New alternating copolymers of fluorene and triphenylamine bearing terthiophene and acceptor groups in the side chains: Synthesis and photovoltaic properties

Article abstract of DOI:10.1016/j.polymer.2011.09.022

A series of conjugated copolymers (P1 and P2-CN-P4-CN) were prepared, in which two kinds of side chains were designed: one was the repeating units similar to that of P3HT, with the aim to increase the compatible with PC ₆₁BM and the hole mobili

Full text of DOI:10.1016/j.polymer.2011.09.022

Polymer 52 (2011) 5302e5311
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Polymer
journal homepage: www.elsevier.com/locate/polymer
New alternating copolymers of fluorene and triphenylamine bearing terthiophene
and acceptor groups in the side chains: Synthesis and photovoltaic properties
,
1
,
Shuang Lia , Zhicai Heb 1, Aoshu Zhonga, Jian Yua, Hongbin Wub, Yue Zhoua, Su’an Chena, Cheng Zhonga,
Jingui Qina, Zhen Lia
,
*
a Department of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials, Wuhan University, Wuhan 430072, China
b College of Materials Science, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of conjugated copolymers (P1 and P2-CNeP4-CN) were prepared, in which two kinds of side
chains were designed: one was the repeating units similar to that of P3HT, with the aim to increase the
compatible with PC61BM and the hole mobility; another one was the acceptor groups connected with the
electron-rich backbone through the conjugated 3,4-ethylenedioxythiophene bridge, in order to broaden
the absorption and lower the LUMO level. By controlling the ratios of these two kinds of side chains, the
absorption band of the resultant conjugated polymers could be fine-tuned, while their energy levels
nearly remained unchanged. As the result, the performance of the corresponding devices increased first,
then decreased, indicating that there would be a balance between the different function of these two side
chains.
Received 22 June 2011
Received in revised form
13 September 2011
Accepted 14 September 2011
Available online 19 September 2011
Keywords:
Conjugated copolymer
Side chains
Ó 2011 Elsevier Ltd. All rights reserved.
Solar cell
1. Introduction
charge mobility, the main reason should be ascribed to the good
compatible of P3HT with PC61BM, which contributed much to the
To resolve the problem of the exhausting of fossil energy sour-
ces, such as petroleum and coal, different strategies are attempted
to develop new renewable energy sources, or convert one kind of
energy to another convenient style, among which, the solar energy
is, perhaps, the most promising one [1,2]. Polymer solar cells (PSCs),
commonly composed of a blend film of conjugated polymer donor
and fullerene derivative (such as [6,6]-phenyl C61-butyric acid
methyl ester (PC61BM)) acceptor sandwiched between an ITO
positive electrode and a low workfunction metal negative electrode
[3e5], have attracted increasing attention as the good candidate to
transform the sunlight to electric energy, due to their unique
advantages including light weight, mechanical flexibility, low-cost
fabrication of large-area devices [6e12], and easy tunability of
chemical structure of the conjugated polymers and the fullerene
derivatives [13e22]. Thanks to the enthusiastic efforts of polymer
chemists, lots of conjugated polymers in different kinds
were designed and prepared. As a typical example, poly(3-
hexylthiophene) (P3HT) was extensively investigated and demon-
strated a PCE value as high as 4e5% [23]. In addition to the high
high quality of their blend film in the PSC device, directly leading to
the high performance.
Despite the above-mentioned lots of advantages, generally, to
realize the practical application of PSCs, the lower power
conversion efficiency (PCE) rather than commercial inorganic solar
cells, still needs to be improved. To achieve the high PCEs, for
chemists, some strategies have been utilized: first, the control of
the band gap of the conjugated polymers lower than 2.0 eV to
match the terrestrial solar radiation as well as possible; second, to
lower the high highest occupied molecular orbital (HOMO) level
of the polymers by structural modification, to boost the magni-
tude of the open-circuit voltage (Voc). However, it is still a big
challenge to obtain low-band gap conjugated polymers exhibiting
both strong and broad absorption with large short-circuit current
density (Jsc) and satisfied energy levels for the high Voc. Some
recent reports proposed one good approach to broaden the
absorption and tune the energy level of the conjugated polymers,
that is, to design and synthesize two-dimensional conjugated
copolymers [24e26], in which the construction blocks in the main
chain act as the donor moieties to mainly determine the HOMO
level, while some acceptors could be introduced at the ends of the
side chain to adjust the LUMO level and broaden the absorption
[27]. Thus, by choosing different main chain and acceptors, the
* Corresponding author. Tel./fax: þ86 27 68756757.
E-mail addresses: lichemlab@163.com, lizhen@whu.edu.cn (Z. Li).
These authors contribute equally to this paper.
1
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.09.022

S. Li et al. / Polymer 52 (2011) 5302e5311
5303
band gap and HOMO level could be conveniently modified at
a large degree, providing more freedom for the polymer chemists
to do the molecular design and achieve possibly high PSC
performance.
