Spatio-Temporal Mapping Uncouples Exciton Diffusion from Singlet-Singlet Annihilation in the Electron Acceptor Y6

Understanding the spatial dynamics of nanoscale exciton energy transport beyond the temporal decay is at the core of photosynthesis and essential to provide a better framework for further improvements of nanostructured optoelectronic devices, such as solar cells. The diffusion coefficient (D) of photovoltaic films has so far only been determined indirectly, from transient singlet-singlet annihilation (SSA) experiments.<br/>Here, we present the full picture of the exciton distribution dynamics, adding the spatial domain to the temporal one, by spatio-temporally resolved photo-luminescence microscopy. In this way, we directly track diffusion and we are able to decouple the real spatial broadening from its overestimation given by SSA. We measured the diffusion coefficient, D = 0.017 ± 0.002 cm²/s, of the non-fullerene electron acceptor Y6, which combined with an exciton lifetime of τ = 840 ps, gives a Y6 film diffusion length of L = (Dτ)^1/2 ≈ 35 nm. Thus, we provide an essential tool that enables a direct and free-of-artifacts determination of diffusion coefficients, which we expect to be at the core of further methodical studies on exciton dynamics in energy materials.