Controllable Preparation And X-Ray Detection Performance Of Metal Halide Perovskite Wafers

X-ray photon detection plays an important role in many fields,from crystal structure analysis to medical radiology to radio astronomy.X-ray detection can be divided into indirect detection and direct detection according to the conversion of X-rays into optical or electrical signals.Compared with the former,the latter goes through fewer imaging steps and has higher resolution.To date,the greatest demand for X-ray detectors is from the medical imaging field,where highly sensitive pixel array direct X-ray detectors based on solid-state semiconductors（such as α-Se,Si,CdTe,etc.）are gradually replacing radiological film.In practical applications,there is often a lack of reliable means of focusing X-rays,which requires that semiconductor materials with suitable thickness can be integrated with the TFT backplane over large areas and the preparation process must be compatible with the TFT backplane,for example,at temperature below 200℃.In recent years,metal halide perovskite,as an emerging semiconductor material,has the advantages of long carrier lifetime,easy low temperature preparation by solution method,high defect tolerance and high carrier mobility lifetime product,which makes it an excellent candidate material in the field of optoelectronics.Because it contains elements with large atomic number such as Pb and I,it has strong attenuation ability to X-ray.Perovskite,with a thickness of several hundred microns to several millimeters,can absorb X-ray efficiently.Therefore,perovskite materials show great application potential in the field of direct X-ray detection.Perovskite single crystals exhibit excellent X-ray response due to low defect density and high carrier mobility lifetime product,and the X-ray detection sensitivity is five orders of magnitude higher than that of commercial amorphous selenium detectors.However,due to the lack of anisotropy of perovskite single crystal growth,it is difficult to prepare large-area wafers with integrated substrate and controllable thickness,which seriously limits the application research of perovskite single crystal X-ray detector in medical imaging.Compared with single crystals,perovskite polycrystals can be prepared in large areas through various methods such as spin coating and scraper coating.Multinational companies such as Samsung and Siemens have realized high resolution imaging based on the thick film of perovskite polycrystals.However,the defect density of perovskite polycrystalline is high,the X-ray response is weak,and the X-ray dose required for medical imaging is still high.Therefore,growing or integrating perovskite wafers with controllable thickness,large size and high X-ray response on the substrate is very important for promoting medical imaging research of perovskite X-ray detectors and reducing radiation dose.Therefore,from the above two aspects,this paper explores the preparation and integration of single crystal wafers and the growth optimization of large-area microcrystal wafers,which is of great significance for the preparation of X-ray detectors with high sensitivity,low detection limit and large-area integration.The specific research contents are as follows:（1）In this chapter,we first grow high-quality MAPbI₃ bulk single crystals by combining continuous mass transfer and reverse temperature crystallization.Subsequently,the single crystal of the block was cut into 1mm thick wafer by cutting,which was post-processed by grinding and polishing for the subsequent integrated use of devices.The integration of the single crystal wafer with the TFT backplane is a key step in the application of perovskite X-ray detectors in medical imaging research.Taking indium tin oxide（ITO）as an example,we attempt to develop effective methods for the integration of perovskite chips with substrates.MAPbI₃ single crystal wafers were integrated onto ITO substrate by "pasting" using the property of MAPbI₃ dissolution and recrystallization induced by MA gas.Specifically,one side of the wafer is first exposed to the atmosphere of MA gas.After the surface perovskite is completely liquefied,it is quickly transferred to ITO substrate modified with hole transport layer.After MA gas is completely volatilized,the polycrystalline layer generated at the interface can closely connect the wafer to the substrate.The thickness of the polycrystalline layer is only 16 microns,and the grain size is relatively large,ranging from several microns to dozens of microns.Through SCLC,the carrier mobility and defect state density of the chip are very close to that of single crystal.In addition,we also optimized the hole transport layer to further reduce the interfacial defect density and improve the wafer adhesion.Finally,the X-ray detection performance of the device was tested,and the sensitivity was up to 1.38×10⁴μC Gyair-1 cm^-2 under an 8V/mm electric field.In addition,the device showed a relatively uniform optical response.（2）The microcrystalline thick film has the advantages of excellent X-ray response of single crystal and large area preparation of polycrystalline,but the surface is rough and porous,making the device preparation difficult.Based on this,in Chapter 4,the highly oriented twodimensional polycrystalline film is used as the growth template,and the inverse temperature crystallization strategy is combined to induce the in situ and large area growth of oriented threedimensional perovskite microcrystal wafers.Oriented microcrystal wafers exhibit improved compactness,relatively smooth surfaces,uniform responses,and maximum grain sizes of more than 100 microns.We further improve the surface smoothness by applying post-sanding treatment.Finally,a self-powered X-ray detector with high sensitivity is constructed based on oriented microcrystal wafers.The sensitivity is up to 4×10⁴μC Gyair^-1 cm^-2 at under 0 bias,and the detection limit is only 40.6 nGyair/s.The device sensitivity and detection limit are comparable to that of single crystal detectors.