Low bandgap polymers with benzo [1,2-b:4,5-b′] dithiophene and bisthiophene-dioxopyrrolothiophene units for photovoltaic applications

Article abstract of DOI:10.1016/j.polymer.2010.11.022

New donor/acceptor polymers PBDTTPT1 and PBDTTPT2 with alternating benzodithiophene (BDT) and bisthiophene-dioxopyrrolothiophene (TPT) units were synthesized by Stille coupling reaction. The polymers had optical bandgaps of 1.78 and 1.82 eV, and HOMO energy levels of -5.30 and -5.35 eV for PBDTTPT1 and PBDTTPT2, respectively. Polymeric solar cell devices based on these copolymers as donors and PC₇₁BM as acceptor showed the highest open circuit voltage of 0.95 V and power conversion efficiency of 2.68% under the illumination of AM 1.5, 100 mW/cm².

Full text of DOI:10.1016/j.polymer.2010.11.022

Polymer 52 (2011) 415e421
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Low bandgap polymers with benzo [1,2-b:4,5-b0] dithiophene and
bisthiophene-dioxopyrrolothiophene units for photovoltaic applications
,
a,
Guobing Zhang a, Yingying Fu b, Zhiyuan Xie b , Qing Zhang
*
*
a Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, China
a r t i c l e i n f o
a b s t r a c t
Article history:
New donor/acceptor polymers PBDTTPT1 and PBDTTPT2 with alternating benzodithiophene (BDT) and
bisthiophene-dioxopyrrolothiophene (TPT) units were synthesized by Stille coupling reaction. The
polymers had optical bandgaps of 1.78 and 1.82 eV, and HOMO energy levels of ꢀ5.30 and ꢀ5.35 eV for
PBDTTPT1 and PBDTTPT2, respectively. Polymeric solar cell devices based on these copolymers as
donors and PC71BM as acceptor showed the highest open circuit voltage of 0.95 V and power conversion
efficiency of 2.68% under the illumination of AM 1.5, 100 mW/cm2.
Received 9 April 2010
Received in revised form
5 October 2010
Accepted 13 November 2010
Available online 22 November 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Conjugated polymer
Polymer solar cells
Benzo [1,2-b:4,5-b0] dithiophene
1. Introduction
Recently, low bandgap polymers with deep HOMO energy level
have been studied for achieving high Voc in PSC devices [17e20].
Research on polymeric solar cells (PSCs) has been intensified in
recent years because PSCs have a potential to generate electricity from
sunlight at low cost [1e8]. Bulk heterojunction (BHJ) structure based
on blending of electron-donating conjugated polymers and high-
electron-affinity fullerene derivatives such as PCBM has become the
most successful device structure for organic photovoltaics (OPVs)
[9,10]. So far, the power conversion efficiency (PCE) of PSCs is still low
incomparisonwiththesiliconbasedsolarcells. Fourstepsareinvolved
the organic photovoltaic phenomena. Theyare photon absorption and
generation of exciton, exciton diffusion, exciton separation and sepa-
rated charge carriers migrate to electrodes. Short circuit current (Jsc),
open circuit voltage (Voc) and fill factor (FF) are three mainparameters
that determine the PCE of the PSC devices. Many examples of low
bandgap polymers, absorbing in the red and near-IR regions of the
solar spectrum, have been developed for maximizing solar photon
harvest, thereby increasing Jsc and PCE [11e14]. Systematic studies on
matching the energy levels of donor polymers with those of PCBM
have been carried out [15,16]. Low bandgap polymers with deep
HOMO energy level will result high Jsc and Voc simultaneously, thereby
maximizing power conversion efficiency.
Polymers containing dioxopyrrolothiophene (DPT) unit have ach-
ieved the Voc of about 0.9 V and PCE around 4e7% [21e25]. In our
previous work, two polymers (PBDTDPTs) based on benzo [1,2-
b:4,5-b0] dithiophene (BDT) and DPT units were synthesized and
were shown relatively high performance in solar cell devices [25].
