Synthesis of N-[4-Octylphenyl]dithieno[3,2-b:2′,3′-d]pyrrole- based broad absorbing polymers and their photovoltaic applications

Article abstract of DOI:10.1016/j.polymer.2013.04.049

Electron rich, fused N-aryl pyrrole based monomer namely 2,6-di(4,4,5,5-tetramethyl-1,3,2-dioxaborolan)-N-[4-octylphenyl]dithieno[3,2-b: 2′,3′-d]pyrrole (N-aryl DTP) was synthesized and copolymerized with electron deficient 4,7-bis(5-bromo-3-octyl-2-thien

Full text of DOI:10.1016/j.polymer.2013.04.049

Polymer 54 (2013) 3198e3205
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of N-[4-Octylphenyl]dithieno[3,2-b:20,30-d]pyrrole-based
broad absorbing polymers and their photovoltaic applications
,
,
Vellaiappillai Tamilavan a 1, Myungkwan Song b 1, Sangjun Kim a, Rajalingam Agneeswari a,
Jae-Wook Kang c, Myung Ho Hyun a
,
*
a Department of Chemistry, Chemistry Institute for Functional Materials, Pusan National University, Busan 690-735, Republic of Korea
b Advanced functional Thin films Department, Korea Institute of Materials Science, Changwon, 641-831, Republic of Korea
c Professional Graduate School of Flexible and Printable Electronics, Department of Flexible and Printable Electronics, Chonbuk National University,
Jeonju 561-756, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Electron rich, fused N-aryl pyrrole based monomer namely 2,6-di(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan)-N-[4-octylphenyl]dithieno[3,2-b:20,30-d]pyrrole (N-aryl DTP) was synthesized and copo-
lymerized with electron deficient 4,7-bis(5-bromo-3-octyl-2-thienyl)-2,1,3-benzothiadiazole (TBT) to
afford alternating polymer PDTPTBT. The absorption band of PDTPTBT was found to cover the entire
visible part of the solar spectrum (300 nme750 nm) and the optical band gap was estimated to be
1.62 eV. In order to extent the absorption of PDTPTBT, another strong electron acceptor unit such as 2,1,3
ꢀbenzothiadiazole or dimethyl-2H-benzimidazole was incorporated in polymer main chain by copoly-
merizing three different comonomers (N-aryl DTP, TBT and 4,7-dibromo-2,1,3ꢀbenzothiadiazole (B) or
4,7-dibromo-2,2-dimethyl-2H-benzimidazole (BI)) at 2:1:1 ratio to afford polymers PDTPTBTB and
PDTPTBTBI, respectively. The absorption band of PDTPTBTB and PDTPTBTBI was found to be extended
up to 1000 nm and 1200 nm, respectively, and the optical band gap was calculated to be 1.33 eV and
1.07 eV, respectively. The HOMO-LUMO energy levels of PDTPTBT, PDTPTBTB and PDTPTBTBI were
found to be suitable for polymer solar cell (PSC) applications. The single layer PSCs fabricated with the
configuration of ITO/PEDOT:PSS/PDTPTBT, PDTPTBTB or PDTPTBTBI:PC60BM(1:3 wt/wt)/LiF/Al showed
maximum power conversion efficiency (PCE) of 2.04%, 1.01% and 0.70%, respectively.
Received 15 February 2013
Received in revised form
17 April 2013
Accepted 20 April 2013
Available online 27 April 2013
Keywords:
Polymer solar cells
Low band gap polymers
Dithieno pyrrole-based polymers
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
[3,4,5,6,7], replacing the existing electron accepting PC60BM or
PC71BM derivatives with new acceptor materials having higher
Polymer based bulk heterojunction solar cells (PSCs) have
attracted much attention due to their advantages in that they are
green energy production techniques and the light weight-flexible
PSC devices can be easily fabricated to large area at low cost by
standard roll-to-roll printing techniques [1,2]. The solar to electrical
energy conversion efficiency (PCE) of the PSCs has been improved
up to 7e9.2% [3,4,5,6,7] for single PSCs and 8e10.6% [8,9,10] for
tandem structured PSCs containing two photoactive layers made
up with two different donor polymers that can absorb the sunlight
at different interval of the solar spectrum. The theoretical studies
implied that the maximum PCE could reach up to 17% for single
layer PSCs and 24% for tandem PSCs [11]. Presently, several at-
tempts such as developing structurally new donor polymers having
the appropriate energy levels with effective light harvesting ability
LUMO energy level [12] and fine tuning the PSC device structure to
inverted or tandem architectures [8,9,10] are in progress with the
aim of enhancing the PSC device performances. In our attempt to
improve the PCE of PSCs, we were interested in preparing new
broad absorbing polymers because the PCE of PSCs strongly de-
pends on the light harvesting ability of the donor polymers. So far,
the broad absorption polymers have been prepared by copoly-
merizing the electron rich and electron deficient unit in alternate
fashion and thereby the band gap of the polymers decreased due to
the combined electronic transitions such as
charge transfer (ICT) between the donor and acceptor moiety
[13,14,15,16,17,18,19,20].
p p* and internal
ꢀ
Recently, we prepared thiophene-(N-aryl)pyrrole-thiophene
(N-aryl TPT) based electron rich monomers and copolymerized them
with various electron rich and electron deficient comonomers
with the aim of preparing N-aryl TPT based polymers showing
dissimilar absorption and energy levels for PSC applications
[21,22,23,24,25,26,27,28]. The photovoltaic studies on N-aryl TPT
* Corresponding author. Tel.: þ82 51 510 2245; fax: þ82 51 516 7421.
E-mail address: mhhyun@pusan.ac.kr (M.H. Hyun).
1
These authors made equal contribution to this work.
