Ultrafast Exciton Transport with a Long Diffusion Length in Layered Perovskites with Organic Cation Functionalization

Layered perovskites have been employed for various optoelectronic devices including solar cells and light-emitting diodes for improved stability, which need exciton transport along both the in-plane and the out-of-plane directions. However, it is not clear yet what determines the exciton transport along the in-plane direction, which is important to understand its impact toward electronic devices. Here, by employing both steady-state and transient photoluminescence mapping, it is found that in-plane exciton diffusivities in layered perovskites are sensitive to both the number of layers and organic cations. Apart from exciton-phonon coupling, the octahedral distortion is revealed to significantly affect the exciton diffusion process, determined by temperature-dependent photoluminescence, light-intensity-dependent time-resolved photoluminescence, and density function theory calculations. A simple fluorine substitution to phenethylammonium for the organic cations to tune the structural rigidity and octahedral distortion yields a record exciton diffusivity of 1.91 cm² s^-1 and a diffusion length of 405 nm along the in-plane direction. This study provides guidance to manipulate exciton diffusion by modifying organic cations in layered perovskites.