However, PBDTDPT polymer films have absorption spectra edged
around 675 nm. Design and synthesis of new polymers with
substantial absorption in longer wavelength will have lower
bandgap and may result better solar photon harvest. Recently, there
are reports on further reduction of polymer bandgaps by incorpo-
ration of additional electron-rich thiophene moieties into existing
donor/acceptor polymers [26e28]. Herein, we report the synthesis
and structure/property relationship on two new low bandgap
polymers based on BDT and DPT units with additional alkyl-
substituted thiophene moieties.
2. Experimental section
2.1. Materials
n-Butyllithium, trimethyltin chloride and triphenylarsine were
obtained from Alfa Aesar Chemical Company. Tris (dibenzylideneace-
tone) dipalladium (Pd2dba3) and tri-o-tolylphosphine were purchased
from SigmaeAldrich Chemical Company. All materials were used as
* Corresponding authors.
E-mail addresses: xiezy_n@ciac.jl.cn (Z. Xie), qz14@sjtu.edu.cn (Q. Zhang).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.11.022

416
G. Zhang et al. / Polymer 52 (2011) 415e421
Scheme 1. Synthesis of monomers and polymer PBDTTPT1 and BDTTPT2.
received without further purification. Tetrahydrofuran (THF) and
toluene were freshly distilled over Na wire under nitrogen prior to use.
follows: a ca. 40-nm-thick PEDOT: PSS (Baytron P AI 4083) was spin-
coated from an aqueous solution onto the pre-cleaned ITO substrates,
and dried at 120 ꢁC for 30 min in air. Then the substrates were
transferred into a nitrogen filled glove box. The prepared solution of
PBDTTPT: PC70BM (5 mg/mL: 10 mg/mL) in chloroform with or
without diiodooctane (2 vol%) was spin-coated on top of the PEDOT/
PSS layer. Finally the samples were transferred into an evaporator
and LiF (1 nm) and Al (100 nm) with area of 0.12 cm2 were
thermally deposited under a vacuum of 10ꢀ6 Torr. The devices were
encapsulated in a glove box and measured in air. Current-voltage
characteristics were measured using a computer controlled Keithley
236 source meter. The photocurrent was measured under AM 1.5G
illumination at 100 mW/cm2 from a solar simulator (Oriel, 91160A-
1000). The external quantum efficiency (EQE) was measured at
a chopping frequency of 280 Hz with a lock-in amplifier (Stanford,
SR830) during illumination with the monochromatic light from
a Xenon lamp.
2.2. Measurements and characterization
Nuclear Magnetic Resonance spectra were recorded on
a Mercury plus 400 MHz machine. Gel permeation chromatography
(GPC) analyses were performed on a Perkin Elmer Series 200 gel
coupled with UVevis detector using tetrahydrofuran as eluent with
polystyrene as standards. Thermogravimetric analysis (TGA) anal-
yses were conducted with a TA instrument QS000IR at a heating rate
of 20 ꢁC minꢀ1 under nitrogen gas flow. Differential scanning calo-
rimetry (DSC) analysis was performed on a TA instrumentation
Q2000 in a nitrogen atmosphere. All the samples (about 10.0 mg in
weight) were first heated up to 300 ꢁC and held for 2 min to remove
thermal history, followed by the cooling rate of 20 ꢁC/min to 20 ꢁC
and then heating rate of 20 ꢁC/min to 300 ꢁC in all cases. UVevis
absorption spectra were recorded on a Perkin Elmer model
l 20
UVevis spectrophotometer. Electrochemical measurements were
conducted with a CHI 600 electrochemical analyzer under nitrogen
in a deoxygenated anhydrous acetonitrile solution of tetra-n-buty-
lammonium hexafluorophosphate (0.1 M). A platinum electrode
was used as a working electrode, a platinum-wire was used as an
auxiliary electrode, and an Ag/Agþ electrode was used as a reference
electrode. Polymer thin films werecoatedonplatinum electrode and
ferrocene was added as reference. A potential scan rate of 100 mV/s
was used for all experiments.
