Ellipsometry Study Of Thin Films In CsPbI_xBr(3-x) Perovskite Solar Cell

All inorganic cesium-lead halide perovskite CsPbI_xBr_3-xhas potential application value in solar cells,light-emitting diodes,photodetectors and other photoelectric devices due to its good thermal stability and photoelectrical properties.However,the current research focus on CsPbI_xBr_3-xthin films is mainly focused on the preparation method,the optimization of the preparation parameters and so on,and the optical properties of CsPbI_xBr_3-xthin films are relatively few.The complex optical constants of thin films can be used to simulate the optical losses caused by reflection,absorption of inactive layers,and the external quantum efficiency of devices,thereby optimizing the structural design of optoelectronic devices,maximizing the external coupling efficiency,and improving device performance.In addition,complex optical constants can also be used to understand interband transitions and further analyze carrier dynamics in materials.As a fast and nondestructive spectral analysis technique,spectroscopic ellipsometry can obtain complex optical constants,thickness,component content,porosity and other information of materials.However,the complex phase segregation,interfacial interface with voids and structure of multiple sublayers in CsPbI_xBr_3-xfilms pose challenges to the ellipsometry analysis.Based on ellipsometry and other auxiliary measurement methods,this paper analyzes the porosity,thickness,optical constant,dielectric constant,and other characteristics of the electron transport layer and the light absorption layer of all inorganic cesium lead halide perovskite batteries.The contents are summarized as follows:（1）Ellipsometry analysis of electron transport layer TiO₂thin films based on different dispersion models.Spectroscopic ellipsometry is carried out by setting up appropriate structural model and choosing appropriate dispersion model for fitting analysis.In this paper,five kinds of commonly used dispersion models such as Cauchy Absorbent and Sellmeier Absorbent are compared to analyze the ellipsometry spectra fitting of TiO₂thin films.The fitting results of each model are verified by the reflection spectra of oblique incidence.The results show that the New-Amorphous,Tauc-Lorentz and Adachi-New Forouhi model are suitable for the ellipsometric spectrum fitting of sol-gel TiO₂film on the whole test band.The Cauchy Absorbent and Sellmeier Absorbent models are suitable for narrow bands and have poor fitting effect in short wave.（2）Ellipsometry analysis of the light absorption layer CsPbI₂Br thin film in perovskite solar cells.The CsPbI₂Br thin films were prepared by one-step spin-coating method.The optical structure model of the thin film,including upper boundary layer/perovskite main layer/SiO₂oxide layer/Sisubstrate,was established by multi-angle ellipsometry analysis.The dielectric constant of CsPbI₂Br films is fitted by Tauc-Lorentz five-oscillator dispersion model.The fitting results show that the porosity of the upper boundary layer of the film is 4%,and the fitting result of CsPbI₂Br band gap is 1.89 e V.The second derivative analysis of the dielectric constant of CsPbI₂Br obtained from the ellipsometry spectral fitting analysis shows that there are obvious photoelectric transition peaks at 2.05 e V,2.8 e V and 3.5 e V,and the sharp photoelectric transition curve at 2.05 e V corresponds to the transition of the exciton line.（3）Ellipsometry analysis of complex optical constants and phase segregation of the light-absorbing layer CsPbIBr₂thin film in perovskite solar cells.CsPbIBr₂films were prepared by one-step spin-coating method,and phase segregation was found in the PL characterization results.The complex optical constants and phase segregation of thin films are analyzed by spectroscopic ellipsometry.The fitting results show that the film is divided into three sub-layers,the iodide rich phase content of the upper boundary phase segregation layer is 28.1%,the porosity is 18.6%.In the lower boundary phase segregation layer,the content of iodine rich phase is 38.1%and that of bromine rich phase is 23.4%.The main perovskite layer contains 18.7%iodine rich phase and 4.2%bromine rich phase,respectively.The fitting band gap of CsPbIBr₂film is 2.07 e V.Further analysis of the second derivative of the dielectric constant shows that there are obvious photoelectric transition peaks at 2.15 e V,2.9 e V and 3.8e V,and the sharp photoelectric transition curve at 2.15 e V corresponds to the transition of the exciton line.Finally,the oblique reflection spectra obtained by ellipsometer fitting data are compared with those measured by experiment.It is found that the position of peaks and the trend of curves of the two spectra are roughly consistent.