The polymerization was carried out under a dry argon atmosphere. Toluene used as reaction solvent was distilled from calcium hydride and stored over activated molecular sieves before use. NMR spectra were measured on Varian 400-MR and Varian System 500 spectrometers (Agilent Technologies, Santa Clara, CA, USA). UV-vis absorption spectra were measured on a SHIMADZU-UV-3150 spectrometer (Shimadzu, Kyoto, Japan). GPC was carried out using serially connected Shodex KF2001 and KF2002 columns (Showa Denko, Tokyo, Japan) and THF as the eluent.
3.2. Device Fabrication
For the fabrication of BHJ-PSCs, patterned ITO glasses were washed sequentially with acetone and 2-propanol under ultrasonication and the glasses were further cleaned by exposure to UV/ozone. A PEDOT-PSS layer (Clevios P VP AI 4083, Heraeus Precious Metals, Leverkusen, Germany) was then formed by spin-coating at 3000 rpm on the ITO glasses followed by baking at 130 °C for 10 min in air. The substrates were transferred to a glove box under a dried nitrogen atmosphere (dew point: −80 °C), and a chlorobenzene solution of pDSBT2-BT:PC71BM blend was spin-coated on the PEDOT-PSS layer at 1500 rpm for 60 s. After drying in vacuo, Ca or LiF, and Al (50 nm) were vapor-deposited through a shadow mask (active area was 0.04 cm2) on the organic active layer at the base pressure of 6 × 10−4 Pa. BHJ-PSCs were encapsulated under nitrogen. The vapor deposition and the encapsulation were successively carried out in the glove box. BHJ-PSCs were tested in air with a computer-programmed Keithley 2611 source meter (TFF, Tokyo, Japan) under a solar simulator (Asahi Spectra HAL-320, Asahi Spectra, Tokyo, Japan) that simulated AM1.5 solar irradiance with the power density of 100 mW/cm2.
3.3. Preparation of pDSBT2-BT
A mixture of DSBT2Sn (0.242 g, 0.198 mmol), 4,7-dibromo-2,1,3-benzothiadiazole (5.83 × 10−2 g, 0.198 mmol), Pd2(dba)3 (9.10 × 10−3 g, 9.90 × 10−3 mmol), (o-tolyl)3P (1.21 × 10−2 g, 3.96 × 10−2 mmol), and toluene (18 mL) was stirred at 70 °C for 5 days. The resulting mixture was allowed to cool to room temperature and an aqueous solution (30 mL) of sodium N,N-diethyldithiocarbamate trihydrate (3.1 g) was added. The mixture was heated to 80 °C for 2 h. The organic layer was separated and washed with water, 3 vol % acetic acid aqueous solution, and then water again, in that order. The organic layer was dried over anhydrous magnesium sulfate and the solvent was removed under vacuum. The residue was reprecipitated using a sequence of solvent mixtures: toluene/methanol, toluene/ethanol, and toluene/ethyl acetate, to provide 108 mg (53% yield) of pDSBT2-BT as a dark purple solid: m.p. >300 °C; 1H-NMR (δ in C6D4Cl2, 500 MHz) 0.87–0.93 (m, 24H), 0.96–1.20 (m, 16H), 1.34–1.46 (m, 16H), 1.50–1.62 (m, 16H), 7.48 (s, 2H), 7.74 (s, 2H), 8.39 (s, 2H); 13C-NMR (δ in CDCl3, 125 MHz) 12.87, 13.49, 13.57, 26.55, 26.62, 27.26, 27.32, 125.28, 134.38, 135.39, 135.63, 136.34, 137.86, 144.78, 146.96, 152.54; GPC Mn 13,000, Mw 24,700, Mw/Mn 1.9; UV-vis abs λmax 599 nm (in C6H5Cl).