2.4. Synthetic procedures
4,8-Dihydrobenzoldithiophene-4,8-dione [29], thiophene-3,4-
dicarboxylic anhydride [30] and 5-tri-n-butylstannyl-3-dode-
cylthiophene [31] were synthesized according to the literature. The
synthesis of monomer M1, M2, M3 and the polymers are showed in
Scheme 1.
2.4.1. 5-Dodecylthieno [3,4] pyrrole-4,6-dione (1)
A solution of thiophene-3,4-dicarboxylic anhydride (3.09 g,
20.05 mmol) and n-dodecylamine (4.09 g, 22.06 mmol) in toluene
(250 mL) was refluxed for 24 h in a flask. After cooled to room
temperature, the reaction mixture was filtrated and was dried
under vacuum to afford a brown solid. Thionyl chloride (200 mL)
2.3. Device fabrication and characterization of solar cells
Polymer solar cells (PSCs) with the device structures of ITO/
PEDOT: PSS/polymer: PC70BM (1:2, w/w)/LiF/Al were fabricated as

G. Zhang et al. / Polymer 52 (2011) 415e421
417
was added and the mixture was refluxed for 5 h. The volatile
was removed under vacuum. The residue was recrystallization
from hexane to afford the titled compound as white crystals
2.4.5. 4,8-Didodecyloxybenzo [1,2-b:4,5-b0] dithiophene (4)
Water (40 mL) was added to benzo [1,2-b:4,5-b0] dithiophene-
4,8-dione (2.20 g, 10.0 mmol), zinc power (1.96 g, 30.0 mmol) and
sodium hydroxide (8.00 g, 200.0 mmol) in a round bottom flask
(150 mL). After refluxing for 1 h, a catalytic amount of tetra-n-
butylammonium bromide (0.032 g, 0.1 mmol) was added. 1-Bro-
modedecane (7.50 g, 30 mmol) was added drop-wise to the flask.
The reaction was refluxed overnight. After cooled to room
temperature, it was poured into water (200 mL). The mixture was
extracted with diethyl ether for three times. The combined organic
layer was dried with anhydrous sodium sulfate. Solvent was
removed under reduced pressure and residue was purified by
flash chromatography on silica gel with dichloromethane/hexane
(1: 15) as eluent to give the titled compound as white solid
(3.25 g, 50.5%). 1H NMR (400 MHz, CDCl3),
d (ppm): 7.82 (s, 2H),
3.60 (t, J ¼ 4.0 Hz, 2H), 1.63 (m, 2H), 1.25 (m, 18H), 0.87 (t, J ¼ 6.8 Hz,
3H).
2.4.2. 1,3-Dibromo-5-dodecylthieno [3,4] pyrrol-4,6-dione (2)
N-Bromosuccinimide (NBS) (6.65 g, 37.33 mmol) was added to
the solution of compound 1 (3.0 g, 9.33 mmol), concentrated
sulfuric acid (12.0 mL) and trifluoroacetic acid (40 mL). The reaction
mixture was stirred at 55 ꢁC for 24 h. The brown solution was
poured into ice water (500 mL) and was extracted with dichloro-
methane. The organic layer was collected and was dried with
anhydrous sodium sulfate. Solvent was removed under reduced
pressure and residue was purified by flash chromatography on
silica gel with diethyl ether/dichloromethane/petroleum ether (1:
1: 15) as eluent to give the titled compound (2.79 g, 62.5%). 1H NMR
(3.61 g, 64.7%). 1H NMR (400 MHz, CDCl3),
d (ppm): 7.48 (d,
J ¼ 5.2 Hz, 2H), 7.36 (d, J ¼ 5.2 Hz, 2H), 4.27 (t, J ¼ 6.4 Hz, 4H),
1.86e1.92 (m, 4H), 1.54 (m, 4H), 1.23e1.37 (m, 32H), 0.88 (t,
J ¼ 6.4 Hz, 6H).
