Active layer
Thickness(Å)
V
oc
(V)

J
sc
(mA/cm
2
)

FF

η
p
(%)

3:1 320 0.66(±0.12) 2.04(±0.56) 0.28(±0.04) 0.37(±0.10)
1:1 400 0.80(±0.02) 3.45(±0.06) 0.31(±0.002) 0.85(±0.02)
1:2 720 0.87(±0.01) 8.37(±0.5) 0.29(±0.06) 2.1(±0.12)
1:3 730 0.84(±0.02) 8.90(±0.60) 0.34(±0.01) 2.55(±0.25)
1:6 760 0.90(±0.02) 9.07(±0.67) 0.34(±0.016) 2.75(±0.09)
Control

SQ(62)/
C
60
(400)
0.80 (±0.04) 8.50(±0.40) 0.57(±0.04) 3.90 (±0.12)

The integrated J
sc
from high EQE photo response of the SQ 1/PC
70
BM (1:6) bulk
cells can reach as high as 11.70 mA/cm
2
at 1 sun. Combined with high FF of 0.57

106

achieved in the planar control cell and high V
oc
of 0.90 V, the SQ 1/PC
70
BM bulk cells
can have efficiency as high as 6%. We anticipate that further understanding of the
nanoscale morphology of small-molecular-weight materials combined with PC
70
BM and
control of the phase separation (such as controlled solvent evaporation after film casting
and mixed solvent) of these two components will realize 6% high efficiency bulk solar
cells from solution processing.
4.3.2 Thermal Annealing Promotes Crystalline Squaraine Surface
Bilayer solar cells have continuous carrier transport pathways, but limited
interface area. While thermal and solvent vapor annealing have been explored for bulk
heterojunction solar cells, the differences in these methods and their impact on crystalline
morphology has not been thoroughly addressed. Here, it is shown that post-annealing
can improve the solution deposited, small molecule film crystallinity and roughness,
thereby providing a route to the optimization leading from planar to BHJ architecture.
This work differs from previous reports of solution processed BHJs, in that rather than
using a homogeneous donor-acceptor mixture, we start with a pure donor film, then
roughen it while also forming an extended crystal structure whose length scales are on the
same order as the exction diffusion length. This is followed by deposition of the C
60

acceptor, resulting in a structure with complete donor-acceptor phase separation. This
creates a low resistance, high efficiency, ordered BHJ.
Squaraine thin films are spin-coated at 3000 RPM on precleaned indium tin oxide
(ITO) substrates, coated with a 80 Å thick layer of MoO
3.
The neat SQ thin films with
thicknesses of 640 Å and 62 Å from 6 mg/ml and 1 mg/ml solutions, respectively, in

107

dichloromethane (DCM) were annealed in nitrogen at temperatures ranging from 50
0
C to
130
0
C. Solar cells were grown with the structure: ITO/MoO
3
(80Å)/SQ
1(62Å)/C
60
(400Å)/ BCP (100Å)/Al (1000Å). After spin-coating the SQ 1 layer, the
substrates were annealed and then transferred through a N
2
glove-box into a vacuum
chamber (base pressure <10
-6
Torr) where C
60
, BCP and the Al cathode were sequentially
thermally deposited.
X-ray data for the films annealed in nitrogen at 110
0
C and 130
0
C for 20 min and
dichloromethane (DCM) vapor annealed in air are shown in Figure 4-17, where
diffraction peaks are observed at 2 θ=7.62 ± 0.07
0
and 2 θ=15.19 ± 0.05
0
, corresponding to
the (011) (d
001
=11.60 ± 0.13Å) and (022) planes, respectively. Solvent annealed SQ 1
films show the strongest (011) and (022) peak intensities, indicating an increase in
crystalline content compared to that of as-cast or thermally annealed films. The mean
crystal size of SQ annealed at 110
0
C and 130
0
C is estimated to be 39 ± 4 nm and 41 ± 2
nm, respectively, inferred from the X-ray peak broadening. Solvent annealing increases
the mean crystallite size to 62 ± 3 nm.
External quantum efficiencies of the as-cast and thermally annealed solar cells are
shown in Figure 4-16(a). The EQE peak at λ=700 nm is due to SQ 1 absorption, whereas
the peaks centered at λ= 430 nm and 470 nm, result from C
60
absorption. Note that the
EQE in the C
60
absorption region increases dramatically with annealing temperature, and
its peak of 43 ± 1% is significantly higher than 28% reported previously for cells with
only 10 min annealing time. For the cell annealed at 130
0
C, the double EQE peak
centered at λ= 653 nm and λ=740 nm is consistent with aggregation (possibly

108

corresponding to dimer absorption) of the SQ 1 molecules. Note that this double peak is
also apparent in the SQ 1 absorption spectrum.
The power conversion efficiency at 1 sun is plotted as a function of annealing
temperature in Figure 4-18(b). Here, η
p
=3.6 ± 0.1%in the as-cast bilayer device (which is
somewhat lower than that previously reported
26
of 3.8 ± 0.2%increases to a maximum of
4.6 ± 0.1% at an annealing temperature of 110
0
C, and then decreases to 2.7 ± 0.1% at
130
0
C due to the reduction in V
oc
from 0.79 ± 0.02V to 0.46 ± 0.01V (these and other
device performance characteristics are provided in Table 4-6), also presumably due to
aggregate formation leading to a discontinuous SQ layer.

Figure 4-17. (a) X-ray-diffraction patterns of squaraine (SQ 1) thin films spin-coated
from dichloromethane (DCM) solvent on indium tin oxide (ITO) coated glass substrates.
The patterns suggest that the neat SQ 1 film annealed at 110
0
C and 130
0
C and DCM
solvent annealed for 20 min, has the (011) and (022) crystal axes oriented normal to the
substrate plane.
6 8 10 12 14 16 18 20 22
110
o
C
130
o
C
Intensity (a.u.)
2 
(011)
SQ (022)
ITO(211)
SQ
DCM solvent anneal

109

The FF is found to roll off significantly at intensities approaching 1 sun. Although
the FF reaches 0.69 ± 0.01 at 0.02 sun intensity after thermal annealing at 110
0
C, it
drops to 0.60 ± 0.02 at 1 sun illumination (100 mW/cm
2
). Annealing up to 110
0
C
increases the FF across the range of power intensities. In contrast, annealing cells at 130
0
C leads to an increase in FF from 0.42 ± 0.02 at 0.002 sun to 0.69 ± 0.01 at 1 sun (Table
4-6), although this increase is accompanied by an even greater differential drop in the
open circuit voltage.

Figure 4-18. (a) External quantum efficiencies (EQE) of the control and five cells
annealed at temperatures shown in legend; (b) The power conversion efficiency ( η
p
)
versus annealing temperature at 1 sun, AM1.5G simulated illumination for a device
structure of ITO/MoO
3
(80Å)/SQ 1(62Å)/C
60
(400Å)/bathocuproine (100 Å)/Al(1000 Å).

400 500 600 700 800
0
5
10
15
20
25
30
35
40
45
50
EQE (%)
Wavelength (nm)
as-cast
50
0
C
70
0
C
90
0
C
110
0
C
130
0
C

20 40 60 80 100 120 140
2.5
3.0
3.5
4.0
4.5
5.0

p
(%) at 1 sun
Annealing temperature (
0
C )

110

Figure 4-19. Atomic force microscope (AFM) images of: (a), (b) as-cast; (c), (d) 110
0
C;
(e), (f) dichloromethane (DCM) solvent-annealed SQ 1 films deposited on indium tin
oxide (ITO) coated glass with a 80 Å thick layer of MoO
3
.
The AFM images of the SQ 1 thin films are shown in Figure 4-19. While the as-
cast SQ 1 film has a root-mean-square (RMS) roughness of 0.9 ± 0.1 nm (Figure 4-19(a)),
the SQ 1 film annealed at 110
0
C shows crystalline features whose roughness is increased
to 1.3 nm ± 0.2 nm (Fig. 4-19(c)). The SQ 1 film after DCM solvent annealing is rougher
still, with RMS =1.9 nm ± 0.2 nm (Fig. 4-19(e)).

Figure 4-20. The dark current saturation current density (Js) and opencircuit voltage
(Voc) measured at 1 sun, AM1.5G illumination vs annealing temperature.
20 40 60 80 100 120 140
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
10
-9
10
-8
10
-7
V
oc
(V) at 1 Sun
Annealing temperature (
0
C )
V
oc
J
s0
(A/cm
2
)
J
s0

As-cast 110°C DCM solvent

111

The exciton diffusion length (L
D
) of thermally and DCM solvent annealed SQ 1
thin films was measured using spectrally resolved photoluminescence quenching (see
Methods).
26
Here, L
D
of SQ 1 thin films annealed at 130
0
C is 1.6 ± 0.2 nm, which is
similar to their roughness of RMS = 1.7 ± 0.2 nm. Increasing the crystallinity through
solvent annealing leads to L
D
= 5.0 ± 0.2 nm, which is three times larger than that of
thermally annealed SQ 1 thin films. However, the efficiency of solvent annealed films is
not as high ( η
p
=1.8 ± 0.3%) as for the polycrystalline films annealed at 110
0
C due to
excessive roughness that may lead to direct contact of the C
60
with the MoO
3
/ITO anode.
Table 4-6. Summary of SQ/C
60
solar cell characteristics under 1 sun, AM1.5G simulated
illumination (solar spectrally corrected) and in the dark.
Annealing
temperature
(
0
C)
V
oc
(V) J
SC

(mA/cm
2
)
FF
P
o
=1 sun
a
J
s

(mA/cm
2
)
η
p
(%) at
P
o
=1 sun
As-cast 0.79(±0.02) 8.60(±0.19) 0.53(±0.01) 5.0×10
-6
3.6(±0.1)
50 0.78(±0.02) 8.26(±0.23) 0.56(±0.02) 3.5×10
-6

3.6(±0.2)
70 0.77(±0.01) 9.36(±0.30) 0.57(±0.01) 5.0×10
-6

4.1 (±0.2)
90 0.76(±0.01) 9.56(±0.34) 0.59(±0.01) 8.2×10
-6

4.5(±0.1)
110 0.76(±0.01) 10.16(±0.22) 0.60(±0.02) 8.5×10
-6

4.6 (±0.1)
130 0.46(±0.01) 8.63(±0.12) 0.69(±0.01) 2.5×10
-4
2.7 (±0.1)
a
Po is the incident power. 1 sun = 100 mW/cm
2
at AM1.5G spectral illumination

112

The reduction in RMS is consistent with the trends in FF, which increases from
0.53 ± 0.01 of as-cast cells to 0.69 ± 0.01 for 130 0C annealed cells measured at 1 sun
intensity. Since FF is influenced by R
S
, we infer that the annealing process significantly
improves charge transport, as noted above. The V
oc
remains larger than 0.76 ± 0.01 V
with a thermal annealing temperature < 110
0
C, and abruptly drops to 0.46 ± 0.01 V at
130
0
C, as shown in Figure 4-20. This change in V
oc
is correlated to changes in the
reverse saturation current density (J
s
) obtained from the diode dark J-V characteristics.
An order of magnitude increase in J
s
is found for cells annealed at 130
0
C, indicating that
roughness increases the leakage current in the diode, again due to the introduction of
isolated regions where the C
60
directly contacts the anode, thereby leading to a reduced
V
oc
.
In conclusion, it is demonstrated that the performance of SQ 1/C
60
photovoltaic
cells is influenced by the morphology and crystallinity of the SQ 1 film controlled via
post-deposition annealing. The rough surface of the annealed SQ 1 film is found to be on
the scale of its exciton diffusion length, thereby leading to efficient exciton dissociation.
The FF increases from 0.53 ± 0.01 to 0.69 ± 0.01, arising from the formation of
crystalline SQ 1 films and continuous hole transport pathways through post-annealing
processes. The improved hole transport leads to a striking increase in EQE arising from
absorption in the C
60
layer. A solar power conversion efficiency of the SQ 1/C
60
planar
devices of η
p
=4.6 ± 0.1% at 1 sun, AM1.5G (correcting for solar mismatch) illumination
is obtained for devices whose SQ 1 layer is thermally annealed at 110
0
C.

113

4.3.3 Solvent Annealed Squariane Bulk Heterojunction OPVs
Efficient BHJ solar cells are characterized by a large interface area between donor
and acceptor materials that ensures efficient photogenerated exciton dissociation into free
charge. The optimal scale of the phase separation between these constituents is that of the
exciton diffusion length, and the separated phases must be contiguous to all for low
resistance charge transport pathways from the photosensitive region to the electrodes.
27-
29
It was found that the annealing roughens the SQ 1 surface, thereby creating a highly
folded BHJ interface with the C
60
, thereby compensating for the very short exciton (1.6 ±
0.2 nm) diffusion length (L
D
) characteristic of the SQ 1 donor. Although L
D
of SQ 1 is
very small, this deficiency is partially compensated by its high absorption coefficient
compared to that of C
60
. This motivates the use of SQ 1: fullerene blends, whereby the
ratio of materials strongly favors that of the fullerene to take advantage of its large L
D
and
low absorption. In previous work this approach has been partially successful, with the
highest EQE under low intensity illumination of SQ 1:PC
70
BM (1:6) blends approaching
50% across the visible spectrum. Unfortunately, devices fabricated using such blends
exhibited exceptionally low fill factors (FF~0.35) due to a large internal series resistance
to charge extraction from the low density of SQ 1 in the mixture. Hence, under standard
simulated solar illumination conditions (100 mW/cm
2
, AM1.5G spectrum), the efficiency
was limited to only ~3%.
In this section, annealing of these SQ 1: PC
70
BM (1:6) blends in solvent vapor to
create continuous crystalline (and hence low resistance) pathways for hole conduction
through the SQ 1 environment are explored. It should be noted that while spin-casting of

114

these mixtures provides a simple means to prepare homogeneous thin films, rapid solvent
evaporation does not allow for sufficient molecular reorganization that is needed to
achieve equilibrium, crystalline and uniformly phase-separated mixture.
30, 31
It is found
that post-annealing through additional extended exposure of the blend to dichloromethane
(DCM) can lead to a more optimized morphology that reduces series resistance, and
hence increases FF to 0.50 ± 0.01 and a power conversion efficiency of η
p
=5.2 ± 0.3% of
the resulting cells under AM1.5G, 1 sun simulated solar emission (corrected for spectral
mismatch). Indeed, our best cells measured reached efficiencies of 5.5% under similar
standard conditions.
Post annealing of SQ 1:PC
70
BM (1:6) blends entails the 6 min to 30 min exposure
of the films to DCM vapors in a closed glass vial enclosed in an ultrahigh purity nitrogen
filled glove box at room temperature. As shown in Figure 4-21, the lack of X-ray
diffraction (XRD) peak for as-deposited SQ 1:PC
70
BM films indicates an amorphous
structure. In contrast, after annealing for 10 min, to do a peak appears at 2 θ=7.80 ± 0.08
0

that increases in intensity when the annealing time is extended to 30 min. This peak is the
(011) reflection of SQ1, corresponding to an intermolecular spacing of 11.26 ± 0.16 Å.
After a 30 min exposure to DCM, a second peak corresponding to the (022) reflection
appears, indicating a continued increase in order. The mean crystal sizes of SQ 1 in the
blends annealed for 12 min and 30 min are estimated to be 2.0 ± 0.2 nm and 51 ± 4 nm,
respectively, inferred from the XRD peak broadening using the Scherrer method.
32

