Syntheses and characterization of carbazole based new low-band gap copolymers containing highly soluble benzimidazole derivatives for solar cell application

Article abstract of DOI:10.1002/pola.24435

Newly designed 2H-benzimidazole derivatives which have solubility groups at 2-position have been synthesized and incorporated into two highly soluble carbazole based alternating copolymers, poly[2,7-(9-(1′-octylnonyl)-9H- carbazole)-alt-5,5-(4′,7′-di(thie

Full text of DOI:10.1002/pola.24435

Syntheses and Characterization of Carbazole Based New Low-Band
Gap Copolymers Containing Highly Soluble Benzimidazole
Derivatives for Solar Cell Application
JAEHONG KIM,1 SUNG HEUM PARK,2 JINWOO KIM,1 SHINUK CHO,3* YOUNGEUP JIN,4 JOO YOUNG SHIM,1
HYUNMIN SHIN,1 SOONCHEOL KWON,2 IL KIM,5 KWANGHEE LEE,2 ALAN J. HEEGER,3 HONGSUK SUH1
1Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University,
Busan 609-735, Republic of Korea
2Department of Material Science and Engineering, Gwangju Institute of Science and Technology,
Gwangju 500-712, Republic of Korea
3Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California 93106
4Department of Industrial Chemistry, Pukyong National University, Busan 608-739, Korea
5The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of polymer Science and Engineering,
Pusan National University, Busan 609-735, Korea
Received 12 June 2010; accepted 3 October 2010
DOI: 10.1002/pola.24435
Published online 23 November 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Newly designed 2H-benzimidazole derivatives which
have solubility groups at 2-position have been synthesized and
incorporated into two highly soluble carbazole based alternating
copolymers, poly[2,7-(9-(10-octylnonyl)-9H-carbazole)-alt-5,5-(40,70-
di(thien-2-yl)-2H-benzimidazole-20-spirocyclohexane)] (PCDTCHBI)
and poly[2,7-(9-(10-octylnonyl)-9H-carbazole)-alt-5,5-(40,70-di(thien-
2-yl)-2H-benzimidazole-20-spiro-400-((2000-ethylhexyl)oxy)-cyclohex-
ane)] (PCDTEHOCHBI) for photovoltaic application. These alter-
nating copolymers show low-band gap properties caused by
internal charge transfer from an electron-rich unit to an electron-
deficient moiety. HOMO and LUMO levels are –5.53 and –3.86 eV
for PCDTCHBI, and –5.49 and –3.84 eV for PCDTEHOCHBI, respec-
tively. Optical band gaps of PCDTCHBI and PCDTEHOCHBI are
1.67 and 1.65 eV, respectively. The new carbazole based the 2H-
benzimidazole polymers show 0.11–0.13 eV lower values of band
gaps as compared to that of carbazole based benzothiadiazole
polymer, poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-
thienyl-20,10,30-benzothiadiazole)] (PCDTBT), while keeping nearly
the same deep HOMO levels. The power conversion efficiencies
of PCDTCHBI and PCDTEHOCHBI blended with [6,6]phenyl-C71
-
butyric acid methyl ester (PC71BM) are 1.03 and 1.15%, respec-
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tively. 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 49: 369–380, 2011
KEYWORDS: 2H-benzimidazole; conjugated polymers; copolymer-
ization; low-band gap polymer; polymer solar cell; thin films
INTRODUCTION Polymer solar cell is one of most focused al-
ternative energy technology for its environmental friendly
characteristics and easy fabrication of devices compared to
the silicon based solar cell. The key technologies of polymer
solar cell are the use of the bulk heterojunction (BHJ) con-
cept,1 mixture of conjugated polymer and fullerene deriva-
tive, and solution process. The BHJ concept provided the
enhancement of power conversion efficiency (PCE) of poly-
mer solar cell and solution process can make polymer solar
cell devices thin, flexible, portable, and low-costed.
performance, the large band gap of P3HT (ꢀ1.9 eV) is not
ideal material for polymer solar cell to improve the solar
spectrum harvest. For the enhanced solar spectrum harvest,
low-band gap polymers, obtained by internal charge transfer
(ICT) from an electron-rich unit to an electron-deficient
moiety, have been used to achieve the high PCE. One of the
most successful low-band gap polymers with ICT strategy
is poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0]-
dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) which
shows ꢀ5.5% PCE with high carrier mobility.6,7
Over the last decade, regioregular poly(3-hexylthiophene)
(P3HT) was intensively investigated for its high carrier
mobility,2 and through the morphology control of P3HT/
[6,6]phenyl-C61-butyric acid methyl ester (PC61BM), PCE
approached near 5%.3–5 Despite P3HT/PCBM show good
In addition to the carrier mobility, the other important factor,
concerned with PCE, is the open circuit voltage (Voc).
Scharber et al. suggested existence of linear relationship
between Voc and the conjugated polymer oxidation potential,
supported by experimental results of 26 conjugated
*Present address: Department of Physics, University of Ulsan, Ulsan 680-749, Korea.
Correspondence to: H. Suh (E-mail: hssuh@pusan.ac.kr)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 369–380 (2011) 2010 Wiley Periodicals, Inc.
NEW LOW-BAND GAP COPOLYMERS, KIM ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
polymers for BHJ solar cells, and also claimed that, to exceed
10% PCE, the polymer must have band gap lower than
1.74 eV and lowest unoccupied molecular orbital (LUMO)
level lower than ꢁ3.94 eV.8
Recently, several low-band gap polymers with deep highest
occupied molecular orbital (HOMO) level were synthesized
to improve the Voc values.9 Among these polymers, poly[N-
90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-
benzothiadiazole)] (PCDTBT) show, through the device opti-
mization, 6.1% PCE.10
unless otherwise indicated. Analytical thin layer chromato-
graphy (TLC) was conducted using Merck 0.20 mm silica gel
60F254 aluminum sheet with fluorescent indicator Spectro-
line ENF-240C. High resolution mass spectra (HRMS) were
recorded on a JEOL JMS-700 mass spectrometer under elec-
tron impact (EI) or fast atom bombardment (FAB) conditions
in the Korea Basic Science Institute (Daegu, Korea). Molecular
weight and polydispersity of the polymer were determined by
gel permeation chromatography (GPC) analysis with a polysty-
rene standard calibration. The differential scanning calorime-
try analysis was performed under a nitrogen atmosphere (50
mL/min) on a TA Instruments DSC 2920 at heating rates of
Diverse efforts for new low-band gap polymers with high PCE
have been focused on the development of new electron-defi-
cient moiety for the optimum ICT, including benzothiadiazoles,
benzo-bis(thiadiazole)s, thienopyrazines, pyrazino-quinoxa-
lines, etc.11–16 The representative material for electron-defi-
cient moiety is 2,1,3-benzothiadiazole (BT) which was utilized
in PCDTBT and PCPDTBT for the state-of-the-art solar cell. De-
spite BT is promising unit, designing of new polymers is quite
restricted by the low solubility resulting from the structure of
BT. Successful synthetic modification to improve the solubility
of BT was accomplished by Bo and Zhang et al.17 Through the
introduction of two octyloxy groups at 5 and 6-positions of
BT, highly soluble BT derivative is prepared, and carbazole
based polymer using this BT derivative show PCE of 5.4%.
