Molecular design of either polymer donors or acceptors is a promising strategy to tune the morphology of the active layer of organic solar cells, enabling a high-performance device. Thereinto, developing novel polymer donors is an alternative method to obtain high photovoltaic performance. Herein, we present a facile side-chain engineering on the dithiophenobenzotriazole (DTBTz) unit of newly-designed polymer donors (named pBDT-DTBTz-EH and pBDT-DTBTz-Me) to boost the performance of non-fullerene solar cells. Compared with pBDT-DTBTz-EH with long N-alkyl side chains, pBDT-DTBTz-Me with a short methyl exhibits stronger molecular aggregation, higher absorption coefficient, and preferred face-on orientation packing. As a consequence, pBDT-DTBTz-Me based devices achieve an optimal power conversion efficiency of 15.31% when donors are paired with the narrow bandgap acceptor Y6, which is superior to that of pBDT-DTBTz-EH based devices (9.17%). Additionally, the pBDT-DTBTz-Me based devices manifest more effective charge separation and transfer than pBDT-DTBTz-EH based devices. These results indicate that fine-tuning side chains of polymer donors provide new insights for the design of high-performance polymer donors in non-fullerene solar cells.