30 cm3/min. Electrochemical cyclic voltammetry was conducted on
a Zahner IM6e Electrochemical Workstation with a Pt disk, a Pt
plate, and a Ag/Agþ electrode as the working electrode, counter
electrode, and reference electrode, respectively, in a 0.1 mol/L tet-
rabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile
solution. Gel permeation chromatography (GPC) was used to
determine the molecular weights of polymers. GPC analysis was
performed on an Agilent 1100 series HPLC system and a G1362A
refractive index detector. Polystyrene standards were used as cali-
bration standards for GPC. THF was used as an eluent and the flow
rate was 1.0 mL/min. Elemental analyses were performed by
Considering all the above points, mainly the construction block
of P3HT, the HOMO and LUMO levels according to the electronic
properties of main chain and the acceptor moieties in the side
chains, we designed a new series of conjugated copolymers
(Chart 1). In these polymers, two kinds of side chains were designed:
one was the repeating units similar to that of P3HT, with the aim to
increase the compatible with PC61BM and the hole mobility; another
one was the acceptor groups connected with the electron-rich
backbone through the conjugated 3,4-ethylenedioxythiophene
bridge, in order to broaden the absorption and lower the LUMO
level. By controlling the ratios of these two kinds of side chains, the
absorption band of the resultant conjugated polymers could be fine-
tuned, while their energy levels nearly remained unchanged. For
comparison, the polymer, P1, only bearing the terthiophene groups
as the side chains was prepared. The obtained results realized our
thought, giving some interesting rules for the further rational design
of this kind of conjugated polymers. Herein, we would like to
present the syntheses, UVevis absorption, energy levels, and
photovoltaic performances of these four copolymers in detail.
a
CARLOERBA-1106 micro-elemental analyzer. Matrix-assisted
laser desorption ionization time-of-flight mass spectra were
measured on a Voyager-DE-STR MALDI-TOF mass spectrometer
(MALDI-TOF MS; ABI, American) equipped with a 337 nm nitrogen
laser and a 1.2 m linear flight path in positive ion mode.
2.2. Solar cell device fabrication and characterization
ITO-coated glass substrates were cleaned by sonication in
detergent, deionized water, acetone, and isopropyl alcohol, and
dried in a nitrogen stream, followed by an oxygen plasma treat-
ment. To fabricate photovoltaic devices, a hole transport thin layer
(ca. 40 nm) of PEDOT:PSS (BaytronPVPAI 4083, filtered at 0.45 mm)
2. Experimental
was spin-coated on the pre-cleaned ITO-coated glass substrates at
3000 rpm and baked at 140 ꢁC for 10 min under ambient condi-
tions. The substrates were then transferred into an argon-filled
glovebox. Subsequently, the polymer: PC61BM active layer was
spin-coated on the PEDOT:PSS layer at 1000 rpm from a homoge-
neous blend solution. The solution was prepared by dissolving the
polymers (0.01 g/mL) and PC61BM (0.02 g/mL) in chlorobenzene
2.1. Materials and instruments
Tetrahydrofuran (THF) was dried over and distilled from KeNa
alloy under an atmosphere of dry nitrogen. N,N-Dimethylforma-
mide (DMF) was dried over and distilled from CaH2. All other
reagents were used as purchased. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolane-2-yl)-9,9-dihexyl- fluorine [29], 1 [30], 2 [31], and 4
[32], were synthesized according to the literature procedures. 1H
and 13C NMR spectra were measured on Varian Mercury300 spec-
solvent and filtered with a 0.2 mm PTFE filter. The substrates were
annealed at 150 ꢁC for 10 min prior to electrode deposition. To
complete device fabrication, the substrates were pumped down to
a high vacuum (1 ꢂ10ꢀ6 Torr), calcium (5 nm) and then aluminum
(100 nm) were thermally evaporated onto the active layer through
shadow masks. The effective devices area was measured to be
0.15 cm2. The current densityevoltage (JeV) characteristics were
recorded with a Keithley 236 source meter. The spectral response
was measured with a commercial photomodulation spectroscopic
setup (Oriel). A calibrated Si photodiode was used to determine the
photosensitivity.
trometer using tetramethylsilane (TMS;
d
¼ 0 ppm) as internal
standard. The Fourier transform infrared (FTIR) spectra were
recorded on a PerkinElmer-2 spectrometer in the region of
3000e400 cmꢀ1 on NaCl pellets. UVevisible spectra were obtained
using a Shimadzu UV-2550 spectrometer. The thermal properties of
the polymers were measured on a Netzsch STA449C thermal
analyzer at a heating rate of 10 ꢁC/min in nitrogen at a flow rate of
C6H13
C6H13
C6H13
C6H13
N
N
x
1-x
n
O
O
"Donor moieties"
S
S
CN
CN
"Acceptor"
S
S
"Similar units of P3HT"
C6H13-n
Chart 1.

5304
S. Li et al. / Polymer 52 (2011) 5302e5311
2.3. Monomer synthesis
was cooled to ambient temperature, it was dropped into methanol
to remove monomers. The obtained solid was dissolved in THF, and
the insoluble solid was filtered out. The filtrate was concentrated,
precipitated into methanol, and the obtained solid was then
washed with a lot of acetone. The solid was dried under vacuum for
1 day to give the product. The yield, 1H NMR, and molecular weight
of the four polymers are as follows.
2.3.1. General procedure for synthesis of compounds 3 and 5
Under an atmosphere of dry nitrogen, Compound 2 or 4 (1.0
equiv) and Compound 1 (1.5 equiv) were dissolved in anhydrous
THF in an ice bath, and the sodium hydride (5.0 equiv) was added in
portion. The reaction mixture was stirred at that temperature for
half an hour, then slowly warmed to room temperature, stirred
overnight. The reaction was quenched with water and the water
phase was extracted twice with dichloromethane. The combined
organic extracts were washed with water, dried over magnesium
sulfate, evaporated, and purified with column chromatography.
