Preparation And Performance Study Of TiO₂-Based CsPbI_3-xBr_x Perovskite Solar Cells

Perovskite Solar Cells（PSCs）have experienced rapid development,currently achieving a power conversion efficiency（PCE）of 25.8%.Despite their high PCE,organic-inorganic hybrid PSCs have poor thermal stability and are rapidly dissolved at high temperatures,limiting their applications.The usage of Cs atoms instead of organic molecules can improve their stability,making them the most promising material and a hotspot for PSC research.Electronic transport layers（ETLs）,a core component of PSCs,play an important role in cell performance.Because of its great thermal stability,low cost,and appropriate energy level,titanium dioxide（TiO₂）is the most favored material for ETLs among PSCs.However,it has some significant drawbacks,including low electron mobility,high electron trap density,and strong catalytic activity.When UV irradiation occurs,charge recombination,instability,and other phenomena occur,leading to poor PCE,severe hysteresis.This paper successfully improved the stability and photoelectric performance of PSCs by using electron beam evaporation to prepare TiO₂ thin films.In this work,the effects of varying TiO₂ ETLs thicknesses and annealing temperatures,as well as surface shapes of bilayer TiO₂,on photovoltaic performance were examined,providing a crucial benchmark for further optimization of PSCs.Ultimately,highly efficient TiO₂-based planar PSCs were successfully obtained,and the mechanisms underlying improvements in the ETL were explored.The main research contents and conclusions were as follows:（1）In this study,the use of electron beam evaporation for preparing TiO₂ ETL was found to produce a more compact and homogeneous film compared to hydrothermal evaporation.The surface rms roughness was approximately one-third that of hydrothermal evaporation,and the average transmittance was measured to be 94.67%.The PCE of TiO₂-based PSCs devices prepared using electron beam evaporation was enhanced by 60%compared to the hydrothermal method.The photoelectric conversion efficiency of the PSCs was found to be significantly influenced by the thickness of the TiO₂ ETL.In this paper,the thickness of TiO₂ ETL（60 nm,70 nm,75 nm,80 nm）was adjusted by the electron beam evaporation method,and at the thickness of 75 nm,the TiO₂ ETL had the densest and homogeneous surface,and the film resistivity was also lower,and the electron extraction ability was stronger,and the PSCs prepared with it obtained the optimal device performance,and its PCE reached 9.00%.（2）To improve the crystalline quality of TiO₂ ETL,the effects of thermal annealing treatment of TiO₂ ETL at different temperatures（non-annealed,200℃,300℃,400℃,450℃,480℃,500℃）on the properties of PSCs were investigated.Results indicated that TiO₂ ETL surface morphology was optimized at 480℃,the least surface defects,the fastest speed of extracting electrons from perovskite films and significantly improving filling factor and conversion efficiency of device.Compared with non-annealed devices,TiO₂ ETL PSCs prepared by surface heat treatment technology based on 480℃ annealing have 3.5 times of filling factor（FF）and 10 times of device efficiency improvement,and the device exhibits the best photovoltaic performance with a PCE of 11.16%,open-circuit voltage（Voc）of 0.84 V,short-circuit current density（Jsc）of 18.33 mA·cm-2,and FF of 73%in the reverse scan.（3）A dual ETLs was designed to improve the hysteresis effect and stability of planar PSC devices.TiO₂ thin films were hydrothermally grown on TiO₂ thin films that had been prepared by electron beam evaporation.To improve electron transport efficiency,high-quality ETLs were prepared by combining the advantages of electron beam evaporation and hydrothermal methods.Results indicated that the surface hydrophilicity of the bilayer TiO₂ ETL was better than that of the monolayer TiO₂ ETL,which was attributed to the higher roughness of the surface of the TiO₂ films prepared by hydrothermal method,which was favorable for the growth of the chalcogenide films.HT-TiO₂（Hydrothermal-TiO₂）bilayer ETLs were grown on 30 nm EB-TiO₂（Electron beam evaporation-TiO₂）films with a relatively uniform surface,and the average film transmittance reached 86.03%,with the perovskite film formation quality being optimal.The steady-state PL test showed that the 30 nm EB-TiO₂/HT-TiO₂ bilayer ETLs were easier to extract electrons and reduced interfacial charge recombination,and the EIS results showed that the PSCs had a smaller recombination rate,and thus the device exhibited the best photovoltaic performance with PCE of 10.14%,Voc of 0.84 V,Jsc of 17.31 mA·cm-2,and FF of 69.88%in the reverse scan.In addition,devices based on 30 nm EB-TiO₂/HT-TiO₂ bilayer ETLs with minimal J-V hysteresis demonstrated the best stability,retaining 47.6%of the initial efficiency after 5 days.These research results provide multiple effective solutions for optimizing the TiO₂ ETL,thus improving the photovoltaic performance of TiO₂-based PSCs.These outcomes not only offer theoretical guidance for the utilization of TiO₂ materials in planar structured PSCs but also present the potential to improve the FF of PSCs.