Optimization Of Cs₂AgBiBr₆ For X-ray Detection And Imaging

X-ray detector has a wide range of applications in the fields of medical imaging,industrial flaw detection and safety inspection.In recent years,metal halide perovskite materials have received extensive attention at home and abroad due to their strong X-ray absorption capacity,high sensitivity,and low-cost preparation.In this paper,Cs₂AgBiBr₆ is selected for research,which contains Bi element,small ionization energy and high carrier mobility and lifetime product(??).In previous study,Cs₂AgBiBr₆ X-ray detector achieved high X-ray sensitivity and low detection limit,which shows great application prospects in X-ray detection and imaging.A Cs₂AgBiBr₆ X-ray flat panel detector needs to solve the problems of signal compatibility with the readout integrated circuit and large area preparation,which includes(1)the instability of baseline and response caused by ion migration;(2)the large dark current;(3)the fabrication of thick-film with large area.In this dissertation,the relationship between the X-ray detection performance and electronic properties of Cs₂AgBiBr₆ is discussed by numerical simulation.Quantitative characterization of ion migration is established and the mechanism of ion migration in the polycrystalline wafer is investigated.Then the ion migration in Cs₂AgBiBr₆ polycrystalline wafer is largely eliminated by introducing heteroepitaxial passivation layers.Finally,an outstanding X-ray detector is fabricated and area array imaging is realized.The details are as follows:(1)The radiation detection performance of Cs₂AgBiBr₆ is studied through numerical simulation and a Cascade Linear System,including theoretical sensitivity,modulation transfer function(MTF)and detective quantum efficiency(DQE).We found that??and dark current are the key parameters that affect the performance of X-ray detectors and imaging.(2)Ion current cause by ion migration is an important part of dark current.The“transient reverse current method”is established to quantitatively characterize the ion migration.The activation energy of ion migration is obtained by eliminating the influence of ion built-in electric field and electronic conductivity.(3)Cs₂AgBiBr₆ wafer is fabricated by using an isostatic-pressing method and a thick film with area of 19.6 cm² is obtained.The activation energy of its ion migration is characterized as 203 me V.Then,we discussed the ion migration mechanism with theoretical calculation,and we found that grain boundary passivation and halogen vacancy passivation are the keys to restrain ion migration.(4)The grain boundary and halogen vacancy are perfectly passivated by introducing bismuth oxybromide(BiOBr)as heteroepitaxial passivation layers in Cs₂AgBiBr₆ wafer.The ion migration is largely eliminated and the average resistivity of the device increases to1.4×10¹⁰?·cm.The??product of Cs₂AgBiBr₆ device reaches 5.51×10^-3 cm²·V^-1.In addition,passivation of the thick film surface by polyimide(PI)further stabilizes the device current baseline and effectively eliminates the 1/f noise of the device.(5)The X-ray detection performance and area array imaging of optimized Cs₂AgBiBr₆wafer are further characterized.The detector achieves outstanding performance with a high sensitivity of 250?C·Gy_air^-1·cm^-2 and a low detection limit of 95.5 n Gy_air·s^-1,which is comparable to the Cs₂AgBiBr₆ single crystal.The large area array imaging device has a sensitivity variation of 1.2%,which shows an excellent uniformity.The spatial resolution reaches 4.9 lp·mm^-1 at MTF=0.2 and a clear X-ray image can be obtained.