Synthesis and photovoltaic properties of new conjugated polymers based on di(2-furyl)thiazolo[5,4-d]thiazole and benzo[1,2-b:4,5-b′]dithiophene

Article abstract of DOI:10.1016/j.polymer.2012.10.025

Di(2-furyl)thiazolo[5,4-d]thiazole (DFTT) has been studied as building block for conjugated polymers. New polymer PBDTODFTT and PBDTTDFTT based on DFTT and two different benzo[1,2-b:4,5-b′]dithiophene (BDT) repeating units have been synthesized and applied as donors in bulk hetero-junction polymer solar cell devices. The polymer PBDTODFTT and PBDTTDFTT have optical bandgap of 2.00 eV and 1.98 eV, and show HOMO energy level of -5.46 eV and -5.36 eV, respectively. The solar cell devices based on PBDTTDFTT:PC₆₁BM blend as active layers have achieved an open-circuit voltage of 0.83 V and a power conversion efficiency of 3.06% under the illumination of AM 1.5 G (100 mW/cm²).

Full text of DOI:10.1016/j.polymer.2012.10.025

Polymer 54 (2013) 1098e1105
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and photovoltaic properties of new conjugated polymers based
on di(2-furyl)thiazolo[5,4-d]thiazole and benzo[1,2-b:4,5-b0]dithiophene
,
a,
Chao Hu a, Zhongwei Wu b, Kangli Cao a, Baoquan Sun b , Qing Zhang
*
*
a School of Chemistry and Chemical Engineering, Shanghai Jiaotong University, 800 Dongchuan Road, Shanghai 200240, China
b Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren’ai Road,
Suzhou 215123, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Di(2-furyl)thiazolo[5,4-d]thiazole (DFTT) has been studied as building block for conjugated polymers.
New polymer PBDTODFTT and PBDTTDFTT based on DFTT and two different benzo[1,2-b:4,5-b0]
dithiophene (BDT) repeating units have been synthesized and applied as donors in bulk hetero-junction
polymer solar cell devices. The polymer PBDTODFTT and PBDTTDFTT have optical bandgap of 2.00 eV
and 1.98 eV, and show HOMO energy level of ꢀ5.46 eV and ꢀ5.36 eV, respectively. The solar cell devices
based on PBDTTDFTT:PC61BM blend as active layers have achieved an open-circuit voltage of 0.83 V and
a power conversion efficiency of 3.06% under the illumination of AM 1.5 G (100 mW/cm2).
Ó 2013 Published by Elsevier Ltd.
Received 11 May 2012
Received in revised form
12 September 2012
Accepted 12 October 2012
Available online 8 November 2012
Keywords:
Conjugated polymers
Furan
Furfural
1. Introduction
significantly improved solubilities [23,30]. Recently, polymers
based on bis(3-alkyithiophen-2-yl)thiazolo[5,4-d]thiazole (BTTT)
Polymer solar cells (PSCs) with advantages such as low cost and
solution fabrication are considered as one of the most promising
technologies for harvesting solar energy in the future [1e8]. Low
bandgap conjugated polymers have attracted much attention for
applications as donor materials in PSC devices recently [9,10]. Many
low bandgap polymers have been synthesized and have achieved
high power conversion efficiencies (PCEs) in bulk hetero-junction
(BHJ) PSC devices [5,6,11e19]. The development of new mono-
mers can lead to synthesis of polymers with tailor-made photo-
physical and electrochemical properties, and may result in new
materials with high performances in PSC devices [4,20e22]. One of
the current developments is using furans as repeating units for the
synthesis of organic semiconducting materials [23]. Furan contains
only the first row elements. Oligofurans have shown tighter
packing and smaller reorganization energy than their thiophene
counterparts [24e29]. The low bandgap conjugated polymers
based on furan as repeating units have shown comparable PSC
device performances as their thiophene analogous and with
and various electron-rich repeating units have been synthesized for
PSC applications [31e34]. Thiazolo[5,4-d]thiazole (TT) consists of
electron-deficient and fused heterocyclic rings which enable
extended intramolecular
p-conjugation and improve intermolec-
ular stacking. Hence, TT based conjugated oligomers and polymers
exhibited relatively high charge carrier mobilities [35,36] and also
displayed relatively good performances in PSC devices [37]. Herein,
we report the synthesis of new conjugated polymers based on 2,5-
di(fur-2-yl)thiazolo[5,4-d]thiazole (DFTT) which combines furans
and thiazolo[5,4-d]thiazole, and the photovoltaic properties of new
polymers.
