Semiconductor Polycrystalline Flakes Prepared By Hot Pressing And Their X-ray Detection Properties

Organic-inorganic hybrid perovskite materials（ABX3,A=MA（CH3NH3+）,FA（CH（NH2）2+）,PEA（C6H5C2H4NH3+）,Cs;B=Pb,Bi;X=Br,I,Cl）not only have the advantages of larger average atomic number,longer carrier lifetime and higher carrier mobility,but also have excellent absorption coefficient to high-energy radiations and good carrier collection efficiency,which makes them ideal materials for radiation detectors.However,under electric field,the halide ions in the perovskite materials are easy to migrate,which will form a reverse electric field in the device to reduce the internal electric filed and the response current.Further,ion migration also easily destroys the ordered crystal structure,causing structural distortion and current baseline drifting.In addition,Pb is a toxicial element and harmful to the environment and organisms.To pratical solve the problems in this kind of device,we have investigated the influences of ion migration on the working mechanism of semiconductor devices,inhibiting ion migration to improve stability,and the application of lead-free semiconductor materials in X-ray detectors.The main achievements we have obtained are as follows:1.The effects of the properties of charge transport layers and ion migration on the X-ray response characteristics of the MAPbI3 wafer based X-ray detecdtors are studied.The specific results have been found:（1）Under reverse bias,the MAPbI3 X-ray detector with p-i-n structure has a larger photocurrent gain factor under X-ray irradiation,up to 411;（2）The properties of p-i-n structured the MAPbl3 X-ray detector are similar to that of photoconductivity detector under reverse bias voltage,in other words,the photocurrent gain increases with the decrease of the light intensity,and the gain coefficient can be reached 11.12 under weak light;（3）The effects of ion migration on the built-in electric field and the energy band structure of the device is studied by numerical simulation approach.It is found that ion migration will cause a larger band bending at the interface btewen perovskite layer and the charge transport layer,which can induce the tunneling current and make the heterojunction lose the junction effect with smaller contact resistance.Thus,p-i-n structured device shows the characteristics of photoconductive devices under reverse bias.（4）It has also been found that the transport layres shows neglibile effects on the performance of perovskite-based X-ray detector,indicating that ion migration has a determinted effect on the charge transport at the interfaces.2.Polycrystalline PEA2PbI4 perovskite wafer was prepared by hot pressing method.The effects of low-dimensional perovskite materials on inhibiting ion migration and improving the stability of X-ray detectors were studied.It has been found:（1）When the molar ratio of PEAI to PbI2 is 2:1,a pure phase perovskite powder can be obtained by ball milling method.This method can be used to prepare powder materials on a large scale;（2）Simultaneous heating and pressing method can promote grain growth to obtain high quality perovskite wafers with obvious orientation.The wafers prepared by simultaneously hot-pressed method and pressed and then heated methods are made into a horizontal structured detector.The product of carrier mobility and carrier lifetime of the two wafers is 6.23 ×10-4 cm2 V-1 and 4.83 ×10-4 cm2 V-1,respectively.The results show that the wafers prepared by simultaneously heating and pressing method have higher carrier collection efficiency.（3）The performance of the wafer X-ray detectors based on the two methods was tested.Under 40 V bias,the sensitivity of the device prepared by simultaneously heating and pressing method was higher to be～1531.6 μC Gy-1air cm-2 and the detection limit was lower to be 351.4 nGy s-1;（4）Under the dark state and long-term X-ray irradiation,the device has a stable dark current and response current.3.The X-ray response characteristics and device stability of Bi-based compounds were studied.It was found that BiVO4 material has excellent environmental stability and excellent X-ray response characteristics.It has been found:（1）BiVO4 polycrystalline wafers were obtained by pressing and sintering method at a high temperature of 800℃;（2）BiVO4 polycrystalline wafers,prepared by sintering in oxygen,air,and nitrogen atmosphere,show similar X-ray response performances.The sensitivities of the devices increased with the increase of the bias voltage.The sensitivity of the device can reach 268.4 μC Gy-1 air cm-2 under 40 V bias,The obatained results indicate that BiVO4 wafers could be prepared in a variety of environments;（3）The X-ray detector assembled with BiVO4 polycrystalline wafer has a lower detection limit of 122.4 nGy s-1;（4）Under 40 V bias voltage and 670.23 μGy s-1 dose rate,the device has a stable dark current and response current.