115

Figure 4-21: The x-ray diffraction patterns for squaraine (SQ 1):PC
70
BM (1:6) films
annealed in dichloromethane (DCM) solvent for 10min, 12 min and 30 min.
The inset shows the molecular structure of SQ 1.
The root-mean-square roughness obtained from the AFM images (Figure 4-22 (a))
of the as-cast film is 0.8 ± 0.1 nm. In contrast, the roughness of the blend after 12 min
solvent annealing increases to 8.4 ± 1.2 nm (Figure 4-22 (b)), indicating substantial
roughening due to the polycrystalline growth of SQ in the mixture. With even longer
annealing of 30 min, the phase separation of SQ 1 and PC
70
BM continues, as indicated by
further roughening to 12.0 ± 1.4 nm (Figure 4-22 (c)).
The spectra in the visible for the as-cast, and four DCM solvent-annealed SQ 1:
PC
70
BM blended films on quartz substrates are shown in Figure 4-22. The absorption
coefficient of SQ throughout the entire observed spectral range increases with annealing
time of up to 8 min, but as time is further increased, the change becomes saturated. Note
also, that the crystalline blend film (DCM 12 min) has less pronounced absorption peak
at λ= 680 nm than in the amorphous films.
6 8 10 12 14 16 18 20
O
-
O
-
N N
2+
OH
OH HO
HO
SQ 1(022)
SQ 1 (011)

Intensity (a.u.)
2 
DCM 10min
DCM 12min
DCM 30min

116

Figure 4-22: The effects of DCM solvent on film morphology. Transmission electron
microscopy and AFM of squaraine (SQ 1):PC
70
BM (1:6) films: (a) as-cast, (b)
annealed in DCM for 12 min, and (c) annealed in DCM for 30 min. The inset
shows the surface images measured by AFM.
The photoluminescence (PL) intensity of a film is quenched in the presence of
charge transfer from photogenerated donor excitons to acceptor molecules. Therefore,
efficient PL quenching in the SQ1:PC
70
BM blends indicates efficient exciton dissociation
due to photogeneration within a distance, L
D
, of an interface.
33, 34

The EQE peak of SQ 1 increases from 26 ± 2% (as-cast) to 60 ± 1% (annealed for
10 min). After a 12 min annealing, the peak EQE is reduced to < 40% across the entire
wavelength range. These results, directly analogous to those obtained in absorption,

117

further indicate that the cell efficiency depends strongly on crystallite size, with the
optimum size comparable to L
D
thereby leading to maximum exciton diffusion to the
dissociating donor/acceptor interface between SQ 1 and PC
70
BM.

Figure 4-23. The effect of DCM solvent annealing as a function of time on squaraine
composite films a) UV-vis absorption, b) photoluminescence (PL), c) EQE,
and d) J-Vcharacteristics of the SQ 1:PC
70
BM (1:6) cells at 1 sun illumination.
400 500 600 700
0
1 x10
4
2 x10
4
3 x10
4
4 x10
4
5 x10
4
6 x10
4
7 x10
4
8 x10
4
9 x10
4
as-cast
DC M 6m in
DC M 8m in
DC M 10m in
DC M 12m in

 (cm
-1
)
W aveleng th (n m )
680 700 720 740
0.2
0.4
0.6
0.8
1.0
1.2
1.4

Normalized PL intensity (a.u.)
Wavelength (nm )
Exc@ 600nm

(a) (b)
300 400 500 600 700
0
8
16
24
32
40
48
56
64

EQE (%)
W avelength (nm)
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-10
-5
0
5
10
15
20

Current density (mA/cm
2
)
Voltage (V)
O ne sun illum ination

(c) (d)

118

Figure 4-24: (a) The power conversion efficiency ( ηp) and (b) fill factor (FF) versus
power intensities as a function of dichloromethane (DCM) solvent annealing
time, for the device structure of ITO/MoO
3
(80 Å)/SQ 1:PC
70
BM (1:6 780
Å)/C
60
(40 Å)/BCP(10 Å)/LiF(8 Å)/Al(1000 Å).
The J-V characteristics in figure 4-23 measured under 1 sun, AM1.5G simulated
solar emission, indicate that the short circuit current density (J
sc
) is substantially
enhanced from 6.9 mA/cm
2
(as-cast) to 12.0 mA/cm
2
(10 min solvent anneal), and then
decreases to 8.3 mA/cm
2
after 12 min exposure to DCM. The FF shows a similar
dependence on annealing time, indicating that the extended order decreases the series
0.1 1 10 100
1
2
3
4
5

p
(%)
Power intensity (mW/cm
2
)
as-cast
DCM 6min
DCM 8min
DCM 10min
DCM 12min

(a)
0.1 1 10 100
0.32
0.36
0.40
0.44
0.48

FF
Power intensity (mW/cm
2
)
ascast
DCM 6min
DCM 8min
DCM 10min
DCM 12min

(b)

119

resistance, as anticipated for crystalline organic materials with improved molecular
packing. However, further increase of DCM annealing time increases the density of
pinholes between active layer and the contacts, leading to shorted diodes.
The optical and electrical changes on annealing lead directly to an increase in η
p
,
as shown in Figure 4-24(a). Here, the as-cast cell η
p
increases slightly with power
intensity, then rolls off to 2.4 ± 0.1% at 1 sun, along with a concomitant decrease in FF
from 0.40 ± 0.02 (at 0.002 sun) to 0.36 ±0.01 (1 sun) (see figure 4-24 (b)). In contrast, for
the 10 min annealed cell the FF increases from 0.42 ± 0.01 (0.002 sun) to 0.50 ± 0.01 (1
sun), while η
p
correspondingly increases from 1.5 ± 0.1% to 5.2 ± 0.3% (1 sun), with a
peak measured value for a cell in this population of 5.5% (J
SC
=12.0 mA/cm
2
, FF=0.5 and
V
oc
=0.92 V). Finally, the 12 min annealed cell shows a roll off in η
p
of 3.2 ± 0.1% , due
to the reduced EQE and FF. In conclusion, DCM solvent annealing leads to control of the
nanoscale phase separation of SQ 1:PC
70
BM (1:6) organic films. Through optimizing
morphology and molecular ordering of the SQ 1:PC
70
BM (1:6) solar cells, a peak power
conversion efficiency of 5.2 ± 0.3% has been achieved in these blended structures, with a
maximum cell performance achieved when the exciton diffusion length is approximately
equal to the mean SQ 1 crystallite size. This precise structural control takes advantage of
the high absorption coefficient yet small diffusion length characteristic of this squaraine
compound, allowing for only very dilute SQ 1:PC
70
BM mixtures to result in high solar
cell efficiency.

120

4.4 Broad Solar Spectral Coverage
Since the absorption peaks of C
60
and SQ 4 (1-NPSQ) lie at wavelengths of λ=450
nm and 710 nm, respectively, and absorption spectrum of squaraine molecule is relativel
y narrow (~100 nm full-width-half-max) compared to many other small molecule donors,
there is a dip in the EQE spectrum between the C
60
and 1-NPSQ peaks, ultimately limitin
g the OPV short circuit current density (J
sc
). Here, we show that blending 1-NPSQ and th
e blue-shifted asymmetric donor, 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl] s
quaraine (SQ 10, DPASQ), can partially fill the gap in EQE between λ=500 nm and 600 n
m, thereby improving device performance compared with single squaraine OPVs. With o
ur collaborator’s work, solvent annealed blended cells can have power conversion efficie
ncies of 5.9±0.3%, representing an improvement of approximately 35% compared to anal
ogous, single donor 1-NPSQ and DPASQ reference devices.
The squaraine donor layers in this work consist of various mixtures of 1-NPSQ,
with an absorption peak of 2.3×10
5
cm
-1
at λ=710 nm, and DPASQ, with an absorption of
2.6×10
5
cm
-1
at λ=530 nm (see Figure 4-25 for the absorption spectra of 1-NPSQ and
DPASQ and their corresponding molecular structural formulae) that fits within the
absorption gap between C
60
and 1-NPSQ. With this combination of materials, in
principle, the OPV cell response can overlap with the solar spectrum from 400 nm to 850
nm, as required for high efficiency solar energy conversion.
35

121

Figure 4-25: Absorption spectra of C
60
, 1-NPSQ, DPASQ and blended 1-NPSQ: DPASQ
(at weight ratio of 1:0.5) films. Inset: (upper) molecular structural formula of
1-NPSQ; and (bottom) DPASQ.

Figure 4-26: Current density vs voltage characteristics under 1 sun, AM1.5G simulated
solar illumination for thermally-annealed, neat 1-NPSQ, DPASQ and blended
donor cells at various weight ratios of 1-NPSQ to DPASQ. Also shown are
characteristics for solvent-annealed (SA), blended cells at a 1:0.5 ratio (here,
CB=compound buffer is used).
-0.2 0.00.2 0.40.6 0.81.0 1.2
-12
-10
-8
-6
-4
-2
0
2
4

Current Density (mA/cm
2
)
Voltage (V)
1-NPSQ
Blend (1:0.2)
Blend (1:0.5)
Blend (1:1)
Blend (1:2)
DPUSQ
Blend (1:0.5 CB)
Blend (1:0.5 CB+SA)
300 400 500 600 700 800 900
0
1
2
3
4
5

Absorption coefficient (x10
5
cm
-1
)
Wavelength (nm)
1-NPSQ
Blend (1:0.5)
DPASQ
C
60
N
OH
OH
N
O
O
2+
N
HO
HO
N
OH
OH O
O
2+

122

Vacuum thermal evaporation was used to deposit an 80 Å-thick molybdenum
oxide (MoO
3
) layer on the ITO surface at a rate of 0.5 Å/s. Next, 1-NPSQ was dissolved
in 1,2-dichlorobenzene (DCB) at 5 mg/ml, and then added to DPASQ powder to achieve
the desired weight ratios. The mixture was stirred at 100
°
C in a glove box filled with
ultrahigh purity N
2
for 12 hrs. After spin-casting the films at a rate of 3000 rpm for 40 s,
the substrates were thermally annealed at 90
°
C

for 6 min in the N
2
atmosphere. The
substrates were once more transferred to the high vacuum chamber for deposition of a
400 Å-thick C
60
film and an 80 Å-thick, 3, 4, 9, 10 perylenetetracarboxylic bis-
benzimidazole (PTCBI) excition blocking and electron transporting layer, both at the rate
of 1 Å/s. A 1000 Å-thick Ag cathode layer was deposited through a shadow mask with 1
mm-diameter circular apertures. The current density–voltage and EQE measurements
were performed in a nitrogen glove box. The J-V characteristics were measured in the
dark and under simulated AM 1.5G solar illumination. The incident power intensity at
one sun (100 mW/cm
2
) was measured using an NREL-calibrated Si reference cell. The
measured photocurrent was corrected by a spectral mismatch factor of between 0.97 and
1.0, depending on the EQE spectrum. Here, EQE was obtained using light from a 200
Hz-chopped and monochromated Xe-lamp.
The J-V characteristics under one sun AM1.5G simulated illumination of blended
OPVs at various weight ratios of 1-NPSQ to DPASQ are presented in Figure 4-25 and
Table 4-7. Here, the neat 1-NPSQ (120 Å thick from a 5mg/ml 1-NPSQ solution) and
DPASQ (85 Å thick from a 1mg/ml DPASQ solution) cells are used as controls. The V
oc

of neat 1-NPSQ and DPASQ cells are 0.92 V and 1.00 V, respectively, whereas V
oc
of the

123

blended cells increases with DPASQ concentration. As shown in Table 4-8, V
oc
increases
to 1.00 V at a 1:2 1-NPSQ:DPASQ, which is the same as for that of the neat DPASQ
cells.

Figure 4-27: Ultraviolet photoelectron spectra of 10Å-thick 1-NPSQ and DPASQ films
on indium-tin-oxide-coated glass substrates. (a) Low energy cutoff; and (b)
high energy cutoff of the films.The dashed line crossings correspond to
intercepts with the energy axis.
To understand the increase in V
oc
of the blended cells, the ultraviolet
photoemission spectroscopy (UPS) was used to measure the highest occupied molecular
orbital (HOMO) energies of 10 Å-thick films of 1-NPSQ and DPASQ. Films were spun
cast onto ITO/glass substrates and then transferred to an ultrahigh vacuum chamber from
an inert N
2
environment. The measurements were performed at a base pressure < 10
-8
torr
using 21.22 eV He-I emission in a Thermo VG scientific Clam 4MCD analyzer system.
There is ~0.1 eV shift of the low energy cutoff between 1-NPSQ and DPASQ films (see
Figure 4-27(a)), while the high energy cutoffs of 1-NPSQ and DPASQ are almost
identical (see Figure 4-27(b)). This suggests that the HOMO energy of DPASQ is 0.1 eV

124

deeper than that of 1-NPSQ; i. e., the HOMO levels of 1-NPSQ and DPASQ are at 5.3 ±
0.1 eV and 5.4 ± 0.1 eV below the vacuum level, respectively. Since V
oc
is related to the
energy difference between the HOMO energy of the donor and the lowest unoccupied
MO (LUMO) of the acceptor, less the polaron binding energy (i.e. the so-called
interfacial energy gap, ∆E
DA
),
32, 36
the deeper HOMO level of DPASQ compared with 1-
NPSQ leads to an increase in V
oc
of the former cell. Assuming a relationship between
∆E
DA
of the blend and the relative concentrations of the two donor constituents follows a
linear relationship, (i.e. E
DA
= E
DA:1-NPSQ
+(1- ) E
DA:DPASQ

,
where  is the weight
ratio of 1-NPSQ to DPASQ, and the energies correspond to the HOMO-LUMO offsets of
the individual constituents with C
60
, respectively), then V
oc
of blended cells should
similarly depend on  at the donor-acceptor interface. For example, for 1:2 1-NPSQ:
DPASQ, the DPASQ concentration at the surface exceeds that of 1-NPSQ, forming a
nearly continuous layer at the donor/acceptor interface. Therefore, the 1:2 blend cells
have V
oc
=1.00 ± 0.02 V, the same as that of neat DPASQ cells.
The J
sc
of the blended cells compared with the neat 1-NPSQ cell increases as 1-
NPSQ: DPASQ increases to 1:0.5, and then deceases as the ratio is further increased. As
expected, this trend is related to changes in the EQE spectrum. As shown in Figure 4-28,
the presence of DPASQ in the blend cells partially fills the gap between λ=500 nm and
600 nm, although the quantum efficiency is significantly lower than its peak for the neat
DPASQ cells. Since the absorption spectrum of DPASQ also overlaps with that of C
60
(see Figure 4-25), the EQE between λ=400 nm and 500 nm also increases. However, the
1-NPSQ peak at λ=700 nm deceases with , since the number of 1-NPSQ molecules

125

within an exciton diffusion length of the donor-acceptor junction is reduced with
increasing DPASQ concentration. When the DPASQ concentration is too high, the
blended film is thicker than the excition diffusion length, leading to the decrease of the
peak in EQE at 400nm< λ<500nm. Therefore, J
sc
reaches a maximum for 1:0.5 blends,
which is ~8% higher when compared with neat 1-NPSQ cells.
Morphology was studied using an atomic force microscope (AFM), with surface
morphologies for several films shown in Figure 4-27.While the neat 1-NPSQ film has a
root-mean-square (rms) roughness of rms=17 ± 1 Å, the surface of the 1:0.5 blend has
rms=8 ± 1 Å, only half that of neat 1-NPSQ films. As the weight ratio of DPASQ further
increases, large crystallites are formed, increasing rms to 12 ± 1 Å at a 1:2 ratio. At high
, 1-NPSQ determines the film surface morphology, while isolated islands of DPASQ
form at the higher weight ratios.
To further understand the performance of the blended cells, the modified ideal
diode equation
,
[exp( ( ) / ) ]
PPd a s
sa s b
PPd eq p
kVJR
JJ qV JR nkT
kR

  

is used to fit the J-V
characteristics in the dark.
33
Here, J
s

is the reverse saturation current density, q is the
electron charge, V
a
is the applied voltage, R
s
is the series resistance, n is the ideality
factor, k
b
is the Boltzmann constant, T is the temperature, and R
p
is the parallel (or shunt)
resistance. Here, the ratio of the polaron pair dissociation rate to its value at equilibrium
is assumed to be k
PPd
/ k
PPd ,eq
 1 for simplicity. Since the use of two donor molecules
creates disorder at the donor-acceptor interface, thereby resulting in an increased ideality
factor compared with that of the neat 1-NPSQ and DPASQ cells, as shown in Table 4-7.

