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Excited state dynamics of organic photovoltaics probed using ultrafast spectroscopy

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Content Excited State Dynamics of Organic Photovoltaics Probed Using
Ultrafast Spectroscopy

by

Saptaparna Das

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 2015
ii

Dedicated to my loving parents, Sunil Ranjan Das and Manashi Das
iii

ACKNOWLEDGEMENTS
Living in Los Angeles and working at USC for last 6 years has been a tremendous
experience. This beautiful journey would not have been possible if I did not have the company of
such helpful and talented friends, colleagues and faculties.
I am greatly indebted to my advisor Steve Bradforth for his continuous guidance and
support throughout my stay at USC. Steve is a highly talented scientist who always know
insightful ways to break down difficult questions into bits addressable in the lab. The best thing I
learnt from him is that the main goal for a scientist is to solve a scientific problem and if one can
solve it using a simple experiment, designing a complicated experiment is superfluous. Steve not
only played a role of an advisor but he also interacted as a friend. I will never forget how I joked
about his growing age and he mocked me with American phrases like "What's shaking". Even
though he was the department chair during my last year of Ph.D., he made sure to interact with
the students regularly. Working in the Bradforth group has made me a better scientist and I am
grateful to Steve for providing me this opportunity.
USC has an incredible set of faculties in Physical Chemistry. The faculties who taught
me during my Ph.D., Steve Bradforth, Alex Benderskii and Daniel Lidar, played an immense
role in providing me with a strong background in spectroscopy and statistical mechanics.
Especially, the non-linear spectroscopy class taught jointly by Alex and Steve was very helpful
for my research. The best part of the Chemistry department at USC is the fact that one can knock
on any faculty's door to ask any question they have.
During my first year in the lab, I was lucky enough to have Diana Suffern as my mentor.
Although she was a final year graduate student, she was kind and patient enough to teach me the
basics of optics in the lab. She planned out small mini projects to get me trained in the lab.
iv

During the end of my first year, I started working with Tom Zhang. Although Tom was a senior
graduate student, he always worked with me as a coworker. And together we tried to solve issues
with the laser and OPA. He played an important role in making me think independently about
research.
In the next two years I was fortunate to have Sean Roberts, the postdoctoral associate, as
my mentor. He taught me how to perform transient absorption and helped me in understanding
the photophysics of organic solar cell materials. He was also very kind to listen to my crazy
proposal ideas and provided me inputs during my qualifying exam. In short, he trained me as a
laser spectroscopist. I am thankful for his help in some of the work shown in Chapter 2 and the
work shown in Chapter 5 is influenced by his published meticulous work.
During my fourth year, Robert Seidel, the German postdoc joined our group and he
brought a social life to the group. From cracking mean jokes to bringing chocolates to
celebrating birthdays, he made sure we don't miss any event. Being German he was very
particular about cars and because of him I was able to travel in various luxury cars. In this
tedious Ph.D. journey, Robert brought lot of happiness and fun. In recent years, I am happy to
met and work with Konstantin Kudinov, Gaurav Kumar, Dhritiman Bhattacharya, and most
recently Jimmy Joy and Laura Estergreen. I was happy to meet Konstantin as he brought the
Russian flavor to the group. Gaurav is the perfectionist in the lab and I am sure that he will reach
new heights during his Ph.D. based on the fact that he built NOPA in just 4 months. Dhritiman is
doing an incredible job as a joint Bradforth-Benderskii student and I am certain that he will be a
true asset for both the labs in future. I will also like to thank Jimmy for performing some of the
TCSPC measurements reported in Chapter 5 and Laura for proof reading Chapter 5. Last but not
the least, I was fortunate to mentor immensely talented undergraduate William Thornbury. I
v

would like to thank him for performing the chemical doping measurements shown in Chapter 2
and 3. Will is an outstanding student who is able to grasp things very quickly and I am very
happy to see his progress in performing ultrafast spectroscopic measurements.
During my Ph.D., I was fortunate to work with various talented collaborators. I would
like to thank Prof. Richard Brutchey and Matt Greaney for the collaborative photophysical
characterization of hybrid photovoltaics. My research would not have been complete without the
collaboration of Prof. Barry Thompson and his students, Petr Khlyabich, Beate Burkhart, Alia
Latif in combined ultrafast characterization and device measurements of polymer:PCBM solar
cells. And finally I really enjoyed working in collaboration with Prof. Mark Thompson and his
students; Cong Trinh, Andrew Bartynskii, Nadia Korovina, Denise Femia, Robert Eric McAnally
in ultrafast photophysics of small molecule organic photovoltaics.
Throughout my stay at USC, a small Indian community developed in Chemistry
department, especially in Physical Chemistry division and they filled my stay with full of joy and
fun. From visiting national parks to celebrating Indian festivals, there was hardly any week
without some fun activity. I will like to thank everyone of this gang, Purnim Dhar, Anirban Roy,
Amit Samanta, Parichita Majumdar, Debasree Ghosh, Suman Chakraborty, Atanu Acharya,
Piyush Deokar, Subodh Tiwari, Chayan Dutta, Subhasish Sutradhar, Gaurav Kumar and
Dhritiman Bhattacharya. Purnim and Anirban for being my best friends for almost eleven years
now, it would have been difficult without you guys around. Amit and Parichita for always being
there, this friendship with them will be for life and I know that they will be just a phone call
away. Lastly a special thank you to our newest member, Anishka Samanta whom we refer to as
"REMPI" for filling life to our parties with her smile. I will miss all of you wherever I go next
and I hope that we all will stay in touch.
vi

This acknowledgement would be incomplete if I don't thank the person who runs the
Chemistry department, Michele Dea. Although she is the busiest person but she still solves all
our problems instantly. Without her I would have been stuck with those administrative issues for
forever. I would also like to thank Katie McKissick and Magnolia Benitez for their advice as
graduate advisers.
Most importantly, I would like to thank my parents who always supported my decision
and provided me the courage to pursue education abroad. My mother always encouraged me to
become independent and I hope that I will fulfill her dreams. My father is my first teacher who
taught me the elemental science and made me enthusiastic about it.
Finally, I would like to acknowledge my best friend and lab partner Anirban Roy for
being always there for me. He joined Bradforth group at the same year as I did. And because of
him I was able to solve many difficult experimental and scientific problems. He was kind enough
to listen to my n number of practice talks for screening and qualifying examination. He is the
person who always heard my issues and cheer me up. I could fill up pages with gratitude towards
him and I hope that this companionship with him stays for forever.
vii

TABLE OF CONTENTS
List of Figures xi
List of Tables xxi
Abstract xxii
Chapter 1. Electronic processes in Organic Photovoltaics 1
1. Current-voltage characteristics of a solar cell 3
2. Electronic and optical processes in organic semiconductor 5
2.1 Optical absorption and exciton generation 7
2.2 Exciton diffusion 8
2.3 Exciton dissociation and charge generation at D-A interface 11
2.4 Charge recombination and charge escape 12
3. Methodology used in this thesis 16
3.1. Time correlated single photon counting (TCSPC) 16
3.2. Femtosecond to nanosecond transient absorption 18
4. Outline of this thesis 20
Chapter 1 Bibliography 22
Chapter 2. Quantifying Charge Recombination in Solar Cells based on Donor-
Acceptor P3HT Analogs
27
1. Introduction 27
2. Experimental section 31
2.1 Synthetic Procedures 31
2.2 Preparation of Thin Films for Femtosecond Transient Absorption 31
2.3 Solar Cell Fabrication 32
2.4 External Quantum Efficiency Measurements 33
2.5 Absolute Absorption and Internal Quantum Efficiency Measurements 33
2.6 Solution Doping 33
2.7 Film Doping 34
2.8 Steady state Photoluminescence 34
2.9 Femtosecond Transient Absorption 34
2.10 Femtosecond to Nanosecond transient absorption using 532 nm 35
3. Results and Discussion 36
3.1. Steady state photophysical properties 36
3.2. Absorption Spectra and Cross-section of Polymer Polaron 41
3.2.1 Neat Polymer film 41
3.2.2 Polymer solution 42
3.3. Femtosecond transient absorption (TA) measurements: 44
viii

3.3.1. Pristine P3HT and P3HT:PCBM bulk heterojunction films 44
3.3.2 Pristine P3HTT-TP-10% and P3HTT-TP-10%:PCBM bulk
heterojunction films
46
3.3.3 Pristine P3HTT-DPP-10% and P3HTT-DPP-10%:PCBM bulk
heterojunction films
49
3.3.4 Excitation density dependent TA measurements of
polymer:PCBM blends
53
3.4 Nanosecond TA measurements 55
3.5. Polaron Geminate Recombination Dynamics 56
3.6. Absolute polaron yield 62
3.7. Device Measurements 64
3.7.1 External quantum efficiency & absolute absorption
measurements
64
3.7.2 Internal quantum efficiency (IQE) measurements 66
4. Conclusion 67
Chapter 2 Bibliography 70
Chapter 3. Influence of strong donor in Polaron dynamics of semi-random
P3HT:PCBM composites
77
1. Introduction 77
2. Experimental Section 79
2.1 Synthetic Procedures 79
2.2 Preparation of Thin Films for Femtosecond Transient Absorption 79
2.3 Solution Doping 80
2.4 Femtosecond Transient Absorption 80
3. Results and Discussion 81
3.1. Steady state photophysical properties 81
3.2. Absorption Spectra and Cross-section of Polymer Polaron 82
3.3 Femtosecond transient absorption 84
3.3.1 Pristine P3HTT-TP-DTP and P3HTT-TP-DTP:PCBM films 84
3.3.2 Pristine P3HTT-DPP-DTP and P3HTT-DPP-DTP:PCBM
films
86
3.4 Polaron Geminate Recombination dynamics 88
3.5 Absolute Polaron Yield 93
4. Conclusion 94
Chapter 3 Bibliography 96
Chapter 4. Symmetry-breaking charge transfer in Zinc dipyrrin derivatives 100
1. Introduction 100
2. Experimental section 103
ix

2.1. Time resolved photoluminescence 103
2.2. Femtosecond transient absorption of zDIP1-3 103
2.3. Nanosecond-to-millisecond transient absorption 105
2.4. Femtosecond transient absorption of ZCl 105
2.5 OPV fabrication and testing 107
3. Results and Discussion 107
3.1. Steady state photophysical properties 107
3.2. Time resolved photoluminescence measurements 110
3.3. Transient absorption measurements 112
3.3.1 Excited state dynamics of zDIP1 113
3.3.2 Excited state dynamics of zDIP2 118
3.3.3 Excited state dynamics of zDIP3 119
3.3.4 Excited state dynamics of ZCl 121
3.3.4.1 In solution 121
3.3.4.2 In solid matrix 124
3.4. Global analysis of femtosecond transient absorption 125
3.5. Comparison between dynamics of zDIP1-3 and ZCl in polar solvents 133
3.6. Application to organic photovoltaic devices 134
4. Conclusion 135
Chapter 4 Bibliography 136
Chapter 5. Singlet fission in covalently linked alkynyltetracene dimer 140
1. Introduction 140
2. Experimental 144
2.1 Synthetic procedure 144
2.2 Preparation of solutions for femtosecond TA measurements 144
2.3 Preparation of thin films for femtosecond TA measurements 144
2.4 Femtosecond transient absorption 145
3. Results 146
3.1 Structure of the dimers 146
3.2 Steady state photophysical properties 147
3.3 Absorption spectra and extinction coefficient for triplet state (T
1
T
n
) 150
3.4 Femtosecond transient absorption (TA) measurements 154
3.4.1. ET-TMS in solution and film 154
3.4.2. BET-B in different media 156
3.4.2.1 BET-B neat film 156
3.4.2.2 BET-B in solution 158
3.4.2.3 BET-B in diphenylanthracene (DPA) &
diphenyltetracene (DPT)
159
3.4.3. BET-X in solution and PMMA 162
x

4. Discussion 163
4.1 Kinetic model for singlet fission in BET-B 164
4.2 Excitation density dependent TA on ET-TMS and BETA neat films 170
4.3 Comparison between neat monomer (ET-TMS) & dimer (BET-B) film 171
5. Conclusion 173
Chapter 5 Bibliography 175

xi

List of Figures
Figure 1.1: a) The energy level diagram of a typical organic solar cell with donor
and acceptor layer. The two device architectures used in the OPVs: b) planar
heterojunction and c) bulk heterojunction. This figure is adapted from ref 12.
2
Figure 1.2: Schematic current voltage characteristic of a solar cell under dark (red
dotted line) and illumination (red solid line). This figure is adapted from reference
12.
3
Figure 1.3: a) Schematic representation of the path length for the AM 1.5 G at
zenith angle of 48.2  from ref 17 with the corresponding photon flux shown in b).
The absorption coefficient spectra for crystalline Si, a typical organic (P3HT)
polymer and small molecule (SubPc).
5
Figure 1.4: a) Electronic state diagram and b) spatial picture to describe the photo-
induced charge-carrier formation mechanism in an organic solar cell. The energy
state diagram is adapted from reference 15.
6
Figure 1.5: Schematic diagram for Förster and Dexter energy transfer. 10
Figure 1.6: Schematic representation of a) singlet exciton, b) CT state and c) free
charges. As one go from exciton to CT state to free charges the distance between
the opposite charges increases due to screening of Coulomb potential by
polarization effects.
11
Figure 1.7: Three possible fates for the charges after generation at D-A interface:
a) intimate /CT state recombination, b) Total escape through diffusion and c)
secondary geminate recombination controlled by diffusion.
14
Figure 1.8: After polaron or charge generation there are two possible pathways for
recombination: geminate and non-geminate recombination.
15
Figure 1.9: Time correlated single photon counting setup in the lab (a) with the
forward and reverse mode described in b) and c), respectively.
17
Figure 1.10: The scheme of transient absorption (top panel) and a typical A
spectrum (bottom panel) consisting of ground state bleach, stimulated emission and
excited state absorption.
19
Figure 2.1: (a) Structure of poly(3-hexylthiophene) (P3HT), poly(3- 37
xii

hexylthiophene-thiophene-thienopyrazine) (P3HTT-TP-10%), poly(3-
hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%) and [6,6]-
phenyl-C
61
-butyric acid methyl ester (PCBM). (b) Absorption spectra of neat
P3HT, P3HTT-TP-10% and P3HTT-DPP-10% films.
Figure 2.2: Steady state photoluminescence spectra of neat a) P3HT film, b)
P3HTT-DPP-10% film and d) P3HTT-TP-10% film following excitation at 525
nm. The discontinuity in the spectra between 1020 and 1085 nm is to remove the
second order scattering from the excitation used at 525 nm. The steady state PL
spectra of neat c) P3HTT-DPP-10% film and d) P3HTT-TP-10% film following
excitation at 700 nm. These spectra are scaled by 2.5 to take account of both
excitation power and optical density at 525 nm and 700 nm.
39
Figure 2.3: Temperature dependent PL spectra for neat a) P3HT film , b) P3HTT-
DPP-10% film and d) P3HTT-TP-10% film following excitation at 525 nm. c) and
e) shows the temperature dependent PL spectra after exciting neat P3HTT-DPP-
10% and P3HTT-TP-10%, respectively at 700 nm.
40
Figure 2.4: Absorption spectra of a) P3HT and b) P3HTT-DPP-10% neat film
before doping (black) and after doping (red) with 20 ppm SbCl
5
solution. The green
line denotes the difference spectrum illustrating the spectrum of positive polaron
band.
42
Figure 2.5: The difference absorption spectra between doped and neat polymer
solution of a) P3HT, b) P3HTT-DPP-10% and c) P3HTT-TP-10%. The inset shows
the determination of the molar extinction coefficient of the polaron band from the
absorption versus the molarity of the SbCl
5
used to dope.
44
Figure 2.6: TA spectra of neat P3HT film (a) and P3HT:PCBM blend (b) with 500
nm excitation. The absorption spectrum of polaron (P
+
) obtained from chemical
doping is shown in black solid line and ground state absorption spectrum for the
sample is shown in black broken line. c) Comparison between time resolved PL
( 
exc
= 500 nm, 
em
= 650 nm, black) and a kinetic trace measured at a probe
wavelength of 1200 nm from TA spectra ( 
exc
= 500 nm, red) of a neat P3HT film.
d) Normalized TA kinetics for S
1
S
n
and positive polaron transition shows the
evolution from S
1
state to polarons.
46
xiii

Figure 2.7: TA spectra of P3HTT-TP-10%:PCBM following 500 nm excitation (a)
and 700 nm excitation (b). The absorption spectrum of polaron (P
+
) obtained from
chemical doping is shown in black solid line and ground state absorption spectrum
for the sample is shown in black broken line. (c) Normalized TA decay of the
polaron band at 900 nm following 500 nm (green) and 700 nm (red) excitation.
47
Figure 2.8: TA spectra of a neat P3HTT-TP-10% in o-DCB following either (a)
400 nm or (b) 700 nm excitation. (c & d) TA spectra of neat P3HTT-TP-10% film
following excitation at 500 nm or 700 nm. The absorption spectra of the sample
studied is shown as a black broken line.
48
Figure 2.9: TA spectra of P3HTT-DPP-10%:PCBM following 500 nm excitation
(a) and 700 nm excitation (b). The absorption spectrum of polaron (P
+
) obtained
from chemical doping is shown in black solid line and ground state absorption
spectrum for the sample is shown in black broken line. The polaron shape shown in
b) is from chemical oxidation of DPP-T2. (c) Comparison between the inverted
ground state bleach of P3HTT-DPP-10%:PCBM after 700 nm excitation with the
ground state absorption spectrum of DPP-T2 oligomer (from ref 66) and PDPP4TP
polymer (from ref 67). (d) The structures of DPP-T2 oligomer and PDPP4TP
polymer from ref 66 and 67, respectively.
50
Figure 2.10: TA spectra of a neat P3HTT-DPP-10% in o-DCB following either (a)
400 nm or (b) 700 nm excitation. (c & d) TA spectra of neat P3HTT-DPP-10% film
following excitation at 500 nm or 700 nm. The absorption spectra of the sample
studied is shown as a black broken line.
51
Figure 2.11: Normalized TA decay of the polaron band at 870 nm following 500
nm (green) and 700 nm (red) excitation.
53
Figure 2.12: Normalized TA decay of the photoinduced polaron absorption band
for (A) P3HT:PCBM, (B) P3HTT-TP-10%:PCBM and (D) P3HTT-DPP-
10%:PCBM as a function of excitation density following 500 nm excitation.
Corresponding traces for P3HTT-TP-10%:PCBM and P3HTT-DPP-10%:PCBM
following 700 nm excitation are shown in (C) & (E), respectively. For
P3HT:PCBM and P3HTT-DPP-10%:PCBM the polaron band dynamics is shown at

probe
= 870 nm and for P3HTT-TP-10%:PCBM blend the polaron band dynamics is
54
xiv

shown at 
probe
= 900 nm.
Figure 2.13: Femtosecond to nanosecond TA of a) P3HT:PCBM and b) P3HTT-
DPP-10%:PCBM film following excitation at 532 nm. The top panel shows the
comparison between the spectral shape of polaron (P
+
) obtained from TA and
chemical doping taken from Figure 2.4.
56
Figure 2.14: Normalized TA decay of polarons for different polymer:PCBM
blends along with the survival probability obtained from the numerical solution of
diffusive geminate recombination (black line) under a Coulomb potential. The fits
are extrapolated to 500 ns for certain polymer:PCBM blends with TA data available
only upto 1ns.
60
Figure 2.15: Spatial distribution of the survival probability density of polarons for
(A) P3HT:PCBM ( 
exc
= 500 nm), (B) P3HTT-DPP-10%:PCBM ( 
exc
= 500 nm),
(C) P3HTT-DPP-10%:PCBM ( 
exc
= 700 nm) and (D) P3HTT-TP-10%:PCBM (for
both 
exc
= 500 nm and 
exc
= 700 nm) at different times following charge
separation. The Coulomb potential corresponding to an Onsager radius of r
C
= 50Å
is shown in cyan.
60
Figure 2.16: Time resolved absolute polaron yield after photoexcitation of
P3HT:PCBM and P3HTT-DPP-10%:PCBM at 532 nm. The polaron absorption
band at 850 nm and 900 nm is used for the absolute polaron yield of P3HT:PCBM
and P3HTT-DPP-10%:PCBM, respectively.
63
Figure 2.17: (a) External quantum efficiency and (b) absorption in integrating
sphere measurements of blended films of the polymers, P3HT (blue), P3HTT-DPP-
10 % (green) and P3HTT-TP-10% (red) with PCBM. c) The calculated parasitic
absorption from transfer matrix using n and k values for each layer. d) The absolute
absorption obtained after subtracting parasitic absorption from measured
absorption. The thickness of the active layer is identical to that used for TA
measurements.
65
Figure 2.18: IQE spectra for BHJ composites of P3HT (blue), P3HTT-DPP-10%
(green) and P3HTT-TP-10% (red) with PCBM calculated using (top) measured
absorption from figure 2.17b and (bottom) calculated absorption from figure 2.17d.
67
Figure 3.1: Structure (a) and absorption spectra (b) of P3HTT-TP, P3HTT-TP- 82
xv

DTP, P3HTT-DPP and P3HTT-DPP-DTP polymers.
Figure 3.2: The absorption spectra of positive polymer polaron for a) P3HTT-TP-
DTP and b)P3HTT-DPP-DTP obtained from the difference spectrum between neat
polymer and polymer doped with aliquots of SbCl
5
. The inset show the variation in
absorption at 900 nm versus the molarity of SbCl
5
solution added.
83
Figure 3.3: TA of P3HTT-TP-DTP:PCBM film following excitation at 500 nm (a)
and 700 nm (c). The TA spectra for corresponding neat P3HTT-TP-DTP films with
b) 500 nm and d) 700 nm pump are also shown. The black broken line shows the
linear absorption spectrum of neat P3HTT-TP-DTP and blue line denotes the
positive polymer polaron absorption spectra obtained from chemically doping the
polymer solution. e) The polaron population kinetics at 900 nm for the P3HTT-TP-
DTP:PCBM (solid) is compared to P3HTT-TP:PCBM (scatter) blend following
excitation at 500 nm (green) and 700 nm (red).
85
Figure 3.4: TA of P3HTT-DPP-DTP:PCBM film following excitation at 500 nm
(a) and 700 nm (b). The black broken line shows the linear absorption spectrum of
neat P3HTT-DPP-DTP and blue line denotes the positive polymer polaron
absorption spectra obtained from chemically doping the polymer solution. c) The
polaron population kinetics at 900 nm for the P3HTT-DPP-DTP:PCBM (solid) is
compared to P3HTT-DPP:PCBM (scatter) blend following excitation at 500 nm
(green) and 700 nm (red).
86
Figure 3.5: TA of P3HTT-DPP-DTP solution in o-dichlorobenzene following
excitation at 500 nm (a) and 700 nm (b). The TA spectra are normalized to the
absorption value at ~680 nm corresponding to the absorption peak of ICT-
transition.
87
Figure 3.6: The best matched polaron survival probability obtained from numerical
solution is shown with the polaron population kinetics of P3HTT-TP-DTP:PCBM
(top) and P3HTT-DPP-DTP:PCBM (bottom).
91
Figure 3.7: Spatial distribution of the survival probability density of polarons for
(a) P3HTT-DPP-DTP:PCBM ( 
exc
= 500 nm), (b) P3HTT-DPP-DTP:PCBM ( 
exc
=
700 nm), (c) P3HTT-TP-DTP:PCBM ( 
exc
= 500 nm) and (d) P3HTT-TP-
DTP:PCBM ( 
exc
= 700 nm) at different times following charge separation. The
92

xvi

Coulomb potential corresponding to an Onsager radius of r
C
= 50Å is shown in
black broken line.
Figure 3.8: The time resolved polaron yield for different polymer:PCBM blends,
1:P3HTT-DPP-DTP:PCBM and 2:P3HTT-TP-DTP:PCBM for both 500 nm and
700 nm excitations.
94
Figure 4.1: Schematic diagram of photocurrent generation via SBCT in an OPV. 102
Figure 4.2: a) Structures of homoleptic zinc dipyrrin derivatives: zDIP1, zDIP2,
zDIP3 and ZCl with their corresponding b) absorption spectra in cyclohexane
solution.
108
Figure 4.3: a) Emission spectra of zDIP1-3 and ZCl in cyclohexane. b) Solvent
polarity dependent emission spectra of zDIP3 in various solvents. c)
Photoluminescence (PL) quantum yield variation of zDIP1 –zDIP3 and ZCl vs.
solvent dielectric constant in the order: cyclohexane, toluene, CH
2
Cl
2
and MeCN.
109
Figure 4.4: Normalized photoluminescence (PL) measurements of a) zDIP1, b)
zDIP2, c) zDIP3 and d) ZCl in various polarity solvents. The emission is collected
at 520 nm and 550 nm for zDIP1-3 and ZCl, respectively following excitation with
500 nm. The instrument response function (FWHM ~ 22 ps) is also shown in red.
The black lines are the fit to the PL measurements with exponential(s) (amplitudes
and time constants are shown in table 1). e) Normalized time resolved PL
measurements of CT emission for zDIP2 and zDIP3 in dichloromethane measured
by collecting emission at 650 nm.
111
Figure 4.5: Femtosecond transient absorption of zDIP1 in (a) cyclohexane, (b)
toluene, (c) dichloromethane and (d) acetonitrile. Excitation at 500 nm, pump
fluence of 160 µJ/cm
2
were used for all except (c) which was performed at 70
µJ/cm
2
.
113
Figure 4.6: a) Spectroelectrochemical data of zDIP1in dichloromethane under
negative applied bias and b) Spectral lineshape comparison between spectro-
electrochemical reduction and TA experiments on zDIP1 in DCM. The encircled
portion in green shows the peak responsible for the difference.
115
Figure 4.7: Nano-to-millisecond transient absorption of zDIP1 in (a) acetonitrile, 117
xvii

(b) different solvents at 0.5 ms and (c) femtosecond transient absorption of zDIP1
in CH
2
Cl
2
:CH
3
I (1:4).
Figure 4.8: Femtosecond transient absorption of zDIP2 in (a) cyclohexane, (b)
toluene and (c) acetonitrile. Excitation at 500 nm, pump fluence of 70 µJ/cm
2
was
used for all.
118-119
Figure 4.9: Femtosecond transient absorption of zDIP3 in (a) cyclohexane, (b)
toluene and (c) acetonitrile. Excitation at 500 nm, pump fluence of 80 µJ/cm
2
was
used for all.
120-121
Figure 4.10: Femtosecond TA of ZCl in a) cyclohexane (CH), acetonitrile (MeCN)
b) at initial delays, c) at long delays, toluene d) at initial delays and e) at long
delays. Excitation pump fluence of 15 µJ/cm
2
was used for all. The red arrow
highlights the change in the transient spectrum.
122-123
Figure 4.11: Femtosecond transient absorption of ZCl in methyl iodide (MeI). The
inset shows the global analysis model used for fitting this TA data. Excitation at
520 nm was performed with a pump fluence of 15 µJ/cm
2
.
124
Figure 4.12: Femtosecond transient absorption of ZCl in PMMA at (a) initial
delays and (b) PMMA at long delays. Excitation pump fluence of 45 µJ/cm
2
was
used. The red arrow highlights the change in the transient spectrum.
125
Figure 4.13: Kinetic model used to fit the transient absorption data for different
polarity solvents of a) zDIP1-3 and b)ZCl.
126
Figure 4.14: (a, c, e, g) SADS used to model the transient spectra of zDIP1. (b, d, f,
h) The fits (black dashed) based on SADS analysis for the time slices taken through
the ground state bleach (red) and spectral range indicative of ICT state (blue).
129
Figure 4.15: (a, c, e, g) SADS used to model the transient spectra of zDIP2. (b, d, f,
h) The fits (black dashed) based on SADS analysis for the time slices taken through
the ground state bleach (red) and spectral range indicative of ICT state (blue).
130
Figure 4.16: (a, c, e, g) SADS used to model the transient spectra of zDIP3. (b, d, f,
h) The fits (black dashed) based on SADS analysis for the time slices taken through
the ground state bleach (red) and spectral range indicative of ICT state (blue).
131
Figure 4.17: (a, c, e, g) SADS used to model the transient spectra of ZCl. (b, d, f,
h) The fits (black dashed) based on SADS analysis for the time slices taken through
132
xviii

the ground state bleach (red), spectral range indicative of CT state (green) and
spectral region for T
1
state absorption (blue).
Figure 4.18: a) Comparison of dynamics for formation of the ICT state (monitored
at 370 nm, 2) (filled) and ground state bleach (open) between zDIP1 (red squares)
and zDIP2 (blue circles) in acetonitrile. b) Alkylation/chlorination leads to similar
dynamics for both the ICT state and ground state bleach for zDIP2, zDIP3 and ZCl.
133
Figure 4.19: Device architecture (a) and I-V (b) for the devices described in the
text. Thickness = 20 nm for DBP, 40 nm for C
60
, and 20 nm for ZCl.
135
Figure 5.1: Structures of the a) monomer ET-TMS and the dimers (b) BET-B and
(c) BET-X.
146
Figure 5.2: Crystal structures of the two BET-X isomers a) and b) in the crystal.
The view chosen here has the xanthene moiety perpendicular to the page. c) Crystal
structure of BET-B isomer showing the overlap over one acene ring.
147
Figure 5.3: Absorption (green) and emission (red) spectra of a THF solution (top
panel) and neat film (bottom panel) for a) ET-TMS, b) BET-B and c) BET-X. The
quantum yields are mentioned in the plots. Due to photodegradation of neat BET-X
film the quantum yield could not be measured.
148
Figure 5.4: Comparison of absorption (green) and emission (red) spectra of the a)
anthracene dimers versus the b) tetracene dimers, with their respective emission
quantum yields listed to the right of the spectra.
150
Figure 5.5: TA spectra of a) 5 mol% Pd(TPBP) in ET-TMS and b) 5 mol%
Pd(TPBP) in BETA films following photoexcitation at 626 nm. c) A close-up of the
induced absorption band between 460 and 600 nm, highlighting the formation of T
1

state from S
1
state of Pd(TPBP) within 10 ps for Pd(TPBP) doped in ET-TMS film.
d) TA spectra of 5 mol% Pd(TPBP) in DPT film with 626 nm excitation.
152
Figure 5.6. Differential extinction spectra obtained for T
1
T
n
transition in ET-
TMS (blue) and BET-B (red).
153
xix

