Side chain engineering of polymer acceptors for all-polymer solar cells with enhanced efficiency

All-polymer solar cells (all-PSCs), which utilize polymer donor (PD)/polymer acceptor (PA) blend films as the photoactive layer, have attracted considerable attention and achieved substantial progress. Nevertheless, only a few PAs have led to power conversion efficiencies (PCEs) >8% in all-PSCs, and most of them are naphthalene diimide-based polymers. Herein, a series of PAs named PBTI2(xDT)-FT have been developed, which consist of a 2,5-bis(2-thienyl)thieno[3,2-b]thiophene diimide (BTI2)-alt-3,4-difluorothiophene polymer backbone, where the ratios of 2-decyltetradecyl (DT) and 2-octyldodecyl (OD) side chains on BTI2 have been varied such that x designates the percentage of BTI2 units with DT side chains (i.e. 0, 30, 50, 70, and 100). The side chain engineering aims to fine-tune the polymer chain packing and thus optimize the film morphology. When applied in all-PSCs through additive-free processing, the PBTI2(50DT)-FT-based cells yield a promising PCE of 8.32% with the well-known PTB7-Th as the polymer donor, substantially higher than those of the two parent copolymers (6.82% for PBTI2(0DT)-FT and 4.30% for PBTI2(100DT)-FT). This PCE is also one of the highest for additive-free all-PSCs. Moreover, the devices show a good stability with 90% of the initial PCE retained after 400 h. This work demonstrates the effectiveness of side chain engineering of PAs on optimizing the film morphology and improving the PCE.