Synthesis and properties of the conjugated polymers with indenoindene and benzimidazole units for organic photovoltaics

Article abstract of DOI:10.1002/pola.26385

A new electron deficient unit, dimethyl-2H-benzimidazole (MBI), and dihydroindeno[2,1-a]indene (ININE) moiety as electron-rich unit were coupled to synthesize the conjugated polymers containing electron donor-acceptor pair for organic photovoltaics. ININE, MBI, and thiophene (or bithiophene) units were incorporated using Stille and Suzuki polymerization to generate poly(2,7-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydro- indeno[2,1-a]indene)- alt-5,5-(4′,7′-di-2-thienyl-2,2-dimethyl-2H-benzimidazole)) (PININEDTMBIs) (or PININEBBTMBIs). In MBI, the sulfur at 2-position of 2,1,3-benzothiadiazole (BT) unit was replaced with dialkyl-substituted carbon, whereas keeping the 1,2-quinoid form, to improve the solubility of the polymers. The field-effect hole mobility of PININEBBTMBI was 3.2 × 10^-4 cm²/Vs which was improved as compared to that of PININEDTMBI (2.7 × 10^-5 cm²/Vs) caused by the introduction of bithiophene units. In case of the most efficient polymer, PININEBBTMBI, the device with the configuration of indium tin oxide (ITO)/poly(3,4- ethylenedioxythiophene) (PEDOT):polystyrene sulfonate (PSS)/polymer:PC ₇₁BM(1:4 w/w)/Al, annealed at 100 C for 10 min demonstrated a open circuit voltage of 0.78 V, a short-circuit current density of 6.66 mA/cm ², and a fill factor of 0.41, leading to the power conversion efficiency of 2.11%, under white-light illumination (AM 1.5 G, 100 mW/cm ²).

Full text of DOI:10.1002/pola.26385

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Synthesis and Properties of the Conjugated Polymers with Indenoindene
and Benzimidazole Units for Organic Photovoltaics
Joo Young Shim,1 Byoung Hoon Lee,2 Suhee Song,1 Heejoo Kim,2 Ju Ae Kim,1
Il Kim,3 Kwanghee Lee,2 Hongsuk Suh1
1Department of Chemistry and Chemistry, Institute for Functional Materials, Pusan National University, Busan 609-735, Korea
2Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea
3The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of Polymer Science and Engineering,
Pusan National University, Busan 609-735, Korea
Correspondence to: H. Suh (E-mail: hssuh@pusan.ac.kr)
Received 30 March 2012; accepted 8 September 2012; published online 12 October 2012
DOI: 10.1002/pola.26385
ABSTRACT: A new electron deficient unit, dimethyl-2H-benzimid-
azole (MBI), and dihydroindeno[2,1-a]indene (ININE) moiety as
electron-rich unit were coupled to synthesize the conjugated
polymers containing electron donor–acceptor pair for organic
photovoltaics. ININE, MBI, and thiophene (or bithiophene) units
were incorporated using Stille and Suzuki polymerization to
generate poly(2,7-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydro-
indeno[2,1-a]indene)-alt-5,5-(40,70-di-2-thienyl-2,2-dimethyl-2H-
benzimidazole)) (PININEDTMBIs) (or PININEBBTMBIs). In MBI,
the sulfur at 2-position of 2,1,3-benzothiadiazole (BT) unit was
replaced with dialkyl-substituted carbon, whereas keeping the
1,2-quinoid form, to improve the solubility of the polymers.
The field-effect hole mobility of PININEBBTMBI was 3.2 ꢀ 10ꢁ4
cm2/Vs which was improved as compared to that of PINI-
NEDTMBI (2.7 ꢀ 10ꢁ5 cm2/Vs) caused by the introduction of
bithiophene units. In case of the most efficient polymer, PINI-
NEBBTMBI, the device with the configuration of indium tin
oxide (ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT):polys-
tyrene sulfonate (PSS)/polymer:PC71BM(1:4 w/w)/Al, annealed
at 100 ꢂC for 10 min demonstrated a open circuit voltage of
0.78 V, a short-circuit current density of 6.66 mA/cm2, and a fill
factor of 0.41, leading to the power conversion efficiency of
2.11%, under white-light illumination (AM 1.5 G, 100 mW/cm2).
C
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2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
51: 241–249, 2013
KEYWORDS: conjugated polymer; copolymerization; synthesis;
dihydroindeno[2,1-a]indene; 2H-benzimidazole
INTRODUCTION One of the most successful approaches for
the organic photovoltaics (OPVs) is bulk heterojunction solar
cell,1–3 which possesses distinctive advantages including
favorable solution processability, such as ink-jet printing4
and roll-to-roll process,5 which could be quite important for
low-cost manufacturing and commercialization.6–8 The disad-
vantage of OPVs including the lower power-conversion effi-
ciency and smaller photocurrent than that of inorganic solar
cells can be overcome by the development of novel materials
with wider coverage of the solar spectrum and higher
absorption coefficients.9–12
ance in the whole visible region.13 New electron deficient
unit, dimethyl-2H-benzimidazole (MBI), to substitute 2,1,3-
benzothiadiazole (BT) unit of poly[N-9-heptadecanyl-2,7-car-
bazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)],
has been designed and utilized for the intramolecular charge
transfer for OPVs.15–17 While keeping the 1,2-quinoid form,
this novel MBI unit has two methyl groups which can supply
higher solubility than those of the BT series.
