Polaron-Mediated Photoconduction in Lead-Free Single-Crystalline Perovskite Thin-Film Devices

The presence of toxic lead and the poor stability of lead-halide perovskites hinder their future commercial development. To address this issue, herein, we developed an all-inorganic lead-free Cs3Bi2I9 perovskite single-crystalline thin film (SCTF) via a space-limited solvent evaporation crystallization method for efficient photodetection. The fabricated photodetector on Cs3Bi2I9 SCTF exhibits responsivity and detectivity of ∼160 mA/W and ∼4.5 × 1013 Jones, respectively, which are comparable to those of other photodetectors, but needs further optimization to compete with commercial silicon and germanium-based photodetectors. The ultrafast transient absorption study reveals the fundamental photophysics in this lead-free Cs3Bi2I9 perovskite. It was observed that the Fröhlich and deformation potential are synergistically coupled to the electronic dynamics, leading to the self-trapping of charge carriers and the subsequent formation of localized polarons within a few picoseconds in the Cs3Bi2I9 perovskite lattice. These self-trapping and localization of charge carriers as small polarons limit carrier transport in Cs3Bi2I9 perovskite. In addition, we observed that the localized small polarons can be absorbed into an excited state by the photon energy of ΔEph = 1.59 eV, which can effectively transfer the charge carriers to a more delocalized eigenstate. Our study provides fundamental insights into the photophysics of lead-free perovskites for optoelectronic applications, which are eventually considered potential future semiconductor materials.