Poly(thieno[3,2-b ]thiophene-alt-bithiazole): A D-A copolymer donor showing improved photovoltaic performance with indene-C ₆₀ bisadduct acceptor

A new D-A copolymer, poly(thieno[3,2-b]thiophene-alt-bithiazole) (PTTBTz), based on thieno[3,2-b]thiophene donor unit and bithiazole acceptor unit, was synthesized by the Pd-catalyzed Stille-coupling reaction for the application as donor material in polymer solar cells (PSCs). PTTBTz film possesses high thermal stability with 5% weight-loss temperature at 450 °C, a lower-lying HOMO energy level at -5.20 eV, a higher hole mobility of 6.45 × 10 ^-3 cm ²/(V s), and a crystalline structure. Photovoltaic performance of the polymer was investigated with [6,6]-phenyl-C ₇₁-butyric acid methyl ester (PC ₇₀BM) or indene-C ₆₀ bisadduct (ICBA) as acceptor and with 3% DIO additive. The power conversion efficiency (PCE) of the PSC based on PTTBTz:ICBA (1:1 w/w) reached 5.35% with a high V _oc of 1.03 V, a J _sc of 8.55 mA/cm ², and an FF of 0.608, whereas the PCE of the PSC based on PTTBTz:PC ₇₀BM (1:1 w/w) was 4.57% with a V _oc of 0.82 V, a J _sc of 9.89 mA/cm ², and an FF of 0.563, under the illumination of AM1.5, 100 mW/cm ². PTTBTz is one of the D-A copolymers that shows better photovoltaic performance with ICBA as acceptor than PC ₇₀BM. PTTBTz/ICBA could be a promising front active layer for high-efficiency tandem PSC because of its high V _oc.