Syntheses and Photovoltaic Properties of Narrow Band Gap Donor

Jul 31, 2014 - Institute of Materials Science, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba,...
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Syntheses and Photovoltaic Properties of Narrow Band Gap Donor− Acceptor Copolymers with Carboxylate-Substituted Benzodithiophene as Electron Acceptor Unit Kosuke Shibasaki,† Kenichi Tabata,† Yohei Yamamoto,‡,∥ Takeshi Yasuda,§,‡ and Masashi Kijima*,‡,∥ †

Institute of Materials Science, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan ‡ Tsukuba Research Center for Interdisciplinary Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan ∥ Division of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan § Photovoltaic Materials Unit, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan S Supporting Information *

ABSTRACT: Stille-coupling of carboxylate-substituted dibrominated benzodithiophene (BDTC) with 2,5-distannylthieno[3,4-b]thiophene gave novel donor−acceptor type alternating copolymers, PBDTC-TT, where BDTC works as an electronaccepting unit in the polymers. They showed broad absorption bands from 500 nm to the near-infrared region, optical band gap (Eg) about 1.5 eV, small π-stacking distance (3.6 Å), and good thermal stability. The hole mobilities of PBDTC-TT determined from performance of their organic field-effect transistors were 3.1−6.9 × 10−4 (cm2 V−1 s−1). The bulk heterojunction (BHJ) solar cells were fabricated with configuration of ITO/PEDOT:PSS/polymer:PC70BM/LiF/ Al. A PBDTC-TT device exhibited photocurrent response upon exposure to light with wavelength of 300−900 nm and incident photon to current conversion efficiency over 40% in the range of 400−750 nm. The power conversion efficiency of the best-performed device reached 3.03% with short-circuit current density of 12.54 mA cm−2, fill factor of 0.48, and open circuit voltage of 0.51 V under illumination of AM 1.5 G/100 mW cm−2. These results show that the BDTC unit can behave as an electron accepting building block for donor−acceptor type narrow band gap polymers, and these types of polymers can be used as a donor material in the active layer for BHJ photovoltaic cells.

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infrared.8 Recently, donor−acceptor (D−A) approaches by incorporating an alternating electron donating (D) unit and an electron accepting (A) unit in the polymer backbone have been examined to narrow the band gap (Eg) of the polymers.9 Above all, benzo[1,2-b:4,5-b′]dithiophene (BDT) has attracted much attention as a D-building block of D−A type polymers. To date, several alkoxy or alkyl substituted BDT based polymers have been synthesized, and their BHJ photovoltaic properties have been reported.10−24 One of the great advantage of BDT polymers is showing good hole mobility due to efficient πstacking of the planar units.14 In addition, weak donor property of the BDT unit could keep the low EHOMO of the polymers.15 Generally, low EHOMO of an electron-donating polymer leads to high Voc.3,6,7 Polymers composed of the D-type dialkoxy substituted BDT unit and the A-type thieno[3,4-b]thiophene

INTRODUCTION The bulk heterojunction (BHJ) photovoltaic cells composed of an electron-donating conjugated polymer and an electronaccepting fullerene derivative in an active layer have been studiously researched as renewable energy source during the past decade. The various electron-donating polymers had been strategically designed and developed to achieve high power conversion efficiency (PCE). They must possess several properties simultaneously such as good solubility, high mobility of charge carriers, appropriate energy levels of highest occupied molecular orbital (EHOMO) and lowest unoccupied molecular orbital (ELUMO) because these properties have relation with characteristic parameters of BHJ solar cells such as short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF).1−7 Among these parameters, Jsc is mainly determined by the amount of absorbed light in an active layer of BHJ solar cell. Therefore, it has been recognized that development of narrow band gap polymers is one of the most effective way to increase PCE because they can capture solar photons extending to near© 2014 American Chemical Society

