Halide perovskites are promising candidates for soft X-ray detection (<80 keV) owing to their high X-ray absorption coefficient, resistivity, and mobility lifetime product. However, the lack of large high-quality single crystals (SCs) renders it challenging to manufacture robust hard X-ray imaging systems (>100 keV) with a low detection limit and stable dark current. Herein, high-quality inch-size two-dimensional (2D) Cs 3 Bi 2 Br 9 (CBB) single crystals are grown from a melt via the Bridgman method. The crystal quality is enhanced by eliminating inclusions of CsBr-rich phases and restraining the trap-state density, leading to an enhanced resistivity of 1.41 × 10 12 Ω cm and a mobility lifetime product of 8.32 × 10 −4 cm 2 V −1 . The Au/CBB/Au singlecrystal device exhibits a high sensitivity of 1705 μC Gy air −1 cm −2 in all-inorganic bismuth-based perovskites and an ultralow detection limit of 0.58 nGy air s −1 in all of the bismuth-based perovskites for 120 keV hard X-ray detection. The CBB detector exhibits high work stability with an ultralow dark current drift of 2.8 × 10 −10 nA cm −1 s −1 V −1 and long-term air environment reliability under a high electric field of 10 000 V cm −1 owing to the ultrahigh ionic activation energy of the 2D structure. The proposed robust imaging system based on CBB SC is a promising tool for X-ray medical imaging and diagnostics. KEYWORDS: lead-free perovskites, two-dimensional perovskites, Cs 3 Bi 2 Br 9 single crystal, ultralow detection limit, X-ray imaging

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Metal halide perovskites have emerged as next-generation semiconductors for X-ray detection because of their excellent photoelectric properties. However, severe ion migration in three-dimensional (3D) halide perovskites usually causes dark current drift in radiation detectors, especially in high electric fields. Here, we report a liquid-phase epitaxial method based on inverse-temperature crystallization (ITC), with which a hybrid/all-inorganic 3D perovskite single-crystal heterojunction is constructed, using an orientated CsPbBr3 single crystal as a substrate. MAPbBr3– n Cl n /CsPbBr3 single-crystal heterojunctions and single-crystal arrays (through the addition of a mask onto the substrate) with good lattice matching were successfully constructed, laying the foundation for high-performance X-ray detectors. In particular, the MAPbBr3/CsPbBr3 heterojunction drastically enhances the X-ray response of the CsPbBr3 single crystal, with a record-high sensitivity of 2.0 × 105 μC Gyair –1 cm–2 and lowest detection limit of 96 nGyair s–1 for 120 keV hard X-ray detection. Moreover, the dark current drift caused by ion migration is suppressed to 3.92 × 10–4 nA cm–1 s–1 V–1 even in a high reverse electric field of −125 V mm–1. Based on the superior detection performance, it is confirmed that a robust and uniform X-ray imaging detector with good resolution was successfully devised using the heterojunction. Our study provides a simple strategy for constructing perovskite single-crystal heterojunction films and arrays that effectively suppress ion migration that otherwise occurs in 3D halide perovskite single crystals to enable the creation of robust ultrasensitive X-ray detection and imaging systems.

The dark current drift caused by severe ion migration in allinorganic perovskite CsPbBr 3 degrades the stability of X-ray detection performance under a high electric field. The halogen doping is an effective method to improve the properties of the perovskites. In this work, the all-inorganic Cl-doped perovskites CsPbBr 3-n Cl n (n = 0, 0.1 and 0.5) single crystals were grown by a modified low-temperature inverse temperature crystallization (ITC) method with the growth temperature lower than the phase transition point. The trap density decreased to 3.05 × 10 10 cm À 3 and the carrier mobility increased to 124.75 cm 2 V À 1 s À 1 for the optimum component of CsPbBr 2.9 Cl 0.1 . Furthermore, by designing the detector with an asymmetric electrode configuration, CsPbBr 2.9 Cl 0.1 detector showed super low dark current and an outstanding hard X-ray induced photoresponse under a high voltage of 200 V. Consequently, the CsPbBr 2.9 Cl 0.1 perovskite detector delivers a high sensitivity (5593.24 μC Gy air À 1 cm À 2 ) and a low detection limit (0.68 μGy air s À 1 ). Our low temperature solution grown Cl-doped all-inorganic perovskite single crystals efficiently improve the photoelectric properties and enhance the X-ray detection performance.

Metal halide perovskite single crystals have become emerging candidates for photovoltaic applications due to their better optoelectronic properties and higher stability than their polycrystalline thin-film counterparts. However, in contrast to the rapid enhancement of power conversion efficiency (PCE), the operational stability of single-crystal perovskite solar cells (PSCs) remains far lagging behind. Herein, it is discovered that widely investigated 20 μm-thick single-crystal PSCs show poor operational stability, which is assigned to low crystal quality of these thin single crystals. Subsequently, the crystal quality of formamidinium0.55methylammonium0.45 lead triiodide (FA0.55MA0.45PbI3) thin single crystals are optimized by adjusting the ion diffusion velocity in confined space, leading to lower trap density, larger ion migration activation energy, and reduced light-induced degradation of material properties. As a result, stable single-crystal PSCs with no efficiency degradation after 330 h of continuous operation at the maximum power point under 1 sun illumination are achieved. Moreover, thickness-dependent device efficiency discloses an ultralong carrier transport length of 200 μm in FA0.55MA0.45PbI3 thin single crystals, which is instructive for developing lateral-structure solar cells.