(400 MHz, CDCl3),
d
(ppm) 3.58 (t, J ¼ 7.2, 2H), 1.62 (m, 2H), 1.25 (m,
18H), 0.87 (t, J ¼ 6.8 Hz, 3H). 13C (100 MHz, CDCl3),
d
(ppm): 160.58,
2.4.6. 4,8-Di(2-ethylhexyloxy)benzo [1,2-b:4,5-b0] dithiophene (5)
Same procedure was used as for compound 4. Compound
used were benzo [1,2-b:4,5-b0] dithiophene-4,8-dione (2.20 g,
10.0 mmol), zinc power (1.96 g, 30.0 mmol), sodium hydroxide
(8.00 g, 200.0 mmol), 1-bromo-2-ethylhexane (5.79 g,
30.0 mmol) and a catalytic amount of tetra-n-butylammonium
bromide (0.032 g, 0.1 mmol). A light yellow oil was obtained
135.00, 113.13, 39.05, 32.14, 29.86, 29.80, 29.67, 29.57 (2C), 29.38,
28.48, 27.02, 22.92, 14.37.
2.4.3. 1,3-Bis(4-n-dodecylthiophene)-5-dodecylthieno [3,4] pyrrol-
4,6-dione (3)
5-Tri-n-butylstannyl-3-dodecylthiophene (5.64 g, 10.43 mmol)
and compound 2 (2.0 g, 4.17 mmol) were dissolved in tetrahydro-
furan (20 mL) in a pressure tube. The solution was degassed by
a nitrogen flow for 30 min. Then Pd2dba3 (0.076 g, 0.0834 mmol)
and tri-o-tolylphosphine (0.051 g, 0.167 mmol) were added into the
solution. The tube was capped and heated to 80 ꢁC for 36 h. After
cooled to room temperature, the mixture was poured into an
aqueous solution of potassium fluoride (100 mL, 1.0 M) and stirred
for 30 min. The mixture was extracted with dichloromethane (3x).
The combined organic layer was dried over anhydrous sodium
sulfate. After removing solvent, the residue was purified by column
chromatography on silica gel using petroleum ether/dichloro-
methane (30: 1) as eluent. A yellow solid was obtained after
removal of the solvent (2.2 g, 64.3%). 1H NMR (400 MHz, CDCl3),
(2.61 g, 58.2%). 1H NMR (400 MHz, CDCl3),
d (ppm): 7.49 (d,
J ¼ 5.2 Hz, 2H), 7.37 (d, J ¼ 5.2 Hz, 2H), 4.19 (d, J ¼ 5.6 Hz, 4H),
1.78e1.86 (m, 1H), 1.66e1.74 (m, 1H), 1.56e1.64 (m,
2H), 1.46e1.56 (m, 2H), 1.25e1.45 (m, 12H), 1.03 (t, J ¼ 7.2 Hz,
6H), 0.95 (t, J ¼ 7.2 Hz, 6H).
2.4.7. 2,6-Bis(trimethyltin)-4,8-didodecyloxybenzo [1,2-b:4,5-b0]
dithiophene (M2)
A solution of n-butyllithium (7.7 ml, 19.25 mmol, 2.5 M in
hexane) was added slowly to compound 4 (4.88 g, 8.75 mmol) in
tetrahydrofuran (120 mL) at ꢀ78 ꢁC. After addition, the mixture
was stirred for 1 h at ꢀ78 ꢁC. Trimethyltin chloride solution
(20.4 mL, 20.4 mmol, 1.0 M in hexane) was added to the mixture.