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.04.049

V. Tamilavan et al. / Polymer 54 (2013) 3198e3205
3199
based homo polymer (PTPT) [21] and copolymers containing phe-
nylene (PPTPT) [21], thiophene (PTTPT) [21], indenofluorene (PIFTPT)
[22], thiadiazoloquinoxaline (TPyTDzQ) [23], 2,1,3ꢀbenzothiadiazole
(TPyTBz) [24,25] or thiopheneꢀ2,1,3ꢀbenzothiadiazoleꢀthiophene
(PTPTTBTand PTTPTTB) [26,27] were found to show maximum PCE of
2.03% with an open-circuit voltage (Voc) of 0.57 V, a short-circuit cur-
rent density (Jsc) of 9.27 mA/cm2, and a fill factor (FF) of 38.4% for
PTTPTTBꢀbased PSC device [27]. The absorption bands of the poly-
mers PTPTTBT and PTTPTTB were found to be quite broad and flat
from 300 nm to 700 nm [26,27]. The broad and flat absorption of
polymers PTPTTBT and PTTPTTB induced us to develop fused thio-
pheneꢀ(N-aryl)pyrroleꢀthiophene (N-aryl-dithiophenepyrrole, N-
aryl DTP) based monomer unit because N-aryl DTP unit is expected to
alcohol. After drying the substrates, a 40 nm thick layer of poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)
(CLEVIOUS, P) was spin-coated (1000/5000 rpm, 30 s) onto the pre-
cleaned and UV-ozone treated ITO substrates and baked in air at
120 ꢁC for 10 min. Subsequently, the 80 nm thickness of active layer
(polymer:PC60BM blend solution) was spin coated (300/600 rpm,
40 s) onto the ITO/PEDOT:PSS substrates. The polymer:PC60BM
blend solution was prepared by mixing each of the polymers and
PC60BM solution in a weight ratio of 1:1, 1:2 or 1:3 in 1,2-
dichlorobenzene. After drying the solvent, 0.7 nm thick layer of
LiF was deposited on the substrates. Subsequently, 120 nm thick
layer of Al was deposited through a shadow mask on top of the ITO/
PEDOT:PSS/polymer:PC60BM/LiF substrates under high vacuum
(1.2 ꢂ 10ꢀ6 torr). The top metal electrode area, comprising the
active area of the solar cells, was found to be 0.36 cm2. The BHJ solar
cell performance was measured using a AM 1.5G solar simulator
(Oriel 300 W) at 100 mW/cm2 light illumination after adjusting the
light intensity using Oriel power meter (model No. 70260 which
was calibrated using laboratory standards that are traceable to the
National Institute of Standards and Technologies, USA). Current-
voltage (JeV) characteristics of the polymer BHJ solar cell devices
show better
p p conjugation and planarity compared with those of
ꢀ
N-aryl TPT units. Recently several electron donor polymers containing
fused electron rich units have been reported to show broad absorption
and high carrier mobility [29]. N-Aryl DTP based polymers are also
expected to show relatively red shifted absorption and better carrier
mobility.
In this study, we prepared a new alternating copolymer, PDTPTBT,
by copolymerizing the newly synthesized N-aryl DTP monomer with
4,7-bis(5-bromo-3-octyl-2-thienyl)-2,1,3-benzothiadiazole
(TBT)
were measured using a standard source measurement unit
monomer and we also incorporated 2,1,3ꢀbenzothiadiazole or 2,2-
dimethyl-2H-benzimidazole unit in the PDTPTBT main chain to
afford polymer PDTPTBTB or polymer PDTPTBTBI, respectively, with
the aim of extending the absorption range. Here, we wish to report
the detailed synthesis, opto-electrical and photovoltaic properties of
three new N-aryl DTPꢀbased polymers including PDTPTBT,
PDTPTBTB and PDTPTBTBI.
(Keithley 236). All fabrication steps and characterization mea-
surements were performed in an ambient environment without a
protective atmosphere. The thickness of the thin films was
measured using a KLA Tencor Alpha-step IQ surface profilometer
with an accuracy of ꢃ1 nm.
2.3. Synthesis of polymers
2. Experimental section
2.3.1. Synthesis of N-[4-octylphenyl]dithieno[3,2-b:20,30-d]pyrrole
(1)
2.1. Materials and instruments
A stirred solution of 3,30-dibromo-2,20-bithiophene (5.2 g,
16 mmol), which was prepared via the known procedure [30], in
40 mL of toluene was purged well with argon for 15 min, and then
NaOtBu (3.7 g, 38.5 mmol), Pd2dba3 (0.4 g, 0.44 mmol), and 1,10-
bis(diphenylphosphine)ferrocene (DPPF, 1.0 g, 1.8 mmol) were
added in sequence. Then, 4-octylaniline (4.0 mL, 18 mmol) was
added, and the mixture was refluxed for 12 h. The mixture was
allowed to cool down, and the solvent was removed by rotary
evaporation. The solid residue was dissolved in dichloromethane
and the insoluble precipitates were filtered off. The organic solution
was washed with water and then brine, and dried over anhydrous
Na2SO4. The solvent was removed by rotary evaporation and the
crude product was purified by column chromatography (silica gel,
hexane:CH2Cl2, 90/10) to afford compound 1 as a white solid. Yield:
The commercially available reagents were received from Aldrich
or TCI chemicals and used without further purification. The com-
mon organic solvents such as dichloromethane, tetrahydrofuran
and diethyl ether were distilled and handled in a moisture-free
atmosphere. The purification of the newly synthesized com-
pounds was performed by column chromatography on silica gel
(Merck Kieselgel 60, 70e230 mesh ASTM). The 1H and 13C NMR
spectra of the compounds were recorded using Varian Mercury
Plus spectrometer (300 MHz and 75 MHz, respectively) in CDCl3.
Gel permeation chromatography (GPC) analyses were conducted
on an Agilent 1200 Infinity Series separation module using poly-
styrene as a standard and chloroform as an eluent to determine the
molecular weight and polydispersity (PDI) of the polymers. Ther-
mogravimetric analyses (TGA) were conducted with a TA instru-
ment Q500 at a heating rate of 10 ꢁC/min under nitrogen. The
photophysical studies of the polymers were performed on JASCO V-
570 spectrophotometer at 25 ꢁC in chloroform or as thin films on
glass. Cyclic voltammetry (CV) measurements were performed
using a CH Instruments Electrochemical Analyzer with Ag/AgCl
reference electrode, platinum as a working and counter electrode in
a 0.1 M tetrabutylammonium tetrafluoroborate (Bu4NBF4) as the
supporting electrolyte at room temperature. Atomic force micro-
scopy (AFM) images of blend films were obtained on a Veeco-
Multimode AFM operating in the tapping mode.