Figure 5.7: Transient absorption spectra of the monomer ET-TMS in a) CHCl
3
and
b) neat film following excitation at 500 nm. The cyan dotted line shows the triplet
spectrum obtained from ET-TMS sensitization measurements with Pd(TPBP). c)
The extinction spectra for S
1
S
n
and T
1
T
n
transitions used to calculate singlet
and triplet populations (d) for ET-TMS. e) Comparison of TA spectra with a fit (red
dotted) consisting of a linear combination of the singlet and triplet spectra of ET-
TMS.
154-155
Figure 5.8. Transient absorption spectra of neat films of a) BET-B with the
corresponding c) singlet and triplet populations calculated using the extinction
spectra shown in b). The cyan dotted line in b) shows the triplet spectrum obtained
from BET-B sensitization experiments. d) Comparison of TA spectra with the fit
(red dotted) consisting of a linear combination of the singlet and triplet spectra of
BET-B.
157
Figure 5.9. Transient absorption spectra of BET-B in a) THF and b) PMMA
solution. The inset in a) shows the solvent polarity independent dynamics at 570 nm
for
1
(T
1
T
1
) state.
158
Figure 5.10. Transient absorption spectra of BET-B doped in a) DPA and b) DPT
on exciting with 550 nm. The cyan dotted line in b) shows the triplet spectrum
obtained from BET-B sensitization. experiments.
160
Figure 5.11: a) UV-visible absorption of neat DPT film (blue) and 10% BET-B
doped DPT film (red) with the excitation spectrum at 550 nm (green) used to pump
in TA measurements. TA spectra of neat DPT film with excitation fluence of b) 26
µJ/cm
2
and c) 156 µJ/cm
2
.
161
Figure 5.12:TA measurements of BETX in a) toluene and b) PMMA following
excitation at 500 nm. c)The dynamics at 400 nm shows no variation in excited state
dynamics with solvent polarity.
163
xx

Figure 5.13. Kinetic model for BET-B in a) solution, PMMA, DPA and b) DPT,
neat film. The notation A, A' denotes the tetracenes in the same dimer and C
denotes the DPT for the BET-B in DPT film or third tetracene in another BET-B
dimer for the BET-B neat film. c) The differential absorption spectra used for S
1

state (black), ¹(T
1
T
1
) state (red) and T
1
state (blue) to simulate the TA data in
different media.
165
Figure 5.14. The TA data for BET-B in different medium (a,c,e) are shown along
with the fits (red dotted line) from the kinetic model and the corresponding relative
concentrations (b,d,f) for each state relative to S
1
state.
166-167
Figure 5.15. The TA data for BET-B in DPT(a) is shown along with the fits (red
dotted line) from the kinetic model and the corresponding relative concentrations
(b) for each state relative to S
1
state.
168
Figure 5.16. The TA data for BET-B neat film (a) is shown along with the fits (red
dotted line) from the kinetic model and the corresponding relative concentrations
(b) for each state relative to S
1
state.
169
Figure 5.17: Excitation fluence dependent a) singlet and b) triplet populations
extracted from the TA measurements on neat ET-TMS film. Similarly, c) and d)
shows the excitation dependent singlet and triplet population from TA
measurements on neat BETA film.
170
Figure 5.18. a) Singlet and triplet population densities determined from TA spectra
of neat ET-TMS and BET-B films. The broken line denotes the fit based on the
kinetic model described before. b) Comparison between time resolved
photoluminescence (PL) measurements on BET-B neat film versus BET-B crystal
to show the non-crystalline nature of the BET-B neat film. The inset shows the time
dynamics a short timescales.
172

xxi

List of Tables
Table 4.1: Fitting parameters for the PL lifetime measurements of zDIP1-3 and ZCl
in different solutions
112
Table 4.2. Kinetic rates for different processes of zDIP1 –zDIP3 and ZCl in different
solvents determined by femtosecond transient absorption measurements based on
kinetic model described in figure 4.13.
127-128
Table 5.1: Time constants ( 
i
=1/k
i
) for the kinetic model used to fit the TA data for
BET-B in different media.
170

xxii

Abstract
Compared to inorganic silicon semiconductors, organic semiconductors possess many
desirable properties such as flexibility, light weight and high absorption coefficient. These
properties lead to low manufacturing and transportation costs for organic solar cells. However,
the maximum efficiency obtained for organic solar cell is ~11% compared to 27% device
efficiency for silicon semiconductor. This low device efficiency can be addressed with proper
understanding of the working principles of organic solar cells. The active layer of an organic
solar cell is composed of an electron donor and an electron acceptor material. Upon light
absorption in the active layer, a series of processes: exciton generation, exciton diffusion, charge
generation and charge recombination occur before the charges are collected at the electrode. The
efficiency of the device depend on the efficiency of each of these processes. In this dissertation
ultrafast spectroscopic techniques are utilized to provide insight into the dynamics and efficiency
of these processes, which will eventually assist in optimizing the device performance for organic
semiconductors.

1

Chapter 1
Electronic processes in Organic photovoltaics
The photovoltaic effect, i.e., conversion of light energy to electrical energy is known
from Becquerel's 1839 work on liquid electrolytes
1
and has since been studied in a wide range of
materials. The first technological advancement in photovoltaic cell was obtained from the 1954
report by Chapin et al. on a silicon based single p–n junction device with a solar power
conversion efficiency of 6%.
2
Since then with improved efficiency (~25%) photovoltaic cells
3

have emerged as a clean and sustainable source of energy to meet the global energy challenge.
Currently, the solar cell technology is dominated by single junction solar cells based on
crystalline Si assembled into large area modules. However, other semiconductor materials are
under active investigation to further reduce the cost of produced electricity by increasing the
power conversion efficiency,
4-5
reducing the amount of absorbing material needed, and lowering
the assembly cost of modules. Over last three decades organic semiconducting materials have
demonstrated high potential for solar cell technology
6-8
with power conversion efficiency
reaching over 11%
9
from the first developed single heterojunction organic photovoltaic cell
reported in 1986 with power conversion efficiency of ~ 1%.
10

The low temperature processing conditions confers organic semiconductors a critical
advantage over the inorganic semiconductors as high temperature processing conditions for latter
restricts the range of substrates that they can be deposited on. On the other hand, organic
semiconductors can be deposited on even a plastic substrate to produce solar cells with lower
cost, flexible forms and light-weight. Further low temperature processing cuts down the
manufacturing costs for organic photovoltaics (OPVs) compared to their inorganic counterparts.
2

These characteristics along with the ability to tune the physical properties of organic molecules
by fine tuning their chemical structure motivate the research and industrial interest in OPVs.
Typical organic solar cells are comprised of an electron donor and an electron acceptor
material sandwiched between the electrodes. Illumination of light leads to generation of singlet
exciton (Coulombically bound e
-
-h
+
pair) typically in the donor followed by diffusion to the
donor-acceptor (D-A) interface where electron transfers to the acceptor.
11-13
The driving force for
this charge transfer comes from the chemical potential gradient present at the D-A interface.
Mainly two architectures for such D –A interface are used in the field: 1) two consecutively
deposited layers of donor and acceptor to form a planar heterojunction (PHJ) (Figure 1.1b) and
2) co-deposition, leading to a D-A bulk heterojunction (BHJ) structure with a much higher
internal interface (Figure 1.1c).
14-16

a)
b) c)
Figure 1.1: a) The energy level diagram of a typical organic solar cell with donor and acceptor
layer. The two device architectures used in the OPVs: b) planar heterojunction and c) bulk
heterojunction. This figure is adapted from ref 12.
E
DA
E
Anode
Cathode
Donor
Acceptor
Active D-A layer
Anode
Cathode
3

1. Current-voltage characteristics of a solar cell:
The most important figure of merit for the performance of solar cell is the power
conversion efficiency, η. The power conversion efficiency of a solar cell can be defined as,

which depends on the open circuit voltage, V
OC
, short circuit current density, J
SC
and fill factor,
FF. The denominator is the incident power, P
in
. These are shown in the current-voltage curve of a
typical solar cell (Figure 1.2). FF is defined as the ratio between the maximum output power,
P
max
, divided by the product of J
SC
and V
OC
(FF=P
max
/J
SC
V
OC
).
12, 16-17
The FF is often represented
as the ratio of area of the filled gray rectangle (P
max
) to unfilled gray rectangle (J
SC
 V
OC
).

Figure 1.2: Schematic current voltage characteristic of a solar cell under dark (red dotted line)
and illumination (red solid line). This figure is adapted from reference 12.
It is clear that to maximize the efficiency each of these parameters need to be optimized. In
systems like OPVs where charge transfer occurs from the donor to the acceptor, the V
OC
is
thermodynamically limited by the energetic offset between the HOMO of the donor and LUMO
-0.5 0.0 0.5 1.0 1.5
J
SC
V
OC
J
SC
V
OC
Current (a.u.)
Voltage (V)
P
max
4

of the acceptor (E
DA
) (Figure 1.1a).
18-19
However, various spectroscopic and temperature-
dependent measurements illustrate that E
DA
is a poor indicator for V
OC
and rather the energy of
the intermolecular D-A charge transfer state (E
CT
) governs the V
OC
.
20-23
E
CT
is limited by E
DA
as
the charge transfer transition occurs when an electron is promoted directly from the HOMO of
the donor to the LUMO of the acceptor. E
CT
correlates linearly with V
OC
with typical energetic
losses of ~0.6 eV due to charge recombination.
24-26
Additional losses between the energy of
excitons in the strongly absorbing neat materials and the V
OC
of a device originate from the
energetic offset (E) required to drive charge formation at the D-A interface (Figure 1.1a). These
losses are discussed in detail in the following sections. Thus one way to increase the V
OC
will be
increasing the E
DA
by increasing the band gap of the materials used. However this can result in
reduction of J
SC
as J
SC
increases with decrease in band gap and it is expressed as,
16

Here, N
ph
( ) is the photon flux density of the incident AM 1.5G solar spectrum (Figure 1.3) at
wavelength, with a total integrated intensity of 100 mW/cm
2
. Air mass 1.5 global (AM1.5G)
simulates the terrestrial solar spectrum on the ground when the sun is at 48.28 zenith angle.
17
The
air mass (AM) represents the proportion of atmosphere that the light must pass through before
striking the Earth relative to the shortest path length when the sun is directly overhead and is
defined as 1/cosθ. The air mass calculates the reduction in the power of light as it passes through
the atmosphere caused by scattering and absorption by air (oxygen and carbon dioxide), dust
particles and/or aerosols in the atmosphere. The number “1.5” indicates that the path of light in
the atmosphere is 1.5 times the shortest length when the sun is at the zenith. The letter “G”
stands for “global” and includes both direct and diffuse radiation. The external quantum
5

efficiency η
EQE
( ) is defined as how efficiently an incident photon gives rise to an electron in a
device throughout the whole spectrum. This η
EQE
is a product of efficiencies associated with
each of the steps happening in a solar cell: absorption, exciton diffusion, exciton dissociation into
free carriers, charge transport and charge collection. The maximum short-circuit current density
can be obtained by integrating from the high photon energy side (short wavelength) of the
spectrum to the wavelength corresponding to the optical band gap of the material. Hence, the
smaller the optical band gap, the larger the maximum short circuit current. It is worth to note that
although maximization of the short-circuit current density requires the use of organic
semiconductors with decreasing optical band gap, the strategy to increase the open-circuit
voltage requires the opposite trend with optical absorption gap.
12

a) b)
Figure 1.3: a) Schematic representation of the path length for the AM 1.5 G at zenith angle of
48.2 from ref 17 with the corresponding photon flux shown in b). The absorption coefficient
spectra for crystalline Si, a typical organic (P3HT) polymer
27
and small molecule (SubPc).
28

2. Electronic and optical processes in organic semiconductor:
Upon light absorption in the active layer of a solar cell, a series of processes: exciton
generation, exciton diffusion, exciton dissociation and charge recombination occur before the
charges are collected at the electrode. The efficiency of OPVs depends directly on the efficiency
6

of each of these individual processes. In this section each of the optical and electronic processes
that govern the operation of a solar cell will be discussed.
a)
b)
Figure 1.4: a) Electronic state diagram and b) spatial picture to describe the photo-induced
charge-carrier formation mechanism in an organic solar cell. The energy state diagram is adapted
from reference 15.
7

2.1. Optical absorption and exciton generation
In organic donor-acceptor solar cells the absorption of solar light is possible by either
/both donor and acceptor organic semiconductors. Organic semiconductors such as a conjugated
polymer (poly(3-hexylthiophene)) or a small molecule like subphthalocyanine often possess very
high absorption coefficients above 10
5
cm
−1
(shown in Figure 1.3b) and thus very low
thicknesses between 100 and 300 nm are sufficient for a good absorption yield in organic
photovoltaic devices. In contrast, solar cells based on the crystalline silicon solar cells need more
than 100 μm.
13
Thus, lower material amounts are needed for organic cells. Unfortunately, most
of the organic semiconductors possess narrow absorption bandwidths and this limits the solar
cell absorption. In contrast the inorganic semiconductors like Si absorb solar spectrum up to
1000 nm (optical bandgap 1.1 eV). Some recent breakthroughs have been achieved in the field of
conjugated polymers with broadened and extended near-IR absorption.
27, 29-32

In organic semiconductors absorption of a photon leads to a singlet exciton as shown in
Figure 1.4. The singlet exciton generated is a Coulomb bound electron hole pair with significant
binding energy 0.5-1 eV as expected from the low dielectric constants (3-4) for organic
materials.
33-34
As this binding energy is higher than the thermal energy at room temperature
(kT~0.026 eV), this is known as Frenkel exciton and the separation between the electron and
hole is typically ~10Å. In contrast, for inorganic semiconductors the binding energy
35
is only
~0.010 eV with a e
-
-h
+
separation distance of ~100Å due to long-range interactions present in
three dimensional lattice of crystalline Si. Thus, optical excitation in organic materials at room
temperature does not lead to free electrons and holes to produce photocurrent. To produce
charges the initial excitation, i.e., the singlet exciton needs to diffuse to the D-A interface to
dissociate. Excitons generated possess a certain lifetime after which they recombine radiatively
8

or non-radiatively. Typically, for most organic materials the exciton generated is a singlet with a
characteristic lifetime on the order of nanoseconds. However, some molecules can also lead to
generation of triplet exciton with a lifetime in the order of microseconds due to spin forbidden
nature of T
1
to S
0
transition. However, generation of triplet exciton via inter-system crossing
leads to energy loss as triplets are generally ~0.3-0.7 eV lower than singlets.
36-37
This energy loss
can be compensated if a singlet exciton on one chromophore interacts with its nearby
chromophores to split its energy to generate two triplets separated on two chromophores via
singlet fission.
38-40
Incorporation of materials capable of undergoing singlet fission into
photovoltaic devices has shown to increase the theoretical upper limit of the power conversion
efficiency from 33.7% to 45 %.
41-42

2.2. Exciton diffusion:
To generate positive and negative charges the exciton needs to migrate to the donor-
acceptor interface. Since the exciton is a neutral species, its motion is random and is not
influenced by the electric field. However, whether the exciton reaches the interface or not is
governed by the lifetime of the exciton. The lifetime () of the exciton and diffusion coefficient
(D) dictates the diffusion length (L=(D)
1/2
) which needs to be comparable to the distance to
interface to be able to dissociate. This restricts the thickness of donor/acceptor in a planar
heterojunction (Figure 1.1b) device as typically the singlet exciton diffusion length of small
molecule organic semiconductors varies from 3-30 nm
43
and for poly (3-hexylthiophene) it is ~ 4
nm.
44
As mentioned in the previous section at least 100-300 nm of active layer thickness is
required for optimum absorption yield but this will mean that only 10% of the excitons generated
will be able to dissociate at the interface. This shows the advantage of bulk heterojunction
(Figure 1.1c) device structure where due to higher internal interface the chances of an exciton to
9

reach the interface is greater than in a planar heterojunction. In principle, for efficient exciton
dissociation the donor-acceptor phase separation should be as small as possible. However, for
subsequent steps like charge generation and charge collection the phase separation need to be as
coarse as possible.
45
Thus the optimum phase separation is decided by these two opposing
requirements. Although a photogenerated singlet exciton in anything other than a very thin
planar heterojunction have rare chances to reach the interface, the triplet exciton due to their long
lifetime (~s) might be expected to be able to reach the D-A interface.
46
The triplet diffusion
also depends on the diffusion coefficient, which is typically smaller for triplets compared to
singlet excitons due to the short range Dexter mechanism adopted by triplets to diffuse.
47
Thus
materials producing triplet excitons can be used in planar heterojunction device where there is
more control over the morphology at D-A interface.
Once an exciton is generated there are two possible mechanisms of exciton diffusion:
Coulombic and electron exchange (Figure 1.5). The Coulombic exchange results from resonant
interaction between transition dipole moments at the initial (excited state) and final (ground
state) sites. This exchange can happen over long-range and it is known as Förster resonant
energy transfer (FRET).
48
The rate constant for this energy transfer from one site to other
site, k
FRET
is given by

where

is the lifetime of the excited donor in the absence of transfer, r is the distance between
the two sites and R
0
is the Förster radius, i.e., the distance at which transfer and spontaneous
decay of the excited donor are equally probable. The rate of energy transfer for triplet excited
state is not described by Förster theory as the dipole matrix element between ground state and
10

triplet excited state is explicitly zero. The other mechanism for exciton diffusion arises from
electron exchange and this is restricted to adjacent sites. This mechanism involves a short range
exchange (Dexter-type) mechanism which occurs through a direct wave function overlap and
thus it is restricted to small length scales.
49
The rate constant is proportional to the spectral
overlap integral and attenuates as k
DET
∝ xp -β Dexter-type mechanism is the primary
mechanism for triplet exciton diffusion. Thus, singlet excitons can move more quickly due to
higher diffusion coefficient than triplets, however the lifetime of singlet exciton is lower than
that for triplet exciton. The efficiency with which singlets and triplets can reach the D-A
interface is system-dependent. In addition molecular packing can also induce small
intermolecular interactions between chromophores in a thin film and thus influence the singlet
and triplet exciton diffusion.

11

Figure 1.5: Schematic diagram for Förster and Dexter energy transfer.
2.3. Exciton dissociation and charge generation at D-A interface:
After the exciton arrives at the D-A interface it first dissociates to form a D
+
-A
-
charge
transfer (CT) state followed by either separation into free charges (Figure 1.4a) or relaxation to
the ground state.
15-16
The initial CT state formed still has some Coulomb attraction between the
electron and hole but it is less than that present in the exciton and higher than those present for
free charges. This is because as the charges move away the electronic polarization effects of the
surroundings screens the Coulomb potential. This is depicted in Figure 1.6.
a) b) c)
Figure 1.6: Schematic representation of a) singlet exciton, b) CT state and c) free charges. As
one go from exciton to CT state to free charges the distance between the opposite charges
increases due to screening of Coulomb potential by polarization effects. This figure is adapted
from ref 15.
Depending on which CT state out of the CT manifold (Figure 1.4a) is populated by the
dissociation of the exciton, there could be two scenarios: 1) The singlet exciton relaxes to the
lowest lying CT
1
state due to fast interconversion from CT
n
to CT
1
or 2) Fast generation of free
charges via hot CT
n
states. If the first case happens then the lowest D
+
-A
-
(CT
1
) state
corresponds to the situation where the hole on the donor is spatially close to the electron on the
acceptor molecule.
50-51
Here, a relatively tight Coulomb binding of the hole and electron is
expected, which is less likely to separate into free charges. The second case corresponds to the
generation of free charges via hot CT states. Several experimental works have shown that indeed
S
1
Free charges
12

charge generation increases on going through hot CT states.
50-52
This is also supported by the
long separation or delocalization distance calculated between electron and hole in higher lying
CT states compared to CT
1
.
50, 53
Thus, having a larger initial separation in hot CT states makes
the mutual escape easier.
Additionally, other processes like energy transfer could be involved before exciton
dissociation. For example, the initial exciton in the donor can first undergo an energy transfer to
generate an exciton in the acceptor followed by formation of a CT state (D
+
-A
-
) by hole transfer
from the acceptor to the donor. Although the final state is same the rates could be different based
on the rate for energy transfer and hole transfer. This has been observed to occur in several
oligothiophen:fullerene dyads.
54

In organic photovoltaics, the terms polaron and polaron pair are very common. A polaron
is a free charge in combination with a distortion or polarization of the environment near the
charge. In a crystalline inorganic material, placing a charge onto a site does not change the
surroundings significantly as the dielectric constant is high and the crystal lattice structure is
rigid. In contrast in many organic semiconductors, a charge onto a certain molecular site can
deform the whole molecule. A polaron pair is a Coulomb-bound pair/CT state of a negative and a
positive polaron either situated in a single material (like D
+
-D
-
) with lower binding energy than
an exciton or on different materials, i.e, the positive polaron on the donor material and the
negative polaron on the acceptor like D
+
-A
-
.
2.4. Charge recombination and charge escape:
After the charge generation occurs at the interface there are three possible fates for those
charges (Figure 1.7). The first one is intimate geminate recombination, i.e., recombination
13

between closely spaced hole in the donor and electron in the acceptor. This can be thought of as
the CT state (D
+
-A
-
) relaxation to the ground state. Geminate recombination implies
recombination between two oppositely charged species generated from same exciton. The second
fate for the charges is to escape the Coulomb potential in the CT state and finally get collected at
the electrodes to generate photocurrent. The efficiency with which the charges can move depends
on their carrier mobility. In crystalline inorganic semiconductors, high rigidity of the lattice and
three dimensional structure ensures wide valence and conduction band to produce carriers with
high mobility (~10
2
-10
3
cm
2
/Vs). On the other hand for organic semiconductors, weak
intermolecular electronic coupling and lack of long-range order leads to low carrier mobilities.
The localization of charge carriers leads to formation of polarons and transportation relies on
polaron hopping from one site to the other site. As a result depending on the morphology the
charge carrier mobilities can vary several order of magnitude from highly disordered material
(~10
-6
-10
-3
cm
2
/Vs) to highly ordered material (~1 cm
2
/Vs). Therefore, based on the mobilities
the charges or polarons diffuse away from the D-A interface to reach their respective electrodes.
Some charges that escape the initial Coulomb potential can also diffuse to each other to have a
second chance for geminate recombination (fate 3, Figure 1.7c). To describe this diffusion
controlled geminate recombination of polarons the solutions need to be obtained by solving the
Debye-Smoluchowki equation,
55

where polarons, whose probability density is given by (r,t), can undergo diffusive
recombination under the influence of a Coulomb potential imposed by the charge of the diffusing
14

particles,

. Experimentally the
measured quantity is polaron population which is the survival probability, Ω (t),

where

is the encounter distance at the D-A interface. Under the assumption that both polaron
recombination and separation can be described by exponential reaction rates,
56
the extensively
used
57-58
Onsager-Braun model is the long time limit
59-60
of equation (5) in the presence of

where

is an external electric field. This diffusion equation assumes the
medium to be isotropic in all directions and therefore any impedance imposed on the diffusion of
electrons and holes due to local variations in morphology of the bulk heterojunction blend is
ignored. A more rigorous way to describe a bulk heterojunction microstructure is obtained by
performing molecular dynamics calculations.
a)
b) c)
CT
state
h
+
e
-
encounter
distance
U(r)
Intimate/CT state Geminate
Recombination
CT
state
h
+
e
-
encounter
distance
U(r)
D
Total escape
CT
state
h
+
e
-
encounter
distance
U(r)
D
Diffusion-controlled
Geminate recombination
15

Figure 1.7: Three possible fates for the charges after generation at D-A interface: a) intimate
/CT state recombination, b) Total escape through diffusion and c) secondary geminate
recombination controlled by diffusion.
In addition there could be another type of recombination between two oppositely charged
carriers generated from different excitons and this is known as non-geminate recombination
(Figure 1.8).
57, 61-62
Since the recombination partners are generated from two different precursors,
the recombination can be described by a bimolecular recombination kinetics.
63
Higher order
processes, for example three body recombination such as Auger or impact recombination, has so
far not been observed for organic semiconductors.
64
The morphology of the active layer plays an
important role in bulk heterojunction solar cells.
65
Due to the tradeoff between exciton
dissociation where a fine donor-acceptor phase segregation is ideal and charge transport where a
bilayer configuration is optimum, the spatial dimensions of the donor–acceptor phases are central
towards the device performance.
66

Figure 1.8: After polaron or charge generation there are two possible pathways for
recombination: geminate and non-geminate recombination.
Geminate Recombination Non-geminate Recombination
16

After the charges escape the next step is charge collection at the electrodes. The
efficiency of the charge collection process is not just determined by the difference between the
work function of the isolated electrode and the energy of the positive or negative carrier. But it
also depends on the interfacial charge-density redistributions and geometry modifications at the
active layer-electrode interface.
67-68

3. Methodology used in this Thesis:
The main spectroscopic techniques used in this thesis are time correlated single photon
counting (TCSPC) and femtosecond-to-microsecond transient absorption.
3.1. Time correlated single photon counting (TCSPC):
TCSPC is a well known technique to measure the fluorescence lifetime. The principle of
TCSPC (Figure 1.9) is based on detection of single photons and the measurement of their arrival
times in respect to a reference signal, usually the light source. TCSPC is a statistical method and
thus requires a high repetitive light source to accumulate sufficient number of photon events for
a required statistical data precision. TCSPC can be compared to a stopwatch where the "start"
signal is provided by the reference signal from the light source and the "stop" signal is provided
by the signal from the fluorescence. The time required for the stop pulse to arrive is measured for
several cycles of data acquisition and this results in a histogram of counts versus time bins
representing the fluorescence intensity versus time. The mode where start signal is provided by
the reference signal and the stop signal from the fluorescence is known as the forward mode
(Figure 1.9b).
69
However, at high repetition rates this mode is disadvantageous as most of the
time the cycle for stopwatch gets started but it never gets stopped by a stop fluorescence signal.
This leads to an increase in the dead time and to avoid this reverse mode is often used. In reverse
17

mode (Figure 1.9c),
69
the start signal is provided by the fluorescence signal and stop signal is
provided by the reference from the light source. In reverse mode the time axis of the memory
histogram is internally reversed in order to see photon events with longer times at the right part
of the time axis. In the experiments described in this thesis the reverse mode is used to be able to
have a good statistical measurement with a high repetition rate laser.
a)
b)
T
1
T
2
T
3
Timing reset
START
STOP
reference
fluorescence
18

c)
Figure 1.9: Time correlated single photon counting setup in the lab (a) with the forward and
reverse mode described in b) and c), respectively.
3.2. Femtosecond to nanosecond transient absorption:
Transient absorption is a very powerful technique to monitor the generation and evolution
of both neutral and charged species. This measurement is a two laser beam experiment where the
pump pulse excites some of the molecules from ground state to the excited state followed by a
probe pulse to measure the absorption of the excited state. The time delay between the pump and
probe pulses helps in monitoring the dynamics of the excited state. The transformation of an
exciton to a charge transfer state or polarons can be monitored by their distinct spectral signature.
Pump-probe measurement is a difference absorption spectrum calculated by subtracting the
absorption spectrum of ground state from the absorption spectrum of the excited state (Δ A)
(Figure 1.10). The first contribution is the ground state bleach (blue broken line, Figure 1.10) as
the number of ground state molecules in the sample excited by pump is less compared to the
non-excited sample. This leads to a negative A spectrum. The second contribution comes from
stimulated emission where the emitted photons from the excited sample is in the same direction
as the probe and thus the intensity of signal hitting the detector is higher and thus also
corresponds to a negative signal (green broken line, Figure 1.10). The third contribution comes
from the excited state absorption. Upon excitation with the pump beam, optically allowed
transitions from the excited states to higher lying excited states of a chromophore can occur at
T-T
1
T-T
2
T-T
3
START
STOP
Delay T Delay T Delay T Delay T
reference
fluorescence
19

multiple probe wavelengths. Consequently, a positive signal in the ΔA spectrum is observed in
the wavelength region of the excited-state absorption (red broken line, Figure 1.10). The
measured spectrum in transient absorption is the sum of all these contributions (black line,
Figure 1.10). In this thesis the pump used is either a femtosecond tunable pulse (500-700 nm,
FWHM~150 fs) or a nanosecond (FWHM~550 ps) pulse at 532 nm. The probe pulse used was a
white light continuum (320-1200 nm) to probe different possible transitions from excited state or
charge carriers. Thus, broadband transient absorption (TA) measurements over the entire time
range (200 fs - 500 ns) helped to monitor the electronic processes from photon absorption to
charge transport processes.

Figure 1.10: The scheme of transient absorption (top panel) and a typical A spectrum (bottom
panel) consisting of ground state bleach, stimulated emission and excited state absorption.

20

Outline of this thesis:
In this thesis the main goal was to measure the dynamics and efficiency for each of the
processes following light absorption, i.e., exciton generation, exciton diffusion, exciton
dissociation and charge recombination. With the insight into the dynamics a physical
understanding for the OPV mechanism can be established. The results obtained assist in
materials design to optimize the photovoltaic performance.
In chapter 1 an overview of different electronic processes in organic solar cells and a
methodology to understand these processes is discussed.
In chapter 2 and 3 the generation and recombination dynamics of polarons in semi-
random donor acceptor P3HT:fullerene blends are studied. The physical reason behind the
device performance is explained from both experimental and simulated polaron population
kinetics. Additionally the polaron yields are quantified to understand the absolute contribution
towards the photocurrent for different donor-acceptor P3HT analogs. In chapter 3 these studies
were further extended to monitor the effect of incorporation of a strong donor monomer unit in
the polymer backbone.
In chapter 4 the dynamics of intramolecular charge-transfer states in a visible light
absorbing system: zinc dipyrrin are studied. The main purpose of the study is to understand how
the energetic driving force to form a charge transfer state can be reduced and thus help in
improving the open circuit voltage of the device. Femtosecond transient absorption and TCSPC
experiments were performed to study the solvent polarity dependent rate for formation and
recombination of charge transfer state in zinc dipyrrin compounds.
21

In chapter 5 the dynamics of singlet fission, i.e., transformation of an excited singlet state
chromophore to form two triplet states on interaction with nearby chromophores is studied. The
main objective of this study is to understand the coupling required for higher singlet fission yield
by evaluating the singlet fission dynamics in a "single molecule" containing two covalently
attached tetracene units. Once this coupling is established, this singlet fission dimer can be doped
in a known singlet fission monomer film to generate more singlet fission sites and thus a higher
singlet fission yield.