ꢂ
10 C/min. Thermogravimetric analysis was performed with a
TA Instruments TGA 2950 in a nitrogen atmosphere at a heat-
ꢂ
ꢂ
ing rate of 10 C/min to 700 C. The UV–vis absorption spec-
tra were recorded by a Varian Cary 1 spectrophotometer.
Cyclic voltammograms of the polymers were obtained with a
solution of tetrabutylammonium tetrafluoroborate (Bu4NBF4)
(0.10 M) in acetonitrile at a scan rate of 80 mV/s at room
temperature under the protection of argon using a Wona-
WPG100 in a three-electrode cell. Polymer films were pre-
pared by dipping platinum working electrodes into the poly-
mer solution, which was prepared with minimum amount of
THF, and then air-drying. A platinum wire and an Ag/AgNO3
electrode were used as the counter electrode and reference
electrode, respectively. Atomic force microscopy (AFM) images
were obtained by Veeco Nanoscope IIIa in tapping mode to
measure the surface morphology of BHJ thin films.
In this article, we report the syntheses and photovoltaic
characteristics of two carbarzole based low-band gap
polymers, poly[2,7-(9-(10-octylnonyl)-9H-carbazole)-alt-5,5-(40,
70-di(thien-2-yl)-2H-benzimidazole-20-spirocyclohexane)]
(PCDTCHBI) and poly[2,7-(9-(10-octylnonyl)-9H-carbazole)-
alt-5,5-(40,70-di(thien-2-yl)-2H-benzimidazole-20-spiro-400-((2000-eth-
ylhexyl)oxy)-cyclohexane)] (PCDTEHOCHBI). These two polymers
contain newly designed highly soluble 2H-benzimidazole
moieties as electron deficient group. The design concept of
the moieties was originated from the intention to introduce
solubility group at the 2-position of the 2,1,3-benzothiadia-
zole (BT). The sulfur at 2-position was substituted with car-
bon, at which alkyl groups can be introduced, to make a
highly soluble electron deficient moiety while keeping the
same electronic properties for the optimum ICT.
Organic Field Effect Transistor Device Fabrication
Field effect transistors (FETs) were fabricated on heavily
n-type doped silicon (Si) wafers with a 200 nm thick ther-
mally grown SiO2 layer. The n-type doped Si substrate func-
tioned as the gate electrode and the SiO2 layer functioned as
the gate dielectric. The semiconducting active layer was
spin-cast from solution at 3000 rpm. Polymer solutions were
prepared at 0.5 wt % concentration in chlorobenzene. Prior
to deposition of active materials, the SiO2 surface was modi-
fied with octyltrichlorosilane (OTS). After active layer deposi-
ꢂ
tion, the films were dried on hot plate stabilized at 80 C for
1 h. Both the OTS treatment and the active layer deposition
were carried out in a controlled atmosphere glove box filled
with N2. Source and drain electrodes (Au) were deposited by
thermal evaporation using a shadow mask. The thickness of
source and drain electrodes was 50 nm. Channel length (L)
and channel width (W) were 50 lm and 2.0 mm, respec-
tively. Electrical characterization was performed under N2
atmosphere using a Keithley semiconductor parametric ana-
lyzer (Keithley 4200). The FET mobilities were calculated in
EXPERIMENTAL
Materials
All reagents were purchased from Aldrich or TCI, and used
without further purification. Solvents were purified by nor-
mal procedure and handled under moisture-free atmosphere.
4,4,5,5-tetramethyl-2-(20-thienyl)-1,3,2-dioxaborolane (9) and
9-(1-octylnonyl)-2,7-bis(40,40,50,50-tetramethyl-10,30,20-dioxabor-
olan-20-yl)-9H-carbazole (14) were prepared according to a
procedure reported in the literature.18,19
the saturation regime using the following equation: Ids
¼
(W/2L) lCi (Vgs ꢁ Vth)2, where Ids is the drain-source cur-
rent in the saturated region, W and L are the channel width
and length, respectively, l is the field-effect mobility, Ci is the
capacitance per unit area of the insulation layer, and Vgs and
Vth are the gate and threshold voltages, respectively.
Instruments
1H and 13C NMR spectra were recorded with a Varian Gem-
ini-300 (300 MHz) spectrometer and chemical shifts were
recorded in ppm units with TMS as the internal standard.
Flash column chromatography was performed with Merck
silica gel 60 (particle size 230–400 mesh ASTM) with ethyl
acetate/hexane or methanol/methylene chloride gradients
Solar Cell Device Fabrication
Solar cell was fabricated on an indium tin oxide (ITO)-coated
glass substrate in the following structure: ITO-coated glass
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ARTICLE
substrate/poly(3,4-ethylenedioxythiophene)
(PEDOT:PSS)/
(s, 2H), 3.89 (br, 4H); 13C NMR (75 MHz, CDCl3) d 134.0,
123.5, 109.9; HRMS, m/e calcd for C6H6Br2N2 263.8898,
measured 263.8901.
polymer-PC71BM mixture/TiOx/Al. The TiOx layer was incor-
porated with thickness of 10 nm as an optical spacer layer
to increase the PCE through adjustment of optical interfer-
ence between the incidental light and the light reflection on
the metal electrode surface.20 The ITO-coated glass substrate
was first cleaned with detergent, ultrasonicated in acetone
and isopropyl alcohol, and subsequently dried overnight in
an oven. PEDOT: PSS (Baytron PH) was spin-cast from aque-
ous solution to form a film of thickness of 40 nm. The sub-
Synthesis of 4,7-Dibromo-2H-benzimidazole-
2-spirocyclohexane (4)
A solution of 3,6-dibromo-1,2-benzenediamine (3.02 g, 11.36
mmol) (3) and cyclohexanone (1.18 mL, 11.36 mmol) in
toluene (75 mL) under argon was stirred at 120 ꢂC for 6 h.