The DFTT can be synthesized from furfural which is an indus-
trial commodity. The furfural production has spread throughout
the world and its precursors range from agriculture and forestry
waste. It has been studied extensively as renewable feedstock for
polymer industry [38,39]. Alkoxy-substituted benzo[1,2-b:4,5-b0]
dithiophene (BDTO) and 2,3-dialkylthiophene-substituted benzo
[1,2-b:4,5-b0]dithiophene (BDTT) have been chosen as electron-
rich units for new conjugated polymers. The BDTs possess rigid
planar structures with extended
p-conjugation. They facilitate p-
electron delocalization in polymer chains and improve
pep
interactions between polymer chains in solid states [40e42]. It
has been found that HOMO energy levels of BDT based conjugated
* Corresponding authors.
E-mail addresses: bqsun@suda.edu.cn (B. Sun), qz14@sjtu.edu.cn (Q. Zhang).
0032-3861/$ e see front matter Ó 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.polymer.2012.10.025

C. Hu et al. / Polymer 54 (2013) 1098e1105
1099
polymers can be fine-tuned by using different substituents on the
central phenyl ring [43]. The BDTT building block has thienyl
groups on the central phenyl ring to provide additional conjuga-
tion. The polymers based on BDTT have shown improved absorp-
tion properties, high charge carrier mobility and deep HOMO
energy position compared with the polymers based on alkoxy-
substituted BDTO [6,43]. Two new polymers, PBDTODFTT and
PBDTTDFTT, have been synthesized and their photovoltaic prop-
erties have been studied in BHJ PCS devices with PC61BM as
acceptors. The devices based on PBDTTDFTT have showed an
open-circuit voltage of 0.83 V and a PCE of 3.06% under the illu-
mination of AM 1.5 G (100 mW/cm2).
For device JeV characterization, a Newport 94023A solar
simulator equipped with a 450 W Xenon lamp and an air mass (AM)
1.5 filter were used to generate simulated AM 1.5 solar spectrum
irradiation source. The irradiation intensity was 100 mw/cm2 cali-
brated by a Newport standard silicon solar cell 91150. A Newport
monochromator 74125 and a power meter 1918 with silicon
detector 918D were used in the EQE measurements. All electrical
data were recorded by a Keithley 2612.
2.3. Materials
Chemicals were purchased from SigmaeAldrich Chemical
Company, Alfa Aesar Chemical Company, Sinopharm Chemical
Reagent Co. Ltd. and Darui Chemical Co. Ltd. Tetrahydrofuran and
toluene were freshly distilled over sodium wire under nitrogen
prior to use. Other materials were used without further
purification.
2. Experiment section
2.1. General information
Nuclear Magnetic Resonance (NMR) spectra were recorded on
a Mercury plus 400 MHz machine. Gel permeation chromatography
(GPC) measurements were performed on a Waters 1515 series GPC
coupled with UVevis detector using tetrahydrofuran as eluent with
polystyrenes as standards. Thermogravimetric analyses (TGA) were
conductedꢀw1 ith a TA instrument QS000IR at a heating rate of
20 ꢁC min under nitrogen gas flow. Differential scanning calo-
rimetry (DSC) analysis was performed on a Perkin Elmer instru-
ment Pyris 1 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 at rate of 20 ꢁC/
min to 20 ꢁC and then heating at rate of 20 ꢁC/min to 300 ꢁC. UVevis
2.4. Synthesis of monomers and polymers
2.4.1. Synthesis of 2,5-di(fur-2-yl)thiazolo[5,4-d]thiazole (DFTT)
[43,44]
A solution of furural (36.1 g, 329 mmol), dithiooxamide (3.61 g,
30.0 mmol) and phenol (10.0 g) was heated to 210 ꢁC for 1 h with
stirring. After cooling to room temperature, the precipitate was
collected through filtration, and was washed thoroughly with
ethanol, ether and hexane successively. The crude product was
recrystallized from chloroform to give the titled product as a brown
solid (3.78 g, 46%). 1H NMR (CDCl3, 400 MHz,
d, ppm): 7.56 (d,
J ¼ 3.6 Hz, 2H), 7.08 (d, J ¼ 3.6 Hz, 2H), 6.58 (m, 2H).
absorption spectra were recorded on a Perkin Elmer model
l 20
UVevis spectrophotometer. Electrochemical measurements were
conducted on a CS-150 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 refer-
ence electrode. Thin film of polymer was coated on the surface of
platinum electrode and ferrocene was chosen as a reference.