Received: May 23, 2014 Revised: July 20, 2014 Published: July 31, 2014 4987

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band gap D−A copolymer, poly[4,8-benzo(1,2-b:4,5-b′)dithiophenedicarboxylate-alt-thieno(3,4-b)thiophene] (PBDTC-TT, Figure 1) by combining BDTC and thieno[3,4b]thiophene (TT) units as A- and D-units, respectively. The fundamental characteristics of PBDTC-TT and photovoltaic properties are investigated.

carboxylate unit (PTB series) are one of the most successful examples of BDT based D−A type polymers that exhibit high PCE.7,11,25−27 In the PTB series, PTB7 shown in Figure 1 has a

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EXPERIMENTAL SECTION

Measurements. 1H NMR and 13C NMR spectra were measured with a JEOL JNM-ECS 400 at resonance frequency of 400 MHz for 1 H and 100 MHz for 13C in CDCl3 at room temperature. The IR spectra were recorded on a NICOLET iS5 FT-IR (Thermo Scientific) and measured on KBr plates. Ultraviolet−visible (UV−vis) spectra of the polymers were measured on a Shimadzu UV-1800 spectrophotometer. The elemental analysis of C, H, and N for the monomers and polymers were performed with a PerkinElmer 2400 CHN elemental analyzer. Thermogravimetric analysis (TGA) was performed under argon at a heating rate of 10 °C min−1 with a Seiko EXSTAR7000. Xray diffraction (XRD) measurements of drop-cast polymer films were performed using a Rigaku MiniFlex600. The number-average molecular weight (Mn), weight-average molecular weight (Mw) and polydispersity index (Mw/Mn) of the polymers were determined by gel permeation chromatography (GPC) calibrated with polystyrene standards using THF as an eluent. The HOMO energy levels of polymers were estimated by photoelectron yield spectroscopy (PYS) using an AC-3 spectrometer (Riken Keiki). The atomic force microscopy (AFM) measurement of the surface morphology of samples was conducted on a Nanocute (SII NanoTechnology Inc.) in tapping mode. Field effect transistor (FET) measurements of spincoated polymer films on 1,1,1,3,3,3-hexamethyldisilazane modified SiO2 (200 nm)/p-Si substrate with source and drain Au electrodes (50 nm) deposited on the polymer films (channel length = 50 μm; channel width = 0.75 mm) were performed under vacuum conditions using GRAIL10-HELIPS-4-HT and Keithley SCS4200 instruments. Materials. All chemicals were purchased from Kanto Chemical Co., Inc., Tokyo Chemical Industry Co., Ltd., Nacalai Tesque Inc., or Sigma-Aldrich Co., LLC., and they were used without further purification unless stated. Toluene and tetrahydrofuran (THF) were purified by distillation according to common methods. BDTC was synthesized according to literature methods31 with a little modification. 2,5-Dibromo thieno[3,4-b]thiophenes having a hexyl side chain (TT1-Br2) and a dodecyl side chain (TT2-Br2) were prepared from 3,4-dibromothiophene via three-steps of (1) selective monosubstitution of an alkynyl group by Sonogashira coupling, (2) cyclization of the alkynyl derivative to TT with elemental sulfur and butyllithium, and (3) bromination of TT by N-bromosuccinimide (NBS), respectively, according to the procedures reported previously.32 The experimental details of BDTC and TT-Br2 are described in Supporting Information. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, CLEVIOS P VP AI 4083) was purchased from Heraeus. [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) (purity 99%) and PTB7 were purchased from Solenne and 1-material Chemscitech Inc., respectively. Synthesis of Didodecyl 2,6-Dibromo-4,8-benzodithiophenedicarboxylate (BDTC-Br2). In a two-necked flask, bromine (618 mg, 3.85 mmol, 5 equiv) was added to a solution of BDTC (0.475 mg, 0.774 mmol, 1 equiv) in dichloromethane (27.2 mL). After small flow of an Ar gas, the solution was stirred at room temperature overnight. Saturated aqueous Na2S2O3 solution was added to the reaction mixture, which was stirred for a few minutes and was extracted with dichloromethane. The organic layer was washed with water and dried over anhydrous Na2SO4. After removal of the solvent, products were purified by recrystallization from dichloromethane. BDTC-Br2 was obtained as a yellow solid (257 mg, 43% yield). 1H NMR (400 MHz, CDCl3, δ/ppm): 0.87 (t, J = 6.8 Hz, 6H), 1.29 (m, 28 H), 1.36 (m, 4H), 1.54 (m, 4H), 1.92 (m, 4H), 4.55 (t, J = 6.8 Hz, 4H), 8.31 (s, 2H). 13C NMR (100 MHz, CDCl3, δ/ppm): 14.11, 22.68, 26.16, 28.66, 29.22, 29.33, 29.50, 29.55, 29.62, 31.91, 66.85, 119.69, 122.75,