P1: Yield: 50%. Mn ¼ 11000; PDI ¼ 1.48.1H NMR (300 MHz, CDCl3,
d): 7.70 (br, 2H), 7.53 (br, 8H), 7.32 (br, 3H), 7.19e6.77 (br, 12H), 6.62
(br, 1H), 2.72 (br, 2H), 1.98 (br, 2H), 1.61 (br,2H), 1.20e1.01 (br, 22H),
0.70 (br, 9H). 13C NMR (75 MHz, CDCl3,
d): 151.93; 147.31; 146.72;
145.97; 140.07; 135.83; 134.67; 132.58; 128.19; 127.58; 125.93;
125.09; 124.94; 124.41; 124.12; 123.85; 123.66; 121.22; 120.67;
120.23;55.49; 40.73; 31.78; 30.42;29.94;28.99; 24.05; 22.81;14.23;
1.25. UVevis (CH3Cl, 1 ꢂ10ꢀ5 mol/L): lmax ¼ 388, 444 nm.
2.3.1.1. 5-Hexyl-5-[2-[4-[N,N-di(4-bromophenyl)amino]phenyl]ethenyl]-
2,2; 5,2-terthiophene(3). Compound 2 (0.14 g, 0.37 mmol), 1 (0.31 g,
0.55 mmol), sodium hydride (0.045 g, 1.85 mmol), THF (20 mL). Using
petroleum ether/chloroform (1:4, v/v) as eluent to obtain 3 as orange
P2: Yield: 61%. Mn ¼ 6800; PDI ¼ 1.76. 1H NMR (300 MHz, CDCl3,
d
): 9.89 (s, eCHO),7.79 (br, AreH), 7.67e7.49 (br, AreH), 7.41 (br,
powder (0.19 g, 60%). 1H NMR (300 MHz, CDCl3,
d
, ppm): 7.35e7.37 (m,
6H), 7.05e7.11 (m, 12H), 7.11e6.79 (m, 2H), 6.68 (d, 1H), 2.77e2.82 (br,
2H), 1.33 (br, 6H), 0.90 (br, 3H). 13C NMR (75 MHz, CDCl3,
, ppm):
AreH), 7.29 (br, AreH), 7.19 (br, AreH), 7.08e6.90 (br, AreH), 6.69
(br, AreH), 4.39 (br, eOCH2eCH2Oe), 2.80 (br, eCH2e), 2.04 (br,
eCH2e), 1.63 (br, eCH2e), 1.31 (br, eCH2e), 1.07 (br, eCH2e), 0.76
d
145.14; 144.72; 140.86; 135.92; 134.76; 134.48; 133.37; 131.40; 131.11;
128.42; 126.35; 125.95; 124.68; 123.82; 123.18; 123.04; 122.83;
122.58; 122.39; 119.63; 114.84; 30.53; 29.18; 27.72; 21.53; 13.04. Anal.
calcd for: C38H33Br2N3: C, 60.08; H, 4.38; N, 1.84. Found: C, 59.19; H,
4.31; N, 1.61. MALDI-TOF MS Calcd for C38H33Br2N3, 759.68; found:
759.86.
(br, eCH3). 13C NMR (75 MHz, CDCl3,
d): 178.22; 150.68; 149.83;
147.72; 146.86; 146.48; 145.26; 144.90; 139.71; 139.21; 138.88;
138.19; 137.38; 137.29; 135.68; 131.40; 130.20; 130.00; 129.79;
128.43; 127.77; 127.57; 126.99; 126.90; 125.98; 125.77; 124.96;
124.63; 124.51; 123.95; 123.09; 122.36; 122.19; 121.98; 121.84;
119.92; 118.93; 118.66; 114.81; 114.52; 114.17; 64.36; 63.55; 54.24;
54.10; 39.40; 30.45; 28.68; 22.74; 22.36; 21.54; 12.99.
2.3.1.2. 2-[2-[4-[N,N-Di(4-bromophenyl)amino]phenyl]ethenyl]3,4-
ethylenedioxythiophene(5). Compound 4 (0.65 g, 3.84 mmol), 1
(2.91 g, 5.76 mmol), sodium hydride (0.37 g, 15.37 mmol), THF
(30 mL). Using petroleum ether/CHCl3 (1:1, v/v) as eluent to
obtain 5 as yellow powder (1.05 g, 46%). 1H NMR (300 MHz, CDCl3,
P3: Yield: 50%. Mn ¼ 3100; polydispersity ¼ 1.70. 1H NMR
(300 MHz, CDCl3,
d): 9.89 (s, eCHO), 7.79e7.60 (br, AreH),
7.62e7.58 (br, AreH), 7.40e7.38 (br, AreH), 7.29 (br, AreH), 7.18 (br,
AreH), 7.10e6.84 (br, AreH), 6.69 (br, AreH), 4.39e4.37 (br,
eOCH2eCH2Oe), 2.82e2.77 (br, eCH2e), 2.05 (br, eCH2e), 1.60 (br,
eCH2e), 1.33 (br, eCH2e), 1.08 (br, eCH2e), 0.76 (br, eCH3). 13C
d
, ppm): 7.36e7.26 (m, 6H), 7.07e6.82 (m, 8H), 6.21 (s, 1H),
4.27e4.20 (m, 4H). 13C NMR (75 MHz, CDCl3,
d
): 146.28; 145.84;
NMR (75 MHz, CDCl3, d):178.14; 157.49; 157.39; 150.71; 147.65;
142.00; 139.05; 132.89; 132.36; 127.15; 125.56; 125.35; 124.24;
117.40; 117.19; 115.66; 99.62; 97.50; 64.80; 64.66.
146.92; 146.55; 145.29; 144.95; 144.72; 138.92; 138.27; 137.38;
137.30; 135.71; 134.61; 133.44; 131.37; 130.25; 130.05; 129.53;
127.73; 126.94; 126.18; 124.98; 124.68; 123.97; 122.87; 122.60;
122.41; 120.49; 119.96; 118.99; 114.90; 114.62; 114.21; 64.37; 63.57;
54.28; 39.47; 30.44; 29.17; 28.68; 27.73; 22.82; 21.51; 12.99; 12.92.
