High-performance Photodetector Based on InSe/Cs2XI2Cl2 ( X =

Recently, all-inorganic two-dimensional (2D) Ruddlesden-Popper (RP) halide perovskites have drawn much attention due to their excellent stability in ambient air. However, their electronic and optical performance resulting from a wide bandgap and low carrier mobility have hindered their use in photodetectors. To overcome these limitations, in this work, taking advantage of hexagonal indium selenide ($\mathrm{In}\mathrm{Se}$) with a high electron mobility, we construct an atomically thin heterostructure. Density-functional-theory (DFT) calculations of the electronic and optical properties are performed for these heterostructures. The results demonstrate that the photodetection response spectrum of the heterostructures is significantly broadened as the bandgap decreases from 2.17 to 0.40 eV for the $\mathrm{In}\mathrm{Se}$/${\mathrm{Cs}}_{2}{\mathrm{Sn}\mathrm{I}}_{2}{\mathrm{Cl}}_{2}$ heterostructure. Moreover, the electron effective mass, ${m}_{e}^{\ensuremath{\ast}}$, is reduced from 1.13 ${m}_{0}$ to 0.41 ${m}_{0}$ for the $\mathrm{In}\mathrm{Se}$/${\mathrm{Cs}}_{2}{\mathrm{Ge}\mathrm{I}}_{2}{\mathrm{Cl}}_{2}$ heterostructure. The significant reductions in both the band gap and effective mass are determined to be related to the type-II band alignment, which favors the carrier separation at the interface. The physical mechanisms related to the usage of this material in photodetectors are also discussed. The proposed III--VI semiconductor InSe and all-inorganic 2D RP perovskite ${\mathrm{Cs}}_{2}X{\mathrm{I}}_{2}{\mathrm{Cl}}_{2}$ (X = $\mathrm{Pb}$, $\mathrm{Sn}$, and $\mathrm{Ge}$) heterostructures provide challenges and opportunities for designing high-performance photodetectors.