2.4.2. Synthesis of 2,5-bis(5-bromofur-2-yl)thiazolo[5,4-d]thiazole
(1)
N-bromosuccinimide (1.48 g, 8.33 mmol) was added into
a solution of DFTT (1.00 g, 3.64 mmol) in DMF (30.0 mL) with stir at
0
ꢁC. The mixture was heated at 90 ꢁC for 1 h and was cooled to
room temperature. The precipitate was collected by filtration and
was washed with water. The residue was recrystallized from
chloroform to give the titled product as a yellow solid (1.38 g, 88%).
1H NMR (CDCl3, 400 MHz,
d
, ppm): 7.03 (d, J ¼ 3.6 Hz, 2H), 6.52 (d,
2.2. Device fabrication and characterization
J ¼ 3.6 Hz, 2H). Anal. Calcd for C12H4Br2N2O2S2 (%): C, 33.35; H,
0.93; N, 6.48. Found: C, 33.58; H, 1.08; N, 6.53.
The photovoltaic properties of polymers were investigated in
the devices with structure of ITO/PEDOT:PSS/Polymer:PC61BM/LiF/
Al. A ca. 45 nm poly(3,4-ethylenedioxylenethiophene):polystyrene
sulfonic acid (PEDOT:PSS) (Baytron P VP AI 4083) layer was spin-
coated on the ITO-coated glass which was pretreated by ultravi-
olet ozone plasma for 15 min. After annealed at 140 ꢁC for 10 min,
the substrates were then taken into a nitrogen-filled glove box for
active layer coating and electrode deposition. The solutions of
polymers with concentration of 10 mg/mL were chosen for active
layer depositions. The solutions of polymers and PC61BM (with
different weight ratios) in chloroform (CF), chlorobenzene (CB) and
mixed solvents (CB and dichlorobenzene) were spin-casted onto
the PEDOT:PSS layer. The thickness of active layer was controlled
by rotational speed of spin coater. After optimization (Table S1),
the rotational speed of 1000 rpm was chosen for spin-casting in
this work. The thickness of active layer fabricated under this
rotational speed was about 65 nm. Then 0.6 nm LiF layer and
100 nm Al layer were evaporated through a shadow mask under
10ꢀ6 Torr. The area of each device was 7.25 mm2 determined by the
shadow mask. The devices fabricated with processing additive of
1,8-diiodoctane (DIO) were made by same method and the pro-
cessing additive was added to the solutions of polymers and
PC61BM before spin-casting.
2.4.3. Synthesis of 2,6-bis(trimethyltin)-4,8-di(2,3-
dihexylthiophen-5-yl)-benzo[1,2-b:4,5-b0]dithiophene (2) [45]
A solution of n-butyllithium (2.89 mL, 6.36 mmol, 2.2 M in
hexane) was added dropwise to 4,8-di(2,3-dihexylthiophen-5-yl)-
benzo[1,2-b:4,5-b0]dithiophene (BDTT) (2.00 g, 2.89 mmol) in
tetrahydrofuran (100.0 mL) at ꢀ78 ꢁC. After addition, the mixture
was warmed to room temperature and was stirred for 1 h. The
reaction mixture was cooled to 0 ꢁC and trimethyltin chloride
(6.4 mL, 6.4 mmol, 1.0 M in hexane) was added. The reaction
mixture was stirred at 0 ꢁC for 1 h and was kept at room temper-
ature overnight. The mixture was poured into water (100 mL) and
extracted with diethyl ether (100 mL) for three times. The
combined organic layer was dried with sodium sulfate and was
concentrated under reduced pressure. The crude product was
recrystallized from iso-propanol to afford yellow crystals (1.94 g,
66%). 1H NMR (CDCl3, 400 MHz,
d, ppm): 7.73 (s, 2H), 7.23 (s, 2H),
2.84 (t, J ¼ 8.0 Hz, 4H), 2.63 (t, J ¼ 8.0 Hz, 4H), 1.63e1.80 (m, 8H),
1.32e1.51 (m, 24H), 0.92 (m, 12H), 0.40 (s, 18H). 13C NMR (CDCl3,
100 MHz, d, ppm): 143.29, 142.17, 140.04, 138.22, 137.36, 135.94,
131.63, 130.04, 122.69, 32.11, 32.08, 30.04, 31.93, 31.05, 29.42, 28.66,
28.26, 22.97, 22.92, 14.46, 14.41, ꢀ8.10. Anal. Calcd for C48H74S4Sn2
(%): C, 56.70; H, 7.34. Found: C, 56.83; H, 7.43.

