Study On Defect Passivation Of Imidazole Ionic Liquids On Perovskite Solar Cells

Organic-inorganic hybrid metal halogen perovskite solar cells（PSCs）have experienced unprecedented rapid development in the past decade.Due to their excellent photoelectric characteristics,low price and flexible preparation,their efficiency has reached 25.5%.However,because of the characteristics of solution preparation and the complexity of precursor composition,there are a lot of defects on the surface and crystal boundary of polycrystalline perovskite thin films,which are not conducive to device performance.In recent years,the excellent and unique physical and chemical properties of ionic liquids have attracted extensive attention in the field of PSCs.In this paper,perovskite layer regulated and modified by imidazole ionic liquids to passivate defects is studied.Ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate（[EMIM]PF₆）was added to perovskite layer,and the best process of regulating perovskite layer by it was explored.When[EMIM]PF₆ is added 0.3 mol%and the annealing temperature of perovskite layer is 150℃,the prepared battery exhibits a better photoelectric conversion efficiency.[EMIM]PF₆ can promote crystal growth,make the film smooth,and enhance the absorbance.Hydrophobic[EMIM]PF₆ can improve the hydrophobicity of perovskite film.The analysis of the intrinsic mechanism of[EMIM]PF₆passivation defects shows that[EMIM]PF₆ produces electrostatic passivation on perovskite,passivates Pb²⁺defects with insufficient coordination and lead clusters.As an ionic liquid compound,[EMIM]PF₆ offers positive and negative charges to perovskite,which improves the electron mobility and passivates various positive and negative defects through charge neutralization.The highest photoelectric conversion efficiency of PSCs prepared in air and doped with[EMIM]PF₆ can reach17.79%,while the efficiency of the control device is only 16.67%,PCE is still higher than that of the original device and close to 83%of the initial value after 60 days in hot and humid atmosphere.1-ethyl-3-methylimidazolium iodide（[EMIM]I）was used to modify the interface of perovskite layer to passivate defects.The best process for preparing the modified layer is as follows:annealing temperature,annealing time and concentration of[EMIM]I are 80℃,10 minutes and 10 mg m L^-1,respectively.It is found that[EMIM]I does not change the crystal structure of perovskite thin film after modification,consumes the redundant and unfavorable PbI₂ on the thin film,and makes the thin film smoother and flatter.Exploring the internal mechanism of[EMIM]I action shows that I⁺in[EMIM]I can transfer to perovskite layer to fill iodine vacancy,which greatly reduces iodide vacancy defects.[EMIM]I also reduces the frequency of non-radiative recombination by providing positive and negative charges to passivate defects in perovskite film.Furthermore,lower work function and more matching energy level with the hole transport layer lead to smaller interface barrier.The extraction and transmission of carriers are effectively promoted.The photoelectric conversion efficiency of the device can reach 18.15%with open circuit voltage V_OCincreasing from 1.06 V to 1.13 V.After 55 days in atmospheric environment,the efficiency is still close to 89%of the initial value,which demonstrates a stable device performance.