π-Extended donor-acceptor conjugated copolymers for use as hole transporting materials in perovskite solar cells

Abstract We introduced 3-octylthiophene (OT) or 3-octylthieno [3,2-b]thiophene (OTT) as π-bridges into PBT (poly(1-(4,8-bis(5- (2-ethylhexyl)selenophen-2-yl)benzo[1,2-b:4,5-b']dithiophen-2-yl)-5-(2-butyloctyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione), which is based on benzodithiophene and thienopyrrolodione units, in order to prepare thiophene-based π-bridging donor-acceptor conjugated copolymers for use as hole-transport layers in perovskite solar cells (PeSCs). The resulting polymers exhibit strengthened intermolecular π-π stacking and increased crystallinity (dominant face-on orientation), which facilitates vertical charge transport and hole mobility. Moreover, the highest occupied molecular orbital level of PBT-OTT is similar to the valence band of the perovskite material. As a result, efficient hole extraction occurs with reduced charge carrier recombination in PBT-OTT-based PeSCs. Therefore, the introduction of π-bridges to extend the π-conjugation length of conjugated polymers is a promising strategy for the development of efficient hole-transporting materials for use in PeSCs.