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 MAPbBr₃ single crystals. Through methylammonium chloride-induced ion exchange, compositionally graded MAPbBr_{3 - x}Cl_x/MAPbBr₃ 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.