Copolymer derived from dihexyl-2H-benzimidazole and carbazole for organic photovoltaic cells

Solar Energy Materials & Solar Cells 95 (2011) 521–528

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Copolymer derived from dihexyl-2H-benzimidazole and carbazole for organic photovoltaic cells Suhee Song a, Sung Heum Park b, Youngeup Jin c, Il Kim d, Kwanghee Lee b, Hongsuk Suh a,n a

Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea c Department of Industrial Chemistry, Pukyong National University, Busan 608-739, Republic of Korea d The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea b

a r t i c l e in f o

abstract

Article history: Received 3 August 2010 Received in revised form 9 September 2010 Accepted 14 September 2010 Available online 20 October 2010

A new accepter unit, dihexyl-2H-benzimidazole, was prepared and utilized for the synthesis of the conjugated polymers containing electron donor–acceptor pair for OPVs. Dihexyl-2H-benzimidazole unit was designed to substitute the BT unit of PCDTBT. A new series of copolymers with carbazole as the electron-rich unit and dihexyl-2H-benzimidazole as the electron-deficient unit are synthesized. To obtain absorption in the longer wavelength region, bithiophene units without any alkyl group are incorporated as one of the monomers, which may result in low solubility of the polymers. In dihexyl-2H-benzimidazole, sulfur at 2-position of BT unit was replaced with dialkyl substituted carbon, while keeping the 1,2-quinoid form, to improve the solubility of the polymers. The spectra of the solid films show absorption bands with maximum peaks at 446–457 nm and the absorption onsets at 527–539 nm, corresponding to bandgaps of 2.30–2.35 eV. Under white light illumination (AM 1.5 G, 100 mW/cm2), the devices with PCBBTHBIs:PCBM layers showed open-circuit voltages (VOC) of 0.13–0.23 V, short-circuit current densities (JSC) of 3.52–5.69 mA/cm2, and fill factors (FF) of 0.36–0.40, giving power-conversion efficiencies of 0.21–0.47%. Crown Copyright & 2010 Published by Elsevier B.V. All rights reserved.

Keywords: Polymer Photovoltaic cells Carbazole 2H-Benzimidazole

1. Introduction Organic semiconducting materials are expected to facilitate the facile manufacture of low cost, flexible, and lightweight devices of OLEDs and solar cells [1–6]. Organic photovoltaics (OPVs) based on composites of a conjugated polymer as electron donor and a fullerene as electron–acceptor has been established to be the most successful of them so far and is advancing rapidly towards commercial viability [7]. Bulk heterojunction polymer solar cells have been the most successful approach for OPVs [8] using the wetprocessing techniques such as roll-to-roll coating or printing [9]. However, the lower power-conversion efficiency and smaller photocurrent than that of inorganic solar cells can be improved by the development of novel materials, which can enhance the coverage of the solar spectrum and the absorption coefficients [10–13], for example, poly(N-90 -heptadecanyl-2,7-carbazolealt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole); PCDTBT) [13–14], containing 2,7-carbozole unit as the electron-rich moiety, shows 6.0% power-conversion efficiency (PCE), one of the highest certified value by the National Renewable Energy Laboratory (NREL) reported to date[14].

n

Corresponding author. E-mail address: [email protected] (H. Suh).

Various low bandgap (1.4–1.9 eV) polymers as efficient electron donors have been investigated in recent years for a better match with the solar spectrum [15]. Many low-bandgap conjugated polymers with excellent efficiencies have electron-deficient heterocycles, such as benzothiadiazole (BT) [16], and electron-rich moieties, such as carbazole [14,17]. Carbazole is a well-known hole-transporting group due to the electron-donating capability of its nitrogen atom for the improved device performance [14,18–20]. The BT unit with 1,2-quinoid form is an excellent electron-deficient moiety but one of its disadvantages is poor solubility in most of the organic solvents [21]. For this reason, it is important to search new strategies to modify the structure of BT so as to improve the solubility of the polymers. In this paper, new conjugated polymers containing electron donor– acceptor pairs for OPV device utilizing a new type of acceptor, dihexyl2H-benzimidazole (HBI) [22], were synthesized by Pd(0)-catalyzed Suzuki or Stille coupling polymerization. To obtain absorption in the longer wavelength region, bithiophene units without any alkyl group are incorporated as one of the monomers, which may result in low solubility of the polymers. In the new acceptor, dihexyl-2Hbenzimidazole, sulfur at 2-position of BT unit was replaced with dialkyl substituted carbon, while keeping the 1,2-quinoid form, to make a highly soluble electron-deficient moiety. The new type of acceptor was introduced in the PCBBTHBI series, alternating copolymer, poly (N-90 -heptadecanyl-2,7-carbazole-alt-5,5-(40 ,70 -bis-2,20 -bithienyl-2,2diethyl-2H-benzimidazole); A-PCBBTHBI), and random copolymers

