Due to the wide application of photodetectors in the fields including optical communication, environmental monitoring, and image sensing, the research for developing highly efficient photodetectors has attracted extensive attention over the past decades. Since 2009, methyl ammonium lead halide (CH₃NH₃PbX₃, X=halogen) perovskites have become one of the hot topics in material science for light collection in photodetectors.

At present, the performance of photodetectors based on perovskite polycrystalline thin films is still at a certain distance from the expected value. One of the main reasons is that the carrier transport at the interface is easily affected by grain boundaries and grain defects. Many research groups have tried to combine perovskite polycrystalline thin films with high-mobility two-dimensional materials to improve device performance and have achieved certain results, but the negative effects of perovskite polycrystalline grain boundaries still remain.

To solve this problem, a team led by Assoc. Prof. YU Weili from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences, and Prof. GUO Chunlei from the University of Rochester, synthesized low surface defect density CH3NH3PbBr3 microplate through the inverse temperature crystallization strategy. They prepared an effective vertical structure photodetector combining high-quality perovskite single crystal with monolayer graphene with high carrier mobility.

The vertical photodetectors made with these CH₃NH₃PbBr₃ microplates and monolayer graphene exhibit excellent light response performance with high light responsivity (≈1017.1 AW^-1), high light detectivity (≈2.02×1013 Jones) and ultrahigh gain of 2.37 × 103 under 532 nm laser irradiation at room temperature. The parameters are approximately one order of magnitude larger than that of the photodetectors without monolayer graphene.

Carrier ultrafast kinetics studies have shown that the improvement in device performance is mainly due to the increased lifetime of perovskite carriers of high-quality perovskite crystals and the effective extraction and transport of free charge by graphene.

The significantly improved device performance after combining perovskite microplate with the two-dimensional graphene are believed closely related to the operational synergy of the two materials in carrier generation and transportation. This emphasizes the role of device structure and energy band design in device optimization, and reveals the mechanism of effective carrier extraction and transmission.

This study, published in Small, provides a novel strategy for the preparation of high-performance perovskite photodetectors, and will play a key role in promoting the research and development of perovskite two-dimensional composite photovoltaic devices.