126

According to Giebink, et al., disorder can lead to increased recombination at the interface
due to the high density of traps that may result. The disorder increases with blend ratio,
thereby leading to a corresponding increase in n. In addition, J
o
of the blended cells is
increased compared with neat 1-NPSQ and DPASQ cells. The series resistance R
s

increases with higher blend ratios indicating reduced hole mobility at high weight ratios.
Nevertheless, PCE at 1 sun intensity for blended cells reaches 5.1 ± 0.2% at =2,
compared with PCE= 4.4 ± 0.1% for the neat 1-NPSQ cells.

Figure 4-28: External quantum efficiency (EQE) spectra of devices in Figure 4-26. Note
that the clearly defined feature due to exciton generation in the DPASQ film
disappears for the CB and CB+SA films due to a significant increase in the
intensity of the C
60
spectra. The presence of DPASQ results in the broadening
of that feature.
To further optimize cell performance, the 1:0.5 film was solvent-vapor annealed
in the presence of dichloromethane vapor for 10 min following deposition of the C
60

layer. In addition, a 1,4,5,8-napthalene-tetracarboxylic-dianhydride (NTCDA) (150
300 400 500 600 700 800 900
0
10
20
30
40
50

EQE (%)
Wavelength (nm)
1-NPSQ
Blend (1:0.2)
Blend (1:0.5)
Blend (1:1)
Blend (1:2)
DPASQ
Blend (1:0.5 CB)
Blend (1:0.5 CB+SA)

127

Å)/PTCBI (50 Å) compound buffer layer was used to cap the blend/C
60
cell, which
further enhances the optical field distribution within the OPV active layer while blocking
excitons from quenching at the cathode.
37
For comparison, analogous thermally-annealed
cells with compound buffer layers were also fabricated.
The short circuit current increases from J
sc
=7.3 ± 0.2 mA/cm
2
for the cells
without compound buffer layers, to J
sc
=7.8 ± 0.2 mA/cm
2
for ones with such buffers,
consistent with previous results. As shown in Figure 4-24 and Table 4-7, the short-circuit
current increases further, to J
sc
=10.5 ± 0.5 mA/cm
2
for solvent-annealed devices due to a
significant increase of EQE in both the C
60
and squaraine absorption regions. The
improvement in EQE (c.f. Figure 4-28) upon solvent-annealing is likely due to the
improvement in crystallinity of both the C
60
and blended layers, which leads to an
increase in exciton diffusion length and charge mobility. We note that this increase in
EQE results in the disappearance of a clearly defined shoulder in the C
60
peak due to
exciton generation in the DPASQ. This is results from the increased C
60
peak intensity,
where the presence of DPASQ now results in an overall broadening of the long
wavelength tail of the C
60
peak. The open circuit voltage declines from V
oc
=0.98 ± 0.02
V for thermal-annealed cells to 0.78 ± 0.02V for solvent-annealed devices, with the fill
factors remaining largely unchanged. The reduction in V
oc
is attributed to the increased
concentration of defects at the donor-acceptor interface during the solvent annealing
process, thereby reducing V
oc
due to enhanced polaron-pair recombination.
38
The power
conversion efficiency at 1 sun intensity (simulated AM 1.5G spectrum) increases from
PCE=5.2 ± 0.2% for thermal-annealed cells to 5.9 ± 0.3% for solvent-annealing.

128

Figure 4-29: Atomic force microscope (AFM) images of a (a) neat 1-NPSQ film; (b) 1-
NPSQ: DPASQ 1:0.5 blend; and (c) a 1:2 blend. Here, RMS indicates the root
mean square roughness of the films in the respective images. Small-size
surface clusters (possibly crystallites) were observed on neat 1-NPSQ film,
which leads to RMS = 17 Å. The surface of 1:0.5 blend is smoother, with
fewer clusters and RMS = 8 Å. The 1:2 blend has large clusters, with RMS =
12 Å.
In conclusion, we have demonstrated that blended 1-NPSQ:DPASQ donor layers
can lead to an increased J
sc
as compared to neat, single component donors. This results
from increased absorption between λ=450 nm and 600 nm, the location of the gap in the
EQE spectrum between C
60
and 1-NPSQ peaks, and enhanced V
oc
due to the deeper
HOMO energy of blended film compared with neat 1-NPSQ. The power conversion
efficiency at 1 sun intensity increases from η
p
=4.4 ± 0.1% for neat 1-NPSQ cells, to
η
p
=5.1 ± 0.2% for blend cells at an optimal 1-NPSQ: DPASQ ratio of 1:0.5. Furthermore,
solvent-annealing a blended device employing a compound, PTCBI/NTCDA exciton
blocking layer, the power conversion efficiency of blended cells increases to 5.9 ± 0.3%
at 1 sun intensity due to a significant increase of J
sc
in spite of a small reduction of V
oc
.

129

But the diminishing of DPASQ response is observed, which might be due to the
decomposition of the DPASQ compound. Currently, a similar DPASQ compound but
more stable is under synthesis and will be analyzed to replace the DPASQ compound.
Table 4-7. Device performance of various 1-NPSQ, DPASQ and blended cells.
*TA: thermal annealed
#
CB: compound buffer
◊
SA: solvent annealed.
Device V
oc
(V)
J
sc

(mA/cm
2
)
FF

ηp(%)

J
s

(mA/cm
2
)
n
1-NPSQ
(TA
*
)
0.92(±0.02) 6.8(±0.1) 0.70(±0.01) 4.4(±0.1) (7.2±0.6)×10
-9
1.72(±0.04)
Blend(1:
0.2, TA)
0.94(±0.02) 7.1(±0.1) 0.71(±0.01) 4.7(±0.1) (1.8±0.2)×10
-8
1.89(±0.04)
Blend(1:
0.5, TA)
0.98(±0.02) 7.3(±0.2) 0.71(±0.01) 5.1(±0.2) (1.3±0.2)×10
-8
1.96(±0.05)
Blend(1:
1, TA)
0.98(±0.02) 7.0(±0.2) 0.7(±0.02) 4.8(±0.2) (6.0±0.5)×10
-8
2.16(±0.07)
Blend(1:
2, TA)
1.00(±0.02) 6.0(±0.2) 0.66(±0.01) 3.9(±0.2) (1.5±0.1)×10
-8
2.23(±0.07)
DPASQ
(TA)
1.00(±0.02) 5.5(±0.1) 0.72(±0.01) 4.0(±0.1)
(1.0±0.1)×10
-
10

1.58(±0.03)
Blend(1:
0.5,
CB
#
+TA
)
0.98(±0.02) 7.8(±0.2) 0.69(±0.01) 5.2(±0.2) (1.4±0.3)×10
-8

1.88(±0.04)
Blend(1:
0.5,
CB+SA
◊
)
0.78(±0.02) 10.5(±0.5) 0.72(±0.01) 5.9(±0.3) (1.5±0.2)×10
-8

1.58(±0.02)

130

4.5 Summary
In this chapter, squaraines’ application in OPVs is discussed including vapor
deposited devices, solution processed bilayer and solution processed BHJ devices. Most
device engineering work is from our collaborators at University of Michigan. From the
first vapor deposited squaraine OPVs, we have learned that squaraine OPVs have high
open circuit voltage mainly due to small dark current and high short circuit current
arising from excellent film absorptivity. However, problems such as low thermal
stability, broad solar spectrum coverage and short exciton diffusion length of squaraines
need to be overcome for further device performance enhancement. These problems are
solved respectively in this chapter. First, squaraine OPVs are explored with solution
process approach which offers the opportunity to study a scope of squaraines. Later,
squaraines are fabricated in BHJ devices by blending with fullerene derivates and also in
nanocrystalline heterojunction structure with acceptor C
60
to overcome short exciton
diffusion length. Such nanocrystalline heterojunction structure is achieved by post
thermal and solvent annealing, which can significantly enhance surface area and have
domain size in the order of exciton diffusion length. The short circuit current is greatly
improved in this structure. Further enhancement in photocurrent can be achieved by
broadband absorption from mixed squaraine blends with different absorption profile.
Strong spectral response from both regions demonstrates the effectiveness of this
approach. By placing compound blocking layer PTCBI and NTCDA, which serve as
electron conducting and exciton blocking layer respectively, squaraine/C
60
based devices
are able to achieve FF as high as 0.7. This directly results in over 25% improvement

131

compared to analogous device using conventional bathocuproine layer. Overall, this
chapter shows engineering tools addressing some current OPVs’ issues like: narrow
overlap with solar spectrum, low charge carrier mobility and short exciton diffusion.
Record device efficiency of 5.9% is achieved. Further enhancement is possible with more
research efforts involved.

132

4.6 Chapter 4: References
1. Tian, M. Q.; Furuki, M.; Iwasa, I.; Sato, Y.; Pu, L. S.; Tatsuura, S., Search for
Squaraine Derivatives That Can Be Sublimed without Thermal Decomposition. J. Phys.
Chem. B. 2002, 106, (17), 4370.

2. Forrest, S. R., Ultrathin Organic Films Grown by Organic Molecular Beam
Deposition and Related Techniques. Chem. Rev. 1997, 97, (6), 1793-1896.

3. Chen, H. J.; Law, K. Y.; Whitten, D. G., Aggregation of Amphiphilic Squaraines
at the Air-Water Interface and in Langmuir-Blodgett Films. J. Phys. Chem. 1996, 100,
(14), 5949-5955.

4. Cornil, J.; Beljonne, D.; Calbert, J. P.; Brédas, J. L., Interchain Interactions in
Organic Pgr-Conjugated Materials: Impact on Electronic Structure, Optical Response,
and Charge Transport. Adv.Mater. 2001, 13, (14), 1053.

5. Bredas, J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R., Chain-Length
Dependence of Electronic and Electrochemical Properties of Conjugated Systems-
Polyacetylene, Polyphenylene, Polythiophene, and Polypyrrole J. Am. Chem. Soc. 1983,
105, (22), 6555-6559.

6. D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.;
Thompson, M. E., Relationship between the Ionization and Oxidation Potentials of
Molecular Organic Semiconductors. Organic Electronics 2005, 6, (1), 11-20.

7. Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.; Thompson, M. E.,
Enhanced Open-Circuit Voltage in Subphthalocyanine/C
60
Organic Photovoltaic Cells. J.
Am. Chem. Soc. 2006, 128, (25), 8108-8109.

8. Sato, N.; Saito, Y.; Shinohara, H., Threshold Ionization Energy of C
60
in the Solid
State Chemical Physics 1992, 162, (2-3), 433-438.

9. Schwedhelm, R.; Kipp, L.; Dallmeyer, A.; Skibowski, M., Experimental Band
Gap and Core Hole Electron Interaction in Epitaxial C
60
Films. Phys. Rev. B 1998, 58,
(19), 13176-13180.

10. Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.; Thompson, M. E.,
Enhanced Open-Circuit Voltage in Subphthalocyanine/C-60 Organic Photovoltaic Cells.
J. Am. Chem. Soc. 2006, 128, (25), 8108.

11. Rand, B. P.; Burk, D. P.; Forrest, S. R., Offset Energies at Organic Semiconductor
Heterojunctions and Their Influence on the Open-Circuit Voltage of Thin-Film Solar
Cells. Phys. Rev. B 2007, 75, (11), 115327.

133

12. Li, N.; Lassiter, B. E.; Lunt, R. R.; Wei, G.; Forrest, S. R., Open Circuit Voltage
Enhancement Due to Reduced Dark Current in Small Molecule Photovoltaic Cells. Appl.
Phys. Lett. 2009, 94, (2), 023307.

13. Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E., Molecular and
Morphological Influences on the Open Circuit Voltages of Organic Photovoltaic Devices.
J. Am. Chem. Soc. 2009, 131, (26), 9281-9286.

14. Westphalen, M.; Kreibig, U.; Rostalski, J.; Luth, H.; Meissner, D., Metal Cluster
Enhanced Organic Solar Cells. Solar Energy Materials and Solar Cells 2000, 61, (1), 97-
105.

15. Naka, S.; Okada, H.; Onnagawa, H.; Yamaguchi, Y.; Tsutsui, T., Carrier
Transport Properties of Organic Materials for El Device Operation. Synthetic Metals
2000, 111, 331-333.

16. Masumoto, Y.; Mori, T., Application of Organic Bathocuproine-Based Alloy Film
to Organic Light-Emitting Diodes. Thin Solid Films 2008, 516, (10), 3350-3356.

17. Salzman, R. F.; Xue, J. G.; Rand, B. P.; Alexander, A.; Thompson, M. E.; Forrest,
S. R., The Effects of Copper Phthalocyanine Purity on Organic Solar Cell Performance.
Organic Electronics 2005, 6, (5-6), 242-246.

18. Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y., Accurate
Measurement and Characterization of Organic Solar Cells. Adv. Funct. Mater. 2006, 16,
(15), 2016-2023.

19. Suemori, K.; Miyata, T.; Yokoyama, M.; Hiramoto, M., Organic Solar Cells
Protected by Very Thick Naphthalene Tetracarboxylic Anhydride Films. Appl. Phys. Lett.
2004, 85, (25), 6269-6271.

20. Peumans, P.; Forrest, S. R., Very High Efficiency Double Heterostructure Copper
Phthalocyanine/C
60
Photovoltaic Cells. Appl. Phys. Lett. 2001, 79, (1), 126-128.

21. Pettersson, L. A. A.; Roman, L. S.; Inganas, O., Modeling Photocurrent Action
Spectra of Photovoltaic Devices Based on Organic Thin Films. J. Appl. Phys. 1999, 86,
(1), 487-496.

22. Boultif, A.; Louer, D., Powder Pattern Indexing with the Dichotomy Method. J.
Appl. Cryst. 2004, 37, 724-731.

23. Moliton, A.; Nunzi, J. M., How to Model the Behaviour of Organic Photovoltaic
Cells. Polym Int 2006, 55, (6), 583-600.