5.4 g (92%). mp 79e80 ꢁC; 1H NMR (300 MHz, CDCl3):
(d, 2H), 7.33 (d, 2 H), 7.17 (s, 4 H), 2.68 (t, 2 H), 1.60e1.74 (m, 2 H),
1.22e1.44 (m,10 H), 0.90 (t, 3 H); 13C NMR (75 MHz, CDCl3):
(ppm)
d (ppm) 7.49
d
144.4, 141.2, 137.8, 129.9, 123.5, 122.8, 116.8, 112.5, 35.8, 32.1, 31.7,
29.7, 29.6, 29.5, 22.9, 14.4; HRMS (EIþ, m/z) [Mþ] Calcd for
C22H25NS2 367.1428, found 367.1434.
2.3.2. Synthesis of 2,6-dibromo-N-[4-octylphenyl]dithieno[3,2-
b:20,30-d]pyrrole (2)
Compound 1 (4.0 g, 10.8 mmol) was dissolved in chloroform
(30 mL) and cooled to 0 ꢁC in ice bath under argon. N-Bromo-
succinimide (NBS) (4.25 g, 23.9 mmol) was added in one portion to
the stirred solution. After 15 min, the mixture was allowed to warm
to room temperature and stirred for 5 h. The organic mixture was
washed with water and then brine and dried over anhydrous
Na2SO4. The solution was filtered and evaporated by rotary evap-
oration and the crude product was purified by column chroma-
tography (silica gel, hexane:CH2Cl2, 90/10) to afford pure
compound 2 as a yellowish solid. Yield: 5.4 g (92%). mp 121e122 ꢁC;
2.2. Device fabrication and characterization of BHJ solar cells
The polymer solar cells were constructed as follows. The
transparent ITO electrode (80 nm thick, 20
was coated on glass substrates and cleaned by ultrasonication
sequentially in detergent, deionized water, acetone, and isopropyl
U/sq sheet resistance)

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V. Tamilavan et al. / Polymer 54 (2013) 3198e3205
1H NMR (300 MHz, CDCl3):
d
(ppm) 7.49 (d, 2H), 7.32 (d, 2 H), 7.16 (s,
1,2-diamine (7.9 g, 30 mmol), which was prepared via the known
procedure [33], in 160 mL of ethyl acetate was added a solution of
copper(II) perchlorate hexahydrate (Cu(II)(ClO4)2.6H2O (22.2 g,
60 mmol) in 90 mL anhydrous ethanol at room temperature. The
mixture was stirred for 30 min and the solid material was filtered
and washed with 100 mL of ethyl acetate:ethanol (8:2) mixture.
The solid material was dried under vacuum to afford pure product
of bis(3,6-dibromobenzene-1,2-diamine)copper(II) perchlorate (3)
as a gray color solid. Yield: 10.5 g (60%). mp 290e291 ꢁC; 1H NMR
2 H), 2.67 (t, 2H), 1.60e1.74 (m, 2 H), 1.22e1.44 (m, 10 H), 0.89 (t,
3H); 13C NMR (75 MHz, CDCl3):
d
(ppm) 142.1, 141.1, 136.8, 130.0,
123.0, 116.6, 115.7, 110.5, 35.7, 32.1, 31.7, 29.7, 29.6, 29.5, 22.9, 14.4;
HRMS (EIþ, m/z) [Mþ] Calcd for C22H23Br2NS2 522.9639, found
522.9647.
2.3.3. Synthesis of 2,6-di(4,4,5,5-tetramethyl-1,3,2-dioxaborolan)-
N-[4-octylphenyl]dithieno[3,2-b:20,30-d]pyrrole (N-aryl DTP)
Under argon atmosphere, a solution of compound 2 (1.84 g,
3.50 mmol) in dry THF (40 mL) was cooled to ꢀ78 ꢁC in a dry ice-
acetone bath for 20 min. To the stirred solution, n-BuLi (3.10 mL,
7.7 mmol, 2.5 M solution in hexane) was added dropwise and
stirred for 20 min in the same bath. Then, the mixture was allowed
to warm to room temperature and stirred for 45 min. The solution
was again cooled to ꢀ78 ꢁC for 20 min and then 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.65 mL, 8.00 mmol) was
added dropwise. The solution was slowly warmed to room tem-
perature and stirred for overnight. The organic mixture was washed
with water and then brine, and dried over anhydrous Na2SO4. The
solvents were filtered and removed by rotary evaporation and the
crude product was purified by column chromatography (silica gel,
hexane:ethyl acetate, 90/10) to afford pure product, N-aryl DTP, as a
light yellow color solid. Yield: 1.01 g (47%). mp 260e261 ꢁC; 1H
(300 MHz, DMSO-d6):
d (ppm) 6.60 (s, 2 H), 4.93(brd s, 8 H).
Compound 3 (8.0 g, 13.5 mmol) and 480 mL of purified acetone
were stirred at room temperature under argon atmosphere. After
6 h, the precipitates were filtered off from the solution and washed
with 200 mL acetone. The solid materials were dried under vacuum
to afford pure product of 4,7-dibromo-2,2-dimethyl-2H-benz-
imidazole copper(I) perchlorate (4) as a dark brown solid. Yield:
3.3 g (52%). mp 239e240 ꢁC; 1H NMR (300 MHz, DMSO-d6):
d
(ppm) 7.45 (s, 2 H), 1.48 (s, 6 H). Compound 4 (2.5 g, 5.35 mmol) in
200 mL of chloroform was added 100 mL of 28e30% aqueous
ammonia solution. The resulting solution was stirred at room
temperature for 1 h. Then, the mixture was poured into water and
the organic layer was separated. The aqueous layer was extracted
one more time with chloroform and the combined organic layer
was washed with water and then brine, and dried over anhydrous
Na2SO4. The solvent was removed by rotary evaporation and the
crude product was purified by column chromatography (silica gel,
hexane:ethyl acetate, 90/10) to afford compound BI as a yellow
solid. Yield: 1.3 g (80%). The characterization data was found to be
identical with the reported data [34].