22

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27

Chapter 2
Quantifying Charge Recombination in Solar Cells based on Donor-Acceptor
P3HT Analogs
1. Introduction
Over the last decade, organic photovoltaics (OPVs) have shown an immense
improvement with power conversion efficiencies reaching over 11%.
1
OPVs are primarily a
blend of an electron donor and acceptor material. The general operational principle involves
photogeneration of an exciton in donor or acceptor phase followed by its diffusion to the
interface where charge generation occurs and finally the charges are transported to the electrodes
through both phases to produce photocurrent.
2-4
These processes can happen at several time
scales, from femtoseconds for exciton dissociation to microseconds for charge collection.
5-8
The
performance of OPVs depend on the efficiency of these individual processes.
At present, the best performing OPVs utilize a bulk heterojunction (BHJ) architecture
9-10

(a bi-continuous composite of an electron donor and acceptor phase) as this aids excitons in
reaching a donor-acceptor interface during their lifetime at which they can undergo charge
separation.
2-3, 11-15
The most commonly studied BHJ OPVs consist of high band gap (~2 eV)
polymers
16
blended with fullerenes that do not harvest the solar spectrum as efficiently as
commercial solar cells that make use of silicon’s indirect bandgap of ~1.1 eV. In recent studies,
near-infrared absorption by OPVs has been achieved by implementing a donor-acceptor
approach, where an alternating pattern of an electron deficient monomer and an electron rich
monomer is used to obtain low band gap polymers.
17-18
These alternating donor-acceptor
28

copolymers have extended NIR absorption as they primarily shift the polymer’s main absorption
band to the red.
8
However, this approach tends to lower the polymer's absorbance in the visible
range and reduces the number of visible solar photons harvested by the polymer.
8, 19-23
Recently,
a new family of semi-random multichromophoric donor-acceptor analogs of regioregular poly(3-
hexylthiophene) (P3HT)
24-28
has been developed, which display exceptionally broad absorption
due to the randomized incorporation of only a small amount (~10% monomer sites) of electron
deficient units into the backbone of P3HT. These analogs retain many of the favorable
characteristics of P3HT, such as preferable mixing with fullerenes at low w/w ratios and high
hole mobility.
Unlike alternating donor-acceptor copolymers, these semi-random donor-acceptor
copolymers display broad absorption features due to the formation of two classes of
chromophores along the polymer backbone.
25-26
The first of these two classes can be broadly
characterized as chromophores that predominantly consist of P3HT-rich backbone segments
whose →
*
transition leads to a strong absorption in the visible (400 - 600 nm), while the
second class consists of intramolecular charge transfer (ICT) chromophores that give rise to
absorption in the NIR (700 - 900 nm).
25-26
ICT chromophores contain at least one electron-
deficient monomer adjacent to electron-rich thiophene or 3-hexylthiophene units that facilitates
the absorption of lower energy photons.
29
The average delocalization length of the exciton for
either of these chromophore classes is unclear and there exists the potential that some segments
contribute to both optical transitions.
The observed spectral characteristics of semi-random polymers make them excellent
candidates for photovoltaic devices. While many of the semi-random polymers studied to date
show high efficiency in BHJ solar cells with [6,6]-phenyl-C
61
-butyric acid methyl ester (PCBM)
29

as an acceptor,
24-25, 27
there are certain cases where the broadened spectral coverage instead lead
to low short circuit current densities (J
sc
) and power conversion efficiencies. As a specific
comparison, diketopyrrolopyrrole (DPP) based polymers (P3HTT-DPP-10%) in BHJ solar cells
show efficiencies approaching 6% and a J
sc
of almost 15 mA/cm
2
,
25
while cells utilizing the
thienopyrazine (TP) containing polymer P3HTT-TP display efficiencies below 1% and
surprisingly low currents of around 3 mA/cm
2
.
26
In contrast, P3HT:PCBM solar cells were found
to give a peak efficiency of 3.89% with a J
sc
of 10.22 mA/cm
2
under identical device efficiency
measurement conditions (i.e. in air).
26
The origin of such large differences in J
sc
caused by subtle
alterations of the polymer chemical composition (for both semi-random polymers only 10% of
the total monomer content is the acceptor unit) is unclear, and understanding the origin of such
differences is crucial for the design of semi-random donor-acceptor polymers for high efficiency
OPVs. The change introduced by adding electron deficient monomers can potentially influence
the delocalization of excited states and thus affect charge generation as the extent of exciton
delocalization has been suggested to influence the spatial separation of electron-hole pairs
following charge transfer.
30-31
Moreover, the presence of an acceptor unit may lead to charge
trapping and enhanced charge recombination within polymer:fullerene composites. The polaron
recombination in OPVs can be broadly classified as either geminate, which is the recombination
of the carriers generated from the same exciton, or non-geminate, defined as the recombination
of the free mobile carriers originating from different initial excitations.
32-34

In this chapter, transient absorption (TA) measurements following excitation of P3HT-
like chromophore and ICT-like chromophore were performed to monitor exciton diffusion,
polaron generation and polaron recombination processes in BHJ thin films of semi-random
donor-acceptor polymer:fullerene blends. These measurements were performed on composites of
30

two new semi-random P3HT analogs with PCBM to understand the nature of the optically
excited states and to investigate the variation in charge generation and recombination dynamics
by incorporation of the electron withdrawing units into the polymer backbone. Two different
electron withdrawing units were studied: DPP and TP, with TP being comparatively stronger in
electron withdrawing strength. As a control, TA measurements are also performed on parent
polymer P3HT blends. Since, both of the semi-random donor-acceptor polymers studied in this
paper contains only 10% of acceptor unit, from now on they will be referred as P3HTT-TP-10%
and P3HTT-DPP-10%. Unlike the parent P3HT:PCBM blend, both semi-random donor-acceptor
polymer:PCBM blends show ultrafast polaron generation indicating exciton diffusion is not the
limiting step for the device efficiency. However, the polaron recombination dynamics is different
for the different polymer:PCBM blends. The modeling of the polaron recombination dynamics
reveal that initial charge separation length at the polymer:fullerene interface plays an important
role in geminate recombination with small charge separation length resulting in high
recombination losses. It was found that poor performance of P3HTT-TP-10%:PCBM is due to
high geminate recombination resulting from low charge separation length present at the
interface. This low charge separation length results from the lower energetic driving force of the
exciton and high degree of morphological disorder present in P3HTT-TP-10%:PCBM.
26
In
contrast, the markedly improved performance of P3HTT-DPP-10%:PCBM can be correlated
with two observations: lower geminate recombination of charges generated from the excitation
of P3HT-like chromophores and some contribution of charges formed by exciting ICT-like
chromophores that escape geminate recombination. The modeling of the polaron recombination
dynamics following the excitation of P3HT-like chromophore reveals similar charge separation
length at the polymer:fullerene interface for both P3HT:PCBM and P3HTT-DPP-10%:PCBM
31

blends. However, the charge separation length obtained for the polaron recombination dynamics
following excitation of ICT-like chromophore is ~1.5 times lower than that obtained for P3HT-
like chromophore excitation. The correlation between excitation-wavelength dependent geminate
recombination dynamics and the internal quantum efficiency (IQE) of the devices illustrates that
although P3HTT-DPP-10%:PCBM solar cells perform better than P3HT:PCBM, there exists
further potential for improvement of the devices by overcoming charge recombination in the
NIR. Nevertheless, the calculated polaron yield using the polaron cross-section from chemical
doping measurements demonstrate that the P3HT-like chromophore of P3HTT-DPP-10%:PCBM
is as efficient as that present for P3HT:PCBM.
2. Experimental section:
2.1 Synthetic Procedures: Procedures for the synthesis of poly(3-hexylthiophene) (P3HT),
poly(3-hexylthiophene-thiophene-thienopyrazine) (P3HTT-TP-10%) and poly(3-hexylthiophene-
thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%) were used without modification as
developed and reported earlier.
25-26

2.2 Preparation of Thin Films for Femtosecond Transient Absorption: Solutions were spin-coated
onto pre-cleaned glass slides from 5 mg/mL chlorobenzene (CB) solutions for P3HT and 5
mg/mL o-dichlorobenzene (o-DCB) solutions for P3HTT-TP-10% and P3HTT-DPP-10% films.
Polymer:PCBM blends at optimal ratios were spun from CB in the case of P3HT:PCBM (w/w
1:1) and o-DCB for P3HTT-TP-10%:PCBM (w/w 1:0.8) and P3HTT-DPP-10%:PCBM (w/w
1:1.3) at 5 mg/mL polymer concentration. P3HT and P3HT:PCBM films were thermally
annealed at 150 °C for 30 min in a nitrogen oven. P3HTT-TP-10%, P3HTT-DPP-10%, P3HTT-
TP-10%:PCBM and P3HTT-DPP-10%:PCBM films were placed in a N
2
cabinet for 30 min. The
32

processing conditions for neat polymers and polymer:PCBM blends were chosen to match the
optimal processing conditions of the corresponding solar cells at which the highest power
conversion efficiencies were obtained.
25-26
The thickness of each film was chosen such that the
peak optical density of the polymer was near 0.2.
2.3 Solar Cell Fabrication: All steps of device fabrication and testing were performed in air. ITO-
coated glass substrates (10 /, Thin Film Devices Inc.) were sequentially cleaned by sonication
in detergent, de-ionized water, tetrachloroethylene, acetone, and isopropyl alcohol, and dried in a
nitrogen stream. A thin layer of PEDOT:PSS (Baytron
®
P VP AI 4083, filtered with a 0.45 μm
PVDF syringe filter – Pall Life Sciences) was first spin-coated on pre-cleaned ITO-coated glass
substrates and baked at 130 ºC for 60 minutes under vacuum. Separate solutions of P3HT in CB
and P3HTT-TP-10%, P3HTT-DPP-10% in o-DCB and PCBM in CB or in o-DCB were
prepared. The solutions were stirred for 24 hrs before they were mixed at desired ratios and
stirred for an additional 24 hrs to form a homogeneous mixture. Subsequently, the
polymer:PCBM active layer was spin-coated (with a 0.45 μm PTFE syringe filter - Pall Life
Sciences) on top of the PEDOT:PSS layer. Concentrations of the polymer:PCBM solutions for
P3HT, P3HTT-TP-10% and P3HTT-DPP-10%, were 5 mg/mL in polymer. The P3HT:PCBM
(1:1) film was spin-coated from CB solution and directly placed in a vacuum chamber for Al
deposition. Upon spin-coating of polymer:PCBM solutions based on P3HTT-TP-10% (1:0.8) and
P3HTT-DPP-10% (1:1.3), films were first placed into the N
2
cabinet for 30 min and then placed
in a vacuum chamber for Al deposition. For deposition of the top electrode, substrates were
pumped down to high vacuum (< 9×10
-7
Torr) and Al (100 nm) was thermally evaporated at 3 –
4 Å/sec using a Denton Benchtop Turbo IV Coating System onto the active layer through
shadow masks to define the active area of the devices as 4.4 mm
2
. Thermal annealing of
33

P3HT:PCBM blends were done by directly placing the completed devices in a nitrogen oven for
30 min at 150 °C. After annealing, the devices were allowed to cool to room temperature before
measurements were performed.
2.4 External Quantum Efficiency Measurements: External quantum efficiency measurements
were carried out using a 300 W Xenon arc lamp (Newport Oriel), chopped at 250 Hz and filtered
through a Cornerstone 260 1/4 m double grating monochromator (Newport 74125) to give
tunable monochromatic light (10 nm FWHM). A light-bias lock-in amplifier was used for
detection while a silicon photodiode calibrated by Newport served as a reference cell.
2.5 Absolute Absorption and Internal Quantum Efficiency Measurements: Thin film spectral
absorbance measurements were performed in transflectance mode on devices consisting of
an ITO/active layer/Al structure in a Perkin-Elmer Lambda 950 UV/vis/NIR spectrophotometer
with a 150 mm diameter integrating sphere, using Al-coated glass as a reference. In the
transflectance mode both the reflection and transmission losses are measured to obtain the
fraction of absorbed photons, η
a
(λ). This way of measuring the absolute number of photons
absorbed by the active layer in the full device geometry is not perfect and more correct estimate
can be provided by using a transfer matrix formalism with the measured values of n and k.
35
The
parasitic absorption (abs
para
( )) due to absorption in the electrodes for P3HT:PCBM, P3HTT-
DPP-10%:PCBM and P3HTT-TP-10%:PCBM is calculated using transfer matrix formalism with
the n and k values reported by Li et al. Internal quantum efficiency was estimated according to
IQE(λ) = EQE(λ)/(η
a
(λ)- abs
para
( )).
36-37

2.6 Solution Doping: 30 mL solutions of P3HT, P3HTT-DPP-10% and P3HTT-TP-10% in o-
dichlorobenzene are prepared with a concentration of 0.08 mg/mL, 0.06 mg/mL and 0.10
34

mg/mL, respectively. The polymer solutions are then transferred to ten vials each containing 3
mL in volume. A stock solution of 0.13 mg/mL SbCl
5
in o-dichlorobenzene is prepared. And to
each vial aliquots of 2 L SbCl
5
solution are added to reach a doping level of 0.1 to 1 wt%
relative to the P3HT polymer solution. For P3HTT-DPP-10% and P3HTT-TP-10% polymer
solution 1.5 L and 2.3 L aliquot of SbCl
5
solution is added to dope them with 0.1 to 1 wt%
ratio. The doping of the polymer solutions were done under nitrogen atmosphere.
2.7 Film Doping: 20 ppm SbCl
5
solution in acetonitrile is prepared to dope the polymer films.
The neat polymer films were dipped in this solution for 1 min and then rinsed with acetonitrile to
rinse any excess SbCl
5
. The complete procedure was done under N
2
atmosphere to avoid any
air/water exposure.
2.8 Steady state Photoluminescence: Steady state photoluminescence measurements on neat
polymer films were performed using Horiba Nanolog Spectrofluorometer. The slit widths used
for the excitation and emission monochromator are 2 mm and 1 mm, respectively. Temperature
dependent photoluminescence spectra are measured by placing the neat polymer film in
evacuated liquid nitrogen controlled cryostat (Janis ST-100).
2.9 Femtosecond Transient Absorption: The apparatus has been described previously.
38-39
In
brief, pump and probe pulses were derived from the output of a Ti:sapphire regenerative
amplifier (Coherent Legend, 1kHz, 4 mJ, 35 fs). Excitation pulses centered at either 500 or 700
nm were generated by pumping a type-II OPA (Spectra Physics OPA-800C) with ~10 % of the
amplifier output. White light supercontinuum probe pulses, spanning the visible (400-950 nm)
and NIR (950-1250 nm) were obtained by focusing a small amount of the amplifier output into a
rotating CaF
2
disk or sapphire window, respectively. To avoid any contribution to the observed
35

dynamics from orientational relaxation,
40
the polarization of the supercontinuum probe was set
at the magic angle (54.7º) with respect to the pump polarization. The probe was collimated and
focused with a pair of off-axis parabolic mirrors into the sample whereas the pump pulse was
focused using a CaF
2
lens. The cross correlation between the pump and probe in a glass substrate
matched to that used to support polymer films had a FWHM of 180 fs averaged across the probe
spectrum for 500 nm excitation. A slightly longer instrument response of 200 fs was found for
700 nm excitation. The supercontinuum probe was dispersed using a spectrograph (Oriel
MS127I) onto either a 256-pixel silicon diode array (Hamamatsu) or InGaAs photodiode array
(Hamamatsu G9213-256S) for visible or NIR detection, respectively. The spot size of the 500
nm pump at the sample had a FWHM of 410 m while the 700 nm pump had a slightly larger
FWHM of 440 m. Transient absorption measurements were performed with a range of pump
pulse energies between 15 nJ to 200 nJ, and over this range the temporal profile was found to
scale linearly with the pump energy.
2.10 Femtosecond to Nanosecond transient absorption using 532 nm: Femtosecond transient
absorption were performed using 532 nm pump generated by pumping an optical parametric
amplifier (Spectra Physics 800 CF) with the fundamental 800 nm output from the ultrafast
amplifier (Coherent Legend, 800 nm, 35fs, 1kHz). A small portion of the fundamental 800 nm is
used to pump CaF
2
and sapphire to generate white light continuum between 320-900 nm and
850-1400 nm, respectively. The white light generated is collimated and focused using pair of
parabolic mirrors at the sample while the pump is focused using a 25 cm CaF
2
lens. The white
light after the sample is dispersed using a monochromator (Oriel MS1271) onto a 256 pixel Si
array and InGaAs array to detect between 400-900 nm and 850-1400 nm, respectively.
Nanosecond transient absorption is done using 532 nm pump generated from a Nd:YAG laser
36

(Alphalas, 532 nm, 700 ps, 1kHz) externally triggered and synchronized with the femtosecond
oscillator. The pump pulse is delayed with respect to the probe pulse using a delay generator DG
645 (Stanford research systems). The transient absorption data reported in Figure 2.13 and the
estimated polaron yield in Figure 2.16 corresponds to excitation density of 3.510
17
cm
-3
and
1.310
17
cm
-3
for P3HT:PCBM and P3HTT-DPP-10%:PCBM, respectively. The TA data were
observed to be excitation density independent for the photoexcitations between (3.5-14) 10
17
cm
-3
and (1.3 - 5.4) 10
17
cm
-3
for P3HT:PCBM and P3HTT-DPP-10%:PCBM, respectively. The
samples were translated perpendicular to the pump and probe path to avoid any photodamage.
3. Results and Discussion:
3.1. Steady state photophysical properties:
The structure and absorption spectra of the polymers P3HT, P3HTT-DPP-10% and
P3HTT-TP-10% are shown in Figure 2.1.
25-26
Unlike P3HT, whose absorption edge falls at 650
nm, the other two semi-random donor-acceptor polymers display strong, broad absorption
features in both the visible and NIR spectral regions and are thus able to absorb more solar
photons than P3HT. The absorption features for semi-random donor-acceptor polymers in the
visible (400-600 nm) and NIR (700-900 nm) is predominantly due to P3HT-like and ICT-like
chromophores, respectively.
37

a)
b)
Figure 2.1: (a) Structure of poly(3-hexylthiophene) (P3HT), poly(3-hexylthiophene-thiophene-
thienopyrazine) (P3HTT-TP-10%), poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole)
(P3HTT-DPP-10%) and [6,6]-phenyl-C
61
-butyric acid methyl ester (PCBM). (b) Absorption
spectra of neat P3HT, P3HTT-TP-10% and P3HTT-DPP-10% films.
The steady state emission of neat P3HT, P3HTT-TP-10% and P3HTT-DPP-10% films
are shown in Figure 2.2. P3HT film (Figure 2.2a) shows an emission between 600-800 nm as
reported previously in literature.
41
The emission measurements on neat P3HTT-DPP-10%
(Figure 2.2b, 2.2c) and P3HTT-TP-10% (Figure 2.2d, 2.2e) were performed following excitation
of both P3HT-like chromophore at 525 nm and ICT-like chromophore at 700 nm. No emission
was observed in the visible region corresponding to that present in P3HT, rather the only
emission observed was in near-IR centered at ~1000 nm. The intensity and the shape of this
emission were observed to be invariant of the excitation wavelength, i.e., exciting both P3HT-
38

like chromophore and ICT-like chromophore results in same emission. This indicates that under
steady state conditions the high energy absorbing P3HT-like chromophores relax to the low
energy ICT-like chromophores and finally the radiative relaxation originates from the ICT-like
chromophore.
a)
b) c)
39

d) e)
Figure 2.2: Steady state photoluminescence spectra of neat a) P3HT film, b) P3HTT-DPP-10%
film and d) P3HTT-TP-10% film following excitation at 525 nm. The discontinuity in the spectra
between 1020 and 1085 nm is to remove the second order scattering from the excitation used at
525 nm. The steady state PL spectra of neat c) P3HTT-DPP-10% film and d) P3HTT-TP-10%
film following excitation at 700 nm. These spectra are scaled by 2.5 to take account of both
excitation power and optical density at 525 nm and 700 nm.
To check the temperature dependence of this energy relaxation, temperature dependent
(78 K - 298 K) photoluminescence (PL) measurements (Figure 2.3) were performed by placing
the films inside a cryostat. This temperature dependent measurements show an increase in the PL
intensity on decreasing the temperature as some thermally activated non-radiative relaxation
pathways vanish at low temperatures. Additionally, the emission spectrum gets more
vibrationally resolved as lowering the temperature leads to a restriction in vibrational motions
(kT being dropped from 200 cm
-1
to 50 cm
-1
).
42-43
This temperature dependent PL measurements
also show an excitation wavelength independent emission spectra and intensity indicating that
the energy transfer from P3HT-like chromophore to ICT-like chromophore in P3HTT-TP-10%
and P3HTT-DPP-10% neat films are so downhill that decreasing the temperature do not affect
the energy transfer process.
40

a)
b) c)
d) e)
Figure 2.3: Temperature dependent PL spectra for neat a) P3HT film , b) P3HTT-DPP-10% film
and d) P3HTT-TP-10% film following excitation at 525 nm. c) and e) shows the temperature
dependent PL spectra after exciting neat P3HTT-DPP-10% and P3HTT-TP-10%, respectively at
700 nm.
41

3.2. Absorption Spectra and Cross-section of Polymer Polaron:
Excitation of the polymer in polymer:PCBM composites lead to generation of an exciton
followed by charge generation at the interface. This leads to the generation of a positive polaron
in the polymer (electron donor material) and negative polaron in the PCBM (electron acceptor
material). In order to investigate the spectral signature for polymer positive polaron spectra, the
polymers were oxidized with SbCl
5
as a chemical oxidative dopant with some modifications to
the reported procedure in literature.
44-45
Furthermore the polaron cross-sections were extracted
based on the known concentration of the dopant added to the polymer solution.
3.2.1 Neat Polymer film:
Neat polymer films of P3HT and P3HTT-DPP-10% were doped by dipping them into a
20 ppm solution of SbCl
5
in acetonitrile. The absorption spectra of the neat films before and after
doping are shown in Figure 2.4. Figure 2.4a shows that after doping the absorption intensity of
P3HT film dropped between 400-600 nm with a concomitant rise of broad induced absorption
band between 650-1050 nm, assigned to oxidized P3HT or positive polaron of P3HT. This
assignment is consistent with the literature assignment for P3HT positive polaron.
44
Similarly,
for P3HTT-DPP-10% film (Figure 2.4b), doping leads to reduction in absorption intensity of
both the absorption bands between 450-550 nm and 600-750 nm, with a simultaneous rise of
broad induced absorption band between 850-1200 nm. This broad absorption band is assigned to
the positive polaron of P3HTT-DPP-10%.
42

a) b)
Figure 2.4: Absorption spectra of a) P3HT and b) P3HTT-DPP-10% neat film before doping
(black) and after doping (red) with 20 ppm SbCl
5
solution. The green line denotes the difference
spectrum illustrating the spectrum of positive polaron band.
3.2.2 Polymer solution:
Polymer solutions were doped by adding low concentration of SbCl
5
to dilute polymer
solutions in o-dichlorobenzene. Figure 2.5 shows the differential steady state absorption
spectrum between the neat polymer solution and series of polymer solutions with increasing
dopant concentration of SbCl
5
(0.1 to 1 wt% of SbCl
5
relative to the polymer). Similar to the
film, doping with SbCl
5
in P3HT solution (Figure 2.5a) leads to decrease in the absorption
intensity between 350-500 nm with subsequent rise of broad induced absorption band between
700-1100 nm due to P3HT positive polaron. An additional peak at ~ 600 nm is observed for the
doped P3HT solution, which was not visible for the doped P3HT film. This additional peak at
600 nm had also been observed previously on doping P3HT solution and it is attributed to the J-
aggregate character of the exciton due to the doping-induced extended chain planarity.
46-47

Similar to P3HT, doping of P3HTT-DPP-10% solution (Figure 2.5b) and P3HTT-TP-10%
solution (Figure 2.5c) with SbCl
5
leads to decrease in the absorption intensity of both the
absorption bands between 350-500 nm and 600-700 nm. This decrease in intensity is
43

accompanied by an increase in broad induced absorption band between 750-1200 nm due to
formation of positive polarons of P3HTT-DPP-10% and P3HTT-TP-10%. The inset in Figure
2.5a, 2.5b and 2.5c shows the increase in the absorption at 850 nm, 900 nm and 900 nm for
positive polaron of P3HT, P3HTT-DPP-10% and P3HTT-TP-10%, respectively as a function of
SbCl
5
concentration (M). A linear fit to this plot provides the molar extinction coefficient of the
positive polaron and then the polaron cross-section,  is estimated using:

where  is the molar extinction coefficient and

is the Avagadro constant. The polaron cross-
section values for P3HT, P3HTT-DPP-10% and P3HTT-TP-10% were found to be 5.32
(±0.1)10
-16
cm
2
, 1.57 (±0.03)10
-15
cm
2
and 1.40 (±0.05)10
-15
, respectively.
a) b)
44

c)
Figure 2.5: The difference absorption spectra between doped and neat polymer solution of a)
P3HT, b) P3HTT-DPP-10% and c) P3HTT-TP-10%. The inset shows the determination of the
molar extinction coefficient of the polaron band from the absorption versus the molarity of the
SbCl
5
used to dope.
3.3. Femtosecond transient absorption (TA) measurements:
3.3.1. Pristine P3HT and P3HT:PCBM bulk heterojunction films:
TA spectra for neat P3HT (Figure 2.6a) and P3HT:PCBM (Figure 2.6b) films were
performed by exciting ππ* transition of P3HT at 500 nm. Photoexciting the neat P3HT film
(Figure 2.6a) at 500 nm leads to immediate appearance of a ground state bleach (400-630 nm)
due to depopulation of the P3HT ground state and a broad photoinduced absorption in the NIR.
To understand the origin of a subset of these broad NIR induced absorption features, time
resolved photoluminescence (PL) measurements were performed on neat P3HT films (Figure
2.6c) following excitation at 500 nm. The PL lifetime of the neat P3HT film was found to be
~290±20 ps. This decay overlaps well with that of the induced absorption band at 1200 nm
observed in transient absorption (TA) spectra (Figure 2.6a) of the neat P3HT film indicating that
this induced absorption feature corresponds to the singlet exciton of P3HT. The two kinetics are
overlaid in Figure 2.6c. Additional broad induced absorption bands between 700-1000 nm is
45

assigned to polymer polarons based on the spectral similarity to the positive polaron band
(Figure 2.4a) obtained from chemical doping measurements on neat P3HT film.
For P3HT:PCBM blend excitation (Figure 2.6b) at 500 nm leads to similar ground state
photobleach (400-630 nm) and broad S
1
S
n
induced absorption between 1000-1200 nm.
However, over time a delayed formation of polymer positive polarons (700-1100 nm) with
concurrent decay of S
1
S
n
induced absorption is observed with a time constant of 6.51.5 ps
(Figure 2.6d). This indicates charge transfer from photoexcited P3HT to PCBM. This
observation is consistent with results from Zhang et. al. and Guo et. al. who observed a similar
delayed rise of polarons following low energy excitation of P3HT:PCBM blend films.
48-49
Guo
et. al. explained the delayed polaron rise by suggesting that exciting the red edge of the P3HT
absorption band predominantly generates excitons in crystalline P3HT regions of the film. This
domains are not adjacent to PCBM and thus forces the excitons to first diffuse to a PCBM
interface to produce polarons.
49
Polymer polarons undergo relatively little charge recombination
in the presence of PCBM over our observation window (survival percentage of 752% after 1 ns,
Figure 2.6d), pointing towards the efficient device performance. Although an induced absorption
band for the PCBM negative polaron is expected at ~1040 nm, it is not observed in our
measurements, due to its low extinction compared to that of P3HT positive polarons.
50-53

a) b)
46

c) d)
Figure 2.6: TA spectra of neat P3HT film (a) and P3HT:PCBM blend (b) with 500 nm
excitation. The absorption spectrum of polaron (P
+
) obtained from chemical doping is shown in
black solid line and ground state absorption spectrum for the sample is shown in black broken
line. c) Comparison between time resolved PL (
exc
= 500 nm,
em
= 650 nm, black) and a
kinetic trace measured at a probe wavelength of 1200 nm from TA spectra (
exc
= 500 nm, red)
of a neat P3HT film. d) Normalized TA kinetics for S
1
S
n
and positive polaron transition shows
the evolution from S
1
state to polarons.
3.3.2 Pristine P3HTT-TP-10% and P3HTT-TP-10%:PCBM bulk heterojunction films:
Using the understanding of the control P3HT:PCBM blend, TA measurements were
performed on blends of semi-random donor-acceptor polymers by exciting either the P3HT-like
absorption feature at 500 nm or the ICT absorption band at 700 nm. Figure 2.7 shows TA spectra
of P3HTT-TP-10%:PCBM (w/w 1:0.8) films after excitation of the polymer at either 500 nm
(Figure 2.7a) or 700 nm (Figure 2.7b). In both cases, immediate formation of a ground state
bleach is observed between 400-550 nm and 650-800 nm. Identical bleaching features were also
observed for both isolated P3HTT-TP-10% chains in solution (Figure 2.8a & 2.8b) and neat
spun-cast films (Figure 2.8c & 2.8d) irrespective of the excitation wavelength used. The rapidity
(< 200 fs) with which the photobleach develop for both the ICT-like and P3HT-like absorption
bands upon excitation of either band demonstrates that regions of the polymer containing TP
units are strongly coupled to neighboring P3HT rich regions. A probable reason for this
47

observation is the ability of TP to impose quinoidal character in the polymer backbone, which
can lead to effective conjugation with P3HT monomers in the ground state.
54-56

a) b)
c)
Figure 2.7: TA spectra of P3HTT-TP-10%:PCBM following 500 nm excitation (a) and 700 nm
excitation (b). The absorption spectrum of polaron (P
+
) obtained from chemical doping is shown
in black solid line and ground state absorption spectrum for the sample is shown in black broken
line. (c) Normalized TA decay of the polaron band at 900 nm following 500 nm (green) and 700
nm (red) excitation.
48

a) b)
c) d)
Figure 2.8: TA spectra of a neat P3HTT-TP-10% in o-DCB following either (a) 400 nm or (b)
700 nm excitation. (c & d) TA spectra of neat P3HTT-TP-10% film following excitation at 500
nm or 700 nm. The absorption spectra of the sample studied is shown as a black broken line.
Following photoexcitation of P3HTT-TP-10%:PCBM at either 500 or 700 nm, a broad
NIR induced absorption (850-1100 nm) (Figure 2.7a & 2.7b) is also observed. This is assigned to
positive polymer polarons based on its spectral similarity to the red-shifted (by 25 nm) positive
polaron spectra obtained from chemical doping measurements (black solid line, Figure 2.7a &
2.7b). Unlike P3HT:PCBM, the majority of the polymer polarons we observe in P3HTT-TP-
10%:PCBM are formed within our instrumental time resolution ( 200 fs). This rapid rise of
polymer polarons may partially result from P3HTT-TP-10%’s propensity to form amorphous
films, which can facilitate the creation of highly mixed polymer PCBM blends.
57-59
Moreover,
due to the presence of the TP unit in the polymer backbone, the exciton generated may possess
49

partial charge-transfer character and thus it can facilitate rapid charge transfer to neighboring
fullerene molecules.
60-65
This prompt formation of polarons suggests that exciton diffusion is not
a major limitation for the device performance of P3HTT-TP-10%:PCBM solar cells. Following
this prompt generation, the decay of the polaron absorption band was found to be independent of
the excitation wavelength (Figure 2.7c). The polaron survival yield is however much lower
(455% after 1 ns) than that observed in P3HT:PCBM (752%), suggesting recombination as a
physical origin for the lower device efficiency of P3HTT-TP-10%:PCBM solar cells.
3.3.3 Pristine P3HTT-DPP-10% and P3HTT-DPP-10%:PCBM bulk heterojunction films:
While P3HTT-TP-10%:PCBM blends display poor photovoltaic performance, P3HTT-
DPP-10%:PCBM blends show markedly better performance relative to P3HT:PCBM. To
understand the physical origin behind this improvement, TA measurements were performed on
P3HTT-DPP-10%:PCBM (w/w 1:1.3) blends (Figure 2.9). When the ICT absorption transition is
excited at 700 nm (Figure 2.9b), the observed ground state bleach has a shape that markedly
differs from the neat polymer’s absorption spectrum (Figure 2.1), being particularly biased
toward the ICT band at 680 nm. The bias of the ground state bleach towards longer wavelength
agrees qualitatively well with absorption spectra of DPP-containing oligomers (DPP-T2)
66
and
polymers (PDPP4TP)
67
(molecular structures are shown in Figure 2.9d), which display a strong
absorption feature peaked at wavelengths ~ 600-650 nm and a weak absorption tail that extends
to bluer wavelengths (Figure 2.9c). This observation suggests that for this polymer, the ICT-like
chromophore is not strongly coupled to the P3HT-like chromophore. In contrast, following 500
nm excitation of the P3HTT-DPP-10%:PCBM, immediate bleaching of both the P3HT-like and
ICT absorption transitions are observed (Figure 2.9a). Now the amplitude of bleach in both these
features appears in a ratio similar to that of the absorption spectra of the polymer blend (dashed
50

line). Interestingly, the shape of the bleach does not change with time, which implies that no
energy relaxation (i.e., via energy transfer) occurs from high energy chromophores absorbing at
500 nm to lower energy ones absorbing at 700 nm over the ps-ns timescale. This result cannot be
associated to very weak coupling between the P3HT-like and ICT-like chromophore as otherwise
the initial bleach shape (at 250 fs) following 500 nm excitation should not show bleach in the
ICT-like absorption band and the bleach evolution of the two bands should have been quite
different. Attempts to clarify the basis for the variation in bleach signature on varying the
excitation energy is performed by TA measurements on neat P3HTT-DPP-10% in both solution
and neat films (Figure 2.10). Qualitatively, similar results were observed and so far no greater
insight into the degree of coupling of the two chromophores has been resolved.
a) b)
c) d)
Figure 2.9: TA spectra of P3HTT-DPP-10%:PCBM following 500 nm excitation (a) and 700 nm
excitation (b). The absorption spectra of polaron (P
+
) obtained from chemical doping is shown in
black solid line and ground state absorption spectra for the sample is shown in black broken line.
51

