A 1,1′-vinylene-fused indacenodithiophene-based low bandgap polymer for efficient polymer solar cells

A 1,1′-vinylene-fused indacenodithiophene (IDTV) donor unit with 22 π-conjugated electrons was synthesized. A ladder-type D–A copolymer PIDTV-ffBT using IDTV as the donor unit and 5,6-difluorobenzothiadiazole (ffBT) as the acceptor unit was developed for application as a donor material in polymer solar cells (PSCs). Compared to other analogue polymers, PIDTV-ffBT possesses a two-dimensional conjugated multi-electron fused ring, excellent planarity and close π–π stacking, leading to a higher light harvesting coefficient, an enhanced charge carrier mobility of 0.032 cm2 V−1 s−1 and improved photovoltaic performance. The PSCs based on PIDTV-ffBT:PC71BM achieved a promising power conversion efficiency (PCE) of 7.3% with a high short-circuit current density (Jsc) of 17.1 mA cm−2. These results indicate that the introduction of the 1,1′-vinylene-fused system into IDTV for ladder-type polymers is an effective strategy to enhance the light absorption coefficient and improve charge carrier mobility for high efficiency PSCs.