Impact of length of branched alkyl side chains on thiazolothiazole-based small molecular acceptors in non-fullerene polymer solar cells

Abstract

It has been reported that the length of branched alkyl side chains on fused-ring electron acceptors confers different impacts on properties versus solubility of BJH blends. However, because this impact on a non-fused acceptor backbone has rarely been studied, we examined the impact of molecular optimization from alkyl chain tuning based on non-fused thiazolothiazole small-molecule acceptors. The length of the side chain on the thiophene bridge was modified from 2-butyloctyl to 2-ethylhexyl, which corresponds to small molecules TTz3(C4C6) and TTz3(C2C4), respectively. Compared with the reported TTz3(C6C8) with long alkyl side chains, TTz3(C4C6) and TTz3(C2C4) exhibited stronger molecular aggregation, higher absorption coefficients, and greater redshifted UV absorption. Unexpectedly, after the alkyl chain was slightly shortened in this type of acceptor system, devices were successfully fabricated, but it was necessary to reduce the blending concentration at low rotation speeds due to the sharp decrease in the solubility of corresponding acceptor materials. Thus, the obtained unfavorable thickness and morphology of the active layer caused a decrease in J_sc and FF. As a consequence, TTz3(C4C6)- and TTz3(C2C4)-based devices showed an unsatisfactory power conversion efficiency of 6.02% and 2.71%, respectively, when donors were paired with the wide bandgap donor J71, which is inferior to that of TTz3(C6C8)-based devices (8.76%). These results indicate that it is challenging to determine the limit of the adjustable range of side chains to modify non-fused thiazolothiazole small-molecule acceptors for high-performance non-fullerene solar cells.