Comprehensive understanding of fluorination-performance relationship: The best-performed A-D-A-type acceptors

Abstract

Acceptor-donor-acceptor (A-D-A)-type nonfullerene acceptors (NFAs) have contributed to an efficiency breakthrough in organic solar cells (OSCs). However, the absence of an in-depth understanding of how to achieve high charge generation probability while guaranteeing low energy loss by molecular design has caused the stagnation of power conversion efficiency (PCE) in A-D-A-type acceptors. The fluorination strategy, as an effective approach to regulating molecular photoelectric and aggregation properties, can significantly affect device performance in OSCs. However, a comprehensive understanding of the relationship between fluorination and photovoltaic performance has been scarcely investigated thus far. Herein, a series of A-D-A-type acceptors, named ZITI-N-nF (n = 2, 4, 6, 8, representing the number of fluorine atoms), were designed and synthesized to reveal the underlying work mechanism of OSCs with low energy loss and efficient charge generation via fine-tuning of the charge-transfer state. The results indicate that ZITI-N-6F-based devices exhibit impressive charge generation probability with low energy loss (E_loss) because of the reduced nonradiative recombination, thus leading to a high PCE of 16.11% in binary OSCs and a PCE of 17.09% in ternary OSCs. Notably, a PCE of 16.6% was verified by the National Institute of Metrology, China, which is the highest certified PCE among OSCs based on A-D-A-type NFAs.