Preparation And Properties Of CsPb(I_1-xBr_x)₃（0≤x≤1） Perovskite Films

In recent years,all-inorganic Cs Pb(I_1-xBr_x)₃（0≤x≤1）perovskite materials have become a research hotspot in the photovoltaic industry due to their excellent optical,electrical properties and high thermal stability.Among them,Cs Pb I₃ has cubic（α）,tetragonal（β）and orthogonal（γ）perovskite phase structures,with phase transition temperatures of 281^oC and 184^oC,respectively;the bandgap is～1.7e V,which makes it suitable to be used as the top cell to construct the tandem cells,so as to further improve the performance of the device.Unfortunately,the perovskite phase of the Cs Pb I₃undergoes severe structural degradation and spontaneously transform into an orthogonal phase（δ）non-perovskite structure at room temperature.Partial or total substitution of I^-with Br^-,shuch as Cs Pb I₂Br,Cs Pb IBr₂,Cs Pb Br₃,etc,can effectively improve the stability of perovskite phase.However,it significantly increases the bandgap and results in inferior photovoltaic performance as compared with the Cs Pb I₃.Thus,it is a challenge to realize the compatibility of stability and device efficiency.In this thesis,on one side the fundamental working mechanisms of Cs Pb I₃-Cs Pb(I_1-xBr_x)₃single-junction gradient-bandgap perovskite solar cells（PSCs）and Cs Pb(I_1-xBr_x)₃/c-Si tandem solar cells（TSCs）were investigated.The influence of absorption layer parameters and structure design on the performance of the device was studied systematically,which provides theoretical guidance for the experiment.On the other hand,the effects of PVP additives on the stability and photoelectric properties of Cs Pb I₃and Cs Pb I₂Br films were investigated,and the mechanism of PVP additive was analyzed.In addition,the effects of Cs Br solution and Pb Br₂ vapor treatments on the performance of Cs Pb(I_1-xBr_x)₃ film were preliminarily studied.The main conclusions are as follows:1.Theoretical investigation of Cs Pb I₃-Cs Pb(I_1-xBr_x)₃ single-junction gradient bandgap PSCs and Cs Pb(I_1-xBr_x)₃/c-Si TSCs:1)The introduction of Br^-doped perovskite thin film on the back of the Cs Pb I₃film,it can not only protect the latter,but also contributes to the formation of abrupt high-low heterojunctions,leading to the reduction of the recombination of carriers at back interface.The simulation analysis found that:（1）The performance of gradient bandgap PSCs is better than the single bandgap device;（2）Under the same thickness for the high bandgap layer,performance of device slightly decreases with the increase of the Br content,but the protective effect increases;（3）Whith the increase of the thickness for the high bandgap layer,the performance of the device decreases slightly.When all factors are considered,the best back layer is Cs Pb Br₃.2)Cs Pb(I_1-xBr_x)₃ thin film is suitable to be used as the top sub-cell for TSCs due to its high bandgap;and c-Si cell is currently the most mature and widely used photovoltaic technology.Therefore,a Cs Pb(I_1-xBr_x)₃/c-Si TSCs was constructed and the device performance was determined by varying the bandgap and thickness of the top battery without altering the structure of the c-Si bottom layer.The simulation analysis found that:（1）When Cs Pb I₃ is used as the top layer,either a high-efficiency two-terminal（2-T）battery or a high-efficiency four-terminal（4-T）battery can be constructed;（2）Cs Pb I₂Br,Cs Pb IBr₂,Cs Pb Br₃ are used as the top battery,4-T device performance is better than 2-T.2.The influence of PVP additives on the stability of Cs Pb(I_1-xBr_x)₃film:1)The addition of PVP additives improves the stability of the Cs Pb I₃ film significantly,as follows:（1）Increasing the nucleation density,shrinking the grain size to the nanometer range,increasing the surface energy,and improving the phase’s stability;（2）PVP molecules are wrapped on the surface of the crystal grains to form a protective layer,which prevents the penetration of water molecules and inhibits the phase change.At the same time,the surface defects of the crystal grains are passivated,the luminous intensity of the material increases,and the carrier lifetime increases.However,due to the relatively high amount of PVP added,it is not conducive to the derivation of carriers.2)Due to the high stability of Cs Pb I₂Br,it is possible to reduce the amount of PVP added to the precursor.However,when a large amount of PVP is added,a large number of voids and cracks in the film surface are generated.Finally,using Ni Ox as the hole transport layer and adding 5 mg of PVP resulted in the formation of a solar cell with an optimum PCE of 10.47%.3.The effect of surface and vapor treatment on the stability of Cs Pb(I_1-xBr_x)₃ film:1)Spin-coating the Cs Br solution on the surface of the Cs Pb I₃ precursor film can cause the I^-part of the back surface layer to be replaced by Br^-,increasing the film’s stability.This effect increases with the number of treatments.2)The Cs Pb I₃ film is treated with Pb Br₂ vapor to allow I^-in the bulk layer to be replaced by Br^-,resulting in the appearance of a two-component film;with increasing temperature and time,the film will degrade to a single-component state.