P4: Yield: 62%. Mn ¼ 5000; polydispersity ¼ 1.48. 1H NMR
2.3.1.3. 2-[2-[4-[N,N-Di(4-bromophenyl)amino]phenyl]ethenyl]3,4-
ethylenedioxythiophen-5-al (6). To
a solution of 5 (1.14 g
2.00 mmol) and anhydrous DMF (1 mL, 13 mmol) in anhydrous
1,2-dichloroethane (50 mL) at 0 ꢁC under a N2 atmosphere was
added POCl3 (0.76 g 5.00 mmol) dropwise, and the mixture was
refluxed for 12 h. After being cooled to room temperature, the
mixture was poured into an aqueous solution of sodium acetate
and then stirred for 2 h. The organic phase was separated by
decantation and the aqueous phase was extracted with CH2Cl2.
The organic phases was collected, dried over MgSO4 and evapo-
rated under vacuum, and purified by column chromatography
(silica gel, petroleum ether/CHCl3, 1:1, as eluent) to obtain 6 as
(300 MHz, CDCl3, d): 9.89 (br, 1H), 7.79e7.76 (br, 4H), 7.65e7.58 (br,
6H), 7.41e7.38 (br, 2H), 7.29 (br, 2H), 7.19e7.16 (br, 2H), 7.10e7.08 (br,
2H), 4.38 (br, 4H), 2.05 (br, 4H),1.58 (br, 4H),1.07 (br,12H), 0.76 (br, 6H).
13C NMR (75 MHz, CDCl3,
d, ppm): 179.51; 151.96; 147.34; 146.76;
146.54; 145.99; 142.44; 140.14; 139.56; 136.63; 135.89; 134.72;
132.61; 131.70; 129.02; 128.20; 127.57; 127.02; 125.91; 125.11;
1224.95;124.41;124.14;123.88;123.69;121.26;120.25;115.83;65.65;
64.85; 55.54; 31.78; 30.46; 29.98; 28.99; 24.10; 22.82; 14.24; 1.26.
pink powder (1.00 g, 83.7%). 1H NMR (300 MHz, CDCl3,
d
): 9.89 (s,
2.3.3. General procedure for synthesis of P2-CNeP4-CN
1H, eCHO), 7.38e7.35 (m, 8H, AreH), 7.05e7.01 (m, 2H, eCH]
CHe), 6.98e6.95 (m, 4H, AreH), 4.38e4.37 (m, 4H, eCH2eCH2e).
To a solution of P2eP4 and malononitrile in THF (10 mL) was
added 0.1 mL of pyridine. The content of malononitrile was
controlled with different ratios for the preparation of different
polymers. The mixture was stirred at room temperature overnight,
then the resultant mixture was poured into methanol, the precipitate
was filtered off and washed with water. The obtained polymer was
purified by repeated precipitation from its THF solution to methanol.
The solid was dried under vacuum for 1 day to yield the product.
P2-CN: P2 (0.132 g), malononitrile(0.103 g). Dark blue powder
13C NMR (75 MHz, CDCl3,
d): 179.47; 148.95; 147.32; 146.19; 138.71;
132.74; 131.41; 131.02; 128.60; 128.19; 126.21; 123.75; 116.46;
116.32; 1115.48; 65.62; 64.81. Anal. calcd for: C27H19Br2NO3S: C,
54.29; H, 3.21; N, 2.34; found: C, 54.88; H, 2.93; N, 2.39.
2.3.2. General procedure for the synthesis of P1eP4
A total of 0.3 mmol of the mixture of Compound 6 (x mmol) and
3 ((0.3 ꢀ x) mmol) was put into a three-neck flask. The content of
each component in the mixture was controlled with different ratios
for the preparation of different polymers. Then THF (14 mL),
potassium carbonate (2.0 M in water, 2.5 mL) and Pd(PPh3)4 (3%
mol) were carefully degassed and charged with nitrogen. The
reaction mixture was stirred at 60 ꢁC for 3 days. After the solution
(0.120 g, 87.8%). 1H NMR (300 MHz, CDCl3,
d): 7.76 (br, AreH),
7.61e7.58(br, AreH), 7.41e7.39(br, AreH), 7.26(br, AreH), 7.17 (br,
AreH), 7.06 (br, AreH), 7.00e6.84 (br, AreH), 6.69 (br, AreH),
4.40e4.36 (br, eOCH2eCH2Oe), 2.79e2.77 (br, eCH2e), 2.05
(br, eCH2e), 1.69e1.66 (br, eCH2e), 1.32 (br, eCH2e), 1.07
(br, eCH2e), 0.76 (br, eCH3). 13C NMR (75 MHz, CDCl3,
d, ppm):

S. Li et al. / Polymer 52 (2011) 5302e5311
5305
151.91; 146.72; 145.93; 142.37; 140.07; 139.53; 136.55; 135.82;
134.68; 128.99; 128.17; 127.53; 125.87; 125.53; 124.90; 124.38;
124.09; 123.84; 123.64; 121.20; 120.20; 68.18; 55.48; 40.74; 31.77;
30.42; 29.95; 28.97; 25.85; 24.03;22.80; 14.23; 1.24 . GPC: (THF,
yields. In the WittigeHornor reaction, here, sodium hydride was
used as the base, instead of the normal t-butoxide potassium, to get
the satisfactory yields. It was easily known that if some other
aldehydes but not 2 and 4 were used in the WittigeHornor reac-
tion, as a result, other monomers rather than 3 and 6 could be
yielded. For example, other thiophene oligomers bearing aldehyde
groups, i.e. quaterthiophene and quinquethiophene, could be
treated as the starting material instead of 2, to adjust the conju-
gated length of the thiophene-based side chain to possibly modify
the quality of the blend films of the polymers and PC61BM. Similar,
other conjugated blocks could replace the substituted thiophene
one in 4 to partially control the intramolecular charge transfer to
regulate the absorption band and LUMO level.
polystyrene standard) Mn ¼ 4.54 kg molꢀ1
,
Mw ¼ 5.97 kg molꢀ1
,
PDI ¼ 1.32. UVevis (CH3Cl,1 ꢂ10ꢀ5 mol/L): lmax ¼ 383, 438, 547 nm.
