Defects Passivation In Perovskite Solar Cells Via Alkali Metal Fluorides

Recently,organic-inorganic halide perovskite materials have attracted great attention due to a series of outstanding photovoltaic properties.Until now,the highest power conversion efficiency（PCE）of perovskite solar cell（PSC）has reached a certified 25.5%,which is comparable to that of crystalline silicon counterparts.Despite the quick development of PSC,there still exist some disadvantages that hinder its further improvement.Among them,point defects,grain boundaries（GBs）and interface recombination have been recognized as the most pronounced.In order to passivate these intrinsic defects,we introduce alkali metal fluoride additives in the perovskite precursor,SnO₂/perovskite interface and SnO₂ precursor,respectively.Finally,high-efficiency and high-stability devices are obtained via these three methods.The specific contents are as follows:（1）Herein,we introduce three different alkali metal fluoride additives in perovskite precursor to modulate the defect behaviors.It is found that the addition of different alkali metal fluorides all demonstrate positive effect on the film quality and device performance,KF in particular.By optimizing the concentration of additives,the perovskite film with 1%KF exhibits greatly enhanced crystallinity and improved morphology.The addition of KF significantly passivates grain boundaries and vacancy defects in bulk perovskite films,leading to increased carrier lifetime and reduced defect state density.Consequently,the PSC device with addition of 1%KF has realized significantly enhanced power conversion efficiency（PCE）of 20.11%with negligible hysteresis.（2）We demonstrate an efficient KF surface modification to simultaneously passivate the SnO₂/Perovskite interface defects and defects within the bulk perovskite films.Owing to the elimination of hydroxyl group defects on the SnO₂ surface via the substitution of terminal hydroxyl group by fluorine,a more hydrophilic SnO₂ surface is obtained,leading to the formation of high quality perovskite films with a better morphology and enhanced crystallinity.In addition,the substitution of the bridge oxygen by fluorine,which is predicted to be n-type doping,leading to the lowered W_Fand preferred energy band alignment.Meanwhile,the K⁺will diffuse into the perovskite film and passivate the grain boundaries and intrinsic I^-vacancy defects,leading to significant reduced trap state density.After KF surface treatment,the non-radiative recombination at the ETL/perovskite interface has been greatly reduced,contributing to more efficient charge extraction.Consequently,the PSC based on SnO₂/KF ETL yields a champion PCE as high as 20.33%with negligible hysteresis.Meanwhile,the device stability is significantly improved.（3）We use commercial aqueous colloidal dispersion to fabricate a SnO₂ layer.The SnO₂-KF composite electron transport layer（ETL）is formed via adding certain amount of KF powder in the SnO₂ precursor.SnO₂-KF composite ETL exhibits more efficient performance in carrier extraction.Therefore,the device based on SnO₂-KF ETL demonstrates superior properties towards control device,contributing to an enhanced short circuit current(J_sc)and a champion PCE as high as 19.73%.