Syntheses and Properties of Random Copolymers Using Thienyl-Thieno-Indole and Bithiophene-Dicarboximide with Different Ratios

We synthesized random copolymers using 6-(2-thienyl)-4H-thieno[3,2-b]indole (TTI) and N-alkyl-2,2′-bithiophene-3,3′-dicarboximide (BTI) as the electron-rich and deficient units for the polymer solar cells (PSCs). Herein, by using different feed ratios of electron-deficient unit, the push-pull type copolymers were synthesized by Stille polymerization of TTI, BTI and thiophene linkage. Out of the series PBTI3 with 7:3 ratio of TTI and BTI showed the highest molar absorption coefficient and mobility, which is favorable for efficiently harvesting solar photons and higher photo-induced carriers to improve the JSC. Among devices of synthesized copolymers, PBTI3/PC71BM (1:2) (with 3% DIO additive) blend-based PSCs showed the best performance with a JSC of 6.78 mA/cm2, a VOC of 0.72 V, a FF of 0.54 and a PCE of 2.64%.