P3-CN: P3 (0.130 g), malononitrile (0.236 g). Dark blue powder
(0.076 g, 61.6%). 1H NMR (300 MHz, CDCl3,
d): 7.81e7.76 (br, AreH),
7.63e7.58 (br, AreH), 7.42e7.40 (br, AreH), 7.26 (br, AreH),
7.19e7.17 (br, AreH), 7.10e6.85 (br, AreH), 6.68 (br, AreH),
4.41e4.37 (br, eOCH2eCH2Oe), 2.80e2.77 (br, eCH2e), 2.06 (br,
eCH2e), 1.66 (br, eCH2e), 1.32 (br, eCH2e), 1.07 (br, eCH2e), 0.76
(br, eCH3). 13C NMR (75 MHz, CDCl3,
d, ppm): 151.99; 146.10;
145.93; 142.37; 140.07; 139.53; 136.55; 135.82; 134.68; 128.99;
128.17; 127.53; 125.87; 125.53; 124.90; 124.38; 124.09; 123.84;
123.64; 121.20; 120.20; 68.18; 55.48; 40.74; 31.77; 30.42; 29.95;
28.97; 25.85; 24.03;22.80; 14.23; 1.24 . GPC: (THF, polystyrene
The polymers, P1 and three precursor polymers, P2, P3 and P4,
were synthesized by Suzuki coupling reaction by adding different
ratios of monomer 3 and 6. The three resultant polymers, P2-CN,
P3-CN and P4-CN, were obtained by a Knoevenagel condensation
between the aldehyde-functionalized conjugated precursor poly-
mer and malononitrile in the presence of pyridine. Just simply
controlling the monomer ratio of 3 and 6 in the polymerization
process, the ratio of x in the polymers could be easily modulated.
This was another advantage of this kind of copolymers, that was,
the concentrations of the different construction blocks in the
resultant polymers could be conveniently handled according to the
different requirements. Since the aldehyde groups could react with
different acceptors, such as 1,3-diethylthiobarbituric acid, rather
than malononitrile used in this paper, thus, other conjugated
copolymers of this kind could be easily prepared if needed. From
this point, P2eP4 could be considered as polymeric reactive
intermediates for the synthesis of other functional polymers.
Further work was still underway in our lab.
standard) Mn ¼ 6.07 kg molꢀ1
,
Mw ¼ 7.92 kg molꢀ1
,
PDI ¼ 1.28.
UVevis (CH3Cl, 1 ꢂ10ꢀ5 mol/L): lmax ¼ 383, 438, 547 nm.
P4-CN: P4 (0.073 g), malononitrile (0.265 g). Dark blue powder
(0.076 g, 60.7%). 1H NMR (300 MHz, CDCl3,
d): 7.81e7.77 (br, 3H),
7.63e7.58 (br, 7H), 7.42 (br, 2H), 7.26 (br, 4H) 7.16e7.10 (br, 4H),
4.42e4.37 (br, 4H), 2.05 (br, 4H),1.65 (br, 4H),1.07 (br,12H), 0.76 (br,
6H). 13C NMR (75 MHz, CDCl3,
d, ppm): 151.97; 140.19; 128.35;
125.54; 120.29; 107.94; 55.51; 40.78; 31.74; 29.96; 24.06; 22.84;
14.29; 1.27. GPC: (THF, polystyrene standard) Mn ¼ 6.18 kg molꢀ1
,
Mw ¼ 7.92 kg molꢀ1, PDI ¼ 1.28. UVevis (CH3Cl, 1 ꢂ10ꢀ5 mol/L):
lmax ¼ 379, 548 nm.
3. Results and discussion
3.1. Synthesis
3.2. Structural characterization
The structures and the synthetic routes to monomers and
the two-dimensional conjugated copolymers were shown in
Schemes 1 and 2. The double bonds in monomer 3 and Compound 5
were formed by WittigeHornor reaction. Then, 5 were subjected to
a Vilsmeier reaction in the presence of DMF and POCl3 in refluxing
1,2-dichloroethane, affording selectively aldehyde 6 in moderate
The monomers and polymers were characterized by spectro-
scopic methods, and all gave satisfactory spectral data. The
monomers 3 and 6 were new compounds, and all the polymers
were not reported yet. Fig. 1 showed the IR spectra of all the
polymers except P1, in which the absorption bands associated with
Scheme 1. Synthetic routes to monomers.

5306
S. Li et al. / Polymer 52 (2011) 5302e5311
Scheme 2. Synthetic routes to polymers.
the aldehyde group were at 1652 cmꢀ1 in P2eP4, while that of
cyano group at about 2222 cmꢀ1 in P2-CNeP4-CN. It was easily
seen that after the conversion to P2-CN, P3-CN, and P4-CN, these
characteristic absorptions of aldehyde moieties disappeared, and
new absorption peaks at 2222 cmꢀ1 (corresponding to C^N
stretching for the three polymers) emerged, indicating the
complete conversion of the aldehyde groups to the cyano ones
within the limits of the sensitivity of FT-IR [24]. This case was
further confirmed by their 1H NMR spectra: there were no signals of
the aldehyde groups observed at about 9.89 ppm in the spectra of
P2-CN, P3-CN, and P4-CN [26,27]. As mentioned above, the ratio of
x in the polymers could be modulated by controlling the monomer
ratio of 3 and 6, the actual values of x were calculated according to
the 1H NMR spectra of the polymers (see Fig. 2). In the 1H NMR
standard, with the results summarized in Table 1. The number-
average molecular weights (Mn) of P1, P2-CN, P3-CN, P4-CN were
found to be 12.1, 4.54, 6.07 and 6.18 kg molꢀ1 respectively, with the
corresponding polydispersity indices of 2.13, 1.32, 1.66 and 1.28. All
the polymers have excellent solubility in common organic solvents
such as THF, chloroform, and chlorobenzene due to its bulky side
chain which could be readily cast into uniform thin films, rendering
them good candidates for the fabrication of PSCs devices. The
thermal property of the copolymers was investigated via thermo-
gravimetric analysis (TGA) under an atmosphere of dry nitrogen.
