Study On Preparation Of All-Inorganic Lead-Free Cs₂AgBiBr₆ Films And Corresponding Solar Cells

In recent years,the organic-inorganic hybrid lead-halide perovskite solar cells（PSCs）have continued to make new breakthroughs and have received growing attention worldwide.To date,the highest power conversion efficiency（PCE）of certified single-junction PSCs has reached 25.5%.However,the stability and lead toxicity are the two major factors limiting their progress toward commercialization.To effectively resolve these two challenges,lead-free Cs₂AgBiBr₆ has recently been found to be a promising alternative material for lead-based perovskites,which has become one of the research highlights in PSCs application due to its steady chemical structure,excellent theoretically photoelectric properties and eco-friendly characteristics.Herein,one-step solution spin-coating method was employed to prepare all-inorganic lead-free Cs₂AgBiBr₆ double perovskite thin films.Using doping and surface passivation techniques to regulate their film quality,and characterized their morphology,structure and photoelectric properties.The solar cells based on lead-free Cs₂AgBiBr₆ double perovskite thin films were obtained employing low-cost and stable carbon as the electrode,and their photoelectric performance and stability were investigated.The main research contents are as follows:（1）A series of Cs₂AgBiBr₆ double perovskite thin films were acquired by changing the doping types and concentrations of alkali metal ions,and then assembled into PSCs.Eventually,the 1%molar content of Li⁺-doped perovskite film become more uniform and compacter,and it possesses larger grain sizes and fewer defects.Compared to the pristine perovskite film,its electron mobility increased to9.5 cm² V^-1 s^-1 and still shows the indirect bandgap with a minimum value of 1.82 e V.The PCE of PSCs prepared by using 1%molar content of Li⁺-doped perovskite film as absorption layer was as high as 2.57%.When the unencapsulated devices were stored for about 90 and 60 days under 5%RH at 25℃and 0%RH at 85℃,respectively,the optimal device could maintain over 80%of its initial efficiency,showing excellent environmental stability.（2）The surface of Cs₂AgBiBr₆ double perovskite thin films and perovskite/Ti O₂interface were modified by introducing polymethyl methacrylate（PMMA）.Compared with the reference perovskite film,the films quality has been remarkably improved,and the pin-holes were filled.Besides,they exhibited reduced leakage current and recombination rate.Simultaneously,the conductivity and electron mobility of the films are also enhanced,which accelerated the migration and extraction of charge carriers.After modification by PMMA,the PCE of assembled device is improved from 1.78%to 2.25%,and it shows an optimal open circuit voltage of 1.18 V.Furthermore,the contact angle between the film and water has greatly increased from 20.1°to 83.7°after PMMA passivation,indicating PMMA-modified film has a good hydrophobic performance,which could effectively inhibit water and oxygen under air permeated into the perovskite layer,and is conducive to enhance the stability of device.The unpackaged devices were stored under 5%RH at 25℃and 0%RH at 85℃air condition for about 80 and 60 days,respectively,the efficiency of the optimal device was almost unchanged,indicating the stability was further improved.