High-Efficiency As-Cast Organic Solar Cells Based on an Asymmetric Acceptor

Herein, three nonfullerene acceptors (L4, L5, and L6) are designed and synthesized. Among them, the introduction of thiophene alkoxy units by asymmetric synthesis can endow the acceptor L5 with a slightly twisted molecular conformation in solution but a planar one in the solid state. Such properties facilitate the solution processing of L5 while maintaining a dense molecular packing in a thin film. Thus, asymmetric acceptor L5-based devices display an appropriate phase morphology and favorable charge transport capability. Hence, as-cast devices based on the asymmetric acceptor L5 generate a high PCE of 15.2%, which is significantly better than devices based on symmetric acceptors L4 (2.35%) and L6 (9.48%). Subsequently, we further fabricated PM6:L5:Y6 ternary devices with a higher PCE of 17.14% than the PM6:Y6-based one (15.68%). This work demonstrates that the introduction of appropriate building blocks through asymmetric molecular design and side-chain engineering can play an important role in improving the performance of solar cell devices.