After removing the solvent under reduced pressure, 4,7-
dibromo-2H-benzimidazole-2(3H)-spirocyclohexane was obtained
as crude product. To a stirred solution of this crude product
in freshly distilled CH2Cl2 (80 mL) at room temperature
under argon was added 85% activated manganese (IV) oxide
(10.92 g, 106.8 mmol). After being stirred at room tempera-
ture for 1 h, the reaction mixture was filtered, diluted with
CH2Cl2 (400 mL), washed with distilled water (3 ꢃ 400 mL),
and dried (MgSO4). After removing the solvent under
reduced pressure, the residue was purified by flash chroma-
tography (40 ꢃ 150 mm column, SiO2, EtOAc/hexane, 1:15)
to give 2.44 g (62%) of compound 4 as a yellow powder;
Rf ¼ 0.3 (SiO2, EtOAc/hexane, 1:15); 1H NMR (300 MHz,
Acetone-D6) d 7.43 (s, 2H), 2.00–1.84 (m, 4H), 1.82–1.70 (m,
2H), 1.70–1.58 (m, 4H); 13C NMR (75 MHz, CDCl3) d 157.1,
136.0, 119.0, 107.5, 32.8, 25.7, 24.7; HRMS, m/e calcd for
ꢂ
strate was dried for 10 min at 140 C in air and then trans-
ferred into a glove box to spin-cast the charge separation
layer. A solution containing a mixture of polymer and
PC71BM in chlorobenzene/dichlorobenzene cosolvent with
concentration of 0.8 wt % was spin-cast on top of the
PEDOT/PSS layer. Dilute titanium tin oxide precursor (TiOx)
was spin-cast onto the polymer-PC71BM mixture layer with a
high spin-speed of 6000 rpm in air. Then, aluminum (Al, 100
nm) electrode was deposited by thermal evaporation in a
vacuum of about 5 ꢃ 10–7 Torr. Current density-voltage (J-V)
characteristics (in dark and illuminated) of the devices were
measured using a Keithley 236 Source Measure Unit. The
illumination was done using an Air Mass 1.5 Global (AM
1.5 G) solar simulator with an irradiation intensity of 1000
W/m2. The processing was carried out in a glove-box under
nitrogen and at room temperature.
C12H12Br2N2 341.9367, measured 341.9365.
Synthesis of 1,4-Dioxaspiro[4.5]decan-8-ol (6)
Synthesis of Monomers and Polymers
To a stirred solution of 1,4-cyclohexanedione mono-ethylene
ketal (5 g, 32.01 mmol) in methyl alcohol (100 mL) at 0 ꢂC
under argon was added portion-wise sodium borohydride
Synthesis of 4,7-Dibromo-2,1,3-benzothiadiazole (2)
To a stirred solution of 2,1,3-benzothiadiazole (10 g, 73.43
mmol) in 48% hydrobromic acid (150 mL, 1.32 mol) at
room temperature under argon was added dropwise Br2
(7.49 mL, 220.29 mmol) in 48% hydrobromic acid (100 mL,
0.88 mol) very slowly. After being stirred at 100 ꢂC for 6 h,
the reaction mixture was cooled to room temperature and
neutralized using saturated aqueous sodium bisulfate
solution. The solid was collected by filtration, washed with
distilled water followed by cold ether, and dried in vacuum
oven to afford 20.05 g (92%) of compound 2, ivory crystal;
Rf ¼ 0.7 (SiO2, EtOAc/hexane, 1:4); 1H NMR (300 MHz,
CDCl3) d 7.73 (s, 2H); 13C NMR (75 MHz, CDCl3) d 153.2,
132.6, 114.2; HRMS, m/e calcd for C6H2Br2N2S 291.8305,
measured 291.8309.
ꢂ
(3.63 g, 32.01 mmol). After being stirred at 0 C for 10 min,
the reaction mixture was sꢂtirred at room temperature for
3 h. After being cooled to 0 C, distilled water (100 mL) was
slowly added to reaction mixture. After removing the solvent
under reduced pressure, the residue was washed with dis-
tilled water (150 mL) followed by ethyl acetate (6 ꢃ 200
mL), and dried (MgSO4). After removing the solvent under
reduced pressure, the residue was purified by flash chroma-
tography (40 ꢃ 120 mm column, SiO2, EtOAc/hexane, 2:1) to
give 4.76 g (94%) of compound 6 as a colorless oil; Rf ¼ 0.3
(SiO2, EtOAc/hexane, 2:1); 1H NMR (300 MHz, CDCl3) d
3.96–3.86 (m, 4H), 3.81–3.71 (m, 1H), 2.01 (br, 1H), 1.90–
1.71 (m, 4H), 1.69–1.47 (m, 4H); 13C NMR (75 MHz, CDCl3)
d 108.5, 68.3, 64.5, 64.4, 32.2, 31.8; HRMS, m/e calcd for
C8H14O3 158.0943, measured 158.0944.
Synthesis of 3,6-Dibromo-1,2-benzenediamine (3)
To a stirred solution of 4,7-dibromo-2,1,3-benzothiadiazole
(3.5 g, 11.91 mmol) (2) in ethyl alcohol (400 mL) at 0 ꢂC
under argon was added portion-wise sodium borohydride
(8.11 g, 214.31 mmol). After being stirred at 0 ꢂC for
10 min, the reaction mixture was stꢂirred at room tempera-
ture for 3 h. After being cooled to 0 C, the reaction mixture
was treated with distilled water (100 mL). After removing
the solvent under reduced pressure, the residue was diluted
with ether (150 mL), washed with saline (3 ꢃ 200 mL), and
dried (MgSO4). After removing the solvent under reduced
pressure, the residue was purified by flash chromatography
(60 ꢃ 150 mm column, SiO2, EtOAc/hexane, 1:4) to give
3.05 g (96%) of compound 3 as a ivory solid; Rf ¼ 0.3 (SiO2,
EtOAc/hexane, 1:4); 1H NMR (300 MHz, CDCl3) d 6.84
Synthesis of 4-[(2-Ethylhexyl)oxy]cyclohexanone (7)
To a stirred mixture of NaH (1.46 g, 36.41 mmol) in DMF
(72 mL) at 0 ꢂC under argon was added dropwise 1,4-diox-
aspiro[4.5]decan-8-ol (4.8 g, 30.34 mmol) (6) in DMF (72
ꢂ
mL). After being stirred at 0 C for 30 min, 2-ethylhexylbro-
mide (8.52 mL, 45.51 mmol) was added, and reaction mix-
ture was stiꢂrred at room temperature for 3 h. After being
cooled to 0 C, distilled water (10 mL) was slowly added to
the reaction mixture to quench the extra of NaH. The reac-
tion mixture was diluted with ethylacetate (200 mL), washed
with distilled water (6 ꢃ 200 mL), and dried (MgSO4). After
removing the solvent under reduced pressure, 8-[(2-
NEW LOW-BAND GAP COPOLYMERS, KIM ET AL.
371

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
ethylhexyl)oxy]-1,4-dioxaspiro[4.5]decane was obtained as
crude product. To a stirred solution of this crudeꢂ product in
acetic acid (5 mL) under argon was stirred at 50 C for 12 h.