4,7-Di(2,20-bithien-5-yl)-2,1,3-benzothiadiazole
(BBTBT)
which has bithiophene unit to substitute the thiophene unit
of 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) provides
longer wavelength absorption as compared to DTBT.18
Poly{5,11-di(1-decylundecyl)indolo[3,2-b]carbazole-3,9-diyl-
alt-4,7-di(2,20-bithien-5-yl)-2,1,3-benzothiadiazole-50,500-diyl}
(PICBBTBT) has higher PCE than poly{5,11-di(1-decylunde-
cyl)indolo[3,2-b]carbazole-3,9-diyl-alt-4,7-dithien-2-yl-2,1,3-
benzothiadiazole-50,500-diyl} (PICDTBT) caused by longer
wavelength absorption.18 In addition, it could be expected
that the introduction of bithiophene unit will improve the
In our previous study, the polymers with dihydroinde-
no[2,1-a]indene moiety (ININE)13,14 as electron-rich unit
show absorption up to the longer wavelength region caused
by the incorporation of one more vinylene unit in the bicy-
clic [2,2,0] system. The polymers incorporating ININE unit
have not only good processability owing to the tetra-alkyl
groups which can be incorporated without generating distor-
tion of the conjugation, but also strong and uniform absorb-
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NEDTMBI) and further synthesized poly[2,7-(5,5,10,10-tetra-
kis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene)-alt-5,5-
(4,7-bis(2,2-bithien-5-yl)-2,2-dimethyl-2H-benzimidazole)]
(PININEBBTMBI) with bithiophene (BBT) to substitute
DT linkage. (Fig. 1) In addition to this, the corresponding
copolymers were also synthesized to check the difference
of the properties. The photovoltaic properties of the poly-
mers were investigated by fabrication of the polymer so-
lar cells with the configuration ITO/PEDOT:PSS/poly-
mer:PC71BM(1:4 w/w)/TiOx/Al.
RESULTS AND DISCUSSION
Synthesis and Characterization
The general synthetic routes toward the monomers and polymers
are shown in Schemes 1 and 2. 4,7-Dibromo-2,2-dimethyl-2H-benz-
imidazole (8) was coupled with tributyl(2-thienyl)stannane20 in tet-
rahydrofuran (THF) by dichlorobis(triphenylphosphine)palladium
(II) to give 2,2-dimethyl-4,7-di(2-thienyl)-2H-benzimidazole (9).
Compound 9 was brominated in THF by using N-bromosuccini-
mide (NBS) to give monomer 4,7-bis(5-bromo-2-thienyl)-
2,2-dimethyl-2H-benzimidazole (10). Compound 8 was coupled
with [2,20]bithiophenyl-5-yl-tributyl-stannane21 in THF by using
dichlorobis(triphenylphosphine)palladium (II) to give 2,2-di-
methyl-4,7-di(2-thienyl)-2H-benzimidazole (11). Compound 11
was brominated in THF by NBS to give monomer
4,7-bis(5-(5-bromothiophen-2-yl)-thiophen-2-yl))-2,2-dimethyl-
2H-benzimidazole (12). Compound 713 and 4,7-bis(5-
bromo-2-thienyl)-2,2-dimethyl-2H-benzimidazole (10) (or
4,7-bis(5-(5-bromothiophen-2-yl)-thiophen-2-yl))-2,2-dimethyl-
2H-benzimidazole (12)) were coupled through Suzuki
FIGURE 1 Structure of PININEDTMBI and PININEBBTMBI.
hole mobility of the polymer caused by the increased pack-
ing ability.19
In this study, we incorporated ININE and MBI units using
dithiophene (DT) linkage, to provide poly(2,7-(5,5,10,10-tet-
rakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene)-alt-5,5-
(40,70-di-2-thienyl-2,2-dimethyl-2H-benzimidazole))
(PINI-
SCHEME 1 Synthetic route for the synthesis of the monomers.
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SCHEME 2 Synthetic route for the synthesis of polymers.
polymerization with Pd(0)-catalyst to yield PININEDTMBI
(or PININEBBTMBI). The copolymers that consist of
2,7-dibromo-5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroin-
deno[2,1-a]indene (6) and 2,5-bis(trimethylstannyl)thiophene
poly dispersity index (PDI, Mw/Mn) values of 2.2, 1.6, 1.7,
and 1.8 were determined by gel permission chromatography
(GPC) for the PININEDTMBI, PININEDTMBI-5, PINI-
NEBBTMBI, and PININEBBTMBI-5, respectively. The thermal
properties of the polymers were characterized by thermal
gravimetric analysis (TGA). Data of TGA of the polymers are
shown in Figure 2 and summarized in Table 1. TGA was per-
formed with a Dupont 951 TGA instrument in a nitrogen
atmosphere at a heating rate of 10 ꢂC/min to 600 ꢂC. TGA
showed that PININEDTMBI, PININEDTMBI-5, PINI-
NEBBTMBI, and PININEBBTMBI-5 are thermally stable with
only about 5% weight loss in air at temperatures of 383,
(13)22 (or 5,50-bis(trimethylstannyl)-2,20-bithiophene (14)23,24
)
as electron-rich units and 4,7-dibromo-2,2-dimethyl-2H-benz-
imidazole (8) as electron-deficient moiety were synthesized by
Pd(0)-catalyzed Stille coupling polymerization in chloroben-
zene. The comonomer feed ratios of 6–8 are 5:5. All of the poly-
mers show good solubility at room temperature in organic sol-
vents such as chloroform, THF, chlorobenzene, and o-
dichlorobenzene (ODCB).
