High‐Performance Vertical‐Structure Photodetector Based on Large Lateral‐Size Lead‐Free Cs ₃ Bi ₂ I ₉ Perovskite Nanoplatelets

Abstract Halide perovskites hold significant promise for photodetection applications due to their high carrier mobility, long carrier diffusion lengths, and solution‐processing compatibility. However, lead toxicity in lead‐based perovskites and the indirect bandgap nature of lead‐free perovskites have limited their practical application. In this work, a solution‐based method is used to synthesize lead‐free Bi‐based Cs ₃ Bi ₂ I ₉ perovskite nanoplatelets (NPs) in situ with large lateral sizes, high crystallinity, and smooth surface. Theoretical calculation results and optical properties indicate that the reduced dimensionality enables Cs ₃ Bi ₂ I ₉ NP with a direct‐indirect bandgap nature, while excitonic absorption dominates the absorption edge and obscures the electronic bandgap. Benefiting from this, as‐fabricated vertical‐structure photodetector (PD) based on Cs ₃ Bi ₂ I ₉ NPs demonstrates outstanding photodetection performance with a high responsivity of 47.4 mA W ⁻¹ and a fast rise/fall time of 0.86/2.21 ms. Besides, the device exhibits excellent long‐term stability, retaining 95% of its initial current density after three months of exposure to the air. This work provides valuable design guidelines and insights into developing lead‐free perovskites for high‐performance PD.