Spectroscopic Study Of Photophysical Processes In CH₃NH₃PbBr₃ Nanocrystals

Lead halide perovskites emerging as good light absorbers and emitters employed in optoelectronic devices have attracted tremendous attention in the field of energy storage and conversion.Despite the continuously reported promising achievements in materials design and devices fabrication,it is not trivial to have a deep understanding of the fundamental issues and novel physical phenomena for better use.Arrays of colloidal nanocrystals?NCs?are known to be promising alternatives to the single-crystal and polycrystalline film semiconductor-based optoelectronics.Unlike the mature research in conventional semiconductors like GaAs,questions remain on how the photo-excited transient species interact,relax and recombine in perovskites NCs and perovskite NCs-based devices.In this thesis,the carrier dynamics,lattice dynamics,and spin dynamics of quasi-2D arrays of methylammonium lead tribromide?CH₃NH₃PbBr₃,also MAPbBr₃?NCs are studied via time-resolved photoluminescence?PL?,temperature-dependent PL,and transient absorption?TA?spectroscopy.The research work in this thesis mainly includes the following parts:1)A modified solution-processed method is presented to acquire high-quality MAPbBr₃ colloidal NCs.A new method is developed to encapsulate perovskite nanocrystalline film to avoid the erosion of water and oxygen,which guarantees the measuring stability and data reliability.2)MAPbBr₃ nanocrystalline film with an average PL lifetime of²0 ns is prepared,and the PL lifetime significantly decreases to¹4 ns when the electron transport layer ZnO applied.With biexponential fitting,PL decays show fast and slow decay components which are assigned to be the surface recombination and bulk recombination respectively.Potential-barrier-induced efficient exciton dissociation is confirmed by analyzing the reapportion of these decay components.The exciton killing by the heterointerface is also observed in steady-state absorption spectra.3)A clear structural phase transition process is firstly demonstrated with increasing temperature from 20 K to 300 K by analyzing the evolution of integrated intensity of PL band within two-photon excitation regime,with which the PL mechanisms in MAPbBr₃NCs are given.It is proved that two-photon PL is much more sensitive than single-photon PL when studying the temperature-dependent lattice dynamics of perovskite NCs.Moreover,the MAPbBr₃/ZnO interface takes effect on these dynamics and significantly shifts the structural phase transitions toward higher temperatures depending on the carrier density.4)The ultrafast hot carrier cooling processes in MAPbBr₃ and MAPbBr₃/ZnO structure are studied by TA spectroscopy.TA spectra have been suggested to originate from the Burstein-Moss shift and band gap renormalization effects which are known to determine the optical band gap upon photoexcitation.The excitation dissociation at MAPbBr₃/ZnO interface is furtherly confirmed by TA spectroscopy.The Rashba effect is firstly observed directly in TA spectra with a giant Rashba parameter?_R up to 2.7 eV??for MAPbBr₃,which can be tuned by the interfacial electric field.The spin-orbit coupling in MAPbBr₃ NCs is studied,and the Rashba splitting in TA spectra exhibits spin-dependent periodic characteristics with varying pump polarization.