0927-0248/$ - see front matter Crown Copyright & 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.09.012

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7-bis(40 ,40 ,50 ,50 -tetramethyl-10 ,30 ,20 -dioxaborolan-20 -yl)-N-900 heptadecanylcarbazole (5) as electron-donating units were co-polymerized through the Suzuki coupling condition with Pd(0)-catalyst to yield poly(N-90 -heptadecanyl-2,7-carbazolealt-5,5-(40 ,70 -bis-2,20 -bithienyl-2,2-diethyl-2H-benzimidazole); A-PCBBTHBI). The copolymers that consist of 2,7-dibromo-9heptadecanyl-9H-carbazole (6) and 5,5’-bis(trimethylstannyl)2,2’-bithiophene (7) as electron-donating units and 4,7-dibromo-2,2-dihexyl-2H-benzimidazole (2) as electron-accepting moieties were synthesized by Pd(0)-catalyzed Stille coupling polymerization in toluene. The actual molar ratios of the HBI units in the copolymers, PCBBTHBIs, were calculated using elemental analysis and found to be very close to the feed molar ratios, as listed in Table 1. Caused by the lower reactivity of the HBI unit as compared to that of carbazole unit in the Stille reaction condition, the actual molar ratios of HBI unit incorporated in the polymers were slightly lower than the feeding ratios. All the polymers show good solubility at room temperature in organic solvents such as chloroform, THF, chlorobenzene, and o-dichlorobenzene (ODCB). To obtain absorption in the longer

(PCBBTHBIs). The photovoltaic properties of the polymers were investigated by fabrication of the polymer solar cells with the configuration of ITO/PEDOT:PSS/polymer:PC71BM/TiOx/Al.

2. Results and discussion 2.1. Synthesis and characterization The general synthetic routes toward the monomers and polymers are outlined in Scheme 1. In the first step, 3, 6-dibromobenzene-1,2-diamine (1) was treated with 7-tridecanone and acetic acid as catalyst to generate 2,2-dihexyl-2Hbenzimidazole (2). Compound (2) was coupled with (2, 2’-bithiophen)-5-yltrimethylstannane in tetrahydrofuran (THF) by dichlorobis(triphenylphosphine)palladium (II) to give 2, 2-dihexyl-4,7-bis(2,20 -bithiophen-5-yl)-2H-benzimidazole (3). Compound (3) was brominated in THF by N-bromosuccinimide (NBS) to give monomer 4,7-bis(50 -bromo(2,20 -bithiophen)-5-yl) -2,2-dihexyl-2H-benzimidazole (4). Compound (4) and 2,

C6H13 H2N

NH2

Br

N

1. acetic acid 7-tridecanone

Br

C6H13

Br

Pd(PPh3)2Cl2 , THF 2

1 C6H13

N

C6H13

N

S

S

Sn(Bu)3

Br

2. MnO2 , THF

C6H13

S

S

N

C6H13

N S

NBS

S

S

Br

N

S

S

S

Br

THF 3 C8H17

C8H17

C8H17 N

O

4

B

4, Pd(PPh3)4

O

O

2M K2CO3 (aq) toluene

5

C6H13

N

N

O B

C6H13

C8H17 S

N

S

S

n A-PCBBTHBI

C8H17

C6H13

C8H17

C6H13

N

N Br

N

Br

Br

Br

Sn

S

Sn

7 C6H13

C8H17

N

N Pd(PPh3)4

S

2

6 C8H17

S

S

S

2M K2CO3 (aq) toluene

C6H13 N S

n

m m:n = 1:9 m:n = 3:7 m:n = 5:5 m:n = 7:3 m:n = 9:1

S

PCBBTHBI-9 PCBBTHBI-7 PCBBTHBI-5 PCBBTHBI-3 PCBBTHBI-1

Scheme 1. Synthetic route for the synthesis of the monomers and polymers.