ꢂ
385, 385, and 378 C, respectively. Differential scanning calo-
Table 1 summarizes the polymerization results including mo-
lecular weights, PDI, and thermal stability of the copolymers.
The number-average molecular weights (Mn) of 17,000,
11,000, 12,000, and 11,500 and weight-average molecular
weights (Mw) of 38,000, 18,000, 21,000, and 21,000 with
rimetry (DSC) revealed no obvious glass transition for poly-
mers up to 300 ꢂC. The high thermal stability of the result-
ing polymers prevents the deformation of the polymer
TABLE 1 Polymerization Results and Thermal Properties of
Polymers
PDIa
TGA (Td)b
a
a
Polymer
Mn
Mw
PININEDTMBI
PININEDTMBI-5
PININEBBTMBI
PININEBBTMBI-5
a
17,000
11,000
12,000
11,500
38,000
18,000
21,000
21,000
2.2
1.6
1.7
1.8
383
385
385
378
Molecular weight (Mw) and PDI of the polymers were determined by
GPC in THF using polystyrene standards.
b
Onset decomposition temperature (5% weight loss) measured by TGA
under N2.
FIGURE 2 Thermogravimetric analysis of the polymers under N2.
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and 637 nm. The PININEDTMBI thin films show two broad
absorption bands with peaks at 415 and 632 nm. The maxi-
mum absorption peak of PININEDTMBI-5 appears at about
457 and 634 nm in ODCB solution. The spectra of the solid
film of PININEDTMBI-5 show absorption bands with maxi-
mum peaks at about 452 and 630 nm. The maximum
absorption peaks of PININEBBTMBI and PININEBBTMBI-5
appear at 438, 634, 463, and 634 nm in ODCB solution,
respectively. The maximum absorption peaks in solid thin
films were at around 435, 632, 460, and 640 nm, respec-
tively. The short-wavelength absorption peaks have been
ascribed to a delocalized excitonic p–p* transition in the
polymer chains and the long-wavelength absorption peaks
attributed to the ICT between the ININE-DT (ININE-BBT)
electron-rich unit and the MBI electron-deficient unit.25 In
the alternating copolymers, PININEDTMBI and PINI-
NEBBTMBI which show stronger long wavelength absorption
peaks caused by the higher ICT effects as compared to the
case of the random copolymers, PININEDTMBI-5 and PINI-
NEBBTMBI-5.
Electrochemical Properties
The electrochemical properties of the polymers were deter-
mined from the band gaps estimated from the absorption
onset wavelength, and the highest occupied molecular orbital
(HOMO) energy levels which were estimated from the cyclic
voltammetry (CV). The CV was performed with a solution of
tetrabutylammonium tetrafluoroborate (Bu4NBF4) (0.10 M)
in acetonitrile at a scan rate of 100 mV/s at room tempera-
ture under argon atmosphere. A platinum electrode (ꢃ0.05
cm2) coated with a thin polymer film was used as the work-
ing electrode. Pt wire and Ag/AgNO3 electrode were used as
the counter electrode and reference electrode, respectively.
The energy level of the Ag/AgNO3 reference electrode (cali-
brated by the Fc/Fcþ redox system) was 4.8 eV below the
vacuum level. The CV spectra are shown in Figure 4, and the
oxidation potentials derived from the onsets of electrochemi-
cal p-doping are summarized in Table 2. HOMO and lowest
FIGURE 3 UV–visible absorption spectra of polymers in ODCB
solution (a) and in the solid state (b).
morphology and is important for organic photovoltaic
applications.
Optical Properties
The solution was prepared using ODCB as a solvent and the
thin film by spin coating on quartz plates from the solution
in ODCB. The UV–visible (vis) absorption spectra of the poly-
mers in solution and as thin films are shown in Figure 3 and
summarized in Table 2. The solution of PININEDTMBI
presents two absorption bands with maximum peaks at 418
unoccupied molecular orbital (LUMO) levels were calculated
ox
according to the empirical formula (EHOMO ¼ -([Eonset
]
þ
red
4.8) eV) and (ELUMO ¼ -([Eonset
]
þ 4.8) eV), respectively.
The polymers, PININEDTMBI, PININEDTMBI-5, PINI-
NEBBTMBI, and PININEBBTMBI-5, exhibited the absorption
TABLE 2 Characteristics of the UV–vis Absorption Spectra, Electrochemical Potentials, and Energy Levels of the Polymers
Optical
Chemical
Band
Gapf (eV)
HOMOc
(eV)
LUMOd
(eV)
Eox
Ered
b
e
e
Band
Gapa (eV)
Soln kmax
Film kmax
Polymers
(nm)
(nm)
(V)
(V)
PININEDTMBI
PININEDTMBI-5
PININEBBTMBI
PININEBBTMBI-5
a
1.69
1.69
1.66
1.66
418, 637
457, 634
438, 634
463, 634
415, 632
452, 629
435, 632
460, 640
ꢁ5.44
ꢁ5.43
ꢁ5.44
ꢁ5.49
ꢁ3.75
ꢁ3.74
ꢁ3.78
ꢁ3.83
0.64
0.63
0.64
0.69
ꢁ1.21
ꢁ1.20
ꢁ1.22
ꢁ1.14
1.85
1.83
1.86
1.83
d
e
Optical energy band gap was estimated from the onset wavelength
Calculated from the HOMO energy levels and Eg.
