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3D metal halide perovskite X‐ray detectors have become the focus of research in recent years due to their superior sensitivity. However, the reported efficient perovskite X‐ray detectors all contain highly toxic lead and worrisome stability, which greatly limits their wide application and promotion. In this study, a distorted 3D germanium perovskite MAGeI3 single crystal (SC) is designed and grown by gradient cooling method. Due to its distorted octahedral structure, the MAGeI3 SCs not only possess high sensitivity but also have superior long‐term working stability. Specifically, the MAGeI3 SC X‐ray detectors achieve a record sensitivity of 42825 µC Gy−1 cm−2 and an impressive low detection limit of 4.1 nGy s−1, both of them are far better than the state‐of‐the‐art lead‐free perovskite SC X‐ray detectors. Moreover, the unencapsulated SC detector still maintains the original response even at high applied electric field and high dose rate X‐ray irradiation for several hours. Eventually, a high resolution of 7.2 lp mm−1 in X‐ray imaging is demonstrated on the MAGeI3 SC detector under a low dose rate of 478 nGy s−1 and at a low electric field of 1 V mm−1. This work provides a new pathway to design high‐performance and low‐toxicity X‐ray detectors. [ABSTRACT FROM AUTHOR]

The development of metal‐halide perovskite nanocrystals (NCs) that yield bright and stable emission is of great importance. Previous reported perovskite NCs are mostly based on APbX3‐type family fabricated via ligand‐ or surfactant‐assisted chemical approaches. However, realizing bright and stable emission remains a challenge because of desorption of ligands/surfactants during long‐term operation. Herein, Ruddlesden–Popper (RP)‐type (A)2(MA)n‐1PbnBr3n+1 NCs with size less than Bohr radius stabilized in mesoporous silica scaffold, which are prepared in situ via physical approach at low temperature are introduced. The RP NCs in mesoporous silica exhibit the formation of spatially and electronically separated quantum wells, efficient energy funneling between different n phases for bright emission (photoluminescence quantum yields of ≈99%), high irradiation stability (T70 = 110 days), and long‐term stability (T90 = 110 days). These RP NCs have broad potential for bright light‐emitting diodes, high‐resolution PL imaging, and waterproof inks. Importantly, for the first time, stretchable perovskite X‐ray scintillator is demonstrated with excellent X‐ray imaging with resolution greater than 14 line pairs mm−1. These findings offer a paradigm to motivate future research toward stable and efficient perovskite optoelectronics. [ABSTRACT FROM AUTHOR]