Record Stability for Fully Passive Perovskite-Based X-Ray Detectors Using Starch as Polymeric Template

Lead-Halide hybrid Perovskites are recently emerging as promising materials for high energy radiation detection thanks to the combination of excellent transport properties, even in polycrystalline films, and their solution processability. Since 2015 hybrid lead-halide perovskites have attracted increasing attention as an interesting alternative to traditional semiconductors used for radiation detection. The presence of heavy elements inside the atomic structure ensures a high X-ray absorption coefficient, together with the high crystal density, make them ideal for direct detection of ionizing radiation. High performance detectors were demonstrated in both polycrystalline and single crystal forms [1], outperforming traditional Si- or CZT-based sensors. Despite top performance of perovskite single crystals, they do not fully exploit the solution processability on flexible substrates, the deposition over large area and the low-voltage operation, typical of thin-film devices. Flexible thin-film perovskite X-ray detectors have been recently reported [2,3]; however, low operational and environmental stability is limiting their further development toward commercialization.<br/>Here we report a strategy based on using starch as a polymeric template for the fabrication of stable thin film perovskite direct X-ray detectors, which confers to the film a series of characteristics that cannot be achieved with pristine perovskites, in particular higher stability in ambient conditions, improved homogeneity of the film also at greater thicknesses, enhanced mechanical flexibility and robustness. The approach is highly reliable and versatile, in fact, we demonstrate it in this work for different perovskite compositions, namely MAPbI₃ and FA_xMA_1-xPbI₃. Moreover, the starch polymer allows excellent tuning of the active layer thickness, since acting as a rheological modifier enhances the viscosity of perovskite precursors’ ink.<br/>The proposed p-i-n photodiodes can operate with no external bias applied (fully passive devices), thus strongly improving the operation stability by decreasing the ion migration. We monitored the current response at 40kVp of X-ray radiation for detectors with different active layers compositions and thicknesses. We found that a 20 wt% starch concentration in the perovskite precursor solution, corresponding to 1050 nm thick perovskite layer provides the best trade-off between X-ray absorption, charge collection efficiency and performance stability. We measured an X-ray sensitivity of 5.5±0.2 μC Gy^-1 cm^-2 (at 0 V), with no degradation on the electric characteristic after 9.4 Gy irradiation.<br/>The device degradation was monitored for samples stored in air for a time window of 630 days. During this period, the samples were subjected to temperatures and relative humidity as high as 30°C and 70% respectively during summer times. After 1.7 years the films retained their dark color and nice dynamics curves under X-rays were measured, demonstrating an exceptional stability: 97% of the initial sensitivity was retained for the best perovskite-starch composite formulation making it the most stable unencapsulated perovskite X-ray detector reported so far.<br/><br/>References<br/>[1] Y. Liu et al, <i>Adv. Mater.</i> <b>2021</b>, <i>33</i>, 2006010.<br/>[2] S. Demchyshyn et al., <i>Adv. Sci.</i> <b>2020</b>, <i>7</i>, 2002586.<br/>[3] A. Ciavatti et al., <i>Adv. Funct. Mater.</i> <b>2021</b>, <i>31</i>, 2009072