The polaron shape shown in b) is from chemical oxidation of DPP-T2. (c) Comparison between
the inverted ground state bleach of P3HTT-DPP-10%:PCBM after 700 nm excitation with the
ground state absorption spectrum of DPP-T2 oligomer (from ref 66) and PDPP4TP polymer
(from ref 67). (d) The structures of DPP-T2 oligomer and PDPP4TP polymer from ref 66 and
67, respectively.
a) b)
c) d)
Figure 2.10: TA spectra of a neat P3HTT-DPP-10% in o-DCB following either (a) 400 nm or
(b) 700 nm excitation. (c & d) TA spectra of neat P3HTT-DPP-10% film following excitation at
500 nm or 700 nm. The absorption spectra of the sample studied is shown as a black broken line.
Similar to P3HTT-TP-10%:PCBM, a broad NIR band (800-1100 nm) for polarons is
generated promptly for both excitation wavelengths. The broad near-IR induced absorption
observed for 500 nm excitation of P3HTT-DPP-10%:PCBM (Figure 2.9a) is assigned to the
polymer positive polaron based on the spectral similarity to the polymer positive polaron spectra
(black solid line) obtained from chemical doping measurements (Figure 2.4b). However, unlike
P3HT:PCBM blends, we do not observe a delayed rise of this band due to the migration of
52

excitons from crystalline regions of the polymer to fullerene rich regions of the film. This is
probably because the average crystallite size in P3HTT-DPP-10%:PCBM films is roughly half of
that present in P3HT:PCBM blends and so the excitons need to migrate a smaller distance to
reach an interface in P3HTT-DPP-10%:PCBM.
25
In contrast, polarons generated upon exciting
the same polymer blend at 700 nm (Figure 2.9b) appear spectrally similar to those created by
chemical oxidation of DPP oligomers in solution (black solid line).
66
Specifically, the polaron
peak at 880 nm and valley at 960 nm overlay well with the absorption spectra of polarons
generated after chemical oxidation of DPP oligomers in solution.
66
However the shape of the
second polaron absorption feature peaked at 1050 nm for chemically doped DPP oligomers
differs in amplitude from that observed for the P3HTT-DPP-10%:PCBM blend upon 700 nm
excitation due to overlap with a S
1
excited state absorption band peaked near 1200 nm. Similar to
P3HTT-TP-10%:PCBM blends, each of these features assigned to polarons, arise within our
experimental time resolution. This rapid polaron generation is probably assisted by both the
partial semi-crystalline morphology of the blend and the charge transfer character of the exciton
generated by excitation of the ICT absorption band at 700 nm.
60-65
Both 500 nm and 700 nm
excitation of P3HTT-DPP-10%:PCBM show prompt polaron generation illustrating that exciton
diffusion does not play a significant role in this blend's device performance. Examining the yield
of polarons that survive recombination shows that after 1 ns survival probability is only 452%
after 700 nm excitation compared to 655% following 500 nm excitation (Figure 2.11).
53

Figure 2.11: Normalized TA decay of the polaron band at 870 nm following 500 nm (green) and
700 nm (red) excitation.
3.3.4 Excitation density dependent TA measurements of polymer:PCBM blends
TA measurements were performed on all three polymer:PCBM blends using a range of
excitation densities to evaluate the role of exciton-exciton annihilation in the TA dynamics.
Figure 2.12 plots the dynamics of the polaron photoinduced absorption for different
polymer:PCBM blends for a series of pump fluences. This range of excitation fluences at 500 nm
corresponds to an excitation density between (0.8 – 3.2)10
18
cm
-3
for P3HT:PCBM, (1.4 –
5.7)10
18
cm
-3
for P3HTT-DPP-10%:PCBM and (1.3 – 5.3)10
18
cm
-3
for P3HTT-TP-
10%:PCBM. For 700 nm excitation, TA data was measured for excitation densities between (2 –
8.3)10
18
cm
-3
for P3HTT-DPP-10%:PCBM and (0.87 – 3.5)10
18
cm
-3
for P3HTT-TP-
10%:PCBM. The polaron temporal profile was invariant and the overall signal level scaled
linearly with the excitation density suggesting little contribution from exciton-exciton
annihilation over the investigated range of excitation densities. Additionally excitation density
independent polaron recombination dynamics illustrate geminate recombination of the polarons.
54

a)
b) c)
d) e)
Figure 2.12: Normalized TA decay of the photoinduced polaron absorption band for (A)
P3HT:PCBM, (B) P3HTT-TP-10%:PCBM and (D) P3HTT-DPP-10%:PCBM as a function of
excitation density following 500 nm excitation. Corresponding traces for P3HTT-TP-
10%:PCBM and P3HTT-DPP-10%:PCBM following 700 nm excitation are shown in (C) & (E),
respectively. For P3HT:PCBM and P3HTT-DPP-10%:PCBM the polaron band dynamics is
shown at
probe
= 870 nm and for P3HTT-TP-10%:PCBM blend the polaron band dynamics is
shown at
probe
= 900 nm.
55

3.4 Nanosecond TA measurements:
Transient absorption measurements on P3HT:PCBM (Figure 2.13a) and P3HTT-DPP-
10%:PCBM (Figure 2.13b) were performed from 200 fs to 500 ns time range following
excitation at 532 nm. Similar to the above discussed fs TA on P3HT:PCBM (Figure 2.6b), an
immediate appearance of ground state bleach between 450-600 nm and broad S
1
S
n
induced
absorption between 1000-1200 nm is observed.
68
With time this S
1
S
n
induced absorption
decreases with concurrent rise in positive polaron band (700-1100 nm) with a time constant of
7±1.5 ps. The spectral shape of this new band is similar to the positive polaron band (blue line)
obtained from chemical doping measurements on neat P3HT film. At longer times (> 50 ps) the
positive polaron band and ground state bleach were observed to relax (Figure 2.13a bottom) to
the ground state due to recombination of the polarons. The polaron dynamics was found to be
invariant of the excitation density for the entire time range (upto 500 ns) suggesting geminate
recombination between the polarons.
TA measurements on P3HTT-DPP-10%:PCBM following excitation at 532 nm (Figure
2.13b) shows immediate appearance of ground state bleach for both P3HT-like chromophore
(400-550 nm) and ICT-like chromophore (600-800 nm). Similar to the previously discussed
results the bleach shape was not observed to change with time suggesting no energy relaxation
from higher energy P3HT-like chromophore to lower energy ICT-like chromophore over ps-ns
timescale.
68
Following photo-excitation at 532 nm, a broad near-IR induced absorption is
observed between 800-1100 nm due to the polymer positive polaron (blue line). Compared to the
delayed polaron generation in P3HT:PCBM, the polaron generation in this blend was observed to
be instantaneous (200 fs). The faster polaron generation rate in P3HTT-DPP-10%:PCBM is due
56

to reduced crystallite size for P3HTT-DPP-10%:PCBM than that present in P3HT:PCBM
25
and
thus the exciton have to migrate less distance to reach the interface to produce charges in
P3HTT-DPP-10%:PCBM. At longer times (Figure 2.13b bottom), the polarons generated
undergo excitation intensity independent recombination dynamics indicating geminate
recombination between the polarons.
a) b)
Figure 2.13: Femtosecond to nanosecond TA of a) P3HT:PCBM and b) P3HTT-DPP-
10%:PCBM film following excitation at 532 nm. The top panel shows the comparison between
the spectral shape of polaron (P
+
) obtained from TA and chemical doping taken from Figure 2.4.
3.5. Polaron Geminate Recombination Dynamics
The recombination dynamics of polarons generated in the polymer:PCBM blends we
have investigated do not follow first order decay kinetics. This is in part due to the fact that
polarons can undergo two forms of geminate recombination following charge transfer to PCBM.
Polymer polarons that are created in intimate contact with the neighboring fullerene layer
experience a strong Coulomb attraction, leading some to never diffuse away from the interface
57

and decay by recombining. While other polaron pairs overcome their Coulomb attraction and
move away from the interface. Of this latter population, some totally escape but others can
diffuse back to the interface and have a second chance at recombination. Although the dynamics
governing the directly recombining fraction can be extracted from an exponential fit, to describe
the latter case where diffusion plays a role we need a function that can incorporate the diffusion
of polarons in the presence of Coulomb attraction. A physical model describing this behavior can
be obtained by solving the Debye-Smoluchowski equation,
69

(1)
where polarons, whose probability density is given by (r,t), can undergo diffusive
recombination under the influence of a Coulomb potential imposed by the charge of the diffusing
particles,

. In our experiment, we record
the survival probability of the polaron population as a function of time. This is simply,

where

is the encounter distance. Under the assumption that both polaron recombination and
separation can be described by exponential reaction rates,
70
the extensively used
32, 36
Onsager-
Braun model is the long time limit
71-72
of equation (2) in the presence of

where

is an external electric field. Here, in order to (i) take explicitly into account the
effect of the diffusion in the separation process and (ii) obtain a time-dependent solution, we use
a numerical solver to find full solutions to equation (1) and (2). Since the TA experiments were
58

performed in the absence of an external electric field, the numerical solution of equation (1) and
(2) was calculated solely with the mutual Coulomb potential.
The physical picture envisioned here is that following charge transfer in the BHJ there
will be a distribution of separation lengths between the positive and negative polarons. The
breadth of the initial distribution of charge separation lengths will depend on the ability of the
parent exciton to overcome the dominant Coulomb interaction via a delocalized charge transfer
state. Therefore, a higher energetic driving force of the parent exciton will result in a broader
distribution of charge separation lengths that will, in turn, help in reducing the extent of charge
recombination.
73-74
In addition, the morphology of the material at the interface plays a critical
role as a highly intermixed blend can lead to lower degree of phase separation and thus can lead
to low charge separation lengths.
75-77

The polarons generated can diffuse by incoherent hole/electron hopping in three-
dimensional space under the influence of a Coulomb potential. Once the two charges reach a
critical encounter distance they are assumed to recombine. The diffusion of the polarons is
controlled by the effective diffusion coefficient, D, obtained from mobility values for holes
moving through the polymers and electrons moving through PCBM using the relation D

. The encounter distance used in the fit was 3.5 Å, which is the distance of closest
approach calculated between a P3HT polymer chain and a PCBM molecule using DFT.
78
The
same encounter distance was used for the semi-random polymer:PCBM blends as most of the
polymer chain consists of 3-hexylthiophene monomers. The boundary condition at the encounter
distance, r
m
is defined by a distance dependent electron transfer rate,
79

xp β

(3)
59

where k
e
is the rate of electron transfer at r
m
and β is inversely proportional to degree of overlap
with values between 0.5 to 1.2 Å
-1
.
79
The exponential factor denotes the decrease in electronic
coupling between the positive polymer polaron and the negative PCBM polaron with increase in
their separation. The calculations shown here are done with β value of 0.5 Å
-1
. The solutions of
Eq. (1) and (2) were calculated to obtain the survival probability of polarons with a numerical
differential equation solver.
80
The initial distribution of polaron separations was assumed to
follow a Gaussian distribution centered at R = 0 Å but with different characteristic widths, L
= for different polymer:PCBM blends. The recombination kinetics of polarons for each
polymer:PCBM blend were fit using a fixed diffusion constant D = 3×10
-5
cm
2
/s as the hole
mobility of all three polymers is similar (
h
= 2×10
-4
cm
2
/Vs)
25-26
and 
e
= 10
-3
cm
2
/Vs for
PCBM.
81
These calculations were carried out assuming that the electrons and holes can diffuse
equivalently in three dimensions. Thus, the isotropic diffusion equations we use ignore any
impedance imposed on the diffusion of electrons and holes due to local variations in morphology
of the BHJ blend. In Figure 2.14, experimental polaron population decay kinetics are compared
to our best-matching model fits of survival probability . Since the polymer polaron band
has a finite rise time for P3HT:PCBM, an exponential rise of ~ 6.5 ps was convoluted with the
survival probability obtained from solving Eq. (2) to fit the overall polaron kinetics. The spatial
distribution of the surviving pairs of polarons for different polymer blends in presence of the
Coulomb potential corresponding to a best-fit is shown in Figure 2.15.
60

Figure 2.14: Normalized TA decay of polarons for different polymer:PCBM blends along with
the survival probability obtained from the numerical solution of diffusive geminate
recombination (black line) under a Coulomb potential. The fits are extrapolated to 500 ns for
certain polymer:PCBM blends with TA data available only upto 1ns.
a) b)
c) d)
61

Figure 2.15: Spatial distribution of the survival probability density of polarons for (a)
P3HT:PCBM (
exc
= 500 nm), (b) P3HTT-DPP-10%:PCBM (
exc
= 500 nm), (c) P3HTT-DPP-
10%:PCBM (
exc
= 700 nm) and (d) P3HTT-TP-10%:PCBM (for both
exc
= 500 nm and
exc
=
700 nm) at different times following charge separation. The Coulomb potential corresponding to
an Onsager radius of r
C
= 50Å is shown in black dotted line.
Reasonable agreement with the data for all polymer:PCBM blends under different
excitations can be obtained using an Onsager radius, r
C
of 50Å (~11) with

=410
14
s
-1
at the
polymer:PCBM interface. Interestingly, the initial width is found to be similar, L~ 48 ± 3Å for
P3HT:PCBM and L~ 45 ± 2Å for P3HTT-DPP-10%:PCBM following excitation of P3HT-like
chromophore at 500 nm/532 nm. This suggests that the initial charge separation length in the
charge transfer state generated following excitation of P3HT-like chromophore is similar for
these two blends. This is because the energetic driving force for the parent exciton in these two
blends is similar (~950 meV) and no energy relaxation is observed for P3HTT-DPP-10%:PCBM.
In contrast, for P3HTT-DPP-10%:PCBM following 700 nm excitation and P3HTT-TP-
10%:PCBM under both 500 nm and 700 nm excitations, a narrower Gaussian distribution with L
~ 312 Å was needed to fit the geminate recombination of polymer polarons. Since the diffusion
coefficient used was identical for all of these blends and was fixed to match bulk mobilities, our
modeling shows that a narrower spatial distribution of charges following charge transfer to
PCBM indeed results in enhanced geminate recombination. The short charge separation length
for P3HTT-TP-10%:PCBM blends can perhaps be attributed to two reasons: (1) a low energetic
driving force for charge separation due to a small LUMO
PCBM
- LUMO
P3HTT-TP-10%
gap (~300
meV vs. ~950 meV in P3HT:PCBM)
26, 82
and (2) a high intermixing of the polymer with
fullerene in amorphous P3HTT-TP-10%:PCBM films leading to smaller length-scale separation
between the two phases. However, for P3HTT-DPP-10%:PCBM films, which have been
demonstrated to adopt a semi-crystalline morphology, the shorter charge separation length we
62

measure following 700 nm excitation likely arises due to a relatively low charge transfer driving
force (~500 meV) compared to that of P3HT-rich chromophores that absorb closer to 500/532
nm (~950 meV). Our conclusions regarding the importance of charge separation distance in
determining the polaron lifetime are qualitatively similar to the work reported by Bakulin et. al.,
which suggested that a large charge separation length is crucial to prevent charge
recombination.
30

3.6. Absolute polaron yield
To further quantify the number of polarons contributing towards photocurrent, absolute
polaron yield was calculated. At first, the number of polarons, N
polarons
generated in each
polymer:PCBM blend can be evaluated from the TA signal using,

(3)
where ∆A(λ
probe
, t) is the TA signal at the probe wavelength corresponding to the absorption of
positive polymer polaron and

,

are the absorption cross-section of positive
polymer polaron and PCBM

. The values for

are reported in section 2.2. The

is obtained from literature, 1.6810
-17
cm
2
at 900 nm.
52
And from the estimated number of
polarons and photoexcitations, the polaron yield can be determined by,

(4)
where, the number of photoexcitations is calculated using the excitation intensity and fraction of
absorption of each polymer:PCBM blend. The time resolved polaron yield for the three blends
are shown in Figure 2.16 except for P3HTT-DPP-10%:PCBM following excitation of ICT-like
63

chromophore as the spectral shape for this polarons corresponds to DPP oligomers with
unknown absorption cross-section. As expected from the TA data, the instantaneous polaron
yield is higher for donor-acceptor copolymer:PCBM compared to P3HT:PCBM. However, the
polaron yield for P3HTT-TP-10%:PCBM is almost a factor of two lower than that present in
P3HTT-DPP-10%:PCBM. This explains the poor device performance of P3HTT-TP-
10%:PCBM. Once the polaron generation is complete , i.e., after 10 ps the polaron yield for both
P3HT:PCBM and P3HTT-DPP-10%:PCBM blends are similar. This indicates that the amount of
polarons contributing towards photocurrent is similar for these two polymer:PCBM blends
following excitation of P3HT-like chromophore. However, the total polaron yield for P3HTT-
DPP-10%:PCBM will also depend on the polarons generated from excitation of ICT-like
chromophore. And TA measurements (Figure 2.11) reveal that there are substantial photo-
charges from ICT-like excitation which leads to better device performance compared to
P3HT:PCBM.

Figure 2.16: Time resolved absolute polaron yield after photoexcitation of P3HT:PCBM and
P3HTT-DPP-10%:PCBM at 532 nm. The polaron absorption band at 850 nm and 900 nm is used
for the absolute polaron yield of P3HT:PCBM and P3HTT-DPP-10%:PCBM, respectively.
64

3.7. Device Measurements:
3.7.1 External quantum efficiency and absolute absorption measurements:
External quantum efficiency (EQE) and absolute absorption measurements were
performed on thin polymer:PCBM films with the same thicknesses as those used for TA
experiments on fully functional devices. EQE is the ratio of the number of charge carriers
collected by a device to the number of photons incident at a given energy. EQE data (Figure
2.17a) shows that P3HT:PCBM device is unable to generate charges beyond 650 nm as photons
at higher energy cannot be absorbed by P3HT. Although both devices made out of the semi-
random donor-acceptor polymer:PCBM, P3HTT-TP-10%:PCBM and P3HTT-DPP-10%:PCBM
show charge generation extended into near-IR, the amount of charges generated is substantially
different. P3HTT-TP-10%:PCBM device shows poor charge generation than even that present in
P3HT:PCBM whereas P3HTT-DPP-10%:PCBM is able to generate charges in substantial
amount in both visible and near-IR region.
Thin film spectral absorbance measurements were performed in transflectance mode on
devices consisting of an ITO/active layer/Al structure in an integrating sphere, using Al-coated
glass as a reference. In the transflectance mode both the reflection and transmission losses are
measured to obtain the fraction of absorbed photons (Figure 2.17b). This measured absorption
profiles although considers the reflection losses but the parasitic absorptions in non active layers
ITO and Al is not considered.
37, 83
These parasitic absorptions can have strong wavelength
dependent properties as shown in Figure 2.17c. These are calculated using transfer matrix
formalism
84
with the reported n and k values for ITO, Al and active layer: P3HT:PCBM,
P3HTT-DPP-10%:PCBM and P3HTT-TP-10%:PCBM.
36
Finally the absolute absorption is
65

calculated by subtracting this parasitic absorption from absolute absorption measured (Figure
2.17d). This method of calculating absolute absorption takes account all important factors like
scattering, surface roughness and parasitic absorptions.
a) b)
c) d)
Figure 2.17: (a) External quantum efficiency and (b) absorption in integrating sphere
measurements of blended films of the polymers, P3HT (blue), P3HTT-DPP-10 % (green) and
P3HTT-TP-10% (red) with PCBM. c) The calculated parasitic absorption from transfer
matrix using n and k values for each layer. d) The absolute absorption obtained after
subtracting parasitic absorption from measured absorption. The thickness of the active layer
is identical to that used for TA measurements.

66

3.7.2 Internal quantum efficiency (IQE) measurements:
To understand the wavelength dependent performance of these semi-random donor-
acceptor P3HT analogs, IQE measurements were performed on fully functional devices. The IQE
of a solar cell is the probability of collecting a charge per photon absorbed by the active layer as
a function of photon energy. For an ideal solar cell without any internal loss pathways, the IQE
should be 100% for all wavelengths absorbed by the active layer. IQE is estimated using IQE(λ)
= EQE(λ )/η
a
(λ), where η
a
(λ) is the fraction of absorbed photons . IQE was determined using two
different absorption values: 1) measured absorption (Figure 2.17b) for the whole device in
integrating sphere and 2) absolute absorption (Figure 2.17d) values calculated after subtracting
parasitic absorption from measured values. Figure 2.18 top displays IQE curves for all the three
polymer blends with measured absorption values from Figure 2.15b. The IQE curves (Figure
2.18 top) indicate that P3HT:PCBM performs proficiently with an IQE of 80% in the spectral
range below 650 nm where the polymer readily absorbs. In contrast, while the broadened
absorption spectrum of P3HTT-TP-10%:PCBM allows the blend to absorb out into the NIR, it
displays an IQE of only ~50% independent of wavelength. Interestingly, the IQE curve for a
P3HTT-DPP-10%:PCBM thin film displays a wavelength dependent IQE with values of 70%
and 55% at 500 nm and 700 nm, respectively. The IQE calculated (Figure 2.18 bottom) using
calculated absolute absorption shows a similar trend with higher values for both P3HT:PCBM
and P3HTT-DPP-10%:PCBM in visible region compared to the device made of P3HTT-TP-
10%:PCBM. Additionally, P3HTT-DPP-10%:PCBM also shows a wavelength dependent IQE
with comparatively lower values in near-IR than in visible. The IQE calculated using measured
absorption are higher because parasitic absorption are not taken into account in the measured
absorption values. Nevertheless, the IQE values clearly show that the excitation wavelength
67

dependent TA measurements is also reflected in the device measurements for P3HTT-DPP-
10%:PCBM. Additionally, wavelength independent IQE for P3HTT-TP-10%:PCBM also
corroborates the TA measurements. This shows that the recombination dynamics observed in TA
measurements on the active layer translates into their device performance.

Figure 2.18: IQE spectra for BHJ composites of P3HT (blue), P3HTT-DPP-10% (green) and
P3HTT-TP-10% (red) with PCBM calculated using (top) measured absorption from figure 2.17b
and (bottom) calculated absorption from figure 2.17d.
4. Conclusion:
In conclusion, we have demonstrated that unlike P3HT:PCBM, these semi-random donor
acceptor polymer:PCBM blends exhibit rapid polaron generation ( 200 fs) that is suggestive of
reduced loss due to exciton diffusion to a polymer:fullerene interface. These results qualitatively
agree with the calculated exciton diffusion efficiencies by Li et al. on the same semi-random
donor acceptor polymers.
36
The polaron survival probability obtained from TA measurements
68

shows that compared to P3HT:PCBM and P3HTT-DPP-10%:PCBM solar cells, the P3HTT-TP-
10%:PCBM solar cells undergo higher geminate recombination losses. This high geminate
recombination loss for P3HTT-TP-10%:PCBM is in good agreement with the low charge
dissociation efficiency calculated by Li et. al.
36
However, in contrast to the single excitation
wavelength result reported by Li et. al. on P3HTT-DPP-10%:PCBM, we observe an excitation
wavelength dependent geminate recombination, with comparatively higher geminate
recombination losses in the NIR than in the visible. The physical reason for high geminate
recombination was understood from the fitting of the polaron survival probability for different
polymer:PCBM blends from TA measurements with a Debye-Smoluchowski model. This
modeling shows that geminate recombination losses can be minimized by achieving a large
charge separation length at the polymer:fullerene interface. In addition, the absolute polaron
yield derived illustrate that lower amount of polarons generated is the reason behind poor device
performance of P3HTT-TP-10%:PCBM compared to P3HT:PCBM. The polaron yield calculated
shows quantitatively similar number of polarons are contributing towards the device
performance for P3HT-like chromophore for both P3HT:PCBM and P3HTT-DPP-10%:PCBM
blends. Lastly, we illustrated that trends in the IQE measurements can be qualitatively correlated
with the wavelength-dependent survival probability density of polarons obtained from time-
resolved TA spectroscopy. This illustrates that both the morphology of the composite at the
interface and the donor-acceptor interactions present in the polymer can influence the charge
separation length and hence charge recombination. Through judicious choice of acceptor
monomer identity, content, and distribution in the semi-random polymer and attention to the
influence on semicrystallinity, it is likely that geminate recombination can be further reduced,
enhancing the performance of this already promising class of conjugated polymers. In future the
69

estimated polaron yield can be directly related to the device photocurrent efficiency by
measuring the transient absorption in a full device geometry, which will lead to valuable
comparison between the spectroscopic and device measurements.