After being poured the reaction mixture cautiously into satu-
rated aqueous sodium bicarbonate solution (50 mL), the
reaction mixture was diluted with ethylacetate (100 mL),
washed with distilled water (3 ꢃ 100 mL), and dried
(MgSO4). After removing the solvent under reduced pressure,
the residue was purified by flash chromatography (40 ꢃ 120
mm column, SiO2, EtOAc/hexane, 1:10) to give 0.35 g (5%)
of compound 7 as a colorless oil; Rf ¼ 0.2 (SiO2, EtOAc/hex-
ane, 1:10); 1H NMR (300 MHz, CDCl3) d 3.68–3.59 (m, 1H),
3.40–3.29 (m, 2H), 2.65–2.48 (m, 2H), 2.30–2.16 (m, 2H),
2.16–2.00 (m, 2H), 1.96–1.59 (m, 2H), 1.56–1.44 (m, 1H),
1.42–1.16 (m, 8H), 0.94–0.78 (m, 6H); 13C NMR (75 MHz,
CDCl3) d 211.9, 72.9, 71.4, 40.2, 37.4, 30.9, 30.8, 30.7, 29.4,
24.2, 23.3, 14.3, 11.4; HRMS, m/e calcd for C14H26O2
226.1933, measured 226.1931.
8.09 (dd, 2H, J ¼ 4.95, 3.84 Hz), 7.39 (dd, 2H, J ¼ 6.31, 5.22
Hz), 7.33 (s, 2H), 7.15 (dd, 2H, J ¼ 8.79, 5.22 Hz), 2.12–1.98
(m, 4H), 1.88–1.70 (m, 6H); 13C NMR (75 MHz, CDCl3) d
157.9, 139.1, 128.8, 128.4, 128.3, 128.2, 127.0, 108.2, 33.6,
26.2, 25.4; HRMS, m/e calcd for C20H18N2S2 350.0911, found
350.0914.
Synthesis of 4,7-Di(20-bromothien-50-yl)-2H-
benzimidazole-2-spirocyclohexane (11)
To a stirred solution of 4,7-di(thien-20-yl)-2H-benzimidazole-
2-spirocyclohexane (500 mg, 1.43 mmol) (10) in CHCl3 (12
mL) protected from light under argon was added NBS (508
mg, 2.85 mmol). After being stirred at room temperature for
3 h, the reaction mixture was diluted with CHCl3 (100 mL),
washed with saline (3 ꢃ 100 mL), and dried (MgSO4). After
removing the solvent under reduced pressure, the residue
was purified by flash chromatography (40 ꢃ 150 mm col-
umn, SiO2, CH2Cl2/hexane, 1:1) to give 1.18 g (83%) of com-
pound 11 as a red solid; Rf ¼ 0.6 (SiO2, CH2Cl2/hexane, 1:1);
1H NMR (300 MHz, CDCl3) d 7.71 (d, 2H, J ¼ 3.85 Hz), 7.23
(s, 2H), 7.09 (d, 2H, J ¼ 4.12 Hz), 2.10–1.94 (m, 4H), 1.88–
1.77 (m, 2H), 1.76–1.64 (m, 4H); 13C NMR (75 MHz, CDCl3)
d 157.4, 140.2, 130.9, 128.2, 127.6, 127.5, 115.5, 108.7, 33.4,
26.0, 25.3; HRMS, m/e calcd for C20H16Br2N2S2 507.9101,
found 507.9109.
Synthesis of 4,7-Dibromo-40-[(2-ethylhexyl)oxy]-
2H-benzimidazole-2-spirocyclohexane (8)
A solution of 3,6-dibromo-1,2-benzenediamine (464 mg, 1.74
mmol) (3) and 4-[(2-ethylhexyl)oxy]cyclohexanone (395 mg,
1.74 mmol) (7) in toluene (11 mL) under argon was stirred
ꢂ
at 120 C for 6 h. After removing the solvent under reduced
pressure, 4,7-dibromo-40-[(2-ethylhexyl)oxy]-2H-benzimida-
zole-2(3H)-spirocyclohexane was obtained as crude product.
To a stirred solution of this crude product in freshly distilled
CH2Cl2 (17 mL) at room temperature under argon was
added 85% activated manganese (IV) oxide (1.44 g, 14.11
mmol). After being stirred at room temperature for 1 h, the
reaction mixture was filtered, diluted with CH2Cl2 (100 mL),
washed with distilled water (3 ꢃ 100 mL), and dried
(MgSO4). After removing the solvent under reduced pressure,
the residue was purified by flash chromatography (40 ꢃ 150
mm column, SiO2, EtOAc/hexane, 1:8) to give 513 mg (62%)
of compound 8 as a yellow oil; Rf ¼ 0.45 (SiO2, EtOAc/hex-
ane, 1:8); 1H NMR (300 MHz, CDCl3) d 7.18 (s, 2H), 3.70–
3.58 (m, 1H), 3.41–3.28 (m, 2H), 2.22–1.96 (m, 6H), 1.56–
1.16 (m, 11H), 0.94–0.78 (m, 6H); 13C NMR (75 MHz, CDCl3)
d 157.4, 157.2, 136.1, 136.0, 119.1, 119.0, 107.2, 74.7, 71.3,
40.3, 30.9, 29.6, 29.5, 29.0, 24.2, 23.3, 14.4, 11.4; HRMS, m/e
calcd for C20H28Br2N2O 470.0568, measured 470.0566.
Synthesis of 4,7-Di(thien-20-yl)-2H-benzimidazole-
2-spiro-400-[(2000-ethylhexyl)oxy]-cyclohexane (12)
To a stirred solution of 4,7-dibromo-40-[(2-ethylhexyl)oxy]-
2H-benzimidazole-2-spirocyclohexane (965 mg, 2.04 mmol)
(8) and 4,4,5,5-tetramethyl-2-(20-thienyl)-1,3,2-dioxaborolane
(2.15 g, 10.21 mmol) (9) in toluene (15 mL) under argon
was added tetrakis(triphenylphosphine) palladium(0) (236
mg, 0.20 mmol) and degassed 2 M K2CO3 (aq) (10.21 mL).
