Effects of Fabrication Processes and Temperature on Exciton Diffusion in Non-Fullerene Acceptors

Abstract

Exciton diffusion to donor–acceptor interfaces plays a critical role in charge generation in organic solar cells, yet its dependence on film fabrication and operating temperature remains poorly understood for high-performance nonfullerene acceptors. Here, femtosecond transient absorption spectroscopy and exciton–exciton annihilation analysis quantify diffusion in Y6 and L8-BO. In Y6 films, processing changes the diffusion coefficient (D) from 0.4 × 10–2 to 1.3 × 10–2 cm2 s–1 and the diffusion length (Ld) from 16.9 to 37.4 nm, whereas L8-BO maintains substantially more stable exciton diffusion across processing conditions. Cooling markedly decreases D and Ld despite a prolonged exciton lifetime, consistent with reduced thermally assisted exciton hopping in an energetically disordered environment. In PM6:Y6 blends, processes related to bimolecular charge recombination slow down with decreasing temperature. These results highlight molecular packing, energetic disorder, and temperature as key factors controlling exciton diffusion and charge dynamics in Y-series acceptors, providing a mechanistic framework for optimizing exciton harvesting in nonfullerene organic solar cells.