70

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77

Chapter 3
Influence of strong donor in Polaron dynamics of semi-random P3HT:PCBM
composites
1. Introduction:
Organic photovoltaics (OPVs) is an emerging technology due to several unique
properties such as light-weight, flexibility, low productivity and manufacturing costs.
1-4
For the
last several years, polymer solar cells have been the major contender in OPVs with power
conversion efficiency reaching ~9% using a single bulk heterojunction device structure.
5-8

Typically polymer solar cells consist of a conjugated polymer as the electron donor material and
a fullerene derivative as the electron acceptor material. In recent times, an alternating donor-
acceptor approach is utilized to extend the polymer absorption in the near infrared, however this
often leads to a decrease in visible absorption by the polymer as alternative donor-acceptor
approach shifts the whole spectrum to the near infrared.
4, 9-10
Recent advances to extend the near-
IR absorption of the conjugated polymers with retention of visible absorption has been
performed by utilizing a semi-random donor acceptor approach on poly-(3-hexylthiophene)
(P3HT) backbone.
4, 11-13

Semi-random donor acceptor analogues of P3HT consist mainly of 3-hexylthiophene
monomers with ~10% incorporation of different acceptor units to extend the near-IR absorption.
This small change to the P3HT polymer backbone have an immense influence on the absorption
properties with absorption onset being pushed upto 1000 nm but with retention of absorption in
the visible.
11, 13
Typically these semi-random donor-acceptor copolymers consist of at least two
78

types of chromophores: 1) P3HT-like chromophore giving rise to ππ* transition between 400-
600 nm and 2) intramolecular-charge transfer (ICT) like chromophore with absorption between
700-900 nm. ICT-like chromophore consist of 3-hexylthiophene monomers covalently linked
with at least one acceptor monomer unit in the polymer backbone. Despite the broadband
absorption exhibited by these semi-random donor-acceptor P3HT analogs, the device
performance for all semi-random polymers are not similar. For example, incorporation of
thienopyrazine (TP) acceptor unit leads to short circuit current density, J
SC
of ~3.2 mA/cm
2
and
photocurrent efficiency (PCE) of ~0.7% for P3HTT-TP:PCBM whereas insertion of
diketopyrrolopyrrole (DPP) acceptor unit leads to high J
SC
~15 mA/cm
2
and PCE ~6% in
P3HTT-DPP-10%:PCBM.
11, 13
This difference in device performance with ~10% variation in the
acceptor monomer content is attributed to the difference in polaron recombination dynamics as a
result of localized charge transfer state formation in P3HTT-TP:PCBM compared to P3HTT-
DPP-10%:PCBM.
14
For simplicity the polymer P3HTT-DPP-10% will be referred in this chapter
as P3HTT-DPP.
As mentioned above, the incorporation of various acceptors in semi-random P3HT
analogs lead to substantial donor-acceptor (D/A) interaction. It is interesting to see how
incorporation of strong donor monomer units influence the D/A effect. A strong donor monomer
unit, dithienopyrroles (DTP) is used to replace 10% of 3-hexylthiophene monomer content to
generate two different donor-acceptor semi-random analogs, P3HTT-TP-DTP and P3HTT-DPP-
DTP.
15
Surprisingly, this incorporation did not seem to influence the band gap compared to their
parent D/A copolymers, P3HTT-TP and P3HTT-DPP. However, the device performance for
P3HTT-TP-DTP:PCBM (PCE~0.2%) was observed to be lower compared to P3HTT-TP:PCBM
(PCE~0.7%) mainly due to decrease in J
SC
.
11, 15
And although P3HTT-DPP-DTP:PCBM had a
79

better device performance (PCE ~3%), compared to P3HTT-DPP:PCBM the device performance
is lower mainly due to decrease in J
SC
.
13, 15
This decrease in J
SC
on incorporation of strong donor
monomer unit DTP suggests that the exciton generated are either not able to undergo charge
transfer or/and are lost due to charge recombination. Transient absorption measurements on these
donor-acceptor polymer:PCBM blends were studied to understand the influence of strong donor
monomer DTP by exciting both P3HT-like chromophore and ICT-like chromophore.
Additionally, the amount of charges generated are quantified using polaron cross-section to
correlate with their device performance.
2. Experimental Section:
2.1 Synthetic Procedures: Procedures for the synthesis of poly(3-hexylthiophene) (P3HT),
poly(3-hexylthiophene-thiophene-thienopyrazine-dithienopyrrole) (P3HTT-TP-DTP) and
poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole-dithienopyrrole) (P3HTT-DPP-DTP)
were used without modification as developed and reported earlier.
15

2.2 Preparation of Thin Films for Femtosecond Transient Absorption: Solutions were spin-coated
onto pre-cleaned glass slides from 10 mg/mL o-dichlorobenzene (o-DCB) solutions for P3HTT-
TP-DTP and P3HTT-DPP-DTP films. Polymer:PCBM blends at optimal ratios were spun from
o-DCB for P3HTT-TP-DTP:PCBM (w/w 1:1) and P3HTT-DPP-DTP:PCBM (w/w 1:1) at 10
mg/mL polymer concentration. P3HTT-TP-DTP, P3HTT-DPP-DTP, P3HTT-TP-DTP:PCBM
and P3HTT-DPP-DTP:PCBM films were placed in a N
2
cabinet for 20 min. The processing
conditions for neat polymers and polymer:PCBM blends were chosen to match the optimal
processing conditions of the corresponding solar cells at which the highest power conversion
80

efficiencies were obtained. The thickness of each film was chosen such that the peak optical
density of the polymer was near 0.2.
2.3 Solution Doping:
30 mL solutions of P3HTT-DPP-DTP and P3HTT-TP-DTP in o-dichlorobenzene are
prepared with a concentration of 0.08 mg/mL and 0.07 mg/mL, respectively. The polymer
solutions are then transferred to ten vials each containing 3 mL in volume. A stock solution of
0.13 mg/mL SbCl
5
in o-dichlorobenzene is prepared. And to each vial aliquots of 1.8 L and 1.6
L of SbCl
5
solution are added to reach a doping level of 0.1 to 1 wt% relative to the P3HTT-
DPP-DTP and P3HTT-TP-DTP polymer solution. The doping of the polymer solutions were
done under nitrogen atmosphere.
2.4 Femtosecond Transient Absorption: The apparatus has been described previously.
16-17
In
brief, pump and probe pulses were derived from the output of a Ti:sapphire regenerative
amplifier (Coherent Legend, 1kHz, 4 mJ, 35 fs). Excitation pulses centered at either 500 or 700
nm were generated by pumping a type-II OPA (Spectra Physics OPA-800C) with ~10 % of the
amplifier output. White light supercontinuum probe pulses, spanning the visible (400-950 nm)
and NIR (950-1250 nm) were obtained by focusing a small amount of the amplifier output into a
rotating CaF
2
disk or sapphire window, respectively. To avoid any contribution to the observed
dynamics from orientational relaxation,
18
the polarization of the supercontinuum probe was set
at the magic angle (54.7º) with respect to the pump polarization. The probe was collimated and
focused with a pair of off-axis parabolic mirrors into the sample whereas the pump pulse was
focused using a CaF
2
lens. The cross correlation between the pump and probe in a glass substrate
matched to that used to support polymer films had a FWHM of 165 fs averaged across the probe
81

spectrum for 500 nm excitation. A slightly longer instrument response of 200 fs was found for
700 nm excitation. The supercontinuum probe was dispersed using a spectrograph (Oriel
MS127I) onto either a 256-pixel silicon diode array (Hamamatsu) or InGaAs photodiode array
(Hamamatsu G9213-256S) for visible or NIR detection, respectively. The spot size of the 500
nm pump at the sample had a FWHM of 400 m while the 700 nm pump had a slightly larger
FWHM of 450 m. Transient absorption measurements were performed with a range of pump
pulse energies between 25 nJ to 400 nJ, and over this range the temporal profile was found to
scale linearly with the pump energy.
3. Results and Discussion:
3.1. Steady state photophysical properties:
The structure and absorption spectra of the donor-acceptor semi-random polymers
studied here, P3HTT-TP-DTP and P3HTT-DPP-DTP are shown in Figure 3.1. For comparison,
the structure and absorption spectra for the corresponding donor-acceptor semi-random polymers
in absence of the strong donor DTP monomer, P3HTT-TP and P3HTT-DPP are also included.
The absorption features in the visible (400-600 nm) and near-IR (700-900 nm) can be attributed
to P3HT-like transition and ICT-like transition, respectively. It is evident from the absorption
spectra that incorporation of strong donor DTP monomer does not seem to influence the band
gap for either of the semi-random copolymers. However, the absorption spectra for P3HTT-TP-
DTP shows a red shift (25 nm) for P3HT-like transition compared to P3HTT-TP due to the
increase in E
HOMO
on incorporation of DTP monomer.
15
The absorption spectra for P3HTT-DPP-
DTP shows increased absorption in the visible region compared to P3HTT-DPP with similar
absorption profile for the ICT-like transition. This shows that replacing small (~10%) portion of
82

monomer units in the polymer backbone induces minor changes in polymer absorption spectrum
compared to the huge change in the device efficiency. Thus, linear absorption spectra alone
cannot explain low device performance compared to parent D/A copolymers.
a)
b)
Figure 3.1: Structure (a) and absorption spectra (b) of P3HTT-TP, P3HTT-TP-DTP, P3HTT-
DPP and P3HTT-DPP-DTP polymers.
3.2. Absorption Spectra and Cross-section of Polymer Polaron:
During OPV mechanism following excitation, an exciton undergoes charge transfer at
donor-acceptor interface to generate polarons. To spectrally assign the polaron absorption spectra
and to further quantify the polaron yield chemical doping the polymers were oxidized with SbCl
5

as a chemical oxidative dopant with some modifications to the reported procedure in literature.
19-
20
Polymer solutions were doped by adding low concentration of SbCl
5
to dilute polymer
83

solutions in o-dichlorobenzene. Figure 3.2 shows the differential steady state absorption
spectrum between the neat polymer solution and series of polymer solutions with increasing
dopant concentration of SbCl
5
(0.1 to 1 wt% of SbCl
5
relative to the polymer). Chemical doping
of P3HTT-TP-DTP solution (Figure 3.2a) and P3HTT-DPP-DTP solution (Figure 3.2b) with
SbCl
5
leads to decrease in the absorption intensity of both the absorption bands between 350-500
nm and 600-700 nm. This decrease in intensity is accompanied by an increase in broad induced
absorption band between 750-1200 nm due to formation of positive polarons of P3HTT-DPP-
DTP and P3HTT-TP-DTP. The inset in Figure 3.2a and 3.2b shows the increase in the absorption
at 900 nm for positive polaron of P3HTT-TP-DTP and P3HTT-DPP-DTP, respectively as a
function of SbCl
5
concentration (M). A linear fit to this plot provides the molar extinction
coefficient of the positive polaron and then the polaron cross-section,  is estimated using:

where  is the molar extinction coefficient and

is the Avagadro constant. The polaron cross-
section values for P3HTT-DPP-DTP and P3HTT-TP-DTP were found to be 4.34 (±0.5)10
-16

cm
2
and 9.23 (±0.2)10
-16
, respectively.
a) b)
84

Figure 3.2: The absorption spectra of positive polymer polaron for a) P3HTT-TP-DTP and
b)P3HTT-DPP-DTP obtained from the difference spectrum between neat polymer and polymer
doped with aliquots of SbCl
5
. The inset show the variation in absorption at 900 nm versus the
molarity of SbCl
5
solution added.
3.3 Femtosecond transient absorption:
3.3.1 Pristine P3HTT-TP-DTP and P3HTT-TP-DTP:PCBM films:
To monitor the polaron dynamics transient absorption (TA) measurements on both
polymer:PCBM blends were performed by exciting P3HT-like transition at 500 nm and ICT-like
transition at 700 nm. Figure 3.3 shows the TA spectra of P3HTT-TP-DTP:PCBM following
excitation at 500 nm (Figure 3.3a) and 700 nm (Figure 3.3c). Both excitations lead to immediate
(200 fs) formation of ground state bleach for both P3HT-like transition (400-600 nm) and ICT-
like transition (700-900 nm). Similar observations were obtained for neat P3HTT-TP-DTP
polymer films (Figure 3.3b and 3.3d). This fast rate of formation of ground state bleach for either
excitations indicates that the two types of chromophores are strongly interacting due to the
quinoidal character imposed by TP unit on the polymer backbone.
21-23
Comparable observations
were acquired previously on P3HTT-TP:PCBM
14
indicating that the interaction between the
P3HT-rich region with the region containing TP unit in the singlet exciton do not get affected by
incorporation of strong donor, DTP in the polymer backbone.
a) b)
85

c) d)
e)
Figure 3.3: TA of P3HTT-TP-DTP:PCBM film following excitation at 500 nm (a) and 700 nm
(c). The TA spectra for corresponding neat P3HTT-TP-DTP films with b) 500 nm and d) 700 nm
pump are also shown. The black broken line shows the linear absorption spectrum of neat
P3HTT-TP-DTP and blue line denotes the positive polymer polaron absorption spectra obtained
from chemically doping the polymer solution. e) The polaron population kinetics at 900 nm for
the P3HTT-TP-DTP:PCBM (solid) is compared to P3HTT-TP:PCBM (scatter) blend following
excitation at 500 nm (green) and 700 nm (red).
Additionally, a broad near IR induced absorption between 850-1200 nm is observed with
spectral profile similar to that of polymer positive polaron obtained from doping P3HTT-TP-
DTP in solution (blue line from Figure 3.2a). For both excitations the polaron band was observed
to rise within 200 fs. This fast polaron generation is mainly due to the presence of mixed
polymer PCBM phase expected from amorphous P3HTT-TP-DTP:PCBM film.
24-26
The
amorphous nature of the polymer is based on the observation of no evident peak from X-ray
diffraction on thin polymer films.
15
Additionally, the partial charge transfer character of the
exciton due to the presence of TP and DTP units in the polymer backbone can also lead to faster
86

polaron generation.
27-32
At longer times, the polaron band was found to relax independent of
excitation wavelength (Figure 3.3e). Compared to previous TA measurements on P3HT-
TP:PCBM,
14
the survival percentage was observed to decrease in P3HTT-TP-DTP:PCBM from
45% to 20% (Figure 3.3e) at 1 ns. This lower survival percentage in presence of the strong donor
DTP unit suggests higher recombination as the physical reason behind reduced device efficiency
of P3HTT-TP-DTP:PCBM compared to P3HT-TP:PCBM.
3.3.2 Pristine P3HTT-DPP-DTP and P3HTT-DPP-DTP:PCBM films:
a) b)
c)
Figure 3.4: TA of P3HTT-DPP-DTP:PCBM film following excitation at 500 nm (a) and 700 nm
(b). The black broken line shows the linear absorption spectrum of neat P3HTT-DPP-DTP and
blue line denotes the positive polymer polaron absorption spectra obtained from chemically
doping the polymer solution. c) The polaron population kinetics at 900 nm for the P3HTT-DPP-
DTP:PCBM (solid) is compared to P3HTT-DPP:PCBM (scatter) blend following excitation at
500 nm (green) and 700 nm (red).
Similarly incorporation of DTP monomer unit in P3HTT-DPP polymer lead to lower
device performance of P3HTT-DPP-DTP:PCBM compared to P3HTT-DPP:PCBM. To
87

understand the reason, TA measurements were performed following excitation of P3HT-like
chromophore at 500 nm (Figure 3.4a) and ICT-like excitation at 700 nm (Figure 3.4b). 700 nm
excitation leads to an immediate ground state bleach similar to the absorption spectral shape of
DPP-containing oligomers and polymers.
33-34
Whereas 500 nm excitation leads to a ground state
bleach corresponding to the absorption profile of the polymer P3HTT-DPP-DTP (black broken
line), where both P3HT-like transition (400-600 nm) and ICT-like transition (650-800 nm) are
bleached. At longer delays (200 fs) the bleach shape following 500 nm excitation was not
observed to evolve to the 700 nm excitation bleach suggesting no energy relaxation from high
energy chromophore to low energy chromophore. In order to solely track the exciton population,
TA on neat P3HTT-DPP-DTP polymer solution were performed following excitation at 500 nm
and 700 nm (Figure 3.5). This shows that in neat solution the 500 nm excitation bleach evolves
towards the 700 nm excitation bleach over 10 ps, indicating energy transfer from higher energy
absorbing P3HT-like chromophore to the lower energy ICT-like chromophore. In the blends this
bleach evolution is not evident because of the presence of other channels with faster rates like
charge transfer to PCBM.
a) b)
Figure 3.5: TA of P3HTT-DPP-DTP solution in o-dichlorobenzene following excitation at 500
nm (a) and 700 nm (b). The TA spectra are normalized to the absorption value at ~680 nm
corresponding to the absorption peak of ICT-transition.
88

The charge transfer to PCBM becomes evident in the blends (Figure 3.4a & 3.4b) from
the generation of near-IR induced absorption (850-1100 nm) assigned to positive polaron with
spectral similarity from chemical doping measurements in solution (blue line). The polarons
dynamics (Figure 3.4c) at 900 nm reveal that 700 nm excitation leads to a prompt (200 fs)
polaron generation whereas 500 nm excitation leads to both prompt and delayed (~300 fs)
polaron generation. The delayed polaron generation for 500 nm excitation is probably due to
comparable semi-crystalline nature of the polymer to that for P3HT (interplane (100) distance is
15.35 Å compared to 16.68 Å present in P3HT). The delayed polaron rise is also observed for
P3HT:PCBM where excitons generated in crystalline P3HT regions of the film that are not
adjacent to PCBM domains, have to first diffuse to a PCBM interface to produce polarons.
35-36

The rapid polaron generation for 700 nm pump is assisted by the charge transfer character of the
exciton generated following ICT-like excitation. At longer times (Figure 3.4c), the polaron
recombination dynamics was found to be excitation wavelength dependent with survival
probability of 50% for 500 nm vs. 30% for 700 nm at 1 ns. Compared to the survival probability
measured previously on P3HTT-DPP:PCBM,
14
incorporation of DTP monomer unit leads to a
15% drop in survival probability for both excitations in P3HTT-DPP-DTP:PCBM. This is the
reason behind lower device performance of P3HTT-DPP-DTP:PCBM compared to P3HTT-
DPP:PCBM.
3.4 Polaron Geminate Recombination dynamics:
The recombination dynamics were observed to be excitation density independent between
(1-5)10
14
cm
-2
indicating the presence of geminate recombination between polarons. Geminate
recombination could be of two types: one involved with intimately placed polarons at the
89

polymer:fullerene interface and the other which first escaped the Coulomb attraction but can
diffuse back to have a second chance for geminate recombination. The first type of
recombination could be described using exponential but for the second type of recombination a
diffusion equation in presence of Coulomb potential is required. Thus to model the diffusion-
controlled polaron dynamics Debye-Smoluchowski equation,
37

(1)
is used where polaron probability density (r,t) undergo diffusion in presence of the Coulomb
potential,

. The long time solution for
equation (1) under the assumptions that charge transfer and charge relaxation are exponential is
the well-known Onsager Braun model.
38-40
The survival probability calculated using polaron
probability density is compared with the polaron population kinetics obtained from TA
measurements.
37

When the exciton generates polarons at the polymer:PCBM interface, the Coulomb
attraction experienced by the parent exciton governs the initial charge separation distance. If the
diffusion constant is same, then lower charge separation distance leads to higher charge
recombination due to Coulomb attraction experienced by the charges. Thus initial charge
delocalization length of the charge transfer state is one of the important factors associated with
charge generation and recombination. Previous experimental and theoretical studies have shown
that energetic driving force of the parent exciton helps to overcome the Coulomb potential and
thus leads to higher charge delocalization length of the charge transfer state.
41-42
In addition a
90

mixed donor acceptor phase morphology is also suggested to contribute towards lower charge
delocalization length.
43-45

Following polaron generation at the polymer:PCBM interface, the polarons can diffuse in
three dimension based on the effective diffusion constant, D calculated from hole mobility of the
polymer and electron mobility of PCBM.
46
In this paper D used was 2.9×10
-5
cm
2
/s and
2.75×10
-5
cm
2
/s based on different reported hole mobilities of P3HTT-DPP-DTP and P3HTT-
TP-DTP, respectively.
15
The encounter distance between the polymer and PCBM is set as 3.5 Å
based on the distance of closest approach calculated between P3HT and fullerene.
47
Since most
of the monomer content is still 3-hexylthiophene in these donor-acceptor semi-random polymers,
the encounter distance used was based on calculations from P3HT. A distance dependent charge
transfer rate
48
is used as a boundary condition at the encounter distance, r
m

xp β

(2)
where k
e
is the rate of electron transfer at r
m
and β value ranges from 0.5 to 1.2 Å
-1
.
48
The exponential
factor illustrate how increase in separation leads to decrease in electronic coupling between the
positive polymer polaron and the negative PCBM polaron. The survival probability of polarons
are calculated by numerically solving equation (1) with boundary condition (2) using a differntial
equation solver.
49
In this paper the polaron dynamics for two polymer:PCBM blends at both
excitations are modeled using k
e
=8×10
12
s
-1
and β =0.8 Å
-1
in presence of Coulomb potential
corresponding to Onsager radius of 50Å. The initial charge separation length between polarons is
assumed to be a Gaussian distribution with a characteristic width L (= ). This characteristic
width, L was varied to get best matching fits for different polaron band dynamics shown in
Figure 3.6. Since following excitation at 500 nm there was a delayed polaron generation of ~ 300
91

fs for P3HTT-DPP-DTP:PCBM, the simulated results are convoluted with an exponential rise of
300 fs.

Figure 3.6: The best matched polaron survival probability obtained from numerical solution is
shown with the polaron population kinetics of P3HTT-TP-DTP:PCBM (top) and P3HTT-DPP-
DTP:PCBM (bottom).
This modeling illustrate that irrespective of the excitation P3HTT-TP-DTP:PCBM
exhibit initial charge separation length of ~14±1Å. Whereas for P3HTT-DPP-DTP:PCBM, initial
charge separation length was calculated to be ~25±2Å for 500 nm excitation to P3HT-like
chromophore and 17±2Å for 700 nm excitation to ICT-like chromophore. Thus for both
polymer:PCBM blends either excitation leads to initial charge separation within the Coulomb
potential (Onsager radius~50Å) and therefore leads to higher charge recombination. The
calculated polaron survival probability density for the polymer:PCBM blends at different delays
are shown in Figure 3.7. For P3HTT-TP-DTP:PCBM this lower charge separation length is
probably due to 1) lower energetic driving force of the parent exciton due to low LUMO
PCBM
-
92

LUMO
P3HTT-TP-DTP
energy gap (500 meV)
15, 50
and 2) mixed polymer:PCBM phase due to
amorphous morphology of P3HTT-TP-DTP:PCBM. The higher charge separation length for 500
nm absorbing P3HT-like chromophore compared to 700 nm absorbing ICT-like chromophore
comes from the higher energetic driving force (950 meV vs. 500 meV) of the parent exciton. The
incorporation of DTP monomer in the polymer backbone results in lower charge separation
length of P3HTT-TP-DTP:PCBM compared to P3HTT-TP:PCBM (L~30±1Å).
14
Similarly the
charge separation length for P3HTT-DPP-DTP:PCBM is lower than the previously reported
lengths for P3HTT-DPP:PCBM (
pump
=500 nm: 45Å and
pump
=700 nm: 30Å).
14
The possible
reason behind this is low hole mobility of polymers on incorporation of DTP monomer unit,
which lowers the escape probability of charges from the interface.
15
Thus, this modeling
illustrates that the initial physical separation length plays an important role in polaron
recombination.
a) b)
c) d)
93

Figure 3.7: Spatial distribution of the survival probability density of polarons for (a) P3HTT-
DPP-DTP:PCBM (
exc
= 500 nm), (b) P3HTT-DPP-DTP:PCBM (
exc
= 700 nm), (c) P3HTT-TP-
DTP:PCBM (
exc
= 500 nm) and (d) P3HTT-TP-DTP:PCBM (
exc
= 700 nm) at different times
following charge separation. The Coulomb potential corresponding to an Onsager radius of r
C
=
50Å is shown in black broken line.
3.5 Absolute Polaron Yield:
The absolute polaron yield calculations can help in quantifying the polarons generated in
the polymer:PCBM blends. To calculate the polaron yield the number of polarons, N
polarons
and
number of photoexcitations, N
photoexcitations
are required. N
polarons
can be calculated from polaron
cross-section and TA data using,

(3)
where ∆A(λ
probe
, t) is the TA signal at the probe wavelength corresponding to the polymer
positive polaron absorption and

,

are the absorption cross-section of positive
polymer polaron and PCBM

radical anion. Reported literature value for

=1.6810
-17
cm
2

at 900 nm is used.
51
The time resolved polaron yield calculated using the polaron cross-section
and TA data are shown in Figure 3.8. Clearly, the calculated polaron yield for P3HTT-TP-
DTP:PCBM shows excitation wavelength independent low polaron yield (~15%) and therefore
this explains the low device performance for this polymer:PCBM blend. On the other hand the
time resolved polaron yield for P3HTT-DPP-DTP:PCBM shows that 500 nm excitation of
P3HT-like chromophore leads to almost double polaron yield compared to 700 nm excitation of
ICT-like chromophore at initial delays (200-400 fs). And in fact at longer delays the polaron
yield for 700 nm excitation of P3HTT-DPP-DTP:PCBM approaches the polaron yield obtained
from P3HTT-TP-DTP:PCBM. This shows that although the P3HT-like chromophore of P3HTT-
94

DPP-DTP:PCBM is pretty efficient, the ICT-like chromophore is not able to contribute enough
towards the device performance.

Figure 3.8: The time resolved polaron yield for different polymer:PCBM blends, 1:P3HTT-
DPP-DTP:PCBM and 2:P3HTT-TP-DTP:PCBM for both 500 nm and 700 nm excitations.
4. Conclusion:
The TA measurements reveal that incorporation of strong donor dithienopyrroles (DTP)
in semi-random donor-acceptor P3HT analogs leads to polaron generation within 300 fs. This
indicates that the donor-acceptor effect due to the presence of DTP and different acceptor
monomer unit in the polymer backbone is advantageous and exciton diffusion process is not
limiting their performance. Once generated, the polaron undergo enhanced recombination
compared to their parent semi-random donor-acceptor copolymers (containing no DTP). The
physical reason behind this high charge recombination is generation of charges with charge
separation length lower than Onsager radius based on Debye-Smoluchowski modeling. This
explains the comparatively low device photocurrent efficiency of DTP containing semi-random
donor-acceptor polymers. However, the high device performance of P3HTT-DPP-DTP:PCBM
compared to P3HTT-TP-DTP:PCBM is evident from the high polaron yield extracted from TA
and chemical doping measurements. This illustrates that small changes to the polymer backbone
95

can play an important role in device performance. In future careful considerations of the
monomer combinations to maintain semi-crystalline nature of P3HT and optimum donor-
acceptor interactions can help to alleviate the geminate recombination to improve the
performance of these semi-random donor-acceptor copolymers.

96

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100

Chapter 4
Symmetry-breaking charge transfer in Zinc dipyrrin derivatives
1. Introduction
Over the last decade significant improvements have been made in organic solar cell
technology with reported power conversion efficiencies reaching over 10%.
1
The power
conversion efficiency of an organic solar cell is defined as η = (J
SC
V
OC
FF)/ P
in
, where P
in
is the
input power in form of solar radiation and the output parameters are short-circuit current density
(J
SC
), open-circuit voltage (V
OC
), and fill factor (FF). J
SC
is related to the product of absorption
intensity and width of the absorption profile for the active layer, where harvesting broader
wavelength range of light can potentially lead to more charge carriers and thus higher J
SC
. Fill
factors are generally related to improved active layer morphologies, balanced charge transport
and extensive device optimization. Most of the recent improvements in organic solar cells have
originated from extending the absorption of the active layer to near-infrared to increase the J
SC
.
2-4

However, the open-circuit voltages (V
OC
) of organic devices are generally low (< 1 V)
5-6
and
serve as a substantial limit to overall device performance.
In organic solar cells the charge separation occurs by electron transfer from the donor to
the acceptor and so V
OC
is thermodynamically limited by the energetic offset of the HOMO of
the donor and the LUMO of the acceptor (E
DA
).
7-8
Thus, the V
OC
is governed by the materials
chosen for the donor and acceptor. Increasing the energetic offset, E
DA
can potentially improve
the V
OC
but it then limits the light absorption to blue and thus leads to lower J
SC
.
9
Based on
spectroscopic and temperature dependent measurements, it has been shown that E
DA
is a poor
101

indicator of V
OC
, and the upper bound for V
OC
is the energy of the ground state to intermolecular
charge transfer state (E
CT
) at the donor/acceptor interface.
10-13
E
CT
is however governed by E
DA

because this transition involves direct excitation of electron from HOMO of the donor to the
LUMO of the acceptor. Reported studies show that there is a linear correlation between E
CT
and
qV
OC
with typical energetic losses of ~0.6 eV due to recombination.
14-16
Additional loss arises
due to the energetic offset required between the LUMO of donor and acceptor to promote charge
generation at the donor/acceptor interface.
17-18

Thus the strategy to improve V
OC
without affecting other parameters like J
SC
and FF, is to
reduce the energetic driving force required for charge generation and to decrease charge
recombination at the donor/acceptor interface. This could potentially be achieved by utilizing
organic molecules which undergo symmetry breaking charge transfer (SBCT). SBCT involves
closely associated pairs of identical molecules or compounds composed of two or more identical
parts, such as covalently bonded organic dimers or metal complexes with two or more identical
ligands. SBCT occurs when an exciton formed initially on one molecule or ligand undergoes
intramolecular charge transfer (ICT), leading to a state in which a hole and an electron are
localized on different molecules or ligands, with very little coupling between the hole and
electron.
19-20
9,9'-Bianthryl derivatives with its D
2d
symmetry in the ground state is the classic
example of the occurrence of symmetry breaking on photoexcitation.
21-23
This consists of poorly
coupled orthogonal identical anthracene chromophores in the ground state and the excited state
photophysics is dictated by the polarity of the medium. In non-polar medium the excited state
photophysics is dictated by a localized excited S
1
state on one chromophore whereas in weakly
polar and polar medium charge transfer occurs from one chromophore to another upon photo-
excitation, producing SBCT.
19, 24
In organic photovoltaics (OPVs), the polar environment at
102

donor acceptor interface can be sufficient to induce SBCT in the chromophore, leading to
spontaneous formation of internal CT excitons. Charge separation between electron donor and
acceptor materials from these SBCT excitons is expected to proceed with lower energetic
requirements than for typical Frenkel excitons found in organic materials.
25-27
Thus, SBCT is an
attractive strategy to achieve charge separation with negligible energetic driving force,
directional specificity and a greatly retarded back-recombination rate due to the presence of
neutral organic ligand spacer. A simple schematic view of how charge transfer and charge
separation involving SBCT could occur at the D/A interface of an OPV is illustrated in Figure
4.1.