The mixture was warmed to room temperature and was stirred
overnight. The reaction was quenched with addition of water
(150 mL) and the mixture was extracted with diethyl ether for
three times. The combined organic layer was dried with anhy-
drous sodium sulfate. Solvent was removed under reduced pres-
sure and residue was purified by recrystallization in isopropanol
to afford a white solid (6.37 g, 82.2%). 1H NMR (400 MHz, CDCl3),
d
(ppm): 7.87 (s, 2H), 7.02 (s, 2H), 3.65 (t, J ¼ 7.6 Hz, 2H), 2.62
(t, J ¼ 7.6 Hz, 4H), 1.65 (m, 6H), 1.23e1.35 (m, 54H), 0.88 (t, J ¼ 6 Hz,
9H). 13C (100 MHz, CDCl3),
(ppm): 162.88, 145.14, 136.97, 132.35,
d
131.28, 128.23, 123.72, 38.80, 32.16, 30.68, 30.64, 29.92,
29.90, 29.85, 29.82, 29.60, 29.58, 29.54, 29.50, 28.78, 27.24, 22.93,
14.37.
2.4.4. 1,3-Bis(4-n-dodecyl-5-bromo-thiophene)-5-dodecylthieno
[3,4] pyrrol-4,6-dione (M1)
d
(ppm): 7.51 (s, 2H), 4.29 (t, J ¼ 6.5 Hz, 4H), 1.86 (m, 4H), 1.57 (m,
4H), 1.23e1.37 (m, 32H), 0.88 (t, J ¼ 6.8 Hz, 6H), 0.45 (s, 18H). 13C
Compound 3 (1.66 g, 2.02 mmol) and tetrahydrofuran (80 mL)
were added into a flask (250 mL) wrapped with aluminum foil. The
solution was stirred and degassed for 30 min. N-Bromosuccinimide
(NBS) (0.79 g, 4.45 mmol) was added in portions over a period of
30 min. The mixture was stirred at room temperature for 2 h, and
then the solution was poured into water (100 mL) and was
extracted with diethyl ether. The combined organic layer was dried
with anhydrous sodium sulfate. Solvent was removed under
reduced pressure and the residue was purified by column chro-
matography on silica gel with petroleum ether/dichloromethane
(30: 1) as eluent. After removing solvent, a yellow solid (1.28 g,
(100 MHz, CDCl3), d (ppm): 143.32, 140.68, 134.27, 133.20, 128.27,
73.87, 32.20, 30.77, 29.96, 29.92, 29.76, 29.63, 26.36, 22.99, 14.43,
ꢀ8.00.
2.4.8. 2,6-Bis(trimethyltin)-4,8-di(2-ethylhexyloxy)benzo
[1,2-b:4,5-b0] dithiophene (M3)
Same procedure was used as for compound M2. Compound
used were n-butyllithium (10.56 ml, 26.4 mmol, 2.5
M in
hexane), compound 5 (5.35 g, 12 mmol), trimethyltin chloride
solution (27.96 mL 27.96 mmol, 1M in hexane). After workup,
product was obtained as pale needle (7.07 g, 76.3%). 1H NMR
65.3%) was obtained. 1H NMR (400 MHz, CDCl3),
d
(ppm): 7.63 (s,
(400 MHz, CDCl3), d (ppm): 7.51 (s, 2H), 4.18 (d, 5.2 Hz, 4H),
2H), 3.64 (t, J ¼ 7.6 Hz, 2H), 2.57 (t, J ¼ 7.6 Hz, 4H), 1.62 (m, 6H),
1.78e1.86 (m, 1H), 1.66e1.74 (m, 1H), 1.56e1.64 (m, 2H),
1.46e1.56 (m, 2H), 1.28e1.45 (m, 12H), 1.02 (t, J ¼ 7.2, 6H), 0.92
1.20e1.35 (m, 54H), 0.87 (t, J ¼ 6.4 Hz, 9H). 13C (100 MHz, CDCl3),
d
(ppm): 162.64, 143.94, 135.66, 132.05, 130.45, 128.47, 113.76,
(t, J ¼ 7.2, 6H), 0.45 (s, 18H). 13C (100 MHz, CDCl3),
d (ppm):
38.90, 32.18, 32.16, 29.92, 29.89, 29.82, 29.75, 29.74, 29.62, 29.48,
29.61, 28.74, 27.22, 22.95, 14.38.
