The Transient Physics Processes In Perovskite Semiconductors

Recently, perovskite lead halide semiconductors have emerged as an excellent family of materials for solar conversion which have attracted tremendous attention for their exceptional performances in solar cells and photo-detectors. Unlike the dramatic improvement in device performance, the mechanism underlying the device performance remains elusive. In the limited number of studies, this new class of semiconductors have shown low density of midgap trap states and long carrier diffusion length, making them also promising for applications in light-emitting diodes and lasers. The emergence of perovskite semiconductors highlights that semiconductors with exceptional optoelectronic performance may be found in halide compounds. However, the physics related to electron, phonon and spin in perovskite semiconductors is not as transparent as that in semiconductors of Ⅳ, Ⅲ-Ⅴ, Ⅱ-Ⅵ compounds. In this thesis, I aim to uncover the physics responsible for the optoelectronic properties in perovskite semiconductors. Using ultrafast spectroscopy, we study the dynamics of excitons, spin and phonons in different time scale. The major findings are summarized as below:1. We have succeeded in demonstrating stable two-photon-pumped lasing at a remarkable low threshold by coupling CsPbBr3 nanocrystals into microtubule resonators. Highly efficient two-photon absorption (with a cross section of 2.7×106 GM) was observed in CsPbBr3 nanocrystals by using Z-scan experiments. Ultrafast transient absorption spectroscopy has been employed to study the dynamics of photo-excited excitons. We observed the development of optical gain in the CsPbBr3 nanocrystals films, and argued that bi-exciton effect may play an important role in the generation of optical gain. We demonstrated two-photon- pumped ASE from CsPbBr3 nanocrystals films and characterized the optical gain coefficient to be～580 cm-1. By coupling such a gain media of CsPbBr3 nanocrystals into microtubule resonators, we achieved untrastable two-photon-pumped lasing with a remarkably low threshold.2. We studied the magneto-photoluminescence in the organic-inorganic hybrid perovskite semiconductors (CH3NH3PbI3) under high magnetic field at low temperture. We observed strong circular polarized photoluminescence emission in the film sample and attributed these responses to magnetic-field-induced spin-mixing of the photo-generated electron-hole pairs with different g-factors—t he Agmodel(△g～0.38). Control experiments suggest that the trap state in CH3NH3PbI3 films plays an unobvious circular polarization from bandgap emission. When increasing the density of photo-excited excitons, the degree of circular polarization drops significantly which we argued to be a result of many-body effect in perovskite semiconductors.3. The stability issue is the major obstacle limiting the perovskite optoelectronic properties. The low thermal conductance has been previously regarded as a major drawback. Here, we report the observation of a remarkable efficient thermal conductance, with a conductivity of 11.2 Wm-1K-1 at room temperature, in densely packed perovskite CH3NH3PbI3 films, via noncontact time-domain thermal-reflectance (TDTR) measurements. The value is comparable to those values in conventional optoelectronic semiconductors, suggesting that perovskite device performance will not be limited by thermal conductance of the materials itself. The temperature-dependent experiments suggest the important roles of organic cations and structural phase transition, which are further confirmed by temperature-dependent Raman spectra and photoluminescence experiments.