Figure 1. D−A structures of PTB7 and PBDTC-TT.

band gap of about 1.7 eV and presented a high PCE over 7%.11,28 Furthermore, it has been reported that BHJ solar cells made from PTB7 have exceeded PCE of 9% by insertion of an interlayer polymer material at the cathode.29 There might be still room for improvement on the PCE by narrowing Eg of a series of the polymers toward 1.5 eV.3,6,15,30 In relation to this, Citterio et al. reported the synthesis of dicarboxylatesubstituted 4,8-benzo[1,2-b:4,5-b′]dithiophene (BDTC), where two alkyl ester side chains, instead of the alkyl or alkoxy side chains, bear at the BDT unit.31 Preliminary examinations of BDTC by the density functional theory (DFT) calculation show rather lower EHOMO and ELUMO than those of the traditional BDT derivatives (Figure 2). From this result, we considered the BDTC unit could serve as an electron-accepting unit in D−A polymers. Thus, in this paper, we design a narrow

Figure 2. HOMO and LUMO energy levels of (a) methoxysubstituted BDT (b) methyl substituted BDT and (c) methoxycarbonyl-substituted BDT (BDTC), optimized by DFT calculation with Gaussian 09 program at the B3LYP/6-31G(d) basis set. 4988

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Scheme 1. Synthetic Pathway for the Polymers

2923, 2852, 1688, 1537, 1502, 1466, 1312, 1220, 1195, 1153, 1052, 812. Anal. Calcd for C54H80O4S4: C, 70.39; H, 8.75. Found: C, 69.17; H, 8.13. Fabrication and Measurement of BHJ Solar Cell Devices. The BHJ solar cells were fabricated in the configuration of ITO/ PEDOT:PSS/BHJ layer/LiF/Al. The patterned ITO (conductivity: 10 Ω/square) glass was precleaned by ultrasonication in acetone and successively in ethanol. The precleaned ITO glass was treated in an ultraviolet-ozone chamber. A thin layer (40 nm) of PEDOT:PSS was spin-coated on the ITO at 3000 rpm and air-dried at 110 °C for 10 min on a hot plate. The substrate was transferred to a glovebox and redried at 110 °C for 10 min on a hot plate under N2 atmosphere. Solutions of the polymers and PC70BM at various blending ratios in odichlorobenzene were subsequently spin-coated on the PEDOT:PSS. The BHJ layers were dried at 110 °C for 10 min. LiF (1 nm) and Al (80 nm) were deposited on the BHJ layer with conventional thermal evaporation at the chamber pressure lower than 5 × 10−4 Pa, which provided the devices with an active area of 2 × 2 mm2. The thickness of BHJ and PEDOT:PSS layers were measured using an automatic microfigure measuring instrument (Surfcorder ET200, Kosaka Laboratory Ltd.). The current density−voltage (J−V) curves were measured using an ADCMT 6244 DC voltage current source/monitor under AM 1.5 solar-simulated light irradiation of 100 mW cm−2 (Pin) (OTENTO-SUN III, Bunkoh-Keiki Co., Ltd.). The PCEs of a solar cells based on the polymers were determined by the equation of PCE = (Jsc × Voc × FF)/(Pin). These organic photovoltaic (OPV) parameters were calculated at the average of the measured results of three OPV cells. The incident photon to current conversion efficiencies (IPCEs) were measured using an SM-250 system (Bunkoh-Keiki Co., Ltd.).