As shown in Table 1 and Figure S2 (see Supporting information), the
5% weight loss temperatures for P1, P2-CN, P3-CN and P4-CN were
in the range of 338e390 ꢁC, which was adequate for the application
of the copolymers as active materials in PSCs and other opto-
electronic devices [25,26,28].
spectrum of P1, there was a peak at
d 2.80 ppm (peak a), which
should be attributed to the hydrogen on the side chain of P1; the 1H
NMR spectrum of P4-CN displayed a new peak at 4.37 ppm (peak
b), which should be assigned to the hydrogen of 3,4-
ethylenedioxythiophene. However, P2-CN and P3-CN have both
peaks. Thus, the ratio of x in P2-CN and P3-CN could be determined
to be 0.33 for P2 and 0.66 for P3, by the ratio of the integral areas of
peak a to peak b in Fig. 2, which were close to the feeding ratios of
the monomer in the reactions (0.33 for P2-CN and 0.75 for P3-CN).
Molecular weights of the polymers were determined by gel
permeation chromatography (GPC) in THF, using polystyrene as
Fig. 3 showed the absorption spectra of P1, P2-CN, P3-CN and
P4-CN, in both chloroform solutions and thin films. In solution, P1
exhibited two distinct absorption peaks: the absorption peak at
about 392 nm originated from the
main chain [24e26]; while that of w439 nm corresponded to the
* transition of the terthiophene moieties in the side chain [31].
pep* transition of its conjugated
pep
After the introduction of the electronic acceptor of dicyano groups,
taking P4-CN as an example, a new absorption peak at the longer
wavelength of 548 nm appeared, which should be attributed to the
strong intramolecular charge transfer (ICT) interaction between the
conjugated main chain and the pendant acceptor moieties [24e27]
and would capture the sunlight at longer wavelength than that for
P3HT to contribute to the performance of the resultant PSC devices.
However, it was noted that in the range of 420e480 nm, P4-CN
demonstrated very weak or nearly no absorption, which would
surely decease the PCE value in some degree. However, in the
spectra of copolymers of P2-CN and P3-CN, the absorption peak of
the terthiophene moieties remained in some extent, while the peak
of ICT interaction at 548 nm appeared, thanks to their special
structure. The good absorption of P3HT in the visible region
contributed to its good PSC efficiency, and the absorption of the
low-band gap conjugated polymers at longer wavelength should be
achieved. Thus, by regulating the ratios of the two components as
shown in P2-CN and P3-CN, the absorption spectra of the resultant
polymers could be easily tuned, to possibly control the
P4
P4-CN
P3
P3-CN
P2
P2-CN
-CN
-CHO
1800 1600 1400 1200 1000 800
2000
Wavenumber (cm )
4000
3000
-1
Fig. 1. FT-IR spectra of the polymers.

S. Li et al. / Polymer 52 (2011) 5302e5311
5307
Fig. 2. 1H NMR spectra of the polymers
performance the corresponding PSC devices. This might be another
advantage of this kind of copolymers. Also, by bonding some other
more stronger acceptor moieties instead of the dicyano groups used
here, the absorption band of ICT interactions could be further red-
shifted [24e26], and the adjustment of the balance between the
absorption of terthiophene moieties and the ICT one should be
more important for the PCE values of the PSC devices. The optical
band gaps (Eogpt) of the four polymers were calculated from the
absorption onset in the solutions. Different from the higher one,
2.34 eV, of P1, P2-CN, P3-CN and P4-CN had the similar value of
w1.82 eV, which was lower than 2.0 eV, for the possibly good
match of the terrestrial solar radiation. By manipulating the
different acceptors in the side chains, the photophysical properties
of this kind of copolymers could be further adjusted according to
the practical requirements. Similar to those reported in the litera-
ture, the polymer films spin-coated on glass substrate showed red-
shifted and broadened absorptions, in comparison with those of
their corresponding dilute solutions, due to the enhanced inter-
chain interactions in the solid state and the probably increased
polarizability of the film. All the above-related data were also
summarized in Table 1.
(Fc/Fcþ) couple, which was used as the internal standard. In the
anodic scan, the initial onset oxidation potential of the conjugated
copolymers were found to occur at 0.93, 0.89, 0.89 and 0.90 V (vs
Ag/Agþ), respectively, which corresponded to HOMO levels
of ꢀ5.30, ꢀ5.26, ꢀ5.26 and ꢀ5.27 eV, respectively, according to the
empirical equation, E(HOMO) ¼ ꢀ(Eox þ 4.37). Since the cathodic
scans for these polymers failed despite many attempts, the LUMO
energy levels of these conjugated copolymer were estimated
from the optical band gaps and the HOMO energies, based on the
relation of Eg ¼ ELUMO ꢀ EHOMO
,
which were calculated to
be ꢀ2.96, ꢀ3.44, ꢀ3.44 and ꢀ3.45 eV, respectively (Table 1). Due to
the same conjugated backbone of these four polymers, their HOMO
energy levels were nearly the same, although there were some
pendant acceptor groups in the side chains of P2-CN, P3-CN and
P4-CN. In accordance with the different structure, the LUMO
energy level of P1 (ꢀ2.96 eV) was different, while those of P2-CN,
P3-CN and P4-CN were similar (wꢀ3.44 eV), which were
decreased after the introduction of the dicyano groups in the
conjugated side chains on the thiophene moieties. It was reported
that the threshold HOMO level for air stable conjugated polymers
was calculated to be about 5.2 eV [33], thus, the little lower HOMO
levels of these four polymers should contribute much to their
chemical stability in ambient conditions, as confirmed by their
relatively good decomposed temperature shown in Table 1. Since
Voc of PSCs was generally proportional to the difference between
the LUMO level of the acceptor and the HOMO level of the donor, it
was reasonable to expect that the deeper HOMO levels of the
polymers would assure the high Voc values of the corresponding
PSCs at a large degree. Also, the big difference (larger than 0.3 eV)
between the LUMO levels of the polymers and PC61BM (ꢀ4.3 eV)
3.3. Electrochemical properties
Cyclic voltametry (CV) was employed to investigate the redox
behavior of the chromophores and estimate their highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels. The CV curves were shown in Fig. 4 and the
corresponding data were summarized in Table 1. All reported
potentials were calibrated against the ferrocene/ferrocenium
Table 1
Molecular weights, thermal properties, electrochemical and optical properties of the polymers.
