End-group modification of non-fullerene acceptors enables efficient organic solar cells

End-group modification of non-fullerene acceptors is an effective means of optimizing the device performance of organic solar cells (OSCs). Here, we design and synthesize two isomeric terminal groups which simultaneously carry an electron-withdrawing fluoro group and an electron-donating phenoxy group. The introduction of a phenoxy group decreases the planarity of the terminal unit, enhances the solubility of acceptors and elevates the energy levels of acceptors. In addition, we find that the substituent position of the phenoxy group of the terminal unit can affect the packing behavior of the acceptors and the morphology of the blend films, thereby affecting the photovoltaic performance of the device. The optimized PBDB-T:Y-G42-based OSCs give a PCE of 15.24%, which is much higher than that of the devices based on PBDB-T:Y-G41 (12.12%) and PBDB-T:Y-F (12.53%). Our results demonstrate that the introduction of bulky substituents to the terminal unit is an effective means of tuning the solubility of NFAs and the morphology of blend films, and ultimately regulating the performance of OSCs.