Long-Range Exciton Diffusion and Efficient Hole Transfer in Molecular Non-Fullerene Acceptors with Strong Resonance Structure

With the rapid development of high-performance non-fullerene acceptor (NFA) materials, bulk-heterojunction organic photovoltaic devices have achieved significant progress, now surpassing 20% power conversion efficiency. Understanding materials with strong resonance characteristics, featuring strong light absorption and low band-tailing absorption, is crucial for elucidating the mechanisms that further enhance photovoltaic efficiency. Here, we uncover the relationship among exciton diffusion length, hole transfer rate, and bond length alternation (BLA) values, which characterize the strong resonance character, in a wide range of Y6-series NFAs (Y5, Y6, L8-BO, and S-CSeF) by using femtosecond transient absorption (fs-TA) measurements. In neat NFA molecular films, we measure exciton lifetimes as a function of excitation density, suggesting that the acceptors with smaller BLA values and stronger resonance characteristics exhibit longer exciton diffusion lengths. In bulk-heterojunction films of donor polymer PM6/D18 and Y6-series NFA, we observe that the interfacial energy offset and BLA values are two key factors that determine the hole transfer rate. With similar interfacial energy offset and driving force values, hole transfer can be accelerated in an acceptor with a smaller BLA value. The resonance structures in Y6-series NFA can support both long-range exciton diffusion in neat films and efficient hole transfer at the interface. Our finding suggests that alternating the carbon-carbon bond lengths and atomic charge distribution in π-conjugated backbones and enhancing the resonance structure of NFAs can provide a promising route for further improving device performance.