Physical and Chemical Reinforcement of Polycrystalline Perovskite Films Enables Robust X‐ray Detectors With Low Detection Limits

Polycrystalline halide perovskite films are highly promising for direct‐conversion X‑ray detection, but their practical application is severely limited by interfacial defects and structural looseness, which cause high leakage current, poor response, severe ion migration, and film fragility. Here, we report a dual‐action strategy that physically and chemically strengthens perovskite films by intercalating a tunneling poly(methyl methacrylate) (PMMA) interfacial layer. The insulating PMMA network fills surface pinholes, drastically increasing overall resistivity, and the detectors exhibit a profoundly suppressed dark current of 458 pA at −1 V and a low limit of detection (LoD) of 3.38 nGy air s −1 . The carbonyl groups in PMMA effectively passivate the uncoordinated Pb 2+ , eliminating non‐radiative recombination and interfacial ion migration, which yields a sensitivity of 1.1 × 10 5 µC Gy air −1 cm −2 at −20 V. Moreover, the polymer mitigates mechanical stress in films and encapsulates them indirectly, enhancing Young's modulus and strain at break while forming a dense barrier against moisture and oxygen. The detectors achieve exceptional operational stability over 15,000 s with negligible current drift, which is also manifested by uniform single‐pixel imaging under low dose rate. This work presents a versatile and effective route to perovskite X‐ray detectors with simultaneously high signal‐to‐noise ratio and long‐term stability.