signals, which offer advantages in terms of high conversion efficiency as well as high spatial resolution. [9] Conventional amorphous selenium (α-Se) has already been commercialized for direct conversion X-ray imaging of mammography. [10] However, its low mobility-lifetime (µτ) product (about 10 −7 cm 2 V −1 ) and the small atomic number generally limited the sensitivity of the detectors as well as their application in higher energy digital radiography and computed tomography scanning. [11,12] In recent years, organic and inorganic halide perovskite (OIHP) materials have been emerging as a new generation of semiconductors that are extensively used in photovoltaics, light-emitting diodes, photodetection, and radiation detection fields. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] Generally, the category of OIHP can be divided into 3D and low-dimensional (LD, including 2D and 0D) ones based on their crystalline structures. Both 3D and LD OIHPs possess unique merits such as the high atomic number and low-temperature solution processibility that are highly preferred for X-ray detectors. [32][33][34][35][36] In addition, the 3D OIHPs usually exhibit a large µτ product up to ≈10 −2 cm 2 V −1 that is beneficial for realizing high sensitivity. [37] Currently, the best sandwich-structured X-ray detectors are based on the 3D triple-cation and mixed-halide OIHP (FA 0.85 MA 0.1 Cs 0.05 PbI 2.55 Br 0.45 , where FA is CH(NH 2 ) 2 , MA is CH 3 NH 3 and Cs is cesium) single crystals (SCs), with a record large sensitivity of (3.5 ± 0.2) × 10 6 µC Gy air −1 cm −2 and a minimum detectable dose of 42 nGy air s −1 at 40 keV X-ray radiation. [38] However, the 3D OIHPs generally suffer from the ion-migration induced dark current and photocurrent drift under a large electric field. In contrast, the LD OIHPs exhibit suppressed ion-migration effect and intrinsically high resistivity, which is beneficial for realizing ultralow noise and stable current output, and hence a low detection limit. [33,36,39] For instance, Zhuang et al. reported a highly sensitive X-ray detector made of layered perovskite-like (NH 4 ) 3 Bi 2 I 9 SC, and demonstrated the unique anisotropic X-ray response, effective suppression of ion migration and a low detection limit of 55 nGy air s −1 . [36] Besides, the 0D MA 3 Bi 2 I 9 SCs are demonstrated to possess ultralow dark carrier concentration of ≈10 6 cm −3 Organic-inorganic halide perovskites have exhibited bright prospects in highsensitivity X-ray detection. However, they generally suffer from the severe field-driven polarization issue that remarkably deteriorates the detection performance. Here, it is demonstrated that the interfacial electrochemical reaction between Au electrodes and halogen in MAPbI 3 single crystals (SCs) is the major source of the dark current polarization in the metal-semiconductor-metal (MSM)-structured perovskite X-ray detectors at the initial stage of biasing. By introducing the p-and n-type charge transport layers to isolate the electrodes from contacting ...