1100
C. Hu et al. / Polymer 54 (2013) 1098e1105
2.4.4. Synthesis of 2,6-bis(trimethyltin)-4,8-di(2-octyldodecyloxy)
benzo-[1,2-b:4,5-b0]dithiophene (3) [46]
was collected (0.38 g, 72.3%). 1H NMR (CDCl3, 400 MHz,
d, PPM):
6.20e7.80 (br, 6H), 4.25 (br, 4H), 0.70e2.10 (br, 78 H). Anal. Calcd for
C62H88N2O4S4 (%): C, 70.68; H, 8.42; N, 2.66. Found: C, 70.01; H,
8.35; N, 2.59. GPC: Mn 22.4 kDa; Mw 55.1 kDa; Mw/Mn 2.46.
A solution of n-butyllithium (3.19 mL, 7.02 mmol, 2.2 M in
hexane) was added dropwise to 4,8-di(2-octyldodecyloxy)-benzo
[1,2-b:4,5-b0]dithiophene (BDTO) (2.50 g, 3.19 mmol) in tetrahy-
drofuran (60 mL) at ꢀ78 ꢁC. After addition, the mixture was stirred
for 1 h at ꢀ78 ꢁC. Trimethyltin chloride (7.10 mL, 7.10 mmol, 1.0 M in
hexane) was added to the mixture. The mixture was warmed to
room temperature and was stirred for 2 h. The reaction was
quenched with addition of water (100 mL) and the mixture was
extracted with diethyl ether (60 mL) for three times. The combined
organic layer was dried with anhydrous sodium sulfate. Solvent
was removed under reduced pressure and the residue was purified
by recrystallization in iso-propanol to afford a white solid (2.87 g,
2.4.6. Synthesis of polymer PBDTTDFTT
Same procedure was used as for polymer PBDTODFTT.
Compounds used were tris(dibenzylideneacetone)dipalladium
(0.0092 g, 0.01 mmol), tri-o-tolylphosphine (0.0122 g, 0.04 mmol),
compound 1 (0.2161 g, 0.50 mmol) and compound 2 (0.5084 g,
0.50 mmol). The crude polymer was purified by washing with
methanol and hexane in a Soxhlet extractor for 24 h each. It was
extracted with hot chloroform in an extractor for 24 h and the
soluble material was collected. After removing solvent, a purplish
81%). 1H NMR (400 MHz, CDCl3,
J ¼ 5.0 Hz, 4H), 1.86 (m, 2H), 1.66 (m, 8H), 1.23e1.56 (m, 56H), 0.89
(m, 12H), 0.45 (s, 18H). 13C NMR (100 MHz, CDCl3,
, PPM): 143.45,
d, ppm): 7.52 (s, 2H), 4.19 (d,
red solid was obtained (0.10 g, 20.8%). 1H NMR (CDCl3, 400 MHz,
d,
PPM): 6.20e8.20 (br, 8H), 2.50e3.20 (br, 8H), 0.60e2.10 (br, 44 H).
Anal. Calcd for C54H60N2O2S6 (%): C, 67.46; H, 6.29; N, 2.91. Found: C,
67.52; H, 6.57; N, 2.52. GPC: Mn 7.87 kDa; Mw 13.0 kDa; Mw/Mn 1.65.
d
140.58, 134.10, 133.13, 128.20, 76.13, 39.42, 32.20, 31.62, 30.45,
30.06, 30.03, 30.00, 29.96, 29.69, 29.65, 27.32, 22.97, 14.43,
14.41, ꢀ8.10.