NMR (300 MHz, CDCl3):
2 H), 2.66 (t, 2 H), 1.58e1.72 (m, 2 H), 1.20e1.40 (m, 34 H), 0.90 (t,
3 H); 13C NMR (75 MHz, CDCl3):
(ppm) 147.8, 141.3, 137.4, 129.8,
d (ppm) 7.66 (s, 2 H), 7.49 (d, 2 H), 7.28 (d,
d
123.1, 121.3, 84.4, 35.7, 32.1, 31.7, 29.7, 29.6, 29.5, 25.0, 22.9, 14.4;
HRMS (EIþ, m/z) [Mþ] Calcd for C34H47B2NO4S2 619.3133, found
619.3139.
2.3.5. General procedure for polymer synthesis
2.3.4. Synthesis of 4,7-dibromo-2,2-dimethyl-2H-benzimidazole
(BI)
A solution of respective monomers (shown in Scheme 2) in
toluene (40 mL) was purged well with argon for 45 min. Then,
Pd(PPh3)4 (0.02 g, 5 mol %) and 2 M K2CO3 solution were added to
the stirred solution and the mixture was heated to reflux under
argon atmosphere. After refluxing for 48 h, 50 mg of phenylboronic
acid was added. The whole mixture was refluxed for 6 h and then
0.1 mL of bromobenzene was added and refluxed again for 6 h.
4,7-Dibromo-2,2-dimethyl-2H-benzimidazole was synthesized
using the similar reported procedures for the preparation of 2,2-
dimethyl-2H-benzimidazole [31,32]. Here, we used 3,6-
dibromobenzene-1,2-diamine as a starting material instead of
benzene-1,2-diamine. A stirred solution of 3,6-dibromobenzene-
Scheme 1. Synthetic route for the synthesis of N-aryl DTP, BI and TBT.

V. Tamilavan et al. / Polymer 54 (2013) 3198e3205
3201
Scheme 2. Synthetic route for the synthesis of PDTPTBT, PDTPTBTB and PDTPTBTBI.
Then, the reaction mixture was cooled to room temperature and
then poured into the mixed solvent of methanol and water
(100 mL:50 mL) with vigorous stirring. The precipitate was recov-
ered by filtration, and then extracted with methanol for 24 h and
acetone for 24 h in a Soxhlet apparatus. PDTPTBT: Black color, Yield
method similar to the reported procedures for the preparation of
2,2-dimethyl-2H-benzimidazole [31,32,33]. To the stirred solution of
3,6-dibromobenzene-1,2-diamine in ethyl acetate was added a so-
lution of copper(II) perchlorate hexahydrate in anhydrous ethanol to
afford compound 3. Compound 3 was stirred with acetone to afford
compound 4. The copper(I) perchlorate in compound 4 was removed
by treating with aqueous ammonia solution to afford BI in good
yield. The method used in this study for the preparation of BI is
expected to be more facile than the method reported recently [34].
The Suzuki polycondensation of the two monomers such as N-aryl
DTP and TBT afforded alternating polymer PDTPTBT. The Suzuki
random copolymerization of the monomers including N-aryl DTP,
TBT and B at 2:1:1 ratio or including DTP, TBT and BI at 2:1:1 ratio
yielded polymers PDTPTBTB and PDTPTBTBI, respectively. The
(0.30 g, 78%). 1H NMR (300 MHz, CDCl3):
d 7.58e7.72 (m, 2 H),
7.44e7.54 (m, 2 H), 7.38 (d, 2 H), 7.26e7.30 (m, 2 H), 7.16e7.26 (m,
2 H), 2.50e2.80 (m, 6 H), 1.50e1.80 (m, 6 H), 1.00e1.50 (m, 30 H),
0.70e1.00 (m, 9 H). PDTPTBTB: Black color, Yield (0.26 g, 46%). 1H
NMR (300 MHz, CDCl3):
d 7.58e7.72 (m, 3 H), 7.46e7.56 (m, 3 H),
7.26e7.42 (m, 6 H), 7.16e7.26 (m, 2 H), 2.50e2.80 (m, 7 H), 1.60e
1.80 (m, 4 H), 1.50e1.60 (m, 3 H), 1.00e1.50 (m, 35 H), 0.70e1.00 (m,
11 H). PDTPTBTBI: Black color, Yield (0.30 g, 46%). 1H NMR
(300 MHz, CDCl3):
d 7.58e7.72 (m, 3 H), 7.46e7.56 (m, 3 H), 7.26e
7.42 (m, 6 H), 7.16e7.26 (m, 2 H), 2.50e2.80 (m, 7 H), 1.60e1.80 (m,
7 H), 1.50e1.60 (m, 3 H), 1.00e1.50 (m, 35 H), 0.70e1.00 (m, 11 H).
3. Results and discussions
3.1. Synthesis and characterization of polymers
The synthetic procedures for the monomers such as N-aryl DTP
and BI and polymers PDTPTBT, PDTPTBTB and PDTPTBTBI are
outlined in Scheme 1 and Scheme 2, respectively. The electron
deficient comonomers such as B and TBT were synthesized by using
the reported procedure [26] as shown in Scheme 1. The electron rich
new N-aryl DTP monomer namely 2,6-di(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan)-N-[4-octylphenyl]dithieno[3,2-b:20,30-d]pyrrole was
synthesized from the similar procedure reported by Koeckelberghs
et al. [30]. The reaction between 3,30-dibromo-2,20-bithiophene and
4-octylaniline in the presence of palladium catalyst afforded com-
pound 1. Compound 1 was then brominated by using NBS to afford
compound 2. The final N-aryl DTP monomer was obtained by
treating compound 2 with n-BuLi followed by 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane. Electron deficient 4,7-dibromo-
2,2-dimethyl-2H-benzimidazole (BI) was synthesized via the
Fig. 1. TGA curves of PDTPTBT, PDTPTBTB and PDTPTBTBI.