S. Song et al. / Solar Energy Materials & Solar Cells 95 (2011) 521–528

523

Table 1 Polymerization results and thermal properties of polymers. Polymer

M na

M wa

PDIa

DSC (Tg)

TGA (Td)b

In the feed composition

In the copolymera

A-PCBBTHBI PCBBTHBI-9 PCBBTHBI-7 PCBBTHBI-5 PCBBTHBI-3 PCBBTHBI-1

7000 11,900 12,900 10,000 10,600 7400

12,000 30,900 21,800 17,900 14,500 10,100

1.7 2.6 1.7 1.8 1.4 1.4

67 91 89 88 80 74

324 415 417 423 426 400

5:5 1:9 3:7 5:5 7:3 9:1

5:5 17.31:82.69 37.35:62.65 53.92:46.08 72.92:27.08 91.63:8.37

Molecular weight (Mw) and polydispersity (PDI) of the polymers were determined by gel permeation chromatography (GPC) in THF using polystyrene standards. Onset decomposition temperature (5% weight loss) measured by TGA under N2.

wavelength region, bithiophene units without any alkyl group are incorporated as one of the monomers, which may result in low solubility of the polymers. In the new acceptor, dihexyl-2Hbenzimidazole, sulfur at 2-position of BT unit was replaced with dialkyl substituted carbon, while keeping the 1,2-quinoid form, to make a highly soluble electron-deficient moiety. Table 1 summarizes the polymerization results, including molecular weight, polydispersity index (PDI), and thermal stability of the polymers. The number-average molecular weight (Mn) of 7000–12,900 and weight-average molecular weight (Mw) of 10,100–30,900 with polydispersity index (PDI, Mw/Mn) of 1.4–2.6 of the resulting polymers were determined by GPC. The thermal properties of the polymers were evaluated by differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) in nitrogen atmosphere. As shown in Fig. 1, their decomposition temperatures (Td), which correspond to a 5% weight loss on heating during TGA, are about 324–426 1C for resulting polymers in N2 atmosphere. The polymers showed good thermal stability with a glass transition temperature (Tg) of 67–91 1C, using DSC performed at a temperature range 30–250 1C. The Tg values of the random copolymers increase with increased amount of HBI unit. The high thermal stability of the resulting polymers prevents the deformation of the polymer morphology and is important for OPVs application. 2.2. Optical properties The optical properties of the polymers were investigated both in ODCB solution and in thin film. The absorption data for the polymers are shown in Fig. 1. Transparent and uniform polymer film was prepared on quartz plates by spin casting from their ODCB solution at room temperature. In solution, the polymers exhibited absorption maxima at about 433–452 nm. The absorption spectra exhibit a gradual red-shift with decreased amounts of the dihexyl2H-benzimidazole contents, that is, from 433 nm with PCBBTHBI-9 to 452 nm with PCBBTHBI-1. The absorption spectra of the thin films show almost the same maximum peaks at about 446–457 nm and the onsets at 527–539 nm, corresponding to bandgaps of 2.30–2.35 eV. The optical bandgaps of the polymers are similar to those with increased amount of contents of dihexyl-2H-benzimidazole unit in the copolymers. The absorption spectra of the polymer films were red-shifted at around 5–14 nm as compared to those of solution, indicating that the inter-chain interaction of polymer chain, which can be attributed to the p–p* transitions of conjugated backbone, increased the planarity of the polymer backbone in the solid state. 2.3. Electrochemical properties The electrochemical properties of the polymers were determined from the bandgaps estimated from the absorption onset wavelengths, and the HOMO energy levels, which were estimated

