Theoretical Study On Doping And Surface Modification To Tune Structures And Properties Of Tin-based And Inorganic Two-dimensional Perovskites

Perovskite solar cells（PSCs）have become one of the most popular photovoltaic research hotspots in the past decade due to their excellent performance such as easy synthesis and low manufacturing cost.And their certified photoelectric conversion efficiency value has exceeded 24%,which is comparable to the widely used silicon-based solar cells,showing strong competitive potential.Although lead-based perovskite solar cells have higher photoelectric conversion efficiency,the biological toxicity of heavy metal lead may bring serious environmental hazards and the poor stability of equipment for long-term operation limits their further large-scale commercial application.Tin-based perovskite formed by replacing lead with tin of the same family as lead is currently the most promising lead-free perovskite,but it suffers from the easily oxidation of tin.Although the two-dimensional perovskite has better stability than the bulk phase,due to its own higher exciton binding energy,the photo-generated carriers are difficult to separate effectively thus are not suitable for photovoltaic applications.The research of this thesis is dedicated to improving the stability of perovskite and the performance of photovoltaic devices,and has conducted theoretical research on the two issues mentioned above:（1）Much effort has been dedicated to boost the development of lead-free perovskite solar cells.However,their performance and stability are still less competitive to the lead-bearing counterparts.By exploiting a mixed Sn-Ge cation strategy for the development of lead-free perovskites,we perform ab initio electronic structure calculations and quantum dynamics simulations on MASn0.5Ge0.5I3 and compare them to MASnI3.The calculations demonstrate that the hybrid cation strategy can improve simultaneously the perovskite stability and the lifetime of charge carriers.The stability increases due to a larger space of possible structures within the favorable range of the structural parameters,such as the Goldschmidt tolerance and octahedron factors.By exploring the larger structure space,mixed perovskites find stable configurations with lower free energies and better fitting components that exhibit reduced fluctuations around the equilibrium geometries.Charge carriers live longer in mixed perovskites because cation mixing results in an additional and moderate disorder that separates electrons and holes,reducing their interactions while still maintaining efficient band-like charge transport.These general and fundamental principles established by the analysis of the simulation results are useful for the design of advanced materials for solar energy and construction of optoelectronic devices.（2）Through Mn²⁺doping and surface attachment of electron-withdrawing groups,the influence of this strategy on the electronic structure and properties of the two-dimensional inorganic perovskite CsPb2I5 was explored.Calculations show that Mn doping and modification of surface organic molecular groups can maintain its original excellent energy band structure suitable for photoelectric material applications,and introduce an impurity energy level near the conduction band at a shallow energy level.This energy level state It may significantly change the original charge distribution and affect the lifetime of carriers,and the mixing effect of surface groups in the deep energy levels of the valence band will have an important impact on the generation of photogenerated carriers in the system.These studies help us understand the design of higher-performance perovskite photovoltaic devices from the principle,and will have valuable guiding significance for those engaged in related fields.