Small molecules containing rigidified thiophenes and a cyanopyridone acceptor unit for solution-processable bulk-heterojunction solar cells

Article abstract of DOI:10.1016/j.dyepig.2015.03.028

We designed two solution processable small molecules, 5-((5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophene-2-yl)methylene)-1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (1) and 5-((6-(4-(diphenylamino)phenyl)-4-(2-ethylhex

Full text of DOI:10.1016/j.dyepig.2015.03.028

Accepted Manuscript
Small molecules containing rigidified thiophenes and a cyanopyridone acceptor unit
for solution-processable bulk-heterojunction solar cells
Akhil Gupta, Abdelselam Ali, Mei Gao, Th. Birendra Singh, Ante Bilic, Scott E.
Watkins, Udo Bach, Richard A. Evans
PII:
S0143-7208(15)00103-5
10.1016/j.dyepig.2015.03.028
DYPI 4709
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 12 September 2014
Revised Date: 5 March 2015
Accepted Date: 26 March 2015
Please cite this article as: Gupta A, Ali A, Gao M, Singh TB, Bilic A, Watkins SE, Bach U, Evans
RA, Small molecules containing rigidified thiophenes and a cyanopyridone acceptor unit for
solution-processable bulk-heterojunction solar cells, Dyes and Pigments (2015), doi: 10.1016/
j.dyepig.2015.03.028.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Small molecules containing rigidified thiophenes and a cyanopyridone acceptor unit for
solution-processable bulk-heterojunction solar cells
a
a
a
c
Akhil Gupta a,b,*, Abdelselam Ali , Mei Gao ,Th. Birendra Singh , Ante Bilic , Scott E.
Watkins a, Udo Bach a,d,e, Richard A. Evans a,*
a CSIRO Manufacturing, Bag 10, Clayton South VIC 3169 Australia; Tel: +61 3 9545 2507; E-
mail: richard.evans@csiro.au; akhil.gupta@csiro.au
b
Medicinal Chemistry, Monash Institute of Pharmaceutical Sciences, Monash University,
Parkville VIC 3052 Australia; akhil.gupta@monash.edu
c CSIRO Manufacturing, Virtual Nanoscience Lab, Parkville Victoria 3052 Australia
d Department of Materials Engineering, Faculty of Engineering, Monash University, Wellington
Road, Clayton VIC 3800 Australia
e
Tech Fellow, The Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton VIC
3168 Australia
Key Words
Donor-π-acceptor, Rigidified thiophenes, Cyanopyridone, Bulk-heterojunction solar cells
Abstract
We
designed
two
solution
processable
small
molecules,
5-((5-(4-
(diphenylamino)phenyl)thieno[3,2-b]thiophene-2-yl)methylene)-1-(2-ethylhexyl)- 4-methyl-
2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (1) and 5-((6-(4-(diphenylamino)phenyl)-
4-(2-ethylhexyl)-4H-dithieno[3,2-b:2ʹ,3ʹ-d]pyrrol-2-yl)methylene)-1-(2-ethylhexyl)-4-
methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile
(2),
consisting
of
a
triphenylamine electron donating group and a cyanopyridone electron accepting group linked

ACCEPTED MANUSCRIPT
by two different rigidified π-spacer thiophenes. Ultraviolet-visible absorption spectra
revealed that the use of three fused rings (dithienopyrrole) as the conjugated π-spacer resulted
in an enhanced intramolecular charge transfer transition and reduction of band gap, when
compared with a non-fused bithiophene and two fused thiophenes (thienothiophene)
analogues. Power conversion efficiencies (PCEs) of 2.39% and 2.14% were achieved for
simple photovoltaic devices based on 1/PC61BM and 2/PC61BM under simulated AM 1.5
illumination (100 mW cm-2), respectively. It has been observed that the degree of conjugation
of the central π-bridge was not a key factor for the enhancement of photovoltaic performance.
1.
Introduction
Bulk heterojunction (BHJ) organic solar cells have attracted a great deal of attention over the
past two decades because of their promise of low-cost fabrication, ease of solution
processability, light weight and potential application in flexible, large-area devices [1]. BHJ
solar cells are comprised of an interpenetrating network of organic donor and acceptor
domains, which is formed during their fabrication via solution processing. Traditionally,
semiconducting polymers such as poly(3-hexylthiophene) (P3HT) as an electron donor
material and a soluble fullerene derivative such as [6,6]-phenyl-C61-butyric acid methyl ester
(PC61BM) as an electron acceptor material have been used [2–5]. However, recent reports of
improved BHJ device performance as a result of using small molecules have attracted much
attention [6–10]. Solution processable small molecules have a number of potential advantages
over polymers such as high absorption coefficients, well defined structures, purification, high
chemical stability and relatively straight-forward synthetic strategies. However, in common
with polymeric structures, the design requirements of small molecules include a low optical
band gap, broad absorption profile, high mobility, multiple reversible redox potentials and

ACCEPTED MANUSCRIPT
appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy levels. It has been established that low band gap materials can be
generated by incorporating an electron donor–π-bridge–electron acceptor (D–π–A or D–A)
motif within the structure [11–16].
Reports of BHJ devices using small organic molecules as donor components have recently
emerged that have utilized squaraine [17], oligothiophene [18], merocyanine dyes [19–20],
diketopyrrolopyrrole derivatives [21], dibenzochrysene [22], and push-pull organic dyes [23–
25]. Progress in the field has resulted in devices exhibiting PCEs in the excess of 9% [26–30].
While this progress is encouraging, considerable scope still exists to develop novel light-
harvesting materials that possess broad and efficient optical absorption, deep HOMO energy
levels (-5.0 to -5.5 eV) and adequate solubility for thin film formation [9,10]. One successful
strategy is the exploration of donor-acceptor molecules with a conjugated π-bridge. Such
structures allow intramolecular charge transfer (ICT) transitions that broaden the absorption
spectrum and narrow the optical band gap.
In our own studies of small molecule chromophores/charge transport materials based on a D–
A design, we have previously shown that the use of a cyanopyridone acceptor fragment has a
positive effect on the optoelectronic properties and material performance in devices [24,31].
However, while the reference dye R1 (see Fig. 1) has almost ideal energy levels, according to
the analysis by Brabec et al.[32], devices based on R1 do not give the predicted power
conversion efficiencies of 8%. This is due to low currents and low fill factors. We therefore
set out to examine what is the effect of changing the chemistry of the central part of the
molecule on their OPV performance. We have synthesized two examples 5-((5-(4-
(diphenylamino)phenyl)thieno[3,2-b]thiophene-2-yl)methylene)-1-(2-ethylhexyl)- 4-methyl-
2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (1) and 5-((6-(4-(diphenylamino)phenyl)-

ACCEPTED MANUSCRIPT
4-(2-ethylhexyl)-4H-dithieno[3,2-b:2’,3’-d]pyrrol-2-yl)methylene)-1-(2-ethylhexyl)-4-
methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (2) along with the reference dye
(R1) such that all the materials have triphenylamine as a common donor and cyanopyridone
as an acceptor fragment. All the molecular structures are shown in Fig. 1. In the reference
dye, a non-fused bithiophene was used as the π-bridge and in this work, we examined the use
of two fused thiophenes (thienothiophene) (target compound 1) and three fused rings
(dithienopyrrole) (target compound 2) as alternative central cores. Our aim was to increase
the absorbance of the compound at higher energy (fill in the gap around 400 nm) and we
hypothesised that using more rigid cores might enhance the intermolecular interactions and
therefore the fill factor of OPV devices. We know that thin films based on R1 are largely
amorphous and that device performance is not improved by thermal annealing [24]. By
introducing more rigid groups, we hoped to improve charge transport.
In this paper, we report the facile synthesis and characterization of the optical,
electrochemical and photovoltaic properties of small molecules 1 and 2. The materials 1 and
2 were synthesized via the Knoevenagel condensation of the appropriate aldehyde with the
active methylene group of the cyanopyridone acceptor and their chemical structures were
1
confirmed by H NMR spectroscopy and mass spectrometry. Knoevenagel condensation of
the aldehyde groups present on the oligothiophenes is an efficient way of generating a double
bond between the π-bridge and an acceptor unit. The use of such condensations is a common
strategy to generate metal-free organic dyes for dye-sensitized solar cells [33]. However, the
use of the same strategy to develop materials for BHJ photovoltaic devices has been limited
[9,10]. In this report, we demonstrate the Knoevenagel condensation reaction between the
aldehyde group present on a rigidified spacer and an aromatizable acceptor. To the best of our
knowledge this is the first time that rigidified thiophenes have been used in conjunction with
the aromatizable cyanopyridone acceptor.

ACCEPTED MANUSCRIPT
FIGURE 1 GOES HERE
2.
Experimental
2.1.
Materials
All reagents and chemicals used, unless otherwise specified, were purchased from
Sigma-Aldrich Co. The solvents used for reactions were obtained from Merck Speciality
Chemicals (Sydney, Australia) and were used as received. 1-(2-ethylhexyl)-4-methyl-2,6-
dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile
and
6-(4-(diphenylamino)phenyl-4-(2-
ethylhexyl)-4H-dithieno[3,2-b:2ʹ,3ʹ-d]pyrrol-2-carbaldehyde were synthesized as per our
previous reports [24,34].
2.2.
Spectroscopic measurements
Unless otherwise specified, all 1H and 13C NMR spectra were recorded using a Bruker
AV400 spectrometer at 400 MHz and 100.6 MHz, respectively, or a Bruker AV200
spectrometer at 200 MHz and 50 MHz, respectively. Chemical shifts (δ) are measured in
parts per million. Thin Layer Chromatography (TLC) was performed using 0.25 mm thick
plates precoated with Merck Kieselgel 60 F254 silica gel, and visualised using UV light (254
nm and 365 nm). Analytical high performance liquid chromatography (HPLC) analyses were
performed using a Waters 2695 alliance system and Waters 2996 photodiode array detector
scanning 190–700 nm using an Alltima HP C18 column at 30 °C with 95% acetonitrile and
5% tetrahydrofuran as mobile phase. Waters Empower 2 data management system was used
for data processing. Melting points were measured using a Gallenkamp MPD350 digital

ACCEPTED MANUSCRIPT
melting point apparatus and are uncorrected. Electron impact (EI) mass spectra were carried
out on a ThermoQuest MAT95XP mass spectrometer using ionisation energy of 70 eV and
employing PerFluoroKerosene (PFK) as a reference sample. Electro spray (ES) mass spectra
were carried out on a Thermo scientific Q-Exactive FTMS. Atmospheric-pressure chemical
ionization (APCI) experiments were carried out on a Thermo Scientific Q-Exactive FTMS,
ionizing by APCI from an ASAP probe [35]. UV–Vis absorption spectra were recorded using
a Hewlett Packard HP 8453 diode array spectrometer. Fluorescence spectra were measured
using a Perkin Elmer LS50B fluorimeter.
2.3.
Electronic measurements
PESA measurements were recorded using a Riken Keiki AC-2 PESA spectrometer
with a power setting of 5 nW and a power number of 0.5. Samples for PESA were prepared
on cleaned glass substrates. Electrochemical measurements were carried out using a
PowerLab ML160 potentiostat interfaced via a PowerLab 4/20 controller to a PC running E-
Chem For Windows version 1.5.2. The measurements were run in argon-purged
dichloromethane with tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting
electrolyte. The cyclic voltammograms were recorded using a standard three electrode
configuration with a glassy carbon (2 mm diameter) working electrode, a platinum wire
counter electrode and a silver wire pseudo reference electrode. The silver wire was cleaned in
concentrated nitric acid followed by concentrated hydrochloric acid and then washed with
deionised water. Cyclic-voltammograms were recorded with a sweep rate of 50 mV s-1. All
the potentials were referred to the E1/2 of ferrocene/ferrocenium redox couple.

ACCEPTED MANUSCRIPT
2.4.
Device fabrication and characterization of photovoltaic devices
Indium tin oxide (ITO)-coated glass (Kintek, 15 Ω/ꢀ) was cleaned by standing in a
stirred solution of 5% (v/v) Deconex 12PA detergent at 90 °C for 20 minutes. The ITO-
coated glass was then successively sonicated for 10 mins each in distilled water, acetone and
isopropanol. The substrates were then exposed to a UV-ozone clean at room temperature for
10 minutes. UV/ozone cleaning of glass substrates was performed using a Novascan PDS-
UVT, UV/ozone cleaner with the platform set to maximum height. The intensity of the lamp
was greater than 36 mW/cm2 at a distance of 10 cm. At ambient conditions the ozone output
of the UV cleaner is greater than 50 ppm. Aqueous solutions of PEDOT/PSS (HC Starck,
Baytron P AI 4083) were filtered (0.2 µm RC filter) and deposited onto glass substrates in air
by spin coating (Laurell WS-400B-6NPP lite single wafer spin processor) at 5000 rpm for 60
sec to give a layer having a thickness of 40 ± 5 nm. The PEDOT/PSS layer was then annealed
on a hotplate in a glove box at 145 °C for 10 min. For OPV devices, the newly synthesized
organic p-type materials and PC61BM (Nano-C) were dissolved in individual vials by
magnetic stirring. Blend ratios and solution concentrations were varied to optimize device
performance. The solutions were then combined, filtered (0.2 µm RC filter) and deposited by
spin coating onto the ITO-coated glass substrates inside a glove box. The coated substrates
were then transferred (without exposure to air) to a vacuum evaporator inside an adjacent
nitrogen-filled glove box. Samples were placed on a shadow mask in a tray. The area defined
by the shadow mask gave device areas of exactly 0.2 cm2. Deposition rates and film
thicknesses were monitored using a calibrated quartz thickness monitor inside the vacuum
chamber. Layers of calcium (Ca) (Aldrich) and aluminium (Al) (3 pellets of 99.999%, KJ
Lesker) having thicknesses of 20 nm and 100 nm, respectively, were evaporated from open
tungsten boats onto the active layer by thermal evaporation at pressures less than 2×10–6
mbar. A connection point for the ITO electrode was made by manually scratching off a small

ACCEPTED MANUSCRIPT
area of the active layers. A small amount of silver paint (Silver Print II, GC Electronics, part
no.: 22-023) was then deposited onto all of the connection points, both ITO and Al. The
completed devices were then encapsulated with glass and a UV-cured epoxy (Summers
Optical, Lens Bond type J-91) by exposing to 365 nm UV light inside a glove box for 10 min.
The encapsulated devices were then removed from the glove box and tested in air within 1 h.
The OPV devices were tested using an Oriel solar simulator fitted with a 1,000W xenon
lamp filtered to give an output of 100 mW/cm2 at simulated AM 1.5. The lamp was calibrated
using a standard, filtered silicon (Si) cell from Peccell Limited which was subsequently cross-
calibrated with a standard reference cell traceable to the National Renewable Energy
Laboratory. The devices were tested using a Keithley 2400 Sourcemeter controlled by
labview software. Film thicknesses were determined using a Dektak 6M Profilometer.
2.5.
Device fabrication and characterization of field effective transistors
A doped (N ~ 3 × 1017 cm-3) silicon (Si) wafer was used as a substrate and as the gate
electrode. Discrete bottom contact OFETs were fabricated on thermally grown smooth silicon
dioxide (SiO2) (230 nm). Interdigitated source and drain electrodes were photo-
lithographically patterned from a 50 nm sputtered gold layer. Channel length, L, of the
devices was 20 ꢀm and the channel width, W, was 2 mm. The SiO2 layer was first cleaned
with acetone, then with 2-propanol and finally treated with UV ozone. Device fabrication was
completed by spin coating the organic semiconductor layers at 1500 rpm in the glove box
from solutions in chloroform. The active layers consisted of pristine compounds or blends
with PC61BM. Without any further treatment, completed devices were transferred (under air-
free conditions) to another glove box fitted to probe the devices using a probe station.
Electrical measurements were carried out using a Keithley 2612 dual channel SMU. The
reproducibility of the organic field effect transistors (OFETs) preparation procedure was high
with a confidence interval for the extracted mobility of ±15% of the values which have been

ACCEPTED MANUSCRIPT
presented. The carrier mobility values presented in this work were generally averaged from
measurements done on at least four devices on the same substrate.
2.6.
Synthesis
New compounds 1 and 2 were synthesized and characterized. The synthetic strategy
for 1 and 2 is represented in schemes 1 and 2 respectively.
Scheme 1 Synthetic strategy for compound 1
2.6.1 Synthesis of thieno[3,2-b]thiophene-2-carbaldehyde (S1)
Thieno[3,2-b]thiophene (3.00 g, 21.4 mmol) was taken in a 100 mL round bottom
flask in ethylene dichloride (75.0 mL) and dimethylformamide (1.65 mL, 21.4 mmol) was

ACCEPTED MANUSCRIPT
added at room temperature. The resulting reaction solution was cooled to 0 °C and POCl3
(5.87 mL, 64.3 mmol) was added drop-wise. The reaction mixture was allowed to warm to
room temperature and heated at reflux overnight. The reaction mixture was treated with a
saturated solution of sodium acetate and the organic layer was separated. The organic layer
was washed with water followed by brine, dried over anhydrous sodium sulphate and the
solvent was evaporated in vacuo to give a crude dark yellow oil, which was purified by
column chromatography on silica (hexane:ethyl acetate 9:1) to give the title compound S1
(2.70 g, 75%) as a light yellow oil. Rf (10% ethyl acetate/hexane) 0.45; 1H NMR (400 MHz,
CD3COCD3, 25 °C, ppm) δ 10.05 (s, 1H), 8.33 (s, 1H), 8.00–7.99 (m, 1H), 7.57–7.55 (m,
13
1H); C NMR (100.6 MHz, CDCl3, 25 °C, ppm) δ 183.7, 145.9, 145.6, 139.4, 134.1, 129.3,
120.3; HRMS (APCI): calcd for C7H5OS2 (M+H)+ 168.9775; found 168.9776.
2.6.2 Synthesis of 5-iodothieno[3,2-b]thiophene-2-carbaldehyde (S2)
Compound S1 (1.50 g, 8.93 mmol) was taken in a 100 mL round bottom flask in an
acetic acid:chloroform 1:1 (v/v) solvent mixture (50.0 mL) and N-iodosuccinimide (2.51 g,
11.2 mmol) was added at room temperature. The resulting reaction mixture was stirred
overnight in the absence of light. The solid that formed in the reaction was collected by
filtration and washed with hexane to obtain the title compound S2 (1.00 g, 64%) as a light
1
green solid. M. Pt.: 185–187 °C; H NMR (200 MHz, CD3COCD3, 25 °C, ppm) δ 10.05 (s,
13
1H), 8.25 (s, 1H), 7.82 (s, 1H); C NMR (100.6 MHz, CDCl3, 25 °C, ppm) δ 183.7, 145.9,
144.9, 143.9, 129.3, 127.7, 83.5; HRMS (APCI): calcd for C7H4IOS2 (M+H)+ 294.8742;
found 294.8743.

ACCEPTED MANUSCRIPT
2.6.3 Synthesis of 5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophene-2-carbaldehyde (S3)
Compound S2 (1.00 g, 3.40 mmol), 4-(diphenylamino)phenylboronic acid (1.50 g,
5.10 mmol), sodium phosphate dodecahydrate (1.55 g, 4.08 mmol) and 10% Pd(C) (200 mg)
were added together in a 250 mL round bottom flask and isopropanol (100 mL) was added at
room temperature. The reaction mixture was heated to 80 °C for 24 h and the reaction
progress was monitored by thin layer chromatography (hexane:ethyl acetate 9:1), which
indicated the consumption of starting aldehyde. The mixture was cooled to room temperature,
filtered through Celite (5.00 g) and eluted with dichloromethane. The solvent was removed
and the residue was purified by flash chromatography eluted with 50% CHCl3/petroleum
ether (60–80 °C) to afford the title compound S3 (1.10 g, 78%) as a yellow solid. M. Pt.:
151–153 °C; Rf (50% CHCl3/petroleum ether [60–80 °C]) 0.30; 1H NMR (400 MHz, CDCl3,
25 °C, ppm) δ 9.93 (s, 1H), 7.88 (s, 1H), 7.51–7.48 (m, 2H), 7.42 (s, 1H), 7.31–7.27 (m, 4H),
7.15–7.13 (m, 4H), 7.10–7.06 (m, 4H); HRMS (APCI): calcd for C25H18NOS2 (M+H)+
412.0824; found 412.0828.
2.6.4 Synthesis of 5-((5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophene-2-yl)methylene)-
1-(2-ethylhexyl)- 4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (1)
Compound S3 (900 mg, 2.18 mmol) and 1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-
tetrahydropyridine-3-carbonitrile (970 mg, 3.72 mmol) were added together in a 250 mL
round bottom flask in chloroform (100 mL) and pyridine (340 mg, 4.36 mmol) was added to
this mixture at room temperature. The resulting reaction mixture was heated at reflux
overnight and the progress of reaction was monitored by thin layer chromatography
(hexane:ethyl acetate 8:2), which indicated the complete consumption of starting compound
S3. The reaction mixture was cooled and the solvent was evaporated in vacuo to give crude
bluish-black oil, which was purified by flash chromatograph eluted with 50%

ACCEPTED MANUSCRIPT
dichloromethane/pet ether (60–80 °C) → 100% dichloromethane to afford the title compound
1 (900 mg, 63%) as a shiny greenish-black powder. Rf (100% dichloromethane) 0.40; HPLC
(5% THF/ACN): 97%; M. Pt.: 208–213 °C; IR (thin solid film, cm-1) 2950, 2924, 2856,
2217, 1678, 1630, 1589, 1570, 1486, 1447, 1413, 1291, 1282, 1171, 783, 751, 694; 1H NMR
(400 MHz, CD2Cl2, 25 °C, ppm) δ 7.96 (s, 1H), 7.91 (s, 1H), 7.54–7.51 (m, 2H), 7.44 (s, 1H),
7.33–7.29 (m, 4H), 7.16–7.05 (m, 8H), 4.01–3.90 (m, 2H), 2.64 (s, 3H), 1.89–1.85 (m, 1H),
13
1.37–1.26 (m, 8H), 0.93–0.85 (m, 6H); C NMR (200 MHz, CDCl3, 25 °C, ppm) δ 163.1,
160.9, 157.9, 156.3, 156.1, 149.4, 146.8, 144.7, 139.2, 138.4, 136.9, 129.5, 127.3, 126.3,
125.3, 124.1, 122.0, 116.2, 115.1, 114.3, 103.3, 44.0, 37.5, 30.5, 28.5, 23.8, 23.1, 18.9, 14.1,
10.6; HRMS (ESI): calcd for C40H37N3O2S2 (m/z) 655.2322; found 655.2311.
O
N
O
CN
S
S
S
S
N
N
CHO
chloroform/pyridine
Reflux
N
N
O
CN
N
O
55%
2
Scheme 2 Synthetic strategy for compound 2
2.6.5 Synthesis
of
5-((6-(4-(diphenylamino)phenyl)-4-(2-ethylhexyl)-4H-dithieno[3,2-
b:2ʹ,3ʹ-d]pyrrol-2-yl)methylene)-1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-
tetrahydropyridine-3-carbonitrile (2)
Compound
6-(4-(diphenylamino)phenyl-4-(2-ethylhexyl)-4H-dithieno[3,2-b:2ʹ,3ʹ-
d]pyrrol-2-carbaldehyde (700 mg, 1.24 mmol) and 1-(2-ethylhexyl)-4-methyl-2,6-dioxo-

ACCEPTED MANUSCRIPT
1,2,5,6-tetrahydropyridine-3-carbonitrile (550 mg, 2.20 mmol) were added together in a 100
mL round bottom flask in chloroform (70.0 mL) and pyridine (147 mg, 1.86 mmol) was
added to this mixture at room temperature. The resulting reaction mixture was heated at
reflux overnight and the solvent was evaporated off to give a crude dark black solid that was
purified by flash chromatograph eluted with 100% dichloromethane to afford the title
compound 2 (550 mg, 55%) as a shiny black powder. Rf (100% DCM) 0.50; HPLC (5%
THF/ACN): 96%; M. Pt.: 137–140 °C; IR (thin solid film, cm-1) 2950, 2924, 2856, 2217,
1
1678, 1630, 1589, 1570, 1486, 1447, 1413, 1291, 1282, 1171, 783, 751, 694; H NMR (400
MHz, CD2Cl2, 25 °C, ppm) δ 7.95 (s, 1H), 7.79 (s, 1H, bd), 7.54–7.52 (m, 2H), 7.33–7.27 (m,
4H), 7.15–7.10 (m, 7H), 7.06–7.04 (m, 2H), 4.09–4.06 (m, 2H), 3.98–3.80 (m, 2H), 2.59 (s,
3H), 2.01–1.95 (m, 1H), 1.89–1.83 (m, 1H), 1.40–1.24 (m, 16H) 0.95–0.81(m, 12H); 13C
NMR (100.6 MHz, CDCl3, 25 °C, ppm) δ 163.2, 161.5, 157.9, 153.8, 152.5, 148.8, 146.9,
146.1, 144.5, 136.1, 134.5, 129.5, 127.5, 126.7, 125.6, 125.1, 123.8, 122.5, 115.8, 114.3,
113.7, 105.1, 100.6, 51.4, 43.9, 40.3, 37.4, 30.6, 30.6, 28.6, 28.5, 24.1, 23.8, 23.1, 22.9, 18.9,
14.1, 13.9, 10.7, 10.6; HRMS (ESI): calcd for C50H54N4O2S2 (m/z) 806.3683; found
806.3646.
3.
Results and discussion
3.1.
Synthesis and characterization
The materials 1 and 2 were synthesized by reacting the aldehyde precursors, 5-(4-
(diphenylamino)phenyl)thieno[3,2-b]thiophene-2-carbaldehyde
and
6-(4-
(diphenylamino)phenyl-4-(2-ethylhexyl)-4H-dithieno[3,2-b:2’,3’-d]pyrrol-2-carbaldehyde, at
reflux with 1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile in

ACCEPTED MANUSCRIPT
chloroform, respectively, in the presence of pyridine as a base. Both the materials were
purified through column chromatography. These solids were characterized spectrally and
their physical properties were investigated. The materials were prepared in moderate to high
yields and were found to be highly soluble in a variety of conventional organic solvents such
as chlorobenzene, chloroform, dichloromethane and toluene. High solubility of organic
materials is an essential criterion for the fabrication of solution-processable organic
photovoltaic devices and newly designed materials 1 and 2 fulfil this criterion. Synthetic
details of R1 have been reported previously [24].
3.2.
Optical properties
The ultraviolet-visible (UV–Vis) spectra of both the new materials 1 and 2 were
measured in chloroform solutions (Fig. 2) and compared with R1. Dye 2 had a significantly
higher extinction coefficient and absorption maximum when compared with 1 or R1. This
bathochromic shift for the solution spectrum of 2 was observed as a result of using
dithienopyrrole as the conjugated π-spacer when compared with either two fused thiophenes
or a non-fused bithiophene, and can be attributed to the extended conjugation for ICT over
the whole molecular backbone. On the contrary, dye 1 exhibited 19 nm of blue shift in the
solution spectrum when compared with R1. Overall, the absorption profile was not
broadened, but it was red-shifted. Absorption maximum (λmax) at 571 nm (ε = 61,011 M-1 cm-
1) with an onset at 706 nm for 1 and λmax at 621 nm (ε = 101,621 M-1 cm-1) with onset at 730
nm for 2 were measured. Peak molar absorptivity for 1 and R1 were similar, whereas we
found ~65% enhancement to the peak molar absorptivity of 2 when compared with 1 or R1.
We further realized that dye 1 may exert solvatochromism for which it is showing inferior
absorption maximum in chloroform, and we decided to conduct its solution spectra in other
solvents such as ethyl acetate (EA) and tetrahydrofuran (THF). The blue shift of 19 nm

ACCEPTED MANUSCRIPT
which was observed for the solution spectrum of 1 in chloroform was reduced to about 5 nm
in both the new solvents when compared with R1. However, considering the empirical
solvent polarity parameter ET(30) we realized that EA and THF have similar ET(30) values.
We decided to choose some more solvents and to compare the solution spectra of 1 and R1 to
elucidate solvatochromism. The solution spectra of 1 and R1 were studied in acetonitrile
(ACN) and dimethylformamide (DMF). We observed that the absorption maxima difference
of 1 and R1 was about 5 nm in both the ACN and DMF solvents, thus indicating that dye 1
certainly exerts solvatochromism. The comparative absorption spectra of dyes 1 and R1 are
depicted in Figures 2a and 2b.
FIGURE 2 GOES HERE
FIGURE 2a GOES HERE
FIGURE 2b GOES HERE
Absorption spectra of 1 and 2 as pristine films (equimolar solutions of 1 and 2 spun at 3000
rpm for 1 minute) were measured on glass substrates (Fig. 3). Generally, for a given
compound, the thin film absorption spectrum will exhibit a bathochromic shift as compared
to its solution spectrum. This was true for 2 where we observed a broader spectrum with
onset extension in comparison to its solution spectrum. So the use of three fused rings as the
conjugated π-spacer could help to absorb a greater amount of solar spectrum. However, film
spectra of 1 indicated a blue shift of about 70 nm when spun-cast from its chloroform
solution. The same observation was found with the use of other solvents such as
chlorobenzene. An explanation to this absorption spectrum modification towards blue shift
can be suggested based on the destruction of ordering of crystal packing.

ACCEPTED MANUSCRIPT
FIGURE 3 GOES HERE
3.3.
Energy levels
Experimental estimation of the HOMO and LUMO energies of compounds 1 and 2 using
Photo Electron Spectroscopy in Air (PESA) and UV–vis spectrometry indicated that the band
gaps of these materials are all in the range required of donor materials for BHJ devices and
are significantly narrower in magnitude than 1.8 eV measured for P3HT (see Fig. 4 for the
energy level diagram). HOMOs for all the compounds were found to be similar. By
comparison with R1, the LUMO of 1 was raised by 0.16 eV as a result of a wider band-gap.
However, the LUMO of 2 was the deepest of all due to the extended red-shift in the film
absorption spectrum when compared with 1 or R1. The optical and electrochemical
properties of all the materials are summarized in Table 1.
FIGURE 4 GOES HERE

ACCEPTED MANUSCRIPT
Table 1
Comparative optical and electronic properties
Dye Absorption
(solution)
Absorption
(film)
Emission
EHOMO Ebandgap ELUMO
(solution) eVc
eVd
eVe
λmaxa/onset/nm λmaxb/onset/nm λmaxa/nm
[ε/(M-1cm-1)]
1
571/706
[61,011]
621/730
[101,621]
590/722
[61,028]
512/722
614/834
584/790
744
720
800
-5.40
-5.40
-5.40
1.72
1.49
1.57
-3.68
-3.91
-3.83
2
R1
a Absorption and emission spectra were measured in chloroform solution.
b Absorption spectra of thin solid films spin-cast (equimolar solutions of 1 and 2 spun at 4000
rpm for 1 minute) from chloroform solutions.
c HOMO levels of the dyes were measured using PESA on thin solid films on glass.
d Energy band gaps were estimated from the absorption onset in thin solid films.
e LUMO levels were calculated from the optical band gaps (film) and HOMO levels (ELUMO
Ebandgap+ EHOMO
=
)
Electrochemistry experiments revealed that both the compounds 1 and 2 undergo reversible
oxidation processes, thus suggesting their suitability as p-type materials in BHJ devices (see
Fig. 5). Cyclic voltammograms indicated that the oxidation of 1 and 2 occurred at a half-
wave potential of around 0.1 eV versus ferrocene/ferrocenium (Fc/Fc+) internal reference

ACCEPTED MANUSCRIPT
(ferrocene potentials: 0.32 eV and 0.24 eV), a finding consistent with the HOMO energy
levels estimated using PESA.
FIGURE 5 GOES HERE
3.4.
Density functional theory calculations
Density functional theory (DFT) calculations using the Gaussian 09 suite of programs [36]
and B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level of theory indicated that the HOMO
electron densities of 1 and 2 were homogeneously populated over the triphenylamine and
central fused conjugated spacers with considerable contribution from the former. The HOMO
was more delocalized over the whole molecular backbone when compared with R1. The
LUMO was delocalized through the spacers and cyanopyridone acceptor fragments with a
sizable contribution from the latter (see Fig. 6).
FIGURE 6 GOES HERE
Furthermore, we thought to elucidate the nature of ICT using time-dependent DFT
calculations (TD-DFT). TD-DFT calculations were carried out using the optimized ground
state geometries of 1 and 2 at the B3LYP/6-311G(d,p) level of theory. The TD-DFT
calculations provide excitation energies and oscillator strengths (denoted by f) of the lowest
singlet states. The computed absorption spectra (see Fig.7) show the first transition peak at
619.39 nm and 589.63 nm, respectively, for 1 and 2. The first peak is quite clearly described
as HOMO → LUMO transition: Excited State 1: 2.0017 eV 619.39 nm f=0.9932,
173(HOMO) → 174(LUMO) 0.70535; Excited State 1: 2.1028 eV 589.63 nm f=1.4739,

ACCEPTED MANUSCRIPT
215(HOMO) → 216(LUMO) 0.69822, for 1 and 2, respectively. Consequently, the
subsequent analysis of the transition density matrix via natural transition orbitals (NTOs),
produces a pair of NTOs with a very similar character as the corresponding HOMO/LUMO
pair (see Fig. 8).
FIGURE 7 GOES HERE
FIGURE 8 GOES HERE
3.5.
Photovoltaic properties
Because of their appropriate optoelectronic properties, solubility and good film forming
capabilities, 1 and 2, were incorporated as p-type semiconducting components with the
soluble fullerene derivative PC61BM as the n-type semiconductor in BHJ photovoltaic
devices and were compared with reference dye R1. BHJ architectures typically deliver higher
device power conversion efficiencies by maximising the surface area of the interface between
the donor and acceptor materials in the active layer. For both the compounds, 1 and 2, the
device structure used was ITO/PEDOT: PSS (38 nm)/active layer/Ca (20 nm)/Al (100 nm)
where the active layer was a solution-processed blend of either of 1 or 2 and the solubilised
fullerene PC61BM. This was done by spin coating mixtures of the appropriate dye with
PC61BM in chlorobenzene under ambient conditions in 1:1 or 1:4 ratios without subsequent
annealing. The optimum layer thickness was found to be in the range of about 60 nm. It was
interesting to see that the BHJ devices based on 1 and 2 were copper-red and blue,
respectively (Fig. 9). The best photovoltaic devices based on compound 1 gave a PCE (η) of
2.39% with 1:4 donor:acceptor blend ratio whereas for compound 2, the optimized blend

ACCEPTED MANUSCRIPT
ratio was found to be 1:1 under which the device afforded 2.14% PCE. The respective
current-voltage (J-V) curves are shown in Fig. 9.
For organic BHJ solar cells, it is documented that Voc is proportional to the energy difference
between HOMO of donor component and LUMO of the acceptor component [37,38]. This
initial screen for the efficacy of target compounds 1 and 2 showed Voc of 0.83 and 0.81V
respectively. These values are consistent with the measured HOMO values and in-line with
the similar HOMO of R1. Device performance showed that with compound 1 we have
significantly improved the FF but the current is slightly reduced by comparison with R1. For
compound 2, the FF is comparable to R1 and the device results are also comparable.
However, the devices based on compound 2 showed a higher photocurrent than either of 1 or
R1 when compared for 1:1 blend, a finding that is consistent with the observed red-shift in
the absorbance for compound 2 when compared with 1 or R1. Table 2 represents the
comparative photovoltaic performance data.
Table 2
Comparative BHJ solar cells performances
Dye Testing
conditions @ thickness
4000 rpm
Blend film Voc
Jsc
FF
Efficiency
(mV)
(mA/cm2)
(η%)
1:1 PC61BMa 64
1:4 PC61BMa 56
1
1
830
5.36
0.45
2.00
830
810
740
872
822
5.44
7.33
5.14
6.77
5.64
0.53
0.36
0.33
0.38
0.32
2.39
2.14
1.27
2.25
1.50
1:1 PC61BMa 62
1:4 PC61BMa 53
1:1 PC61BMa 55
2
2
R1
R1
1:4 PC61BMa
65

ACCEPTED MANUSCRIPT
a BHJ devices with specified weight ratio. Device structure is ITO/PEDOT: PSS (38
nm)/active layer/Ca(20 nm)/Al (100 nm) with an active layer thickness of around 60 nm.
FIGURE 9 GOES HERE
The incident photon to current conversion efficiency (IPCE) curves of the best BHJ devices
based on these small molecules under monochromatic light are shown in Fig. 10. For dye 1,
the IPCE curve is blue-shifted relative to the thin film absorption spectrum. The IPCE shows
a maximum of ~42% at 505 nm. By contrast, the IPCE spectrum of compound 2 shows a
broad response covering 400–800 nm, with a maximum value of ~41% at 640 nm. The IPCE
measurements reported here are comparable to the film absorptions of 1 and 2 which indicate
that the current arises mainly from the p-type donor material.
FIGURE 10 GOES HERE
With regards to the processing conditions of blend solutions, we [22] and others [6] have
shown in previous studies that there exists a strong effect of solvent and the degree of
crystallization on the cell performance. Attempts to fabricate the devices using low boiling
solvents, such as chloroform, resulted in very poor photovoltaic performance. This was
mainly due to poor film quality. The finding that the OPV devices comprising 1 and 2
perform better with high-boiling solvents is significant as the use of high-boiling solvents is
preferable from a processing point of view.
To gain an insight into the effective charge carrier mobilities in active blends, we examined

ACCEPTED MANUSCRIPT
the field effect mobility, µe,h, of charge carriers in devices fabricated using 1 and 2 from their
corresponding OFETs. In this study, bottom gate bottom contact OFETs were fabricated by
depositing the organic semiconductor from solution by spin casting onto a lithographically
patterned substrate (see experimental for details). To our surprise, we were unable to measure
either hole or electron mobilities for the blend films, both being ≤ 10-7 cm2/Vs. We did find a
hole mobility of the order of 1.2 x 10-6 cm2/Vs for 1 in its pristine state while no mobility was
observed for 2 in its pristine state. This unbalanced charge carrier transport, unlike polymer
BHJ [39] may be responsible for the lower FF values observed in the present system. For R1,
hole mobility of the order of 1.8 x 10-6 cm2/Vs was observed in its pristine state with no
electron mobility.
To examine the physical microstructure of the surface we used atomic force microscopy
(AFM) in tapping mode. Figure 11 displays the topographic and phase images of 1:4 blends
of 1 and 2 with PC61BM. 1 appears to have a worm-like morphology with larger domains
and better phase separation with surface roughness of ~ 2.8 nm, whereas blend films of 2 are
fairly smooth, with a roughness of ~ 0.33 nm. This difference in morphology is like the origin
of why the devices of 1 outperform those of 2, even though 1 has a lower extinction
coefficient and λmax. Compound 1 displays superior morphological properties that had a
greater effect on efficiency than the light absorption properties of that dye. The presence of
two ethylhexyl substituents on 2 may be hindering the formation of favourable microstructure
due excessive compatibility of 2 with PC61BM. The use of shorter alkyl groups on the
dithienopyrrole may improve its performance by decreasing its solubility.
FIGURE 11 GOES HERE

ACCEPTED MANUSCRIPT
4.
Conclusions
In conclusion, we have demonstrated the use of D–π–A small molecules containing rigidified
π-spacer thiophenes in organic solar cells. The new materials 1 and 2 using thienothiophene
and dithienopyrrole respectively as π-spacers were soluble in common organic solvents and
were applied as p-type semiconducting components along with the n-type material PC61BM
in BHJ photovoltaic devices. By comparison with a reference dye using bithiophene as the π-
spacer (R1), compound 1 shows a larger band gap, similar extinction coefficient and blue
shifted absorption in solution and thin films whereas 2 had a significantly smaller band gap, a
large red-shift in absorbance and a larger extinction coefficient suggesting greater visible
light harvesting capacity. When these compounds were used in BHJ devices, better
performance was observed with 1. Efforts are ongoing to further improve the power
conversion efficiency of devices and future studies will look at exploiting the fused
thiophenes unit in higher efficiency D–π–A materials.
Acknowledgements
This research was funded through the Flexible Electronics Theme of the CSIRO Future
Manufacturing Flagship and was also supported by the Victorian Organic Solar Cell
Consortium (Victorian Department of Primary Industries, Sustainable Energy Research and
Development Grant, Victorian Department of Business and Innovation, Victoria’s Science
Agenda Grant and the Australian Renewable Energy Agency (ARENA)) and the Australian
Centre for Advanced Photovoltaics (ACAP).

ACCEPTED MANUSCRIPT
References
[1] Yu G, Gao J, Hummelen JC, Wudl F, Heeger AJ. Polymer photovoltaic cells:
enhanced efficiencies via a network of internal donor-acceptor heterojunctions. Science
1995;270:1789–91.
[2]
Facchetti A. π-Conjugated polymers for organic electronics and photovoltaic cell
applications. Chem Mater 2011;23:733–58.
[3] Boudreault P-LT, Najari A, Leclerc M. Processable low-bandgap polymers for
photovoltaic applications. Chem Mater 2011;23:456–69.
[4]
Gupta A, Watkins SE, Scully AD, Singh ThB, Wilson GJ, Rozanski LJ, Evans RA.
Band-gap tuning of pendant polymers for organic light-emitting devices and photovoltaic
applications. Synth Met 2011;161:856–63.
[5]
He Y, Li Y. Fullerene derivative acceptors for high performance polymer solar cells.
Phys Chem Chem Phys 2011;13:1970–83
[6]
Walker B, Kim C, Nguyen T-Q. Small molecule solution-processed bulk
heterojunction solar cells. Chem. Mater. 2011;23:470–82
[7]
Li Y, Guo Q, Li Z, Pei J, Tian W. Solution processable D–A small molecules for
bulk-heterojunction solar cells. Energy Environ Sci 2010;3:1427–36.
[8]
Anthony JE. Small-molecule, nonfullerene acceptors for polymer bulk heterojunction
organic photovoltaics. Chem Mater 2011;23:583–90
[9]
Mishra A, Bäuerle P. Small molecule organic semiconductors on the move: promises
for future solar energy technology. Angew Chem Inter Ed 2012;51:2020–67

ACCEPTED MANUSCRIPT
[10] Lin Y, Li Y, Zhan X. Small molecule semiconductors for high-efficiency organic
photovoltaics. Chem. Soc. Rev. 2012;41:4245–72.
[11] Thompson BC, Frechet JM. Polymer–fullerene composite solar cells. Angew Chem
Int Ed 2008; 47: 58–77.
[12] Günes S, Neugebauer H, Sariciftci NS. Conjugated polymer-based organic solar cells.
Chem Rev 2007;107:1324–38.
[13] Dennler G, Scharber MC, Brabec CJ. Polymer-fullerene bulk-heterojunction solar
cells. Adv Mater 2009;21:1323–38
[14] Chen J, Cao Y. Development of novel conjugated donor polymers for high-efficiency
bulk-heterojunction photovoltaic devices. Acc Chem Res 2009;42:1709–18.
[15] Cheng Y-J, Yang S-H, Hsu C-S. Synthesis of conjugated polymers for organic solar
cell applications. Chem Rev 2009;109:5868–5923.
[16] Li Y, Zou Y. Conjugated polymer photovoltaic materials with broad absorption band
and high charge carrier mobility. Adv Mater 2008;20:2952–58.
[17] Wei GD, Wang SY, Sun K, Thompson ME, Forrest SR. Solvent-annealed crystalline
squaraine:PC70BM (1:6) solar cells. Adv Energy Mater 2011;1:184–7.
[18] Liu Y, Wan X, Wang F, Zhou J, Long G, Tian J, You J, Yang Y, Chen Y. Spin-coated
small molecules for high performance solar cells. Adv Energy Mater 2011;1:771–5.
[19] Burckstummer H, Kronenberg NM, Gsanger M, Stolte M, Meerholz K, Wurthner F.
Tailored merocyanine dyes for solution-processed BHJ solar cells. J Mater Chem
2010;20:240–3.

ACCEPTED MANUSCRIPT
[20] Burckstummer H, Kronenberg NM, Meerholz K, Wurthner F. Near-infrared
absorbing merocyanine dyes for bulk heterojunction solar cells. Org Lett 2010;12:3666–9.
[21] Walker B, Tomayo AB, Dang XD, Zalar P, Seo JH, Garcia A, Tantiwiwat M, Nguyen
T-Q. Nanoscale phase separation and high photovoltaic efficiency in solution-processed,
small-molecule bulk heterojunction solar cells. Adv Funct Mater 2009;19:3063–9.
[22] Winzenberg KN, Kemppinen P, Fanchini G, Bown M, Collis GE, Forsyth CM,
Hegedus K, Singh ThB, Watkins SE. Dibenzo[b,def ]chrysene derivatives: Solution-
processable small molecules that deliver high power-conversion efficiencies in bulk
heterojunction solar cells. Chem Mater 2009;21:5701–3.
[23] Ko HM, Choi H, Paek S, Kim K, Song K, Lee JK, Ko J. Molecular engineering of
push-pull chromophore for efficient bulk-heterojunction morphology in solution processed
small molecule organic photovoltaics. J Mater Chem 2011;21:7248–7253.
[24] Gupta A, Ali A, Bilic A, Gao M, Hegedus K, Singh B, Watkins SE, Wilson GJ, Bach
U, Evans RA. Absorption enhancement of oligothiophene dyes through the use of a
cyanopyridone acceptor group in solution-processed organic solar cells. Chem Commun
2012;48:1889–91.
[25] Gupta A, Armel V, Xiang W, Fanchini G, Watkins SE, MacFarlane DR, Bach U,
Evans RA. The effect of direct amine substituted push-pull oligothiophene chromophores on
dye-sensitized and bulk heterojunction solar cells performance. Tetrahedron 2013;69:3584–
92.
[26] Li Z, He G, Wan X, Liu Y, Zhou J, Long G, Zuo Y, Zhang M, Chen Y. Solution
processable rhodanine-based small molecule organic photovoltaic cells with a power
conversion efficiency of 6.1%. Adv Energy Mater 2012;2:74–7.

ACCEPTED MANUSCRIPT
[27] Vander Poll TS, Love JA, Nguyen T-Q, Bazan GC. Non-basic high-performance
molecules for solution-processed organic solar cells. Adv Mater 2012;24:3646–49.
[28] Zhou J, Wan X, Liu Y, Zuo Y, Li Z, He G, Long G, Ni W, Li C, Su X, Chen Y. Small
molecules based on benzo[1,2-b:4,5-b']dithiophene unit for high-performance solution-
processed organic solar cells. J Am Chem Soc 2012;134:16345−51.
[29] Gupta V, Kyaw AKK, Wang DH, Chand S, Bazan GC, Heeger AJ. Barium: an
efficient cathode layer for bulk-heterojunction solar cells. Sci Rep 2013;3:1965−70.
[30] Zhou J, Zuo Y, Wan X, Long G, Zhang Q, Ni W, Liu Y, Li Z, He G, Li C, Kan B, Li
M, Chen Y. Solution-processed and high-performance organic solar cells using small
molecules with a benzodithiophene unit. J Am Chem Soc 2013;135:8484–87.
[31] Gupta A, Ali A, Singh TB, Bilic A, Bach U, Evans RA. Molecular engineering for
panchromatic absorbing oligothiophene donor-acceptor organic semiconductors. Tetrahedron
2012;68:9440–47.
[32] Scharber MC, Mühlbacher D, Koppe M, Denk P, Waldauf C, Heeger AJ, Brabec CJ.
Design rules for donors in bulk-heterojunction solar cells-towards 10% energy-conversion
efficiency. Adv Mater 2006;18:789–94.
[33] Xiang W, Gupta A, Kashif MK, Duffy N, Bilic A, Evans RA, Spiccia L, Bach U.
Cyanomethylbenzoic acid: an acceptor for donor–π–acceptor chromophores used in dye-
sensitized solar cells. ChemSusChem 2013;6:256–60.
[34] Gupta A, Kelson MMA, Armel V, Bilic A, Bhosale SV. N-alkyl- and N-aryl-
dithieno[3,2-b:2’,3’-d]pyrrole-containing organic dyes for efficient dye-sensitized solar cells.
Tetrahedron 2014;70:2141–50.

ACCEPTED MANUSCRIPT
[35] McEwen CN, McKay RG, Larsen BS. Analysis of solids, liquids, and biological
tissues using solids probe introduction at atmospheric pressure on commercial LC/MS
instruments. Anal Chem 2005;77:7826–31.
[36] Frisch MJ et al. Gaussian 09, Revision D.01. Wallingford CT: Gaussian Inc.; 2013.
[37] Blouin N, Michaud A, Gendron D, Wakim S, Blair E, Neagu-Plesu R, Bellette M,
Durocher G, Tao Y, Leclerc M. Toward a rational design of poly(2,7-carbazole) derivatives
for solar cells. J Am Chem Soc 2008;130:732–42.
[38] Gadisa A, Svensson M, Andersson MR, Inganäsa O. Correlation between oxidation
potential and open-circuit voltage of composite solar cells based on blends of polythiophenes/
fullerene derivative. Appl Phys Lett 2004;84:1609–11.
[39] Bijleveld JC, Zoombelt AP, Mathijssen SG, Wienk MM, Turbiez M, De Leeuw DM,
Janssen RAJ. Poly(diketopyrrolopyrrole−terthiophene) for ambipolar logic and photovoltaics.
J Am Chem Soc 2009;131:16616–7.