| Recently,the fossil energy crisis and environmental pollution increasingly threaten the security and stability of international community.In order to alleviate energy and environmental problems,most countries are vigorously developing clean and renewable energy.Solar energy is the most abundant renewable energy on earth,and its full utilization can meet the needs of human daily life and industrial production.Hence,solar cell is expected as one of the important technologies to solve energy and environmental problems.Nevertheless,the high cost of commercial silicon-based solar cells limits its further development.Metal halide perovskites solar cells(HPSCs)with superior performance(The photoelectric conversion efficiency is 25.7%,which is comparable to silicon-based solar cells),simple process and low cost have been developed rapidly,which is expected as the promising candidate of traditional silicon-based solar cells.However,organicinorganic hybrid Pb-based halide perovskites with superior performance could be easily degraded by thermal excitation,inducing lead pollution and harming human health and environmental safety.Replacing organic component and Pb by stable Cs cation(Cs+)and nontoxic Sn cation(Sn2+)could address the issues of thermal stability and toxicity,respectively.Accordingly,the obtained CsSnI3 perovskite,which has achieved the best performance(10.1%)among all-inorganic lead-free HPSCs,is the most promising material for fabricating nontoxic,stable and highperformance HPSCs.In the CsSnI3 perovskites family,orthorhombic phase(γ)CsSnI3 exhibits superior photoelectric properties and nontoxicity,indicating attractive development prospect.Nevertheless,γ-CsSnI3 could be easily transformed into nonperovskite phase(yellow phase,Y)CsSnI3 with poor photoelectric properties.Moreover,water would degrade y-CsSnI3 in the nature of ionic crystal and Sn2+is oxidized to Sn4+by oxygen molecules,which causes a mass of surface point defects,rapid degradation of γ-CsSnI3 surface structure and attenuation of device performance.The issue of energy level alignment between band-edge structure of γ-CsSnI3 and charge transportation layer limits photo-generated carriers transportation in the HPSCs and leads to severe loss-in potential of photovoltaic device,which is one of the key factors for the poor performance.Therefore,the poor phase stability,surface stability and performance of photovoltaic devices caused by band-edge structure are the critical bottleneck limiting the commercialization of γ-CsSnI3 HPSCs.Researchers have developed kinds of methods to solve these problems.However.the surface stability could not be well improved by element doping and dimensional engineering improves the stability at the expense of photoelectric properties and performance of HPSCs.Surface passivation exhibits the giant potential for optimizing the stability and photoelectric properties of γ-CsSnI3 simultaneously.To data,the under-lying mechanism for regulating stability and band-edge structure through surface passivation are not revealed,and the mechanism for screening passivators is unclear.Trial and error method is inefficient and high-cost for the development of stable and efficient y-CsSnI3,as well as design and screening of passivators.Therefore,it is urgent to investigate the mechanism of stability and band-edge structure regulated by surface passivation,which guide the optimization of photoelectric properties and research and development of stable and highperformance of γ-CsSnI3 HPSCs.Hence,this paper adopts first-principles calculations,ab-initio molecular dynamics simulation and transition-states search.Based on the design of passivators configurations(chain length and halogenation)and stacking mode(molecular orientation and surface coverage),the under-lying mechanism at the atomic level on regulating phase and surface stability of γ-CsSnI3 through surface passivation is investigated.After systematic investigation,theoretical guidance could be provided for the research and development of stable and high-performance γ-CsSnI3,as well as design and screening of surface passivators.The detailed research is as follows:(1)Phase stability regulation.This paper focuses on the easily phase transition of γ-CsSnI3.The physical mechanism on regulation of γ-CsSnI3 phase stability by surface passivation is investigated.The spontaneous distortion of[SnI6]octahedra caused by the too small ionic radius of Cs+is the fundamental reason of phase transition.Based on the regulation of molecular orientation and halogenated configurations of phenylethylammonium,surface passivation significantly improves the phase stability of γ-CsSnI3.The constructed inter-molecular hydrogen bonds by designing the stacking modes of aniline passivators could control steric hindrance among passivators.Therefore,the under-lying mechanism on regulating y-CsSnI3 phase stability by surface passivation is revealed.The steric hindrance among passivators could suppress the distortion of passivators.The distortion of[SnI6]octahedra is effectively suppressed owing to the formed H-I bonds between passivators and y-CsSnI3.Therefore,surface passivation could effectively improve phase stability of γ-CsSnI3.(2)Surface stability regulation.This paper concentrates on the problem that the surface structure of process γ-CsSnI3 is easily degraded by water and oxygen molecules.The under-lying mechanism on enhancing surface stability of γ-CsSnI3 through surface passivation is studied.The fundamental reason for surface degradation by water and oxygen molecules is the hydrophilia and oxytropism ofγ-CsSnI3.After surface passivation by aminoethyl thiophene with long chain length,the diffuse energy barrier of water molecules is introduced and the adsorption of oxygen molecules is impeded.The precondition for the degradation of surface structure is eliminated.Halogenated phenylethylammonium and pseudo-halide with high surface coverage could effectively suppress the surface defects activity of y-CsSnI3,hindering the degradation of surface structures.Therefore,surface passivation could improve the surface stability by impressing the diffuse of water and oxygen molecules and surface point defect activity.(3)Band-edge structure regulation.This paper aims at the issue of band-edge structure of y-CsSnI3.The physical mechanism on regulating band-edge structure and photoelectric properties of y-CsSnI3 by surface passivation is investigated.phenylethylammonium and its halogenated derivatives could linearly tailor work function of γ-CsSnI3 and controllably adjust the conduction band-edge structure of passivators and γ-CsSnI3.The anti-bonding orbital hybridization between pseudohalide and Sn ions regulates valance band-edge structure.Therefore,the interfacial energy level alignment between γ-CsSnI3 and charge transportation layer is optimized.Moreover,the regulation of valance band-edge structure significantly enhances the band dispersion of passivators and photoelectric properties,such as absorption coefficients and photo-generated carriers transportation.(4)Design and screening of passivators.This paper aims at the problem of inefficient design and screening of passivators.The direction for designing and screening passivators is guided at the atomic level.Through improving the steric hindrance and dipole moments of passivators by increasing chain length and surface coverage,the stability of γ-CsSnI3 is enhanced and the conduction band-edge structure is regulated.The anti-bonding orbital hybridization could regulate the valance band-edge structure and optimize photoelectric properties.Accordingly,the simulated photoelectric conversion efficiency of γ-CsSnI3 HPSCs could be improved from 13.3%to 20.84%.In conclusion,the designed and screened passivators should have high steric hindrance and the ability of anti-bonding orbital hybridization,and the surface coverage should be high enough to improve the stability and performance of y-CsSnI3 HPSCs. |
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