Efficiency enhancement in an indacenodithiophene and thieno[3,4-c]pyrrole-4,6-dione backbone photovoltaic polymer with an extended thieno[3,2-b]thiophene π-bridge

Two novel donor–π–acceptor (D–π–A) type polymers based on indacenodithiophene (IDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD) with thiophene (PIDT-t-TPD) or thieno[3,2-b]thiophene (PIDT-tt-TPD) as π-bridges were designed and synthesized. The π-conjugated backbone length in the repeating unit of the polymer chain was varied by different π-bridges. Both polymers exhibit planar molecular structures, similar broad optical band gaps (Eoptg) and low-lying highest occupied molecular orbital (HOMO) energy levels. However, PIDT-tt-TPD with the more extended π-conjugated backbone shows higher hole mobility and better miscibility with (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM). Polymer solar cells (PSCs) based on PIDT-tt-TPD:PC71BM show the highest power conversion efficiency (PCE) of 5.23% with a short-circuit current density (Jsc) of 11.22 mA cm−2. Considering the limited absorption of PIDT-tt-TPD (400–650 nm), the Jsc value is very impressive. In contrast, PIDT-t-TPD: PC71BM only shows a PCE of 2.47% with a low Jsc of 5.52 mA cm−2. The lower Jsc value of PIDT-t-TPD is consistent with its limited external quantum efficiency (EQE) response (lower than 40%). The present results demonstrate that extending the length of a π-conjugated backbone by an appropriate π-bridge in the D–A polymer improves the photovoltaic performance of PSCs.