Formamidinium Lead Halide Perovskite Crystals with Unprecedented Long Carrier Dynamics and Diffusion Length

TLDR

State‑of‑the‑art perovskite solar cells achieve record efficiencies by replacing methylammonium with formamidinium, yet FA‑based films suffer severe structural disorder and high trap densities that obscure their intrinsic properties. The study aims to characterize the optical and electrical properties of FAPbX3 (X = Br⁻, I⁻) single crystals. The authors investigate these properties using detailed optical and electrical measurements on FAPbX3 single crystals. FAPbX3 single crystals exhibit markedly improved transport, with FAPbBr3 showing a five‑fold longer carrier lifetime, a ten‑fold lower dark carrier concentration, and diffusion lengths of 6.6 µm (FAPbI3) and 19.0 µm (FAPbBr3)—among the longest reported for perovskites—highlighting their promise for integrated applications.

Abstract

State-of-the-art perovskite solar cells with record efficiencies were achieved by replacing methylammonium (MA) with formamidinium (FA) in perovskite polycrystalline films. However, these films suffer from severe structural disorder and high density of traps; thus, the intrinsic properties of FA-based perovskites remain obscured. Here we report the detailed optical and electrical properties of FAPbX3 (where X = Br– and I–) single crystals. FAPbX3 crystals exhibited markedly enhanced transport compared not just to FAPbX3 polycrystalline films but also, surprisingly, to MAPbX3 single crystals. Particularly, FAPbBr3 crystals displayed a 5-fold longer carrier lifetime and 10-fold lower dark carrier concentration than those of MAPbBr3 single crystals. We report long carrier diffusion lengths—much longer than previously thought—of 6.6 μm for FAPbI3 and 19.0 μm for FAPbBr3 crystals, the latter being one of the longest reported values in perovskite materials. These findings are of great importance for future integrated applications of these perovskites.