Figure 4.1: Schematic diagram of photocurrent generation via SBCT in an OPV.
Although 9,9'-Bianthryl derivatives show SBCT, they do not absorb visible light and thus
they are not suitable to harvest the solar energy. Indeed, very few organic dyes that absorb in the
visible region undergo SBCT processes.
28-33
Herein, we investigate the photophysics of
homoleptic zinc dipyrrin complexes (Figure 4.2) that exhibit intense visible absorption in a range
of organic solvents. These compounds have structural features similar to 9,9’ -bianthryl (i.e.,
poorly coupled orthogonal chromophores) in the ground state.
24
Zinc dipyrrins and analogous
103

compounds are attractive because, in addition to strong absorption in the visible region of the
spectrum, their syntheses are easy and scalable. The excited state photophysics studied by
transient absorption (TA) and time correlated single photon counting (TCSPC) measurements
reveal that SBCT in polar solvents is an effective nonradiative pathway for the electronic excited
states of homoleptic zinc dipyrrins. In nonpolar solvents such as cyclohexane, these complexes
do not undergo SBCT and thus exhibit higher fluorescence quantum yields.
2. Experimental section:
2.1. Time resolved photoluminescence. Fluorescence lifetime measurements of zinc dipyrrin
derivatives in different polarity solvents were carried out using an excitation wavelength of 500
nm obtained from an optical parametric amplifier (Coherent OPA 9450) pumped by a 250 kHz
Ti:sapphire amplifier (Coherent RegA 9050). The emission was collected at 520 nm for S
1
state
and 645 nm for the CT state of zDIP1-3. The emission from S
1
state of ZCl is measured at 550
nm. The emitted photons are detected using a R3809U-50 Hamamatsu PMT with a Becker and
Hickl SPC 630 detection module (22 ps time resolution).
2.2. Femtosecond transient absorption of zDIP1-3. Pump and probe pulses were obtained from
the output of a Ti:Sapphire regenerative amplifier (Coherent Legend, 1kHz, 4 mJ, 35 fs). The
excitation pulses centered at 500 nm were generated by pumping a type-II OPA (Spectra Physics
OPA-800C) with ~10 % of the amplifier 800 nm output and mixing the resulting OPA signal
output with the residual 800 nm pump in a type-II BBO crystal. White light supercontinuum
probe pulses, spanning the visible (320-950 nm) were obtained by focusing a small amount of
the amplifier output onto a rotating CaF
2
disk. The supercontinuum probe was collimated and
focused with a pair of off-axis parabolic mirrors into sample whereas the pump was focused
104

before the sample position using a 25 cm CaF
2
lens. To avoid any contribution to the observed
dynamics from orientational relaxation, the polarization of the supercontinuum was set at the
magic angle (54.7º) with respect to the pump polarization. The cross correlation between pump
and probe in a thin 1mm quartz substrate gave a fwhm of 180 fs for 500 nm excitation. The
supercontinuum probe was dispersed using a spectrograph (Oriel MS127I) onto a 256-pixel
silicon diode array (Hamamatsu) for multiplexed detection of the probe.
Samples containing zDIP1, zDIP2 or zDIP3 dissolved in cyclohexane, toluene,
dichloromethane, or acetonitrile were placed in a close capped 1 mm quartz cuvette. The
concentration of each sample was adjusted to give an optical density between 0.11 and 0.18 at
500 nm. The solutions were deaerated by bubbling with N
2
prior to analysis. During data
collection, the samples were slowly oscillated perpendicular to the pump and probe to reduce
photodamage to the sample by the pump. At early time delays, a strong non-resonant signal from
the sample cell and solvent is observed. The solvent response is found to relax within 180 fs for
cyclohexane, dichloromethane and acetonitrile while in toluene this signal was stronger and
obscured the first 300 fs. To effectively remove this non-resonant signal, a second measurement
of the neat solvent was performed in an identical cuvette under same excitation conditions. The
transient signal resulting from the solvent was then subtracted from the zinc dipyrrin solution
signal. The non-resonant solvent response did however give a useful measure of the temporal
dispersion of the supercontinuum after propagating through the CaF
2
plate and sample. The
presented data have been corrected to account for this dispersion. Transient absorption
measurements were performed with pump fluences varying between 70 and 300

µJ/cm
2
. Over
this range, the signal was found to scale linearly with the pump energy.
105

2.3. Nanosecond-to-millisecond transient absorption. Samples were prepared in a nitrogen
glovebox with dry solvents, such that the maximum absorbance was approximately OD = 1.
These samples were sealed in 1-cm x 1-cm quartz cuvettes with Kontes valves to keep the
solution air-free. The third harmonic of a 10 Hz Q-switched Nd:YAG laser (Spectra-Physics
Quanta-Ray PRO-Series, pulse width: 8 ns) was used to pump an optical parametric oscillator
(Spectra-Physics Quanta-Ray MOPO-700), tunable in the visible region. The excitation
wavelength for each sample was chosen such that OD (at λ excitation) = 0.3 –0.4, and the laser
power was attenuated to 3 mJ/pulse using a half-wave plate and polarizer combination. For
single-wavelength transient absorption kinetics measurements, probe light was provided by a
75 W arc lamp operated in either continuous or pulsed mode. Single wavelengths were selected
by a double monochromator with 1 mm slits, detected by a photomultiplier tube, and amplified
and recorded with a transient digitizer. Single-wavelength traces were acquired at approximately
5 nm increments over the range of 350–595 nm, on a 2 μs, 100 μs, or 10 ms timebase window,
averaging over 300 laser pulses. A reference wavelength (400 nm) was acquired as every third
trace, to take into account photo-degradation of the sample. Data were converted to units of
ΔOD = -log
10
(I/I
0
). Kinetics traces at each wavelength were scaled based on the intensity of the
previous reference trace. Decays were fit to a single or double exponential with a long-time
offset using Matlab (version 2010b) curve fitting software. To generate transient absorption
profiles at various time points, the ΔOD at that time point was taken from the exponential fit of
each single-wavelength kinetics trace.
2.4. Femtosecond transient absorption of ZCl. Femtosecond pump and probe pulses were
derived from the output of a Ti:Sapphire regenerative amplifier (Coherent Legend, 1kHz, 4 mJ,
35 fs). Approx. 10% of the amplifier 800 nm output was used to pump a type-II OPA (Spectra
106

Physics OPA-800C) to generate a signal at ~1540 nm and this OPA signal output was mixed
with the residual 800 nm pump in a type-II BBO crystal to generate the 520 nm excitation pulses.
At the sample position the excitation pulse is focused to ~400 m (FWHM) using a CaF
2
lens.
White light supercontinuum probe pulses between 320-950 nm were obtained by focusing a
small amount of the amplifier output on a rotating CaF
2
disk. The supercontinuum probe was
collimated and focused with a pair of off-axis parabolic mirrors into sample. To suppress the
scattering from the excitation pulse, a perpendicularly oriented pump and probe were used to
collect the data by passing the probe through an analyzing polarizer after the sample. The cross
correlation between pump and probe in a thin 1mm quartz substrate gave a fwhm of 150 fs for
520 nm excitation. The supercontinuum probe was dispersed using a spectrograph (Oriel
MS127I) onto a 256-pixel silicon diode array (Hamamatsu) for multiplexed detection of the
probe.
The solutions of ZCl in cyclohexane, toluene and acetonitrile were placed in a screw-capped
1 mm quartz cuvette. The concentration of ZCl in cyclohexane, toluene and acetonitrle was
adjusted to give an optical density of 0.21, 0.18 and 0.16, respectively at 520 nm. The solutions
were deaerated by bubbling with N
2
prior to analysis. The solid film of ZCl in PMMA was
prepared by spin coating on a quartz substrate to reach an optical density of 0.1 at 520 nm. The
film also had an additional quartz window on top surface and the outer edges were sealed with
epoxy under N
2
atmosphere. During data collection, the samples were slowly oscillated
perpendicular to the pump and probe to reduce photodamage to the sample by the pump.
Transient absorption measurements were performed with pump fluences varying between 5.7
and 40

µJ/cm
2
. Over this range, the signal was found to scale linearly with the pump energy.
107

2.5. OPV fabrication and testing. ZCl was synthesized according to published procedures.
34

DBP was obtained from Lumtec. C
60
was obtained from MER. BCP was obtained from Sigma-
Aldrich. Al (99.999 %) was obtained from Alfa. All organic materials were purified by gradient
sublimation before use. The devices were deposited on ITO precleaned with tergitol and organic
solvents. All layers were deposited by vacuum thermal evaporation (system base pressure of
1−3 × 10
−6
Torr) at rates between 0.02 and 0.2 nm s
−1
. I-V measurements were performed in air
at 25 °C using a Keithley 2420 Sourcemeter (sensitivity = 100 pA) in the dark and under ASTM
G173-03 spectral mismatch corrected 1000 W/m
2
white light illumination from an AM1.5G
filtered 300 W xenon arc lamp (Asahi Spectra HAL-320W). Routine spectral mismatch
correction was performed using a silicon photodiode (Hamamatsu S1787−04,8RA filter)
calibrated at the National Renewable Energy Laboratory (NREL). Chopped and filtered
monochromatic light (250 Hz, 10 nm fwhm) from a Cornerstone 260 1/4 M double grating
monochromator (Newport 74125) was used in conjunction with an EG&G 7220 lock-in amplifier
to perform all spectral responsivity and spectral mismatch correction measurements.
35

3. Results and Discussion:
3.1. Steady state photophysical properties:
The molecular structure and absorption spectra of different zinc dipyrrin derivatives
studied are shown in Figure 4.2. Single crystal X-ray analysis shows a distorted tetrahedral
configuration adopted by the zinc center in all the complexes with the two ligands held nearly
perpendicular to each other. The dihedral angles between mean planes of the two dipyrrin
ligands in zDIP1, zDIP2, zDIP3 and ZCl are 83.4°, 87.9°, 82.1° and 87.6°, respectively.
34, 36

However, no clear correlation is apparent between the degree of alkylation/chlorination and
108

variation of the dihedral angles, indicating that the distortions are likely dictated by crystal
packing forces. Although the absorption spectra of zDIP1-3 are almost identical with maximum
absorption between 485-493 nm, the absorption of ZCl is red-shifted by ~ 27 nm due to electron
withdrawing nature of the Cl atoms. The absorption spectra (Figure 4.2) of homoleptic zinc
dipyrrin (ML
2
) derivatives is similar to those containing only one dipyrrin ligand (MLX) and
thus it indicates that the absorption transition only involves one dipyrrin ligand.
36
This suggests
little to no electronic coupling between the two dipyrrin ligands in homoleptic zinc dipyrrin
derivatives.
a) b)
Figure 4.2: a) Structures of homoleptic zinc dipyrrin derivatives: zDIP1, zDIP2, zDIP3 and ZCl
with their corresponding b) absorption spectra in cyclohexane solution.
All the complexes in cyclohexane display emission spectra (Figure 4.3a) with similar Stokes
shift (~550 cm
-1
) indicating emission from a localized excited S
1
state. However, with increase in solvent
polarity the shape of the emission spectra changes (Figure 4.3b) and a steady decrease in
photoluminescence quantum yield is observed (Figure 4.3c). Representative spectra in different
polarity solvents for zDIP3 is shown in Figure 4.3b. With increasing polarity, for all zinc
dipyrrin complexes except zDIP1 a broad emission band emerges at ~650 nm.
36
This red shifted
emission band do not originate from triplet state as it is spectrally different from the reported
phosphorescence emission measured at 77 K.
36
Since this 650 nm emission band becomes more
109

pronounced with increased solvent polarity, it is assigned to emission from charge transfer state,
similar to that reported for 9,9’ -bianthryl derivatives
37
and meso-coupled boron dipyrrin
compounds.
33
It is interesting to note that no low energy emission is detected from non-alkylated
zDIP1 in polar solvent probably due to formation of a dark charge transfer state. The
luminescence quantum yield ( <0.001) of 650 nm emission for both zDIP2 and zDIP3 indicate
a very weak oscillator strength for this transition. The decrease in photoluminescence quantum
yield with increasing polar solvent indicates that locally excited state in zinc dipyrrin derivatives
deactivate to a weak or non-emissive charge transfer state in polar solvents.
a) b)
c)
Figure 4.3: a) Emission spectra of zDIP1-3 and ZCl in cyclohexane. b) Solvent polarity
dependent emission spectra of zDIP3 in various solvents. c) Photoluminescence (PL) quantum
110

yield variation of zDIP1–zDIP3 and ZCl vs. solvent dielectric constant in the order: cyclohexane,
toluene, CH
2
Cl
2
and MeCN.
3.2. Time resolved photoluminescence measurements:
To understand the physical reason behind the solvent polarity dependent decrease in
quantum yield, photoluminescence (PL) lifetime measurements (Figure 4.4) were performed for
all the derivatives. The PL lifetime data of zinc dipyrrin complexes in cyclohexane shows a
mono-exponential decay with lifetimes 3.7, 4.8, 1.4 and 2.5 ns for zDIP1, zDIP2, zDIP3 and
ZCl, respectively. This confirms that the excited S
1
state can only deactivate to S
0
state and no
other intermediate state is involved in cyclohexane. With increasing solvent polarity, the excited
state decays with multi-exponential lifetimes with increasing amplitude for sub-nanosecond
component. The sub-nanosecond component is faster than our instrument’s response time
(<22 ps). This multi-exponential decay for the excited state S
1
indicates that with increase in
solvent polarity, majority of the S
1
population is going to a different state and only some are
relaxing to the ground state by the radiative process (Table 4.1). The amplitude for the faster
component in polar solvents increases on alkylation/chlorination, i.e., moving from zDIP1 to
zDIP3 and ZCl the amount of S
1
population going to a different state increases. This is probably
because of the restricted orthogonal geometry of the ground state achieved by
alkylation/chlorination. The lifetime data for weak low energy emission peak at ~ 650 nm
(Figure 4.4e) shows a mono-exponential decay from charge transfer state with a lifetime of 2.2
and 2.5 ns for zDIP2 and zDIP3, respectively.
111

a) b)
c) d)
e)
Figure 4.4: Normalized photoluminescence (PL) measurements of a) zDIP1, b) zDIP2, c) zDIP3
and d) ZCl in various polarity solvents. The emission is collected at 520 nm and 550 nm for
zDIP1-3 and ZCl, respectively following excitation with 500 nm. The instrument response
function (FWHM ~ 22 ps) is also shown in red. The black lines are the fit to the PL
measurements with exponential(s) (amplitudes and time constants are shown in table 4.1). e)
112

Normalized time resolved PL measurements of CT emission for zDIP2 and zDIP3 in
dichloromethane measured by collecting emission at 650 nm.
Table 4.1: Fitting parameters for the PL lifetime measurements of zDIP1-3 and ZCl in different
solutions
Solvents a
1 
1
(ns) a
2

2
(ps)*
zDIP1 Cyclohexane 1 3.7
Dichloromethane 0.227 2.2 0.773 < 22
Acetonitrile 0.013 1.7 0.987 < 22
zDIP2 Cyclohexane 1 4.8
Dichloromethane 0.181 2.5 0.819 < 22
Acetonitrile 0.089 1.4 0.910 < 22
zDIP3 Cyclohexane 1 1.4
Dichloromethane 0.150 2.4 0.850 < 22
Acetonitrile 0.067 1.4 0.933 < 22
ZCl Cyclohexane 1 2.5
Dichloromethane 0.12 0.29 0.88 <22
Acetonitrile 0.04 0.85 0.96 <22
* The faster component was faster than our instrument response function (FWHM ~22 ps).
3.3. Transient absorption measurements:
The decrease in PL quantum efficiency and increase in amplitude for faster deactivation
(<22 ps) for homoleptic zinc dipyrrin complexes in polar solvents, with simultaneous appearance
of an additional broad peak at longer wavelengths in the emission spectra, suggest a deactivation
pathway to a weakly emissive state. This state most likely has charge transfer character and is
113

formed via a symmetry-breaking mechanism similar to that reported for bichromophoric systems
like 9,9'-bianthryl, meso-linked BODIPY dyads, etc.
33, 37
To know the dynamics for the
intramolecular charge transfer (ICT) state formation, femtosecond transient absorption (TA)
measurements were performed in solvents with different polarity.
3.3.1 Excited state dynamics of zDIP1
a) b)
c) d)
Figure 4.5: Femtosecond transient absorption of zDIP1 in (a) cyclohexane, (b) toluene, (c)
dichloromethane and (d) acetonitrile. Excitation at 500 nm, pump fluence of 160 µJ/cm
2
were
used for all except (c) which was performed at 70 µJ/cm
2
.
Femtosecond TA of zDIP1 solution (Figure 4.5) in cyclohexane, toluene,
dichloromethane and acetonitrile were performed following excitation at 500 nm. In cyclohexane
(Figure 4.5a) excitation of zDIP1 leads to an immediate appearance of ground state bleach
between 430-500 nm (compare to absorption in Figure 4.2) due to depopulation of chromophores
114

from ground state. Simultaneously, excited state absorption at 345 nm and stimulated emission
between 520-600 nm (compare to emission in Figure 4.3a) are observed due to population of
localized excited S
1
state. At later times, this localized excited state S
1
relaxes to the ground state
with an excited state lifetime of ~4.5 ns comparable to the measured fluorescence lifetime (~3.7
ns). In polar solvents like dichloromethane (Figure 4.5c) and acetonitrile (Figure 4.5d), excitation
with 500 nm at early times leads to the formation of similar ground state bleach, stimulated
emission and excited state absorption, indicating population of localized excited S
1
state.
However, within 4-6 ps, the excited state absorption and stimulated emission disappear with
concurrent rising bands at 370 nm and 517 nm. The induced absorption at 517 nm is similar to
the zDIP1 anion obtained from spectro-electrochemical reduction experiments in
dichloromethane (Figure 4.6a). In Figure 4.6b the two absorption profiles are overlaid to make a
direct comparison between the two experiments. Although the peak at 517 nm matches perfectly,
the bleach shape looks different between the two experiments. This is mainly due to the presence
of an additional peak between 370-470 nm (encircled in green) in the spectro-electrochemical
reduction experiments. The absence of this peak in TA data is probably because the molecule
forms both a zDIP1 radical cation and radical anion in its excited state (as probed in TA
experiment) whereas in the reduction experiments there only zDIP1 radical anion is present. It is
possible that the total absorptions for the radical anion is obscured by the presence of radical
cation, and the anion signal may be overwhelmed by the bleach signal in TA data which is
doubled due to removal of two chromophores (one forming the anion and the other forming the
cation). Another possible reason could be a two electron reduction, i.e. reduction of both dipyrrin
ligands, in spectro-electrochemical experiments.
115

a) b)
Figure 4.6: a) Spectroelectrochemical data of zDIP1in dichloromethane under negative applied
bias and b) Spectral lineshape comparison between spectro-electrochemical reduction and TA
experiments on zDIP1 in DCM. The encircled portion in green shows the peak responsible for
the difference.
Since in both polar solvents (Figure 4.5c & 4.5d) the induced absorption peak at 370 nm
evolves with similar kinetics to that at 517 nm, we assign the 370 nm peak to the new excited
state as well. This state is assigned as the intramolecular charge transfer (ICT) species and the
absence of characteristic ICT absorptions in cyclohexane indicates that stabilization by polar
solvents is required to favor ICT state over the local excited S
1
state. The increase in the ground
state bleach during the first 10 ps (Figure 4.5c and 4.5d) is a direct consequence of symmetry
breaking charge transfer (SBCT). This is because initially upon photoexcitation only one of the
two DIP units is bleached and later upon SBCT the other DIP unit is also bleached and thus the
bleach increases approx. by a factor of 2.
Similar to cyclohexane, in weakly polar solvent like toluene (Figure 4.5b), excitation
with 500 nm leads to immediate appearance of ground state bleach, stimulated emission and
excited state absorption from population of localized S
1
state. However, within 10 ps the excited
state absorption at 370 nm evolves with retention of S
1
stimulated emission. The induced
116

absorption at 370 nm indicates SBCT of zDIP1 in toluene; however, the induced absorption at
517 nm is hidden due to the overlap with the stimulated emission band. Additionally, the
stimulated emission persists over the probing time (1.1 ns), which is much longer than that in
acetonitrile or dichloromethane. The quantum yield of zDIP1 in toluene is also much higher than
that in acetonitrile and dichloromethane (Figure 4.3c). These observations can be explained by
the presence of an equilibrium between the local excited state S
1
and ICT states in toluene. This
is probably because stabilization by weakly polar toluene lowers the energy of the ICT state
close to that of the locally excited S
1
state. A similar equilibrium between locally excited S
1
and
I T states of 9,9’ -bianthryl was reported in weakly polar media.
38-40
In more polar solvents, the
ICT states are stabilized and thus shifts the equilibrium to formation of the charge transfer
species.
Once the ICT state is generated since the ICT state is weakly emitting, it can either
recombine non-radiatively to S
0
state or to T
1
state. Previous studies on 9,9’ -bianthryl reported
that the ICT state either recombined radiatively to the ground state or nonradiatively to triplet
states in polar solvents.
38
Deactivation of the ICT states were further probed by performing
nano-to-microsecond TA measurements on zDIP1 in different solvents (Figure 4.7a & 4.7b). The
induced absorption peaks from the ICT state was not observed within the instrument response
time (~500 ns) and new induced absorption bands appeared between 350–450 nm. To elucidate
the origin of these new induced absorption features, femtosecond TA of zDIP1 was measured in
a dichloromethane/methyl iodide (CH
2
Cl
2
:CH
3
I=1:4) solvent mixture (Figure 4.7c) to accelerate
intersystem crossing from the excited S
1
state to the T
1
state.
41
The induced absorption observed
for this mixture spectrally matches well with microsecond TA spectrum of zDIP1 in acetonitrile
(Figure 4.7a). Thus, the induced absorption feature observed in acetonitrile from 500 ns to
117

milliseconds is assigned exclusively to the triplet state. Similar results were obtained in toluene.
The triplet state was also observed in cyclohexane; however, the signal intensity is much weaker
than that observed in more polar solvents (Figure 4.7b). This suggests that formation of the
triplet state via S
1
T
1
intersystem crossing is less efficient in cyclohexane than via
S
1
ICTT
1
recombination in toluene and acetonitrile. However, the branching between
ICTS
0
and ICTT
1
transitions was not clearly studied as the recombination dynamics of ICT
state between 1 ns to 500 ns was not measured.
a) b)
c)
Figure 4.7: Nano-to-millisecond transient absorption of zDIP1 in (a) acetonitrile, (b) different
solvents at 0.5 ms and (c) femtosecond transient absorption of zDIP1 in CH
2
Cl
2
:CH
3
I (1:4).

118

3.3.2 Excited state dynamics of zDIP2
Similar to zDIP1 in non-polar solvents like cyclohexane, excitation of zDIP2 (Figure 4.8)
leads to immediate formation of ground state bleach between 450-510 nm (compare to
absorption in Figure 4.2) with a concurrent appearance of simulated emission (510-560 nm,
compare Figure 4.3) and excited state absorption at 352 nm from localized excited S
1
state. At
longer times this excited S
1
state is observed to decay with a time constant of 4.8 ns (comparable
to fluorescence lifetime). In highly polar solvent like acetonitrile (Figure 4.8c) at initial times
formation of localized S
1
state is evident from the excited state absorption (peak at 352 nm) and
stimulated emission bands. However, within 2 ps these bands start to disappear with
simultaneous appearance of new induced absorption peaks at 380 nm and 540 nm. Based on the
similarity in spectral position of these peaks with the ICT state of zDIP1 in polar solvents (Figure
4.5), these are assigned to the ICT state of zDIP2. At longer times (> 500 ps), the ICT state is
observed to relax with simultaneous repopulation of ground state bleach. This indicates that
recombination of ICT state to S
0
state is more prominent in zDIP2 than that present in zDIP1.
a) b)
119

c)
Figure 4.8: Femtosecond transient absorption of zDIP2 in (a) cyclohexane, (b) toluene and (c)
acetonitrile. Excitation at 500 nm, pump fluence of 70 µJ/cm
2
were used for all.
In weakly polar solvent like toluene (Figure 4.8b) excitation of zDIP2 with 500 nm pump
leads to immediate appearance of ground state bleach, stimulated emission and excited state
absorption (~352 nm) due to population of the S
1
state. At longer times (> 5 ps), the ICT peaks at
380 and 540 nm appear but unlike acetonitrile, the PL quantum yield of zDIP2 in toluene is
substantial and thus the stimulated emission stays for a longer time. Therefore in weakly polar
solar solvent like toluene an equilibrium between S
1
state and ICT state is proposed to explain
both ICT state formation and PL quantum yield. In highly polar solvent, this equilibrium shifts to
exclusively form ICT state due to the stabilization of ICT state.
3.3.3 Excited state dynamics of zDIP3
In cyclohexane, zDIP3 (Figure 4.9a) show similar dynamics to that observed for zDIP1
and zDIP2, with immediate appearance of stimulated emission (510-580 nm, compare Figure
4.3) and excited state absorption (peak at 356 nm) due to formation of localized S
1
state.
However, the relaxation of the S
1
excited state to the ground state for zDIP3 is faster (~1.4 ns)
compared to zDIP1 (4.5 ns) and zDIP2 (4.8 ns). This dramatic decrease in S
1
excited state
lifetime for zDIP3 is consistent with the PL lifetime measurements (shown in Figure 4.4) and it
120

is probably due to presence of higher non-radiative processes as observed from low PL quantum
yield (Figure 4.3).
36

In toluene, excitation of zDIP3 (Figure 4.9b) initially leads to formation of S
1
state
followed by relaxation of S
1
state to generate ICT state (indicated by the induced absorption at
380 nm). However, some population from S
1
state can also relax back to the ground state via
stimulated emission as this is observed till 1 ns. Thus, an equilibrium exists between the ICT
state and S
1
state similar to that observed in zDIP1 and zDIP2 in toluene. Unlike toluene, in polar
solvents such as acetonitrile initially populated S
1
state of zDIP3 (Figure 4.9c) evolves to ICT
state within 1 ps due to the stabilization of the ICT states by the large solvation energy of the
acetonitrile. Both the ICT peaks at 380 nm and 550 nm for acetonitrile solution is evident with
almost no stimulated emission after 5 ps. The SBCT mechanism for the formation of the ICT
state is also evident from the increase in ground state bleach over 10 ps as initial photoexcitation
bleaches only one of the two DIP units and later upon SBCT the other DIP unit is also bleached.
a) b)
121

c)
Figure 4.9: Femtosecond transient absorption of zDIP3 in (a) cyclohexane, (b) toluene and (c)
acetonitrile. Excitation at 500 nm, pump fluence of 80 µJ/cm
2
were used for all.
3.3.4 Excited state dynamics of ZCl
3.3.4.1 In solution
The TA of ZCl in cyclohexane (Figure 4.10a) shows that initial excitation by 520 nm
leads to immediate depopulation of the ground state (ground state bleach between 450-550 nm,
compare Figure 4.2) with simultaneous appearance of the stimulated emission between 530-
600 nm (compare Figure 4.3) and excited state absorption at 360 nm from a localized excited
state S
1
. At later times, this localized excited state S
1
relaxes to the ground state with an excited
state lifetime of ~2.5 ns (similar to fluorescence lifetime). Similar to cyclohexane, in strongly
polar solvent like acetonitrile (Figure 4.10b) excitation to the S
1
state leads to immediate
formation of excited state absorption (360 nm) and stimulated emission (530-600 nm). However,
within 2 ps, the ICT state (rising peaks at 415 nm and 450 nm) develops with simultaneous decay
of stimulated emission and excited state absorption. These new bands are assigned to ICT states
based on similar observations on zDIP1-zDIP3 in high dielectric solvents. The increase in
ground state bleach over 2 ps is also a clear consequence of SBCT in acetonitrile. At longer
times (Figure 4.10c), this ICT state decays and a new peak arises at 450 nm with a time constant
122

of 550 ps. To elucidate the origin of these new induced absorption feature, femtosecond TA of
ZCl was measured in methyl iodide (MeI) (Figure 4.11), a solvent that is expected to accelerate
intersystem crossing from the excited singlet state to the triplet state.
42
However, for a
chromophore whose excited state dynamics is solvent polarity dependent, using a polar solvent
such as MeI (
r
= 6.97) can also lead to formation of ICT state in addition to enhanced ISC to T
1

state from the excited singlet state. Figure 4.11 shows that after excitation, within 3 ps the
excited state absorption and stimulated emission disappears followed by generation of ICT
induced absorption at 415 nm. At longer times, a new induced absorption feature at 450 nm is
observed to grow within 100 ps for ZCl dissolved in MeI. Since this new absorption feature at
450 nm grows in MeI faster than that observed for toluene and acetonitrile, the induced
absorption feature at 450 nm can be assigned to the T
1
state. Once the T
1
state is generated, no
further decay of T
1
state was observed in the 1 ns time window.
a)
123

b) c)
d) e)
Figure 4.10: Femtosecond TA of ZCl in a) cyclohexane (CH), acetonitrile (MeCN) b) at initial
delays, c) at long delays, toluene d) at initial delays and e) at long delays. Excitation pump
fluence of 15 µJ/cm
2
were used for all. The red arrow highlights the change in the transient
spectrum.
In a weakly polar medium such as toluene (Figure 4.10d), stimulated emission and
excited state absorption from S
1
state appear immediately after the excitation of ZCl. However,
within 6 ps, these bands start to disappear with concurrent rising bands at 415 nm and 545 nm.
The low PL quantum yield (Figure 4.3c) of ZCl in toluene (~1%) suggests that most of the
population from S
1
state transfers to the ICT state. Thus unlike the non-chlorinated zinc dipyrrin
complexes (zDIP1-3) the ICT state in ZCl is not in equilibrium with S
1
state. As time increases,
this ICT state decays to form a T
1
state with a time constant of ~ 450 ps (Figure 4.10e).
124

Figure 4.11: Femtosecond transient absorption of ZCl in methyl iodide (MeI). The inset shows
the global analysis model used for fitting this TA data. Excitation at 520 nm was performed with
a pump fluence of 15 µJ/cm
2
.
3.3.4.2 In solid matrix
To examine whether intramolecular charge transfer happens in a solid film, TA
measurements were also performed for ZCl dispersed in PMMA film. Similar to toluene and
acetonitrile, the TA measurements of ZCl in PMMA (Figure 4.12) shows the evolution of similar
ICT states (rising bands at 415 nm and 545 nm) with simultaneous decay of stimulated emission
and excited state absorption from S
1
state. Interestingly compared to the toluene, the generation
rate of ICT state is faster (~ 0.7 ps) but the amplitude of the bands at 415 nm and 545 nm are
lower in PMMA. The lower production is probably the consequence of less stabilization of the
ICT state via solvation due to restricted reorientation in the PMMA matrix. At longer times
(Figure 4.12b) the ICT state relaxes to form T
1
state within 70 ps.
125

a) b)
Figure 4.12: Femtosecond transient absorption of ZCl in PMMA at (a) initial delays and (b)
PMMA at long delays. Excitation pump fluence of 45 µJ/cm
2
was used. The red arrow highlights
the change in the transient spectrum.
This fast generation rate (0.7-6 ps) of the ICT state in PMMA matrix (
r
~ 3.5) and
toluene (
r
= 2.38) is highly beneficial for charge separation at the D/A interface. The polar
nature of the D/A interface in an OPV may prove sufficient to induce SBCT to form ICT state
and thus can be expected to markedly affect the open circuit voltage by modulating the charge
transfer and separation at the D/A interface.
3.4. Global analysis of femtosecond transient absorption
The transient absorption on homoleptic zinc dipyrrin complexes show a solvent polarity
dependent kinetics, where in non-polar solvent only localized S
1
state is populated and in weakly
polar to polar solvents population transfer is observed from S
1
state to ICT state. In addition, the
rate for formation of ICT state and its subsequent relaxation were found to be dependent on the
polarity of the surrounding medium. To quantify the rate of decay of these states in different
polarity solvents, TA spectra were fit with a linear decomposition model described elsewhere in
detail.
43
In brief, the signal can be described by:

