Low Noise X-ray Flat Panel Detector Based On Perovskite Single Crystal

In today's society,X-ray has important applications in medical,industrial flaw detection,aerospace and other fields,and detection and resolution of X-ray is an important part of the application of X-ray,so the development of high-performance,low-noise X-ray detector is the most important application of X-ray.Generally speaking,X-ray detection methods can be divided into indirect detection and direct detection.The indirect X-ray detection method based on scintillation crystal is to convert the X-ray into visible light,and then convert the visible light signal into electrical signal through CMOS or CCD image sensor.Due to the scattering of visible light generated in scintillation crystals and the intermediate step of converting X-ray photons into visible photons,indirect X-ray detectors have the disadvantages of low spatial resolution and low quantum efficiency.Direct x-ray detectors based on semiconductors convert high-energy photon energy directly into photogenerated carrier signals through semiconductor active materials,which may obtain high spatial resolution and quantum efficiency.Silicon,germanium,selenium and other semiconductor materials are usually used for X-ray direct detection.However,due to the low atomic number of these elements and their low absorption coefficient for X-ray,high-energy x-ray photon radiation can easily damage the lattice of these materials.Therefore,most of the direct x-ray imaging devices composed of these materials can only be used for low-energy X-ray imaging.For X-ray with high photon energy,Cd Zn Te Single Crystal is usually used to detect.However,these inorganic active crystals have high cost and low yield,and it is difficult to integrate with array readout circuit to form high-resolution imaging plate.Therefore,it is of great academic value and application prospect to study new active materials for X-ray detection and to prepare low-noise detectors..Organic inorganic lead halide perovskite(MAPb X₃,MA=CH₃NH₃,X=Cl,Br,I)has become a global research hotspot in recent years due to its advantages of simple preparation and high photoelectric conversion efficiency,and it is also considered as a new semiconductor material suitable for X-ray detection.However,there are some problems in perovskite based X-ray detectors,such as large dark state current,large detector noise,and difficult to combine with silicon-based circuits.There is still a large gap between the practical application of X-ray detectors.The research results are as follows:1.In this dissertation,MAPb Cl_0.5Br_2.5 single crystal with low defect density and high carrier mobility was grown by adjusting the ratio of halogen group elements in perovskite single crystal.It is found that the defect density of perovskite crystal can be changed by adjusting the ratio of halogen group elements.Seven kinds of perovskite single crystals with different ratios of halogen elements were grown by solution method.The defect density and carrier mobility of these perovskite crystals were calculated by space charge limited current method and fly over time method.Through comparative analysis,it is found that when the ratio of bromine to chlorine is 5:1,the carrier mobility of the crystal reaches the highest,which is 170.5cm²v^-1s^-1(electron),181.2cm²v^-1s^-1(hole).The defect density of the crystal is the lowest,which is 1.93×10⁹cm^-3.The work was published in Materials Letters(2019,Vol.236,26-29).2.In this dissertation,we use the method of liquid phase epitaxy to epitaxial the lattice matched functional layer on the crystal surface to improve the interface problem,and to prepare the X-ray detector with low noise and high detection efficiency.In this dissertation,lattice matched MAPb Br₃ and MAPb Cl₃ layers were epitaxed on the top and bottom surfaces of MAPb Cl_0.5Br_2.5 by liquid phase epitaxy(LPE).The X-ray detector with the structure of Au/MAPb Br₃/MAPb Cl_0.5Br_2.5/MAPb Cl₃/Ag was obtained.The average noise of dark current and photocurrent is 3.24n A/Hz^0.5 and 0.25n A/Hz^0.5 under bias voltage of-200V,which are far lower than those of X-ray detector with spin coating organic functional layer at the same bias voltage.At the same time,this kind of low noise X-ray detector is also used to realize the mechanical scanning X-ray imaging and take the X-ray imaging photos.The work was published in CHINESE PHYSICS LETTERS(2020,Vol.37(0181011))3.A new technique of bonding perovskite crystal to TFT readout circuit substrate is proposed,which not only does not need complex pattern alignment,but also ensures the spatial resolution of X-ray imaging devices.A kind of crystal adhesive with the composition of mapbbr3 precursor+terpineol+ethyl cellulose(terpineol 35%,terpineol/ethyl cellulose=3:1)was prepared.After processing,the adhesive not only has viscosity,but also contains abundant mapbbr3 perovskite polycrystalline particles.Using this adhesive to connect the perovskite crystal with the readout circuit substrate can ensure a good carrier transport channel between the perovskite crystal and the pixel electrode,and make the adjacent detection pixels have a large impedance.This technique can be used in the fabrication of high resolution perovskite X-ray imaging devices.4.In order to fabricate large area X-ray flat panel imaging devices,the same perovskite crystals with similar size were seamlessly spliced by homogeneous liquid phase epitaxy.Homoepitaxy refers to epitaxial layer which is the same as the substrate.The homogeneous epitaxial layer not only has the same material composition with the substrate,but also has the same crystal plane orientation as the epitaxial substrate.Using this characteristic,we can connect two crystal planes with the same orientation through the homogeneous epitaxial layer.In the liquid phase growth of crystal,the crystal will preferentially precipitate on the surface of the existing seed,which makes the size of the seed increase continuously.Therefore,LPE can not only realize the seamless joining of the same perovskite crystals,but also effectively increase the size of the bonding products in this dissertation,two MAPb Cl_0.5Br_2.5 single crystals with the size of 0.5cm×0.5cm×0.1cm were grown by homo epitaxy for 36 hours.Finally,a cuboid MAPb Cl_0.5Br_2.5 single crystal with the size of 2.5cm×1.5cm×0.5cm was obtained.