Study On Perovskite Photodetectors Based On Mixed Cation Perovskite/Tin Oxide Heterostructure

In recent years,organic-inorganic hybrid halide perovskite materials（OIHPs）have shown outstanding performance in laser applications,photodetectors（PPDs）,solar cells（PSCs）,and light-emitting diodes（PLEDs）.OIHPs have many excellent properties,such as high photoelectric conversion efficiency,wide absorption spectrum,low cost,and easy preparation.Perovskite photodetectors are a new type of optoelectronic devices developed in recent years.They have a wide range of applications in biochemical detection,environmental monitoring,optical fiber communication,image sensing,and other fields.With the development of various solution synthesis processes and thin film preparation technologies,the control technology of the structure and morphology of halide perovskite materials is becoming more practiced,and high-performance halide perovskite materials emerge in endlessly.The research of high-performance photodetectors based on perovskite materials has attracted more and more attention.Utilize a heterojunction structure is one of the common and effective methods to improve the performance of photodetector.This work uses the excellent photoelectric properties of halide perovskite materials.Tin oxide（SnO₂）was combined with perovskite materials to study the effect of heterostructure on the performance improvement of photodetectors.A kind of photoelectric detector based on halide perovskite materials is demonstrated,and enhanced performance of photodetector based on halide perovskite materials is demonstrated.This paper consists of three parts,including the preparation and characterization of SnO₂ thin films,the preparation and characterization of organic-inorganic halide perovskite thin films,and the preparation and characteristics of SnO₂/MA_0.7FA_0.3Pb I₃high-performance perovskite photodetectors.The main contents of this study are as follows:1.Preparation and characterization of SnO₂ thin films:the characterization of SnO₂films deposited on glass substrates by magnetron sputtering and spin coating were studied respectively.The films were characterized by UV-Vis Spectrum,X-ray diffraction,and SEM.The experimental data show that the smooth,uniform and reproducible SnO₂ thin films can be formed by magnetron sputtering.Two kinds of SnO₂ thin films with different thicknesses were prepared by magnetron sputtering.The results show that when the sputtering time is 20 minutes,the light absorption of the film increases with the decrease of the wavelength.The absorption band edge of the films sputtered for 20 minutes changed from 340 nm to 375 nm,and the absorption intensity was higher than that of the films sputtered for 10 minutes.This shows that increasing the thickness of the film can increase the absorption of the film and improve the photoelectric performance.The SEM surface morphology of SnO₂ thin films prepared by magnetron sputtering shows that the surface of SnO₂ films is smooth and uniform,which is consistent with the amorphous phase observed by XRD.For SnO₂ prepared by spin coating,the speed of spin coating is inversely proportional to the increase of film thickness.The films with different thicknesses were prepared at 3000,4000 and 5000rpm.The absorption test shows that the absorption edge of the film spun at 3000 rpm slightly deviates.Compared with the films rotating at 4000 and 5000 rpm,the film has higher absorption intensity,which also indicates that the increase of film thickness leads to the increase of absorption.In addition,the X-ray diffraction patterns show that the SnO₂ thin films prepared by magnetron sputtering are similar to those prepared by the spin coating method,with only SnO₂（110）and（101）broad peaks,and no other narrow diffraction peaks.This indicates that the two SnO₂ films are amorphous in nature.Therefore,it is necessary to anneal at 450 ^OC or higher to obtain SnO₂ crystals with significant X-ray diffraction peaks.2.Preparation and characterization of organic-inorganic halide perovskite films:MA_0.7FA_0.3Pb I₃ and MAPb I₃ perovskite films were successfully prepared by a one-step spin coating method.The optical properties,surface morphology,and crystallinity of MA_0.7FA_0.3Pb I₃ and MAPb I₃ perovskite films were compared.The characterization of the