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V. Tamilavan et al. / Polymer 54 (2013) 3198e3205
and PDTPTBTBI showed good solubility in chlorobenzene and
dichlorobenzene, but relatively poor solubility in chloroform and
tetrahydrofuran at room temperature. The GPC analysis revealed
that the weight average molecular weights (Mw) of polymers
PDTPTBT, PDTPTBTB and PDTPTBTBI were 2.21 ꢂ 104, 9.06 ꢂ 103
and 1.40 ꢂ 104 and their polydispersities were 1.95, 1.45, and 1.82,
respectively. The thermal stability of the polymers PDTPTBT,
PDTPTBTB and PDTPTBTBI was determined by using thermogravi-
metric analysis (TGA), and the 5% decomposition temperature was
found to be 407 ꢁC, 377 ꢁC and 376 ꢁC, respectively. Fig.1 displays the
TGA curve of polymers PDTPTBT, PDTPTBTB and PDTPTBTBI.
3.2. Optical properties
The UV-visible-NIR absorption spectra of PDTPTBT, PDTPTBTB
and PDTPTBTBI in a chloroform solution and as thin film on glass
are shown in Fig. 2. All absorption spectra of PDTPTBT, PDTPTBTB
and PDTPTBTBI showed their first absorption maxima almost at
the same wavelength, but the second absorption maxima were
found to be significantly red shifted in the order of
PDTPTBT < PDTPTBTB < PDTPTBTBI. The absorption maxima
(lmax) of PDTPTBT were observed at 423 nm and 533 nm in chlo-
roform and at 441 nm and 597 nm as thin film while those of
random polymer PDTPTBTB were observed at 417 nm and 624 nm
in chloroform and at 430 nm and 653 nm as thin film and those of
PDTPTBTBI were observed at 423 nm and 703 nm in chloroform
and 431 nm and 716 nm as thin film. The film state absorption
bands of PDTPTBT, PDTPTBTB and PDTPTBTBI were found to be
slightly broader than the solution state bands and their corre-
sponding absorption maxima were red shifted compared to those
observed in solution state. The later phenomenon indicates that the
polymers are well oriented (better
p p interchain interaction) in
ꢀ
film state than those in solution state. In our previous study, we
demonstrated that polymer PTPTTBT incorporating both of N-aryl
TPT and TBT units shows a broad and flat absorption maximum
resulting from the overlap of the two
originated from the N-aryl TPT and TBT units [26,27]. Similarly, the
absorption spectrum of PDTPTBT is expected to be resulted from
p p* electronic transitions
ꢀ
Fig. 2. UVdvisibleeNIR absorption spectra of PDTPTBT, PDTPTBTB and PDTPTBTBI in
chloroform and thin film on glass.
the overlap of the two
the N-aryl DTP and TBT units. On the other hand, the absorption
bands of PDTPTBTB and PDTPTBTBI are expected to be attributed
p p* electronic transitions originated from
ꢀ
careful 1H NMR analysis of the polymers PDTPTBTB and PDTPTBTBI
revealed the presence of all three monomers such as N-aryl DTP, TBT
and B or BI in their main chain at different ratios. The two different
repeating unit [TBT based repeating unit (m) and 2,1,3-
to the combination of the
p p* electronic transitions and the
ꢀ
donoreacceptor internal charge transfer (ICT) between N-aryl DTP
and 2,1,3ꢀbenzothiadiazole or dimethyl-2H-benzimidazole. The
electron attracting ability of the electron deficient units used in this
study is expected in the following order of TBT < B < BI, and,
consequently, the absorption bands of the polymers were found to
be red shifted in the order of PDTPTBT < PDTPTBTB < PDTPTBTBI.
The optical band gaps of PDTPTBT, PDTPTBTB and PDTPTBTBI
were calculated from the onset wavelength of the optical absorp-
tion in thin film to be 1.62 eV, 1.33 eV, and 1.07 eV, respectively. As
benzothiadiazole
or
2,2-dimethyl-2H-benzimidazole
based
repeating unit (n)] ratios of both polymers, PDTPTBTB and
PDTPTBTBI, were determined to be 1:0.5. The solubility of the
polymers was tested in the common organic solvents such as chlo-
roform, tetrahydrofuran, chlorobenzene and dichlorobenzene. The
solubility of PDTPTBT was found to be excellent in all common
organic solvents mentioned above, whereas polymers PDTPTBTB
Table 1
Polymerization results, thermal, optical and electrochemical properties of polymers PDTPTBT, PDTPTBTB and PDTPTBTBI.
PDIa
TGAb (ꢁC)
lmax in solution (nm)c
lmax as film (nm)d
Eg, opt (eV)e
HOMO (eV)f
LUMO (eV)f
Eg, elc (eV)g
a
Mw
PDTPTBT
PDTPTBTB
PDTPTBTBI
2.21 ꢂ 104
9.06 ꢂ 103
1.40 ꢂ 104
1.95
1.45
1.82
407
377
376
423, 533
417, 624
423, 703
441, 597
430, 653
431, 716
1.62
1.33
1.07
ꢀ5.27
ꢀ5.01
ꢀ4.94
ꢀ3.57
ꢀ3.66
ꢀ3.76
1.70
1.35
1.18
a
Weight average molecular weight (Mw) and polydispersity (PDI) of the polymers were determined by GPC using polystyrene standards.
5% weight loss temperature measured by TGA under N2.
Measurements were performed in chloroform solution.
b
c
d
e
f
Measurements in thin film were performed on glass.
Band gap estimated from the onset wavelength of the optical absorption in thin film.
The HOMO and LUMO levels of the polymers were estimated from cyclic voltammetry analysis.