a
polymers
Mn (kg/mol)
Mw (kg/mol)
PDI (Mw/Mn)
Td (ꢁC)
labs (nm)
labs (nm)
Egb (eV)
EHOMOc (eV)
ELUMOd (eV)
Solution
Film
P1
12.1
4.54
6.07
6.18
25.9
5.97
10.1
7.92
2.13
1.32
1.66
1.28
361
390
349
338
392,439
388,444
391,548
384,547
379,549
2.34
1.82
1.82
1.82
ꢀ5.30
ꢀ5.26
ꢀ5.26
ꢀ5.27
ꢀ2.96
ꢀ3.44
ꢀ3.44
ꢀ3.45
P2-CN
P3-CN
P4-CN
383,438 547
383,438 547
379,548
a
5% Weight loss temperature measured by TGA under N2.
Band gap estimated from optical absorption band edge of the film.
Calculated from the onset oxidation potentials of the polymers.
Estimated using empirical equations ELUMO ¼ EHOMO þ Eg.
b
c
d

5308
S. Li et al. / Polymer 52 (2011) 5302e5311
1.0
0.8
0.6
0.4
0.2
0.0
1.0
P1
P1
P2-CN
P3-CN
P4-CN
P2-CN
P3-CN
P4-CN
0.8
monomer 3
0.6
0.4
0.2
0.0
320
420
520
620
720
320
420
520
620
720
Wavelength (nm)
Wavelength (nm)
Fig. 3. UVevis absorption spectra of the polymer in the CHCl3 solutions and films.
would contribute much to the charge transfer from the polymers to
PC61BM to benefit the device performance.
noticeable effect on the electronic distribution properties of the
polymer systems. The calculated frontier orbitals for the repeating
units of these polymers were shown in Figure S3 (see Supporting
information). Although there were some discrepancies existed
between the calculation and experimental results, we could still
obtain some information from Figure S3 (see Supporting
information). As shown in Figure S3a (see Supporting
information) and 6c, when the chain length of P1 and P4-CN was
2, their HOMOs were located throughout the whole conjugated
system, resulting in their similar HOMO values (Table 1). Interest-
ingly, their LUMO values were different at a relatively large degree,
no matter the computed LUMOs localized mainly on the side
chains. This was reasonable. As mentioned above, the LUMO levels
of this kind of copolymers were almost determined by the acceptor
moieties in the side chains, thus, after malononitrile was intro-
duced to P4-CN, its LUMO level should surely be lowered. Unlike
those of P1 and P4-CN, the electron density distributions at HOMO
comprised part of the main chain and the terthiophene group,
while those of LUMO were highly localized near the acceptor
moieties in the side chains. The well-separated distribution of
HOMO and LUMO levels disclosed there was a charge-transfer
transition between them, partially indicating the charge separa-
tion through sequential transfer for electrons from the conjugated
donor to the side chain acceptor groups and then to PC61BM in the
bulk-heterojunction PSCs. If some other stronger acceptors were
used, it was expected that even lower LUMO energy levels could be
obtained.
3.4. Theoretical calculations
To get some insight into the fundamentals of molecular archi-
tecture, density functional theory (DFT) calculations were per-
formed at the B3LYP/6-31G(d) level using Gaussian 09, Revision
A.02 program [34] with a chain length of n ¼ 2 for P1 and P4-CN
(while 1 for the repeating unit in P2-CN and P3-CN) for their
good comparison, and the corresponding KohneSham orbitals
were obtained at the same level of theory. To simplify the calcu-
lating process, aliphatic side chains of the fluorine moieties in the
polymers were replaced by methyl groups. This is a reasonable
approach since the substituents in the 9-position of the fluorene
ring were not in conjugation with the aromatic system and had no
P1
P2-CN
P3-CN
P4-CN
3.5. Photovoltaic properties
Bulk-heterojunction PSCs using the prepared four conjugated
polymers P1, P2-CN, P3-CN and P4-CN as donor and PC61BM as
acceptor were investigated with the device configuration of ITO/
PEDOT:PSS/Polymer:PC61BM(1:2,
w/w)/Ca/Al.
Photovoltaic
performance characteristics of the PSCs were summarized in
Table 2. Fig. 5 showed the current densityevoltage (JeV) curves
of the PSCs based on the copolymers/PC61BM (1:2, w/w),
measured under the illumination of simulated AM 1.5 G condi-
tions (900 W/m2). All the devices exhibited a high Voc (in the
range of 0.80e0.90 V), due to their relatively low HOMO level,
which was mainly determined by their main-chain donor. P3-CN
demonstrated better photovoltaic performance than the other
three polymers with Jsc of 3.76 mA/cm2, FF of 0.36, Voc of 0.90,
corresponding to a PCE of 1.34%.
0.0
0.5
1.0
1.5
2.0
Potential (V vs Ag/Ag )
Fig. 4. Cyclic voltammograms of the copolymer thin films on platinum electrode in
0.1 mol/L Bu4NPF6, CH3CN solution.

