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

Abstract

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 (E^opt_g) 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 (J_sc) of 11.22 mA cm⁻². Considering the limited absorption of PIDT-tt-TPD (400–650 nm), the J_sc value is very impressive. In contrast, PIDT-t-TPD: PC₇₁BM only shows a PCE of 2.47% with a low J_sc of 5.52 mA cm⁻². The lower J_sc 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.