126.20, 137.06, 42.62, 165.72. Anal. Calcd for C36H52Br2O4S2: C, 55.96; H, 6.78. Found: C, 55.83; H, 6.62. Synthesis of 2-Hexyl-4,6-bis(trimethylstannyl)thieno[3,4-b]thiophene (TT1-Sn2). In a two-necked flask, a solution of TT1-Br2 (1.08 g, 2.83 mmol) in dry THF (20.2 mL) was cooled at −78 °C under an Ar atmosphere. n-BuLi (1.65 mol dm−3 (M) in hexane, 3.75 mL) was added dropwise to the reaction mixture, which was stirred for 1.5 h at −78 °C. Trimethyltinchloride (1.00 mol dm−3 (M) in hexane, 7.07 mL) was added dropwise to the reaction mixture, which was stirred for 30 min. The resulting solution was gradually warmed to room temperature, and was stirred overnight. The solution was slowly added to water at 0 °C. The mixture was extracted with diethyl ether, and the organic layer was washed with water and was dried over anhydrous Na2SO4. After removal of the solvent, the product was obtained as yellow oil without further purification by column chromatography (1.07 g, 69% yield), which was satisfactorily pure for synthetic use. 1H NMR (400 MHz, CDCl3, δ/ppm): 0.37 (s, 9H), 0.39 (s, 9H), 0.85 (t, J = 6.8 Hz, 3H), 1.25−1.29 (m, 4H), 1.36 (m, 2H), 1.65 (m, 2H), 2.73 (t, J = 7.3 Hz, 2H), 6.58 (s, 1H). 13C NMR (100 MHz, CDCl3, δ/ppm): −8.52, −8.07, 14.09, 22.577, 28.84, 30.55, 31.57, 32.22, 114.21, 127.18, 128.77, 148.37, 152.69, 155.93. Anal. Calcd for C18H32S2Sn: C, 39.31; H, 5.86. Found: C, 39.59; H, 5.88. Synthesis of 2-Dodecyl-4,6-bis(trimethylstannyl)thieno[3,4b]thiophene (TT2-Sn2). TT2-Sn2 was similarly prepared from TT2Br2 as TT1-Sn2, giving yellow oil in 60% yield. 1H NMR (400 MHz, CDCl3, δ/ppm): 0.37 (s, 9H), 0.39 (s, 9H), 0.85 (t, J = 6.8 Hz, 3H), 1.20−1.40 (m, 18H), 1.65 (m, 2H), 2.73 (t, J = 7.3 Hz,2H), 6.58 (s, 1H). 13C NMR (400 MHz, CDCl3, δ/ppm): −8.52, −8.07, 14.11, 22.69, 29.17, 29.35, 29.37, 29.55, 29.64, 29.66, 30.60, 31.92, 32.22, 114.21, 127.18, 128.76, 148.37, 152.71, 155.93. Anal. Calcd for C24H44S2Sn: C, 45.46; H, 6.99. Found: C, 46.43; H, 7.12. Synthesis of PBDTC-TT1. In a two-necked flask, BDTC-Br2 (108 mg, 0.140 mmol), TT1-Sn2 (770 mg, 0.140 mmol), dry toluene (4.52 mL), and tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (6.78 mg) were mixed with stirring at 120 °C for 36 h under an Ar atmosphere. After cooling to room temperature, the polymeric products were precipitated from methanol. The precipitates were successively extracted with acetone, hexane and chloroform by Soxhlet extraction. The chloroform extract was again precipitated from methanol. PBDTC-TT1 was obtained as a black solid (84.0 mg, 72% yield). 1H NMR (400 MHz, CDCl3, δ/ppm): 0.67−2.50 (br, 57H), 2.84 (br, 2H), 4.64 (br, 4H), 6.85 (br 1H), 7.75 (br, 2H). FTIR (KBr, νmax/cm−1): 3123, 2923, 2852, 1688, 1536, 1501, 1466, 1313, 1220, 1195, 1154, 1053, 819. Anal. Calcd for C48H68O4S4: C, 68.85; H, 8.19. Found: C, 69.12; H, 7.88. Synthesis of PBDTC-TT2. PBDTC-TT2 was obtained in 75% yield from BDTC-Br2 and TT2-Sn2 in the same way for PBDTC-TT1. 1 H NMR (400 MHz, CDCl3, δ/ppm): 0.67−3.00 (br, 71H), 4.57 (br, 4H), 6.84 (br 1H), 7.60 (br, 2H);. FTIR (KBr, νmax/cm−1): 3123,