(1)
126

where the time dependent TA signal, S( ,t), can be decomposed into a set of time independent
species associated difference spectra (SADS), 
n
( ), and time dependent populations, c
n
(t). Each
SADS corresponds to the characteristic TA spectrum of an excited configuration of the system
and consist of both positive features due to excited state absorption and negative features due to
stimulated emission and ground state bleach. c
n
(t) is calculated assuming that the evolution of the
initially excited population is governed by a series of sequential first order rate processes. Based
on the solvent polarity dependent TA data, two different kinetic models (Figure 4.13) were used:
a) zDIP1-3 with no triplet state involved and b) ZCl with triplet state. The chlorination of zinc
dipyrrin induces triplet state formation within 500 ps due to heavy atom effect, probably due to
increase in rate of population transfer from
1
(ICT) to
3
(ICT) state followed by relaxation to
3
T
1

state.
44
Another difference between the kinetic model for ZCl and other homoleptic zDIP1-3
complexes is the absence of the backward rate (
CR
) from ICT state to S
1
state. In zDIP1-3
complexes the model accounts for an equilibrium between the local excited state S
1
and the ICT
state because the ICT induced absorption is observed even with high PL quantum yield (>15%)
in toluene. However, in ZCl the PL quantum yield is < 1% and thus only a forward rate (
CT
)
from S
1
state to ICT state is considered.
a) b)
127

Figure 4.13: Kinetic model used to fit the transient absorption data for different polarity solvents
of a) zDIP1-3 and b)ZCl.
In the model, 
S1
is the first order time constant for the deactivation from S
1
state, the
only possible deactivation pathway for S
1
state in cyclohexane. The first order time constants,


and 

denotes the formation time constant of the ICT state from S
1
state and
recombination time constant of the ICT state to the S
0
state (zDIP1-3) or T
1
state (ZCl),
respectively. The first order time constant for the decay of the T
1
state, was not captured in the
time range measured upto 1 ns and so this was kept as an offset for fitting the TA data for weakly
polar and polar solvents. These time constants are listed in Table 4.2. The SADS required to
model polarity dependent TA data of zDIP1, zDIP2, zDIP3 and ZCl are shown in Figure 4.14,
4.15, 4.16 and 4.17, respectively. These SADS are the characteristic transient spectral signature
for the initially populated S
1
state (
1
), ICT state (
2
) and T
1
state (
3
) (only present in ZCl). The
corresponding fits to the time slices of the ground state bleach, ICT state induced absorption and
triplet induced absorption (only present in ZCl) for zDIP1, zDIP2, zDIP3 and ZCl are also shown
in Figure 4.14, 4.15, 4.16 and 4.17, respectively.
Table 4.2. Kinetic rates for different processes of zDIP1–zDIP3 and ZCl in different solvents
determined by femtosecond transient absorption measurements based on kinetic model described
in figure 4.13.
Solvent 
CT
(ps) 
CR
(ps) 
rec
(ns) 
S1
(ns)
zDIP1 CycHex - - - 4.5 ± 0.1
toluene 14 ± 1 22 ± 2 3.5 ± 0.2 3.0 ± 0.3
CH
2
Cl
2
5.5 ± 0.5 - 3.3 ± 0.3 -
MeCN 3.6 ± 0.5 - 2.1 ± 0.2 -
128

zDIP2 CycHex - - - 4.8 ± 0.3
toluene 8.6 ± 2 15.5 ± 2 4.0 ± 0.5 3.9 ± 0.3
MeCN 1.1 ± 0.3 - 0.9 ± 0.1
zDIP3 CycHex - - - 1.4 ± 0.3
toluene 2.3 ± 1 11.2 ± 3 2.5 ± 0.3 2.1 ± 0.2
MeCN 1.0 ± 0.5 - 1.4 ± 0.1 -
ZCl CycHex - - - 2.5 ± 0.2
toluene 6±0.5 - 0.45 ± 0.02 2.1 ± 0.2
MeCN 1.5 ± 0.3 - 0.55 ± 0.01 -
PMMA 0.7±0.2 - 0.07±0.005 -
129

Figure 4.14: (a, c, e, g) SADS used to model the transient spectra of zDIP1. (b, d, f, h) The fits
(black dashed) based on SADS analysis for the time slices taken through the ground state bleach
(red) and spectral range indicative of ICT state (blue).
130

Figure 4.15: (a, c, e, g) SADS used to model the transient spectra of zDIP2. (b, d, f, h) The fits
(black dashed) based on SADS analysis for the time slices taken through the ground state bleach
(red) and spectral range indicative of ICT state (blue).
131

Figure 4.16: (a, c, e, g) SADS used to model the transient spectra of zDIP3. (b, d, f, h) The fits
(black dashed) based on SADS analysis for the time slices taken through the ground state bleach
(red) and spectral range indicative of ICT state (blue).
132

133

Figure 4.17: (a, c, e, g) SADS used to model the transient spectra of ZCl. (b, d, f, h) The fits
(black dashed) based on SADS analysis for the time slices taken through the ground state bleach
(red), spectral range indicative of CT state (green) and spectral region for T
1
state absorption
(blue).
3.5. Comparison between dynamics of zDIP1-3 and ZCl in polar solvents:
Compared to zDIP1 (
CT
= 4 ps and 
rec
= 2.1 ns), femtosecond TA of zDIP2 in
acetonitrile (Figure 4.18a) show faster rates for both formation and recombination of the ICT
state (
CT
= 1.1 ps and 
rec
= 0.9 ns). The ICT state dynamics was however observed to be similar
for zDIP2, zDIP3 and ZCl in acetonitrile (Figure 4.18b). All three show faster rates for both
formation and recombination of the ICT state (
CT
= 1.1, 1.0, 1.5 ps and 
rec
= 0.9, 1.4, 0.6 ns for
zDIP2, zDIP3 and ZCl, respectively), although in ZCl the relaxation of the ICT state occurs to
the T
1
state. The two dipyrrin ligands of zDIP2, zDIP3 and ZCl are expected to have less
torsional freedom around the Zn center relative to zDIP1 due to the presence of methyl groups or
l atoms at the α -positions. This steric hindrance constrains the ligands to adopt a more
orthogonal configuration as expected from X-ray crystal structure. Without these steric
impediments, the ligands can twist in the excited state, promoting inter-ligand π-π interactions.
This π-π interaction will lower the energy of the excited state, making the S
1
state less
energetically favorable to form the ICT state in zDIP1.
a) b)
134

Figure 4.18: a) Comparison of dynamics for formation of the ICT state (monitored at 370 nm,
2) (filled) and ground state bleach (open) between zDIP1 (red squares) and zDIP2 (blue circles)
in acetonitrile. b) Alkylation/chlorination leads to similar dynamics for both the ICT state and
ground state bleach for zDIP2, zDIP3 and ZCl.
3.6. Application to organic photovoltaic devices:
In organic photovoltaics (OPVs), C
60
is a very common acceptor due to its high electron
mobility and ability to form efficient heterojunctions with a wide variety of donor materials. The
LUMO energy for ZCl was measured to be similar to that for C
60
and so the idea was to evaluate
ZCl as an alternative acceptor material.
45
To study the performance of ZCl in devices, vapor
deposited planar-heterojunction OPVs were fabricated, where the donor is
tetraphenyldibenzoperyflanthrene (DBP) and the acceptor is either ZCl or C
60
. The open circuit
voltage (V
OC
) along with other device parameters for these two devices are shown in Figure 4.19.
Based on similar LUMO energies, the thermodynamic limit for V
OC
obtained from the
LUMO
acceptor
 HOMO
donor
energy gap (E
DA
) is observed to be same ~1.5 eV for both DBP/ZCl
and DBP/C
60
devices.
45
However, the V
OC
for the ZCl device is significantly larger compared to
DBP/C
60
device. This increase in V
OC
is probably due to weak electronic coupling between donor
and acceptor materials in the ZCl devices caused by incorporation of SBCT into the charge
separation process. This is because initially intramolecular charge transfer occurs via SBCT in
ZCl followed by hole transfer to DBP to form an oxidized donor (DBP
+
) and reduced dipyrrin
ligand separated by a neutral dipyrrin ligand. This increases the separation length between the
two opposite charges and thus the recombination decreases.
135

a) b)
Figure 4.19: Device architecture (a) and I-V (b) for the devices described in the text.
Thickness = 20 nm for DBP, 40 nm for C
60
, and 20 nm for ZCl.
4. Conclusion
In conclusion, it was shown that homoleptic zinc dipyrrin complexes undergo SBCT to
form ICT state within 1 ps in strongly polar medium. The ICT state formation in rigid PMMA
(
S
~3) matrix indicates that SBCT can also happen in weakly polar medium without the
stabilization provided by solvation. This is promising as then the polarity of donor-acceptor
interface is sufficient to induce SBCT. Thus, the energetic driving force required for charge
generation can be reduced and presence of a neutral ligand spacer between donor and acceptor
leads to weak coupling of the donor and acceptor to reduce charge recombination losses. This
proposition is supported by the high V
OC
measurements for ZCl devices. In future, the
recombination of ICT to T
1
state needs to be further reduced probably by reducing the heavy
atom effect without effecting the LUMO position required to make devices. These SBCT
materials can be very useful as interface materials in OPVs, where a thin layer will be placed at
the donor acceptor interface to significantly enhance V
OC
.

0.0 0.5 1.0 1.5
-6
-4
-2
0
Current density (mA/cm
2
)
Voltage (V)
DBP/C
60
DBP/ZCl
136

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140

Chapter 5
Singlet Fission in covalently linked alkynyltetracene dimer
1. Introduction:
The Shockley-Queisser limit states that the maximum efficiency obtained by a single
junction solar cell cannot exceed 33%.
1
One of the dominant loss mechanism contributing
towards this limit is thermalization loss, i.e., energy absorbed above the band gap is dissipated to
heat. Multiexciton generation is one of the processes to increases the limit to 45%,
2
in which the
high energy photon splits into two, producing two excitons that are both capable of charge
separation, while lower-energy photons are used in the usual fashion. Multiexciton generation
has been observed for various colloidal inorganic nanocrystals.
3-4
With molecular chromophores,
such excitation splitting is known as singlet fission. Singlet fission is a process in which a
chromophore in its singlet excited state interacts with an adjacent molecule in ground state to
form two triplet states. The process is illustrated in equation 1. In the first step the S
1
+S
0
state
transforms to a correlated pair of triplets,
1
(T
1
T
1
) with overall singlet character. Later, this
transient state loses spin coherence and diffuse to generate two uncorrelated triplets.
5

141

The conditions required for a useful singlet fission material are: (1) satisfying the
thermodynamic energy requirement, E(S
1
) ≥ 2E(T
1
), (2) presence of significant electronic
coupling between the S
0
S
1
and
1
(T
1
T
1
) states, (3) faster rate of singlet fission compared to other
excited state relaxation pathways, and (4)finally the correlated triplet pair should be able to lose
coherence and diffuse to form separated triplets.
6-7
Extensive research has been done to
understand and engineer materials with these criteria fulfilled.
6, 8
Singlet fission has been studied
extensively in thin films or crystals of polyacenes containing four rings (tetracene) and greater
with triplet yield reaching upto 200%.
9-13
Certain biradicaloid materials like 1,3-
diphenylisobenzofuran have also been shown to fulfill the energy requirement with ~200%
triplet yield.
14
However, the factors driving the second step of singlet fission, i.e., the separation
of the triplets out of the coupled pair,
1
(T
1
T
1
) is comparatively less explored. This is because
most of the experiments performed till date observe direct transformation of singlet to a triplet
state with no distinct intermediate coupled triplet pair state except some experiments where a
superposition of S
1
and
1
(T
1
T
1
) state is proposed to be observed.
15-16
Therefore, it has been
difficult to independently study the generation and decay of
1
(T
1
T
1
) state. Theoretical studies
have suggested that entropy can be one of the driving force for separation of the triplets.
16-17

Too weak electronic coupling between two chromophores can be insufficient to permit
fast formation of a correlated triplet-pair state compared to other deactivation pathways, and
coupling stronger than kT can lead to a bound
1
(T
1
T
1
) state such that the triplets cannot separate
on the timescale available.
7
An extreme example of intramolecularly strongly coupled
chromophores is 1,3-butadiene where the
1
(T
1
T
1
) state is identical to the molecular
2
Ag state.
18

Additionally, if the coupling opens new deactivation channels like excimer or charge transfer
formation, the singlet fission process can get modulated. However, excimer has been
142

suggested
19-21
and disputed
22
as an intermediate step during singlet fission of pentacene. This
coupling is related with the relative orientation of the chromophores and ideally it needs to fulfill
all the above mentioned criteria. Recent studies on two different polymorphs of 1,3-
diphenylisobenzofuran illustrate that depending on the orientation, the singlet fission yield can
vary from 125% to 10%.
23
In another example, attaching phenyl rings to tetracene (5,12
diphenyltetracene:DPT) leads to an amorphous film compared to the polycrystalline film for
tetracene.
13
This translates to a triplet yield of 125% with two triplet rise times for DPT
13

compared to ~200% triplet yield with ~200 ps rise time for tetracene
11
thin film. The proposed
interpretation is that singlet fission only occurs at a subset of sites in the disordered DPT film,
those with a favorable relative orientation of neighboring DPT molecules. When one of these
sites is excited directly, the fast triplet rise time (~800 fs) is observed whereas the slow rise time
(~100 ps) results from the time required for singlet excitons to diffuse to a singlet fission active
site before producing triplets. These results highlights the dependence of the singlet fission rate
on the relative chromophore orientation.
In most of the reported singlet fission studies the coupling between the chromophores is
controlled by the morphology of the film or crystal. In order to gain a fundamental understanding
of the orientation effects on the rate of singlet fission, covalently linked dimers have been studied
in solution where the geometry and strength of intermolecular interactions can be carefully tuned
through chemical synthesis. Linearly-linked tetracene and isobenzofuran dimers produce a small
number of triplets (~3-9%) in solution,
24-26
but it cannot be conclusively shown that the triplets
originate from singlet fission in these systems due to such low yields. Cofacial perylene-
3,4:9,10-bis(dicarboximide) (PDI) dimers have been observed to relax into an excimer upon
excitation followed by relaxation to the ground state.
27-28
Recent studies on donor-acceptor
143

polymers show the first observation of intramolecular singlet fission for polymers isolated in
solution with a triplet yield of ~170% where the strong intrachain donor-acceptor interactions
provides the required coupling by forming an intermediate of charge transfer state character.
29
A
more recent dimer based on molecular chromophores like two pentacenes linked via a phenylene
spacer in meta-position feature triplet quantum yields as high as 156%.
30
This shows that with
right orientation and coupling between intramolecular chromophores the singlet fission yield for
an isolated dimer in solution is getting close to the values reported for thin films with
intermolecular coupling.
In this chapter excited state dynamics of two co-facial dimers of tetracene (Figure 1) are
studied. The motivation is to optimize the chromophore orientation for the tetracene dimer and
then dope them into other monomer singlet fission host of low film crystalline quality like DPT
to produce more triplets due to production of more singlet fission sites. Thus, this will relax the
constraints placed on the film manufacturing process to achieve low-cost, high efficiency singlet
fission based OPVs. In one dimer, bis-ethynyltetracenyl-xanthene (BET-X) the overlap of the
tetracenes is maximum, while in the other bis-ethynyltetracenyl-benzene (BET-B) the tetracenes
are still cofacial but the overlap is minimum. These dimers are characterized by both steady state
and time resolved absorption spectroscopies in several media. It was observed that the amount of
π overlap has a large impact on the excited state dynamics of the dimers, such that one dimer
relaxes via excimer-like interactions, while the other relaxes into a
1
(T
1
T
1
) state in several
picoseconds. Additionally, the role of entropy in driving the transformation from
1
(T
1
T
1
) state to
uncorrelated triplets in BET-B have been illustrated with experimental evidence. Finally, a single
rate for the triplet formation in the BET-B dimer film with triplet yield of 154±10% compared to
144

two rise times for triplets in monomeric tetracene derivative with triplet yield of 90±8%
demonstrate the presence of intramolecular singlet fission in BET-B dimer film.
2. Experimental Section:
2.1 Synthetic Procedures: Procedures for the synthesis of bis-ethynyltetracenyl-xanthene (BET-
X) and bis-ethynyltetracenyl-benzene (BET-B) were used without modification as developed and
reported earlier.
31

2.2 Preparation of Solutions for Femtosecond Transient Absorption: Samples containing
BET-X and BET-B dissolved in cyclohexane, toluene, chloroform, THF, or acetonitrile were
placed in a close capped 1 mm quartz cuvette. The concentration of each sample was adjusted to
give an optical density between 0.11 and 0.18 at 500 nm. The solutions were prepared in the
glovebox to prevent any quenching by oxygen.
2.3 Preparation of Thin Films for Femtosecond Transient Absorption: Solutions were spin-
coated onto pre-cleaned quartz slides from tetranydrofuran (THF) solutions for BET-B ad ET-
TMS neat film. The thickness of the BET-B and ET-TMS films were ~66 nm and 100 nm,
respectively with an optical density of ∼0.3 and ~0.2 at 500 nm. For the sensitization
measurements ~5 mol% of acenes (ET-TMS or BET-B or DPT) are doped in Pd(TPBP) film. For
the PMMA films ~1-2 wt% of BET-B and BET-X dimers are dispersed in the polymer solution.
And for BET-B doped in DPA and DPT films 17.6 and 11.9 mol%, respectively of BET-B is
used. Film samples used for TA studies had an additional quartz window placed on top of the
film, and the outside edges were sealed with epoxy under a N2 atmosphere to prevent sample
photooxidation.
145

2.4 Femtosecond Transient Absorption: The apparatus has been described previously.
13, 32
In
brief, pump and probe pulses were derived from the output of a Ti:sapphire regenerative
amplifier (Coherent Legend, 1kHz, 4 mJ, 35 fs). Excitation pulses centered at either 500 or 550
nm were generated by pumping a type-II OPA (Spectra Physics OPA-800C) with ~10 % of the
amplifier output. White light supercontinuum probe pulses, spanning the visible (320-950 nm)
were obtained by focusing a small amount of the amplifier output into a rotating CaF
2
disk. To
avoid any contribution to the observed dynamics from orientational relaxation,
33
the polarization
of the supercontinuum probe was set at the magic angle (54.7º) with respect to the pump
polarization. The probe was collimated and focused with a pair of off-axis parabolic mirrors into
the sample whereas the pump pulse was focused using a CaF
2
lens. The cross correlation
between the pump and probe in a quartz substrate matched to that used to support films had a
FWHM of 150 fs averaged across the probe spectrum for 500 nm excitation. A slightly longer
instrument response of 170 fs was found for 550 nm excitation. The instrument response for the
solution measurements are slightly higher ~180 fs. The supercontinuum probe was dispersed
using a spectrograph (Oriel MS127I) onto either a 256-pixel silicon diode array (Hamamatsu) for
visible detection. Spectra were measured for a range of excitation fluences from 12 to 96 μJ/cm
2

for ET-TMS neat film and 8 to 64 μJ/cm
2
for BET-B film. The TA data shown in Figure 5.7 and
5.8 are reported for 24 μJ/cm
2
and 16 μJ/cm
2
for ET-TMS and BET-B, respectively. The solution
measurements were performed with a pump fluence of ~25 μJ/cm
2
. Samples were slowly
translated perpendicular to the path of the pump and probe by a linear stage to prevent
photodamage. The dwell time used for each position on the sample is less than 1 ms (time
between two consecutive pulses) to make sure each laser pulse hits a fresh spot.

146

3. Results:
3.1. Structure of the dimers:
In designing the covalent dimers for the singlet fission study, the important criterion for
the monomer selection is absence of fast non-radiative deactivation pathways from the singlet
excited state in solution. A good starting point is a monomeric chromophore whose fluorescence
quantum yield (Φ
fl
) is close to unity. A chemically stable, TMS-capped analog of ethynyl-
tetracene (ET-TMS) (Figure 5.1a) is chosen as the monomer model system. Two different dimers
of ethynyl-tetracence are prepared to examine the effect of orientation between the two
chromophore units on singlet fission and other photophysical processes. One of the dimers was
built on a benzene core (BET-B) with a smaller amount of cofacial overlap between the two ET
units, while the second dimer (BET-X) was built from a xanthene core with the goal to maximize
the cofacial π orbital overlap of the acenes (Figure 5.1b & 5.1c).
a) b) c)
Figure 5.1: Structures of the a) monomer ET-TMS and the dimers (b) BET-B and (c) BET-X.
The overlap between the acenes is confirmed from the crystal structure of BET-X and
BET-B (Figure 5.2). The BET-X dimer has two structurally distinct forms. In the first BET-X
dimer (Figure 5.2a) the acenes are slanted at ~50 angle relative to the xanthene core, resulting in
a slip-stacked geometry with three acene rings overlapping. In the other BET-X dimer (Figure
147

5.2b) the acenes are nearly perpendicular to the xanthene core, with all four rings overlapping. In
both of the BET-X dimers the distance between the tetracenes is approximately 3.4 Å. In BET-B
dimer the tetracene moieties are oriented at ~46 angle with respect to each other (Figure 5.2c).
This conformer of BET-B exhibits approximately one ring of overlap between the tetracenes
(Figure 5.2c). The distance between the tetracenes within BET-B is 3.1 Å at the closest point of
interaction, and 3.8 Å at the farthest point.
a) b) c)
Figure 5.2: Crystal structures of the two BET-X isomers a) and b) in the crystal. The view
chosen here has the xanthene moiety perpendicular to the page. c) Crystal structure of BET-B
isomer showing the overlap over one acene ring.
3.2. Steady state photophysical properties:
The steady state absorption and emission spectra for ET-TMS, BET-B and BET-X in
solution and neat film are shown in Figure 5.3. The absorption spectra for ET-TMS in solution
(Figure 5.3a top panel) looks spectrally similar to the tetracene derivatives reported in the
literature.
11, 13
The emission spectra for ET-TMS in solution mirrors its absorption with a Stokes
shift of ~460 cm
-1
. The fluorescence lifetime of ET-TMS in THF was found to be 17 ns with a
fluorescence quantum yield (QY) of 95%. The absorption spectrum of neat ET-TMS film is
broader and 20 nm red-shifted compared to the solution (Figure 5.3a bottom panel). The
emission properties of the neat ET-TMS film are drastically different from those in solution. The
148

neat film emission line-shape is broad and featureless with a significant Stokes shift of ~2500
cm
-1
suggesting that emission is originating from an excimer state. The intensity of the neat film
emission is very low with a QY of 0.6 %. This suggests that in neat film some additional
relaxation channel opens up for the excited state S
1
populations.
a) b)
c)
Figure 5.3: Absorption (green) and emission (red) spectra of a THF solution (top panel) and neat
film (bottom panel) for a) ET-TMS, b) BET-B and c) BET-X. The quantum yields are mentioned
in the plots. Due to photodegradation of neat BET-X film the quantum yield could not be
measured.
Compared to the monomer ET-TMS, the absorption spectra of both dimers BET-B and
BET-X (Figure 5.3b & 5.3c top panel) in solution exhibit an intense 0-1 transition compared to
0-0 transition. This feature has previously been observed in covalent dimers with cofacially
oriented chromophores and has been attributed to Davydov interactions.
27-28, 34
In addition
149

compared to the monomer ET-TMS, the BET-B and BET-X dimers exhibit a red shift of 16 nm
and 9 nm, respectively in solution. In the neat film the characteristic absorption shape is similar
to solution with a red shift of ~10 nm for both dimers. However, the emission properties (Figure
5.3b & 5.3c bottom panel) in solution are drastically different for both the dimers. BET-B dimer
in THF exhibits a vibrationally resolved S
1
emission with a Stokes shift of ~ 650 cm
-1
whereas
BET-X dimer exhibit a broad emission with a Stokes shift of ~2800 cm
-1
. The emission shape in
BET-X is indicative of an excimeric excited state, however, the emission is not from a fully
formed excimer, as the lineshape exhibits some vibronic structure. Similar emission
characteristics have been observed for an isolated, co-facially stacked pair of tetracenes at
15 K.
35
Nevertheless, the emission QY for the dimers were found to decrease compared to ET-
TMS (QY~95%) with QY of 2.6% for BET-X and 0.6 % for BET-B in solution. This decrease in
QY indicates that compared to the monomer, the covalently tethered dimers exhibit additional
relaxation pathways from the S
1
excited state.
To illustrate the differences of the xanthene and benzene cores the ethynyl-tetracene
dimers (BET-B and BET-X) are compared to their ethynyl-anthracene analogs (BEA-B and
BEA-X) (Figure 5.4). The energy requirements for singlet fission (S
1
 2 T
1
) are met for the
ethynyl-tetracene dimer analogs but not for ethynyl-anthracene analogs. When the acenes are
cofacially bound on the xanthene moiety, their predominant excited state relaxation pathway is
via an excimer. The low intensity, excimer-like emission is exhibited by both of the xanthene
based ethynyl-anthracene and ethynyl-tetracene dimers, Figure 5.4(a) and (b) (top). The
quantum efficiency of the emission in these dimers is fairly low, 1.6% and 2.6%, respectively.
In contrast, the difference in emission quantum yields of the anthracene (BEA-B) and tetracene
(BET-B) with benzene core is drastic, 68% versus 0.6%, respectively. This indicates that for the
150

dimers attached to benzene core, the lower E(T
1
)/E(S
1
) energy of tetracene opens a channel of
excited state deactivation which is inaccessible in the anthracene dimer with a high E(T
1
)/E(S
1
)
ratio.
a) b)
Figure 5.4: Comparison of absorption (green) and emission (red) spectra of the a) anthracene
dimers versus the b) tetracene dimers, with their respective emission quantum yields listed to the
right of the spectra.
3.3. Absorption spectra and extinction coefficient for triplet state (T
1
T
n
):
The absorption spectra and extinction coefficient for T
1
T
n
transition of ET-TMS and
BET-B is obtained from triplet sensitization measurements using a triplet sensitizer doped in neat
film of acene. The BET-X films were observed to photodegrade and so experiments could not be
done on them. Triplet sensitization of the monomer ET-TMS and BETA were performed by
using a small portion (~ 5 mol%) of palladium tetraphenylbenzoporphyrin (Pd(TPBP)) blended
into the film of acenes. Pd(TPBP) was chosen as a sensitizer because Pd porphyrins are known to
undergo rapid (picosecond) intersystem crossing with high ~97% triplet yield.
36
The triplet
energy of 1.59 eV for Pd(TPBP)
37-38
makes triplet energy transfer from T
1
-Pd(TPBP) to T
1
of
acenes energetically favorable as the triplet energy for analogous tetracene derivatives is ~1.20-
151

1.28 eV.
39
The transient absorption measurements were performed by selectively exciting the
Pd(TPBP) molecules doped in films of ET-TMS and BETA using 626 nm excitation. Figure 5.5
shows that exciting Pd(TPBP) leads to an immediate photobleach of its Q (640 nm) and Soret
(445 nm) bands, indicating a depopulation of the Pd(TPBP) ground state. In the spectral range
between these two transitions, an induced absorption is observed whose shape evolves with time.
Figure 5.5c highlights the spectral evolution during the first 10 ps after photoexcitation and
reveals the presence of an isosbestic point at 508 nm. Based on the similarity of the signal at this
delay to the reported triplet spectrum of Pd(TPBP) measured before in TA experiments,
38
the
isosbestic point observed at early delays shows the ISC between the S
1
and T
1
states of
Pd(TPBP). However, at longer times (>10 ps), the photobleaches of both the Soret and Q bands
of Pd(TPBP) decay, indicating repopulation of the porphyrin's ground state (Figure 5.5a and
5.5b). Concomitant with the photobleach relaxation, the induced absorption band between 440
and 600 nm evolves from a shape indicative of Pd(TPBP)'s T
1
state into a narrower band with
peaks located at 460 and 506 nm for ET-TMS (Figure 5.5a) and band between 400-490 nm for
BETA (Figure 5.5b). This bands are assigned to the T
1
T
n
transition for ET-TMS (Figure 5.5a)
and BETA (Figure 5.5b) as following generation this bands do not decay within the ns time
window and the long time peaks obtained for ET-TMS matches well with that reported for
T
1
T
n
transition for tetracene derivatives.
32, 39
With this spectral assignment in hand, the TA
spectra in Figure 5.5a and 5.5b are fitted using a three-state kinetic model,
S
1, Pd(TPBP)

T
1, Pd(TPBP)

T
1, acene

where k
ISC
is the ISC rate of Pd(TPBP) and k
TT
is the rate of triplet energy transfer from
Pd(TPBP) to acene hosts. The optimum value of 1/k
ISC
was calculated to be ~3 ps for both doped
152

films, while the best fit for 1/k
TT
was estimated to be 50 ps and 20 ps for ET-TMS and BETA,
respectively.
a) b)
c) d)
Figure 5.5: TA spectra of a) 5 mol% Pd(TPBP) in ET-TMS and b) 5 mol% Pd(TPBP) in BETA
films following photoexcitation at 626 nm. c) A close-up of the induced absorption band
between 460 and 600 nm, highlighting the formation of T
1
state from S
1
state of Pd(TPBP)
within 10 ps for Pd(TPBP) doped in ET-TMS film. d) TA spectra of 5 mol% Pd(TPBP) in DPT
film with 626 nm excitation.
In addition to the spectral assignment for T
1
T
n
transition for ET-TMS and BETA, to
calculate the extinction spectra for this acenes a similarly (~ 5 mol%) doped film of Pd(TPBP) in
5,12 diphenyltetracene (DPT) film was studied to produce DPT triplets with known extinction
spectra. The TA spectra for Pd(TPBP) doped in DPT following excitation of Pd(TPBP) at 626
nm are shown in Figure 5.5d. Similar to previously reported
32
triplet spectra narrower band with
peaks located at 455 and 490 nm are observed to evolve within 50 ps. With the known extinction
spectra and TA data at long time (800-900 ps) the triplet transfer yield was calculated to be
153

93±4%. Since the ISC rate from S
1
(Pd(TPBP)) to T
1
(Pd(TPBP)) and T
1
(Pd(TPBP)) to
S
0
(Pd(TPBP)) is similar for all three doped acenes, at long times (longer than triplet transfer
time), ~800-900 ps it was assumed that amount of triplets produced in DPT will be same as that
produced in similarly doped ET-TMS and BETA films. Based on this assumption, the triplet
extinction spectra can be calculated using,

where X refers to the acene host, in this case ET-TMS or BETA. The A signals used here were
long time (800-900 ps) data where the triplet energy transfer from Pd(TPBP) is complete. This
timescale is reasonable as 1/k
TT
for DPT, ET-TMS and BETA was calculated to be 40 ps, 50 ps
and 20 ps, respectively. The l(X:Pd) and l(DPT:Pd) refer to the length of the Pd(TPBP) doped
acene (ET-TMS or BETA) and DPT films, respectively. The thickness for all the three Pd(TPBP)
doped films were similar ~65 nm. Therefore using the equation (1) the triplet extinction spectra
for ET-TMS and BETA are calculated from the long time data (800-900 ps) TA data on
Pd(TPBP) doped acene films (Figure 5.6).