3. Results and discussion
2.4.5. Synthesis of polymer PBDTODFTT
3.1. Synthesis and characterization
Tris(dibenzylideneacetone)dipalladium (0.0092 g, 0.01 mmol),
tri-o-tolylphosphine (0.0122 g, 0.04 mmol) were added to a solu-
tion of compound 1 (0.2161 g, 0.50 mmol) and compound 3
(0.5545 g, 0.50 mmol) in toluene (15.0 mL) under nitrogen. The
solution was subjected to three cycles of evacuation and admission
of nitrogen. The mixture was heated to 110 ꢁC for 48 h. After cooling
to room temperature, the mixture was poured into methanol
(100 mL) and was stirred for 1 h. The precipitate was collected by
filtration. The crude product was purified by washing with meth-
anol and hexane successively in a Soxhlet extractor for 24 h each. It
was extracted with hot chloroform in a Soxhlet extractor for 24 h.
After removing solvent under reduced pressure, a purplish red solid
The synthetic routes for monomers are shown in Scheme 1. 2,5-
di(furan-2-yl)thiazolo[5,4-d]thiazole was synthesized in 46% yield
by reaction of furural and dithiooxamide [43,44]. The DFTT was
dibrominated with N-bromosuccinimide (NBS) in N,N-dime-
thylformamide (DMF) with 88% yield. The BDTO, BDTT and the
corresponding ditin monomers were synthesized according to the
literature methods [45,46]. Polymer PBDTODFTT and PBDTTDFTT
were synthesized by Stille cross-coupling reaction in presence of
tris(dibenzylideneacetone)dipalladium as catalyst and tri-o-tolyl-
phosphine as ligand (Scheme 2). The polymers were purified by
washing with methanol and hexane successively in a Soxhlet
Scheme 1. Synthesis of monomers.

C. Hu et al. / Polymer 54 (2013) 1098e1105
1101
Scheme 2. Synthesis of polymers.
extractor for 24 h each, and then were extracted with hot chloro-
form in a Soxhlet extractor for 24 h. After removing solvent under
reduced pressure, purplish red solids were collected. Polymer
PBDTODFTT showed better solubility than polymer PBDTTDFTT
because of the large branched alkyl chains. PBDTODFTT was
soluble in common organic solvent such as dichloromethane,
chloroform, toluene, chlorobenzene and dichlorobenzene. Polymer
PBDTTDFTT was soluble in chloroform and chlorobenzene.
dialkylthienyl group. The similar phenomenon had been reported
before [42].
3.4. Electrochemical properties
The cyclic voltammograms of the polymers are displayed in
Fig. 2, and the electrochemical properties are summarized in
Table 2. Both polymers showed profiles with semi-reversible oxide
cycles and semi-reversible reduce cycles. The HOMO and LUMO
levels were estimated from the onset oxidation and reduction
potentials, respectively. The HOMO energy levels of PBDTODFTT
and PBDTTDFTT were at ꢀ5.46 eV and ꢀ5.36 eV, respectively. The
LUMO energy levels of PBDTODFTT and PBDTTDFTT were
at ꢀ2.86 eV and ꢀ2.91 eV, respectively. The HOMO energy level of
the polymer based on alkoxy-substituted BDT was slightly lower
than the polymer based on thienyl-substituted BDT in current
system. This result was different from previous report [42].
3.2. Thermal stability
Thermogravimetric analyses of the polymers are shown in
Fig. S9 (see SI). Both polymers showed good thermal stabilities.
PBDTODFTT had a decomposition temperature over 300 ꢁC, and
PBDTTDFTT had a decomposition temperature over 400 ꢁC. The
relatively poor thermal stability of PBDTODFTT was due to the
alkoxy groups in the polymer [47,48]. Neither polymers displayed
noticeable glass transition in differential scanning calorimetry
(DSC) analysis.
3.5. Photovoltaic properties
3.3. Optical properties
PSC devices with polymer PBDTODFTT and PBDTTDFTT as
electron donors and PC61BM as electron acceptor were fabricated.
The device structure was ITO/PEDOT:PSS/polymer:PC61BM/LiF/Al.
The optimized active layer thickness was chosen as 65 nm (spin-
coated at 1000 rpm) (Table S1). The devices were optimized with
different processing solvents (Table 3 and Table S2). Chloroform
was found to be the best solvent for processing these polymers. The
weight ratio of polymer to PC61BM in active layer was adjusted to
optimize the PV performance (Table 3). The optimized device based
on PBDTODFTT had a thickness of 65 nm and polymer/PC61BM
weight ratio of 1:1. The device gave a short circuit current (Jsc) of
1.4 mA/cm2, a fill factor (FF) of 0.46, an open-circuit voltage (Voc) of
0.84 V and resulted in a PCE of 0.54%. The optimized device based
on PBDTTDFTT had a thickness of 65 nm and polymer/PC61BM
weight ratio of 1:1. The device gave a Jsc value of 6.97 mA/cm2, a FF
value of 0.46, an Voc value of 0.86 V and resulted in a PCE of 2.52%.