Band gap estimated from the HOMO and LUMO levels estimated from the cyclic voltammetry analysis.
g

V. Tamilavan et al. / Polymer 54 (2013) 3198e3205
3203
our expectation, polymer PDTPTBT showed significantly broader
absorption band than the reported polymer PTPTTBT and the op-
tical band gap of PDTPTBT was found to be 0.32 eV lower than that
of PTPTTBT [26]. The optical properties of PDTPTBT, PDTPTBTB and
PDTPTBTBI are summarized in Table 1.
Fig. 3. Cyclic voltammograms of PDTPTBT, PDTPTBTB and PDTPTBTBI as films on
platinum working electrode in 0.1 M Bu4NBF4/acetonitrile at 100 mV/s, potential vs.
Ag/AgCl.
Fig. 4. J-V characteristics of PSCs prepared from ITO/PEDOT:PSS/PDTPTBT or
PDTPTBTB or PDTPTBTBI:PC60BM/LiF/Al under AM 1.5 irradiation (100 mW/cm2).

3204
V. Tamilavan et al. / Polymer 54 (2013) 3198e3205
Table 2
three polymers were located above the conduction band of the
PC60BM, which ensure the possibility of the efficient electron
transfer from the polymer to PC60BM. The HOMO and LUMO energy
levels of PDTPTBT, PDTPTBTB and PDTPTBTBI were included in
Table 1.
Solar cell performance of PDTPTBT, PDTPTBTB and PDTPTBTBI as electron donors
with PC60BM as an electron Acceptor in ITO/PEDOT:PSS/Polymer:PC60BM/LiF/Al
device.
Photoactive active layer
Jsc
Voc
FF (%)c
PCE
(%)d
(mA/cm2)a
(V)b
PDTPTBT : PC60BM (1:1 wt/wt)
PDTPTBT : PC60BM (1:2 wt/wt)
PDTPTBT : PC60BM (1:3 wt/wt)
PDTPTBTB : PC60BM (1:1 wt/wt)
PDTPTBTB : PC60BM (1:2 wt/wt)
PDTPTBTB : PC60BM (1:3 wt/wt)
PDTPTBTBI : PC60BM (1:1 wt/wt)
PDTPTBTBI : PC60BM (1:2 wt/wt)
PDTPTBTBI : PC60BM (1:3 wt/wt)
5.47
7.16
8.91
5.06
6.00
6.48
3.74
4.97
5.62
0.52
0.53
0.55
0.44
0.47
0.48
0.19
0.44
0.44
36.67
36.57
42.09
26.30
27.46
32.73
26.80
26.80
28.20
1.04
1.38
2.04
0.58
0.77
1.01
0.19
0.59
0.70
3.4. BHJ solar cell properties
Each of polymers PDTPTBT, PDTPTBTB and PDTPTBTBI were
used as an electron donor material in PSC device fabrication. The
PSC devices were fabricated with the structure of ITO/PEDOT:PSS/
PDTPTBT or PDTPTBTB or PDTPTBTBI:PC60BM (1:1, 1:2 or 1:3 wt/
wt)/LiF/Al. The active layer of the PSCs was sandwitched between
the hole injecting (PEDOT:PSS) and electron transporting (LiF)
layers and covered with ITO anode and Al cathode. The current
density-voltage (JeV) characteristics of the PSC devices were
measured under the illumination of AM 1.5 G (100 mW/cm2) solar
simulator and their corresponding JeV curves are shown in Fig. 4
and their corresponding characteristic parameters such as Voc, Jsc,
FF and PCE are summarized in Table 2. In every case, the PCE of the
device fabricated from polymer:PC60BM (1:3 wt/wt) as an active
layer was higher than that of the device fabricated from poly-
mer:PC60BM (1:1 wt/wt) or polymer:PC60BM (1:2 wt/wt) active
layer under the identical condition. From the preliminary OPV
studies, the maximum PCE of 2.04% was obtained with the device
fabricated from PDTPTBT:PC60BM (1:3 wt/wt) with a Voc of 0.55 V, a
Jsc of 8.91 mA/cm2, and a FF of 42.09%. On the other hand, the PSC
devices fabricated from PDTPTBTB:PC60BM (1:3 wt/wt) and
PDTPTBTBI:PC60BM (1:3 wt/wt) showed relatively decreased PCE
of 1.01% (Voc of 0.48 V, Jsc of 6.48 mA/cm2 and FF of 32.73%) and
0.70% (Voc of 0.44 V, Jsc of 5.62 mA/cm2 and FF of 28.20%), respec-
tively, compared to that of the device fabricated from
PDTPTBT:PC60BM (1:3 wt/wt). The higher content of PC60BM is
expected to be more favorable for the efficient light harvesting and
electronꢀhole separation and, consequently, the improved photo-
voltaic parameters such as Jsc, Voc and FF increase the overall
photovoltaic performances. However, when the content of PC60BM
was increased further to 1:4 wt/wt and 1:5 wt/wt in poly-
mer:PC60BM blends, the photovoltaic performances (PCE) for all
three polymers were found to decrease with relatively lower Jsc and
FF values compared to those of the device prepared from 1:3 wt/wt
polymer:PC60BM blends. The HOMO energy level of the polymers
PDTPTBT, PDTPTBTB and PDTPTBTBI was found to be deeper in
the order of PDTPTBT > PDTPTBTB > PDTPTBTBI, which is
correlated well with the obtained Voc of the PSC obtained in this
study. In contrast, the Jsc of the PSC devices prepared from the
synthesized polymers were expected to be in the order of
PDTPTBT < PDTPTBTB < PDTPTBTBI, but the experimental results
a
Short-circuit current density.
Open-circuit voltage.
Fill factor.