1.2 A-PCBBTHBI PCBBTHBI-9 PCBBTHBI-7 PCBBTHBI-5 PCBBTHBI-3 PCBBTHBI-1

1.0 UV Intensity (a.u.)

b

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

Wavelength (nm)

1.2 A-PCBBTHBI PCBBTHBI-9 PCBBTHBI-7 PCBBTHBI-5 PCBBTHBI-3 PCBBTHBI-1

1.0 UV Intensity (a.u.)

a

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

Wavelength (nm) Fig. 1. UV–visible absorption spectra of polymers in ODCB solution (a) and in the solid state (b).

from the cyclic voltammetry (CV). The CV spectra are shown in Fig. 2, and the oxidation potentials derived from the onsets of electrochemical p-doping are summarized in Table 2. HOMO levels were calculated based on the empirical formula (EHOMO ¼ –([Eonset]ox +4.8)) (eV). The polymers, PCBBTHBIs, exhibited the absorption onset wavelengths of 527–538 nm in solid thin film, which correspond to almost the same bandgaps of 2.30–2.35 eV. The polymers exhibit irreversible processes in an oxidation scan. The oxidation onsets of the polymers were estimated to be

524

S. Song et al. / Solar Energy Materials & Solar Cells 95 (2011) 521–528

wavelengths of the absorption spectra. The optical bandgap of the alternating copolymer, A-PCBBTHBI, is very well matched with the electrochemical bandgap. The electrochemical bandgaps of the random copolymers decrease with increased amount of contents of dihexyl-2H-benzimidazole unit in the copolymers. Fig. 3 illustrates the energy levels of the polymers as compared to those of PC71BM and the work functions of indium tin oxide (ITO), poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and aluminum (Al) used in OPVs.

0.55–0.73 V, which correspond to HOMO energy levels of  5.35 to 5.53 eV. The LUMO energy levels of the polymers can be calculated with the HOMO energy levels and optical bandgaps. The LUMO energy levels of the polymers were thus determined to be  3.03 to  3.22 eV. The reduction potential onsets of the polymers are  1.28 to  1.66 V. The electrochemical bandgaps, calculated from cyclic voltammetry data, are 1.99–2.36 eV, somewhat lower than the optical bandgaps estimated from the onset

A-PCBBTHBI PCBBTHBI-9 PCBBTHBI-7 PCBBTHBI-5 PCBBTHBI-3 PCBBTHBI-1

1.0

Current

0.5

2.4. Polymer photovoltaic properties The OPVs were fabricated by spin casting of ODCB solution of PC71BM/polymers. All the polymers were applied as the donors in the conventional BHJ type OPVs with PC71BM as the acceptor, which has been widely used for this purpose. Typical J–V characteristics and IPCE values of the devices with the configuration of ITO/PEDOT: PSS/polymer:PC71BM(1:4)/TiOx/Al under AM 1.5 G irradiation (100 mW/cm2) are depicted in Figs. 4 and 5, respectively. The photovoltaic parameters of the polymers, including open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and power-conversion efficiency (PCE) are summarized in Table 3. The devices with PCBBTHBIs:PC71BM layers showed an open-circuit voltage (VOC) of 0.13–0.23 V, a short-circuit current density (JSC) of 3.52–5.69 mA/cm2, and a fill factor (FF) of 0.36–0.40, giving a power-conversion efficiency of 0.21–0.47%. The device with A-PCBBTHBI:PC71BM blends demonstrated a VOC value of 0.12 V, a JSC value of 5.18 mA/cm2, and a FF of 0.33, leading to the PCE value of 0.27%. VOC is approximately a measure of the difference between the

0.0

-0.5

-1.0 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Voltage (V) Fig. 2. Electrochemical properties of polymers.

