Research On X-ray Detector Based On Metal Halide Pressed Wafer

Since most semiconductor materials with good charge transport characteristics cannot effectively absorb high-energy radiation,finding an ideal X-ray photosensitive conductor has always been a topic of interest.In recent years,metal halide perovskite materials have been used in the production of X-ray detection due to their low dark current,high carrier mobility,long carrier life,low defect state density,large atomic number and adjustable band gap.The device has attracted extensive attention from researchers,and has been proved to be a promising candidate semiconductor material for X-ray detection by a large number of experiments.Due to the strong penetrating ability of X-ray photons,a thicker active layer is needed to absorb X-rays,so high requirements are placed on the transport performance of the active layer materials.At present,most of the research groups reported perovskite X-ray detection which are made of single crystal materials.Although single crystals have the advantages of low trap density,high mobility,and large carrier diffusion length,the preparation of large-area lamellar perovskite single crystals is still a big challenge.In addition,due to the large atomic number of elements such as lead,iodine,bromine,etc.,the semiconductor material composed of them can produce strong absorption of X-ray high-energy photons.In recent years,the methylamine lead halide（MAPbX₃,X=Cl,Br,or I）material has exhibited excellent photoelectric performance.The prepared X-ray detector has made significant progress,but the methylamine-based perovskite material is unstable.Easy to decompose is also a problem that needs to be overcome.In order to better solve the above problems,in this paper we have prepared two direct X-ray detectors based on different materials:First,in order to solve the disadvantages of devices made of methylamine halide materials that are unstable and easily decomposed,we first tried to replace methylammonium with formamidinium.We found that the single crystal was grown in the process of growing formamidinium lead bromide（FAPbBr₃）.The growth conditions are strict,and there are problems such as long growth time,small size,and irregular shape.Here we use a tableting process to prepare FAPbBr3 polycrystalline tablets with controllable size and thickness to replace single crystals.X-ray diffraction pattern analysis showed that the crystallinity of the tablet was good,and the surface and cross section of the wafer were observed by scanning electron microscope（SEM）to see that the contact between the grains was good.However,through testing,it was found that the device made of pure FAPbBr3 tablet as a material did not respond to X-rays.To solve this problem,we doped it with different proportions of Csx FA1-x PbBr3（x=0.1、0.2、0.3）.When x=0.2,the device has excellent performance and high X-ray sensitivity.When the bias voltage is 20 V and the X-ray intensity is 505μGy/s,the response current can reach 110 nA,which corresponds well with the volt-ampere characteristic curve.In addition,we also studied the visible light response performance of the above detectors,and the experimental results confirmed that the Cs_x FA_1-x-x PbBr₃ tablet also showed a strong response to visible light.Through X-ray diffraction pattern analysis,it is found that after cesium is added,a part of two-dimensional layered crystal structure of mixed cationic perovskite is generated.This two-dimensional structure of perovskite may be beneficial to improve the performance of the detector.Second,although organic and inorganic lead halide perovskites have achieved excellent results on X-ray detectors,lead element is a heavy metal that seriously harms human health,and the use of lead-based technologies will inevitably be restricted in the future.Therefore,the development of lead-free perovskite is imminent.As an alternative element of lead,bismuth also has a high atomic number,and bismuth-based halide semiconductors can also be used to make X-ray detectors.In this work,we prepared Cs3Bi2I9 microcrystalline tablet for X-ray detector.After testing,under 200V bias voltage and 50 keV X-ray,the device dark current is 1.39 nA,and the sensitivity reaches 14μC Gy^-1 cm^-2.By changing the ratio of cesium iodide and bismuth iodide,we obtained a CsBi₃I₁₀ component perovskite tablet,and tried to prepare an X-ray detector with the CsBi₃I₁₀ tablet as the active layer.Under the bias voltage and X-ray intensity,the device dark current is reduced to 0.197 nA and the sensitivity is increased to 41μC Gy^-1 cm^-2.