Figure 5.6. Differential extinction spectra obtained for T
1
T
n
transition in ET-TMS (blue) and
BET-B (red).
154

3.4. Femtosecond transient absorption (TA) measurements:
3.4.1. ET-TMS in solution and film
Transient absorption spectra of ET-TMS molecules isolated in solution resemble those of
the monomeric tetracenes.
13, 32
Upon excitation with 500 nm light, transient absorption spectra
of ET-TMS in solution (Figure 5.7a) show S
1
S
n
induced absorption features (~ 407 nm)
similar to those observed for tetracene derivatives,
13, 32
with no spectral evolution over 1 ns. The
S
1
S
n
induced absorption decays with the same rate as that for fluorescence decay (~17 ns). As
anticipated from the steady state spectral data, the excited state dynamics of ET-TMS neat film
differ significantly from those in solution. The TA data of neat ET-TMS film in Figure 5.7b
shows an immediate appearance of S
1
S
n
induced absorption peak at ~ 407 nm following
excitation at 500 nm. However, within 1 ps this peak decreases and a new induced absorption
feature grows at ~506 nm. The absorption profile (peaks at 460 and 506 nm) from the transient
spectrum of neat ET-TMS film at long delays match with the T
1
spectrum of ET-TMS from
sensitization measurements (Figure 5.7b, dotted cyan line and Figure 5.5a).
a) b)
155

c) d)
e)
Figure 5.7: Transient absorption spectra of the monomer ET-TMS in a) CHCl
3
and b) neat film
following excitation at 500 nm. The cyan dotted line shows the triplet spectrum obtained from
ET-TMS sensitization measurements with Pd(TPBP). c) The extinction spectra for S
1
S
n
and
T
1
T
n
transitions used to calculate singlet and triplet populations (d) for ET-TMS. e)
Comparison of TA spectra with a fit (red dotted) consisting of a linear combination of the singlet
and triplet spectra of ET-TMS.
The triplet yield calculations were performed using the procedure reported by Roberts et.
al.,
13
where the time dependent singlet and triplet populations are extracted by fitting the TA data
with a linear combination of the extinction spectra for the S
1
and T
1
state using,

Here, a
S
(t), a
T
(t) refers to the time dependent singlet and triplet populations and 
S
( ),
T
( )
are the differential extinction spectra for S
1
S
n
and T
1
T
n
transitions. These two spectra
(Figure 5.7c) are determined experimentally from TA measurements independent of those
performed on neat ET-TMS film. The lineshape for S
1
state of ET-TMS is obtained by red-
156

shifting the transient absorption spectrum of ET-TMS in solution at initial delays by 18 nm. This
shift corresponds to the red-shift in the ground-state absorption spectra (Figure 5.3) on going to
neat film from solution. Comparison of the amplitude of the ground-state bleach that appears in
this spectrum with the extinction spectra of ET-TMS ground state allows to scale the amplitude
of the basis spectrum such that it represents the change in the molar absorptivity of the acene
following excitation to its S
1
state. The triplet differential extinction spectra are obtained using
the sensitization experiments described in the previous section. Figure 5.7e compares the
transient spectrum measured for neat ET-TMS film with fits comprised of a linear combination
of the S
1
and T
1
basis spectra. The agreement between the fit and the transient data is able to
reproduce both the decay of the S
1
→S
n
transition and the growth of the T
1
→T
n
transition. Using
the extinction spectra for S
1
S
n
and T
1
T
n
(Figure 5.7c) transitions, the TA data for ET-TMS
film was fit to extract the singlet and triplet populations (Figure 5.7d) and the maximum triplet
yield was determined to be ~ 90±8% at 45 ps (efficiencies here are quotes as (triplets
produced)/(initial # singlets)). These experiments show that the energy level criterion is fulfilled
for the ET-TMS molecule and thus the E(T
1
)/E(S
1
) ratio in the covalent dimers of ET-TMS
should be sufficient for them to undergo singlet fission.
3.4.2. BET-B in different media
3.4.2.1 BET-B neat film
TA spectra of neat BET-B film are shown in Figure 5.8. The excitation with 500 nm
(Figure 5.8a) leads to immediate ground state bleach between 475-575 nm and S
1
S
n
induced
absorption at 420 nm. However, within 1 ps the peak due to S
1
S
n
decreases and a new induced
absorption feature grows in at 450 nm. This new induced absorption feature is assigned to
T
1
T
n
absorption, based on the sensitization measurements of BET-B film with Pd(TPBP)
157

(broken cyan line and Figure 5.5b). The triplet yield calculations were performed using the
procedure described above for ET-TMS neat. Using the extinction spectra for S
1
S
n
and T
1
T
n

(Figure 5.8b), the TA data for neat BET-B was fit to extract the singlet and triplet populations
(Figure 5.8c) and a maximum triplet yield of 154±10% at 10 ps was obtained. Once the triplets
are generated, they decayed with a rate of ~ 400 ps. This lifetime is significantly shorter than the
triplet state produced via sensitization (3 µs) and is independent of the excitation power
(described later); which suggests geminate triplet-triplet annihilation as the dominant decay
pathway in neat BET-B. The fits to the TA data for BET-B neat film are shown in Figure 8d.
a) b)
c) d)
Figure 5.8. Transient absorption spectra of neat films of a) BET-B with the corresponding c)
singlet and triplet populations calculated using the extinction spectra shown in b). The cyan
dotted line in b) shows the triplet spectrum obtained from BET-B sensitization experiments. d)
Comparison of TA spectra with the fit (red dotted) consisting of a linear combination of the
singlet and triplet spectra of BET-B.
158

3.4.2.2 BET-B in solution
TA measurements on solutions of BET-B in THF and PMMA were performed using 500
nm excitation (Figure 5.9). TA data shows immediate appearance of ground state bleach between
450-550 nm and S
1
S
n
induced absorption between 350-680 nm (peak a 400 nm). The induced
absorption at ~420 nm looks similar in width compared to that of ET-TMS (60 nm vs. 50 nm),
suggesting that the induced absorption at ~ 400 nm originates from the S
1
state. As the time
delay increases, the S
1
S
n
absorption decreases and a new transient state (with induced
absorption at ~560 nm) appears within 1 ps. However, this state does not exhibit the
characteristic spectral line shape of the BET-B T
1
T
n
absorption and decays with a lifetime of
700 ps. The rate and magnitude of its formation is independent of solvent polarity and
polarizability (Figure 5.9a, inset), ruling out a charge transfer state. Since no excimer-like
emission is observed from steady state emission spectra for solution (Figure 5.3b), the excimer
formation is also ruled out. The spectral shape for this new transient state looks like a
combination of T
1
T
n
and S
1
S
n
spectral shapes for BET-B. So the other possibility for this
transient state is
1
(T
1
T
1
) state and further evidence for this assignment is described later.
a) b)
Figure 5.9. Transient absorption spectra of BET-B in a) THF and b) PMMA solution. The inset
in a) shows the solvent polarity independent dynamics at 570 nm for
1
(T
1
T
1
) state.
159

3.4.2.3 BET-B in diphenylanthracene (DPA) and diphenyltetracene (DPT):
The previous experiments suggest that although in neat BET-B film the triplets are
observed to evolve, isolated BET-B in solution do not show triplet generation following
excitation to the singlet state. Rather a new state,
1
(T
1
T
1
) evolves in solution with a different
lineshape from that of uncorrelated triplets in BET-B. To determine if a triplet from the
1
(T
1
T
1
)
state on an isolated BET-B is transferred to another molecule to give two uncorrelated triplets,
we doped BET-B into 5,12-diphenyltetracene, DPT, a material that can potentially accept a
triplet from
1
(T
1
T
1
) state. DPT’s S
1
energy is higher than that of BET-B, therefore BET-B can be
selectively excited, and no singlet energy transfer should occur. The T
1
state of DPT is also
expected to be higher than that of BET-B, but considering the long lifetime of BET-B’s
1
(T
1
T
1
)
state, triplet energy may transfer from the
1
(T
1
T
1
) state of BET-B to DPT. A sample of BET-B
doped into 9,10-diphenyl-anthracene, DPA, was prepared as a control. In the DPA based film the
host’s T
1
state energy is markedly higher than that of BET-B, which will preclude any triplet
transfer from the BET-B molecule. To get reasonable optical density to photoexcite in TA
measurements, 18 and 12 mol% of BET-B is doped in DPA and DPT. At this doping
concentration there is possibility of two adjacent BET-B molecules, however, the BET-B doped
DPA transient absorption data shows that such occurrence is rare. The pump wavelength
(550 nm) was chosen such that only the BET-B molecules are selectively excited, avoiding the
excitation of the host molecules, DPA or DPT. BET-B doped in DPA (Figure 5.10a) exhibits
similar transient behavior as BET-B in solution. Upon initial excitation to the S
1
state, the same
transient
1
(T
1
T
1
) state with induced absorption at ~580 nm was populated within 1 ps, followed
by relaxation to the ground state in ~500 ps. BET-B doped into DPT (Figure 5.10b) exhibits
similar spectral and kinetic behavior to the BET-B in solution and PMMA until 2 ps, however, at
160

longer time delays (>10 ps), the spectral feature for the T
1
of BET-B (similar to that observed for
neat BET-B film) becomes distinct. Simultaneously, the ground state bleach (sharp peaks at 440,
470 and 500 nm) corresponding to the DPT host emerges, suggesting an energy transfer to the
DPT. The energy transfer is not the singlet energy transfer as the characteristic S
1
S
n
induced
absorption for the DPT is not observed
13
and the linear absorption spectra also suggest that the S
1

of BET-B is lower in energy compared to the S
1
of DPT. This indicates a triplet energy transfer
to the DPT host from the
1
(T
1
T
1
) state initially formed in BET-B. Thus, at longer delays the
1
(T
1
T
1
) state of BET-B relaxes to generate one T
1
(BET-B) and one T
1
(DPT). The combined
triplet yield was found to be 140±10% at 1 ns based on the extinction spectra for BETA and DPT
T
1
T
n
transitions. This yield calculation is based on the fact that the triplet transfer from
1
(T
1
T
1
)
state to one T
1
(BET-B) and one T
1
(DPT) is complete within 1 ns. The validity of this
assumption is verified in the kinetic model described later.
a) b)
Figure 5.10. Transient absorption spectra of BET-B doped in a) DPA and b) DPT on exciting
with 550 nm. The cyan dotted line in b) shows the triplet spectrum obtained from BET-B
sensitization. experiments.
To confirm that no DPT molecules in BET-B doped DPT film is photoexcited, TA
measurements were also performed on neat DPT film under similar excitation conditions. The
comparison between UV-visible absorption of neat DPT film and 10% BET-B doped in DPT
161

film is shown in Figure 5.11a. The neat DPT film was prepared with similar amount of DPT as
that present in BET-B doped DPT film. The pump spectrum used for TA measurements on BET-
B doped DPT film and neat DPT film is also shown in Figure 5.11a. The linear absorption
spectrum indicates that the excitation of DPT molecules is very lean due to low absorption value
(0.008 O.D.) for neat DPT film at 550 nm. To exclude the possibility of DPT excitation in BET-
B doped DPT film, TA measurements were performed on neat DPT film (Figure 5.11b) with
similar pump fluence (26 J/cm
2
) as that used for BET-B doped DPT film reported in Figure
5.10b. This excitation condition did not result any detectable signal from TA measurements.
However, with 6 times higher pump fluence (156 J/cm
2
) at 550 nm the TA measurements on
neat DPT film (Figure 5.11c) reproduced the reported TA spectrum of neat DPT film,
13
i.e., over
the course of ∼100 ps, the strong S
1
→S
n
transition at 420 nm decays with concurrent growth of
T
1
→T
n
transition between 470 and 520 nm.
a)
b) c)
162

Figure 5.11: a) UV-visible absorption of neat DPT film (blue) and 10% BET-B doped DPT film
(red) with the excitation spectrum at 550 nm (green) used to pump in TA measurements. TA
spectra of neat DPT film with excitation fluence of b) 26 J/cm
2
and c) 156 J/cm
2
.
3.4.3. BET-X in solution and PMMA
On the other hand TA spectra of BET-X in toluene and PMMA (Figure 5.12) show no
distinct spectral feature for triplet and
1
(T
1
T
1
) state. Following the 500 nm excitation in both
cases, a photo-induced absorption between 380-700 nm (with the peak at ~400 nm) and a ground
state bleach between 450-550 nm were immediately ( 200 fs) observed. Interestingly,
compared to the S
1
S
n
induced absorption (peak at ~407 nm) of ET-TMS (Figure 5.7), the
induced absorption for BET-X was found to be broader (100 nm vs. 50 nm). The broadening
indicates that the tetracenes within BET-X are coupled strongly to each other in the excited state,
suggesting an excimer interaction between them. This suggests formation of intramolecular
excimer-like state (380-700 nm) from the S
1
state within 200 fs as the acenes in BET-X are pre-
oriented for efficient excimer interactions. With increasing time delay, the induced absorption
peak diminishes with concurrent recovery of the ground state bleach, albeit the dynamics are
slower in the PMMA sample of BETX. During this period no new spectral features similar to the
triplets for ET-TMS or BET-B is observed in the probe window. Attempts to measure the triplet
spectra for BET-X using sensitization were unsuccessful due to photodegradation of BET-X
films. Additionally, the spectral shape and dynamics were invariable with solvent polarity and
polarizability (Figure 5.12c), indicating no charge transfer state involvement in the excited state
dynamics of BET-X.
163

a) b)
c)
Figure 5.12:TA measurements of BETX in a) toluene and b) PMMA following excitation at 500
nm. c)The dynamics at 400 nm shows no variation in excited state dynamics with solvent
polarity.
4. Discussion
The steady state and time resolved photophysical data of BET-X indicate that cofacial
orientation of the acenes is not ideal for singlet fission in substituted tetracenes, as excimer-like
interactions become the predominant excited state relaxation pathway. The excited state
dynamics of BET-B are significantly different from those of BET-X. Unlike BET-X, BET-B
does not exhibit excimer-like emission. The steady state emission spectra of BET-B in solution
shows ~650 cm
-1
Stokes shift. Additionally, the photophysical behavior of the neat films of BET-
B and BET-X are different. The neat film of BET-X photodegrades even under an inert
atmosphere, whereas BET-B undergoes rapid singlet fission in the solid state.
164

4.1 Kinetic model for singlet fission in BET-B
Based on the transient absorption studies of BET-B in several media, a simplified scheme
for the excited state kinetics (Figure 5.13a & b) is proposed where the time dependent
concentration of each transient state can be calculated by solving the coupled rate equations:

where k
1
is the rate of formation of ¹(T
1
T
1
) state from S
1
state and k
3
is the relaxation of ¹(T
1
T
1
)
state to S
0
state. k
2
represents the rate of formation of uncorrelated triplets T
1
state from the
correlated triplet pair ¹(T
1
T
1
) state. The time dependent populations obtained from solving these
equations are multiplied with corresponding extinction spectra of S
1
state, ¹(T
1
T
1
) state and T
1

state to simulate the TA data in different media. For solution and PMMA, the extinction spectra
for S
1
state and ¹(T
1
T
1
) state (Figure 5.13c) is obtained from the initial time (200-300 fs) and
long time (800-900 ps) data of BET-B in solution, respectively. For neat BET-B film and BET-B
doped into DPA and DPT films (Figure 5.13d), the extinction spectra of S
1
state from solution is
red shifted based on the UV-Vis and ¹(T
1
T
1
) state is obtained from long time (800-900 ps) data of
BET-B in DPA. While the extinction spectra for T
1
state of BET-B and DPT are obtained from
triplet sensitized measurements described in section 3.3.
165

a) b)
c) d)
Figure 5.13. Kinetic model for BET-B in a) solution, PMMA, DPA and b) DPT, neat film. The
notation A, A' denotes the tetracenes in the same dimer and C denotes the DPT for the BET-B in
DPT film or third tetracene in another BET-B dimer for the BET-B neat film. c) The differential
extinction spectra used to simulate the TA data of BET-B in c) solution and PMMA and d) DPA,
DPT, neat film.
Since no distinct transient triplet signature was observed in TA data for BET-B in
solution, PMMA and DPA the rate k
2
was kept zero for these three media. In these media, TA
shows that upon photoexcitation of BET-B the S
1
interconverts into a new state, which has been
assigned as the
1
(T
1
T
1
) state. Although the
1
(T
1
T
1
) assignment could not be tested directly,
experiments were performed to rule out other possibilities. BET-B excited state dynamics were
found to be invariant to solvent polarity or polarizability and thus the charge transfer character of
the transient state was ruled out. Furthermore, the emission properties of BET-B indicate that the
emitting state is not an excimer. Therefore, the TA data of BET-B in solution, PMMA and DPA
were fit using the scheme a) (Figure 5.13a) to obtain the rate constants for the formation (k
1
) and
relaxation (k

) of
1
(T
1
T
1
) state with rate of formation (k
2
) of separated triplets fixed at zero. The
166

fits to the TA spectra with the concentration for each transient state is shown in Figure 5.14. The
values for k
1
and k
3
are reported in Table 5.1. The formation rate (k
1
) of
1
(T
1
T
1
) state was found
to be similar in all three media, however the relaxation rate (k
3
) of
1
(T
1
T
1
) state to S
0
state was
found to be slower in PMMA probably due to restricted motion available in the PMMA matrix.
The triplets do not form in solvated BET-B because there is no driving force for separating the
triplets from the triplet pair state,
1
(T
1
T
1
). Within the BET-B molecule the triplets are strongly
coupled to each other in the
1
(T
1
T
1
) pair, and an entropic or enthalpic driving force is necessary
to separate the triplets. In substituted tetracenes singlet fission is not significantly exothermic,
therefore an entropic driving force is necessary for singlet fission to take place.
16-17
In other
words, once the
1
(T
1
T
1
) state has formed, an energy transfer onto another chromophore is
required in order to split the triplets from the coherent pair into distinct triplets.
a) b)
c) d)
167

e) f)
Figure 5.14. The TA data for BET-B in different medium (a,c,e) are shown along with the fits
(red dotted line) from the kinetic model and the corresponding relative populations (b,d,f) for
each state relative to S
1
state population.
BET-B doped into DPT illustrates the need for a third acene to give the uncorrelated
triplets, i.e. T
1
(BETA) + T
1
(DPT). The selective excitation of BET-B leads to the population of
BET-B’s S
1
state, followed by relaxation into the
1
(T
1
T
1
) state. From 20 ps distinct transient
T
1
T
n
absorption spectral features of BET-B and DPT were observed. The kinetic model for
singlet fission in this sample (Figure 5.13b) assigns two rate constants to the singlet fission
process. The first rate constant (k
1
) represents the conversion from S
1
-(BET-B) to

1
(T
1
T
1
)-(BET-B), while the second (k
2
) represents the
1
(T
1
T
1
)-(BET-B) to T
1
-(BET-B) +
T
1
-(DPT) transformation. These values are reported in Table 5.1. The rate for formation (k
1
) and
relaxation (k
3
) of
1
(T
1
T
1
) state were found to be similar to those obtained for BET-B in solution
and DPA. The rate of formation (k
2
) of individual triplets on BET-B and DPT from the
1
(T
1
T
1
)
state was estimated to be (125 ps)
-1
. The fits to the TA data along with the concentration for the
transient states are shown in Figure 5.15. The first rate constant (k
1
) is dependent on the
electronic coupling between the two states involved, which in turn depends on the relative
orientation of the pair of chromophores. These results indicate that the relative tetracene
orientation in BET-B allows for sufficient coupling between these states for the first step to
168

proceed efficiently. The second rate constant (k
2
) is dependent on both enthalpy and entropy.
The triplet energy of DPT is close or slightly greater than that of BET-B, such that
1
(T
1
T
1
)-(BET-B) to T
1
-(BET-B) + T
1
-(DPT) transformation is energetically uphill and the
1
(T
1
T
1
)-(BET-B) state lives long enough to be experimentally detected. The driving force for this
transformation is thus entropic in nature. Assuming that each BET-B molecule is surrounded by
multiple DPT molecules to which the energy transfer can occur, the entropy of the T
1
-(BET-B) +
T
1
-(DPT) state is greater than that of
1
(T
1
T
1
)-(BET-B), as there are several microstates which can
represent the T
1
-(BET-B) + T
1
-(DPT) state, while
1
(T
1
T
1
)-(BET-B) can only be described by one
microstate. Once the triplets, T
1
-(BET-B) and T
1
-(DPT) are generated, no relaxation was
observed within the 1 ns time window.
a) b)
Figure 5.15. The TA data for BET-B in DPT(a) is shown along with the fits (red dotted line)
from the kinetic model and the corresponding relative populations (b) for each state relative to S
1

population.
To fit the TA data of the neat BET-B film similar kinetic model (Figure 5.13b) is used
with comparable rates for formation (k
1
) and relaxation (k
3
) of
1
(T
1
T
1
) state, but to account for
the fast formation of separated triplets a faster rate of formation (k
2
~(0.23 ps)
-1
) of individual
triplets from the
1
(T
1
T
1
) state is required. This faster k
2
leads to a maximum of ~10% population
in
1
(T
1
T
1
) state in presence of ~36% population in T
1
(BET-B) state at 600 fs and thus
1
(T
1
T
1
)-(BET-B) state is not experimentally distinct in the TA data for neat BET-B film. This is
169

probably because the
1
(T
1
T
1
)-(BET-B) to T
1
-(BET-B) + T
1
-(BET-B) transformation is not as
energetically uphill as in the case of BET-B doped into DPT. Therefore, effectively the singlet
fission kinetics from S
1
-(BET-B) to T
1
-(BET-B) + T
1
-(BET-B) becomes faster in neat BET-B
film. Additionally to fit the TA data for neat BET-B film, the relaxation of triplets generated are
modeled by an initial exponential decay (k
4
=(400 ps)
-1
) followed by an offset to consider long
time (s) dynamics. This fast exponential relaxation of the triplets is probably due to geminate
triplet-triplet annihilation between triplets present on adjacent BET-B molecules and later when
the triplets get enough time to diffuse the recombination dynamics gets slower. The fits to the
TA spectra and the concentrations obtained for the transient sates are shown in Figure 5.16.
a) b)
Figure 5.16. The TA data for BET-B neat film (a) is shown along with the fits (red dotted line)
from the kinetic model and the corresponding relative populations (b) for each state relative to S
1

state population.
170

4.2. Excitation density dependent TA on ET-TMS and BETA neat films:
To determine if singlet-singlet annihilation strongly affects the TA spectra of neat ET-TMS
and BETA films, transient spectra were measured for a series of pump fluences between 8-96
J/cm
2
. Figure 5.17 shows the extracted normalized time dependent singlet and triplet
populations obtained for neat ET-TMS (Figure 5.17a & b) and BETA (Figure 5.17c & d). This
shows that at high fluence (>24 J/cm
2
for ET-TMS and >16 J/cm
2
for BETA) the singlet
population dynamics (Figure 5.17a, 5.17c) becomes faster due to initiation of singlet-singlet
annihilation, however the triplet population kinetics (Figure 5.17b, 5.17d) remain unaffected.
This also leads to a decrease in the singlet fission yield from 110% to 82% for ET-TMS and
160% to 140% for BETA films. The relaxation rate for the BETA triplets (Figure 5.17d) was
observed to be excitation density independent indicating that this relaxation is originating from
geminate triplet-triplet annihilation.
Table 5.1: Time constants (
i
=1/k
i
) for the kinetic model used
to fit the TA data for BET-B in different media.
BET-B in 
1
(ps) 
2
(ps) 
3
(± 50 ps)
THF 2 ± 0.5 -- 500
PMMA 1.7 ± 0.3 -- 1000
DPA 2 ± 0.2 -- 500
DPT 1.4 ± 0.3 125 ± 1 500
neat 0.8 ± 0.2 0.23 ± 0.05 500

171

a) b)
c) d)
Figure 5.17: Excitation fluence dependent a) singlet and b) triplet populations extracted from the
TA measurements on neat ET-TMS film. Similarly, c) and d) shows the excitation dependent
singlet and triplet population from TA measurements on neat BETA film.
4.3. Comparison between neat monomer (ET-TMS) and dimer (BET-B) film
The time dependent singlet and triplet populations extracted from the TA data of neat ET-
TMS and neat BET-B film are shown in Figure 5.18a. Both the singlet and triplet populations of
neat ET-TMS film show that triplet generation occurs over two timescales: one with exponential
time scale of 0.25 ps followed by a delayed exponential rise of 3 ps. The maximum triplet yield
was found to be ~90±8% at 50 ps. However, the rate of T
1
formation was found to be faster
(k
1
=(0.8 ps)
-1
and k
2
=(0.23 ps)
-1
) in a neat BET-B film with maximum triplet yield of ~154±10%
at 10 ps. This is also evident from the time dependent singlet populations, where the singlet
population in neat BET-B shows a single exponential relaxation compared to the presence of
both prompt and delayed relaxation of the singlet population in neat ET-TMS. This higher and
172

faster singlet fission suggests that the coupling is greater or that singlet fission is more
exothermic in BET-B. It is more likely that the increase in the rate stems from the relative
geometry of the acenes, and therefore the coupling, being more favorable in BET-B than ET-
TMS film. The thermodynamic argument is unlikely because the energies of the triplet states
typically are not significantly affected by Davydov interactions, whereas the S
1
energy is
decreased by 117 meV in BET-B compared to ET-TMS due to dimerization. Therefore, the
S
1
/T
1
energy ratio is expected to be lower in BET-B than in ET-TMS. Additionally, the triplet
yield in a neat BET-B film is dramatically larger than in neat ET-TMS (154% vs. 90%). This
increase in yield and faster rate of singlet fission do not arise from the crystalline morphology of
the film as time-resolved fluorescence measurements (Figure 5.18b) on neat BET-B film and
BET-B crystal clearly shows that BET-B neat film is not crystalline. This high and fast singlet
fission occurs because BET-B exhibits intramolecular singlet fission, and most of the molecules
are pre-oriented to undergo singlet fission, whereas in ET-TMS neat film not all chromophore
pairs are ideal for singlet fission. At longer times (> 10 ps), 50% of the triplets produced in neat
BET-B film decay with a rate of 400 ps and the rest follows the long (~s) temporal dynamics
similar to the triplets produced from neat ET-TMS film. This shows that although higher number
of triplets are generated initially in neat BET-B film, majority of the triplets undergo geminate
triplet-triplet annihilation compared to long lifetime of the triplets generated in the monomer ET-
TMS film. This suggests that increase in coupling between two tetracenes increase both
formation and annihilation rate of separated triplets. However, if the triplets could be separated
in a different host material like DPT then due to decrease in coupling the annihilation rate could
be reduced as observed from the long time triplet kinetics of BET-B in DPT film (Figure 5.15b).
173

a) b)
c)
Figure 5.18. Singlet and triplet population densities determined from TA spectra of neat ET-
TMS (a) and BET-B (b) films. The broken line denotes the fit based on the kinetic model
described before. b) Comparison between time resolved photoluminescence (PL) measurements
on BET-B neat film versus BET-B crystal to show the non-crystalline nature of the BET-B neat
film. The inset shows the time dynamics a short timescales.
5. Conclusion
The steady state and time resolved measurements on two different covalently attached
tetracene dimers, BET-X and BET-B shows that relative orientation of two teteracene
chromophores can affect the excited state deactivation pathway. BET-X with cofacially oriented
tetracenes exhibit excited sate deactivation mainly through an excimer-like state. Whereas, BET-
B with an angular orientation shows the formation of
1
(T
1
T
1
) state in solution with no further
evolution of separated triplets. The experiments on BET-B in different media illustrate that two
chromophores are not sufficient to generate uncorrelated triplets from the correlated triplet pair
174

1
(T
1
T
1
) and a third molecule with low E
T
is required to separate the triplets by triplet energy
transfer. The high and fast singlet fission in neat BET-B film compared to the monomer ET-TMS
film illustrate the presence of intramolecular singlet fission in BET-B and thus leads to an
increase in singlet fission sites. In future, this singlet fission dimer can be doped in a known
singlet fission monomer film to generate more singlet fission sites and thus higher singlet fission
yield. Thus, this will relax the constraints placed on the film manufacturing process to achieve
low-cost, high efficiency singlet fission based OPVs.

175

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Abstract (if available)

Abstract Compared to inorganic silicon semiconductors, organic semiconductors possess many desirable properties such as flexibility, light weight and high absorption coefficient. These properties lead to low manufacturing and transportation costs for organic solar cells. However, the maximum efficiency obtained for organic solar cell is ~11% compared to 27% device efficiency for silicon semiconductor. This low device efficiency can be addressed with proper understanding of the working principles of organic solar cells. The active layer of an organic solar cell is composed of an electron donor and an electron acceptor material. Upon light absorption in the active layer, a series of processes: exciton generation, exciton diffusion, charge generation and charge recombination occur before the charges are collected at the electrode. The efficiency of the device depend on the efficiency of each of these processes. In this dissertation ultrafast spectroscopic techniques are utilized to provide insight into the dynamics and efficiency of these processes, which will eventually assist in optimizing the device performance for organic semiconductors.

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Creator Das, Saptaparna (author)

Core Title Excited state dynamics of organic photovoltaics probed using ultrafast spectroscopy

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 08/04/2015

Defense Date 06/12/2015

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

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