The regulation of π-bridge of indacenodithiophene-based donor-π-acceptor conjugated polymers toward efficient polymer solar cells

The instinct feature of pi-bridges of donor-pi-acceptor (D-pi-A) conjugated polymers plays a critical role in tailoring the polymer basic properties as well as the photovoltaic performance. In this work, we designed two new D-pi-A polymers PSeBDDIDT and PTzBDDIDT based on donor unit indacenodithiophene (IDT), acceptor unit benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD) with varying pi-conjugated bridges of selenophene and thiazole units. Compared to the counterpart polymer PThBDDIDT with thiophene pi-bridge, polymers PSeBDDIDT and PTzBDDIDT exhibited enhanced intermolecular interactions, expanded absorption range and better molecular planarity arising from the suppressed steric hindrance between the respective bridge and the adjacent BDD moiety. Conventional polymer solar cells were fabricated with PC71BM as the acceptor. The results witness a greatly improved efficiency for PSeBDDIDT based solar cell compared to PThBDDIDT based counterpart device (7.04%). As a result, the champion PCE of 8.65% was recorded with Voc of 0.899 V, Jsc of 16.04 mA cm(-2) and FF of 0.60. It is worth highlighting that this is the highest efficiency among all IDT-based polymeric donors reported till now. As a contrast, the devices fabricated from PTzBDDIDT demonstrated a low efficiency of 2.75%, with increased Voc of 0.968 V, Jsc of 8.12 mA cm(-2) and FF of 0.35. The inferior photocurrent and fill factor of PTzBDDIDT based device was in accord with the poor exciton dissociation probability (38%) and low hole and electron charge transport inside the polymer solar cell. In conclusion, this work demonstrates a state of the art IDT-based polymeric donor with selenophene pi-bridge and the importance of rational regulation of pi-bridges on polymer properties and photovoltaic efficiency of polymer solar cells.