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RESULTS AND DISCUSSION Synthesis and Properties of PBDTC-TT. The synthetic pathway for the polymers is shown in Scheme 1. BDTC-Br2 was obtained by dibromination of BDTC with Br 2 . Distannylation of TT-Br2 with Me3SnCl was instantly carried out as soon as corresponding TT-Br2 were prepared from TT with NBS for the instability. PBDTC-TT1 and PBDTC-TT2 were obtained in good yields over 70% yields by Stille coupling between corresponding TT-Sn2 and BDTC-Br2. They were soluble in common organic solvents such as dichloromethane, chloroform, and o-dichlorobenzene at room temperature. Number-average molecular weight (Mn) and polydispersity (Mw/Mn) of PBDTC-TT1 and PBDTC-TT2 were 10.1 kg/ mol, 12.5 kg/mol, 1.18, and 1.48, respectively. The molecular weights of the polymers were comparable each other and not so 4989

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Figure 3. UV−vis absorption spectra of PBDTC-TT and PTB7 (a) in CHCl3 and (b) in the film state.

of regioregular poly(3-hexylthiophene) (P3HT) (EHOMO = −4.70 eV, Eg = 1.90 eV) and higher than that of PTB7 (EHOMO = −5.15 eV, Eg = 1.69 eV). The Voc is directly related to difference between the EHOMO of a donor and the ELUMO of an acceptor such as PCBM, i.e., basically lower EHOMO of the donor can contribute to obtain higher Voc. The ELUMO of the PBDTC-TT1 and PBDTC-TT2 estimated from their EHOMO and Eg were situated at about −3.33 and −3.27 eV, respectively. It is generally recognized that ELUMO of the photoactive donor material should be at least 0.3 eV higher than ELUMO of PCBM (−4.3 eV) for efficient charge separation of generated excitons.3,6,38 Accordingly, it is assumed that the ELUMO of the polymers are sufficiently high for the charge separation. X-ray Diffraction Analysis. The X-ray diffraction (XRD) analyses were performed to investigate structural order of the polymers in the film state. PBDTC-TT1 and PBDTC-TT2 showed a diffraction peak at 2θ = 3.05, 3.57° (d1 = 24.7, 29.0 Å) and a weak diffraction at about 24.5−24.8° (about 3.6 Å), respectively, as shown in Figure 4. The former diffractions

high but enough to investigate their basic properties and to examine basic characteristics of BHJ solar cell devices.33,34 Thermogravimetric analysis of PBDTC-TT1 and PBDTCTT2 showed that temperatures of 5 wt % loss of them were 362 and 359 °C, respectively (Figure 1S in the Supporting Information) and were higher than those of a series of PTB based polymers (about 320 °C).35,36 This result suggests that PBDTC-TT has good thermal stability as well as the PTB polymers Figure 3 shows the absorption spectra of PBDTC-TT1 and PBDTC-TT2 with a reference data of PTB7, and the results are summarized in Table 1. The optical characteristics of PBDTCTable 1. Optical Properties and Energy Levels of the Polymers UV−vis absorption

polymer

λmax (nm) in CHCl3

PBDTC-TT1

702

PBDTC-TT2

692

λmax (nm) film

λonset (nm) film

EHOMOa (eV)

ELUMOb (eV)

Egc (eV)

707, 757 707, 757

827

−4.83

−3.33

1.50

810

−4.80

−3.27

1.53

a

Estimated by photoelectron spectroscopy in the atmosphere. bELUMO = Eg + EHOMO. cEstimated from the absorption band edge of the films.