Onset oxidation and reduction potential measured by
of the optical absorption. Eg ¼ 1240/konset
.
b
Solution absorption in o-chlorobenzene.
Calculated from the oxidation potentials.
cyclic voltammetry.
c
f
Calculated from the Eox and Ered.
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FIGURE 4 Electrochemical properties of polymers.
FIGURE 6 Current density–potential characteristics of the poly-
mer:PC71BM(1:4) blend under the illumination of AM 1.5, 100
mW/cm2.
onset wavelengths of 733, 735, 749, and 748 nm in solid
thin film, which corresponds to band gaps of 1.69, 1.69,
1.66, and 1.66 eV, respectively. The polymers exhibit irre-
versible processes in an oxidation scan. The oxidation onsets
of the polymers, PININEDTMBI, PININEDTMBI-5, PINI-
NEBBTMBI, and PININEBBTMBI-5, were estimated to be
0.64, 0.63, 0.64, and 0.69 V, which correspond to HOMO
energy levels of ꢁ5.44, ꢁ5.43, ꢁ5.44, and ꢁ5.49 eV, respec-
tively. The HOMO energy level of PININEBBTMBI-5 is higher
than that of PININEDTMBI-5, which can be attributed to the
introduction of the electron-donating bithiophene to the
backbone. The reduction potential onsets of the polymers,
PININEDTMBI, PININEDTMBI-5, PININEBBTMBI, and PINI-
NEBBTMBI-5, are ꢁ1.21, ꢁ1.20, ꢁ1.22, and ꢁ1.14 eV,
respectively, which corresponds to LUMO energy levels of
ꢁ3.75, ꢁ3.74, ꢁ3.78, and ꢁ3.83 eV, respectively. The electro-
chemical band gaps, calculated from CV data, are about 1.85,
1.83, 1.86, and 1.83 eV, somewhat higher than the optical
band gaps estimated from the onset wavelengths of the
absorption spectra.
transfer characteristics of the polymer devices of the octyltri-
chlorosilane (OTS)-modified SiO2. The hole mobilities of the
polymers were calculated from the transfer characteristics of
the OFETs. The field-effect hole mobilities of PININEDTMBI,
PININEDTMBI-5, PININEBBTMBI, and PININEBBTMBI-5 are
2.7 ꢀ 10ꢁ5, 3.1 ꢀ 10ꢁ5, 3.2 ꢀ 10ꢁ4, and 1.1 ꢀ 10ꢁ4 cm2/Vs,
respectively.
Polymer Photovoltaic Properties
The photovoltaic cells were fabricated by spin coating the
bulk heterojunction films from ODCB solutions of poly-
mers:PC71BM(1:4 w/w). PC71BM as the electron-accepting
component has been widely used in OPV devices. Typical
current density–voltage (J–V) characteristics of the devices
with the configuration of ITO/PEDOT:PSS/polymer:
PC71BM(1:4 w/w)/Al under AM 1.5 G irradiation (100 mW/
cm2) are shown in Figure 6. The photovoltaic parameters for
all the polymers, including open circuit voltage (VOC), short-
circuit current density (JSC), fill factor (FF), and power con-
version efficiency (PCE) are summarized in Table 3. The
devices with PININEDTMBI and PININEDTMBI-5 with
PC71BM showed VOC values of 0.80 and 0.81 V, JSC values of
4.06 and 1.74 mA/cm2, and FF of 0.34 and 0.40 giving
power conversion efficiencies of 1.09 and 0.56%, respec-
tively. The devices based on PININEBBTMBI and PINI-
NEBBTMBI-5, using bithiophene unit, with PC71BM with
Hole Mobility from Field-Effect Transistor
Characteristics
The field-effect carrier mobilities of the polymers were eval-
uated by fabricating thin-film field-effect transistors (FETs)
based on the top-contact geometry. Figure 5 shows the FET
TABLE 3 Photovoltaic Properties and TFT Characteristics of the
Polymer Solar Cells
Mobility
Polymer: VOC JSC
PC71BM (V) (mA/cm2) FF (%) Vs)
PCE (cm2/
Polymers
PININEDTMBI 1:4
0.80 4.06
0.81 1.74
0.34 1.09 2.7 ꢀ 10ꢁ5
0.40 0.56 3.1 ꢀ 10ꢁ5
PININEDTMBI- 1:4
5
PININEBBTMBI 1:4
0.78 6.66
0.80 5.38
0.41 2.11 3.2 ꢀ 10ꢁ4
0.37 1.58 1.1 ꢀ 10ꢁ4
FIGURE 5 Transfer characteristics of top-contact OFETs from
PININEBBTMBI- 1:4
5
polymers.
