Abstract Low‐dimensional metal halide perovskites exhibit exceptional photoelectronic properties and intrinsic stability, positioning them as a promising class of semiconductor materials for light‐emitting devices and photodetectors. In this work, we present a millimeter‐scale single crystal of mixed low‐dimensional (one‐dimensional–zero‐dimensional [1D–0D]) organic lead iodide with well‐defined crystallinity. The fabricated single‐crystal devices demonstrate high‐sensitivity photoresponse and x‐ray detection performance. By spatially isolating organic molecules to form the mixed 1D–0D crystal structure, ion migrations is effectively suppressed, resulting in a remarkable three orders of magnitude reduction in the dark current (56.4 pA @200 V) of the single‐crystal devices. Furthermore, by enhancing the background characteristics, we achieved an impressive low x‐ray detection limit of 154.5 nGys−1 in the single‐crystal device. These findings highlight that the mixed 1D–0D organic lead iodide configuration efficiently controls ion migration within the crystal structure, offering a promising avenue for realizing high‐performance perovskite‐based photodetectors and x‐ray detectors.
