Improvement Of The Performance Of CsPbBr₃ Perovskite Solar Cells By The Ferroelectric Material PbTiO₃

In the past decade,the photoelectric conversion efficiency of organic-inorganic hybrid perovskite as a solar cell with light absorption layer has reached 25.7%.However,the organic components in perovskite are prone to volatilization at high temperatures,susceptible to oxidation by oxygen,and also influenced by ultraviolet radiation,which have hindered the process of commercial development.All inorganic CsPbBr₃perovskite solar cells（PSCs）,which are prepared by cesium ion（Cs⁺）replacing organic ion（MA⁺）,have attracted attention because of easy preparation and good stability.Nevertheless,crystal defects of CsPbBr₃perovskite materials and non-radiative recombination of carriers lead to low power conversion efficiency（PCE）of the cells.Furthermore,the relatively small built electric field cannot provide enough intensity to completely separate the photoexcitons and make the charge escape from the deep-level defect state.Therefore,enhancing the intensity of built-in electric field of the cell to promote the separation and transmission of electrons and holes is an effective way to achieve high efficiency of CsPbBr₃ PSCs.In this paper,the study mainly aims to enhance the built-in electric field of the cells,inhibit the non-radiative recombination between the carriers,and improve the carrier separation efficiency by inducing an ultra-thin ferroelectric oxide PbTiO₃layer between the electron transport material Ti O₂ and CsPbBr₃,which could generate additional electric field after polarization to increase the open circuit voltage(V_oc)and short circuit current(J_sc)of the cells.Subsequently,the Fermi level and charge transport characteristics of Ti O₂/PbTiO₃ layer had been investigated by doping PbTiO₃ thin film with strontium（Sr）,which could increase the conductivity and melting point of PbTiO₃,and also improve the durability and stability of the PbTiO₃ film,as well as the crystal phase transition temperature and structure,and finally achieve the purpose of improving the photoelectric performance of CsPbBr₃ PSCsThe specific research contents are as follows:（1）Improvement of the performance of CsPbBr₃ PSCsby ferroelectric PbTiO₃ layer:ferroelectric PbTiO₃ thin films were grown by in-situ reaction of lead acetate solution（Pb C₄H₆O₄）with the Ti source of titanium dioxide（Ti O₂）dense films at 450°C.Then using multi-step spin coating method for the preparation of the FTO/Ti O₂/PbTiO₃/CsPbBr₃/C structure of the inorganic CsPbBr₃ PSCs,the photoelectric performance tests show that the transmission properties of carrier have been improved in the ferroelectric PbTiO₃ thin layer device.and of the cell are better.The open circuit voltage(V_oc)had been increased from 1.42 V of Ti O₂devices to 1.48 V of Ti O₂/0.02M-PT devices,the short circuit current(J_sc)had been increased from 7.01m A/cm² to 7.50 m A/cm²,and PCE had been increased from 7.70%to 9.05%（The conversion efficiency of the cell had been improved about 17 percent）.UPS and Mott-Schottky were used to analyze the energy band structure and internal electric field of the cell.It was found that the energy band structure of CsPbBr₃ PSCscould be changed under the influence of polarization electric field due to the incorporation of ferroelectric PbTiO₃ layer,which enlarged the Fermi energy level difference between the absorptive layer and the electron layer of CsPbBr₃ and enhanced the internal electric field of the cell.The result indicated that the non-radiative recombination between carriers have been inhibited and the photoelectric performance of the cell device have been improved.（2）Effect of Sr-doped PbTiO₃thin film on CsPbBr₃ PSCscells:Based on the research of PbTiO₃ thin film improving the performance of CsPbBr₃ PSCs,0.02M PT device with the best performance was selected,and strontium acetate(（C₂H₃O₂）₂Sr with different contents was added to its precursor solution to react with the surface of Ti O₂ layer at high temperature to form Pb-Sr Ti O₃ thin film.The successful doping of Sr into PbTiO₃ thin films was confirmed by XPS and EDS.The UPS test of Ti O₂/Pb-Sr（5%）Ti O₃ showed that a small amount of Sr again reduced the work function of Ti O₂/PbTiO₃ layer and expanded the Fermi energy level difference between the light-absorbing layer and the electron layer,which could further improve the V_oc of CsPbBr₃ PSCsdevices.The J-V test on the cell device shows that when the content of Sr is 5%of Pb,the V_oc and PCE of the cell increase to 1.50 V and 9.25%on the basis of 0.02M PT device.However,when the concentration of Sr continues to increase,the quality and ferroelectricity of PbTiO₃ film are affected.As a result,the J_scand PCE of the cell are decreased,and the carrier separation and transmission capability shown by PL and TSPV are also consistent with the law of J-V.