Ellipsometry Spectroscopy Study Of Cesium Lead Bromine Spin-coated Film

Cs Pb Br₃is a kind of all-inorganic perovskite material with good resistance to moisture and heat.It is expected to be applied to photovoltaic devices such as solar cells,photodetectors and light-emitting diodes due to its good photoelectric performance and phase stability.At present,most researches focus on the interface modification and element doping of Cs Pb Br₃grains and the improvement of preparation methods,and relatively few studies on the optical constants of Cs Pb Br₃films,and the optical constants of Cs Pb Br₃films are usually related to the film preparation technology.The analysis of its optical constants is helpful to improve the preparation technology of thin films and optimize the structure of optoelectronic devices.Ellipsometer spectroscopy is one of the important means to measure the optical constants of thin films.It uses the change of polarization state of incident light after reflection to characterize the optical properties of the sample.It has the advantages of fast,sensitive and non-destructive.In this paper,the optical properties of Cs Pb Br₃thin film prepared by multi-step spin coating method are studied based on the measurement and analysis of ellipsoid spectrum,combined with other auxiliary testing methods.Furthermore,Cs Pb Br₃thin film is used as the optical absorption layer for the design and simulation of solar cells.Cs Pb₂Br₅phase impurities are easy to appear in Cs Pb Br₃thin films prepared by spinning coating method,and it has been found that the existence of a small amount of Cs Pb₂Br₅phase has a positive effect on the performance improvement of photoelectric devices.Therefore,Cs Pb₂Br₅thin films are further prepared by multi-step spinning coating method.The optical properties and heterogeneous components were analyzed by ellipsometry spectroscopy.The main research contents are summarized as follows:（1）The difference characteristics of micromorphology and structure of Cs Pb Br₃films prepared by two step immersion method and multiple step spin coating method are studied.It is found that the multiple step spin coating method is easier to obtain a smooth film.Cs Pb Br₃perovskite thin films were prepared on monocrystalline silicon and FTO substrates by multi-step spin coating method.Ellipsometry spectrometer was used to measure Cs Pb Br₃perovskite thin films prepared on silicon wafers at multiple angles（60°,65°and 70°）.The complex dielectric dispersion,band gap,surface roughness and thickness of Cs Pb Br₃were obtained by using the dispersion model of Tanguy and Tauc-Lorentz3.The band gap value was 2.36 e V,which was consistent with the 526 nm band fluorescence peak in the fluorescence spectrum.A Glass/Sn O₂:F（350 nm）/Ti O₂（20 nm）/Cs Pb Br₃/Spiro-OMe TAD（200 nm）/Au optical model of perovskite solar cells was constructed,and the conversion efficiency was 14.36%.（2）Cs Pb₂Br₅thin films were prepared on monocrystal silicon substrate by multi-step spin-coating method.The Cs/Pb/Br atomic ratio of Cs Pb₂Br₅thin films was≈1/2.3/7.3,and the contents of Pb and Br elements were slightly higher,indicating that there were other products in the prepared Cs Pb₂Br₅thin films besides the main component Cs Pb₂Br₅.According to the fluorescence emission spectrum,the prepared Cs Pb₂Br₅film only has a very weak luminescence peak at 526 nm,and the position of the peak is the same as that of Cs Pb₂Br₅material.Combined with the results of EDS spectra,it is inferred that the film contains a small amount of Cs Pb Br₃phase.According to the absorption spectrum,the band gap of Cs Pb₂Br₅film is calculated to be 3.45 e V.The elliptical polarization spectrometer is used to measure the Cs Pb₂Br₅film prepared on silicon wafers from multiple angles,and a three-layer optical structure model of Cs Pb₂Br₅film is established.The bottom layer is Si O₂natural oxide layer,and the Cs Pb₂Br₅layer is divided into main layer and rough layer.The main layer contains 97.2%Cs Pb₂Br₅phase and 2.8%lead-rich phase,while the rough layer contains 82%Cs Pb₂Br₅phase,16.2%cesium-rich phase and 1.8%pores.The optical constants of Cs Pb₂Br₅films were fitted with Tauc-Lorentz3 dispersion model,and the band gap was about 3.00 e V.The difference between the band gap values calculated by absorption spectrum and ellipsoid spectrum further indicated the existence of cesium-rich phase and lead-rich phase in the prepared Cs Pb₂Br₅films.