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ascribed to its highest hole mobility out of the series. The
atomic force microscopy image of the PININEDTM-
BI:PC71BM(1:4 w/w) composites shows large domain
(Fig. 8) size which results in the exciton recombination and
consequently reduces the charge separation efficiency.26 For
this reason, PININEBBTMBI with smaller domain size has
the highest PCE value out of the series. Although PINI-
NEDTMBI-5 and PININEBBTMBI-5 have smoother morphol-
ogy, the random copolymers have lower efficiency caused by
poor ICT effect. The incident photon-to-current efficiency
(IPCE) spectra of the photovoltaic devices from poly-
mer:PC71BM(1:4w/w) blends are presented in Figure 7. The
IPCE spectra of the polymers show maxima of 22.4% at 480
nm for PININEDTMBI, 16.6% at 470 nm for PININEDTMBI-
5, 45.6% at 470 nm for PININEBBTMBI, and 43.6% at 480
nm for PININEBBTMBI-5. The enhanced efficiency of PINI-
NEBBTMBI results from the higher IPCE values between
300 and 600 nm as compared to the cases of other
polymers.
FIGURE 7 IPCE curves of polymer:PCBM (1:4) blend under the
illumination of AM 1.5, 100 mW/cm2.
thermal treatment showed power conversion efficiencies of
up to 2.11 and 1.58%. The devices had VOC values of 0.78
and 0.80 V, JSC values of 6.66 and 5.38 mA/cm2, and FF of
0.41 and 0.37, respectively. Although PININEBBTMBI (or
PININEBBTMBI-5) has a similar molecular weight as com-
pared to PININEDTMBI (or PININEDTMBI-5), JSC value of
PININEBBTMBI (or PININEBBTMBI-5) is higher than that of
PININEDTMBI (or PININEDTMBI-5) as summarized in Table
3. The highest PCE value of PININEBBTMBI could be
CONCLUSIONS
ININE and MBI units were incorporated to provide PINI-
NEDTMBI using DT linkage and PININEBBTMBI using BBT
moiety and corresponding copolymers were synthesized to
generate PININEDTMBI-5 and PININEBBTMBI-5. The
FIGURE 8 Atomic force microscopy images of PININEDTMBI/PC71BM (a), PININEDTMBI-5/PC71BM (b), PININEBBTMBI/PC71BM (c),
and PININEBBTMBI-5/PC71BM (d).
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PININEBBTMBI thin film shows two broad absorption peaks
at 438 and 634 nm and an absorption onset at 748 nm, cor-
responding to a band gap of 1.66 eV. The oxidation onset of
PININEBBTMBI was estimated to be 0.64 V, which corre-
sponds to a HOMO energy level of ꢁ5.47 eV. The device fab-
ricated with a PININEBBTMBI:PC71BM(1:4 w/w) blend dem-
onstrated VOC value of 0.78 V, JSC value of 6.66 mA/cm2, and
FF of 0.41, leading to a power conversion efficiency of
2.11%. The higher efficiency of PININEBBTMBI as compared
to those of other polymers is owing to the high JSC, which is
attributed to the small domain size with PC71BM in blend
film and higher hole mobility.
ous solution to form a film of 40 ꢂnm thickness. The sub-
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 poly-
mer:PC71BM(1:4 w/w) in dichlorobenzene solvent with con-
centration of 7 wt/mL % was then spin cast on top oꢂf the
PEDOT/PSS layer. The film was dried for 60 min at 70 C in
the glove box. Then, an aluminum (Al, 100 nm) electrode
was deposited by thermal evaporation in a vacuum of about
5 ꢀ 10ꢁ7 Torr. Current density–voltage (J–V) characteristics
of the devices were measured using a Keithley 236 Source
Measure Unit. Solar cell performance was measure by using
an Air Mass 1.5 Global (AM 1.5 G) solar simulator with an
irradiation intensity of 1000 W mꢁ2. An aperture (12.7
mm2) was used on top of the cell to eliminate extrinsic
effects such as crosstalk, waveguiding, shadow effects, and
so on. The spectral mismatch factor was calculated by com-
parison of solar simulator spectrum with AM 1.5 spectrum
at room temperature.
EXPERIMENTAL
General
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.
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
unless otherwise indicated. Analytical thin-layer chromato-
graphy was conducted using Merck 0.25-mm silica gel 60F
precoated aluminum plates with fluorescent indicator
UV254. High-resolution mass spectra (HRMS) were recorded
on a JEOL JMS-700 mass spectrometer under electron impact
(EI) conditions in the Korea Basic Science Institute (Daegu).
Molecular weight and polydispersity of the polymer were
determined by GPC analysis with a polystyrene standard cali-
bration. The UV–vis absorption spectra were recorded by a
Varian 5 E UV/VIS/NIR spectrophotometer. The FETs were
fabricated on heavily doped n-type silicon (Si) wafers each
covered with a thermally grown silicon dioxide (SiO2) layer
with thickness of 200 nm. The active layer was deposited by
spin coating at 3000 rpm. Prior to active layer deposition,
SiO2 surfaces were treated with OTS to make surface hydro-
phobic. All solutions were of 1 vol % concentration in 1,2-
dichlorobenzene. The thickness of the deposited films was
about 120 nm. Prior to deposition of source drain electrodes,
the films were dried on hot plate stabilized at 80 ꢂC for 10
min. All fabrication processes were carried out in the glove
box filled with N2. Source and drain electrodes using Au
were deposited by thermal evaporation using shadow mask.