143.46, 140.81, 134.10, 133.10, 128.19, 75.84, 40.88, 30.75, 29.47,
24.12, 23.42, 14.45, 11.60, ꢀ8.11.

418
G. Zhang et al. / Polymer 52 (2011) 415e421
2.4.9. Polymer PBDTTPT1
Tris(dibenzylideneacetone)dipalladium (0.015 g, 0.016 mmol)
and triphenylarsine (0.0098 g, 0.032 mmol) were added to a solu-
tion of M1 (0.78 g, 0.80 mmol) and M2 (0.71, 0.80 mmol) in toluene
(20 mL) under nitrogen. The solution was subjected to three cycles
of evacuation and admission of nitrogen. The mixture was heated to
110 ꢁC for 72 h. After cooled to room temperature, the mixture was
poured into methanol (100 mL) and stirred for 2 h. A purple
precipitate was collected by filtration. The product was purified by
washing with methanol and hexane in a Soxhlet extractor for 24 h
each. The residue was extracted with hot chloroform in an extractor
for 24 h. After removing solvent, a purple solid was collected
(0.98 g, 89.5%). Mn ¼ 16.8 kDa; PDI ¼ 1.59. 1H NMR (400 MHz,
CDCl3),
d (ppm): 7.98 (br, 2H), 7.27 (br, 2H), 4.25 (br, 4H), 3.71(br,
2H), 2.5e2.7(br, 4H), 1.02e2.02 (br, 100H), 0.7e0.95 (br, 15H).
2.4.10. Polymer PBDTTPT2
PBDTDPT2 are synthesized according to the same procedure as
for PBDTDPT1. Compound used were Pd2dba3 (0.015 g, 0.016 mmol),
triphenylarsine (0.098 g, 0.032 mmol), M1 (0.78 g, 0.80 mmol) and
M3 (0.62 g, 0.80 mmol). After workup, a purple solid was obtained
(0.87 g, 86.3%). Mn ¼ 21.5 kDa; PDI ¼ 1.84.1H NMR (400 MHz, CDCl3),
Fig. 1. TGA curves of the polymer PBDTTPT1 and PBDTTPT2.
3.3. Optical property
d
(ppm): 7.95 (br, 2H), 7.28 (br, 2H), 4.15 (br, 4H), 3.65 (br, 2H),
The UVevis absorption spectra of the copolymers in chloroform
solution and as thin film were shown in Fig. 2. The optical
absorption properties of the copolymers were listed in Table 1. The
2.50e2.90 (br, 4H), 0.98e2.05 (br, 78), 0.70e0.95 (br, 21H).
3. Results and discussion
3.1. Synthesis of polymers
The synthetic routes for the monomers M1, M2, M3 and poly-
mers were illustrated in Scheme 1. The N-alkylated imide (1) was
reacted with an excess amount of N-bromosuccinimide (NBS) to
give the dibrominated imide (2) as a pale solid. The dibrominated
imide (2) was reacted with 5-tri-n-butylstannyl-3-dodecylth-
iophene to give 3 by Stille coupling reaction. The compound 3 was
halogenated by NBS in tetrahydrofuran to afford M1 in 65.3% yield.
The synthesis of benzo-dithiophenes were also outlined in Scheme
1. The tin containing monomers M2 and M3 were synthesized from
compound 5 and 6 by lithiumation with n-butyllithium and
quenching with trimethyltin chloride. The alternating polymer
PBDTTPT1 and PBDTTPT2 were prepared by Stille coupling reac-
tion with 1:1 monomer ratio in the presence of Pd2dba3 as catalyst
and triphenylarsine as ligand to give PBDTTPT1 (yield 89.5%) and
PBDTTPT2 (yield 86.3%). The crude copolymers were purified by
precipitating in methanol and by washing with methanol and
hexane in a Soxhlet. The number average molecular weights (Mn) of
the copolymers were 16.8 kDa and 21.5 kDa with a polydispersity of
1.59 and 1.84 for PBDTTPT1 and PBDTTPT2, respectively. Both
copolymers showed good solubility in common organic solvents,
such as tetrahydrofuran, dichloromethane and toluene.
