Effect of Alkyl Side Chains of Polymer Donors on Photovoltaic Performance of All-Polymer Solar Cells

In this manuscript, we investigate the effect of alkyl side chains of polymer donors on photovoltaic performance of all-polymer solar cells (all-PSCs) based on polymer acceptors containing a boron–nitrogen coordination bond (B ← N). We develop three polymer donors, CD-C12, CD-C16, and CD-C20, with 4,8-di(thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophen-alt-5,6-difluoro-7-(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole as the main chain and 2-butyloctyl, 2-hexyldecyl, and 2-octyldodecyl as the side chains, respectively. The three polymer donors exhibit similar LUMO/HOMO energy levels and similar absorption spectra. However, they show different aggregation tendency in solution and different solid stacking properties. Among the three polymers, CD-C16 containing medium-length alkyl side chains exhibits the strongest aggregation behavior in solution and much enhanced crystallinity in films. The three polymer donors are blended with a B ← N-based polymer acceptor PBN-14 to prepare all-PSCs. The device of CD-C16 with medium-length alkyl side chains shows small-size phase separation, which leads to a power conversion efficiency (PCE) of up to 9.8%. In comparison, the active layers of CD-C12 and CD-20 show large-scale phase separation and decreased PCEs of the devices. The optimal all-PSC device performance of CD-16 is ascribed to the strong aggregation behavior of polymer backbones in solution and the high crystallinity, which inhibit the oversized crystals of the polymer acceptor and lead to ideal phase separation morphology of the donor/acceptor blend.