The thickness of source and drain electrodes was 50 nm.
Channel length (L) and channel width (W) were 50 mm and
1.5 mm, respectively. Electrical characterization was per-
formed using a Keithley semiconductor parametric analyzer
(Keithley 4200) under N2 atmosphere. Solar cells were fabri-
cated on an ITO-coated glass substrate with the following
structure; ITO-coated glass substrate/(PEDOT:PSS)/polymer:
PC71BM(1:4 w/w)/TiOx/Al. 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-
Synthesis of 2,2-Dimethyl-4,7-di(2-thienyl)-2H-
benzimidazole (9)
To a stirred solution of 4,7-dibromo-2,2-dimethyl-2H-benzim-
idazole (8) (2.7 g, 8.9 mmol) and tributyl(2-thienyl)stannane
(16.6 g, 44.4 mmol) in 40 mL of THF at room temperature
was added dichlorobis(triphenylphosphine)palladium(II) (2
ꢂ
mol %). The reaction mixture was stirred for 12 h at 80 C,
concentrated under reduced pressure, and purified by flash
column chromatography to give compound 9 as red solid
ꢂ
(mp, 172 C).
1H NMR (300 MHz, CDCl3): d (ppm) 1.67 (s, 6H), 7.13 (d of
d, 2H, J ¼ 3.9 and 5.4 Hz), 7.30 (s, 2 H), 7.38 (d of d, 2H. J ¼
1.1 and 4.9 Hz), 8.03 (d of d, 2 H, J ¼ 1.1 and 3.5 Hz); 13C
NMR (75 MHz, CDCl3): d (ppm) 22.38, 105.40, 126.85,
128.24, 128.28, 128.80, 128.99, 138.94, 157.94. HRMS (m/z,
EIþ) calcd C17H14N2S2 310.0598, found 310.0600.
Synthesis of 4,7-Bis(5-bromo-2-thienyl)-2,2-dimethyl-2
H-benzimidazole (10)
To a stirred solution of 2,2-dimethyl-4,7-di(2-thienyl)-2 H-
benzimidazole (8) (1.3 g, 4.19 mmol) in THF (20 mL) at
room temperature was added NBS (1.52 g, 8.6 mmol). After
3 h at room temperature, water and ethyl acetate were
added. The mixture was washed with 3 ꢀ 100 mL of water.
The organic layer was concentrated under reduced pressure
and purified by flash column chromatography to give 1.25 g
ꢂ
(64%) of compound 10 as a red solid (mp, 207 C).
1H NMR (300 MHz, CDCl3): d (ppm) 1.58 (s, 6H), 7.08 (d,
2H, J ¼ 3.9 Hz), 7.21 (s, 2H), 7.69 (d, 2 H, J ¼ 3.9 Hz); 13C
NMR (75 MHz, CDCl3): d (ppm) 22.02, 105.59, 115.11,
127.56, 127.90, 128.08, 130.72, 139.86, 157.45. HRMS (m/z,
EIþ) calcd for C17H12Br2N2S2 465.8809, found 465.8810.
Synthesis of 4,7-Bis(2,2-bithien-5-yl)-2,2-dimethyl-2
H-benzimidazole) (11)
To a stirred solution of 4,7-dibromo-2,2-dimethyl-2 H-benz-
imidazole (8) (5.0 g, 16.5 mmol) and [2,20]bithiophenyl-5-yl-
tributyl-stannane (16.1 g, 49.4 mmol) in 40 mL of THF at
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room temperature was added dichlorobis(triphenylphosphi-
ne)palladium(IIꢂ) (2 mol %). The reaction mixture was stirred
for 12 h at 80 C, concentrated under reduced pressure, and
purified by flash column chromatography to give compound
4b,5,9b,10-tetrahydroindeno[2,1-a]inden-2-yl]-1,3,2-dioxaboro-
lane (7) (400 mg, 0.44 mmol), 4,7-bis(5-(5-bromothiophen-2-
yl)-thiophen-2-yl))-2,2-dimethyl-2H-benzimidazole) (12) (280
mg, 0.44 mmol) and (PPh3)4Pd(0) (3 mol %) were mixed with
a mixture of toluene and an aqueous solution of 2 M K2CO3.
After reflux for 72 h under argon atmosphere, the reaction
mixture was treated with phenylboronic acid. After additional
reflux for 12 h, the reaction mixture was treated with 0.1 mL
of bromobenzene followed by reflux overnight to complete the
end-capping reaction. After cooling to room temperature, the
reaction mixture was poured into methanol. The precipitated
material was recovered by filtration. The resulting solid mate-
rial was reprecipitated using 100 mL of THF/1.0 L of methanol
several times to remove catalyst residues and to provide the
polymer, PININEBBTMBI with 50% yield. The resulting poly-
mer was soluble in THF, CHCl3, ODCB, and toluene.
ꢂ
11 as shiny purple solid (mp, 190 C).
