Randomly Embedded Quasi‐2D Micro‐Islands in Perovskite Single Crystal Enables High Performance Photodetection

ABSTRACT Lead‐halide perovskite single crystals (SCs) are premier candidates for broadband photodetection and radiation imaging, yet they face persistent trade‐offs between efficient carrier transport and low dark current, as well as performance versus environmental stability. While 2D perovskite SCs offer enhanced stability via insulating organic spacers, they inherently impede charge transport, thereby compromising performance. Here, we resolve these conflicts by engineering a quasi‐2D micro‐islands‐embedded 3D perovskite SC (MIESC) architecture by adding the trifunctional modulator didodecyldimethylammonium bromide (DDAB), which simultaneously modulates dimensional control, defect passivation, and surface stabilization. Constructed Type‐II heterojunctions at the interfaces of micro‐islands act as isotropic energy barriers that suppress dark current transport while separating and transporting photo‐generated carriers, suggesting excitation‐modulated carrier transport. MIESC exhibits an ultralow defect density of 1.45 × 10 8 cm −3 and remains stable for 310 days, demonstrating its high quality and exceptional stability. Enabled by this excitation‐modulated carrier‐transport model, the MIESC‐based photodetector exhibits a 67‐fold enhancement in responsivity at 405 nm, yielding a wide linear dynamic range of 166 dB at 650 nm. MIESC‐based X‐ray detector achieves an ultralow detection limit of 16.7 nGy air ·s −1 , 1/344 of the standard medical diagnostic rate. This work establishes a robust paradigm for high‐performance perovskite SCs and related photodetectors.