S. Li et al. / Polymer 52 (2011) 5302e5311
5309
Table 2
Fig. 6 demonstrated the comparison of the PCE values of the
polymers using P1 as reference. It was seen that the PCE values
increased first, then decreased, accompanying with the increasing
Photovoltaic properties of the polymer solar cells.
polymers
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
concentration of the DepeA ones in the polymers, visually
exhibiting the changing trend of these four polymers discussed
above. Since we only prepared two polymers, P2-CN and P3-CN,
which contained both the terthiophene moieties and the DepeA
P1
0.80
0.85
0.90
0.90
1.04
1.73
3.76
2.73
34
37
36
32
0.32
0.61
1.34
0.86
P2-CN
P3-CN
P4-CN
ones, there might be other polymers of this kind with different
ratios of these two components exhibiting even better results.
However, the preliminary experimental results could give some
clues for the design of conjugated copolymers for the PSCs: the
prolonging of the absorption band to the long wavelength by the
It seemed that there were no rules present in the tested results
at the first glance. If analyzing their Jsc values in detail, it was found
that from P1 to P2-CN and further to P3-CN, the Jsc values increased
from 1.04 to 1.73 then to 3.76 mA/cm2. This result should be
ascribed to the progressively improved absorption spectra in the
long wavelength as the result of the appeared new absorption peak
at about 547 nm brought by the enhanced ICT interaction. However,
the Jsc value of P4-CN decreased to 2.73 mA/cm2, no matter its
absorption band with the peak centered at about 548 nm further
enhanced, due to the increased concentration of the repeating units
bearing the acceptors. This was reasonable. Generally, the Jsc value
was mainly determined by the absorption behavior of the conju-
gated polymers. As discussed in the part of the absorption behavior
of these four conjugated polymers, although P4-CN absorbed very
well at the longer wavelength from 480 to 610 nm, it demonstrated
very weak or nearly no absorption in the range of 420e480 nm,
which would surely decease the PCE value in some degree.
However, in the spectra of copolymers of P2-CN and P3-CN, the
absorption peak of the terthiophene moieties remained in some
extent with the absorption band shorter than about 480 nm, while
the peak of ICT interaction at 548 nm appeared, thanks to their
special structure. Thus, the remained absorption peak originated
from the terthiophene moieties in the range of 420e480 nm,
contributed much to the good performance of P3-CN, indicating
that the absorption band at the short wavelength could not be
ignored in the design of the conjugated polymers. So, considering
the results carefully, there would be a balance between the
absorptions at short wavelengths and the longer ones.
introduction of the DepeA structure, could not be built on the
victim of the sharply decreased absorption in the short wave-
length. On the other hand, the obtained results indicated the
advantage of the structure of P2-CN and P3-CN: the functional
units with different absorption behavior could be integrated into
the copolymers. It should be pointed out that the PSC devices
were not well optimized, thus, better performance might be
achieved.
With the aim to know some information about the morphology
of copolymer: PC61BM blends, AFM was used to characterize their
films, with the obtained topography images shown in Fig. 7. It was
easily seen that the surface of P1:PC61BM and P3-CN:PC61BM blend
films were quite smooth, with root-mean-square roughness (rms)
of 0.474 and 0.324 nm, respectively. Also, the surface of P2-
CN:PC61BM blend films was still good, with rms of 0.648. This
indicated the good miscibility between P1, P2-CN, P3-CN, and
PC61BM without large phase separation. However, the surface of
P4-CN:PC61BM were very rough, with rms of 3.735 nm. Comparing
their structure carefully, in P4-CN, there was only one kind of side
chains with the terminal acceptors, but no side chains of the ter-
thiophene groups. As discussed in the introduction part, the
introduction of the terthiophene groups in the side chains, was to
increase the compatibility of the resultant polymers with PC61BM.
Thus, the AFM results confirmed the role of the terthiophene
groups, without their presence, the compatibility was much worse.
0.0
-0.5
-1.0
-1.5
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
-2.0
P1
P2-CN
P3-CN
P4-CN
-2.5
-3.0
0.0
0.2
0.4
0.6
0.8
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)
The ratio of malononitrile substituted side chain.
Fig. 5. JeV characteristics of the devices with the structure of ITO/PEDOT:PSS/Poly-
Fig. 6. The comparison of the PCE values of the polymers using P1 as reference. Line is
mer:PC61BM (1:2, w/w)/Ca/Al.
included as a guide to the eye.

5310
S. Li et al. / Polymer 52 (2011) 5302e5311
Fig. 7. AFM images (2 ꢂ 2
m
m2) of the active layers from the four polymers (polymer: PC61BM ratio of 1:2)
This indicated that in the design of new polymers with better
photovoltaic performance, perhaps, the introduction of some
thiophene oligomer blocks could contribute to the compatibility
with PC61BM.
modifying the structure, other copolymers with better perfor-
mance should be obtained.
Acknowledgements
4. Conclusions
We are grateful to the National Science Foundation of China (no.
21034006) and the National Fundamental Key Research Program
(no. 2011CB932702) for financial support.
In summary, a series of conjugated copolymers were success-
fully prepared, and well characterized, which were soluble in
common polar solvents and exhibited good thermal stability. The
preliminary study demonstrated the following.
Supporting information
The performance of the copolymers in the PSC devices did not
always increase accompanying the increasing concentration of the
malononitrile substituted side chain. Thus, there should be
a balance between the absorptions at short wavelengths and the
longer ones. This might be a new strategy to design conjugated
copolymers for PSC applications with adjustable performance, into
which the functional units with different absorption behavior could
be integrated.
Supporting information associated with this article can be
found, in the online version, at doi:10.1016/j.polymer.2011.09.022.
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