Table 2 Electrochemical potentials and energy levels of the polymers. Polymers

Optical bandgapa (eV)

HOMOb (eV)

LUMOc (eV)

Eoxd (V)

Eredd (V)

Chemical bandgape (eV)

A-PCBBTHBI PCBBTHBI-9 PCBBTHBI-7 PCBBTHBI-5 PCBBTHBI-3 PCBBTHBI-1

2.35 2.30 2.34 2.32 2.31 2.32

 5.46  5.51  5.50  5.51  5.53  5.35

 3.11  3.21  3.16  3.19  3.22  3.03

0.66 0.71 0.70 0.71 0.73 0.55

 1.66  1.28  1.38  1.56  1.63  1.51

2.32 1.99 2.08 2.27 2.36 2.06

a

Optical energy bandgap was estimated from the onset wavelength of the optical absorption. Calculated from the oxidation potentials. Calculated from the HOMO energy levels and Eg. d Onset oxidation and reduction potential measured by cyclic voltammetry. e Calculated from the Eox and Ered. b c

-3.03 eV -3.11 eV

-3.21 eV

-3.16 eV

-3.19 eV

-3.21 eV

PCBM Al (4.3 eV) (4.3 eV) ITO (4.8 eV) PEDOT (5.0 eV)

2.35 eV

2.30 eV

2.34 eV

2.32 eV

2.31 eV

2.32 eV

-5.35 eV -5.50 eV -5.59 eV -5.53 eV PCBBTHBI-1 A-PCBBTHBI PCBBTHBI-7 PCBBTHBI-3 PCBBTHBI-9 PCBBTHBI-5 6.1 eV -5.46 eV

-5.51 eV

Fig. 3. Energy-level diagram showing the HOMO and LUMO energy levels of polymers and PC71BM.

S. Song et al. / Solar Energy Materials & Solar Cells 95 (2011) 521–528

5  5 mm2 by the tapping mode. The height images of the polymer:PC71BM films show clearly that the surface of the polymer film is very smooth (rms roughness values are only 0.58–1.23 nm as shown in Table 3, and there is no aggregation formed.

Current density (mA/cm2)

0

3. Conclusions

A-PCBBTHBI PCBBTHBI-9 PCBBTHBI-7 PCBBTHBI-5 PCBBTHBI-3 PCBBTHBI-1

-2

0.2

0.0

0.4

0.6

Voltage (V) Fig. 4. Current density–potential characteristics of the polymer solar cells under the illumination of AM 1.5, 100 mW/cm2.

A-PCDDTHBI PCDDTHBI-9 PCDDTHBI-7 PCDDTHBI-5 PCDDTHBI-3 PCDDTHBI-1

IPCE (%)

20

New polymers, PCPDTDTHBIs utilizing dihexyl-2H-benzimidazole, were synthesized to show good solubility at room temperature in organic solvents, even with two bithiophene units in the structures. To obtain absorption in the longer wavelength region, two bithiophene units without alkyl group are incorporated, which may result in low solubility of the polymers. In the new acceptor, HBI unit, sulfur at 2-position of BT unit was replaced with dialkyl substituted carbon, while keeping the 1,2-quinoid form, to make a highly soluble electron-deficient moiety. The advantage of HBI unit compared to the benzothiadiazole moiety of PCDTBT is to improve the solubility of the polymer while keeping the coplanarity of the backbone. The spectra of the solid films show absorption bands with maximum peaks at 446–457 nm and the absorption onsets at 527–539 nm, corresponding to the bandgaps of 2.30–2.35 eV. Under white light illumination (AM 1.5 G, 100 mW/cm2), the devices with PCBBTHBIs:PC71BM layers showed an open-circuit voltage (VOC) of 0.13–0.23 V, a short-circuit current density (JSC) of 3.52–5.69 mA/cm2, and a fill factor (FF) of 0.36–0.40, giving a power-conversion efficiency of 0.21–0.47%.

4. Experimental section

10

0 400

500

600

700

800

Wavelength (nm) Fig. 5. IPCE curves of the polymer solar cells under the illumination of AM 1.5, 100 mW/cm2.

