High‐Performance Flexible Self‐Powered Photodetectors Utilizing Spontaneous Electron and Hole Separation in Quasi‐2D Halide Perovskites

Abstract Although there are recent advances in many areas of quasi‐2D halide perovskites, photodetectors based on these materials still cannot achieve satisfactory performance for practical applications where high responsivity, fast response, self‐powered nature, and excellent mechanical flexibility are urgently desired. Herein, utilizing one‐step spin‐coating method, self‐assemble quasi‐2D perovskite films with graded phase distribution in the order of increasing number of metal halide octahedral layers are successfully prepared. Gradient type‐II band alignments along the out‐of‐plane direction of perovskites with spontaneous separation of photo‐generated electrons and holes are obtained and then employed to construct self‐powered vertical‐structure photodetectors for the first time. Without any driving voltage, the device exhibits impressive performance with the responsivity up to 444 mA W −1 and ultrashort response time down to 52 µs. With a bias voltage of 1.5 V, the device responsivity becomes 3463 mA W −1 with the response speed as fast as 24 µs. Importantly, the device's mechanical flexibility is greatly enhanced since the photocurrent prefers flowing through the metal halide octahedral layers between the top and bottom contact electrodes in the vertical device structure, being more tolerant to film damage. These results evidently indicate the potential of graded quasi‐2D perovskite phases for next‐generation optoelectronic devices.