Non-equivalent D-A copolymerization strategy towards highly efficient polymer donor for polymer solar cells

D-A copolymerization is a broadly utilized molecular design strategy to construct high efficiency photovoltaic materials for polymer solar cells (PSCs), and all the D-A copolymer donors reported till now are the alternate D-A copolymers with equal D-and A-units. Here, we first propose a non-equivalent D-A copolymerization strategy with unequal D- and A-units, and develop three novel non-equivalent D-A copolymer donors (PM6-D1, PM6-D2 and PM6-D3 with D/A unit ratio of 1.1:0.9, 1.2:0.8 and 1.3:0.7, respectively) by inserting more D units into the alternate D-A copolymer PM6 backbone to finely tune the physicochemical and photovoltaic properties of the polymers. The three non-equivalent D-A copolymers show the down-shifted highest occupied molecular orbital (HOMO) energy levels, higher hole mobility, higher degree of molecular self-assembly and higher molecular crystallinity with the increase of D-unit ratio in comparison with the alternate D-A copolymer PM6. As a result, all the three non-equivalent D-A copolymer-based PSCs with Y6 as acceptor achieve improved power conversion efficiency (PCE) with higher Voc, larger Jsc and higher FF simultaneously. Particularly, the PM6-D1:Y6 based PSC achieved a high PCE of 17.71%, which is significantly higher than that (15.82%) of the PM6:Y6 based PSC and is one of the highest performances in the binary PSCs.