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

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.