Self-Trapped Exciton Diffusion in a Lead-Free Two-Dimensional Hybrid Perovskite

Lead halide perovskites have paved the way for main-group metal halide materials to emerge as strong contenders for next-generation optoelectronics, including applications like solar cells, LEDs, lasers, sensors, and photocatalysis. Efficient light emission in these materials is often attributed to self-trapped excitons (STEs), where excitations create transient defects that trap excitons within the crystal lattice. While STEs are typically considered immobile, exciton diffusion is crucial for many optoelectronic applications.

In this work, we explore the optical properties and exciton transport in the lead-free hybrid perovskite [CMA]₄[Bi₂Cl₁₀] ○ H₂O. This material exhibits a UV band gap of 3.5 eV and photoluminescence centered at 2 eV with a large full width at half maximum of 0.5 eV, covering much of the visible spectrum. We attribute the broadband emission to STE formation, driven by the strong electron-phonon coupling, resulting in bright and efficient light emission. Using temperature-dependent, time- and spatially- resolved photoluminescence spectroscopy, we uncover an unexpected diffusive transport regime for the STEs with diffusion coefficients on the order of 0.1 cm² s⁻¹. Notably, the diffusion accelerates at higher temperatures, yielding diffusion lengths exceeding 10 nm, supported by a long carrier lifetime of 400 ns.

These findings are the first to demonstrate STE diffusion in a two-dimensional hybrid perovskite, challenging the conventional view of STE as immobile transient defects. Our results provide new insights into the mechanisms of exciton transport and emission in hybrid halide perovskites, advancing the design principles for future optoelectronic devices.