Defect Passivation Of All-inorganic Perovskite Thin Films For High-Performance Solar Cells

Organic-inorganic perovskite solar cells?PSCs?are featured with their high power conversion efficiency?PCE?and low-cost solution processing.However,organic cations in the organic-inorganic lead halide perovskite materials are sensitive to light,heat and moisture,which lead to poor long-term stability of organic-inorganic lead halide PSCs and hinder their commercialization process.Therefore,all-inorganic cesium lead halide?CsPbX₃,X=halide?perovskite materials without organic cations have received increasing attention.However,the CsPbX₃ films prepared through solution process are polycrystalline,which can form numerous defects at surfaces and grain boundaries.Nonradiative recombination of charge carriers occurred in these defects,will affect performance of the resulting devices.Thus,how to obtain CsPbX₃thin films with reduced defects is one of key problems to be solved in this field.Therefore,a series of CsPbX₃ films with reduced defects were prepared by defect passivation in this thesis.The influence of passivation treatment on the structures and properties of films were discussed in depth.All-inorganic CsPbX₃ PSCs with high efficiency and good stability were fabricated based on the optimized thin films.The research contents of this thesis can be divided into the following two parts:?1?Polyethylene glycol?PEG?was used as polymer additive to prepare high-quality CsPbIBr₂ perovskite thin film for solar cells.The CsPbIBr₂ perovskite thin film prepared by the one-step solution method has many voids in the grain boundaries and low coverage on the substrate.We added a proper amount of PEG into the CsPbIBr₂precursor solution so as to improve the quality of thin film.PEG can slow down crystal growth and restrain aggregation of perovskite crystals during the process of perovskite phase formation.Then,the defect states at grain boundaries and surface of CsPbIBr₂thin film were passivated effectively and thus reduced probability of nonradiative recombination,thereby leading to a more uniform film with fewer voids.Consequently,PSCs based on CsPbIBr₂ with PEG-passivation exhibited enhanced photovoltaic performance with an enlarged open-circuit voltage(V_OC)up to 1.28 V and an enhanced PCE of 7.31%in comparison with the pristine CsPbIBr₂ PSCs with a lower V_OC of 1.10V and an inferior PCE of 6.36%.Furthermore,the PEG-passivated PSC showed better environmental stability.?2?Zinc cations(Zn²⁺)were used as a dopant to prepare high-quality CsPbI₂Br perovskite thin film for solar cells.The effect of different Zn²⁺dopant content on the morphology,structure,optical and electrical properties as well as energy level of CsPbI₂Br film were investigated.It is found that Zn²⁺-doped CsPbI₂Br film showed more uniform morphology,fewer trap states,improved charge transportation and enhanced light-harvesting ability.Meanwhile,Zn²⁺-doped CsPbI₂Br film exhibited tunable energy levels.Time-resolved photoluminescence results showed that the Zn²⁺doping leads to perovskite film with extended photoluminescence lifetime which means a longer diffusion length and subsequently enhanced photocurrent and V_OC.In particular,the best-performance PSC based on CsPbI₂Br with the optimal Zn²⁺doping showed a higher PCE of 12.16%with a larger V_OC of 1.236 V,an improved J_SC of 15.61 mA cm^-2 in comparison with the control device based on pure CsPbI₂Br which exhibited a PCE of 10.21%with a V_OC of 1.123 V,a J_SC of 13.27 mA cm^-2.At the same time,improved hysteresis and storage stability were also obtained for the PSC based on CsPbI₂Br with the optimal Zn²⁺doping compared with the control dopant-free device.