Disentangling Polaron Formation Pathways in Y6 by Probing Noninteracting and Interacting Chromophores via Transient Absorption Spectroscopy

Abstract

In highly efficient organic solar cells, the optoelectronic properties of the non‐fullerene acceptor Y6 are governed by the molecular packing of noninteracting and interacting molecules. In this study, steady‐state and transient absorption spectroscopy are employed to disentangle the spectral signatures and excited‐state dynamics of interacting and noninteracting Y6 in solution and thin films. By controlling aggregation in dilute solution, we identify localized Frenkel‐type excitons (LE) on noninteracting Y6 and aggregate excitons (AE) on intermolecular interacting Y6. The AEs exhibit strongly fluence‐dependent lifetimes (30–200 ps) governed by singlet–singlet annihilation, whereas localized excitons decay monoexponentially on a nanosecond timescale. Furthermore, long‐lived polaronic states are observed to form on aggregated Y6 within tens of picoseconds and persist into the nanosecond regime. Comparison with thin‐film measurements reveals similar processes with an additional ultrafast energy transfer from noninteracting chromophores to aggregated domains. These results establish a consistent framework for assigning transient absorption features in Y6 and suggest that, in neat films of Y6, the pairwise formation of polarons arises from an Onsager‐like autoionization process of delocalized excitons in domains of interacting Y6 with moderate yield. This provides central insight into charge‐generation mechanisms in Y6‐based photovoltaic systems.