134

24. Uchida, S.; Xue, J. G.; Rand, B. P.; Forrest, S. R., Organic Small Molecule Solar
Cells with a Homogeneously Mixed Copper Phthalocyanine: C
60
Active Layer. Appl.
Phys. Lett. 2004, 84, (21), 4218-4220.

25. Zhang, F. L.; Jespersen, K. G.; Bjorstrom, C.; Svensson, M.; Andersson, M. R.;
Sundstrom, V.; Magnusson, K.; Moons, E.; Yartsev, A.; Inganas, O., Influence of Solvent
Mixing on the Morphology and Performance of Solar Cells Based on Polyfluorene
Copolymer/Fullerene Blends. Adv. Funct. Mater. 2006, 16, (5), 667-674.

26. Lunt, R. R.; Giebink, N. C.; Belak, A. A.; Benziger, J. B.; Forrest, S. R., Exciton
Diffusion Lengths of Organic Semiconductor Thin Films Measured by Spectrally
Resolved Photoluminescence Quenching. J. Appl. Phys. 2009, 105, (5), 053711.

27. P. Peumans, S. U., S. R. Forrest, Bulkheterojunction Photovoltaic Cells Using
Small-Molecular-Weight Organic Thin Films. Nature 2003, 425, 158.

28. Walker, B.; Tomayo, A. B.; Dang, X.-D.; Zalar, P.; Seo, J. H.; Garcia, A.;
Tantiwiwat, M.; Nguyen, T.-Q., Nanoscale Phase Separation and High Photovoltaic
Efficiency in Solution-Processed, Small-Molecule Bulk Heterojunction Solar Cells. Adv.
Funct. Mater. 2009, 19, (19), 3063-3069.

29. Peumans, P.; Uchida, S.; Forrest, S. R., Efficient Bulk Heterojunction
Photovoltaic Cells Using Small-Molecular-Weight Organic Thin Films. Nature 2003,
425, (6954), 158-162.

30. Yao, Y.; Hou, J.; Xu, Z.; Li, G.; Yang, Y., Effect of Solvent Mixture on the
Nanoscale Phase Separation in Polymer Solar Cells. Adv. Funct. Mater. 2008, 18, (12),
1783-1789.

31. Silvestri, F.; Irwin, M. D.; Beverina, L.; Facchetti, A.; Pagani, G. A.; Marks, T. J.,
Efficient Squaraine-Based Solution Processable Bulk-Heterojunction Solar Cells. J. Am.
Chem. Soc. 2008, 130, (52), 17640-17641.

32. Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stuhn, B.; Schilinsky, P.;
Waldauf, C.; Brabec, C. J., Correlation between Structural and Optical Properties of
Composite Polymer/Fullerene Films for Organic Solar Cells. Adv. Funct. Mater. 2005,
15, (7), 1193-1196.

33. Kim, J. S.; Park, Y.; Lee, D. Y.; Lee, J. H.; Park, J. H.; Kim, J. K.; Cho, K.,
Poly(3-Hexylthiophene) Nanorods with Aligned Chain Orientation for Organic
Photovoltaics. Adv. Funct. Mater. 2005, 20, (4), 540-545.

135

34. Lunt, R. R.; Benziger, J. B.; Forrest, S. R., Relationship between Crystalline
Order and Exciton Diffusion Length in Molecular Organic Semiconductors. Adv.Mater.
2010, 22, (11), 1233-1236.

35. Giebink, N. C.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest, S. R.,
Thermodynamic Efficiency Limit of Excitonic Solar Cells. Phys. Rev. B 2011, 83, (19),
195326.

36. Clarke, T. M.; Ballantyne, A. M.; Tierney, S.; Heeney, M.; Duffy, W.;
McCulloch, I.; Nelson, J.; Durrant, J. R., Charge Photogeneration in Low Band Gap
Polyselenophene/Fullerene Blend Films. J. Phys. Chem. C 2010, 114, (17), 8068-8075.

37. Sharma, G. D.; Suresh, P.; Sharma, S. S.; Vijay, Y. K.; Mikroyannidis, J. A.,
Effect of Solvent and Subsequent Thermal Annealing on the Performance of
Phenylenevinylene Copolymer:Pcbm Solar Cells. ACS Appl. Mater. Interfaces 2010, 2,
(2), 504-510.

38. Sievers, D. W.; Shrotriya, V.; Yang, Y., Modeling Optical Effects and Thickness
Dependent Current in Polymer Bulk-Heterojunction Solar Cells. J. Appl. Phys. 2006,
100, (11), 114509.

136

Chapter 5. Large Scale Synthesis of Squaraines
5.1 Why Large Scale Synthesis?
From previous chapters, squaraines are discussed with synthesis, optical and
electronic properties. Chapter 4, includes squaraines application in various types of
devices: vapor deposited OPVs, solution processed bilayer and BHJ OPVs. By applying
post thermal
1
and solvent annealing,
2
the photocurrent could be significantly enhanced. In
addition, a squaraine device with broadband absorption is acquired by mixed squaraines
from different absorption regions.
3
With suitable device structure design by incorporating
an optical spacer, a decent FF of 0.7 is obtained.
4
With all these tools available, a
squaraine device with record performance over 6% was demonstrated.
5
In all, squaraine
materials are successful donors in OPVs. Further improvement is expected to make when
more intensive research is involved.
There are a number of chemistry and engineering research groups actively
working on various aspects of OPV devices. However, most engineering groups lack
access to the newest high performance materials. In reality, they strongly depend on the
commercial available sources in the market.
In order to understand what is going on behind the commercialization of an OPV
molecule of interest and to make the novel squarines we developed in lab, available to
researchers all over the world, I have undertaken a summer internship opportunity at
Sigma Aldrich Corp. Sigma Aldrich Corp. is the leader in fine chemical and biochemicals
supply business. Accordingly, as part of this internship program at Sigma Aldrich, I
decided to explore the opportunity of commercializing the high performance squaraines.

137

For me, this is a great opportunity to be involved in the process of large scale synthesis
and commercialization products. During my 10 weeks summer internship at Sigma
Aldrich, five compounds were synthesized in over hundred-gram quantities. The
following is the product information.

Figure 5-1. The structures of squaraines and precursors made in Simga Aldrich.

2,4-bis[4-(N,N-diIisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ)

2,4-bis[4-(N,N-dibenzylamino)-2,6-dihydroxyphenyl] squaraine (BZSQ)

2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl] squaraine (DPSQ)

3.5-dimethoxy-N,N-diphenylaniline

5-(diphenylamino)benzene-1,3-diol

138

Three squaraines are selected as they represent different classes of squaraines
with different synthetic approaches. SQ is representative of alkylanilino squaraines, while
DPSQ is successful example of arylanilino squaraines. SQ could be made via two types
of nucleophilic substitution while the precursor of DPSQ is made via Buckwald-Hartwig
amination protocol.
5.2 Difference between Academic Labs and Industry
During the course of this summer internship program, I recognized few different
approaches between academia and industry. In this section, I tried to summarize my
observations.
5.2.1 Safety
Safety takes the utmost priority in industrial setting. Before any new lab person is
allowed to run experiment, thorough 4-stage training is provided. This included, reading
the standard operating procedures and guidelines; watching a senior lab person (trainer)
doing the lab work; carrying out laboratory work, while the trainer watches closely.
When these steps are successfully completed, only then I was able to perform my
research. I realized this is important and needed, as the scale of synthesis is several folds
larger and any accident at that scale would be devastating to the safety of people and
property.
Like all chemistry labs, people without eye protection like goggles are not
allowed in the labs. In addition, all active lab personnel were encouraged and mandated
to wear and to avoid the chemical contamination of personal clothes. This uniform is

139

washed on a daily basis. In addition, metal toe shoes are provided and enforced to wear to
protect any unexpected glass drop. Half mask respirator is used when necessary.
Besides physical protection by personal wear, a secondary container is always
needed for all reaction set ups and solution transfer. This is understandable because
glassware handled here is much larger in size. Any equipment failure leading to spilling
on reaction container will be recovered by secondary container.
In a typical reaction set up, four apparatus are necessary: thermometer, air driven
mechanical stir bar, condenser and addition funnel. All reactions were carried in inert
atmosphere (nitrogen), unless the chemistry demands the supply of air. The thermometer
is always set up for monitoring and recording the reaction temperature. This active
reaction temperature monitoring avoids any runaway reactions. All exothermic reactions
and high energy reactions were also monitored using reaction calorimetry to pin down the
safe reaction conditions.
During a reaction, stir process is also important to guarantee uniform heat
distribution. Unlike the normal magnetic spin bar used in small scale synthesis, only air
driven mechanical stirrer is utilized for stirring purpose in large scale reactions. The
powerful mechanical stirrer excludes the possibility of overheat inside reaction flask.
Solvent choices: Solvent selection is one of the most important steps in any
chemical synthesis. In large scale settings this is even more important as large amount of
volatile solvent could easily build up pressure and catch fire. Accordingly, the reactions
were run under explosion-proof conditions. All the equipment is grounded to avoid any
static discharge. Also, all solvents prone to develop static electricity or highly volatile in

140

nature are avoided. For example, n-heptane or cyclohexane is used in place of n-hexane
and tert-butyl methyl ether is used in place of diethyl ether.
Solution transfer: unlike the small scale reaction, a pressure/vacuum transfer is the
most common way in solution/solvent transfer in a large scale reaction. This is an
effective and fast way to transfer large amount of solution. Most of the transfers were
done under nitrogen pressure.
5.2.2 Ideas to Real Products
Sigma Aldrich is a specialty chemical company selling products in life sciences
and chemistry. Aldrich specializes in supplying chemicals for chemistry and material
science areas. In order to keep up with the ever growing synthetic needs of chemists and
material scientists, Aldrich introduces close to 1500 new products every year. Normally,
new products are planned one year ahead.
This new product introduction involves several steps. First step is to collect new
product ideas. This could come from synergistic effort from R&D scientists, marketing
group and reach out with field experts. In the early idea collecting process, this could
come across from literature reports, conference talks and/or technology licenses.
Once all raw ideas are collected, a detailed technical assessment is performed.
During this process, scalability, safety and environmental issues are considered. The
ideas after technical assessment will be evaluated by marketing group. Not all novel ideas
could become good selling products in real world. In this sense, the real value of idea
should be carefully reviewed. How much does it cost to produce it? How novel is the

141

idea? What is the long term profit? In industry, only the products that pass both technical
and market evaluation will be considered for real production.
Real production includes two parts – R&D run and Production scale up. R&D
scientists are responsible for providing a detailed written report for scalable synthesis
procedure with listed observations. Before passing this report to the production group, a
one hundred gram batch of product with required purity must be produced. Only at this
point, the new product will be further scaled up in production department depending on
the market demand. This is an effective way to reduce production cost since it is more
expensive for R&D scientists to explore a cost effective large scale synthesis scheme.
Once a complete report with synthesis procedure and purification methods is finished, it
is easier for production to reproduce the reaction.
Quality assurance (QA) is also critical for commercialized products. It is
important to supply product that meets the customer requirement consistently every time.
Accordingly, the purity of the product is determined by an independent quality control
department. The product would be made available to the customers, only when the
product meets required specifications.
Product commercialization and promotion is the last step. With the help of R&D
Scientists, marketing would prepare promotional material with details of application data
or any other relevant information.
5.2.3 Detailed reaction plan and notebook record
Transfer of learning from R&D department to the production department is
important for meeting the market demand for the products, as production department is

142

producing the products developed in R&D. Electronic notebooks are used to record such
details as reaction progress, color changes, temperature changes, and any special
techniques used for isolation and purification of product. At the end of product
development, a detailed report is prepared and given to production department. This is
many times more detailed than any procedure one can see in literature. Besides setting up
reaction, details like calculating reagent moles, and the cost of each material is also
recorded. This way, an estimate of cost for each step is on file. This is valuable for final
product cost evaluation as well. Every single detail about reaction should be written down
from how to set up the reaction or any unexpected circumstance. Such a detailed report
make it possible for our production group to repeat recipe. This is an effective way to
save the cost of labor.
5.3 Large Scale Process of Squaraines
For the large scale squaraine synthesis conducted at Aldrich, the synthesis scheme
is very similar with the one in our lab. In all, the handling of the reaction and the choice
of reagents are the major differences in large scale squaraine synthesis, the choice of
reagents or reaction handling.
In large scale reactions, purification by column chromatography is not preferred
in terms of energy and cost. To purify intermediate diisobutylaniline, I used to run flash
column to purify this oily crude mixture. At Aldrich, the scale of intermediate is about
300 g. It is not practical to separate crude product on a column considering the quantity.
Instead, the crude mixture was purified by washing with chloroform solvent.

143

5.3.1 An Air Stable Ligand
Here I would like to discuss how to choose the right reagent in large scale
reactions. The compound DPSQ has been made in our research lab in ~2 g quantity. The
precursor synthesis used Buckwald-Hartwig amination, which involved with P(tBu)
3
.
The ligand is normally used to activate the Pd
2
(dba)
3
catalyst. However, P(tBu)
3
is quite
air sensitive and pyrophoric. It should be stored in glove box and could easily undergo
oxidation once exposed to air. Thus, an air stable replacement is desired to run a reaction
on hundreds of grams scale.
In the literature, Netherton et.al
6
proposed to replace P(tBu)
3
with salt
[HP(tBu)
3
]BF
4.
The idea is to have phosphonium salt undergo in-situ deprotection under
the base Na(OtBu). Then the phosphine ligand will be automatically released during the
reaction. More importantly, the salt [HP(tBu)
3
]BF
4
is demonstrated to be an air stable and
thus easy to handle. No degradation is observed in air for two months. This replacement
salt could be easy to handle in air during large scale process.

Figure 5-2. The process of converting air sensitive phosphines to air stable phosphonium
salts and how phosphine is released from phosphonium salts.

144

The figure 5-2 shows the mechanism of converting air sensitive phosphines into
air stable phosphonium salts. By following the scheme 5-1, I was able to use the above
conditions to make over 300 g of the precursor N-(3,5-dihydroxyphenyl)diphenylamine
via one step purification process.

Scheme 5-1. Synthesis scheme for DPSQ.
5.3.2 The Purity of Precursor and Final Product
The purity of crude DPSQ depends on the methods of purification of triarylamine
precursors and is variable in the range 88-99% by HPLC analysis. Comparing the use of
N-(3,5-dihydroxyphenyl)diphenylamine
5
purified by column chromatography on silica
gel (in our lab) gives DPSQ with lower purity (ca. 88.1% by HPLC) whereas use of N-
(3,5-dihydroxyphenyl)diphenylamine obtained by crystallization (at Aldrich) from the
reaction mixture gives DPSQ with higher purity (ca. 99.3% by HPLC). Based on mass,
UV/vis spectroscopy and HPLC data, impurities in the samples of crude DPSQ can be
identified as the corresponding regioisomeric DPSQs where one or both carbon atoms of
the four membered rings attached to N-phenyl ring instead of N-(3,5-dihydroxyphenyl)

145

rings of the triarylamine moiety. This may due to the modified reaction conditions,
forming less impurity and making the purification process simpler.
5.3.3 Purity Characterization
The characterization was performed by Shimadzu Prominance-LCMS 2020
equipped with a column oven (T = 40 °C), a PDA photodetector (200800 nm), and an MS
spectrometer (LCMS 2020; m/z range: 0-2000; ionization modes: ESI/APCI).
1. DPSQ synthesized in Aldrich from crystallized precursor N-(3,5-
dihydroxyphenyl)diphenylamine with purity of 99.3%.

