Gradient 2D/3D Perovskite Films Prepared by Hot‐Casting for Sensitive Photodetectors

2D Ruddlesden–Popper perovskites have attracted wide attention recently because of tunable optoelectronic properties and have been used as alternatives to their 3D counterparts in various optoelectronic devices. Here, a series of (PEA)(2)(MA)(n) (−1)Pb(n)I(3) (n) (+1) perovskite thin films is designed and fabricated by a convenient hot‐casting method to obtain gradient n in the films, which leads to the formation of vertical heterojunctions that can enhance charge separation in the films under light illumination. Based on a single gradient perovskite film, a highly sensitive and stable photodetector with a responsivity up to 149 AW(−1) and a specific detectivity of 2 × 10(12) Jones is obtained. This work paves a way to realizing high‐performance optoelectronic devices with enhanced charge separation by introducing compositional gradient in a perovskite film.