The UVevis absorption spectra of polymers with same
concentration in dilute chloroform solution are shown in Fig. 1 top
and the spectra of thin films are shown in Fig. 1 bottom, and the
optical properties are summarized in Table 1. The onset absorption
edges of the dilute polymer solutions (6.25 ꢂ 10ꢀ6 M) were at
605 nm and 600 nm for PBDTODFTT and PBDTTDFTT, respectively.
The onset absorption edges of the thin films were at 620 nm and
625 nm for PBDTODFTT and PBDTTDFTT, respectively. The onset
absorptions of both polymers were slight red shift in thin films
compared with those of solutions. The red shifts of thin film
spectra indicated interchain aggregations in solid state. Although
the two polymers had similar bandgap, PBDTTDFTT had a broad-
ened absorption spectra compared with PBDTODFTT, especially in
the range of 300 nme500 nm. The improved absorption in that
region resulted from the replacement of alkoxy chain with

1102
C. Hu et al. / Polymer 54 (2013) 1098e1105
Fig. 2. Cyclic voltammograms of PBDTODFTT and PBDTTDFTT films in acetonitrile/
0.1 M [n-Bu4N]þ[PF6]ꢀ with scan rate of 50 mV sꢀ1
.
a device with an active layer thickness of 65 nm, polymer/PC61BM
weight ratio of 1:1, chloroform as the solvent for spin-coating,
and addition of DIO (2.0% by volume) as processing additive
showed the best device performance. The optimized device gave an
Voc value of 0.83 V, a Jsc value of 7.67 mA/cm2, a FF of 48% and
resulted in a PCE value of 3.06%. The currentevoltage curves and
external quantum efficiency (EQE) curves for the devices based on
PBDTTDFTT are displayed in Fig. 4. The addition of processing
additive increased the EQE value of the device in the region of 325e
600 nm. This corresponded to the improvement in Jsc value and PCE
of the devices.
The relatively higher performances of polymer PBDTTDFTT
based devices than polymer PBDTODFTT based devices mainly
resulted from higher Jsc values of PBDTTDFTT based devices than
those of PBDTODFTT based devices. The absorption spectra of
PBDTTDFTT were broadened compared to that of PBDTODFTT,
especially in the 300 nme500 nm region. The absorption spectra
and the EQE curves of the D/A blends are displayed in Fig. S10.
The better absorption of PBDTTDFTT likely resulted in higher Jsc,
and thus larger PCE.
Fig. 1. UVevis absorption spectra of PBDTODFTT and PBDTTDFTT in chloroform
solution (6.25 ꢂ 10ꢀ6 M) (top) and as thin films (bottom).
The effect of processing additive on the device performance was
also studied. The results are summarized in Table 3 and Table S1. For
the PBDTODFTT polymer, a device with an active layer thickness of
65 nm, polymer/PC61BM weight ratio of 1:1, chloroform as the
solvent for spin-coating, and addition of DIO (1.0% by volume) as
processing additive showed the best device performance. The
optimized device gave an Voc of 0.75 V, a Jsc of 5.2 mA/cm2, a FF of
48%, and resulted in a PCE of 1.87%. The currentevoltage curves and
external quantum efficiency (EQE) curves for the devices based on
PBDTODFTT are displayed in Fig. 3. The addition of processing
additive significantly increased the EQE value of the device in the
region of 300e600 nm. This resulted in large improvement on Jsc
value and PCE of the devices. For the PBDTTDFTT polymer,
Hole-only devices were fabricated with the configuration of ITO/
PEDOT:PSS/polymer/MoO3/Al. The hole mobilities were measured
by space charge limited current (SCLC) method (details in SI). The
hole mobilities of PBDTODFTT/PC61BM and PBDTTDFTT/PC61BM
were 1.95 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1 and 3.05 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1
,
respectively. It was reported that polymers with alkylthienyl-
substituted repeating units such as BDTT had higher hole mobil-
ities than polymers with alkoxy-substituted repeating units such as
BDTO [36]. The high hole mobility of PBDTTDFTT may also
contribute to the improvement of device performances.