Table 3 Photovoltaic properties of the polymer solar cells. Polymers

VOC (V)

JSC (mA/cm2)

FF

PCE (%)

rms (nm)a

A-PCBBTHBI PCBBTHBI-9 PCBBTHBI-7 PCBBTHBI-5 PCBBTHBI-3 PCBBTHBI-1

0.12 0.21 0.23 0.13 0.19 0.26

5.18 4.02 4.16 3.94 3.52 4.69

0.30 0.37 0.36 0.40 0.40 0.38

0.27 0.30 0.35 0.21 0.26 0.47

1.18 0.59 0.75 0.85 0.58 1.23

a

525

Root-mean-square (rms) roughness from AFM measurement.

oxidation potential of the donor and the reduction potential of the acceptor (PC71BM). Even though PCBBTHBIs have similar HOMO levels, all devices with the polymers demonstrated quite low VOC values, out of which any clear trend cannot be found. The lower PCE values of the PCBBTHBI devices are attributed to the less efficient ICT and smaller red-shift of the absorption spectra. The AFM topographies of polymer blends (PCBBTHBIs:PC71BM¼ 1:4 w/w) were investigated by the casting films of ODCB solutions as shown in Fig. 6, where the images were obtained in a surface area of

General. All reagents were purchased from Aldrich or TCI, and used without further purification. Solvents were purified by the normal procedure and handled under the moisture-free atmosphere. 1H and 13C NMR spectra were recorded with a Varian Gemini-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) and ethyl acetate/hexane or methanol/methylene chloride gradients unless otherwise indicated. Analytical thin layer chromatography (TLC) was conducted using Merck 0.25 mm silica gel 60F pre-coated aluminum plates with fluorescent indicator UV254. High resolution mass spectra (HRMS) were recorded on a JEOL JMS-700 mass spectrometer under fast atom bombardment (FAB) conditions in the Korea Basic Science Institute (Daegu). Molecular weight and polydispersity of the polymer were determined by gel permeation chromatography (GPC) analysis with a polystyrene standard calibration. The UV–vis absorption spectra were recorded by a Varian 5E UV/VIS/ NIR spectrophotometer, while the Oriel InstaSpec IV CCD detection system with a xenon lamp for the photoluminescence and electroluminescence spectra measurements. 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 temperature in argon atmosphere. A platinum electrode (  0.05 cm2) coated with a thin polymer film was used as the working 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 (calibrated by the Fc/Fc + redox system) was 4.8 eV below the vacuum level. Solar cells were fabricated on an indium tin oxide (ITO)-coated glass substrate with the following structure; ITO-coated glass substrate/poly(3,4ethylenedioxythiophene) (PEDOT:PSS)/polymer: PC71BM/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

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S. Song et al. / Solar Energy Materials & Solar Cells 95 (2011) 521–528

Fig. 6. Atomic force microscopy images of A-PCBBTHBI/PC71BM (a), PCBBTHBI-9/PC71BM (b), PCBBTHBI-7/PC71BM (c), PCBBTHBI-5/PC71BM (d), PCBBTHBI-3/PC71BM (e), and PCBBTHBI-1/PC71BM (f).

from aqueous solution to form a film of 40 nm thickness. The substrate was dried for 10 min at 140 1C in air and then transferred into a glove box to spin cast the charge separation layer. A solution containing a mixture of polymer:PC71BM (1:4) in dichlorobenzene solvent with concentration of 7 wt/ml% was then spin cast on top of the PEDOT/PSS layer. The film was dried for 60 min at 70 1C in the glove box. The TiOx precursor solution diluted by 1:200 in methanol was spin cast in air on top of the polymer:PC71BM layer (5000 rpm for 40 s). The sample was heated at 80 1C for 10 min in air. Then, an aluminum (Al, 100 nm) electrode was deposited by thermal evaporation in a vacuum of about 5  10  7 Torr. The current

density–voltage (J–V) characteristics of the devices were measured using a Keithley 236 Source Measure Unit. Solar cell performance utilized an Air Mass 1.5 Global (AM 1.5 G) solar simulator with an irradiation intensity of 1000 Wm  2. An aperture (12.7 mm2) was used on top of the cell to eliminate the extrinsic effects such as cross-talk, waveguiding, shadow effects, etc. The spectral mismatch factor was calculated by comparison of solar simulator spectrum with AM 1.5 spectrum at room temperature. Synthesis of 4,7-dibromo-2,2-dihexyl-2H-benzimidazole (2). To a stirred solution of 3,6-dibromobenzene-1,2-diamine (10 g, 37.65 mmol), 7-tridecanone (10 mL) and acetic acid (10 mL)