Power conversion efficiency.
b
c
d
3.3. Electrochemical properties
Determination of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) energy
levels of the synthesized polymers is essential to evaluate their
suitability for BHJ solar cell applications. For the efficient charge
separation at the donor (polymer)ꢀacceptor (PC60BM) interface,
the LUMO level of the polymer should be above the LUMO level of
PC60BM by at least 0.2e0.3 eV [35]. In addition, to achieve higher
voltage (Voc) in BHJ solar cells, the deeper HOMO energy level of the
electron donor material is preferable since the theoretical Voc is
defined as the energy difference between the HOMO level of the
polymer and LUMO level of the acceptor [35]. The HOMO and
LUMO energy levels of the polymers were estimated from the cyclic
voltammetry (CV) analysis. Fig. 3 represents the cyclic voltamo-
grams of PDTPTBT, PDTPTBTB and PDTPTBTBI. All three polymers
were showed reversible electrochemical behaviors. The HOMO and
LUMO energy levels of the polymers were calculated from their
onset oxidation (Eox, onset) and reduction (Ered, onset) potential values
by using the following equations, EHOMO ¼ ꢀ(Eox, onset þ 4.4) eV and
ELUMO ¼ ꢀ(Ered, onset þ 4.4) eV [23,36]. The Eox, onset and Ered, onset
potentials of PDTPTBT, PDTPTBTB and PDTPTBTBI were deter-
mined by the carful CV analysis to be 0.87 V, 0.61 V, 0.54 V
and ꢀ0.83 V, ꢀ0.74 V, ꢀ0.64 V, respectively. The HOMO and LUMO
energy levels of the polymers were calculated to
be ꢀ5.27 eV, ꢀ5.01 eV, ꢀ4.94 eV and ꢀ3.57 eV, e3.66 eV, e3.76 eV,
respectively. The electrochemical band gaps calculated from the
HOMO-LUMO levels of polymers PDTPTBT, PDTPTBTB and
PDTPTBTBI were 1.70 eV, 1.35 eV and 1.18 eV, respectively. The
resulting electrochemical band gap values of PDTPTBT, PDTPTBTB
and PDTPTBTBI are quite similar to the optical band values calcu-
lated from the onset of absorption. The conduction bands of the
showed
that
the
order
is
reversed
as
like
PDTPTBT > PDTPTBTB > PDTPTBTBI. Though, the absorption band
Fig. 5. AFM image obtained by tapping-mode on the surface for ITO/PEDOT:PSS/PDTPTBT or PDTPTBTB or PDTPTBTBI:PC60BM (1:3 wt/wt) subtracts.

V. Tamilavan et al. / Polymer 54 (2013) 3198e3205
3205
of PDTPTBTB and PDTPTBTBI was found to be broader than that of
Acknowledgments
PDTPTBT, relatively low molecular weight and lower band gap of
the polymers are expected to decrease the carrier transport and
charge dissociation at DꢀA interfaces [37,38]. Consequently, the
photovoltaic parameters such as Jsc and FF were decreased for the
devices prepared from the polymers PDTPTBTB and PDTPTBTBI.
However, the overall PCE of PDTPTBTB and PDTPTBTBI was found
to be quite better than previously reported near infrared (NIR)
absorption polymers [23,36]. In addition, recently, NIR absorption
polymers showing low band gap (1.30 eV) and energy levels similar
to those of polymers PDTPTBTB and PDTPTBTBI were reported to
give quite high PCE (around 9.5%) when they were used in triple
layer solar cells along with relatively large band gap polymers even
though they gave low PCE in single layer solar cells [39,40]. In this
instance, polymers PDTPTBTB and PDTPTBTBI would be quite
attractive for tandem solar cell applications.
This research was supported by the New & Renewable Energy
program of the Korea Institute of Energy Technology Evaluation
and Planning (KETEP) grant (No. 20103020010050) funded by the
Ministry of Knowledge Economy, Republic of Korea.
References
[1] Park HJ, Kang M-G, Ahn SH, Guo LJ. Adv Mater 2010;22:E247e53.
[2] Søndergaard R, Hösel M, Angmo D, Larsen-Olsen TT, Krebs FC. Mater Today
2012;15:36e49.
[3] Chen H-Y, Hou J, Zhang S, Liang Y, Yang G, Yang Y, et al. Nat Photonics 2009;3:
649e53.
[4] Liang Y, Xu Z, Xia J, Tsai S-T, Wu Y, Li G, et al. Adv Mater 2010;22:E135e8.
[5] Chu T-Y, Lu J, Beaupre S, Zhang Y, Pouliot J-R, Wakim S, et al. J Am Chem Soc
2011;133:4250e3.
[6] Son HJ, Wang W, Xu T, Liang Y, Wu Y, Li G, et al. J Am Chem Soc 2011;133:
1885e94.
The study on the surface of the active layer of PSCs is quite
much important due to their strong influences on the photovoltaic
performances [41]. The surface morphologies such as the donor-
acceptor blending nature and smoothness of the active layers
are usually studied by AFM analysis. In this instance, the surfaces
of ITO/PEDOT:PSS/PDTPTBT, PDTPTBTB or PDTPTBTBI:PC60BM
(1:3 wt/wt) substrates were subjected to AFM analysis and the
AFM images of the active layers are shown in Fig. 5. The root-
mean-square (rms) roughness of the active layers was deter-
mined to be 0.56, 0.95 and 1.40 nm, respectively and the blending
between the polymer and PC60BM was found to better in the order
of PDTPTBT > PDTPTBTB > PDTPTBTBI. From the AFM analysis,
the overall energy conversion efficiency of the synthesized poly-
mers PDTPTBT, PDTPTBTB and PDTPTBTBI is expected in the
order of PDTPTBT > PDTPTBTB > PDTPTBTBI, and the PCE ob-
tained in this study was found to be correlated well with the AFM
results.
[7] Price SC, Stuart AC, Yang L, Zhou H, You W. J Am Chem Soc 2011;133:
4625e31.
[8] Small CE, Chen S, Subbiah J, Amb CM, Tsang S-W, Lai T-H, et al. Nat Photonics
2012;6:115e20.
[9] Dou L, You J, Yang J, Chen C-C, He Y, Murase S, et al. Nat Photonics 2012;6:
180e5.
[10] You J, Dou L, Yoshimura K, Kato T, Ohya K, Moriarty T, et al. Nat Commun
2013. http://dx.doi.org/10.1038/ncomms2411.