After being stirred at 80 ꢂC for 12 h, the reaction mixture
was diluted with ethyl acetate (100 mL), washed with
distilled water (3 ꢃ 100 mL), and dried (MgSO4). After
removing the solvent under reduced pressure, the residue
was purified by flash chromatography (40 ꢃ 150 mm col-
umn, SiO2, CH2Cl2/hexane, 1:1) to give 0.82 g (84%) of com-
pound 12 as a red oil; Rf ¼ 0.4 (SiO2, CH2Cl2/hexane, 1:1);
1H NMR (300 MHz, CDCl3) d 8.15 (dd, 1H, J ¼ 3.85,
4.95 Hz), 8.06 (dd, 1H, J ¼ 3.85, 4.95 Hz), 7.40–7.34 (m, 2H),
7.31 (s, 2H), 7.16–7.10 (m, 2H), 3.76–3.65 (m, 1H), 3.52–
3.39 (m, 2H), 3.32–2.10 (m, 5H), 1.66–1.28 (m, 12H), 0.95 (t,
6H, J ¼ 7.42 Hz); 13C NMR (75 MHz, CDCl3) d 158.2, 158.1,
139.0, 139.0, 128.9, 128.7, 128.6, 128.5, 128.4, 128.3, 128.2,
128.0, 126.9, 126.8, 107.6, 76.0, 71.6, 40.2, 30.9, 30.4, 30.2,
29.4, 24.2, 23.4, 14.4, 11.4; HRMS, m/e calcd for
Synthesis of 4,7-Di(thien-20-yl)-2H-benzimidazole-
2-spirocyclohexane (10)
To a stirred solution of 4,7-dibromo-2H-benzimidazole-2-spi-
rocyclohexane (4.2 g, 12.21 mmol) (4) and 4,4,5,5-tetra-
methyl-2-(20-thienyl)-1,3,2-dioxaborolane (8.98 g, 42.73
mmol) (9) in toluene (42 mL) under argon was added tetra-
kis(triphenylphosphine) palladium(0) (1.41 g, 1.22 mmol)
and degassed 2 M K2CO3 (aq) (61.04 mL). After being stirred
at 80 ꢂC for 12 h, the reaction mixture was diluted with
ethyl acetate (100 ml), washed with distilled water (3 ꢃ 100
mL), and dried (MgSO4). After removing the solvent under
reduced pressure, the residue was purified by flash chroma-
tography (40 ꢃ 150 mm column, SiO2, CH2Cl2/hexane, 1:1)
to give 2.15 g (50%) of compound 10 as a red solid; Rf ¼
0.4 (SiO2, CH2Cl2/hexane, 1:1); 1H NMR (300 MHz, CDCl3) d
C28H34N2OS2 478.2113, measured 478.2109.
Synthesis of 4,7-Di(20-bromothien-50-yl)-2H-benzimidazole-
2-spiro-400-[(2000-ethylhexyl)oxy]-cyclohexane (13)
To a stirred solution of 4,7-di(thien-20-yl)-2H-benzimidazole-
2-spiro-400-[(2000-ethylhexyl)oxy]-cyclohexane (711 mg, 1.49
mmol) (12) in CHCl3 (18 mL) protected from light under
argon was added NBS (449 mg, 2.52 mmol). After being
stirred at room temperature for 3 h, the reaction mixture
was diluted with CHCl3 (100 mL), washed with saline (3 ꢃ
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ARTICLE
100 mL), and dried (MgSO4). The solvent was removed
under reduced pressure, the residue was purified by flash
chromatography (40 ꢃ 150 mm column, SiO2, CH2Cl2/hex-
ane, 1:2) to give 863 mg (91%) of compound 13 as a red
for 24 h to remove the oligomers and catalyst residue. After
being dried under reduced pressure at room temperature,
350 mg of poly[2,7-(9-(10-octylnonyl)-9H-carbazole)-alt-5,5-
(40,70-di(thien-2-yl)-2H-benzimidazole-20-spiro-400-((2000-ethylhex-
yl)oxy)-cyclohexane)] (16, PCDTEHOCHBI) was obtained as
solid; Rf
¼
0.4 (SiO2, CH2Cl2/hexane, 1:2); 1H NMR
1
(300 MHz, CDCl3) d 7.76 (d, 1H, J ¼ 4.12 Hz), 7.67 (d, 1H, J
¼ 4.12 Hz), 7.20 (d, 2H, J ¼ 1.10 Hz), 7.08 (d, 2H, J ¼ 4.12
Hz), 3.76–3.65 (m, 1H), 3.50–3.39 (m, 2H), 2.30–2.06 (m,
5H), 1.64–1.22 (m, 12H), 1.00–0.84 (m, 6H); 13C NMR (75
MHz, CDCl3) d 157.7, 157.6, 140.1, 131.0, 130.8, 128.2,
128.0, 127.8, 127.7, 127.4, 115.5, 115.4, 108.1, 75.8, 71.7,
40.2, 30.9, 30.3, 30.0, 29.4, 24.2, 23.4, 14.4, 11.4; HRMS, m/e
calcd for C28H32Br2N2OS2 634.0323, measured 634.0321.
dark blue powder; H NMR (300 MHz, CDCl3) d 8.27 (br, 1H),
8.22–8.01 (m, 3H), 7.85 (br, 2H), 7.76–7.32 (m, 6H), 4.67 (br,
1H), 3.77 (br, 1H), 3.57–3.40 (m, 2H), 2.59–2.13 (m, 7H), 2.02
(br, 4H), 1.79 (br, 2H), 1.46–0.69 (m, 44H).
RESULTS AND DISCUSSION
Synthesis and Characterization of Polymers
The general synthetic routes toward the monomer and poly-
mers are outlined in Schemes 1 and 2. 2,1,3-Benzothiadia-
zole was brominated using bromine and 48% hydrobromic
acid to afford 4,7-dibromo-2,1,3-benzothiadiazole (2), which
was converted to 3,6-dibromo-1,2-benzenediamine (3) by
sodium borohydride. Compound 3 was coupled with cyclo-
hexanone in toluene to give 4,7-dibromo-2H-benzimidazole-
2(3H)-spirocyclohexane, which was oxidized with activated
manganese (IV) oxide to obtain 4,7-dibromo-2H-benzimida-
zole-2-spirocyclohexane (4). Commercially available 1,4-
cyclohexanedione mono-ethylene ketal (5) was treated with
sodium borohydride in methanol to generated 1,4- dioxaspir-
o[4.5]decan-8-ol (6). Compound 6 was alkylated with 2-eth-
ylhexyl bromide using sodium hydride to afford 8-[(2-ethyl-
hexyl)oxy]-1,4-dioxaspiro[4.5]decane, which was deketalated
with acetic acid to obtain 4-[(2-ethylhexyl)oxy]cyclohexanone
(7). The resulting compound 7 was coupled with compound
3 in toluene to give 4,7-dibromo-40-[(2-ethylhexyl)oxy]-2H-
benzimidazole-2(3H)-spirocyclohexane, which was oxidized
with activated manganese (IV) oxide to afford 4,7-dibromo-
40-[(2-ethylhexyl)oxy]-2H-benzimidazole-2-spirocyclohexane
(8). Compound 4 was coupled with 4,4,5,5-tetramethyl-2-(20-
thienyl)-1,3,2-dioxaborolane (9) using Suzuki coupling
condition21 to generate 4,7-di(thien-20-yl)-2H-benzimidazole-
2-spirocyclohexane (10), which was brominated with NBS in
chloroform to obtain 4,7-di(20-bromothien-50-yl)-2H-benzim-
idazole-2-spirocyclohexane (11). Compound 8 was coupled
with compound 9 using Suzuki coupling condition to obtain
4,7-di(thien-20-yl)-2H-benzimidazole-2-spiro-400-[(2000-ethylhex-
yl)oxy]-cyclohexane (12), which was brominated using NBS
in chloroform to afford 4,7-di(20-bromothien-50-yl)-2H-benz-
imidazole-2-spiro-400-[(2000-ethylhexyl)oxy]-cyclohexane (13).
