High-Performance Vertical-Structure Photodetector Based on Large Lateral-Size Lead-Free Cs₃Bi₂I₉ Perovskite...

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 Cs₃Bi₂I₉ 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 Cs₃Bi₂I₉ 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 Cs₃Bi₂I₉ 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.