1H NMR (300 MHz, Benzene-d6): d (ppm) 1.50 (s, 6H), 6.59
(d of d, 2H, J ¼ 3.6 Hz and 4.8 Hz), 6.67 (d, 2H, J ¼ 4.4 Hz),
6.74 (s, 2H), 7.00 (d, 2 H. J ¼ 3.6 Hz), 7.04 (d, 2H. J ¼ 3.8 Hz),
8.00 (d, 2H, J ¼ 3.5 Hz).13C NMR (75 MHz, CDCl3): d (ppm)
22.46, 105.53, 124.41, 124.93, 125.14, 128.23, 128.29, 128.35,
129.21, 137.52, 137.82, 138.88, 158.03. HRMS (m/z, EIþ)
calcd for C25H18N2S4474.0350, found 474.0353.
Synthesis of 4,7-Bis(5 -(5-bromothiophen-2-yl)-thiophen-
2-yl))-2,2-dimethyl-2H-benzimidazole) (12)
To a stirred solution of 4,7-bis(2,2-bithien-5-yl)-2,2-dimethyl-
2H-benzimidazole (11) (0.9 g, 1.90 mmol) in THF at 0 ꢂC
was added NBS (0.67 g, 3.80 mmol). After 3 h at room tem-
perature, water and ethyl acetate were added. The mixture
was washed with 3 ꢀ 100 mL of water. The organic phase
was concentrated under reduced pressure and purified by
flash column chromatography to give compound 12 as shiny
Polymerization of Poly(5,5,10,10-tetrakis(2-ethylhexyl)-
5,10-dihydroindeno[2,1-a]indene-co-5,5-(40,70-di-2-
thienyl-2,2-dimethyl-2H-benzimidazole))-5
Carefully purified 4,7-dibromo-2,2-dimethyl-2H-benzimida-
zole (8) (149 mg, 0.5 mmol), 2,7-dibromo-5,5,10,10-tetra-
kis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene (6) (400
mg, 0.5 mmol), 2,5-bis(trimethylstannyl)thiophene (13) (404
mg, 1.0 mmol), tri(o-tolyl)phosphine (3.2 mg, 4 mol %) and
Pd2(dba)3(3 mol %) were dissolved in dry chlorobenzene.
After reflux for 36 h under argon atmosphere, the reaction
mixture was treated with trimethyl(phenyl)tin. After addi-
tional reflux for 12 h, the reaction mixture was treated with
0.1 mL of bromobenzene followed by reflux overnight to
complete the end-capping reaction. After cooling to room
temperature, the reaction mixture was poured into methanol.
The precipitated material was recovered by filtration. The
resulting solid material was reprecipitated using 100 mL of
THF/1.0 L of methanol several times to remove catalyst resi-
dues and to provide the polymer, PININEDTMBI-5 with 50%
yield. The resulting polymer was soluble in THF, CHCl3,
ODCB, and toluene.
ꢂ
purple solid (mp, 210 C).
1H NMR (300 MHz, CDCl3): d (ppm) 1.58 (s, 6H), 7.00 (s,
4H), 7.11 (d, 2H, J ¼ 3.8 Hz), 7.25 (s, 2H), 7.91 (d, 2H, J ¼
4.1 Hz); 13C NMR (75 MHz, CDCl3): 22.39, 105.65, 111.79,
124.42, 125.06, 128.35, 128.40, 129.06, 131.03, 137.94,
138.15, 138.99, 157.91.HRMS (m/z, EIþ) calcd for
C25H16Br2N2S4631.8564, found 631.8543.
Polymerization of Poly(2,7-(5,5,10,10-tetrakis
(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene)-alt-
5,5-(40,70-di-2-thienyl-2,2-dimethyl-2H-benzimidazole))
Carefully purified 4,4,5,5-tetramethyl-2-[5,5,10,10-tetrakis(2-
ethylhexyl)-7-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-4b,
5,9b,10-tetrahydroindeno[2,1-a]inden-2-yl]-1,3,2-dioxaborolane
(7), 4,7-bis(5-bromo-2-thienyl)-2,2-dimethyl-2H-benzimidazole
(10) (440 mg, 0.89 mmol) and (PPh3)4Pd(0) (3 mol %) were
mixed with a mixture of toluene and an aqueous solution of 2
M K2CO3. After reflux for 72 h under argon atmosphere, the
reaction mixture was treated with phenylboronic acid. After
additional reflux for 12 h, the reaction mixture was treated
with 0.1 mL of bromobenzene followed by reflux overnight to
complete the end-capping reaction. After cooling to room tem-
perature, the reaction mixture was poured into methanol. The
precipitated material was recovered by filtration. The resulting
solid material was reprecipitated using 100 mL of THF/1.0 L
of methanol several times to remove catalyst residues and to
provide the polymer, PININEDTMBI with 50% yield. The
resulting polymer was soluble in THF, CHCl3, ODCB, and
toluene.