[11] Lunt RR, Osedach TP, Brown PR, Rowehl JA, Bulovic V. Adv Mater 2011;23:
5712e27.
[12] Meng X, Zhang W, Tan Z, Li Y, Ma Y, Wang T, et al. Adv Funct Mater 2012;22:
2187e93.
[13] Li W, Qin R, Zhou Y, Andersson M, Li F, Zhang C, et al. Polymer 2010;51:
3031e8.
[14] Wang L, Fu Y, Zhu L, Cui G, Liang F, Guo L, et al. Polymer 2011;52:
1748e54.
[15] Dong Y, Cai W, Hu X, Zhong C, Huang F, Cao Y. Polymer 2012;53:1465e72.
[16] Wang M, Li C, Lv A, Wang Z, Bo Z, Zhang F. Polymer 2012;53:324e32.
[17] Sun Y, Lin B, Yang H, Gong X. Polymer 2012;53:1535e42.
[18] Zeigler DF, Chen K-S, Yip H-L, Zhang Y, Jen AK-Y. J Polym Sci Part A: Polym
Chem 2012;50:1362e73.
[19] Seo Y-H, Lee W-H, Park J-H, Bae C, Hong Y, Park J-W, et al. J Polym Sci Part A
Polym Chem 2012;50:649e58.
[20] Jiang J-M, Yang P-A, Lan S-C, Yu C-M, Wei K-H. Polymer 2013;54:155e61.
[21] Tamilavan V, Song M, Jin S-H, Hyun MH. J Polym Sci Part A Polym Chem
2010;48:5514e21.
4. Conclusions
In this study, we prepared three new polymers such as
PDTPTBT, PDTPTBTB and PDTPTBTBI and applied them to the
fabrication of bulk heterojunction PSCs. The alternating polymer
PDTPTBT was prepared by copolymerizing N-aryl DTP and TBT
units. The optical and electrochemical studies of PDTPTBT indi-
cated that the absorption band is extended from 300 nm to 750 nm
and their HOMO and LUMO energy levels are located at ꢀ5.21 eV
and ꢀ3.57 eV, respectively. As an effort to extend the absorption of
PDTPTBT, we incorporated 2,1,3ꢀbenzothiadiazole or 2,2-
dimethyl-2H-benzimidazole in polymer PDTPTBT main chain to
afford polymers PDTPTBTB and PDTPTBTBI, respectively. The ab-
sorption band of PDTPTBTB and PDTPTBTBI was found to be
extended up to 1000 nm and 1200 nm, respectively. The solution
processed PSC devices prepared from each of the three polymers
with the configuration of ITO/PEDOT:PSS/polymer:PC60BM (1:1 wt/
wt and 1:2 wt/wt and 1:3 wt/wt)/LiF/Al showed maximum PCE of
2.04%, 1.01% and 0.70%, respectively, for 1:3 wt/wt donor-acceptor
blend ratio. Even though the absorption ability of PDTPTBTB and
PDTPTBTBI is better than that of PDTPTBT, their too low band gaps
and relatively low molecular weights are expected to be the reason
of their poor photovoltaic performances. We believe that the low
band gap polymers PDTPTBTB and PDTPTBTBI can be promising
candidates for tandem structured PSCs and polymer PDTPTBT is
expected to show increased performance under the condition of
optimized photovoltaic device structure.
[22] Tamilavan V, Sakthivel P, Li Y, Song M, Kim C-H, Jin S-H, et al. J Polym Sci Part
A: Polym Chem 2010;48:3169e77.
[23] Tamilavan V, Song M, Jin S-H, Hyun MH. Synth Met 2011;161:1199e206.
[24] Tamilavan V, Song M, Jin S-H, Hyun MH. Curr Appl Phys 2012;12:S124e30.
[25] Tamilavan V, Song M, Ban T-W, Jin S-H, Hyun MH. Polym Bull 2012;69:
439e54.
[26] Tamilavan V, Song M, Jin S-H, Hyun MH. Polymer 2011;52:2384e90.
[27] Tamilavan V, Song M, Jin S-H, Hyun MH. Macromol Res 2013;21:406e13.
[28] Tamilavan V, Song M, Jin S-H, Hyun MH. Bull Korean Chem Soc 2012;33:
3845e8.
[29] Duan C, Huang F, Cao Y. J Mater Chem 2012;22:10416e34.
[30] Vanormelingen W, den Bergh KV, Verbiest T, Koeckelberghs G. Macromole-
cules 2008;41:5582e9.
[31] Hati S, Patra GK, Naskar JP, Drew MGB, Datta D. New J Chem 2001;25:218e20.
[32] Duff EJ. J Chem Soc (A) 1968:434e7.
[33] Naef R, Balli H. Helv Chim Acta 1978;61:2958e73.
[34] Song S, Jin Y, Park SH, Cho S, Kim I, Lee K, et al. J Mater Chem 2010;20:
6517e23.
[35] Zhu Z, Waller D, Gaudiana R, Morana M, Muhlbacher D, Scharber M, et al.
Macromolecules 2007;40:1981e6.
[36] Tamilavan V, Song M, Jin S-H, Park HJ, Yoon UC, Hyun MH. Synth Met
2012;162:1184e9.
[37] Schilinsky P, Asawapirom U, Scherf U, Biele M, Brabec CJ. Chem Mater
2005;17:2175e80.
[38] Moet DJD, Lenes M, Kotlarskia JD, Veenstra SC, Sweelssen J, Koetse MM, et al.
Org Elect 2009;10:1275e81.
[39] Bijleveld JC, Zoombelt AP, Mathijssen SGJ, Wienk MM, Turbiez M, de
Leeuw DM, et al. J Am Chem Soc 2009;131:16616e7.
[40] Li W, Furlan A, Hendriks KH, Wienk MM, Janssen RAJ. J Am Chem Soc http://
dx.doi.org/10.1021/ja401434x.
[41] Kastner C, Susarova DK, Jadhav R, Ulbricht C, Egbe DAM, Rathgeber S, et al.
J Mater Chem 2012;22:15987e97.