Air‐Stabilized Lead‐Free Hexagonal Cs₃Bi₂I₉ Nanocrystals for Ultrahigh‐Performance Optical Detection

Abstract Broadband photodetectors possess enormous potentials for diverse applications while it is generally subjected to a lack of active materials with high performances under user‐friendly conditions. Bismuth‐based perovskites are promising and demonstrate better air stability than lead halide perovskites against moisture, heat, and light. In the present work, lead‐free all‐inorganic hexagonal Cs₃Bi₂I₉ perovskite nanocrystals are successfully fabricated via a modulated colloidal synthetic route. The Cs₃Bi₂I₉ nanocrystals are monodispersed with a size of ≈28 nm, which are applied for the construction of flexible Cs₃Bi₂I₉ detector and vertical heterogeneous structured photodetector of p‐Si/Cs₃Bi₂I₉/graphene. The flexible detector not only demonstrates excellent detective performance, but also yields high‐resolution photographs as an imaging sensor. The p‐Si/Cs₃Bi₂I₉/graphene photodetector exhibits broad spectral response from ultraviolet to infrared light (254−1064 nm) with microsecond response time (31/34 µs). In particular, it performs with a high responsivity of 23.6 A W⁻¹ and a high specific detectivity of 1.75 × 10¹³ Jones under 650 nm illumination even in weak light intensity as low as 1 µW cm⁻², and exhibits excellent stability in ambient air, which is obviously ahead of the previously reported work. In addition, finite element analysis further demonstrates the excellent optoelectronic properties of Cs₃Bi₂I₉ materials and the broad spectral response of vertical heterojunctions.