Investigation On Photophysical Properties Of Metal Halide Perovskite By Microscopic Spectroscopy

Metal halide perovskites have been one of the most promising alternative semiconductors in photovoltaic and light-emitting devices due to their excellent optoelectronic properties including high absorption coefficients,low exciton binding energy,long carrier diffusion length and tunable bandgap properties.Besides,facile fabrication processes of the materials also determine their low cost in future production.Until now,rapid progresses of metal halide perovskite have been achieved.According to the latest report from National Renewable Energy Laboratory,the best power conversion efficiency of perovskite based solar cells has reached 25.2%.And the external quantum efficiency of light-emitting diodes has also been reported to exceed20%.However,besides rushing for higher efficiency and better performance,the fundamental understandings including defect properties,recombination processes of the photogenerated carriers and ion migration processes are still lacking of investigation.Along with the inherent poor stability of the materials,further development of the application is greatly limited.Due to the direct connection between photoluminescence?PL?signal and photophysical properties as well as the composition and crystal structure transformation processes,the study of PL signal should be a promising and convenient approach to investigate the fundamentals of the metal halide perovskite materials,and will be of great help to promote the understanding of their physical processes and find out effective methods to solve the problems such as stability,defect properties,optical physical impairments and photoinduced phase segregation in metal-halide perovskite crystals which severely limit their applications.In this work,with the spatial and time resolution of the microscopic spectroscopy technique,we investigated the defect properties of the organic-inorganic hybrid perovskite including the distribution and chemical compositions of defects,revealed the light propagating processes in hexagonal organic-inorganic perovskite crystals and their effect on recombination processes of photo-generated carriers,and verified the inhibition of light induced phase segregation processes in all-inorganic mixed-halide perovskite via B-site doping.The main research projects in this thesis are listed as follows:?1?We first used methylammonium lead triiodide perovskite?MAPb I₃?to study the crystal defects in these materials.By real-time monitoring the micro-absorption spectra during the PL variation including increase and decrease in PL intensity,PL variation by atmosphere and excitation power,and PL decline due to degradation by microscopic spectroscopy technology,we successfully separated the contribution from PL quantum efficiency and the absorption ability.Due to that variation of PL quantum efficiency was mostly connected with the defect concentration in semiconductors,we tried to characterize the defects in MAPb I₃ based on the excitation power dependence of PL intensity.By using wide-field fluorescence microscope,we introduced spatial resolution and correlated the concentration of defects with location,size and morphology of the MAPb I₃ crystals.In addition,we also observed that relatively higher concentration of Pb I₂ precursor will generate more defects during the preparation.Finally,we observed both PL enhancement and PL decline on the same MAPb I₃ crystal.By detailed studying the PL behaviors from different aspects including PL spectra and lifetime,reversibility of the PL decline and excitation power dependence,and combining the experimental results with theoretical calculations we first identified that interstitial defects of I and Pb are respectively responsible for the PL enhancement and PL decline.?2?The dynamic properties of the photo-generated carriers in metal-halide perovskite are not only influenced by defect properties,but also correlated with the geometry of the crystals.Thus,we studied the PL spectra at different locations on a hexagonal-shaped MAPb I₃ micro crystal and verified that the extra red-shifted band emitting from crystal edges was due to the light propagating processes involving light propagation,total reflection,self-absorption,and charge carrier redistribution.We also built a decay model to explain how the light propagating progresses influenced the recombination processes of the photogenerated carriers in MAPb I₃ crystals which resulted in the significantly different time-resolved dynamics.?3?Besides the poor stability of metal-halide perovskite under heat and moisture,the mixed-halide perovskite also suffers from the phase segregation problems which are due to the ion migration induced by photoirradiation.Thus,to understand the mechanisms of photo-induced phase segregation and find out effective solutions,we studied all inorganic mixed-halide perovskite due to their inherently improved stability against heat and moisture which can help to rule out the influences of organic ions'poor stability.By real-time monitoring the PL spectra of Cs Pb IBr₂ and B-site doped mixed-halide perovskite by Sn²⁺or Mn²⁺,we clearly explained the mechanisms of the phase segregation existing under light irradiation and verified that B-site doping in mixed-halide perovskite can efficiently prevent the phase separation processes and maintain stable emitting wavelength.Moreover,by replacing Mn²⁺for Sn²⁺,we also found that materials'tolerance to ambient atmosphere could be significantly improved indicating an effective optimization method.