All Inorganic Perovskite Light Emitting Diodes Based On In-situ Dynamic Thermal Crystallization

Metal halide perovskite materials have been expected due to their special optical and electrical characteristic.Perovskite materials can be divided into organic-inorganic hybrid perovskite materials and all inorganic perovskite materials.Due to the volatilization of organic components in organic-inorganic hybrid perovskite materials,their light stability and heat stability are relatively poor compared with all inorganic perovskite materials.However,in order to develop all inorganic perovskite materials,it is necessary to solve the problem of solubility of some precursors in organic solvents.In this paper,the in-situ assisted crystallization temperature of CsPbBr₃ was studied.It was found that the level of defect density was related to the in-situ assisted crystallization temperature and precursor solvent.Therefore,we developed an in-situ thermally assisted crystallization process based on in-situ preparation technology,which improved the crystallinity of the film and reduced the defect density.The research work is divided into the following three parts:1.In order to solve the problem of solubility of cesium halide in organic solvents,we deposited CsBr(in vacuum thermal evaporation)on the surface of PbBr₂ films(prepared by spin coating method).CsPbBr₃ films were prepared by solid-solid diffusion method(two-step method).On this basis,the surface morphology of PbBr₂ was controlled by in-situ thermally assisted crystallization process,which increased the specific surface area of PbBr₂ film and promoted the solid-solid diffusion rate.The maximum current efficiency of 7.1 cd/A was obtained under the driving voltage of 5 V,which was 200% higher than that of the device using the traditional crystallization process.The results show that the in-situ thermal assisted crystallization process can effectively improve the quality of CsPbBr₃ inorganic perovskite film.2.In the process of preparing CsPbBr3 films by solid-solid diffusion method,we found that due to the limitation of PbBr₂ solution and equipment,it is difficult to make PbBr₂ films thinner.However,the luminescent properties of perovskite light-emitting devices are sensitive to the film thickness,which limits the performance of CsPbBr₃ light-emitting devices based on in-situ thermally assisted crystallization.In the study of PbBr₂ films,we found that in-situ thermally assisted crystallization process will effectively reduce the surface defects of CsPbBr₃ films,so we tried to apply in-situ thermally assisted crystallization process in one-step spin coating method.By analyzing the morphology and characteristics of CsPbBr₃ films at different temperatures,we found that the CsPbBr₃ films prepared at appropriate auxiliary crystallization temperature have better crystallinity and lower defect density.And it will avoid the generation of CsPbBr₃ derivatives,which can effectively improve the photoelectric performance of light-emitting devices.On this basis,the performance of CsPbBr₃ light-emitting devices was further improved.3.Although in-situ thermally assisted crystallization process can play an important role in the film-forming process of perovskite materials,it cannot achieve accurate control the thickness of CsPbBr₃ films.Therefore,we developed in-situ thermally assisted crystallization process based on vacuum thermal evaporation method and constructed the growth model of CsPbBr₃ films.This process not only assist the crystallization of CsPbBr₃,but also save the post annealing process of perovskite film,which is helpful to improve the stability of the devices.The maximum brightness of CsPbBr₃ light-emitting devices based on this technology can reach 17350 cd/m² at 6.5 V.At the same time,the maximum current efficiency of 10.3 cd/A can be obtained.In addition,CsPbBr₃ light-emitting devices based on this method show excellent stability,and its half-life is more than 400 minutes.After 24 days of storage in air,the luminescence stability has no obvious change.Through the above researches,we draw a preliminary conclusion: in solution method,the in-situ thermally assisted crystallization process is limited by solvent volatility,although it can reduce the defect density,but its electroluminescence performances are still not good enough.In the vapor deposition method,the in-situ assisted crystallization process can overcome the problem of solvent volatilization and realize the in-situ crystallization of high-quality CsPbBr3 films,resulting in uniform grain size and low density of defect.Therefore,vacuum thermal evaporation technology based on in-situ thermally assisted crystallization can play a positive role in improving the performance of all inorganic perovskite light-emitting devices represented by CsPbBr₃.