Phase Modulation Two-Photon Microscopy of Hybrid Halide Perovskite

Abstract: Recently, intense research activities have been focused on solution-processed hybrid lead halide perovskite semiconductors which are being developed for the solar-cell, light emitting diode (LED), and lasing applications. To optimize and fabricate a competitive perovskite-based optoelectronic device, the fundamental studies of the electronic structure, photo-generated carrier dynamics as well as the photoluminescence (PL) & photocurrent (PC) response are necessary and of great significance. In this thesis, we have used the temperature dependent phase modulation two-photon PL and PC microscopy to investigate fundamental photophysics of the hybrid halide perovskite. Firstly, we investigate the carrier-phonon scattering process in methylammonium lead bromide (MAPbBr3) and formamidinium lead bromide (FAPbBr3) perovskites directly after the excitation by the sunlight or optical lasers. We found that the charge carrier coupling to the longitudinal optical (LO) phonons is stronger than the exciton-acoustic phonon coupling in both MAPbBr3 and FAPbBr3 perovskites. The large LO phonon - free charge carrier coupling strength in MAPbBr3 thin films (TFs) at room temperature hinders the carrier mobility and lowers the PC yield. The charge carriers recombine via exciton or free charge carrier recombination. By separately quantifying spatially resolved emission from excitons and free charge carriers in MAPbBr3 bulk crystals, we found that the emission from excitons is predominant in the small isolated localized spot of the crystal while the emission from the free charge carriers is predominant in the main bulk crystal. The larger band structure deformation surrounding the localized spot protects the photo-generated carriers and promotes the first order (1O) recombination. Considering the charge accumulation effect in the high repetition rate excitation system, the trap states and their dependence on phase transition at low temperature are explored. We found that the areas with less traps have more second order recombination emission. With decreasing temperature, the 1O recombination emission is increasing due to the increased exciton population and the related emission. Besides, the lifetime related to the accumulation effect is decreasing since the electrons cannot de-trap from the deeper traps. In addition, the studies on the photo physics processes may help to develop highly efficient MAPbBr3 perovskites solar cells and photodetectors. The relationship between the PC performance with the different emissive states is investigated in a MAPbBr3 solar cell. The results suggest that the high emissive state at 2.34 eV, corresponding to the band to band recombination, strongly contributes to the PC, while the lower emissive state at 2.28 eV does not significantly contribute to the PC since it is a trap state. When working as a MAPbBr3 microcrystal (MC)-based photodetector, the carrier transport length can vary from 5.7 μm (0.02 V) due to diffusion to 23.2 μm (2 V) due to the drift. For the solar cell and photodetector application, the inhomogeneous distribution of the trap states introduces spatially inhomogeneous distribution of PC as well as PL.