Figure 5-3. The HPLC analysis of DPSQ synthesized in our lab.

146

2. In comparison, here is the HPLC result of DPSQ synthesized in our lab via
the column purified precursor N-(3,5-dihydroxyphenyl)diphenylamine. This one is
with 88.1 % purity.

Figure 5-4. The HPLC analysis of DPSQ synthesize in Sigma Aldrich.
For the low purity DPSQ synthesized in our lab, further purification like
crystallization from hot tolune/chlorobenzene could improve purity to 99.6%.
In conclusion, a clean precursor purified from crystallization is preferred for
making a high quality squaraine compound. This could make final purification much
easier and being efficient for large scale production as well.

147

Figure 5-5. The HPLC analysis of DPSQ recrystallized from hot chlorobenzene.
5.4 Summary
As part of this internship program, five new products were made in large scale
and high quality. The synthesis is versatile which could be applied to make several other
squaraine derivatives. The procedure developed is now transferred to manufacturing
group for further production and scale up of these products. These products are available
for sale from Sigma-Aldrich. Commercial availability of good quality squaraines would
certainly help advancing research in OPV area.

148

5.5 Chapter 5: References

1. Wei, G.; Wang, S.; Renshaw, K.; Thompson, M. E.; Forrest, S. R., Solution-
Processed Squaraine Bulk Heterojunction Photovoltaic Cells. Acs Nano 4, (4), 1927-
1934.

2. Wei, G.; Wang, S.; Sun, K.; Thompson, M. E.; Forrest, S. R., Solvent-Annealed
Crystalline Squaraine: Pc(70)Bm (1:6) Solar Cells. Adv. Energy Mater. 2011, 1, (2), 184-
187.

3. Xiao, X.; Wei, G.; Wang, S.; Zimmerman, J. D.; Renshaw, C. K.; Thompson, M.
E.; Forrest, S. R., Small-Molecule Photovoltaics Based on Functionalized Squaraine
Donor Blends. Adv. Mater. 2012, 24, (15), 1956-1960.

4. Lassiter, B. E.; Wei, G.; Wang, S.; Zimmerman, J. D.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R., Organic Photovoltaics Incorporating Electron Conducting Exciton
Blocking Layers. Appl. Phys. Lett. 2011, 98, (24), 243307.

5. Wei, G.; Xiao, X.; Wang, S.; Sun, K.; Bergemann, K. J.; Thompson, M. E.;
Forrest, S. R., Functionalized Squaraine Donors for Nanocrystalline Organic
Photovoltaics. Acs Nano 2011, 6, (1), 972-978.

6. Netherton, M. R.; Fu, G. C., Air-Stable Trialkylphosphonium Salts:Simple,
Practical, and Versatile Replacements for Air Sensitive Trialkylphosphines. Application
in Stoichiometric and Catalytic Processes. Org. Lett. 2001, 3, (26), 4295-4298.

149

Bibliography
- Ashwell, G. J.; Bahra, G. S.; Brown, C. R.; Hamilton, D. G.; Kennard, C. H. L.;
Lynch, D. E., 2,4-Bis[4-(N,N-Dibutylamino)Phenyl] Squaraine: X-Ray Crystal Structure
of a Centrosymmetric Dye and the Second-Order Non-Linear Optical Properties of Its
Non-Centrosymmetric Langmuir-Blodgett Films. J. Mater. Chem. 1996, 6, (1), 23-26.

- Augustine, G.; Rohatgi, A.; Jokerst, N. M.; Dhere, R., Contentration-Dependent
near and above Band-Edge Asorption in Doped Inp and Its Effect on Solar Cell
Modeling. J. Appl. Phys. 1995, 78, (4), 2666-2670.

- Bagnis, D.; Beverina, L.; Huang, H.; Silvestri, F.; Yao, Y.; Yan, H.; Pagani, G.
A.; Marks, T. J.; Facchetti, A., Marked Alkyl- Vs Alkenyl-Substitutent Effects on
Squaraine Dye Solid-State Structure, Carrier Mobility, and Bulk-Heterojunction Solar
Cell Efficiency. J. Am. Chem. Soc. 2010, 132, (12), 4074-4075.

- Bailey-Salzman, R. F.; Rand, B. P.; Forrest, S. R., Near-Infrared Sensitive Small
Molecule Organic Photovoltaic Cells Based on Chloroaluminum Phthalocyanine. Appl.
Phys. Lett. 2007, 91, (1), 013508.

- Banerjee, A. N.; Chattopadhyay, K. K., Recent Developments in the Emerging
Field of Crystalline P-Type Transparent Conducting Oxide Thin Films. Prog. Cryst.
Growth Charact. Mater. 2005, 50, (1-3), 52-105.

- Barth, S.; Bassler, H., Intrinsic Photoconduction in Ppv-Type Conjugated
Polymers. Phys. Rev. Lett. 1997, 79, (22), 4445-4448.

- Beverina, L.; Abbotto, A.; Landenna, M.; Cerminara, M.; Tubino, R.; Meinardi,
F.; Bradamante, S.; Pagani, G. A., New Pi-Extended Water-Soluble Squaraines as Singlet
Oxygen Generators. Org. Lett. 2005, 7, (19), 4257-4260.

- Beverina, L.; Ruffo, R.; Patriarca, G.; Angelis, F. D.; Roberto, D.; Righetto, S.;
Ugo, R.; Pagani, G. A., Second Harmonic Generation in Nonsymmetrical Squaraines :
Tuning of the Directional Charge Transfer Character in Highly Delocalized Dyes. J.
Mater. Chem. 2009, 19, 8190-8197.

- Beverina, L.; Salice, P., Squaraine Compounds: Tailored Design and Synthesis
Towards a Variety of Material Science Applications. Eur. J. Org. Chem. 2010, (7), 1207-
1225.

- Bigelow, R. W.; Freund, H. J., An Mndo and Cndo/S(S + Des Ci) Study on the
Structural and Electronic-Properties of a Model Squaraine Dye and Related Cyanine.
Chem. Phys. 1986, 107, (2-3), 159-174.

150

- Boultif, A.; Louer, D., Powder Pattern Indexing with the Dichotomy Method. J.
Appl. Cryst. 2004, 37, 724-731.

- Brabec, C. J.; Shaheen, S. E.; Winder, C.; Sariciftci, N. S.; Denk, P., Effect of
Lif/Metal Electrodes on the Performance of Plastic Solar Cells. Appl. Phys. Lett. 2002,
80, (7), 1288-1290.

- Bredas, J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R., Chain-Length
Dependence of Electronic and Electrochemical Properties of Conjugated Systems-
Polyacetylene, Polyphenylene, Polythiophene, and Polypyrrole J. Am. Chem. Soc. 1983,
105, (22), 6555-6559.

- Brumbach, M.; Placencia, D.; Armstrong, N. R., Titanyl Phthalocyanine/C-60
Heterojunctions: Band-Edge Offsets and Photovoltaic Device Performance. J. Phys.
Chem. C 2008, 112, (8), 3142-3151.

- Buerckstuemmer, H.; Kronenberg, N. M.; Gsaenger, M.; Stolte, M.; Meerholz,
K.; Wuerthner, F., Tailored Merocyanine Dyes for Solution-Processed Bhj Solar Cells. J.
Mater. Chem. 20, (2), 240-243.

- Chen, H. J.; Law, K. Y.; Whitten, D. G., Aggregation of Amphiphilic Squaraines
at the Air-Water Interface and in Langmuir-Blodgett Films. J. Phys. Chem. 1996, 100,
(14), 5949-5955.

- Chiu, S.-W.; Lin, L.-Y.; Lin, H.-W.; Chen, Y.-H.; Huang, Z.-Y.; Lin, Y.-T.; Lin,
F.; Liu, Y.-H.; Wong, K.-T., A Donor-Acceptor-Acceptor Molecule for Vacuum-
Processed Organic Solar Cells with a Power Conversion Efficiency of 6.4%. Chem.
Commun. 48, (13), 1857-1859.

- Chung, S.-J.; Zheng, S.; Odani, T.; Beverina, L.; Fu, J.; Padilha, L. A.; Biesso, A.;
Hales, J. M.; Zhan, X.; Schmidt, K.; Ye, A.; Zojer, E.; Barlow, S.; Hagan, D. J.; Van
Stryland, E. W.; Yi, Y.; Shuai, Z.; Pagani, G. A.; Bredas, J.-L.; Perry, J. W.; Marder, S.
R., Extended Squaraine Dyes with Large Two-Photon Absorption Cross-Sections. J. Am.
Chem. Soc. 2006, 128, (45), 14444-14445.

- Clarke, T. M.; Ballantyne, A. M.; Tierney, S.; Heeney, M.; Duffy, W.;
McCulloch, I.; Nelson, J.; Durrant, J. R., Charge Photogeneration in Low Band Gap
Polyselenophene/Fullerene Blend Films. J. Phys. Chem. C 2010, 114, (17), 8068-8075.

- CornelissenGude, C.; Rettig, W.; Lapouyade, R., Photophysical Properties of
Squaraine Derivatives: Evidence for Charge Separation. J. Phys. Chem. A 1997, 101,
(50), 9673-9677.

151

- Cornil, J.; Beljonne, D.; Calbert, J. P.; Brédas, J. L., Interchain Interactions in
Organic Pgr-Conjugated Materials: Impact on Electronic Structure, Optical Response,
and Charge Transport. Adv.Mater. 2001, 13, (14), 1053.

- Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.; Dyer, D. L.; Ehrlich, J. E.;
Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Lee, I. Y. S.; McCord-Maughon, D.; Qin, J.
Q.; Rockel, H.; Rumi, M.; Wu, X. L.; Marder, S. R.; Perry, J. W., Two-Photon
Polymerization Initiators for Three-Dimensional Optical Data Storage and
Microfabrication. Nature 1999, 398, (6722), 51-54.

- Curtis, M. D.; Cao, J.; Kampf, J. W., Solid-State Packing of Conjugated
Oligomers: From Pi-Stacks to the Herringbone Structure. J. Am. Chem. Soc. 2004, 126,
(13), 4318-4328.

- D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.;
Thompson, M. E., Relationship between the Ionization and Oxidation Potentials of
Molecular Organic Semiconductors. Org. Electron. 2005, 6, (1), 11-20.

- D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P. I.; Polikarpov, E.;
Thompson, M. E., Relationship between the Ionization and Oxidation Potentials of
Molecular Organic Semiconductors. Org. Electron. 2005, 6, 11-20.

- De Arco, L. G.; Zhang, Y.; Schlenker, C. W.; Ryu, K.; Thompson, M. E.; Zhou,
C., Continuous, Highly Flexible, and Transparent Graphene Films by Chemical Vapor
Deposition for Organic Photovoltaics. Acs Nano 2010, 4, (5), 2865-2873.

- Denk, W.; Strickler, J. H.; Webb, W. W., 2-Photon Laser Scanning Fluorescence
Microscopy. Science 1990, 248, (4951), 73-76.

- Dirk, C. W.; Herndon, W. C.; Cervanteslee, F.; Selnau, H.; Martinez, S.;
Kalamegham, P.; Tan, A.; Campos, G.; Velez, M.; Zyss, J.; Ledoux, I.; Cheng, L. T.,
Squarylium Dyes - Structural Factors Pertaining to the Negative 3rd-Order Nonlinear-
Optical Response. J. Am. Chem. Soc. 1995, 117, (8), 2214-2225.

- Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E., Measurement of
the Lowest Unoccupied Molecular Orbital Energies of Molecular Organic
Semiconductors. Org. Electron. 2009, 10, (3), 515-520.

- Dong, H.; Zhu, H.; Meng, Q.; Gong, X.; Hu, W., Organic Photoresponse
Materials and Devices. Chem. Soc. Rev. 2012, 41, (5), 1754-1808.

- Ehrlich, J. E.; Wu, X. L.; Lee, I. Y. S.; Hu, Z. Y.; Rockel, H.; Marder, S. R.;
Perry, J. W., Two-Photon Absorption and Broadband Optical Limiting with Bis-Donor
Stilbenes. Opt. Lett. 1997, 22, (24), 1843-1845.

152

- Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stuhn, B.; Schilinsky, P.;
Waldauf, C.; Brabec, C. J., Correlation between Structural and Optical Properties of
Composite Polymer/Fullerene Films for Organic Solar Cells. Adv. Funct. Mater. 2005,
15, (7), 1193-1196.

- Fitzner, R.; Reinold, E.; Mishra, A.; Mena-Osteritz, E.; Ziehlke, H.; Koerner, C.;
Leo, K.; Riede, M.; Weil, M.; Tsaryova, O.; Weiss, A.; Uhrich, C.; Pfeiffer, M.;
Baeuerle, P., Dicyanovinyl-Substituted Oligothiophenes: Structure-Property
Relationships and Application in Vacuum-Processed Small-Molecule Organic Solar
Cells. Adv. Funct. Mater. 21, (5), 897-910.

- Forrest, S. R., The Limits to Organic Photovoltaic Cell Efficiency. MRS Bulletin
2005, 30, 28-32.

- Forrest, S. R., Ultrathin Organic Films Grown by Organic Molecular Beam
Deposition and Related Techniques. Chem. Rev. 1997, 97, (6), 1793-1896.

- Friedlingstein, P.; Houghton, R. A.; Marland, G.; Hackler, J.; Boden, T. A.;
Conway, T. J.; Canadell, J. G.; Raupach, M. R.; Ciais, P.; Le Quéré, C., Update on Co
2

Emission. Nature Geosci 2010, 3, 811-812.

- Giebink, N. C.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest, S. R.,
Thermodynamic Efficiency Limit of Excitonic Solar Cells. Phys. Rev. B 2011, 83, (19),
195326.

- Goetzberger, A.; Hebling, C.; Schock, H. W., Photovoltaic Materials, History,
Status and Outlook. Mater. Sci. Eng., R 2003, 40, (1), 1-46.

- Green, M.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E., Solar Cell
Efficiency Tables (Version 38) Progress in Photovoltaics 2011, 19, (5), 565-572.

- Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D., Solar Cell
Efficiency Tables (Version 39). Progress in Photovoltaics 2012, 20, (1), 12-20.

- Groves, C.; Reid, O. G.; Ginger, D. S., Heterojeneity in Polymer Solar Cells:
Local Morphology and Performance in Organic Photovoltaics Studeid with Scanning
Probe Microscopy. Acc. Chem. Res 2010, 43, (5), 612-620.

- Havinga, E. E.; Tenhoeve, W.; Wynberg, H., Alternate Donor-Acceptor Small-
Band-Gap Semiconducting Polymers - Polysquaraines and Polycroconaines. Synth. Met.
1993, 55, (1), 299-306.

153

- He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y.,
Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and
Fill Factor in Polymer Solar Cells. Adv. Mater. 2011, 23, (40), 4636-4643.

- Heliatek GmbH, t., Heliatek Sets New World Record Efficiency of 10.7 % for Its
Organic Tandem Cell 2012.

- Herzog, A. V.; Lipman, T. E.; Kammen, D. M., Renewable Energy Sources.
2001.

