Transient Spectroscopy of Photoexcitations and Morphology Control of Organometal Trihalide Perovskites

We studied the photoexcitation dynamics in various hybrid perovskites by using broadband ps transient photomodulation (PM) spectroscopy and variable stripe length (VSL) technique. We observed both excitonic and free carriers spectral features in MAPbI$_{3}$ but mainly excitonic transition in MAPbI$_{1.1}$Br$_{1.9}$ and MAPbI$_{3-x}$Cl$_{x}$ films. We also fabricated MAPbBr$_{3}$ films with nano-crystal pinning (NCP) treatment, which allows for smaller crystalline grain size. The transient spectra show a narrower and longer-lived photobleaching band in NCP treated films consistent with the increase in the photoluminescence efficiency. In addition the net optical gain measured by VSL is markedly increased up to 300 cm$^{-1}$, and the lasing threshold is concurrently reduced. Measurement of the waveguide losses in the NCP films shows that the improvement in lasing properties can partly be attributed to the reduced optical scattering.