Evidence of Cation Symmetry Reduction Induced Bulk Photovoltaic Effect in Metal‐Free Perovskite for Efficient Self‐Powered X‐Ray Detection

Abstract Metal‐free perovskite (MFP) X‐ray detectors have attracted attention due to biocompatibility and synthesizability. However, the necessity of high voltages for MFP X‐ray detectors affects stability and safety. Although, the bulk photovoltaic effect (BPVE) with spontaneous electric field is a potential alternative for X‐ray detection without high voltage, the constitutive relationship of BPVE in MFP remains unclear. Herein, the relationship between BPVE and cation symmetry is explored, and a self‐powered X‐ray detector is realized by BPVE in MFP for the first time. Theoretical studies show that cation symmetry reduction can distort the halide octahedron in one direction, which increases the dipole moment and crystal polarity to induce BPVE. The electric field from crystal polarity can drive the defect passivation by the equilibrium carrier and enhance the nonequilibrium carrier performance for BPVE. Then, polar MFP (mPAZE‐NH 4 Br 3 H 2 O) with excellent BPVE is designed. Due to the nonlinear response, the detector obtains a numerically recorded equivalent sensitivity (≈10 3 µC Gy air −1 cm −2 ) at 0 V. Moreover, the imaging performance is demonstrated and two image convolution kernels for it are constructed. Finally, it features continuous operation (20000 s) and temperature stability (‐55–250 °C). It is believed that the method will further drive the application of MFP for X‐ray detectors.