TT1 and PBDTC-TT2 were basically same. Interestingly, the wavelengths of absorption maximum (λmax) and absorption onset (λonset) of PBDTC-TT1 in CHCl3 were observed at 702 and 827 nm, respectively, which were significantly red-shifted compared with those of PTB7 (Figure 3a). It is considered that intramolecular charge transfer effectively occurs between the Dtype TT unit and A-type BDTC unit in PBDTC-TT. Thin films of PBDTC-TT showed two λmax at 707 and 757 nm, a broad absorption band from 500 to 900 nm, and λonset almost the same as those in solution. The absorption spectrum extending to the near-infrared region is overlapped with solar photon flux around 700 nm,4,37 which might enhance exciton generation. The Eg of PBDTC-TT1 and PBDTC-TT2 determined from λonset were 1.50 and 1.53 eV, respectively. These values are almost the same as an ideal Eg of 1.5 eV for electron-donating materials to be performed BHJ solar cells efficiently.3,6,15 Energy Levels of the Polymers. The EHOMO values of PBDTC-TT1 and PBDTC-TT2 were estimated from their onset of photoelectron spectra in the atmosphere (Figure 2S in the Supporting Information), and the results are summarized in Table 1. PBDTC-TT1 and PBDTC-TT2 showed EHOMO of −4.83 and −4.80 eV, respectively, which were lower than that

Figure 4. X-ray diffraction patterns of drop cast films of the polymers.

correspond to interchain distances between conjugated polymer backbones while the latter is attributed to π-stacking distance between planar polymer backbones as summarized in Figure 5. PBDTC-TT1 that has a hexyl chain at TT showed that the interchain distance observed by XRD is almost equivalent to length of the dodecyl ester chain at BDT estimated by the DFT calculation, whereas PBDTC-TT2 that has a dodecyl chain at TT showed the rather longer interchain distance in the XRD result. These results suggest that a hexyl chain at TT in PBDTC-TT1 is put in the interdigitated polymer main-chain packing separated by the dodecyl ester spacer but the dodecyl chain at TT in PBDTC-TT2 disturbs to form the dense packing, broadening the space adequately. Polymer Solar Cell. BHJ solar cells were fabricated by spin coating of o-dichlorobenzene solutions of PBDTC-TT 4990

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Figure 6. J−V curves of BHJ solar cell of PBDTC-TT1:PC70BM (2:3) and PBDTC-TT2:PC70BM (1:1) under illumination of AM 1.5 G.

large heterojunction interfaces and sufficient percolation paths for electrons or holes to reach the electrodes in the blend film, thereby decreasing photocurrent recombination and leading to the highest FF and Voc of the three OPVs. The observed Voc is consistent with a predicted value of Voc (0.5 V) estimated from difference between EHOMO of PBDTC-TT1 (−4.8 eV) and ELUMO of PC70BM (−4.3 eV) although it is not high. The PCE decreased by making the active layer thicker. The decrease mainly caused by lowering FF, which is ascribed to increasing photocurrent recombination in the thicker BHJ layer. The optimized BHJ OPV consisting of PBDTC-TT2 and PC70BM (1:1) showed a PCE of 1.71% with Jsc of 7.13 mA cm−2, FF of 0.47, and Voc of 0.51 V as shown in Table 2, which was lower than that of the optimized BHJ OPV with PBDTCTT1 and PC70BM. The different length of the side chain at TT between PBDTC-TT1 and PBDTC-TT2 might affect miscibility of the components and the domain size of the BHJ layer. However, the surface image of the blend film of PBDTCTT2:PC70BM (1:1) observed by AFM was the aggregate of spherical domains similar to that of PBDTC-TT1:PC70BM (2:3) (Figure 3S in the Supporting Information). The lower PCE of PBDTC-TT2:PC70BM than PBDTC-TT1:PC70BM is mainly attributed to the lower Jsc. To investigate the difference of Jscs in the two BHJ OPVs, the hole mobility of the polymers was measured by the method of organic field effect transistors (OFETs). PBDTC-TT1 and PBDTC-TT2 showed hole mobilities of 6.9 × 10−4 cm2 V−1 s−1 and 3.1 × 10−4 cm2 V−1 s−1, respectively (Figure 4S in the Supporting Information). So, the lower Jsc of the BHJ devices of PBDTC-TT2 could be due to the lower hole mobility compared to PBDTC-TT1. The extended interchain spacing of PBDTC-TT2 shown in Figure 5 could lead to the lower hole mobility. The IPCEs of the devices of PBDTC-TT1:PC70BM (2:3) and PBDTC-TT2:PC70BM (1:1) were measured and the IPCE curves are shown in Figure 8 with a reference result using