3.2. Thermal stability
The thermal stabilities of copolymers were investigated by
thermogravimetric analysis (TGA). The TGA plots of PBDTTPT1, and
PBDTTPT2 were showed in Fig. 1. The point of five percent weight
loss was selected as the onset decomposition point. The thermal
decomposition temperatures for PBDTTPT1 and PBDTTPT2 were
determined around 352 ꢁC under N2 atmosphere. This indicates
that the two copolymers have adequate thermal stability for
application in polymer solar cells and other optoelectronic devices.
Neither polymers displayed noticeable glass transition in differ-
ential scanning calorimetry (DSC) analysis.
Fig. 2. Normalized UVevis spectra of PBDTTPT1 and PBDTTPT2 (a) in chloroform
solution and (b) as thin film.

G. Zhang et al. / Polymer 52 (2011) 415e421
419
Table 1
Optical and redox properties of PBDTTPT1 and PBDTTPT2.
Polymer
Solution
Film
Film
abs
max
abs
max
abs
onset
Egopt (eV)
E
(V)
HOMO (eV)a
LUMO (eV)c
ox
red
E
(V)b
onset
l
(nm)
l
(nm)
l
(nm)
onset
PBDTTPT1
PBDTTPT2
531
511
576
535
695
680
1.78
1.82
0.59
0.64
ꢀ5.30
ꢀ5.35
ꢀ1.19
ꢀ1.18
ꢀ3.52
ꢀ3.53
a
ox
HOMO ¼ ꢀð4:71 þ E
Þ.
onset
E
¼ E
ꢀ Egopt
.
b
c
red
ox
onset
onset
red
LUMO ¼ ꢀð4:71 þ E
Þ.
onset
PBDTDPT1 and PBDTDPT2 (both of 1.84 eV) [25]. Unlike other
reported system [26,27], incorporation of additional thiophene
moieties into BDT and DPT system did not show significant change
on the bandgaps of polymers. The steric hindrance of the alkyl side
chains on thiophenes may introduce additional distortion on the
main chain of polymers and disrupt conjugation.
3.4. Electrochemical property
The electrochemical properties of the copolymers were inves-
tigated by cyclic voltammetry (CV). The onset oxidations (HOMO
energy level) were determined by CV. The CV curves of two
copolymers film in acetonitrile containing tetra-n-butylammonium
hexafluorophosphate (0.1 M) at a potential scan rate of 100 mV/s
were shown in Fig. 3. The potentials were referenced to the ferro-
cene/ferrocenium redox couple (Fc/Fcþ). It was assumed that the
redox potential of Fc/Fcþ had an absolute energy level of ꢀ4.8 eV to
vacuum [32]. The potential of Fc/Fcþ was measured under the same
conditions and located at 0.09 V related to the Ag/Agþ electrode.
The CV data were summarized in Table 1. The onset oxidation
potential of PBDTTPT1 and PBDTTPT2 were located at 0.59 and
0.64 eV, respectively. The HOMO energy levels of copolymer
PBDTTPT1 and PBDTTPT2 were calculated to be ꢀ5.30 and
ꢀ5.35 eV. The HOMO energy levels of PBDTTPT1 and PBDTTPT2
are slightly higher than polymer PBDTDPTs (ꢀ5.42 and 5.44 eV)
[25]. The small increasing of HOMO energy levels was resulted from
the addition of electron rich alkyl-substituted thiophene units. The
similar results were reported before [27,33]. The relatively low
HOMO energy level of two copolymers are desirable for achieving
higher open-circuit voltage when they are used as donors blended
with PCBM in PSCs. The relative lower HOMO energy levels in these
new polymers can be attributed to electron deficient imide group
Fig. 3. Cyclic voltammograms of polymers thin film on
Bu4NPF6/MeCN (0.1 M) at a scan rate of 100 mV/s.