- Hiramoto, M.; Suezaki, M.; Yokoyama, M., Effect of Thin Gold Interstitial-Layer
on the Photovoltaic Properties of Tandem Organic Solar-Cell. Chem. Lett. 1990, (3), 327-
330.

- Hirose, Y.; Kahn, A.; Aristov, V.; Soukiassian, P.; Bulovic, V.; Forrest, S. R.,
Chemistry and Electronic Properties of Metal-Organic Semiconductor Interfaces: Al, Ti,
in, Sn, Ag, and Au on Ptcda. Phys. Rev. B 1996, 54, (19), 13748-13758.

- Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K., Energy Level Alignment and Interfacial
Electronic Structures at Organic Metal and Organic Organic Interfaces. Adv. Mater.
1999, 11, (8), 605-625.

- Jadhav, P. J.; Mohanty, A.; Sussman, J.; Lee, J.; Baldo, M. A., Singlet Exciton
Fission in Nanostructured Organic Solar Cells. Nano Letters 11, (4), 1495-1498.

- Jyothish, K.; Arun, K. T.; Ramaiah, D., Synthesis of Novel Quinaldine-Based
Squaraine Dyes: Effect of Substituents and Role of Electronic Factors. Org. Lett. 2004, 6,
(22), 3965-3968.

- Kawazoe, H.; Yasukawa, M.; Hyodo, H.; Kurita, M.; Yanagi, H.; Hosono, H., P-
Type Electrical Conduction in Transparent Thin Films of Cualo2. Nature 1997, 389,
(6654), 939-942.

- Khan, S. M.; Kaur, M.; Heflin, J. R.; Sayyad, M. H., Fabrication and
Characterization of Zntpp:Pcbm Bulk Heterojunction (Bhj) Solar Cells. J. Phys. Chem.
Solids 72, (12), 1430-1435.

- Kim, J. S.; Park, Y.; Lee, D. Y.; Lee, J. H.; Park, J. H.; Kim, J. K.; Cho, K.,
Poly(3-Hexylthiophene) Nanorods with Aligned Chain Orientation for Organic
Photovoltaics. Adv. Funct. Mater. 2005, 20, (4), 540-545.

- Kopidakis, N.; Mitchell, W. J.; van de Lagemaat, J.; Ginley, D. S.; Rumbles, G.;
Shaheen, S. E.; Rance, W. L., Bulk Heterojunction Organic Photovoltaic Devices Based
on Phenyl-Cored Thiophene Dendrimers. Appl. Phys. Lett. 2006, 89, (10), 103524.

154

- Lassiter, B. E.; Wei, G.; Wang, S.; Zimmerman, J. D.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R., Organic Photovoltaics Incorporating Electron Conducting Exciton
Blocking Layers. Appl. Phys. Lett. 2011, 98, (24), 243307.

- Law, K. Y., Organic Photoconductive Materials: Recent Trends and
Developments. Chem. Rev. 1993, 93, (1), 449-486.

- Law, K. Y., Squaraine Chemistry - Effects of Structural-Changes on the
Absorption and Multiple Fluorescence Emission of Bis[4-
(Dimethylamino)Phenyl]Squaraine and Its Derivatives. J. Phys. Chem. 1987, 91, (20),
5184-5193.

- Law, K. Y., Squaraine Chemistry - Effects of Solvent and Temperature on the
Fluorescence Emission of Squaraines. J. Photochem. Photobiol. A-Chem. 1994, 84, (2),
123-132.

- Law, K. Y.; Bailey, F. C., Squaraine Chemistry-a New Approach to Symmetrical
and Unsymmetrical Photoconductive Squaraines-Characterization and Solid State
Properties of These Materials Can. J. Chem. 1993, 71, (4), 494-505.

- Law, K. Y.; Bailey, F. C., Squaraine Chemistry. Synthesis of Bis(4-
Dimethylaminophenyl)Squaraine from Dialkyl Squarates. Mechanism and Scope of the
Synthesis. Can. J. Chem. 1986, 64, (12), 2267-2273.

- Leliege, A.; Blanchard, P.; Rousseau, T.; Roncali, J.,
Triphenylamine/Tetracyanobutadiene-Based D-a-D Pi-Conjugated Systems as Molecular
Donors for Organic Solar Cells. Org. Lett. 13, (12), 3098-3101.

- Li, G.; Yao, Y.; Yang, H.; Shrotriya, V.; Yang, G.; Yang, Y., "Solvent
Annealing" Effect in Polymer Solar Cells Based on Poly(3-Hexylthiophene) and
Methanofullerenes. Adv. Funct. Mater. 2007, 17, (10), 1636-1644.

- Li, N.; Lassiter, B. E.; Lunt, R. R.; Wei, G.; Forrest, S. R., Open Circuit Voltage
Enhancement Due to Reduced Dark Current in Small Molecule Photovoltaic Cells. Appl.
Phys. Lett. 2009, 94, (2), 023307.

- Li, W.; Du, C.; Li, F.; Zhou, Y.; Fahlman, M.; Bo, Z.; Zhang, F.,
Benzothiadiazole-Based Linear and Star Molecules: Design, Synthesis, and Their
Application in Bulk Heterojunction Organic Solar Cells. Chem. Mater. 2009, 21, (21),
5327-5334.

- Lin, L.-Y.; Chen, Y.-H.; Huang, Z.-Y.; Lin, H.-W.; Chou, S.-H.; Lin, F.; Chen,
C.-W.; Liu, Y.-H.; Wong, K.-T., A Low-Energy-Gap Organic Dye for High-Performance
Small-Molecule Organic Solar Cells. J. Am. Chem. Soc. 133, (40), 15822-15825.

155

- Lipomi, D. J.; Tee, B. C. K.; Vosgueritchian, M.; Bao, Z., Stretchable Organic
Solar Cells. Adv. Mater. 2011, 23, (15), 1771-1775.

- Lloyd, M. T.; Anthony, J. E.; Malliaras, G. G., Photovoltaics from Soluble Small
Molecules. Materials Today 2007, 10, (11), 34-41.

- Loser, S.; Bruns, C. J.; Miyauchi, H.; Ortiz, R. P.; Facchetti, A.; Stupp, S. I.;
Marks, T. J., A Naphthodithiophene-Diketopyrrolopyrrole Donor Molecule for Efficient
Solution-Processed Solar Cells. J. Am. Chem. Soc. 133, (21), 8142-8145.

- Lunt, R. R.; Benziger, J. B.; Forrest, S. R., Relationship between Crystalline
Order and Exciton Diffusion Length in Molecular Organic Semiconductors. Adv. Mater.
2010, 22, (11), 1233-1236.

- Lunt, R. R.; Giebink, N. C.; Belak, A. A.; Benziger, J. B.; Forrest, S. R., Exciton
Diffusion Lengths of Organic Semiconductor Thin Films Measured by Spectrally
Resolved Photoluminescence Quenching. J. Appl. Phys. 2009, 105, (5), 053711.

- Luo, C.; Zhou, Q.; Jiang, G.; He, L.; Zhang, B.; Wang, X., The Synthesis and
1
o
2

Photosensitization of Halogenated Asymmetric Aniline-Based Squaraines. New J. Chem.
2011, 35, (5), 1128-1132.

- Ma, B.; Woo, C. H.; Miyamoto, Y.; Frechet, J. M. J., Solution Processing of a
Small Molecule, Subnaphthalocyanine, for Efficient Organic Photovoltaic Cells. Chem.
Mater. 2009, 21, (8), 1413-1417.

- Ma, C.-Q.; Fonrodona, M.; Schikora, M. C.; Wienk, M. M.; Janssen, R. A. J.;
Baeuerle, P., Solution-Processed Bulk-Heterojunction Solar Cells Based on
Monodisperse Dendritic Oligothiophenes. Adv. Funct. Mater. 2008, 18, (20), 3323-3331.

- Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J., Thermally Stable,
Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network
Morphology. Adv. Funct. Mater. 2005, 15, (10), 1617-1622.

- Marrocchi, A.; Silvestri, F.; Seri, M.; Facchetti, A.; Taticchia, A.; Marks, T. J.,
Conjugated Anthracene Derivatives as Donor Materials for Bulk Heterojunction Solar
Cells: Olefinic Versus Acetylenic Spacers. Chem. Commun. 2009, (11), 1380-1382.

- Masumoto, Y.; Mori, T., Application of Organic Bathocuproine-Based Alloy Film
to Organic Light-Emitting Diodes. Thin Solid Films 2008, 516, (10), 3350-3356.

- Mayer, A. C.; Lloyd, M. T.; Herman, D. J.; Kasen, T. G.; Malliaras, G. G.,
Postfabrication Annealing of Pentacene-Based Photovoltaic Cells. Appl. Phys. Lett. 2004,
85, (25), 6272-6274.

156

- Mayerhoeffer, U.; Deing, K.; Gruss, K.; Braunschweig, H.; Meerholz, K.;
Wuerthner, F., Outstanding Short-Circuit Currents in Bhj Solar Cells Based on Nir-
Absorbing Acceptor-Substituted Squaraines. Angew. Chem. Int. Ed. Engl. 2009, 48, (46),
8776-8779.

- McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; Macdonald, I.; Shkunov,
M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W. M.; Chabinyc, M. L.; Kline, R. J.;
McGehee, M. D.; Toney, M. F., Liquid-Crystalline Semiconducting Polymers with High
Charge-Carrier Mobility. Nature Materials 2006, 5, (4), 328-333.

- Mei, J.; Graham, K. R.; Stalder, R.; Reynolds, J. R., Synthesis of Isoindigo-Based
Oligothiophenes for Molecular Bulk Heterojunction Solar Cells. Org. Lett. 12, (4), 660-
663.

- Meyers, F.; Chen, C.-T.; Marder, S. R.; Brédas, J.-L.; Meyers, F., Electronic
Structure and Linear and Nonlinear Optical Properties of Symmetrical and
Unsymmetrical Squaraine Dyes. Chem. Eur. J. 1997, 3, (4), 537.

- Mihailetchi, V. D.; Xie, H. X.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M.,
Charge Transport and Photocurrent Generation in Poly (3-Hexylthiophene):
Methanofullerene Bulk-Heterojunction Solar Cells. Adv. Funct. Mater. 2006, 16, (5),
699-708.

- Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.; Behera, G. B., Cyanines
During the 1990s: A Review. Chem. Rev. 2000, 100, (6), 1973-2011.

- Moliton, A.; Nunzi, J. M., How to Model the Behaviour of Organic Photovoltaic
Cells. Polym Int 2006, 55, (6), 583-600.

- Morel, D. L.; Ghosh, A. K.; Feng, T.; Stogryn, E. L.; Purwin, P. E.; Shaw, R. F.;
Fishman, C., High Efficiency Orgaic Solar Cells. Appl. Phys. Lett. 1978, 32, 495.

- Moule, A. J.; Meerholz, K., Morphology Control in Solution-Processed Bulk-
Heterojunction Solar Cell Mixtures. Adv. Funct. Mater. 2009, 19, (19), 3028-3036.

- Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.; Thompson, M. E.,
Enhanced Open-Circuit Voltage in Subphthalocyanine/C
60
Organic Photovoltaic Cells. J.
Am. Chem. Soc. 2006, 128, (25), 8108-8109.

- Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.; Thompson, M. E.,
Enhanced Open-Circuit Voltage in Subphthalocyanine/C-60 Organic Photovoltaic Cells.
J. Am. Chem. Soc. 2006, 128, (25), 8108.

157

- Naka, S.; Okada, H.; Onnagawa, H.; Yamaguchi, Y.; Tsutsui, T., Carrier
Transport Properties of Organic Materials for El Device Operation. Synth. Met. 2000,
111, 331-333.

- Netherton, M. R.; Fu, G. C., Air-Stable Trialkylphosphonium Salts:Simple,
Practical, and Versatile Replacements for Air Sensitive Trialkylphosphines. Application
in Stoichiometric and Catalytic Processes. Org. Lett. 2001, 3, (26), 4295-4298.

- Odom, S. A.; Webster, S.; Padilha, L. A.; Peceli, D.; Hu, H.; Nootz, G.; Chung,
S.-J.; Ohira, S.; Matichak, J. D.; Przhonska, O. V.; Kachkovski, A. D.; Barlow, S.;
Bredas, J.-L.; Anderson, H. L.; Hagan, D. J.; Van Stryland, E. W.; Marder, S. R.,
Synthesis and Two-Photon Spectrum of a Bis(Porphyrin)-Substituted Squaraine. J. Am.
Chem. Soc. 2009, 131, (22), 7510-7511.

- Ojala, A.; Petersen, A.; Fuchs, A.; Lovrincic, R.; Poelking, C.; Trollmann, J.;
Hwang, J.; Lennartz, C.; Reichelt, H.; Hoeffken, H. W.; Pucci, A.; Erk, P.; Kirchartz, T.;
Wuerthner, F., Merocyanine/C60 Planar Heterojunction Solar Cells: Effect of Dye
Orientation on Exciton Dissociation and Solar Cell Performance. Adv. Funct. Mater. 22,
(1), 86-96.

- P. Peumans, S. U., S. R. Forrest, Bulkheterojunction Photovoltaic Cells Using
Small-Molecular-Weight Organic Thin Films. Nature 2003, 425, 158.

- Pandey, R.; Holmes, R. J., Graded Donor-Acceptor Heterojunctions for Efficient
Organic Photovoltaic Cells. Adv. Mater. 22, (46), 5301-5305.

- Park, H.; Rowehl, J. A.; Kim, K. K.; Bulovic, V.; Kong, J., Doped Graphene
Electrodes for Organic Solar Cells. Nanotechnology 2010, 21, (50).

- Perez, M. D.; Borek, C.; Djurovich, P. I.; Mayo, E. I.; Lunt, R. R.; Forrest, S. R.;
Thompson, M. E., Organic Photovoltaics Using Tetraphenylbenzoporphyrin Complexes
as Donor Layers. Adv. Mater. 2009, 21, (14-15), 1517-1520.

- Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E., Molecular and
Morphological Influences on the Open Circuit Voltages of Organic Photovoltaic Devices.
J. Am. Chem. Soc. 2009, 131, (26), 9281-9286.

- Pettersson, L. A. A.; Roman, L. S.; Inganas, O., Modeling Photocurrent Action
Spectra of Photovoltaic Devices Based on Organic Thin Films. J. Appl. Phys. 1999, 86,
(1), 487-496.

- Peumans, P.; Forrest, S. R., Very High Efficiency Double Heterostructure Copper
Phthalocyanine/C
60
Photovoltaic Cells. Appl. Phys. Lett. 2001, 79, (1), 126-128.

158

- Peumans, P.; Uchida, S.; Forrest, S. R., Efficient Bulk Heterojunction
Photovoltaic Cells Using Small-Molecular-Weight Organic Thin Films. Nature 2003,
425, (6954), 158-162.

- Peumans, P.; Yakimov, A.; Forrest, S. R., Small Molecular Weight Organic Thin
Film Photodetectors and Solar Cells. J. Appl. Phys. 2003, 93, (7), 3693-3723.

- Potocnik, J., Renewable Energy Sources and the Realities of Setting an Energy
Agenda. Science 2007, 315, (5813), 810-811.

- Rand, B. P.; Burk, D. P.; Forrest, S. R., Offset Energies at Organic Semiconductor
Heterojunctions and Their Influence on the Open-Circuit Voltage of Thin-Film Solar
Cells. Phys. Rev. B 2007, 75, (11), 115327.