Figure 5. Schematic image of main-chain separation by side-chains and π-stacking distance of PBDTC-TT. The distances in black are XRD results and those in red are estimated values by the DFT calculation.

containing PC70BM on an ITO glass coated with PEDOT:PSS and successive depositions of LiF and Al to make the device structure of ITO/PEDOT:PSS/polymer:PC70BM/LiF/Al. Characteristics of these devices were investigated by measuring J−V curves under AM 1.5 solar-simulated light irradiation of 100 mW cm−2. Typical results are summarized in Table 2 and J−V curves for the PBDTC-TT1:PC70BM and PBDTCTT2:PC70BM devices with the highest PCE are shown in Figure 6. When composition ratio of PBDTC-TT1 to PC70BM was 1:1, PCE of the device was 2.45%. PCE decreased to 2.35% when the ratio was 1:2, while it increased to 3.03% at the composition ratio of 2:3 with Jsc of 12.54 mA cm−2, FF of 0.48, and Voc of 0.51 V. The PCEs exhibit a distinct maximum as a function of the PC 70 BM concentration in PBDTCTT1:PC70BM BHJ OPVs; the PCE has a maximum at a PBDTC-TT1:PC70BM ratio of 2:3 and decreases at both lower and higher concentrations of PC70BM. To investigate the origin of this PC70BM composition dependence of the PCEs, the morphologies of the PBDTC-TT1:PC70BM blends were observed by AFM. The AFM images presented in Figure 7 show that the PBDTC-TT1 and PC70BM phases are separated into globular domains in the sizes of 50−300 nm, which grow in numbers with increasing fraction of PC70BM. This indicates that the globular domains are PC70BM. The PBDTCTT1:PC70BM (1:2) blend film (Figure 7c) showed pronounced phase separation, which certainly reduced the heterojunction interface area for exciton dissociation and consequently led to the lowest Jsc of the three OPVs. On the other hand, the PBDTC-TT1:PC70BM (2:3) film (Figure 7b) might have the Table 2. Characteristicsa of PBDTC-TT Based Solar Cellsb. polymer

mixture ratioc

thicknessd (nm)

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

PBDTC-TT1 PBDTC-TT1 PBDTC-TT1 PBDTC-TT1 PBDTC-TT1 PBDTC-TT2 PBDTC-TT2

1:1 1:2 2:3 2:3 2:3 1:1 2:3

81 86 76 148 224 72 90

12.40 10.47 12.54 12.08 12.36 7.13 6.81

0.47 0.49 0.51 0.48 0.47 0.51 0.49

0.42 0.45 0.48 0.40 0.32 0.47 0.44

2.45 2.35 3.03 2.34 1.88 1.71 1.49

a

The average value calculated from the results of three OPV cells. bDevice structure of ITO/PEDOT:PSS (40 nm)/polymer:PC70BM/LiF (1 nm)/ Al (80 nm). cBlending ratio (w/w) of the polymer and PC70BM. dThickness of the active layer. 4991