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were dissolved in diethyl ether (200 mL) and heated to 50 1C overnight. After the reaction mixture was cooled to room temperature, water and ethyl acetate were added. The aqueous phase was extracted with ethyl acetate and combined organic layers were dried over MgSO4. The solvent was removed from the vacuum and the residue was purified by column chromatography to give 4,7-dibromo-2,2-dihexyl-2,3-dihydro-1H-benzimidazole. To a stirred solution of given 4,7-dibromo-2,2-dihexyl-2,3-dihydro1H-benzimidazole in 50 mL of tetrahydrofuran (THF) at room temperature, MnO2 (10 g) was added. After 3 h, the solid was filtered and washed with THF. The combined organic phase was concentrated under reduced pressure and purified by flash column chromatography to give 4.8 g of compound (3) as yellow powder. mp 60 1C; 1H NMR (300 MHz, CDCl3): d (ppm) 0.83 (t, 6 H, J¼6.87 Hz), 0.92–0.88 (m, 4 H), 1.25–1.17 (m, 12 H), 2.24–2.18 (m, 4 H), 7.20 (s, 2 H); 13C NMR (75 MHz, CDCl3): d (ppm) 14.26, 22.78, 23.72, 29.62, 31.74, 35.77, 109.92, 118.42, 136.12, 157.87. HRMS (m/z, FAB + ) calcd. for C19H29Br2N2 443.0697, found 443.0695. Synthesis of 2,2-dihexyl-4,7-bis((2,2’-bithiophen)-5-yl)-2H-benzimidazole (3). To a stirred solution of 4,7-dibromo-2,2-dihexyl-2Hbenzimidazole (2) (2.5 g, 5.62 mmol) and (2,2’-bithiophen)-5yltributylstannane (12.8 g, 28.1 mmol) in 40 mL of THF at room temperature were added dichlorobis(triphenylphosphine)palladium(II) (2 mol%). The reaction mixture was stirred for 12 h at 80 1C, concentrated in reduced pressure, and purified by flash column chromatography to give compound (3) (1 g) as violet solid. mp 102 1C; 1H NMR (300 MHz, CDCl3): d (ppm) 0.82 (t, 6 H, J¼6.7 Hz), 1.22–1.77 (m, 20 H), 6.22 (d, 2 H, J ¼3.9 Hz), 6.72 (t, 2 H, J¼3.6 Hz), 6.81 (d, 2 H, J¼ 3.8 Hz), 6.91 (d. 2 H, J¼5.4 Hz), 7.26 (d, 2 H, J ¼4.1 Hz), 7.61 (s, 2 H); 13C NMR (75 MHz, CDCl3): d (ppm) 14.09, 22.77, 26.60, 31.11, 31.72, 35.15, 37.13, 101.95, 125.29, 128.32, 128.75, 129.00, 129.12, 133.34, 133.57, 142.53, 145.66, 149.84, and 160.82. HRMS (m/z, FAB + ) calcd. for C35H39N2S4 615.1996, found 615.1995. Synthesis of 2,2-dihexyl-4,7-bis(5’-bromo-(2,2’-bithiophen)-5-yl)-2Hbenzimidazole (4). To a stirred solution of 2,2-dihexyl-4,7-bis((2,2’bithiophen)-5-yl)-2H-benzimidazole (3) (1 g, 1.63 mmol) in THF at room temperature was added N-bromosuccinimide (NBS; 0.58 g, 3.25 mmol). After 3 h at room temperature, water and ethyl acetate were added. The combined organic phase was concentrated under reduced pressure and purified by flash column chromatography to give compound (4) (420 mg) as violet solid. mp 132 1C; 1H NMR (300 MHz, CDCl3): d (ppm) 0.87 (t, 6 H, J¼ 6.7 Hz), 1.29–1.77 (m, 20 H), 6.29 (d, 2 H, J ¼4.1 Hz), 6.72 (d, 2 H, J¼4.0 Hz), 7.17 (d, 2 H, J¼3.9 Hz), 7.27 (d, 2 H, J ¼4.1 Hz), 7.59 (s, 2 H); 13C NMR (75 MHz, CDCl3): d (ppm) 14.06, 22.75, 26.55, 30.98, 31.65, 35.10, 37.14, 101.93, 116.58, 129.01, 130.20, 130.78, 131.11, 133.33, 134.45, 142.53, 144.69, 154.43, 160.80. HRMS (m/z, FAB + ) calcd. for C35H37N2S4 771.0206, found 771.0211. Synthesis of poly(N-90 -heptadecanyl-2,7-carbazole-alt-5,5-(40 ,70 -bis0 2,2 -bithienyl-2,2-diethyl-2H-benzimidazole); A-PCBBTHBI). Carefully purified 2,2-dihexyl-4,7-bis(5’-bromo-(2,2’-bithiophen)-5-yl)-2Hbenzimidazole (4), 2,7-bis(40 ,40 ,50 ,50 -tetramethyl-10 ,30 ,20 -dioxaborolan-20 -yl)-N-900 -heptadecanylcarbazole (5), and (PPh3)4Pd(0) (3 mol%) were dissolved in a mixture of toluene and 2 M aqueous Na2CO3. The mixture was refluxed with vigorous stirring for 3 days in argon atmosphere. After 72 h, phenylboronic acid was added to the reaction, then 12 h later, bromobenzene was added and the reaction mixture was refluxed overnight to complete the endcapping reaction. After the mixture was cooled to room temperature, it was poured into methanol. The precipitated material was recovered by filtration using the Buchner funnel with perforated plate. The resulting solid material was reprecipitated using 1.0 L of methanol several times to remove catalyst residues and to generate polymers. The resulting polymers were soluble in THF, CHCl3, odichlorobenzene (ODCB), and toluene.

