Stability Of Hole Transport Layers In Perovskite Solar Cells

As a representative of emerging photovoltaic cells,perovskite solar cells（PSCs）have developed rapidly in recent years,and its photoelectric conversion efficiency（PCE）has repeatedly reached new highs,which has received extensive attention from academia and industry.However,the low stability of PSCs is a major obstacle to its commercialization.Generally,PSCs consist of hole transport layers（HTLs）,perovskite layers,electron transport layers,and electrodes.HTLs directly contact with perovskite,and its stability significantly affects the stability of PSCs.Therefore,studying the stability of the HTLs is of great significance to promote the commercialization of PSCs.This thesis has conducted a detailed study on the stability of HTLs.The main research content includes additives,dopants and doping strategy.Specific research content is as follows:（1）Aiming at the problems of Li⁺migration,Li-TFSI moisture absorption,and t BP volatilization in the Li-TFSI/t BP based doping system,a Li-TFSI/12-crown-4 based doping system was developed by replacing t BP with a crown ether additive called 12-crown-4 for Spiro-OMe TAD.Studies have shown that in addition to the doping process under aerobic conditions,Spiro-OMe TAD can be doped with Li⁺（12-crown-4）under anaerobic conditions,which further improves the electrical properties of HTLs.The Li-TFSI/12-crown-4 doped PSCs achieve a PCE of 20.19%.In addition,the chelating effect of 12-crown-4 with Li⁺will inhibit the moisture absorption of Li⁺and the migration of Li⁺within the device,the hydrophobic skeleton of 12-crown-4 can also improve the moisture resistance of HTLs.Meanwhile,the solvation effect of 12-crown-4 can significantly improve the solubility of Li-TFSI in chlorobenzene,resulting in more uniform and dense HTLs.PSCs based on Li-TFSI/12-crown-4 still maintain 82%of the initial PCE after aging in air for 30 days,while the PCE of the PSCs based on Li-TFSI/t BP decreases to 37%.（2）Li-TFSI-doped Spiro-OMe TAD usually requires a slow and uncontrollable exposure process to obtain good electrical properties,which affects the stability and reproducibility of the PSCs.Therefore,a pre-doping method based on perfluorooctane is developed for classical hole transport material Spiro-OMe TAD.Perfluorooctane can dissolve O₂ and CO₂ in large quantities,and has strong electron-withdrawing groups.The doping process of Spiro-OMe TAD can be completed in the gas bubble stage of the solution,so the slow and uncontrollable exposure process is avoidable.Studies have shown that CO₂ can oxidize Spiro-OMe TAD more efficiently than O₂.PSCs prepared by the new method achieve the highest PCE of 20.04%,which is higher than PSCs prepared by the traditional method（18.66%）,and has excellent stability and repeatability.In addition,due to the hydrophobic structure of perfluorooctane and the ability to filter out some harmful by-products,HTLs prepared by new method have a smoother morphology and better stability compared with the traditional HTLs.（3）The traditional dopant Li-TFSI brings problems such as moisture absorption and ion migration,and needs to introduce additives to improve its solubility and film-forming properties However,extra additives increase the cost of the device and may introduce new problems.Thus,a novel hydrophobic dopant PFB-TFSI for classical hole transport material Spiro-OMe TAD was developed to replace the traditional dopant Li-TFSI.Different from Li-TFSI,PFB-TFSI can oxidize Spiro-OMe TAD directly because of its appropriate HOMO and LUMO level.Therefore,compared with Li-TFSI doped HTLs,the hole extraction and transmission ability of PFB-TFSI doped HTLs are strengthened.Besides,due to the outstanding hydrophobicity and solubility of PFB-TFSI,HTLs doped with PFB-TFSI have excellent film morphology and stability.Finally,the PFB-TFSI doped PSCs achieve a PCE of 19.16%,which remain 88%of the initial PCE after 30 days,while the Li-TFSI doped PSCs only remain 35%of the initial PCE.