Table 1
Table 2
Optical properties of polymers.
Electrochemical properties of polymers.
Copolymers
lonset (nm)
Eogpt (eV)b
Eoonxset (V)/HOMO (eV)a
E
(V)/LUMO (eV)b Egelectrochem (eV)c
onset
Copolymer
red
in solutiona
as film
PBDTODFTT 0.75/ꢀ5.46
PBDTTDFTT 0.65/ꢀ5.36
ꢀ1.85/ꢀ2.86
ꢀ1.80/ꢀ2.91
2.60
2.45
PBDTODFTT
PBDTTDFTT
605
600
620
625
2.0
1.98
a
b
c
onset
HOMO ¼ ꢀe(E
þ 4.71).
þ 4.71).
ox
onset
a
In chloroform solution.
LUMO ¼ ꢀe(E
red
onset onset
b
Eogpt ¼ 1240/lonset (in film).
E
¼ e(E
ꢀ Eorendset).
red
ox

C. Hu et al. / Polymer 54 (2013) 1098e1105
1103
Table 3
Solar cell characteristic of polymers: PC61BM blends.
Copolymers Device
Polymer/
Voc (V) Jsc (mA/m2) FF PCE (%)
Solution DIO ratio
(vol%)
PC61BM w/w
PBDTODFTT 1:1
CF
CF
CF
CF
CF
CF
CF
0
1
2
0
0
0
2
0.84
0.75
0.74
0.86
0.86
0.73
0.83
1.4
5.2
0.46 0.54
0.48 1.87
0.43 1.57
0.35 1.54
0.42 2.52
0.40 1.90
0.48 3.06
1:1
1:1
4.93
5.13
6.97
6.51
7.67
PBDTTDFTT 1:0.6
1:1
1:2
1:1
The devices based on PBDTODFTT and PBDTTDFTT showed
similar Voc. The Voc of BHJ PCS devices with PC61BM as acceptor is
related to the difference between the HOMO level of the donor
polymers and the LUMO level of the PC61BM. Two new polymers
displayed almost same HOMO energy levels, and thus resulted in
similar Voc. However, the addition of DIO brought a decreased Voc
with different degree in devices based on PBDTODFTT and
PBDTTDFTT respectively. This phenomena which associated with
processing additive also emerged in some other systems [49,50],
Fig. 4. (Top) JeV curves and (bottom) EQE spectra of PSC devices based on
PBDTTDFTT/PC61BM (1:1) fabricated from chloroform before and after addition of DIO
(2% by volume).
and can be ascribed to the strong interchaineinterlayer interaction
caused by the addition of DIO, leading to a reduced effective
“bandgap” relative to the HOMOeLUMO difference [51].
The devices based on new conjugated polymer PBDTTDFTT
showed similar Voc, but low FF, low Jsc and low PCE compared with
devices based on previous reported thiophene analogue, PBDTTTZ
[37]. However, the rigorous comparison between the device
performance of new polymer PBDTTDFTT and that of PBDTTTZ is
not possible because the device fabrication processes are different
in two works. The different device fabrication processes can affect
the device performance.
3.6. Morphology of the active layer
Control over the morphology is crucial for the performance of
bulk hetero-junction solar cells. The morphologies of active layer
with PBDTTDFTT/PC61BM weight ratio (1:1) processed from chlo-
roform without and with addition of DIO (2.0% by volume) as
processing additive were investigated by atomic force microscopy
(AFM) (Fig. 5). The images of blended films without and with DIO
additive showed root mean square roughness of 0.51 nm and
0.68 nm, respectively. Addition of DIO promoted phase separation
in the PBDTTDFTT/PC61BM blend.
Fig. 3. (Top) JeV curves and (bottom) EQE spectra of PSC devices based on
PBDTODFTT/PC61BM (1:1) fabricated from chloroform before and after addition of DIO
(1% by volume).

1104
C. Hu et al. / Polymer 54 (2013) 1098e1105
Fig. 5. AFM topography images (a, c) and phase images (b, d) of the PBDTTDFTT:PC61BM (1:1, w/w) film processed from chloroform without and with DIO (2% by volume). The size
of images is 1 m.
m ꢂ 1
m
m
4. Conclusions
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2012.10.025.

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