a platinum electrode in
maximal absorption of PBDTTPT1 and PBDTTPT2 were at 531 and
511 nm in solution, respectively. The two copolymers exhibited
broad absorption between 350 and 700 nm, which resulted from
intramolecular charge transfer (ICT) between the donor and
acceptor units. The film absorption spectra of copolymers were
depicted in Fig. 2b, the absorption maximums of PBDTTPT1 and
PBDTTPT2 solid films were at 576 and 535 nm, respectively. The
thin film absorption spectra of PBDTTPT1 and PBDTTPT2 were
broadened. The absorption maximums were red-shifted about 35
and 24 nm in thin film comparing with those of the solution
spectra. These indicated that the strong intermolecular interaction
and aggregation occurred in solid-state of those polymers. The
absorption spectra of PBDTTPT1 (with all straight side chains) were
red-shifted both in solution and as thin film compared with those
of PBDTTPT2 (with two ethylhexyl side chains). The branch alkyl
chains of PBDTTPT2 might prevent effective conjugation of the
polymer. The optical bandgap (Egopt) was calculated from the
absorption edge of solid-state film. The optical bandgap was 1.78 eV
(absorption edge at 695 nm) for PBDTTPT1 and 1.82 eV (absorption
edge at 680 nm) for PBDTTPT2. Only a slight decrease in bandgap
was observed compared with our previous reported polymer
Table 2
Devices performance of polymers solar cells.
Weight ratio
PBDTTPT1: PC71BM
PBDTTPT2: PC71BM
1:1
1:2
1:3
2:1
1:1
1:2
1:3
2:1
Jsc (mA/cm2)
Voc (V)
FF
3.04
0.89
0.62
1.68
2.06
0.88
0.60
1.09
1.73
0.88
0.58
0.89
2.18
0.92
0.53
1.08
3.93
0.92
0.57
2.05
3.12
0.93
0.56
1.62
2.20
0.93
0.56
1.14
2.49
0.95
0.50
1.18
Fig. 4. Current-voltage characteristics of polymers/PC71BM solar cells under illumi-
PCE (%)
nation of AM 1.5G, 100 mW/cm2.

420
G. Zhang et al. / Polymer 52 (2011) 415e421
Table 3
EQE of devices processed with DIO and without DIO were very
different in the region of 350e700 nm. The EQE values of devices with
DIO processing additives are much higher in this region and result the
higher Jsc.
Devices performance of polymers solar cells with and without processing additive.
PBDTTPT1:
PC71BM ¼ 1: 1
PBDTTPT2:
PC71BM ¼ 1: 1
No DIO
2% DIO
No DIO
2% DIO
Jsc (mA/cm2)
Voc (V)
FF
3.04
0.89
0.62
1.68
4.75
0.82
0.62
2.43
3.93
0.92
0.57
2.05
5.28
0.83
0.61
2.68
4. Conclusions
We have successfully synthesized two new low bandgap copol-
ymers by incorporation of additional alkylthiophene moieties into
the BDT and DPT system. The new polymers show good solubility
in common organic solvent, high thermal stability and broad
absorption in the region from 350 to 700 nm. The effect of incor-
poration of additional alkylthiophene units on the optical, electro-
chemical and photovoltaic properties of polymers were studied.
Only a slight decreasing in bandgap was observed in new polymers
compared with our previous reported copolymer PBDTDPTs. The
devices based on the blends of new copolymers and PC71BM
exhibited Voc of 0.82e0.95 V. The PCEs of devices with the copol-
ymer: PC71BM (wt. ratio of 1: 1) achieved 2.43% for PBDTTPT1 and
2.68% for PBDTTPT2 under the illumination of AM 1.5G, 100 mW/
cm2. The results indicate that these new copolymers with high Voc
might become useful materials for organic solar cells applications.
PCE (%)
Acknowledgement
This work was supported by National Nature Science Founda-
tion of China (NSFC Grant no. 20674049 and Grant no. 20834005),
and Shanghai municipal government (Grant no. B202 and Grant no.
10ZZ15).
Fig. 5. External quantum efficiency of polymer/PC71BM blends.
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