| X-ray detectors are widely used in medical,military,material analysis,security and industrial testing fields.However,X-rays,because of their high energy,can directly damage human's gene and cause cancer.For example,in the field of medical treatment,to get a clearer imaging and early diagnosis of diseases,people had to increase radiation doses.Therefore,it is of great significance to study the X-ray detector with low detection limit,which is conducive to reducing radiation damage and achieving clearer imaging.Organic-inorganic hybrid halide perovskites,represented by methylammonium lead bromide(CH3NH3PbBr3),have made remarkable progress in the field of X-ray detection due to their advantages such as strong X-ray absorption,excellent photoelectric properties and low-cost preparation.However,the long-term stability of hybrid halide perovskite is insufficient due to the presence of volatile organic components.The all-inorganic halide perovskite,represented by cesium lead bromine(CsPbBr3)and cesium silver bismuth bromide(Cs2AgBiBr6)and so on,has the advantages of better X-ray attenuation capability and better thermal stability,showing a good application prospect.The related research is in the initial stage before the beginning of this topic:the relationship between composition,structure and performance of X-ray detector is not clear,the understanding of what determines the detection limit is insufficient,and how to achieve a low detection limit is a huge chanllenge.Focusing on the above problems,this paper aims at reducing the minimum detection limit of all-inorganic perovskite X-ray detector and understanding the physical threshold of the detection limit.I mainly focus on material selection,preparation process optimization and device structure design to suppress device noise and improve the carrier collection efficiency.The main contents are as follows:(1)Solvent volatilization in the thick film prepared by solution method leads to amounts of holes in the thick film.Defects at grain boundaries would increase the signal noise and deteriorate the detection limit of the device.In this chapter,the four-step hot pressing method was adopted to avoid the use of solvent to prepare high quality CsPbBr3 thick film without holes,which can effectively reduce material defects and improve the efficiency of carrier collection.The CsPbBr3 thick film X-ray detector with Schottky structure was prepared by high work function metal Au to suppress the dark current,At the same time,the shallow vacancy defect VBr produced by bromine volatilization was used to improve the photoconductance gain of the device.The sensitivity reached 55 684?C·Gy-1air·cm-2(the recorded sensitivity at that time),and the minimum detection limit is 215 n Gyair·s-1(2)In order to further reduce noise and lower the minimum detection limit of the device,high-quality CsPbBr3 single crystals were grown by Bridgman method.We then employ COMSOL to get a modeling and simulation approach of the single crystal grown process and clarify the influence of temperature field setting and pulling speed on the quality of crystal.Then a high crystallinity,low defect concentration(9.8×109 cm-3),high??product(5.62×10-2 cm2·V-1),and weak p-type CsPbBr3 single crystal was obtained.The X-ray detector based on CsPbBr3 single crystal showed an average noise current of?3.54×10-13A·Hz-1/2,effectively suppresses the white noise and 1/f noise.Thanks to the low noise,the minimum detection limit of the device was further reduced to 136.4 n Gyair·s-1.(3)Compared with CsPbBr3,Cs2AgBiBr6 has higher resistivity,which is conducive to preparing low noise device,at the same time,Cs2AgBiBr6has greater density,smaller bandgap and electron hole ionization energy,these properties make it has better X-ray attenuation and ionization ability.Also,as an indirect bandgap semiconductor,Cs2AgBiBr6has long carrier lifetime.Considering all these features,X-ray detector based on Cs2AgBiBr6 single crystal has opportunity to further reduce the minimum detection limit.Firstly,nucleation temperature and metastable zone are employed to direct the growth of Cs2AgBiBr6 single crystals.Then,promoting the sequence of Ag+/Bi3+and reduce Ag Biand Bi Ag defects in the crystal by annealing at 423 K for 1h.Next,the surface was treated with isopropyl alcohol to remove the surface states.The optimized Cs2AgBiBr6 crystals exhibited low defect concentration(2.3×109 cm-3),high mobility(16 cm2·V-1·s-1)and high resistivity(1.60×1011?·cm).The noise current of Cs2AgBiBr6 single crystal X-ray detector was as low as?9.55×10-16 A·Hz-1/2,and the 1/f noise was almost completely suppressed.Finally,the ultralow detection limit of 59.7 n Gyair·s-1 was achieved,and the working stability was good at the same time. |
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