optical absorption spectrum shows that,compared with MAPb I₃ film,the mixed cation perovskite film has higher absorption intensity from UV to the visible region,and has an absorption band edge at about 798 nm.MAPb I₃ thin films exhibit similar broadband absorption,and the absorption edge is about 780 nm.In addition,the absorption peaks of the two perovskite films are the same as those of the perovskite films with improved photovoltaic performance reported before,which verifies the successful preparation of perovskite materials.The results of SEM show that the MA_0.7FA_0.3Pb I₃ thin films show more compact and uniform morphology,and the grain size is larger(in the experiment,the large grain size MA_0.7FA_0.3Pb I₃ thin films are prepared by adding Pb（SCN）₂ components).UV-Vis absorption test also showed that Pb（SCN）₂ did not change the bandgap of the perovskite absorption layer.As a result,the grain boundaries with less horizontal orientation can block charge carriers.Compared with the perovskite film with smaller grain size,the larger grain size of MA_0.7FA_0.3Pb I₃ perovskite film is expected to reduce the carrier recombination rate,which will enhance the performance of perovskite photodetectors.In addition,MA_0.7FA_0.3Pb I₃ film has good compactness and covers the entire substrate,which is conducive to carrier transport and improves the performance of photodetectors.During the preparation of MAPb I₃ film,SEM results show that the grains fall off partially and cannot cover the entire substrate surface.Some pinholes can be found in the film,which reduce the performance of the detector.The X-ray diffraction patterns of MAPb I₃perovskite thin films show obvious diffraction peaks at 14.04°,28.24°,and 31.96°,respectively,which correspond to（110）,（220）and（310）crystal planes,respectively.No obvious diffraction peak of Pb I₂ was detected,indicating that Pb I₂ was completely transformed into MAPb I₃.However,in the XRD images of MA_0.7FA_0.3Pb I₃ thin films,in addition to the above diffraction peaks,a characteristic diffraction peaks of Pb I₂ were observed at 12.4°due to the increase of Pb I₂ content in the composition.3.Preparation and characterization of SnO₂/MA_0.7FA_0.3Pb I₃ mixed cation perovskite photodetectors and SnO₂/MAPb I₃ perovskite photodetectors:two kinds of perovskite photodetectors were successfully prepared,and their properties were characterized.Both photodetectors have a wide absorption range of 400 to 800 nm.For the photodetector with SnO₂ layer prepared by the sputtering method,the photodetector with SnO₂ layer sputtered for 20 min has a high responsivity of 0.63 A/W,a high detectivity measurement of 1.22×10¹² Jones,and fast response time（rise time is 5.5ms,and the fall time is 6.1 ms）.The enhanced performance is attributed to the improvement of crystallinity and morphology of MA_0.7FA_0.3Pb I₃ film on the SnO₂ layer and the existence of heterojunction.The electron transfer from MA_0.7FA_0.3Pb I₃ to SnO₂enhances the exciton separation efficiency in the photosensitive layer.For the photodetectors with SnO₂ layer prepared by the spin coating method,through optimizing the spin-coating speed of SnO₂ layer,high absorption,and high-performance photodetectors can be obtained by 3000 rpm process.However,compared with the photodetectors with SnO₂ layer prepared by magnetron sputtering,the performance of the devices prepared by the solution method is poor,which is mainly due to the high carrier mobility and compactness of SnO₂/MA_0.7FA_0.3Pb I₃ film prepared by the sputtering method,which forms high-quality heterojunction structure.In this thesis,the mechanism of the heterojunction structure,improving the performance of photodetectors is analyzed.Firstly,electrons are transferred from the perovskite layer to the SnO₂,which leads to the accumulation of electrons in the SnO₂ layer,and the built-in electric field is formed at the interface between SnO₂ and perovskite.The built-in electric field will promote the separation of electron-hole pairs generated by the photoelectric effect,enhance the carrier concentration of photodetectors,and then improve the responsivity performance of the photodetectors.The results show that the SnO₂/MA_0.7FA_0.3Pb I₃heterojunction structure is expected to be used to construct high-performance perovskite photodetectors.