The Influence Of Self-trapped Excitons On The Photoelectric Properties Of Semiconductors: First-principles Studies

The strong exciton-phonon interaction in material will cause self-trapping of free excitons and the accompanying local lattice distortion.Self-trapped excitons（STE）have a significant effect on the photoelectric properties of semiconductors,such as large stocks-shift in luminescent materials,broad band light emission,and open circuit voltage loss in photovoltaic devices.STE in alkali-halogen compounds,organic molecular crystals and solid rare gases have been studied and widely reported.In recent years,STE is found in some new semiconductor optoelectronic materials,such as metal halide perovskites（MHPs）and binary semiconductor Sb₂S₃,but the influence of STE on their photoelectric properties is not yet fully understood.1)Experiments show that the self-trapping process of free excitons in MHPs is affected by chemical composition,doping elements,and crystal structure and so on.However,the micro-mechanism of the formation of STE is still unclear,which is the limitation of development and application of MHPs-based broad band white light sources.2)Sb₂S₃ semiconductor is considered a new type of solar cell material,but exciton self-trapping will cause serious open-circuit voltage loss and restrict the improvement of its energy conversion efficiency.Although several experiments report that the exciton trapping in Sb₂S₃ is an intrinsic trapping process,there is a lack of theoretical confirmation.Therefore,exploring the microscopic mechanism of STE in these semiconductor materials is of great significance for developing new optoelectronic devices.In this thesis,based on density functional theory（DFT）calculations,the exciton trapping behavior in MHPs and Sb₂S₃ was studied,as well as its influence on the luminescence and photovoltaic properties of the materials.Our main research contents are as follows:Firstly,by doping Yb³⁺ions in 3D pure inorganic perovskite Cs Pb X₃（X=Br,Cl）to achieve low temperature STE luminescence,and the full width at half maximum（FWHM）is more than 200 nm and generates a Stokes-shift of nearly 1e V.Both results of experiment and theory show that the introduction of Yb³⁺ions preferentially replace Pb²⁺ions and promotes the formation of ortho-halogen vacancies.This composite defect will form a defect state under conduction band,which can effectively capture the carriers in the host lattice,so that the interaction between carriers and lattice is enhanced,which promote the formation of STE.In addition,the introduction of Yb³⁺ions and the formation of halogen vacancies can reduce the deformation energy of Pb X₆octahedron,so that structure distortion is more likely to occur to accommodate STE.Our study provides a new idea for the formation of non-intrinsic STE and broad-band luminescence induced by rare earth ion doped 3D pure inorganic perovskite.Then,based on first-principles calculations,we investigate the microcosmic mechanism for the formation of intrinsic STE in 1D-C₄N₂H₁₄Pb Cl₄ system,and further discuss the relationship between electric polarization and luminescence properties.The results show that the 1D-C₄N₂H₁₄Pb Cl₄system has a low electron dimension（flat-band characteristic）,which provides a prerequisite for the formation of STE.Based on the?SCF method,the self-trapping structure is optimized and its structure deformation is deriving from the non-Jahn-Teller distortion of the PBCl₆ octahedron.Corresponding density of states（DOS）and partial charge density further confirm that intrinsic STE exist in 1D-C₄N₂H₁₄Pb C₄ system.Transition probability before and after lattice distortion reveal that central symmetry breaking of structure will destroy the forbidden transition.In addition,the formation of STE will accompany the electric polarization,and the transition probability is proportional to the electric polarization,which can effectively increase the luminous intensity.Finally,based on the Delta-Self-Consistent Field（ΔSCF）method and PBEsol and HSE06+D2 functions,we reported a theoretical study on the origin of large Stokes-shift in Sb₂S₃.Both of intrinsic and extrinsic STEs were considered as possible sources of large Stokes-shift.On the one hand,the results show that there is no intrinsic STE in Sb₂S₃.On the other hand,by comparing the intrinsic defects,we find that V_Sand S_idefects are energetically accessible,and their holes have a strong local distribution.The corresponding DOS further confirms the formation of hole polaron.By mean of exciton binding energy,exciton self-trapping energy and lattice deformation energy,the open-circuit voltages for both V_S and S_i were quantitatively estimated,which were close to 0.6 e V.The results fully support the phenomenon observed in experiment that hole polaron induces the formation of STE.These simulation results show that large Stokes-shift（open-circuit voltage）can be avoided,and the energy conversion efficiency of Sb₂S₃thin film solar cells can be effectively improved by inhibiting the formation of defects and STE in the experiment of preparation of Sb₂S₃,which is even expected to approach the theoretical limit value of 28.6%.In this thesis,the effects of STE effect on the photoelectric properties of new semiconductor are analyzed in detail from both intrinsic and non-intrinsic aspects.First,we reveal the microcosmic mechanism of STE formation in perovskite system and binary semiconductor Sb₂S₃ systems,and provide a theoretical support for understanding the phenomenon of broad-band luminescence and large Stoke-shift in the experiment.Secondly,we further discuss the relationship between polarization and luminescence properties.These results provide a new idea and theoretical basis for doping induced broadband luminescence,electric polarization regulating luminescence intensity,and further improving energy conversion efficiency.