Polymerization of Poly(5,5,10,10-tetrakis(2-ethylhexyl)-
5,10-dihydroindeno[2,1-a]indene-co-5,5-(40,70-bis-2,20-
bithienyl-2,2-dimethyl-2H-benzimidazole))-5
Carefully purified 4,7-dibromo-2,2-dimethyl-2H-benzimida-
zole (8) (149 mg, 0.5 mmol), 2,7-dibromo-5,5,10,10-tetra-
kis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene (6) (400
mg, 0.5 mmol), 5,50-bis(trimethylstannyl)-2,20-bithiophene
(14) (484 mg, 1.0 mmol), tri(o-tolyl)phosphine (3.2 mg, 4
mol %), and Pd2(dba)3(3 mol %) were dissolved in dry
chlorobenzene. After reflux for 36 h under argon atmos-
phere, the reaction mixture was treated with trimethyl(phe-
nyl)tin. After reflux for 12 h, the reaction mixture was
treated with 0.1 mL of bromobenzene followed by reflux
overnight to complete the end-capping reaction. After cooling
to room temperature, the reaction mixture was poured into
methanol. The precipitated material was recovered by filtra-
tion. The resulting solid material was reprecipitated using
100 mL of THF/1.0 L of methanol several times to remove
Polymerization of Poly(2,7-(5,5,10,10-tetrakis
(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene)-alt-
5,5-(4,7-bis(2,2-bithien-5-yl)-2,2-dimethyl-
2H-benzimidazole))
Carefully purified 4,4,5,5-tetramethyl-2-[5,5,10,10-tetrakis(2-
ethylhexyl)-7-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
catalyst
residues
and
to
provide
the
polymer,
248
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6 Sacriciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Sci-
ence 1992, 258, 1474–1476.
PININEBBTMBI-5 with 50% yield. The resulting polymer
was soluble in THF, CHCl3, ODCB, and toluene.
7 Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Sci-
ence 1995, 270, 1789–1791.
Poly(2,7-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroin-
deno[2,1-a]indene)-alt-5,5-(40,70-di-2-thienyl-2,2-di-
methyl-2H-benzimidazole))
8 Nie, Y.; Zhao, B.; Tang, P.; Jiang, P.; Tian, Z.; Shen, P.; Tan,
S.; J. Polym. Sci. Part A: Polym. Chem. 2011, 49, 3604–3614.
Anal. Cald for C65H86N2S2: C, 81.36; H, 9.03; N, 2.92; S, 6.68.
Found: C, 74.54; H, 8.36; N, 2.68; S, 6.02.
9 Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Wal-
dauf, C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18,
789–794.
Poly(2,7-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroin-
deno[2,1-a]indene)-alt-5,5-(40,70-di-2-thienyl-2,2-di-
methyl-2H-benzimidazole))-5
Anal. Cald for C65H86N2S2: C, 81.36; H, 9.03; N, 2.92; S, 6.68.
Found: C, 74.50; H, 8.08; N, 2.76; S, 6.10.
10 Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.;
Heeger, A. J.; Bazan, G. C. Nat. Mater. 2007, 6, 497–500.
11 Kim, J. Y.; Kim, S. H.; Lee, H. H.; Lee, K.; Ma, W.; Gong, X.;
Heeger, A. J. Adv. Mater. 2006, 18, 572–576.
12 Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2006, 19,
2295–2300.
Poly(2,7-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroin-
deno[2,1-a]indene)-alt-5,5-(40,70-di-2-thienyl-2,2-di-
methyl-2H-benzimidazole))
Anal. Cald for C73H90N2S4: C, 78.02; H, 8.07; N, 2.49; S,
11.41. Found: C, 73.17; H, 7.48; N, 2.10; S, 9.81.
13 Song, S.; Jin, Y.; Kim, S. H.; Moon, J.; Kim, K.; Kim, J. Y.;
Park, S. H.; Lee, K.; Suh, H. Macromolecules 2008, 41,
7296–7305.
14 Kim, S. H.; Hwang, I. W.; Jin, Y.; Song, S.; Moon, J.; Suh,
H.; Lee, K. Sol Energy Mater. Sol. Cells 2011, 95, 361–364.
Poly(2,7-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroin-
deno[2,1-a]indene)-alt-5,5-(40,70-di-2-thienyl-2,2-di-
methyl-2H-benzimidazole))-5
Anal. Cald for C73H90N2S4: C, 78.02; H, 8.07; N, 2.49; S,
11.41. Found: C, 74.47; H, 8.14; N, 2.84; S, 6.29.
15 Song, S.; Jin, Y.; Park, S. H.; Cho, S.; Kim, I.; Lee, K.;
Heeger, A. J.; Suh, H. J. Mater. Chem. 2010, 20, 6517–6523.
16 Song, S.; Park, S. H.; Jin, Y.; Park, J.; Shim, J. Y.; Kim, I.;
Lee, H.; Lee, K.; Suh, H. J. Polym. Sci. Part A: Polym. Chem.
2010, 48, 4567–4573.
17 Song, S.; Park, S. H.; Jin, Y.; Kim, I.; Lee, K.; Suh, H. Poly-
mer 2010, 51, 5385–5391.
ACKNOWLEDGMENTS
18 Zhou, E.; Yamakawa, S.; Zhang, Y.; Tajima, K.; Yang, C.;
Hashimoto, K. J. Mater. Chem. 2009, 19, 77302–7737.
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MEST)
(No. 2010-0015069).
19 Jeong, H.-G.; Lim, B.; Na, S.-I.; Baeg, K.-J.; Kim, J.; Yun, J.-
M.; Kim, D.-Y. Macromol. Chem. Phys. 2011, 212, 2308–2318.
20 Yin, B.; Jiang, C.; Wang, Y.; La, M.; Liu, P.; Deng, W.; Synth.
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