The polymerization of monomer 11 and 14 was achieved
under Suzuki coupling condition using palladium catalyst to
yield alternating copolymer poly[2,7-(9-(10-octylnonyl)-9H-
carbazole)-alt-5,5-(40,70-di(thien-2-yl)-2H-benzimidazole-20-spi-
rocyclohexane)] (15, PCDTCHBI). The polymerization of mono-
mer 13 and 14 was achieved under Suzuki coupling condition
using palladium catalyst to yield alternating copolymer poly[2,7-
(9-(10-octylnonyl)-9H-carbazole)-alt-5,5-(40,70-di(thien-2-yl)-
2H-benzimidazole-20-spiro-400-((2000-ethylhexyl)oxy)-cyclohexane)]
(16, PCDTEHOCHBI).
Synthesis of Poly[2,7-(9-(10-octylnonyl)-9H-carbazole)-alt-
5,5-(40,70-di(thien-2-yl)-2H-benzimidazole-20-spirocyclo-
hexane)] (15, PCDTCHBI)
To a stirred solution of 9-(1-octylnonyl)-2,7-bis(40,40,50,50-
tetramethyl-10,30,20-dioxaborolan-20-yl)-9H-carbazole
(306
mg, 0.465 mmol) (14) and 4,7-di(20-bromothien-50-yl)-2H-
benzimidazole-2-spirocyclohexane (237 mg, 0.465 mmol)
(11) in toluene (8 mL) under argon was added tetrakis(tri-
phenylphosphine) palladium(0) (16.1 mg, 0.014 mmol) and
degassed 2 M K2CO3 (aq) (2.3 mL). After being stirred at
room temperature for 5 min, the reaction mixture was
ꢂ
stirred at 80 C for 3 days. The reaction mixture was poured
into methanol/H2O (10:1) (500 mL), and the dark blue pre-
cipitates were collected by filtration. The resulting polymer
was dissolved again in THF, and obtained polymer solution
was slowly poured into intensely stirred methanol (500 mL).
The precipitated polymer was collected by filtration, and
purified by Soxhlet extraction in methanol for 24 h to remove
the oligomers and catalyst residue. After being dried under
reduced pressure at room temperature, 360 mg of poly[2,7-
(9-(10-octylnonyl)-9H-carbazole)-alt-5,5-(40,70-di(thien-2-yl)-2H-
benzimidazole-20-spirocyclohexane)] (15, PCDTCHBI) was
obtained as dark blue powder; 1H NMR (300 MHz, CDCl3) d
8.28–7.98 (m, 4H), 7.85 (br, 2H), 7.76–7.31 (m, 6H), 4.66 (br,
1H), 2.38 (br, 2H), 2.08 (br, 6H), 1.85 (br, 6H), 1.38–1.05 (m,
24H), 0.80 (br, 6H).
Synthesis of Poly[2,7-(9-(10-octylnonyl)-9H-carbazole)-alt-
5,5-(40,70-di(thien-2-yl)-2H-benzimidazole-20-spiro-400-
((2000-ethylhexyl)oxy)-cyclohexane)] (16, PCDTEHOCHBI)
To a stirred solution of 9-(1-octylnonyl)-2,7-bis(40,40,50,50-tet-
ramethyl-10,30,20-dioxaborolan-20-yl)-9H-carbazole (400 mg,
0.608 mmol) (14) and 4,7-di(20-bromothien-50-yl)-2H-benz-
imidazole-2-spiro-400-[(2000-ethylhexyl)oxy]-cyclohexane (387
mg, 0.608 mmol) (13) in toluene (8 mL) under argon was
added tetrakis(triphenylphosphine) palladium(0) (21 mg,
0.018 mmol) and degassed 2 M K2CO3 (aq) (3 mL). After
being stirred at room temperature for 5 min, the reaction
mixture was stirred at 80 ꢂC for 3 days. The reaction mix-
ture was poured into methanol/H2O (10:1) (500 ml), and
the dark blue precipitates were collected by filtration. The
resulting polymer was dissolved again in THF, and obtained
polymer solution was slowly poured into intensely stirred
methanol (500 mL). The precipitated polymer was collected
by filtration, and purified by Soxhlet extraction in methanol
Molecular weights and polydispersities of the polymers were
determined by GPC analysis with a polystyrene standard cali-
bration. The number-average molecular weights (Mn) of
NEW LOW-BAND GAP COPOLYMERS, KIM ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 The synthetic routes for the monomers.
SCHEME 2 The synthetic routes for the PCDTCHBI and PCDTEHOCHBI.
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TABLE 1 Physical Properties of the PCDTCHBI and
PCDTEHOCHBI
PDIa
Tg (ꢂC)b Td (ꢂC)c
a
a
Polymer
Mn
22,000 105,000 4.9
Mw
PCDTCHBI
85
77
438
414
PCDTEHOCHBI 31,000 158,000 5.1
a
Number-average molecular weight (Mn), weight-average molecular
weight (Mw), and polydispersity of the polymers were measured by gel
permeation chromatography (GPC) in THF using polystyrene standards.
b
Glass transition temperature (Tg) measured by DSC under N2.
Onset decomposition temperature (Td) (5% weight loss) measured by
c
TGA under N2.
620 nm in solution, respectively, and 623 and 641 nm in the
film, respectively. As compared to that of PCDTBT (kfilm,max
¼ 576 nm),19 the absorption maxima of PCDTCHBI and
PCDTEHOCHBI exhibited bathochromic shift of 47 and 65
nm, respectively. Moreover, absorption onsets of PCDTCHBI
and PCDTEHOCHBI were 743 and 752 nm, respectively
which are red shifted by 83 and 92 nm, respectively, com-
pared to that of PCDTBT (kfilm,onset ¼ 660 nm). As a result,
PCDTCHBI and PCDTEHOCHBI showed reduced optical band
gap values of 1.67 and 1.65 eV, respectively.