- Reidy, K.; Campanile, C.; Muff, R.; Born, W.; Fuchs, B., Mthpc-Mediated
Photodynamic Therapy Is Effective in the Metastatic Human 143b Osteosarcoma Cells.
Photochem. Photobiol. 2012, 88, (3), 721-727.

- Reis, L. V.; Serrano, J. P.; Almeida, P.; Santos, P. F., The Synthesis and
Characterization of Novel, Aza-Substituted Squarylium Cyanine Dyes. Dyes and
Pigments 2009, 81, (3), 197-202.

- Rettig, W., Charge Separation in Excited-States of Decoupled Systems - Tict
Compounds and Implications Regarding the Development of New Laser-Dyes and the
Primary Processes of Vision and Photosynthesis. Angew. Chem. Int. Ed. Engl. 1986, 25,
(11), 971-988.

- Ronchi, E.; Ruffo, R.; Rizzato, S.; Albinati, A.; Beverina, L.; Pagani, G. A.,
Regioselective Synthesis of 1,2- Vs 1,3-Squaraines. Org. Lett. 2011, 13, (12), 3166-3169.

- Roquet, S.; de Bettignies, R.; Leriche, P.; Cravino, A.; Roncali, J., Three-
Dimensional Tetra(Oligothienyl)Silanes as Donor Material for Organic Solar Cells. J.
Mater. Chem. 2006, 16, (29), 3040-3045.

- Rousseau, T.; Cravino, A.; Bura, T.; Ulrich, G.; Ziessel, R.; Roncali, J., Bodipy
Derivatives as Donor Materials for Bulk Heterojunction Solar Cells. Chem. Commun.
2009, (13), 1673-1675.

- Rowell, M. W.; Topinka, M. A.; McGehee, M. D.; Prall, H.-J.; Dennler, G.;
Sariciftci, N. S.; Hu, L.; Gruner, G., Organic Solar Cells with Carbon Nanotube Network
Electrodes. Appl. Phys. Lett. 2006, 88, (23).

159

- Salzman, R. F.; Xue, J. G.; Rand, B. P.; Alexander, A.; Thompson, M. E.; Forrest,
S. R., The Effects of Copper Phthalocyanine Purity on Organic Solar Cell Performance.
Org. Electron. 2005, 6, (5-6), 242-246.

- Sato, N.; Saito, Y.; Shinohara, H., Threshold Ionization Energy of C
60
in the Solid
State Chem. Phys. 1992, 162, (2-3), 433-438.

- Scherer, D.; Dorfler, R.; Feldner, A.; Vogtmann, T.; Schwoerer, M.; Lawrentz,
U.; Grahn, W.; Lambert, C., Two-Photon States in Squaraine Monomers and Oligomers.
Chem. Phys. 2002, 279, (2-3), 179-207.

- Schiermeier, Q.; Tollefson, J.; Scully, T.; Witze, A.; Morton, O., Energy
Alternatives: Electricity without Carbon. Nature 2008, 454, 816-823.

- Schlenker, C. W.; Thompson, M. E., Current Challenges in Organic Photovoltaic
Solar Energy Conversion. Top Curr Chem 2012, 312, 175-212.

- Schlenker, C. W.; Thompson, M. E., The Molecular Nature of Photovoltage
Losses in Organic Solar Cells. Chem. Commun. 2011, 47, (13), 3702-3716.

- Schwedhelm, R.; Kipp, L.; Dallmeyer, A.; Skibowski, M., Experimental Band
Gap and Core Hole Electron Interaction in Epitaxial C
60
Films. Phys. Rev. B 1998, 58,
(19), 13176-13180.

- Service, R. F., Is It Time to Shoot for the Sun. Science 2005, 309, (5734), 548-
551.

- Service, R. F., Solar Energy: Can the Upstarts Top Silicon? Science 2008, 319,
718-720.

- Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.;
Hummelen, J. C., 2.5% Efficient Organic Plastic Solar Cells. Appl. Phys. Lett. 2001, 78,
(6), 841-843.

- Sharma, G. D.; Suresh, P.; Sharma, S. S.; Vijay, Y. K.; Mikroyannidis, J. A.,
Effect of Solvent and Subsequent Thermal Annealing on the Performance of
Phenylenevinylene Copolymer:Pcbm Solar Cells. ACS Appl. Mater. Interfaces 2010, 2,
(2), 504-510.

- Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y., Accurate
Measurement and Characterization of Organic Solar Cells. Adv. Funct. Mater. 2006, 16,
(15), 2016-2023.

160

- Sievers, D. W.; Shrotriya, V.; Yang, Y., Modeling Optical Effects and Thickness
Dependent Current in Polymer Bulk-Heterojunction Solar Cells. J. Appl. Phys. 2006,
100, (11), 114509.

- Silvestri, F.; Irwin, M. D.; Beverina, L.; Facchetti, A.; Pagani, G. A.; Marks, T. J.,
Efficient Squaraine-Based Solution Processable Bulk-Heterojunction Solar Cells. J. Am.
Chem. Soc. 2008, 130, (52), 17640-17641.

- Smits, E. C. P.; Setayesh, S.; Anthopoulos, T. D.; Buechel, M.; Nijssen, W.;
Coehoorn, R.; Blom, P. W. M.; de Boer, B.; de Leeuw, D. M., Near-Infrared Light-
Emitting Ambipolar Organic Field-Effect Transistors. Adv. Mater. 2007, 19, (5), 734-
738.

- Sprenger, H.; Ziegenbein, W., Condensation Products of Squaric Acid and
Tertiary
Aromatic Amines. Angew. Chem. Int. Ed. 1966, 5, 894.

- Steinberger, S.; Mishra, A.; Reinold, E.; Levichkov, J.; Uhrich, C.; Pfeiffer, M.;
Baeuerle, P., Vacuum-Processed Small Molecule Solar Cells Based on Terminal
Acceptor-Substituted Low-Band Gap Oligothiophenes. Chem. Commun. 47, (7), 1982-
1984.

- Stubinger, T.; Brutting, W., Exciton Diffusion and Optical Interference in Organic
Donor-Acceptor Photovoltaic Cells. J. Appl. Phys. 2001, 90, (7), 3632-3641.

- Suemori, K.; Miyata, T.; Yokoyama, M.; Hiramoto, M., Organic Solar Cells
Protected by Very Thick Naphthalene Tetracarboxylic Anhydride Films. Appl. Phys. Lett.
2004, 85, (25), 6269-6271.

- Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J.,
Solution-Processed Small-Molecule Solar Cells with 6.7% Efficiency. Nature Materials
2011, 11, (1), 44-48.

- Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.;
Gershenson, M. E.; Rogers, J. A., Elastomeric Transistor Stamps: Reversible Probing of
Charge Transport in Organic Crystals. Science 2004, 303, (5664), 1644-1646.

- Tamayo, A. B.; Dang, X.-D.; Walker, B.; Seo, J.; Kent, T.; Nguyen, T.-Q., A Low
Band Gap, Solution Processable Oligothiophene with a Dialkylated Diketopyrrolopyrrole
Chromophore for Use in Bulk Heterojunction Solar Cells. Appl. Phys. Lett. 2009, 94,
(10), 103301.

- Tang, C. W., Two Layer Organic Photovoltaic Cell. Appl. Phys. Lett. 1986, 48,
(2), 183-185.

161

- Tang, C. W.; Albrecht, A. C., Photovoltaic Effects of Metal Cholorophyll-a-Metal
Sandwich Cells. J. Chem. Phys. 1975, 62, (6), 2139-2149.

- Tenent, R. C.; Barnes, T. M.; Bergeson, J. D.; Ferguson, A. J.; To, B.; Gedvilas,
L. M.; Heben, M. J.; Blackburn, J. L., Ultrasmooth, Large-Area, High-Uniformity,
Conductive Transparent Single-Walled-Carbon-Nanotube Films for Photovoltaics
Produced by Ultrasonic Spraying. Adv. Mater. 2009, 21, (31), 3210-3216.

- Tian, M. Q.; Furuki, M.; Iwasa, I.; Sato, Y.; Pu, L. S.; Tatsuura, S., Search for
Squaraine Derivatives That Can Be Sublimed without Thermal Decomposition. J. Phys.
Chem. B 2002, 106, (17), 4370-4376.

- Tian, M. Q.; Furuki, M.; Iwasa, I.; Sato, Y.; Pu, L. S.; Tatsuura, S., Search for
Squaraine Derivatives That Can Be Sublimed without Thermal Decomposition. J. Phys.
Chem. B. 2002, 106, (17), 4370.

- Treibs, A., Pyrrole Dyes. Chimia 1966, 20, (9), 329.

- Treibs, A.; Jacob, K., Cyclotrimethine Dyes Derived from Squaric Acid. Angew
Chem., Int. Ed. Engl. 1965, 4, 694.

- Uchida, S.; Xue, J. G.; Rand, B. P.; Forrest, S. R., Organic Small Molecule Solar
Cells with a Homogeneously Mixed Copper Phthalocyanine: C
60
Active Layer. Appl.
Phys. Lett. 2004, 84, (21), 4218-4220.

- Umezawa, K.; Cittierio, D.; Suzuki, K., Water-Soluble Nir Fluorescent Probes
Based on Squaraine and Their Application for Protein Labeling. Anal. Sci. 2008, 24, (2),
213-217.

- Volkova, K. D.; Kovalska, V. B.; Losytskyy, M. Y.; Bento, A.; Reis, L. V.;
Santos, P. F.; Almeida, P.; Yarmoluk, S. M., Studies of Benzothiazole and
Benzoselenazole Squaraines as Fluorescent Probes for Albumins Detection. J Fluoresc
2008, 18, 877-882.

- Wöbkenberg, P. H.; Labram, J. G.; Swiecicki, J.-M.; Parkhomenko, K.;
Sredojevic, D.; Gisselbrecht, J.-P.; de Leeuw, D. M.; Bradley, D. D. C.; Djukic, J.-P.;
Anthopoulos, T. D., Ambipolar Organic Transistors and near-Infrared Phototransistors
Based on a Solution-Processable Squarilium Dye. J. Mater. Chem. 2010, 20, (18), 3673-
3680.

- Wagner, J.; Gruber, M.; Hinderhofer, A.; Wilke, A.; Broeker, B.; Frisch, J.;
Amsalem, P.; Vollmer, A.; Opitz, A.; Koch, N.; Schreiber, F.; Bruetting, W., High Fill
Factor and Open Circuit Voltage in Organic Photovoltaic Cells with Diindenoperylene as
Donor Material. Adv. Funct. Mater. 2010, 20, (24), 4295-4303.

162

- Walker, B.; Tomayo, A. B.; Dang, X.-D.; Zalar, P.; Seo, J. H.; Garcia, A.;
Tantiwiwat, M.; Nguyen, T.-Q., Nanoscale Phase Separation and High Photovoltaic
Efficiency in Solution-Processed, Small-Molecule Bulk Heterojunction Solar Cells. Adv.
Funct. Mater. 2009, 19, (19), 3063-3069.

- Wang, C.-L.; Zhang, W.-B.; Van Horn, R. M.; Tu, Y.; Gong, X.; Cheng, S. Z. D.;
Sun, Y.; Tong, M.; Seo, J.; Hsu, B. B. Y.; Heeger, A. J., A Porphyrin-Fullerene Dyad
with a Supramolecular "Double-Cable" Structure as a Novel Electron Acceptor for Bulk
Heterojunction Polymer Solar Cells. Adv. Mater. 2011, 23, (26), 2951-2956.

- Wei, G.; Wang, S.; Renshaw, K.; Thompson, M. E.; Forrest, S. R., Solution-
Processed Squaraine Bulk Heterojunction Photovoltaic Cells. Acs Nano 2010, 4, (4),
1927-1934.

- Wei, G.; Wang, S.; Renshaw, K.; Thompson, M. E.; Forrest, S. R., Solution-
Processed Squaraine Bulk Heterojunction Photovoltaic Cells. Acs Nano 4, (4), 1927-
1934.

- Wei, G.; Wang, S.; Sun, K.; Thompson, M. E.; Forrest, S. R., Solvent-Annealed
Crystalline Squaraine: Pc(70)Bm (1:6) Solar Cells. Adv. Energy Mater. 2011, 1, (2), 184-
187.

- Wei, G.; Xiao, X.; Wang, S.; Sun, K.; Bergemann, K. J.; Thompson, M. E.;
Forrest, S. R., Functionalized Squaraine Donors for Nanocrystalline Organic
Photovoltaics. Acs Nano 2011, 6, (1), 972-978.

- Wei, G. D.; Wang, S. Y.; Renshaw, K.; Thompson, M. E.; Forrest, S. R.,
Solution-Processed Squaraine Bulk Heterojunction Photovoltaic Cells. ACS Nano 2010,
4, (4), 1927-1934.

- Westphalen, M.; Kreibig, U.; Rostalski, J.; Luth, H.; Meissner, D., Metal Cluster
Enhanced Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2000, 61, (1), 97-105.

- Wolf, J.; Law, K. Y.; Myers, A. B., Hole-Burning Subtracted Fluorescence Line-
Narrowing Spectroscopy of Squaraines in Polymer Matrices. J. Phys. Chem. 1996, 100,
(29), 11870-11882.

- Xiao, X.; Wei, G.; Wang, S.; Zimmerman, J. D.; Renshaw, C. K.; Thompson, M.
E.; Forrest, S. R., Small-Molecule Photovoltaics Based on Functionalized Squaraine
Donor Blends. Adv. Mater. 2012, 24, (15), 1956-1960.

- Xue, J. G.; Rand, B. P.; Uchida, S.; Forrest, S. R., A Hybrid Planar-Mixed
Molecular Heterojunction Photovoltaic Cell. Adv. Mater. 2005, 17, (1), 66-71.

163

- Yao, Y.; Hou, J.; Xu, Z.; Li, G.; Yang, Y., Effect of Solvent Mixture on the
Nanoscale Phase Separation in Polymer Solar Cells. Adv. Funct. Mater. 2008, 18, (12),
1783-1789.

- Yoo, S.; Domercq, B.; Kippelen, B., Efficient Thin-Film Organic Solar Cells
Based on Pentacene/C-60 Heterojunctions. Appl. Phys. Lett. 2004, 85, (22), 5427-5429.

- Zhang, F. L.; Jespersen, K. G.; Bjorstrom, C.; Svensson, M.; Andersson, M. R.;
Sundstrom, V.; Magnusson, K.; Moons, E.; Yartsev, A.; Inganas, O., Influence of Solvent
Mixing on the Morphology and Performance of Solar Cells Based on Polyfluorene
Copolymer/Fullerene Blends. Adv. Funct. Mater. 2006, 16, (5), 667-674.

- Zhang, Q. T.; Tour, J. M., Alternating Donor/Acceptor Repeat Units in
Polythiophenes. Intramolecular Charge Transfer for Reducing Band Gaps in Fully
Substituted Conjugated Polymers. J. Am. Chem. Soc. 1998, 120, (22), 5355-5362.

164

Appendix A: Chapter 2 Supplemental Information

Figure A-1.
1
H-NMR spectrum of (SQ) 2 in CDCl
3
at 60 ºC.