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Figure 7. AFM images of the blend films of PBDTC-TT1:PC70BM (a) (1:1), (b) (2:3), and (c) (1:2).

based on PBDTC-TT1 and PBDTC-TT2 achieved efficient sensitization of the whole visible range and the near-IR region as far as 900 nm. These results show that the BDTC unit can behave as an electron-accepting unit of D−A type conjugated polymers, and also demonstrates that the series of PBDTC-TT is a candidate of new D−A type narrow band gap polymers for BHJ solar cells. The narrower Eg of PBDTC-TT compared with the series of PTB based polymers is an advantage for BHJ solar cell applications. For future study, the results open up a new way to increase IPCE and PCE by designing new PBDTC-TT based polymers with appropriate EHOMO, which have dense packed-structures using appropriate solubilizing side chains in the film state.

Figure 8. IPCE of the devices of PBDTC-TT1:PC70BM (2:3), PBDTC-TT2:PC70BM (1:1), and PTB7:PC70BM(2:3).

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PTB7. BHJ OPVs based on PBDTC-TT1 and PBDTC-TT2 achieved efficient sensitization of the whole visible range and the near-IR region as far as 900 nm. These results are consistent with the absorption spectra of the polymers. PBDTC-TT based BHJ OPVs showed higher IPCE values than the PTB7-based OPV in the 750−900 nm region, although the IPCE values in the range of 300−750 nm were lower than the reference device of PTB7:PC70BM (2:3) with PCE = 6.2% (Jsc = 14.64 mA cm−2, Voc = 0.72 V, and FF = 0.61).39 The device of PBDTCTT1 displayed higher IPCE response (maximum IPCE = 50%) than that of PBDTC-TT2 (maximum IPCE = 33%). The difference of IPCE between both BHJ devices is responsible for the difference of their Jsc. It is thought that narrowing of the interchain spacing of PBDTC-TT should be an effective factor to increase Jsc as well as PCE from considerations of the results of XRD and OFET as mentioned above.

ASSOCIATED CONTENT

* Supporting Information S

Synthetic procedures of TT-Br and BDTC, NMR spectra of all compounds and polymers, thermogravimetric analysis charts, photoemission yield spectra in the atmosphere, AFM images of PBDTC-TT2:PC70BM, and FET characteristics of the PBDTCTT1 and PBDTC-TT2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(M.K.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Tsukuba Nanotechnology Human Resource Development Program, University of Tsukuba, and we thank a guest Prof. Dr. N. Ota, a coordinator, and a guest Prof. Dr. M. Kitajima, University of Tsukuba, for helpful discussion through the program. We thank Prof. Dr. H. Goto, University of Tsukuba, for the use of GPC, and Chemical Analysis Division, Research Facility Center for Science and Technology, University of Tsukuba, for facilities of the NMR, elemental analysis, and TGA.

CONCLUSIONS In summary, thieno[3,4-b]thiophene (TT) and benzodithiophene dicarboxylate (BDTC)-based new donor−acceptor conjugated polymers were synthesized by Stille cross-coupling polymerization. They possess the narrow band gap of ∼1.5 eV, small π-stacking distance, high lying EHOMO and good solubility to common organic solvents. The BHJ solar cells using PBDTC-TT1 exhibited the PCE value of 3.03% with Jsc of 12.54 mA cm−2, FF of 0.48, and Voc of 0.51 V, while that using PBDTC-TT2 showed PCE of 1.71% with Jsc of 7.13 mA cm−2, FF of 0.47, and Voc of 0.51 V. The higher performance of PBDTC-TT1:PC70BM (2:3) than PBDTC-TT2:PC70BM (1:1) was almost ascribed to the higher Jsc of the former device. According to XRD analysis and OFET measurements, the denser packing structure of PBDTC-TT1 results in the higher hole mobility, which leads to the higher PCE value in comparison with the device using PBDTC-TT2. BHJ OPVs

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