FIGURE 1 TGA and DSC data of the PCDTCHBI and PCDTE-
HOCHBI. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
PCDTCHBI and PCDTEHOCHBI were 22,000 and 31,000,
respectively, and polydispersity index (PDI, Mw/Mn) values of
PCDTCHBI and PCDTEHOCHBI were 4.9 and 5.1, respectively.
The polymers were soluble in organic solvents such as
chloroform, tetrahydrofuran, dichloromethane, chlorobenzene,
and orthodichlorobenzene (ODCB). The thermal properties
of the polymers were evaluated by differential scanning
calorimetry (DSC) and thermo gravimetric analysis (TGA) in
nitrogen. The decomposition temperatures (Td) determined
by TGA, which correspond to a 5% weight loss upon heating,
of PCDTCHBI and PCDTEHOCHBI were 438 and 414 ꢂC,
respectively, and the glass transition temperatures (Tg) of
PCDTCHBI and PCDTEHOCHBI were 85 and 77 ꢂC, respec-
tively. TGA and DSC curves of PCDTCHBI and PCDTEHOCHBI
are shown in Figure 1, and the physical properties of the
polymers are summarized in Table 1.
Optical Properties
The UV–visible absorption spectra of the polymers measured
both in ODCB solution and the film state on glass slides are
shown in Figure 2 and summarized in Table 2. The absorp-
tion maxima of PCDTCHBI and PCDTEHOCHBI were 612 and
FIGURE 2 Normalized UV-Vis absorption data of the PCDTCHBI
and PCDTEHOCHBI. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
NEW LOW-BAND GAP COPOLYMERS, KIM ET AL.
375

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Optical Properties of the PCDTCHBI and
TABLE 3 Electrochemical Potentials and Energy Levels of the
PCDTEHOCHBI
PCDTCHBI and PCDTEHOCHBI
kmax (nm)
konset (nm)
Eox
Ered
HOMO LUMO Eg,elec Eg,opt
Polymer
(V)a (V)a
(eV)b
(eV)c
(eV)d
(eV)e
a
In
ODCB
FWHMfilm
(nm)
In
ODCB Film
PCDTCHBI
0.74 ꢁ1.20 ꢁ5.53
PCDTEHOCHBI 0.70 ꢁ1.22 ꢁ5.49
ꢁ3.86
ꢁ3.84
1.94
1.92
1.67
1.65
Polymer
Film
PCDTCHBI
404, 612 407, 623 173
710
726
743
752
a
Oxidation and reduction onset potentials measured by cyclic
voltammetry.
PCDTEHOCHBI 403, 620 406, 641 176
b
a
Calculated by the oxidation onset potential relative to ferrocene (Fc) of
Estimated from the absorption peaks of PCDTCHBI and PCDTE-
HOCHBI, at 623 and 643 nm, respectively.
which the ionization potential value is ꢁ4.8 eV for ferrocene/ferroce-
nium (Fc/Fcþ) redox system.
c
Calculated by the HOMO energy levels and Eg,opt
Electrochemical band gaps calculated by Eox and Ered values.
Optical band gaps estimated by the onset wavelengths of absorption
.
d
e
The sulfur atom in BT moiety of PCDTBT has the lone pairs
which may give some electron releasing resonance effect, but
alkylated carbons in newly designed electron-deficient
moieties of the PCDTCHBI and PCDTEHOCHBI do not have
the lone pair which can generate resonance effect. As a
result, electron deficiency of the electron-deficient moiety of
the PCDTCHBI and PCDTEHOCHBI is higher than that of BT
of the PCDTBT, and elevated electron deficiency of them
should lower the LUMO energy level which results in the
reduced band gaps as compared to that of PCDTBT. There-
fore, introduction of 2H-benzimidazole as electron deficiency
moiety for ICT polymer can be regarded as a good way to
reduce the band gap of polymers.
of polymer films.
of the transfer curve at constant Vds ¼ ꢁ60 V are shown in
Figure 4 and the FET properties of the polymers are sum-
marized in Table 4. The field-effect mobilities and on/off
ratios at a drain voltage of Vds ¼ ꢁ60 V were calculated in
the saturation regime.23 The FET mobilities of PCDTCHBI
and PCDTEHOCHBI were 1.31 ꢃ 10ꢁ4 and 8.57 ꢃ 10ꢁ5
cm2/Vs, respectively, and Vth of PCDTCHBI and PCDTE-
HOCHBI were ꢁ18 and ꢁ20 V, respectively. With the substitu-
tion of the sulfur of BT with alkylated carbon to improve the
solubility, PCDTCHBI and PCDTEHOCHBI showed slightly
decreased carrier mobility, compared to PCDTBT.
Electrochemical Properties
The LUMO energy levels of the polymers were determined
from the optical band gaps which were estimated from the
absorption edges, and the HOMO energy levels which were
estimated from the cyclic voltammetry. All measurements
were calibrated against ferrocene, as an internal standard, of
which ionization potential value is ꢁ4.8 eV for ferrocene/fer-
rocenium (Fc/Fcþ) redox system.22 The cyclic voltammo-
grams and energy level diagram of the polymers are shown
in Figure 3 and the oxidation potentials of polymers derived
from the onset of electrochemical p-doping are summarized
in Table 3. The oxidation onset potentials of PCDTCHBI and
PCDTEHOCHBI were 0.74 and 0.70 eV, which corresponded
to the HOMO energy levels of ꢁ5.53 and ꢁ5.49 eV, respec-
tively. The LUMO energy levels of PCDTCHBI and PCDTE-
HOCHBI, calculated from the values of the optical band gaps
and HOMO energy levels, were ꢁ3.86 and ꢁ3.84 eV,
respectively.
Photovoltaic Properties and Morphologies
The J-V characteristics of solar cell based on blends of
PCDTCHBI and PCDTEHOCHBI with PC71BM using Air Mass
1.5 Global (AM 1.5 G) solar simulator with an irradiation
intensity of 1000 W/m2, are shown in Figure 5 and summar-
ized in Table 5. Out of the devices of PCDTCHBI with
increased w/w ratios of PC71BM from 1:2 to 1:4, the Voc, the
short circuit current densities (Jsc), and the fill factors (FF)
were about same to provide PCEs of 1.06 and 1.03%, respec-
tively. On the other hand, increasing w/w ratios of PC71BM
from 1:2 to 1:4 in the devices of PCDTEHOCHBI enhanced
the Jsc and FF values to improve the PCEs from 0.34 to
1.15%. As newly designed electron-deficient moieties of the
PCDTCHBI and PCDTEHOCHBI are bulkier than BT of the
PCDTBT caused by the cyclohexane units which may prohibit
better stacking between polymer chains as compared to the
case of PCDTBT, this causes lower carrier mobilities of the
PCDTCHBI and PCDTEHOCHBI, which results in lower PCE
of the devices.
