The Effect of Air Ionization in Testing Perovskite-Based Direct Conversion X-ray Detectors

sı Supporting Information X -ray detectors have a wide range of applications, including medical diagnostics, industrial product inspection, and scientific investigations.Direct X-ray detectors using a photoconductive material offer highresolution imaging with high sensitivity.1 Conventional photoconductive materials suffer from limitations such as low attenuation coefficient (a-Se) which results in low sensitivities and high production costs (CdTe).In recent years, various alternative materials, including HgI 2 , 2 PbI 2 , 3 and metal halide perovskites, [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] have been explored to address these challenges.Perovskites are intensively investigated due to their high attenuation coefficient, excellent charge carrier transport properties, and low production cost thanks to solution processability.[24][25][26][27][28] Going forward, the field is focusing on instability and reliability of perovskite materials and corresponding devices.[29][30][31] With new materials being developed and tested for sensitive X-ray detection, it is important to select the most promising materials with standardized and quantitative measuring techniques.For instance, when measuring the sensitivity of a device (one of the most important figures of merit of an X-ray detector), under atmospheric conditions, the current produced by the ionized air must be properly considered.Air is in fact used in X-ray detectors in ion chambers.32 We noticed that while some of the perovskite X-ray detectors 4,5 offer response currents on the order of microamperes, many others 6-22 report nanoamperes or even lower response currents, where the contribution of air ionization current may become nonnegligible (Figure 1).This raises concerns as air ionization can interfere with sensitivity estimation, necessitating the dissemination of appropriate evaluation methods for X-ray detectors.In this Viewpoint, we show examples of sensitivity overestimation by disregarding air ionization in device structures typically used in the field.In addition, we summarize instances of potential overestimation in the literature and propose possible proper evaluation methods. Evaluation of Air Ionization Current.To investigate how much current is generated from air ionization and how to eliminate it, we prepared a parallel plate device consisting of two Cu electrodes (Figure 2a) and tested the device inside a homemade vacuum chamber (Figure S1a).The electrodes were 5 mm apart and no other material was present between them except for air.Note that the irradiation field cross section

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