Non-radiative recombination energy losses in Y-series asymmetric acceptor-based organic solar cells.

The molecular engineering of Y6 has marked a paradigm shift in the development of non-fullerene acceptors (NFAs) for organic solar cells (OSCs), enabling remarkable enhancements in power conversion efficiencies (PCEs) through its distinct A-DA'D-A architecture and optimized intermolecular packing. However, further advancement is hindered by persistent non-radiative recombination energy losses (ΔE3), which predominantly originate from molecular relaxation and sub-gap charge recombination pathways. Among various structural optimization strategies, the implementation of molecular asymmetry has recently emerged as a promising approach to suppress ΔE3 without compromising light absorption or charge transport. This review systematically summarizes recent progress in asymmetric Y-series NFAs, including modifications involving central cores, terminal groups, side chains, and multi-site asymmetrization. Emphasis is placed on the mechanistic understanding of how specific asymmetries influence molecular energetics, exciton dynamics, and non-radiative decay processes. Theoretical models and empirical correlations are discussed to elucidate the structure-ΔE3 relationship. Finally, key challenges and prospective design principles for the rational development of next-generation asymmetric NFAs are outlined.