The two polymers exhibited similar HOMO and LUMO energy
levels, but the values of PCDTEHOCHBI were slightly higher
than those of PCDTCHBI. The up-shifted values of HOMO and
LUMO levels indicate that the electron withdrawing ability of
PCDTEHOCHBI is weaker than that of PCDTCHBI due to the
introduction of alkoxy group at cyclohexane of the 2H-benz-
imidazole. However, as compared to the case of PCDTBT, the
same or deeper HOMO energy levels and lower LUMO
energy levels of PCDTCHBI and PCDTEHOCHBI are more
ideal for the polymer solar cell device.8
The absorption spectra of PCDTCHBI and PCDTEHOCHBI
blend films with PC71BM are shown in Figure 6. As the ratio
of PC71BM in BHJ films is increased, the intensity of the
TABLE 4 FET Characteristics of the PCDTCHBI and
PCDTEHOCHBI
Polymer
Vth (V)
l (cm2/Vs)
On/Off Ratio
OFET Properties
The FET devices fabricated using PCDTCHBI and PCDTE-
HOCHBI showed typical p-channel FET characteristics. Plots
PCDTCHBI
ꢁ18
ꢁ20
1.31 ꢃ 10ꢁ4
8.57 ꢃ 10ꢁ5
103
103
PCDTEHOCHBI
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ARTICLE
FIGURE 3 Cyclic voltammograms and energy diagram of the PCDTCHBI and PCDTEHOCHBI. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
absorption is decreased, caused by the reduced relative con-
centration of PCDTCHBI and PCDTEHOCHBI in BHJ films.
The incident photon-to-current efficiency (IPCE) spectra of
polymer BHJ photovoltaic devices composed of polymer and
PC71BM are shown in Figure 6. The IPCE spectra of the solar
cell devices show maxima of 28.9% at 555 nm for
PCDTCHBI:PC71BM (1:2) and 31.1% at 365 nm for PCDTE-
HOCHBI:PC71BM (1:4). The film surface morphologies exam-
ined by AFM in tapping mode are shown in Figure 7. The
root mean square (RMS) roughness of PCDTCHBI:PC71BM
FIGURE 4 Output characteristics at different gate biases (left) and transport characteristics at constant Vds ¼ ꢁ60 V (right) [semilo-
1/2
garithmic plot of ꢁIds versus Vgs (left axis) and plot of (ꢁIds
)
versus Vgs (right axis)] for FET devices using PCDTCHBI and PCDTE-
HOCHBI. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
NEW LOW-BAND GAP COPOLYMERS, KIM ET AL.
377

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 J-V characteristics of PCDTCHBI:PC71BM (1:2) and
PCDTEHOCHBI:PC71BM (1:4) BHJ solar cell devices in dark and
under white light illumination (AM 1.5 conditions).
FIGURE 6 Absorption spectra of
a PCDTCHBI and PCDTE-
HOCHBI blend films with PC71BM, and IPCE spectra.
blend (1:2) and (1:4) were 0.684 and 0.921 nm, respectively,
and those of PCDTEHOCHBI:PC71BM blend (1:2) and (1:4)
were 1.412 and 0.768 nm, respectively. In case of PCDTE-
HOCHBI, the reduced roughness of 1:4 blend film morpho-
logy, compared with the case of 1:2 ratio, improved the PCE
caused by enhancement of Jsc.
were introduced into the electron-deficient moiety,
PCDTCHBI and PCDTEHOCHBI showed typical low-band gap
properties and good electronic properties. HOMO and LUMO
energy levels of PCDTCHBI and PCDTEHOCHBI were ꢁ5.5
and ꢁ3.8 eV, respectively, with band gaps of just about 1.67
eV. As compared to the case of PCDTBT, PCDTCHBI and
PCDTEHOCHBI have the same or deeper HOMO energy levels
and lower LUMO energy levels, which is ideal for the poly-
mer solar cell device. Among the polymer blends, PCDTE-
HOCHBI:PC71BM (1:4) exhibited best photovoltaic perform-
ance, with Voc ¼ 0.72 V, Jsc ¼ 4.31 mA/cm2, and FF ¼ 0.37,
giving a PCE of 1.15%. Considering their desirable electronic
CONCLUSIONS
Newly designed 2H-benzimidazole derivatives, which substi-
tuted the sulfur of the BT unit of PCDTBT with alkylated car-
bon to improve the solubility of the polymers while keeping
the 1,2-quinoid form, were synthesized as the electron-defi-
cient moiety for ICT copolymers. Despite cyclohexane groups
TABLE 5 Photovoltaic Performances of the Device with the Configuration of ITO/PEDOT:PSS/
Polymer-PC71BM/TiOx/Al
Polymer
Acceptor Ratio Solvent
Jsc (mA/cm2) Voc (V) FF
Efficiency (%)
PCDTCHBI
PCDTCHBI
PC71BM
PC71BM
1:2
1:4
1:2
1:4
CBþDCB 4.94
0.61
0.62
0.60
0.72
0.32 1.06
0.30 1.03
CBþDCB 5.57
CBþDCB 2.42
CBþDCB 4.31
PCDTEHOCHBI PC71BM
PCDTEHOCHBI PC71BM
0.24 0.34
0.37 1.15
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ARTICLE
FIGURE
7 AFM
topographic
images (5 ꢃ 5 lm) of BHJ thin
films (a) PCDTCHBI:PC71BM (1:2),
(b) PCDTCHBI:PC71BM (1:4), (c)
PCDTEHOCHBI:PC71BM (1:2), and
(d) PCDTEHOCHBI:PC71BM (1:4).
[Color figure can be viewed in
the online issue, which is avail-
able at wileyonlinelibrary.com.]
3 Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery,
properties and high solubilities in common organic solvents,
the electron-deficient 2H-benzimidazole derivatives can pro-
vide the fine-tuning of the HOMO and LUMO energy levels of
ICT copolymers and expand the design scope of carbazole
based structures for ICT copolymers with their improved
solubility. Moreover, with our new 2H-benzimidazole deriva-
tives, it is possible to make highly soluble electron-deficient
moieties while keeping the characteristics of BT for ICP
polymer, which will enable the syntheses of the polymers
incorporating electron-rich moieties with extremely low
solubilities.
K.; Yang, Y. Nat Mater 2005, 4, 864–868.
4 Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Adv Funct
Mater 2005, 15, 1617–1622.
5 Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.;
Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-
S.; Ree, M. Nat Mater 2006, 5, 197–203.
6 Muhlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller,
¨
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This work was supported by the National Research Founda-
tion of Korea (NRF) grant funded by the Korea government
(MEST) (2010-0015069). Research at UCSB was supported
by the International Cooperation Research Program (Global
Research Laboratory, GRL) of the Ministry of Education,
Science and Technology of Korea (K20607000004).
8 Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf,
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