the coincidence efficiency. This indicates that the solid angle controlled by the PET system and the efficiency of the intrinsic detector are greater. [11,12] More significantly, the Compton events in the pulse height spectrum are mainly caused by scattering in the scintillator. Therefore, these deficiencies can be avoided by lowering the threshold. However, the signal provided by Compton scattering quanta in adjacent crystals may be higher than the threshold, and thus may affect the position resolution and make it worse. In X-ray computed tomography (CT), the body is mainly irradiated continuously from multiple directions by fan-shaped X-rays, while attenuation profiles are recorded, and finally reconstructed from the cross-sectional view of the body. [13] This technology mainly uses complex scanning methods in the field of clinical practice. At present, the realized way of X-rays detector can be roughly divided into two types, direct and indirect. The former one directly absorbs incident X-rays, generates electronic signals through semiconductors or engenders chemical signals through thin films. [14-17] This method can directly convert X-rays into visible light without going through other processes. Therefore, an X-ray detector with a wide linear response range, fast pulse rise time, high-energy resolution, and spatial resolution can be obtained, which makes it copiously applied in X-ray detection. [18-21] However, X-ray detectors based on semiconductors face the challenges of high cost and low efficiency. In addition, although the film is cheap, it is difficult to apply in digital form, which may limit its further development. The latter one refers to the conversion of X-rays (in the highenergy radiation, in addition to X-rays, α, β, and γ rays are also included) into ultraviolet visible (UV) light through scintillators which can be further captured by optical devices. [22-24] It consists of a scintillator and an array photodiode (PD). In contrast, since the scintillator that converts X-rays indirectly is cheap, it is easier to implement in the industry than direct detectors, and has the characteristics of low cost and abundant options, stability, and flexible conversion rate. [25,26] At present, indirect X-ray detectors are widely used in ordinary flat panel X-ray detectors. Moreover, it can be flexibly combined with commercially mature sensor arrays (e.g., amorphous silicon PDs, thin film transistor arrays, photomultiplier tubes, complementary metal oxide semiconductors, silicon avalanche PDs, and X-ray imaging charge-coupled devices [CCD]), therefore, they have attracted more attention. Scintillator is a unique class of luminescent materials, which is extensively used in many fields such as nondestructive testing, medical imaging, space probes, etc. Compared with the disadvantages of traditional scintillator materials which have high cost, are fragile, have long response time and low spatial resolution disadvantages, metal halide perovskites are considered to be the most potential scintillator materials in ...

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Although lead‐based perovskite solar cells (PSCs) are highly efficient, the toxicity of lead (Pb) limits its large‐scale commercialization. As such, there is an urgent need to find alternatives. Many studies have examined tin‐based PSCs. However, pure tin‐based perovskites are easily oxidized in the air or just in glovebox with an ultrasmall amount of oxygen. Such a characteristic makes their performance and stability less ideal compared with those of lead‐based perovskites. Herein, how to address the instability of tin‐based perovskites is introduced in detail. First, the crystalline structure, optical properties, and sources of instability of tin‐based perovskites are summarized. Next, the preparation methods of tin‐based perovskite are discussed. Then, various measures for solving the instability problem are explained using four strategies: additive engineering, deoxidizer, partial substitution, and reduced dimensions. Finally, the challenges and prospects are laid out to help researchers develop highly efficient and stable tin‐based perovskites in the future.

The highest certified power conversion efficiency (PCE) of black phase based CsPbI3 perovskite solar cells has exceeded 18%, and become a hotspot in recent progress. However, the black phase of CsPbI3 rapidly transforms to yellow phase in ambient conditions due to its thermodynamic instability. Here, a Ruddlesden–Popper 2D structure is introduced into γ‐CsPbI3 film to stabilize the black phase via reducing dimensionality. It is found that a judicious amount of phenylethylammonium iodide can adjust the dimensionality of γ‐CsPbI3 film from 2D to quasi‐2D and 3D phase. Comprehensive consideration to obtain both the stability and high PCE, quasi‐2D (n = 40) γ‐CsPbI3 delivers a reproducible PCE of 13.65% with negligible hysteresis. By utilizing femtosecond transient absorption and time‐resolved PL decay, similar carrier kinetics in n = 40 and ∞ samples are observed, meaning an efficient charge extraction. More importantly, when the device is placed at 80 °C in N2 condition or in air with RH of 25–30%, its PCE keeps ≈88% and ≈89% of its initial PCE after 12 days, respectively. Such results are better than the 3D one (≈69% and ≈16%, respectively).

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