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General procedure of copolymerization. Carefully purified 4,7-dibromo-2,2-dihexyl-2H-benzimidazole (2), 2,7-dibromo-9heptadecanyl-9H-carbazole (6), 5,5’-bis(trimethylstannyl)-2,2’bithiophene (7), and (PPh3)4Pd(0) (3 mol%) were dissolved in a mixture of toluene and DMF. After refluxing for 72 h, trimethyl (phenyl)tin was added to the reaction mixture. After refluxing for additional 12 h, the reaction mixture was treated with bromobenzene and refluxed overnight to complete the end-capping. After cooling to room temperature, the mixture was poured into methanol. After filtration using the Buchner funnel with perforated plate, the resulting solid material was reprecipitated using 1.0 L of methanol several times to remove the catalyst residues. The resulting polymers were soluble in THF, CHCl3, o-dichlorobenzene (ODCB), and toluene. The ratio of 5,5’-bis(trimethylstannyl)-2,2’bithiophene (7) (1.0 equivalent) vs. dibromides (4,7-dibromo-2,2dihexyl-2H-benzimidazole (2) 2,7-dibromo-9-(1-octylnonyl)9H-carbazole (6)) (1.0 equivalent) was kept always as (7):(2+6) ¼1:1. The comonomers feed ratios of compounds (2) to (6) are 9:1, 7:3, 5:5, 3:7, and 1:9, and the corresponding copolymers were named as PCBBTHBI-9, 7, 5, 3, and 1, respectively. PCBBTHBI-9. Analysis. Calcd. for C28H33.3N1.9S2.0: C, 73.01; H, 7.29; and N, 5.78. Found: C, 70.11; H, 7.06; and N, 5.31. PCBBTHBI-7. Anal. Calcd. for C30H35.9N1.7S2.0: C, 74.37; H, 7.47; and N, 4.92. Found: C, 71.33; H, 7.23; and N, 4.40. PCBBTHBI-5. Anal. Calcd. for C32H38.5N1.5S2.0: C, 75.61; H, 7.64; and N, 4.14. Found: C, 73.48; H, 7.51; and N, 3.81. PCBBTHBI-3. Anal. Calcd. for C34H41.1N1.3S2.0: C, 76.74; H, 7.79; and N, 3.42. Found: C, 74.65; H, 7.68; and N, 3.09. PCBBTHBI-1. Anal. Calcd. for C36H43.7N1.1S2.0: C, 77.77; H, 7.92; and N, 2.77. Found: C, 73.24; H, 7.47; and N, 2.32.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (2010-0015069).

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