Halide‐Gradient Sidewall Heterojunctions for Lateral Charge Confinement in Perovskite Single Crystal x‐ray Detectors

Halide perovskite single crystals are promising active materials for direct x‐ray detectors because of their strong x‐ray absorption, efficient charge transport, and solution‐processable crystal growth. However, in thick single‐crystal detectors, lateral side surfaces can act as conductive leakage pathways, increasing dark current, inducing baseline drift, and generating transient overshoot and undershoot during x‐ray irradiation. Previous passivation and heterojunction strategies have mainly focused on metal–semiconductor interfaces, leaving side‐surface transport pathways insufficiently controlled. Here, a selective side‐surface ion‐exchange strategy is developed to construct halide‐gradient sidewall heterojunctions in MAPbBr 3 single crystals. Through methylammonium chloride‐induced ion exchange, compositionally graded MAPbBr 3 − x Cl x /MAPbBr 3 multi‐heterojunctions are formed along the crystal sidewalls, creating wide‐bandgap lateral barriers that confine charge carriers within the Br‐rich bulk region. Spatial conductivity mapping and current‐distribution simulations reveal that the graded sidewalls suppress edge‐dominated conductivity and homogenize vertical charge transport without requiring guard‐ring electrodes. Consequently, side‐surface leakage, baseline drift, and transient undershoot are strongly suppressed, yielding reproducible and distortion‐free x‐ray responses. The treated detectors exhibit a 4.7‐fold enhancement in sensitivity, a 2.85‐fold reduction in detection limit, and markedly improved low‐dose x‐ray imaging contrast. This work establishes sidewall‐selective halide‐gradient engineering as a scalable route for lateral charge confinement in perovskite single‐crystal radiation detectors.