Figure A-2.
13
C-NMR spectrum of (SQ) 2 in CDCl
3
at 60 ºC.

ppm (t1)
3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0
0
10
20
30
40
50
60
N
CH
3
HO
HO
N
H
3
C
OH
OH O
O
2+
ppm (t1)
100 150
0
50
100
150
184.447
163.246
158.470
158.450
158.380
130.094
127.678
127.032
95.493
86.476
53.292
N
CH
3
HO
HO
N
H
3
C
OH
OH O
O
2+

165

Figure A-3.
1
H-NMR spectrum of (SQ) 3 in CDCl
3
at 25 ºC.

Figure A-4.
13
C-NMR spectrum of (SQ) 3 in CDCl
3
at 25 ºC.

ppm (t1)
6.0 7.0 8.0 9.0 10.0 11.0
0
50
100
150
200
N
HO
HO
N
OH
OH O
O
2+
ppm (t1)
100 150
0
5000
10000
15000
20000
25000
30000
35000
181.363
163.501
163.058
159.508
144.081
129.807
127.569
127.050
104.954
98.746
N
HO
HO
N
OH
OH O
O
2+

166

Figure A-5.
1
H-NMR spectrum of (SQ) 4 in CDCl
3
at 60 ºC.

Figure A-6.
1
H-NMR spectrum of aromatic region of (SQ) 4 in CDCl
3
at 60 ºC.

ppm (t1)
6.0 7.0 8.0 9.0 10.0 11.0
0
500
1000
N
HO
HO
N
OH
OH O
O
2+
ppm (t1)
7.20 7.30 7.40 7.50 7.60 7.70 7.80 7.90
0
100
200
300
400
500
600
N
HO
HO
N
OH
OH O
O
2+

167

Figure A-7.
13
C-NMR spectrum of (SQ) 4 in CDCl
3
at 60 ºC.

Figure A-8.
1
H-NMR spectrum of (SQ) 5 in CDCl
3
at 25 ºC.

ppm (t1)
100 150
0
100
200
300
400
500
181.339
163.776
163.254
160.311
144.152
140.101
134.987
130.481
129.712
128.831
128.691
127.574
127.313
126.770
126.743
126.400
126.031
123.077
104.870
98.273
N
HO
HO
N
OH
OH O
O
2+
ppm (t1)
6.0 7.0 8.0 9.0 10.0 11.0
0
500
N
HO
HO
N
OH
OH O
O
2+

168

Figure A-9.
1
H-NMR spectrum of aromatic region of (SQ) 5 in CDCl
3
at 25 ºC.

Figure A-10.
13
C-NMR spectrum of (SQ) 5 in CDCl
3
at 25 ºC.

ppm (t1)
7.20 7.30 7.40 7.50 7.60 7.70 7.80 7.90
0
50
100
150
200
250
300
N
HO
HO
N
OH
OH O
O
2+
ppm (f1)
100 150
0
100
200
300
400
500
181.591
164.097
163.455
159.839
144.443
141.780
134.007
132.135
129.902
129.860
127.857
127.812
127.695
127.118
126.928
126.594
125.862
125.580
105.449
99.326
N
HO
HO
N
OH
OH O
O
2+

169

Figure A-11.
1
H-NMR spectrum of (SQ) 6 in CDCl
3
at 60 ºC.

Figure A-12.
1
H-NMR spectrum of (SQ) 7 in CDCl
3
at 60 ºC.
ppm (t1)
6.0 7.0 8.0 9.0 10.0 11.0
0
50
100
150
N
HO
HO
N
OH
OH O
O
2+
ppm (t1)
6.0 7.0 8.0 9.0 10.0 11.0
0
500
1000
1500
N
HO
HO
N
OH
OH O
O
2+

170

Figure A-13.
1
H-NMR spectrum of aromatic region of (SQ) 7 in CDCl
3
at 60 ºC.

Figure A-14.
13
C-NMR spectrum of (SQ) 7 in CDCl
3
at 60 ºC.

ppm (t1)
6.50 7.00
0
500
1000
N
HO
HO
N
OH
OH O
O
2+
N
HO
HO
N
OH
OH O
O
2+
ppm (f1)
100 110 120 130 140 150 160 170 180
0
5000
10000
15000
20000
25000
30000
181.444
163.112
163.057
140.262
140.149
139.622
131.436
129.908
128.553
128.246
127.382
127.268
127.017
104.757
98.004

171

Figure A-15.
1
H-NMR spectrum of (SQ) 8 in CDCl
3
at 60 ºC.

Figure A-16.
1
H-NMR spectrum of aromatic region of (SQ) 8 in CDCl
3
at 60 ºC.

ppm (t1)
6.0 7.0 8.0 9.0 10.0 11.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
N
HO
HO
N
OH
OH O
O
2+
ppm (t1)
7.50 8.00
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
N
HO
HO
N
OH
OH O
O
2+

172

Figure A-17.
13
C-NMR spectrum of (SQ) 8 in CDCl
3
at 60 ºC.

Figure A-18.
1
H-NMR spectrum of (SQ) 9 in CDCl
3
at 25 ºC.

ppm (f1)
100 110 120 130 140 150 160 170 180
0
10000
20000
30000
40000
50000
60000
70000
181.523
164.169
163.634
160.781
144.701
137.324
131.296
131.011
129.790
129.435
128.392
127.071
126.986
126.710
126.618
126.089
125.873
125.841
124.734
122.068
105.278
98.794
N
HO
HO
N
OH
OH O
O
2+
ppm (t1)
5.0 10.0
0
500
1000
1500
2000
N
HO
HO
N
O
O
2+

173

Figure A-19.
13
C-NMR spectrum of (SQ) 9 in CDCl
3
at 25 ºC.

Figure A-20.
1
H-NMR spectrum of (SQ) 10 in CDCl
3
at 25 ºC.

ppm (t1)
50 100 150 200
0
100
200
300
400
175.587
163.586
139.977
129.174
128.066
125.313
93.818
60.248
27.563
20.156
N
HO
HO
N
O
O
2+
ppm (t1)
6.0 7.0 8.0 9.0 10.0 11.0 12.0
0
50
100
N
HO
HO
N
O
O
2+

174

Figure A-21.
1
H-NMR spectrum of (SQ) 10 in CDCl
3
at 25 ºC.

Figure A-22.
13
C-NMR spectrum of (SQ) 10 in CDCl
3
at 25 ºC.
ppm (t1)
7.200 7.250 7.300 7.350 7.400 7.450 7.500
0
10
20
30
40
50
60
N
HO
HO
N
O
O
2+
ppm (t1)
100 150
0
50
100
150
200
250
300
350
175.487
163.553
144.502
139.647
129.702
129.260
128.655
127.643
126.560
125.421
98.281
N
HO
HO
N
O
O
2+

175

Appendix B: Chapter 3 Supplemental Information
400 500 600 700 800 900
0.0
0.2
0.4
0.6
0.8
1.0
SQ 1
SQ 2
SQ 3
SQ 4
SQ 5
SQ 6
SQ 7
SQ 8
SQ 9
SQ 10
Absorbance (au)
Wavelength (nm)

Figure B-1. Absorption spectra of squaraines (SQ) 1–10 in CH
2
Cl
2
solution.
400 500 600 700 800 900
0.0
0.2
0.4
0.6
0.8
1.0
SQ 1
SQ 3
SQ 4
SQ 5
SQ 6
SQ 7
SQ 8
SQ 9
SQ 10
Absorbance (a.u)
Wavelength(nm)

Figure B-2. Absorption spectra of films of squaraines (SQ) 1, 3–10 spin cast from
CH
2
Cl
2
.

176

-3 -2 -1 0 1
-5.0x10
-5
0.0
5.0x10
-5
1.0x10
-4
-1.15
-1.39
Current (A)
Potential (V)
SQ 1
0.36
0.60

Figure B-3. CV trace of squaraine (SQ) 1 in CH
2
Cl
2
. The large signal at 0 V is from the
ferrocene used as an internal reference.

-2 -1 0 1
-3.0x10
-4
-2.0x10
-4
-1.0x10
-4
0.0
1.0x10
-4
2.0x10
-4
3.0x10
-4
0.48
Current (A)
Potential (V)
SQ 2
0.60
-1.25

Figure B-4. CV trace of squaraine (SQ) 2 in CH
2
Cl
2
. The large signal at 0 V is from the
ferrocene used as an internal reference.

177

-2 -1 0 1
-1.0x10
-4
0.0
1.0x10
-4
Current (A)
Potential (V)
SQ 3
0.63
-1.08
0.52

Figure B-5. CV trace of squaraine (SQ) 3 in CH
2
Cl
2
. The large signal at 0 V is from the
ferrocene used as an internal reference.
-2 -1 0 1
-3.0x10
-4
-2.0x10
-4
-1.0x10
-4
0.0
1.0x10
-4
2.0x10
-4
3.0x10
-4
SQ 4
Current (A)
Potential (V)
0.81
0.47
-1.24

Figure B-6. CV trace of squaraine (SQ) 4 in CH
2
Cl
2
. The large signal at 0 V is from the
ferrocene used as an internal reference.

178

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-2.0x10
-4
-1.0x10
-4
0.0
1.0x10
-4
2.0x10
-4
Current (A)
Potential (V)
SQ 5
0.65
0.52
-1.11

Figure B-7. CV trace of squaraine (SQ) 5 in CH
2
Cl
2
. The large signal at 0 V is from the
ferrocene used as an internal reference.

-1 0 1
-5.0x10
-5
0.0
5.0x10
-5
Current (A)
Potential (V)
SQ 6 no ferrocene
0.92
0.81
-0.71

Figure B-8. CV trace of squaraine (SQ) 6 in CH
2
Cl
2
.The large signal at 0 V is from
ferrocene used as an internal reference.

179

-2 -1 0 1
-1.0x10
-4
0.0
1.0x10
-4
-1.07
Current (A)
Potential (V)
SQ 7 0.67
-1.16
0.54

Figure B-9. CV trace of squaraine (SQ) 7 in CH
2
Cl
2
. The large signal at 0 V is from the
ferrocene used as an internal reference.
-2 -1 0 1
-1.0x10
-4
0.0
1.0x10
-4
2.0x10
-4
3.0x10
-4
Current (A)
Potential (V)
SQ 8
0.91
0.39
0.98

Figure B-10. CV trace of squaraine (SQ) 8 in CH
2
Cl
2
. The signal at 0 V is from the
ferrocene used as an internal reference.

180

-2 -1 0 1
-2.0x10
-4
0.0
2.0x10
-4
Current (A)
Potential (V)
SQ 9
0.69
-1.79
-1.6
0.53

Figure B-11. CV trace of squaraine (SQ) 9 in CH
2
Cl
2
. The large signal at 0 V is from the
ferrocene used an internal reference.
-2 -1 0 1
-0.00004
0.00000
0.00004
Current (A)
Potential (V)
SQ 10
0.76
0.64
-1.53
-1.38

Figure B-12. CV trace of squaraine (SQ) 10 in CH
2
Cl
2
. The large signal at 0 V is from
the ferrocene used an internal reference.

Abstract (if available)

Abstract As the worldwide demand for energy continues to rise, coupled with the problem of the depletion of energy resources and concerns about the environmental impact of fossil fuels, the search for cost-effective clean energy has become a growing global priority. Chief among current clean energy sources, solar power has the biggest potential to satisfy our energy demands. Currently, commercially available inorganic solar cells have reasonable efficiency in the range of 15%-20%. Although their prices have been dropping steadily, the high manufacturing feestill remain a limitationof inorganic photovoltaics (IPV). Instead, organic photovoltaics (OPVs) have attracted tremendous attention from both academia and industry due to the advantages of abundant materials, along with inexpensive manufacturing and installation.. Perceived as a viable alternative to IPVs, the performance of OPVs (~10%) is quickly catching up with its inorganic counterparts. However, to reach commercialization bar of 15%, some issues need to be resolved for organic materials, such as high absorbance in broader region of solar spectrum, low charge carrier mobility and short exciton diffusion length. New materials, optimization of process conditions, and novel device architecture are key factors to address these issues. ❧ The aim of this dissertation is to explore a new type of functional material for high performance OPVs. By introducing a new library of squaraines with varying substituents, tunable absorption throughout visible and near-infrared, and energy levels can be achieved. A range of photophysical properties have been studied to obtain more useful information about this new type of material. The application of new squaraine materials as active donors is covered in chapter 4. With good thermal stability and solution processability, squaraine thin films can be fabricated by both vacuum deposition and spin-casting methods. Squaraines have been utilized as donor materials in highly efficient OPVs. A new approach of blending squaraines with complementary absorption is proposed to achieve broadband absorption with strong spectral response from 500 nm to 850 nm. In collaboration with Prof. Stephen Forrest’s group at University of Michigan, squaraine-based OPVs have achieved marked increase in device performance. With post-thermal and solvent annealing processes, squaraine-based nanocrystalline heterojunction devices can lead to a significant increase in surface area and enhanced photocurrent. Utilizing compound blocking layer can result in enhancing performance by over 25%. ❧ The last chapter describes the synthesis of representative functional squaraine materials on an industrial scale. These materials are commercially available at the time of the preparation of this dissertation.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

Pyrrolic squaraines and energy management in organic photovoltaics

PDF

Organic photovoltaics: from materials development to device application

PDF

Properties and applications of dipyrrin-based molecules in organic photovoltaics

PDF

Development of a family of semi-random multichromophoric polymers for application in organic solar cells

PDF

Molecular and polymeric donors for bulk heterojunction organic solar cells

PDF

Porphyrin based near infrared‐absorbing materials for organic photovoltaics

PDF

New materials and fabrication techniques and their utilization in organic photovoltaic devices

PDF

Singlet fission in disordered acene films: from photophysics to organic photovoltaics

PDF

Molecular aspects of photoconversion processes in organic solar cells

PDF

Molecular and morphological effects on the operational parameters of organic solar cells

PDF

Structure-function studies of random and semi-random poly(3-hexyl thiophene) copolymers

PDF

The influence of CdSe surface ligands on hybrid inorganic/organic solar cell performance

PDF

Improving the field of organic photovoltaics through the development of new active layer materials with unique photophysical properties

PDF

Expanding the library of surface ligands for semiconductor nanocrystal synthesis and photovoltaic applications

PDF

Development of N-type chromophores for organic photovoltaics, and thermally activated delayed fluorescence NHC complexes for organic light-emitting diodes

PDF

Organic solar cells: molecular electronic processes and device development

PDF

Improving the efficiency and stability of organic solar cells through ternary strategies

PDF

High energy hosts and blue emitters for phosphorescent organic light emitting diodes

PDF

Influence of polymer structure on electronic properties and performance as donor materials in bulk heterojunction solar cells

PDF

Excited state dynamics of organic photovoltaics probed using ultrafast spectroscopy

Asset Metadata

Creator Wang, Siyi (author)

Core Title Squaraines and their applications to organic photovoltaics

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 07/26/2014

Defense Date 06/14/2012

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag OAI-PMH Harvest,organic photovoltaics,squaraines

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark E. (committee chair), Haas, Stephan W. (committee member), Prakash, G. K. Surya (committee member)

Creator Email siyiwang@usc.edu,siyiwang17@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-70786

Unique identifier UC11289444

Identifier usctheses-c3-70786 (legacy record id)

Legacy Identifier etd-WangSiyi-1035.pdf

Dmrecord 70786

Document Type Dissertation

Rights Wang, Siyi

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA