Synthesis and characterization of alkoxyphenylthiophene substituted benzodithiophene-based 2D conjugated polymers for organic electronics applications

Article abstract of DOI:10.1016/j.dyepig.2015.07.032

In order to establish the correlation between intramolecular non-covalent interactions in the polymer backbone and the position/nature of alkoxy side chains in electron rich unit, two new series of low band gap polymers (P1-P4; P5-P8) were synthesized. We

Full text of DOI:10.1016/j.dyepig.2015.07.032

Dyes and Pigments 123 (2015) 100e111
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and characterization of alkoxyphenylthiophene substituted
benzodithiophene-based 2D conjugated polymers for organic
electronics applications
Kakaraparthi Kranthiraja a, Kumarasamy Gunasekar a, Nallan Chakravarthi a,
,
, *
Myungkwan Song b **, Jong Hun Moon c, Jin Yong Lee c, In-Nam Kang d, Sung-Ho Jin a
a Department of Chemistry Education, Graduate Department of Chemical Materials, BK 21 PLUS Team for Advanced Chemical Materials, and Institute for
Plastic Information and Energy Materials, Pusan National University, Busan 609-735, Republic of Korea
b Surface Technology Division, Korea Institute of Materials Science, Changwon 641-831, Republic of Korea
c Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Republic of Korea
d Department of Chemistry, The Catholic University of Korea, Bucheon, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
In order to establish the correlation between intramolecular non-covalent interactions in the polymer
backbone and the position/nature of alkoxy side chains in electron rich unit, two new series of low band
gap polymers (P1eP4; P5eP8) were synthesized. We observed significant changes in photophysical
properties upon changing the position and nature of the alkoxy side chain with respect to their intra-
molecular non-covalent interactions in the backbone, which are reflected in their field-effect transistors
mobilities and photovoltaic properties. The UveVis spectra of polymers containing a thieno[3,4-c]pyr-
role-4,6-dione unit shows a vibronic shoulder, and the intensity of vibronic shoulder varies upon
changing the position and nature of alkoxy side chain, in contrast, this type of absorption shoulder was
not observed in polymer series containing a benzo[c][1,2,5]-thiadiazole moiety. In the thieno[3,4-c]
pyrrole-4,6-dione series, a maximum power conversion efficiency of 4.19% with an open-circuit voltage
of 0.91 V was noted. The power conversion efficiency of a benzo[c][1,2,5]-thiadiazole containing poly-
mers was improved upon addition of 1,8-diiodooctane with maximum of 5.26% being recorded for
polymer solar cells.
Received 19 March 2015
Received in revised form
24 July 2015
Accepted 25 July 2015
Available online 1 August 2015
Keywords:
Low band gap polymers
Non-covalent interactions
Planar backbone
Polymer solar cells
Polymer field effect transistors
Side chain
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
graft them with different alkyl chains [2]. In order to improve the
device performance of polymer solar cells or polymer field effect
Donoreacceptor (DeA) conjugated polymers have received
great amount of interest in optoelectronics such as polymer solar
cells, and polymer field effect transistors due to their advantage
over their counterparts, (inorganic silicon materials), such as low
cost, light weight, flexibility, and also possibility of large scale roll-
to-roll processing of printed electronics [1]. Until today a huge
number of DeA conjugated polymers have been designed and
synthesized to improve the device performance of polymer solar
cells and polymer filed effect transistors. But, the studies are mainly
focused on the design of various types of conjugated materials and
transistors further it is crucial to study structure property re-
lationships of conjugated polymers, such as side chain engineering,
planarity of backbone and their effect on morphology which can
strongly influence the performance of devices [3]. Recently few
research groups have studied all the above properties and analyzed
their effect on device performances. Particularly, the device per-
formance, mainly depending on the planarity of the backbone of
conjugated polymers [4,5].
In order to improve the planarity of backbone, many possible
ways have been anticipated, such as synthesizing highly fused
conjugated aromatic ring system, introduction of conjugated
p-
spacers between the electron rich and deficient units and the
introduction of either planar aromatic electron rich or electron
deficient units [6e8]. In some rare cases, the nature of the alkyl
chain either on the electron rich unit or electron deficient unit has
* Corresponding author.
** Corresponding author.
E-mail addresses: smk1017@kims.re.kr (M. Song), shjin@pusan.ac.kr (S.-H. Jin).
http://dx.doi.org/10.1016/j.dyepig.2015.07.032
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

K. Kranthiraja et al. / Dyes and Pigments 123 (2015) 100e111
101
also played a role on the planarity of the polymer backbone [9]. In
recent times another way of improving the planarity of the back-
bone has become interesting, i.e. the intramolecular non-covalent
interactions between the electron rich and electron deficient
units of conjugated polymers, which can forcefully make the
backbone planar [10]. It has been proven that these intramolecular
non-covalent interactions greatly influence the bulk properties of
conjugated polymers such as high mobility, better optical proper-
ties and suitable energy levels. These factors greatly influence the
performance of polymer solar cells and polymer filed effect tran-
sistors in many cases [11]. According to the previous reports, the
side chains grafted on the polymer backbone not only induce the
solubility but also can modulate the morphology of the active layer
when it blends with phenyl-C71-butyric acid methyl ester (PC71BM)
as an acceptor in bulk hetero junction polymer solar cells [12]. To
this end, we attempt to investigate the correlation between intra-
molecular non-covalent interactions in the backbone and the na-
ture/position of alkyl chains on an electron rich unit of conjugated
polymers to improve the planarity of the backbone.
Here, we have synthesized two series of conjugated polymers
P1eP4 and P5eP8 containing the alkoxyphenylthiophene (APTh)
linked benzodithiophene (BDT) as an electron rich and 1,3-di(2-
bromothien-5-yl)-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione
(TPD) and [5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]
thiadiazole] (BT) as an electron deficient units as shown Scheme 1,
respectively. We have analyzed the influence of nature (ethylhexyl
to hexyldecyl) and position para (p) to meta (m) of the alkyl chain in
APTh on the planarity of backbone with respect to their intra-
molecular non-covalent interactions. The intramolecular non-
covalent interactions between the carbonyl oxygen and thio-
phene sulfur is stronger than the alkoxy oxygen and thiophene
sulfur [13]. As we expected, there are significant differences in its
photophysical, photovoltaic and transistor characteristics between
the two series as well as within the series of polymers. The alkoxy
side chains in APTh of all the polymers will be represented
p-hexyldecyloxyphenylthiophene
hexyloxyphenylthiophene (m-EHPTh), m-hexyldecyloxyphenylth-
iophene (m-HDPTh), respectively.
(p-HDPTh)
and
m-ethyl-
2. Experimental
2.1. Materials and characterization
All chemicals and reagents were purchased from SigmaeAldrich
Chemical Co. Ltd, TCI, and Alfa Aesar, and were used without further
purification. 1H and 13C NMR spectra were recorded on a Varian
Mercury Plus 300 MHz, 600 MHz spectrometers in CDCl3 using
tetramethylsilane (TMS) as an internal standard. The FTIR spectra
were measured using a JASCO FTIR spectrometer. The UveVis ab-
sorption spectra were recorded on a JASCO V-570 spectrophotom-
eter at room temperature. TGA was carried out on a Mettler Toledo
TGA/SDTA 851e under N2 atmosphere at a heating rate of 10 ꢀC/min.
Polymerization was conducted in CEM focused microwave TM syn-
thesis system (CEM Discover-S). Weight average molecular weight
(Mw), number average molecular weight (Mn), and polydispersity
index (PDI) were determined against polystyrene as standard by GPC
using PL gel 5 mm MLXED-C column on an Agilent 1100 series liquid
chromatography system with tetrahydrofuran as an eluent. Cyclic
voltammetry measurements were carried out in a 0.1 M solution of
tetrabutylammonium tetrafluoroborate in anhydrous acetonitrile at
a scan rate of 100 mV/s using CHI 600C potentiostat (CH In-
struments); three electrode cell with platinum electrode as the
working electrode, Ag/AgCl as the reference electrode and a plat-
inum (Pt) wire as the counter electrode were used. Polymer thin
films were coated on Pt electrode and dried before the experiment.
2.2. Polymer field effect transistors fabrication and characterization
Bottom contact geometry polymer filed effect transistors was
fabricated to examine the charge-transport properties of polymers.
The photoactive layers were deposited by spin coating a 5 mg/mL
as
follwos:
p-ethylhexyloxyphenylthiophene
(p-EHPTh),
Scheme 1. Structures of polymers.

102
K. Kranthiraja et al. / Dyes and Pigments 123 (2015) 100e111
polymer solution in chloroform under a N2 atmosphere onto n-
octyltrichlorosilane, n-doped silicon wafer with a 300 nm thickness
of SiO2 dielectric layer. The devices were then annealed for
10 min at 140 ꢀC in a glove box under N2 atmosphere. The polymer
filed effect transistors characteristics were determined under
ambient conditions, and no precautions were taken to insulate the
materials or devices from exposure to air, moisture, or light. The
field effect mobility was calculated in the saturation regime by
J ¼ 6 Hz, 2H), 1.7 (m, 1H), 1.33e1.55 (m, 8H), 0.9 (m, 6H). 13C NMR
(300 MHz, CDCl3):
d (ppm) 159.33, 144.77, 128.13, 127.42, 127.30,
123.94, 122.22, 115.15, 70.90, 39.67, 30.82, 29.37, 24.15, 23.32, 14.35,
11.38. HRMS (EIþ, m/z) [Mþ] calcd for C18H24OS 288.15; found,
288.12.
2.4.2. Synthesis of 2-(4-(2-hexyldecyloxy)phenyl)thiophene
(colorless oil 2, yield 68%)
using the equation IDS ¼ (
source current, is the field effect mobility, W is the channel width
(120 m), L is the channel length (12 m), Ci is the capacitance per
m
WCi/2L)(VGeVT)2, where IDS is the drain-
1H NMR (300 MHz, CDCl3):
d (ppm) 7.46e7.58 (m, 2H), 7.16e7.22
m
(m, 2H), 7.02e7.08 (m, 1H), 6.86e6.94 (m, 2H), 3.80e3.90 (m, 2H),
1.70e1.80 (m, 1H), 1.10e1.40 (m, 24H), 0.80e0.90 (m, 6H). 13C NMR
m
m
unit area of the gate dielectric layer, and VG is the gate voltage.
(300 MHz, CDCl3):
d (ppm) 159.32, 144.76, 128.10, 127.40, 127.26,
123.91, 122.18, 115.14, 71.31, 38.22, 32.16, 32.11, 31.64, 30.27, 29.94,
29.84, 29.58, 28.52, 27.10, 27.08, 27.01, 22.92, 17.51, 14.34, 13.82.
HRMS (EIþ, m/z) [Mþ] calcd for C26H40OS 400.28; found, 400.20.
2.3. Fabrication and characterization of the bulk heterojunction
polymer solar cells
The ITO and glass substrates that have been used for fabrication
were ultrasonically cleaned with detergent, water, acetone, and
isopropyl alcohol. Then, a 40 nm thick layer of PEDOT:PSS was
coated on the electrodes by spin coating a solution of PEDOT:PSS
diluted with isopropyl alcohol with the dilution ratio being 1:2 and
annealed at 150 ꢀC for 10 min in oven. A 90 10 nm thick active
layer of P1eP8:PC71BM was spin coated using a mixture of poly-
mer:PC71BM (8 mg:16 mg) that was dissolved in 1 mL of chloro-
benzene and further mixed with 1e3 vol% 1,8 diiodooctane (DIO). A
0.7 nm thickness of LiF and 100 nm thick Al cathode was deposited
via evaporation on the active layer. The performances of the bulk
heterojunction polymer solar cells were measured under simulated
AM 1.5G illumination (100 mW/cm2). The irradiance of the sunlight
simulating illumination was calibrated using a standard Si photo-
diode detector fitted with a KG5 filter. The performance of the bulk
heterojunction polymer solar cells were measured using calibrated
air mass (AM) 1.5G solar simulator (Oriel® Sol3A™ Class AAA solar
simulator, models 94043A) with a light intensity of 100 mW/cm2
adjusted using a standard PV reference cell (2 cm ꢁ 2 cm mono-
crystalline silicon solar cell, calibrated at NREL, Colorado, USA) and
a computer controlled Keithley 2400 source measure unit. The
incident photon to current conversion efficiency spectrum was
measured using Oriel® IQE-200™ equipped with a 250 W quartz
tungsten halogen lamp as the light source and a monochromator,
an optical chopper, a lock-in amplifier, and a calibrated silicon
photodetector. The thickness of the thin films was measured using
a KLA Tencor Alpha-step IQ surface profilometer with an accuracy
of 1 nm. AFM images were acquired with a XE-100 (park system
corp.) in tapping mode.
2.4.3. Synthesis of 2-(3-(2-ethylhexyloxy)phenyl)thiophene
(colorless oil 3, yield 70%)
1H NMR (300 MHz, CDCl3):
d (ppm) 7.27e7.31 (m, 3H), 7.14e7.20
(m, 2H), 7.05e7.09 (m, 1H), 6.81e6.84 (m, 1H), 3.84e3.90 (m, 2H),
1.58e1.75 (m, 1H), 1.33e1.53 (m, 8H), 0.91e0.97 (m, 6H). 13C NMR
(300 MHz, CDCl3):
d (ppm) 160.24, 144.64, 135.89, 130.02, 128.10,
124.98, 124.98, 123.43, 118.54, 113.78, 112.57, 70.80, 39.67, 30.79,
29.34, 28.49, 26.97, 24.13, 23.27,17.51,14.28,13.78,11.35. HRMS (EIþ,
m/z) [Mþ] calcd for C18H24OS 288.15; found, 288.14.
2.4.4. Synthesis of 2-(3-(2-hexyldecyloxy)phenyl)thiophene
(colorless oil 4, yield 73%)
1H NMR (300 MHz, CDCl3):
d (ppm) 7.28e7.34 (m, 3H), 7.1e7.2
(m, 2H), 7.02e7.08 (m, 1H), 6.80e6.88 (m, 2H), 3.8e3.9 (m, 2H), 1.8
(m, 1H), 1.2e1.4 (m, 24), 0.8e0.95 (m, 6H). 13C NMR (300 MHz,
CDCl3):
d (ppm) 160.06, 150.59, 144.67, 135.91, 130.02, 129.09,
128.10, 126.18, 124.95, 123.42, 123.27, 118.52, 113.79, 112.59, 108.61,
71.20, 38.28, 36.67, 32.18, 32.13, 28.54, 27.02, 22.94, 19.67, 17.74,
15.34, 13.82. HRMS (EIþ, m/z) [Mþ] calcd for C26H40OS 400.28;
found, 400.24.
2.5. Synthesis of compound 5e8
The compounds 5e8 were synthesized by applying the similar
procedure. The synthetic procedure for 5 is as follows: Compound 1
(2.59 g, 9 mmol) in dry tetrahydrofuran (25 mL), n-BuLi (3.6 mL,
9 mmol) was added dropwise at 0 ꢀC, then reaction mixture was
heated to 50 ꢀC for 2 h and then 4,8-dihydrobenzo[1,2-b:4,5-b]
dithiophen-4,8-dione (1 g, 4.53 mmol) was added to reaction
mixture at same temperature and then continued the heating for
2 h and the reaction mixture was cooled to room temperature and
then SnCl2$2H2O (4.08 g, 18 mmol) in 10 mL (H2O:HCl 7:3 v/v) was
added to reaction mixture and then stirred for overnight. The re-
action mixture was then poured into cold water and extracted with
diethyl ether and dried over anhydrous MgSO4 and then evaporated
the organic layer and the resulted crude product was purified by
column chromatography on silica gel with (hexane: methylene
chloride 9:1 v/v) to furnish a yellow colored solid.
2.4. Synthesis of compounds 1e4
The compounds 1e4 were synthesized by applying the similar
procedure. The representative procedure for compound 1 is as
follows: 4-bromophenyl-2-ethylhexyl ether (6 g, 21 mmol) and 2-
tributylstannylthiophene (10 mL, 31.5 mmol) were added to dry
toluene (30 mL) and purged with N2 for 20 min and then Pd(PPh3)4
(1.2 g, 1.05 mmol) was added to the reaction mixture and refluxed
overnight. After completion of reaction, the reaction mixture was
cooled to room temperature and the solvent was removed. Crude
product was extracted with methylene chloride (2 ꢁ 100 mL),
washed with water, brine solution, and dried over anhydrous
MgSO4. The resulted crude was purified by column chromatog-
raphy on silica gel with hexane as an eluent.
2.5.1. Synthesis of 4,8-Bis(2-(4-(2-ethylhexyloxy)phenyl)-5-thienyl)
benzo[1,2-b:4,5-b0]dithiophene (yellow color solid 5, 28.9%). mp
132e134 ꢀC
1H NMR (300 MHz, CDCl3):
d
(ppm) 7.72 (d, J ¼ 6 Hz, 2H), 7.61 (d,
J ¼ 6 Hz, 4H), 7.44e7.50 (m, 4H), 7.32 (d, J ¼ 3 Hz, 2H), 6.94e6.97 (m,
4H), 3.88e3.90 (m, 4H), 1.72e1.77 (m, 2H), 1.36e1.54 (m, 16H),
2.4.1. Synthesis of 2-(4-(2-ethylhexyloxy)phenyl)thiophene
(colorless oil 1, yield 66%)
0.93e0.98 (m, 12H). 13C NMR (300 MHz, CDCl3):
d (ppm) 159.57,
1H NMR (300 MHz, CDCl3):
7.19e7.25 (m, 2H), 7.02e7.08 (m, 1H), 6.90 (d, J ¼ 9 Hz, 2H), 3.85 (d,
d
(ppm) 7.51 (d, J ¼ 6 Hz, 2H),
145.69, 139.31, 138.08, 136.83, 129.311, 127.922, 127.36, 126.91,
124.09, 123.61, 122.45, 115.25, 70.95, 39.64, 30.78, 29.33, 24.12,

K. Kranthiraja et al. / Dyes and Pigments 123 (2015) 100e111
103
23.27, 14.29, 11.34. HRMS (EIþ, m/z) [Mþ] calcd for C46H50O2S4
762.27; found, 762.25.
2.6.2. Synthesis of 2,6-Bis(trimethyltin)-4,8-bis(2-(4-(2-
hexyldecyloxy)phenyl)-5-thienyl)benzo[1,2-b:4,5-b0]dithiophene
(yellow oil D2, 75%)
1H NMR (300 MHz, CDCl3):
d (ppm) 7.82 (s, 2H), 7.66e7.69 (m,
2.5.2. Synthesis of 4,8-Bis(2-(4-(2-hexyldecyloxy)phenyl)-5-
thienyl)benzo[1,2-b:4,5-b0]dithiophene (yellow color solid 6, 40%).
mp 99e101 ꢀC
4H), 7.51e7.53 (m, 2H), 7.38e7.39 (m, 2H), 6.98e7.01 (m, 4H),
3.91e3.93 (m, 4H), 1.85 (m, 2H), 0.86e0.89 (m, 48H), 0.37e0.55 (m,
18H). 13C NMR (300 MHz, CDCl3): 159.55, 145.58, 143.68, 142.97,
138.98, 137.72, 131.34, 129.25, 127.39, 122.48, 115.29, 71.39, 38.27,
32.20, 30.31 29.62, 27.1422.97, 14.40. FTIR (KBr, cmꢂ1) 2956, 2928,
2847, 1607. HRMS (EIþ, m/z) [Mþ] calcd for C68H98O2S4Sn2 1314.45;
found, 1314.40.
1H NMR (300 MHz, CDCl3):
d
(ppm) 7.71 (d, J ¼ 5.7 Hz, 2H), 7.61
(d, J ¼ 8.4 Hz, 4H), 7.44e7.52 (m, 4H), 7.33e7.34 (m, 2H), 6.90e6.98
(m, 4H), 3.9 (m, 4H), 1.9 (m, 2H), 1.1e1.4 (m, 48H), 0.9 (m, 12H). 13
NMR (300 MHz, CDCl3): (ppm) 159. 58, 145.70, 139.31, 138.08,
C
d
136.82, 129.31, 127.92, 127.92, 127.34, 126.90, 124.09, 123.62, 122.44,
115.25, 71.37, 38.22, 32.14, 31.63, 27.08, 22.90, 14.32. HRMS (EIþ, m/
z) [Mþ] calcd for C62H82O2S4 986.52; found, 986.50.
2.6.3. Synthesis of 2,6-Bis(trimethyltin)-4,8-bis(2-(3-(2-
ethylhexyloxy)phenyl)-5-thienyl)benzo[1,2-b:4,5-b0]dithiophene
(yellow solid D3, 64%). mp 131e133 ꢀC
1H NMR (300 MHz, CDCl3):
d (ppm) 7.73 (s, 2H), 7.31e7.48 (m,
2.5.3. Synthesis of 4,8-Bis(2-(3-(2-ethylhexyloxy)phenyl)-5-
thienyl)benzo[1,2-b:4,5-b0]dithiophene (yellow solid 7, yield: 26%).
mp 100e103 ꢀC
10H), 6.84e6.90 (m, 2H), 3.92e3.94 (m, 4H), 1.74e1.80 (m, 2H),
1.34e1.53 (m, 16H), 0.92e0.99 (m, 12H), 0.32e0.42 (m, 18H). 13C
NMR (300 MHz, CDCl3):
d (ppm) 160.12, 145.21, 143.66, 143.16,
1H NMR (300 MHz, CDCl3):
d
(ppm) 7.70 (d, J ¼ 6 Hz, 2H),
140.05, 137.69, 135.75, 131.17, 130.13, 129.19, 123.76, 122.36, 118.52,
114.07, 112.51, 70.93, 39.69, 30.82, 29.34, 24.16, 23.27, 14.28, 11.35.
FTIR (KBr, cmꢂ1) 2964, 2918, 2866, 1597. HRMS (EIþ, m/z) [Mþ]
calcd for C52H66O2S4Sn2 1090.20; found, 1090.19.
7.44e7.51 (m, 6H), 7.28e7.33 (m, 6H), 6.82e6.90 (m, 2H), 3.90e3.92
(m, 4H), 1.74e1.80 (m, 2H), 1.35e1.51 (m, 16H), 0.91e0.99 (m, 12H).
13C NMR (300 MHz, CDCl3):
d (ppm) 160.13, 145.56, 139.36, 136.85,
135.56, 130.15, 129.30, 128.04, 124.06, 123.06, 118.47, 114.16, 112.49,
70.89, 39.70, 31.07, 29.35, 24.15, 23.27, 14.28, 11.36. HRMS (EIþ, m/z)
[Mþ] calcd for C46H50O2S4 762.27; found, 762.22.
2.6.4. Synthesis of 2,6-Bis(trimethyltin)-4,8-bis(2-(3-(2-
hexyldecyloxy)phenyl)-5-thienyl)benzo[1,2-b:4,5-b0]dithiophene
(yellow oil D4, 68%)
1H NMR (300 MHz, CDCl3):
d (ppm) 7.73 (s, 2H), 7.48e7.50 (m,
2.5.4. Synthesis of 4,8-Bis(2-(3-(2-hexyldecyloxy)phenyl)-5-
4H), 7.31e7.46 (m, 6H), 3.91e3.95 (m, 2H), 1.83 (m, 2H), 1.08e1.48
thienyl)benzo[1,2-b:4,5-b0]dithiophene (yellow oil 8, 40%)
(m, 48H), 0.69e0.88 (m, 12H),0.32e0.51 (m, 18H). 13C NMR
1H NMR (300 MHz, CDCl3):
d (ppm) 7.68e7.74 (m, 2H), 7.42e7.52
(300 MHz, CDCl3):
d (ppm) 160.12, 145.56, 143.26, 143.15, 140.04,
(m, 4H), 7.20e7.36 (m, 8H), 6.82e6.90 (m, 2H), 3.89e3.91 (m, 4H),
1.81 (m, 2H), 1.29e1.54 (m, 48H), 0.9 (m, 12H). 13C NMR (300 MHz,
CDCl3): 160.14, 145.57, 139.36, 139.18, 136.85, 135.56, 130.16, 129.31,
128.05, 126.12, 124.06, 123.77, 123.55, 118.46, 114.17, 112.49, 71.28,
38.27, 32.10, 31.66, 30.26, 29.93, 29.82, 29.56, 27.10, 22.90, 12.32.
HRMS (EIþ, m/z) [Mþ] calcd for C62H82O2S4 986.52; found, 986.53.
139.57, 139.18, 138.26, 137.68, 136.85, 136.24, 135.73, 131.17, 130.14,
129.20, 128.07, 123.77, 122.36, 118.49, 114.06, 112.48. FTIR (KBr,
cmꢂ1) 2965, 2929, 2865, 1593. HRMS (EIþ, m/z) [Mþ] calcd for
C68H98O2S4Sn2 1314.45; found, 1314.42.
2.7. General polymerization procedure
All the polymers were synthesized by applying microwave-
assisted Stille polymerization. The representative procedure for
the synthesis of polymer P1 is as follows: To a 10 mL microwave
tube, compound D1 (300 mg, 0.275 mmol), and 1,3-di(2-
bromothien-5-yl)-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione
(A1) (161 mg, 0.275 mmol), Pd2 (dba)3 (5.4 mg, 2 mol %), and (o-
tol)3P (16 mg, 16 mol %) were dissolved in anhydrous chloroben-
zene, 5 mL). The reaction mixture was purged with N2 for 15 min.
The microwave tube was placed into the reactor and heated to
120 ꢀC for 30 min. After cooling to room temperature reaction
mixture was poured into methanol to obtain precipitate. The
resulted precipitate was purified by soxhlet extraction method
using methanol, hexane, acetone, and chloroform. The chloroform
fraction was evaporated to get P1.
2.6. Synthesis of D1eD4
Monomers D1eD4 was synthesized by applying the similar
procedure. The synthetic procedure for D1 is as follows: A solution
of compound 5 (0.6 g, 0.78 mmol) in dry tetrahydrofuran (20 mL)
at ꢂ78 ꢀC under N2 atmosphere, tert-BuLi (0.92 mL) was added
dropwise stirred for 30 min at same temperature and trimethyltin
chloride (0.312 g, 1.57 mmol) was added at one shot to reaction
mixture at ꢂ78 ꢀC and then stirred the reaction mixture at room
temperature for overnight. Aqueous sodium carbonate solution was
added slowly to reaction mixture and then extracted with diethyl
ether and dried over anhydrous MgSO4 to get yellow solid upon
recrystallization with ethanol.
Poly[4,8-bis(2-(4-(2-ethylhexyloxy)phenyl)-5-thienyl)benzo[1,2-
b:4,5-b0]dithiophene-alt-1,3-di(2-bromothien-5-yl)-5-(2-ethylhexyl)
thieno[3,4-c]pyrrole-4,6-dione] (P1, 58%). Mn and PDI are 26,000 g/
2.6.1. Synthesis of 2,6-Bis(trimethyltin)-4,8-bis(2-(4-(2-
ethylhexyloxy)phenyl)-5-thienyl)benzo[1,2-b:4,50]dithiophene
(yellow solid D1, 70%). mp 179e181 ꢀC
mol and 1.2. 1H NMR (300 MHz, CDCl3):
d (ppm) 7.5e7.68 (br, 4H),
1H NMR (300 MHz, CDCl3):
d
(ppm) 7.75 (s, 2H), 7.63 (d, J ¼ 9 Hz,
7.19e7.39 (br, 10H), 6.92 (br, 4H), 3.87 (br, 4H) 3.53 (br, 2H) 1.76 (br,
4H), 7.46 (d, J ¼ 3 Hz, 2H), 7.35 (d, J ¼ 3 Hz, 2H), 6.95e7.01 (m, 4H),
3.89e3.94 (m, 4H), 1.74e1.78 (m, 2H), 1.36e1.52 (m, 16H),
0.93e0.98 (m, 12H), 0.31e0.54 (m, 18H). 13C NMR (300 MHz,
3H), 1.25e1.55 (br, 16H), 0.85e0.95 (br, 18H). 13C NMR (600 MHz,
CDCl3):
d (ppm) 127.17, 120.99, 114.85, 70.63, 39.42, 30.53, 29.68,
29.12, 26.03, 23.86, 23.07, 22.64, 14.09, 11.12. FTIR (KBr, cmꢂ1) 3430,
3071, 2927, 2847, 1751, 1681. Anal. Calcd for C68H69NO4S7: C, 68.71;
H, 5.85; N, 1.18. Found C, 68.55; H, 6.06; N, 1.41.
CDCl3):
d (ppm) 159.50, 145.35, 143.62, 142.95, 138.95, 137.66,
131.28, 129.19, 127.36, 127.10, 122.44, 115.24, 70.96, 39.65, 30.79,
29.34, 24.13, 23.26, 14.29, 11.35. FTIR (KBr, cmꢂ1) 2964, 2918, 2862,
1601. HRMS (EIþ, m/z) [Mþ] calcd for C52H66O2S4Sn2 1090.20;
found, 1090.18.
Poly[4,8-bis(2-(4-(2-hexyldecyloxy)phenyl)-5-thienyl)benzo[1,2-
b:4,5-b0]dithiophene-alt-1,3-di(2-bromothien-5-yl)-5-(2-ethylhexyl)
thieno[3,4-c]pyrrole-4,6-dione] (P2, 59%). Mn and PDI are 27,000 g/

104
K. Kranthiraja et al. / Dyes and Pigments 123 (2015) 100e111
mol and 1.3. 1H NMR (300 MHz, CDCl3):
d
(ppm) 7.60e7.77 (br, 4H),
1H NMR (300 MHz, CDCl3):
d (ppm) 8.48 (br, 2H), 7.70e7.90 (br, 2H),
7.25e7.27 (br,10H), 6.94 (br, 4H), 3.87 (br, 4H), 3.52 (br, 2H),1.76 (br,
7.49 (br, 6H), 7.25e7.30 (br, 4H), 6.89 (br, 4H), 4.14 (br, 4H), 3.91 (br,
3H), 1.31e0.56 (br, 48H), 0.89 (br, 18H). 13C NMR (600 MHz, CDCl3):
4H), 1.81e1.91 (br, 2H), 1.28e1.53 (br, 76H), 0.86 (br, 18H). 13C NMR
d
(ppm) 38. 00, 31.91, 31.37, 29.69, 26.85, 22.68, 17.34, 14.11. FTIR
(KBr, cmꢂ1) 3441, 3057, 2917, 2858, 1756, 1695. Anal. Calcd for
84H101NO4S7: C, 71.39; H, 7.20; N, 0.99. Found C, 71.74; H, 7.30; N,
1.28.
(600 MHz, CDCl3): d (ppm) 159.89, 138.75, 135.29, 129.92, 123.25,
118.25, 113.92, 112.18, 102.42, 71.04, 38.03, 31.91, 29.70, 26.88, 14.2.
C
FTIR (KBr, cmꢂ1) 3444, 3064, 2923, 2847, 1729, 1597. Anal. Calcd for
C92H118N2O4S7. C, 71.73; H, 7.72; N, 1.82. Found. C, 71.43; H, 7.75; N,
Poly[4,8-bis(2-(3-(2-ethylhexyloxy)phenyl)-5-thienyl)benzo[1,2-
b:4,5-b0]dithiophene-alt-1,3-di(2-bromothien-5-yl)-5-(2-ethylhexyl)
2.18.
thieno[3,4-c]pyrrole-4,6-dione] (P3, 64%). Mn and PDI are 30,000 g/
3. Results and discussion
3.1. Synthesis and characterization
mol and 1.2. 1H NMR (300 MHz, CDCl3):
d
(ppm) 7.19e7.47 (br, 10H),
6.88 (br, 8H), 3.9 (br, 4H), 3.49 (br, 2H), 1.9 (br, 3H), 1.35e1.54 (br,
24H), 0.93 (br, 18H). 13C NMR (600 MHz, CDCl3):
(ppm) 162.34,
d
159.79, 145.12, 140.22, 139.28, 138.47, 137.95, 136.67, 135.27, 135.21,
132.10, 130.32, 129.84, 128.25, 127.56, 125.63, 123.75, 122.92, 119.51,
118.25, 113.76, 112.03, 76.78, 70.53, 42.52, 39.43, 38.09, 30.54, 29.10,
28.50, 23.87, 14.09, 11.15, 10.43. FTIR (KBr, cmꢂ1) 3446, 3057, 2917,
2847, 1756, 1691. Anal. Calcd for C68H69NO4S7: C, 68.71; H, 5.85; N,
1.18. Found C, 68.97; H, 6.15; N, 1.31.
All the monomers and polymers were synthesized according to
the route shown in Scheme 2. The m and p-alkoxy substituted
bromophenyl compounds were synthesized by Williamson's aryl
ether synthesis from their corresponding bromophenols followed
by Stille coupling with 2-tributylstannylthiophene to yield the
conjugated APTh (1e4). These compounds were treated with 4,8-
dihydrobenzo[1,2-b:4,5-b0]dithiophene-4,8-dione in the presence
Poly[4,8-bis(2-(3-(2-hexyldecyloxy)phenyl)-5-thienyl)benzo[1,2-
b:4,5-b0]dithiophene-alt-1,3-di(2-bromothien-5-yl)-5-(2-ethylhexyl)
thieno[3,4-c]pyrrole-4,6-dione] (P4, 62%). Mn and PDI are 28,000 g/
of n-BuLi to give
p-conjugated side chain linked BDT derivatives
(5e8) followed by reaction with trimethyltin chloride in the pres-
ence of tert-BuLi yielded the monomers D1eD4. Additionally, the
monomers A1 and A2 were synthesized according to the literature
procedures [14,15]. Finally, all the polymers (P1eP8) were syn-
thesized by microwave-assisted Stille polymerization. All the new
monomer structures were confirmed by 1H NMR, 13C NMR, HRMS,
FTIR spectroscopies. And the polymers were characterized by 1H
NMR, 13C NMR spectroscopy, FTIR spectroscopy, and elemental
analysis. The NMR and FTIR spectra of the final compounds are
shown in Fig. S1eS8. All the polymers are readily soluble in com-
mon organic solvents such as tetrahydrofuran, chloroform, and
chlorobenzene. The thermal properties of P1eP8 were measured
by thermal gravimetric analysis (TGA) at heating rate of 10 ꢀC/min
(Fig. S9) and the data are listed in Table S1. The onset of decom-
position temperature (Td, corresponding to 5% weight loss) of
P1eP8 is in the range of 330e430 ꢀC indicating their very high
thermal stability. Particularly, the Td for P1eP4 is higher than that
of P5eP8 indicating better close packing arrangement of polymer
backbone in the former than that of in the latter series.
mol and 1.2. 1H NMR (300 MHz, CDCl3):
d (ppm) 7.6e7.9 (br, 4H),
7.23e7.46 (br, 10H), 6.88 (br, 4H), 3.91 (br, 4H), 3.5 (br, 2H), 1.82 (br,
3H), 1.29e1.58 (br, 48H), 0.88 (br, 18H). 13C NMR (600 MHz, CDCl3):
d
(ppm) 162.49, 159.82, 145.58, 139.56, 137.00, 136.31, 135.33,
132.61, 130.68, 129.83, 126.69, 123.78, 118.41, 113.91, 112.16, 71.03,
42.48, 38.08, 31.41, 30.49, 29.35, 23.78, 10.37. FTIR (KBr, cmꢂ1) 3437,
3077, 2917, 2847, 1751, 1691. Anal. Calcd for C84H101NO4S7. C, 71.39;
H, 7.20; N, 0.99. Found C, 71.62; H, 7.49; N, 1.27.
Poly[4,8-bis(2-(4-(2-ethylhexyloxy)phenyl)-5-thienyl)benzo[1,2-
b:4,5-b0]dithiophene
-alt-5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)
benzo[c][1,2,5]thiadiazole] (P5, 62%). Mn and PDI are 33,000 and 1.2.
1H NMR (300 MHz, CDCl3):
(ppm) 8.48 (br, 2H), 7.68e7.80 (br, 2H),
d
7.48e7.49 (br, 6H), 7.25e7.36 (br, 4H), 6.90 (br, 4H), 4.13 (br, 4H),
3.89 (br, 4H), 1.77e1.92 (br, 2H), 1.25e1.54 (br, 44H), 0.95 (br, 18H).
13C NMR (600 MHz, CDCl3):
d (ppm) 159.33, 150.15, 145.48, 138.52,
129.08, 128.10,122.24,114.99, 70.68, 39.41, 31.76, 30.54, 29.11, 25.91,
23.88, 22.68. 14.10, 11.14. FTIR (KBr, cmꢂ1) 3435, 3077, 2913, 2847,
1729, 1592. Anal. Calcd for C76H86N2O4S7. C, 69.36; H, 6.59; N, 2.13;
Found C, 69.97; H, 6.92; N, 2.31.
Poly[4,8-bis(2-(4-(2-hexyldecyloxy)phenyl)-5-thienyl)benzo[1,2-
b:4,5-b0]dithiophene-alt-5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)
benzo[c][1,2,5]thiadiazole] (P6, 62%). Mn and PDI are 35,000 and 1.3.
3.2. Optical properties
The UveVis absorption spectra of P1eP4 and P5eP8 in chlo-
roform solution and film are shown in Fig. 1a, c and b, d, respec-
tively; it clearly shows that there is a strong influence of nature/
position of alkyl chain in APTh with respect to intramolecular non-
covalent interactions. All the polymers show dual absorption
characteristics; the bands in the lower energy region between 450
and 700 nm are correspond to the intramolecular charge transfer
(ICT) interaction between electron rich and deficient units and the
bands in the higher energy region between 300 and 400 nm are
1H NMR (300 MHz, CDCl3):
d (ppm) 8.44 (br, 2H), 7.36e7.63 (br, 8H),
7.25e7.36 (br, 4H), 6.89 (br, 4H), 4.13 (br, 4H), 3.87 (br, 4H),
1.81e1.91 (br, 2H), 1.28e1.53 (br, 76H), 0.90 (br, 18H). 13C NMR
(600 MHz, CDCl3):
d (ppm) 159.37, 129.26, 127.09, 115.00, 71.11,
38.00, 31.91, 29.70, 26.87, 22.68, 14.12. FTIR (KBr, cmꢂ1) 3439, 3069,
2918, 2852, 1714, 1611. Anal. Calcd for C92H118N2O4S7. C, 71.73; H,
7.72; N, 1.82. Found C, 72.02; H, 8.06; N, 1.78.
Poly[4,8-bis(2-(3-(2-ethylhexyloxy)phenyl)-5-thienyl)benzo[1,2-
b:4,5-b0]dithiophene
-alt-5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)
benzo[c][1,2,5]thiadiazole] (P7, 52%). Mn and PDI are 24,000 and 1.3.
1H NMR (300 MHz, CDCl3):
(ppm) 8.48 (br, 2H), 7.68e7.83 (br, 2H),
attributed to the pep* transition of polymer backbone. The
calculated molar extinction coefficients of former series P1eP4
(~6 ꢁ 104 Mꢂ1 cmꢂ1) are higher than those of latter series P5eP8
(~5 ꢁ 104 Mꢂ1 cmꢂ1). Importantly, the ICT band in P1eP4 has
d
7.48e7.49 (br, 6H), 7.25e7.43 (br, 4H), 6.90 (br, 4H), 4.14 (br, 4H),
3.92e3.94 (br, 4H), 1.77e1.92 (br, 2H), 1.25e1.54 (br, 44H),
greater absorptivity than their
p
ep
* band. In contrast, the ICT band
0.82e0.98 (br, 18H). 13C NMR (600 MHz, CDCl3):
d
(ppm) 159.90,
145.36, 138.88, 135.30, 129.94, 123.58, 118.26, 113.87, 112.20, 81.79,
70.58, 39.45, 31.79, 30.57, 25.97, 22.69, 14.11, 11.16. FTIR (KBr, cmꢂ1
in P5eP8 has lesser absorptivity than their pep*. This difference in
absorptivity suggests that the former series has more planar
backbone than the latter because the extent of ICT interactions
mainly depends on the planarity of the backbone. Interestingly,
P1eP4 shows a pronounced vibronic shoulder peak between 590
and 620 nm both in solution and film due to the presence of strong
intramolecular non-covalent interactions between sulfur atom of
thiophene unit and the oxygen atom of carbonyl group; in contrast,
)
3435, 3064, 2918, 2852, 1733, 1601. Anal. Calcd for C76H86N2O4S7. C,
69.36; H, 6.59; N, 2.13. Found C, 69.72; H, 6.73; N, 2.17.
Poly[4,8-bis(2-(3-(2-hexyldecyloxy)phenyl)-5-thienyl)benzo[1,2-
b:4,5-
benzo[c][1,2,5]thiadiazole] (P8, 59%). Mn and PDI are 26,000 and 1.3.
b0]dithiophene-alt-5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)

K. Kranthiraja et al. / Dyes and Pigments 123 (2015) 100e111
105
Scheme 2. Synthetic route for monomers.
P5eP8 does not show such a shoulder peak which may be due to
the weak intramolecular non-covalent interactions between the
alkoxy oxygen and thiophene sulfur. Particularly, vibronic shoul-
ders in P1eP4 are well pronounced in the film state, which further
confirms the strong intramolecular non-covalent interactions.
Particularly, P3 shows almost equal relative intensity of the ICT
band and the shoulder peak confirms the existence of more planar
backbone than their counterparts. This can be further supported by
measuring various concentration UveVis absorption spectra of
P1eP4 in chloroform. As shown in Fig. S10, the intensity of the
shoulder peak is well pronounced upon increasing the concentra-
tion of polymer. In contrast, P5eP8 does not show any such pro-
nounced shoulder peaks with similarities. Collectively we conclude
that the polymers (P1eP4) have more planar backbone than the
polymers (P5eP8) due to the presence of strong intramolecular
non-covalent interactions in the former than in the latter, which
would lead to better charge transporting properties of P1eP4 than
that of P5eP8. In P1eP4, the ICT band of P2 (p-HDAPTh) appears
broader than that of in P1 (p-EHAPTh), in contrast, the P3 (m-
EHAPTh) shows broader ICT band than that of in P4 (p-HDAPTh).
Particularly, the P2 and P3 have a more pronounced vibronic
shoulder than that in P1 and P4, a feature which reveals that the
position and nature of the alkoxy chains play a key role in the
planarization of the polymer backbone. The longer branched alkyl
chain in p-position (P2) and shorter branched alkyl chain in m-
position (P3) show similar absorption patterns with slight red shift
of p-substituent; the similar absorption patterns also persist in the
thin film. Among the p-substitution (P1 and P2), the longer alkyl
chain (P2) shows more red shifted ICT band than the shorter one
(P1), in contrast, among the m-substitution (P3 and P4), the shorter
alkyl chain (P3) shows more red shifted ICT band than the longer
one (P4) this might be due to longer alkyl chain in m-position leads
to more steric hindrance. In the case of P5eP8, among the p-sub-
stitution (P5 and P6) the shorter alkyl chain shows red shifted ICT

106
K. Kranthiraja et al. / Dyes and Pigments 123 (2015) 100e111
Fig. 1. UveVis absorption spectra of (a) P1eP4 in chloroform solution (b) P5eP8 in chloroform solution (c) P1eP4 film (d) P5eP8 film.
band (P5) than the longer one (P6), in contrast, among the m-
substitution, the longer one shows red shifted ICT band (P8) than
the shorter one (P7). Finally, these findings reveal that the position
and nature of alkoxy group in APTh have a significant influence on
planarization of polymer backbone with respect to their intra-
molecular non-covalent interactions in both the series of polymers.
were determined from their onset of first oxidation potentials.
Although, all the polymers show relatively deep HOMO energy
level, the P1eP4 possess a deeper HOMO than the P5eP8, which
may enable a high open-circuit voltage (Voc) for the former, since
the Voc in mainly depends on the difference between the HOMO of
the donor and lowest unoccupied molecular orbital (LUMO) of the
acceptor materials. This may be due to the existence of higher
electron withdrawing nature of the TPD unit in P1eP4 than that of
the BT in P5eP8. Polymers P1eP8 show the HOMO energy level
below the air oxidation threshold limit (5.2 eV), which indicate
their air stability. The LUMO energy level of all polymers was
calculated from their corresponding optical band gap (Egopt) and
HOMO energy levels. The HOMO and LUMO energy levels of the
P1eP8 clearly indicating their good air stability and efficient
exciton dissociation at donor/acceptor interfaces, respectively [16].
3.3. Electrochemical properties
The electrochemical properties of P1eP8 were measured using
cyclic voltammetry (CV) and the corresponding cyclic voltammo-
grams are shown in Fig. S11 and the data are listed in Table 1. The
highest occupied molecular orbital (HOMO) energy levels of P1eP8
Table 1
Optical and electrochemical properties of P1eP8.
Polymer
Solution
Film
Egopt
HOMO
(eV)
LUMO
(eV)
3.4. Computational study
lmax (nm)a
lmax (nm)b
(eV)c
P1
P2
P3
P4
P5
P6
P7
P8
526, 601
564, 615
548, 607
516, 583
574
546
531
546
549, 602
564, 612
556, 607
545, 601
587
576
567
570
1.81
1.82
1.76
1.84
1.68
1.68
1.74
1.76
ꢂ5.36
ꢂ5.40
ꢂ5.42
ꢂ5.47
ꢂ5.30
ꢂ5.37
ꢂ5.34
ꢂ5.39
ꢂ3.55
ꢂ3.58
ꢂ3.66
ꢂ3.63
ꢂ3.62
ꢂ3.69
ꢂ3.60
ꢂ3.63
Density functional theory (DFT) calculations were carried out
using a suite of Gaussian 09 programs to further inspect the elec-
trochemical properties of P1eP8 and analyze the planarity of
polymer backbone. Becke's three parameterized Lee-Yang-Parr
exchange functional (B3LYP) and 6-31G* basis sets were used
[17]. Optimized structures, HOMO and LUMO wave functions of
P1eP8 monomers were calculated as shown in Fig. S12. Compared
to the LUMOs, the HOMOs are populated widely on the BDT units.
As for the LUMOs, orbitals are mainly distributed over TPD or BT
units, and show clear charge transfer from the donor to the
acceptor. The HOMO energy levels of P1eP4 were calculated to
a
Absorption maxima measured from UveVis absorption spectrum in chloroform
solution.
b
Absorption maxima measured from UveVis absorption spectrum in thin film
state.
c
Estimated from the onset of the absorption in thin films (Eogpt ¼ 1240/ lonset).

K. Kranthiraja et al. / Dyes and Pigments 123 (2015) 100e111
107
Fig. 2. Transfer characteristic curves of polymer field-effect transistors based on P1eP8.

108
K. Kranthiraja et al. / Dyes and Pigments 123 (2015) 100e111
Fig. 3. JeV characteristics of P1eP8 without and with DIO.
be ꢂ4.89, ꢂ4.86, ꢂ4.97, and ꢂ4.96 eV, respectively, while those of
P5eP8 were ꢂ4.76, ꢂ4.77, ꢂ4.80, and ꢂ4.81 eV, respectively. These
values are in agreement with experimental observations. The
planarity of the optimized structures of the repeating units of
P1eP8 was also observed through the dihedral angles between the
donor and the acceptor units. P1eP4 possess the dihedral angles
ranging from 0ꢀ to 15ꢀ, while P5eP8 ranging from 15ꢀ to 30ꢀ
conform that the polymer P1eP4 have a more planar backbone
than that of in P5eP8. As for electrostatic interactions, calculated
Mulliken charge distribution shows an almost equal degree of
charge separation between the sulfur atom of thiophene and the
oxygen atom of electron deficient units of both the series of poly-
mers. The larger dihedral angle in the latter series might originated
from steric repulsions between the lengthy hydrocarbon chain of
electron deficient unit BT and the APTh of donor.
behavior for derived polymer solar cells. Predictably we observed
considerable difference in hole mobilities of P1eP8 depending on
their side chain nature and position. P1eP4 show higher hole
mobilities than P5eP8. This may be due to the enhanced planarity
in P1eP4. Particularly, P3 showed the hole mobility in the range of
10ꢂ3 cm2/V$s. This means that the loss of photocurrent is reduced
and thus high photovoltaic performance will be expected for the
derived polymer solar cells.
3.6. Photovoltaic properties
To investigate the impact of the nature and position of alkoxy
group in the BDT of P1eP8 on their photovoltaic properties, the
bulk heterojunction polymer solar cells were fabricated with a
Table 2
3.5. Polymer field effect transistors mobility
Photovoltaic properties of optimized pristine polymer solar cells of P1eP8.
Polymer
Ratio
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
The field effect hole mobilities of the P1eP8 were measured in a
P1
P2
P3
P4
P5
P6
P7
P8
1:2
1:3
1:2.5
1:2
1:2
1:2
1:2
1:2
5.48
5.42
9.50
6.72
6.77
8.46
7.81
6.90
0.94
0.89
0.91
1.00
0.69
0.72
0.71
0.72
43.99
47.32
48.31
46.29
39.96
40.42
33.85
34.24
2.27
2.30
4.19
3.13
1.86
2.45
1.88
1.69
bottom contact geometry device on a silicon wafer with channel
lengths (12 mm) and width (120 mm) under a nitrogen (N2) atmo-
sphere. All devices were fabricated with polymer thin films as
active layer, which were annealed at 140 ꢀC. The transfer charac-
teristics of the devices were shown in Fig. 2 and corresponding data
are summarized in Table S2. P1eP8 delivered prominent field-
effect hole mobilities, indicating that they possess typical p-type

K. Kranthiraja et al. / Dyes and Pigments 123 (2015) 100e111
109
Table 3
coverage of P1eP4 are almost similar, which can be realized by the
existence of a highly planar backbone in P3. Whereas P5eP8
exhibited Jsc in the range of 6.77e8.46 mA/cm2 with a maximum of
8.46 mA/cm2 for P6, which is higher than that of P5. Although P5
has broader absorption coverage than that of P6, the polymer P6 is
showing higher Jsc may be due to the morphological mismatches.
Considering the Voc of the polymers, P1eP4 show higher Voc than
that of P5eP8 due to the deeper HOMO of the former. The Voc
values are very similar in P1eP4 and in P5eP8 with a maximum of
1 V and 0.72 V for P4 and P6/P8, respectively. It suggests that the Voc
value of pristine blend films mainly correlate with the energy gap
between the HOMO of the donor polymer and LUMO of the PC71BM
in the active layer rather than the morphology between them.
Additionally, P1eP4 also show higher fill factor (FF) values than
that of P5eP8 which might be due to the high hole mobility and
better morphological nature, hence, P1eP4 show higher power
conversion efficiency (PCE) than the P5eP8. Particularly, P3 shows
a PCE of 4.19% for pristine blend film due to the higher Jsc, Voc, and
FF values than those of its counterparts. This can be realized by very
high planar backbone of P3 that leads to nanoscale phase separa-
tion in the active layer along with the very high Voc of 0.91 V.
Among P5eP8, the polymer P6 displays a better photovoltaic
performance with a maximum PCE of 2.45%. Finally, it is believed
that m-EHAPTh (P3) unit is highly suitable for enhancing the
planarity of backbone among P1eP4. It is well known that upon
addition of a certain amount of solvent additive to the active layer
Photovoltaic properties of optimized polymer solar cells of P1eP8 with DIO.
Polymer
Ratio
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
P1c
P2a
P3b
P4b
P5c
P6c
P7c
P8c
1:2
1:3
1:2.5
1:2
1:2
1:2
1:2
1:2
8.15
7.98
8.89
5.72
9.23
12.73
12.37
11.15
0.89
0.85
0.84
1.00
0.67
0.72
0.72
0.72
49.89
51.46
53.72
39.73
51.57
57.39
51.00
46.82
3.61
3.51
4.03
2.28
3.18
5.26
4.50
3.78
a
2% DIO.
1% DIO.
3% DIO.
b
c
configuration of ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al and
measured under the illumination of AM 1.5G illumination
(100 mW/cm2). The active layer of the device was spin-coated from
the chlorobenzene solution of the corresponding polymer and
PC71BM blend. Several device processing conditions such as poly-
mer to PC71BM ratio, film thickness, and external additive con-
centration were carefully optimized to get best device performance.
Fig. 3a and b shows the current densityevoltage (JeV) curves of
optimized devices of a pristine blend of P1eP4 and P5eP8,
respectively, and the data are summarized in Table 2. Interestingly,
P3 shows higher short-circuit current density (Jsc) of 9.50 mA/cm2
than their counterparts (P1, P2 and P4) although the absorption
Fig. 4. EQE spectra of P1eP8 without and with DIO.

110
K. Kranthiraja et al. / Dyes and Pigments 123 (2015) 100e111
of bulk heterojunction polymer solar cells, the photovoltaic prop-
erties are improved due to the enhancement of phase separation in
morphology. In this regard, the optimized photovoltaic properties
of all the polymers were further analyzed in the presence of DIO.
The photovoltaic properties of P1eP4 and P5eP8 based devices
in the presence of DIO are listed in Table 3. Upon addition of DIO to
the active layers of two series of polymers, latter series (P5eP8)
showed improved device performance compared to the former
series (P1eP4) This is due to the fact that the DIO could able to
improve the active layer morphology in case of P5eP8 based de-
vices whereas, in P1eP4 based devices, it would have destroyed the
already existed good phase separation in the active layer. In addi-
tion to this, the influence of DIO is also depends on the nature and
polymers. Even though all devices could absorb the photons be-
tween 300 and 700 nm, the percentage of EQE varies significantly
between the polymers. This is due to the nature of the donor and
acceptor materials and the morphology between them in the active
layer. Upon considering the EQE of P1eP4, the polymer P3 shows
very broad EQE with a maximum of ~60% consistent with the
higher Jsc that leads to higher PCE. Upon addition of DIO to the
active layer of P1eP4, the EQE value of P1 and P2 is increased, but
for P3 and P4 is decreased as shown in Fig. 4c due to the
morphological mismatching in the active layer, which is consistent
with the photovoltaic performances. In case of P5eP8, the EQE is
increased for all the polymers as shown in Fig. 4d upon addition of
DIO which may be due to the improved phase separated morpho-
logical nature than their pristine counterparts. The Jsc values
calculated from integration of the EQE spectra were well matched
with Jsc obtained from the JeV measurements.
position of the alkoxy group in
p-conjugated side chain linker of
BDT in P5eP8. Especially the Jsc and PCE of P6 is increased from
8.46 mA/cm2 and 2.45% to 12.73 mA/cm2 and 5.26%, respectively.
However in the case of P1eP4 based devices, upon addition of DIO
to the active layers the photovoltaic performance of P1, P2 is
slightly increased and P3, P4 is decreased.
3.7. Morphological studies
Fig. 4a and b shows the external quantum efficiency (EQE)
curves of P1eP4 and P5eP8, optimized devices without DIO. All
devices show a broad response in the range of 300e700 nm, which
are consistent with absorption spectra of the corresponding
To analyze further the difference between the photovoltaic
properties of polymers, the morphology of the active layer of the
corresponding polymers was investigated using tapping-mode
atomic force microscopy (AFM). Fig. 5 shows the representative
Fig. 5. AFM images of active layers based P1eP8 without and with DIO.

K. Kranthiraja et al. / Dyes and Pigments 123 (2015) 100e111
111
AFM images of optimized blend films of P1eP8. In the case of
P1eP4, the active layer of P3 shows well organized nanoscale
morphology better than that of P1, P2, and P4 and is consistent
with the superior photovoltaic properties. This is caused by the
well-organized planar backbone of P3. Upon addition of DIO to the
active layer of P1eP4, the surface roughness is decreased for P1 and
P2 and increased for P3 and P4. This is consistent with the higher
device performance of P1 and P2 and lesser performance of P3 and
P4 in the presence of DIO than their pristine counterparts,
respectively. Very interestingly, the AFM images of pristine active
layers of P3 and P4 shows well organized nano structures, a feature
which is consistent with the superior photovoltaic properties in
neat blends. Upon addition of DIO, the root mean square (RMS)
roughness of the image is increased due to the disturbance of the
present nano structure. This indicates that the well-organized
polymer backbone of P3 and P4 are completely disturbed by DIO,
which leads to reduce the photovoltaic performance. On the con-
trary, in the case of P5eP8, the surface roughness of all the poly-
mers is decreased upon addition of DIO. Thus, P5eP8 displays
superior photovoltaic performance than their pristine counter-
parts. This indicates that the DIO modulates the morphology of the
active layer for better charge transportation that leads to improved
photovoltaic performance. Due to this reason the PCE of P6 and P7
is increased from 2.45 to 1.88% to 5.26 and 4.50%, respectively.
side chain nature and position in APTh of polymer backbone and
how it influences the bulk properties of resulted polymers.
Acknowledgments
This work was supported by grant fund from the National
Research Foundation (NRF) (2011-0028320) and the Pioneer
Research Center Program through the NRF (2013M3C1A3065522)
by the Ministry of Science, ICT & Future Planning (MSIP) of Korea.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.07.032.
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