Free Excitons and Exciton-phonon Coupling in CH_3NH_3PbI_3 Perovskite Single Crystals at Low Temperatures

摘要

Hybrid lead halide perovskites, such as methylammonium lead halides CH_3NH_3PbX_3 (MAPbX_3, X = I, Br, Cl), are emerging as an excellent class of materials for various optoelectronic applications including solar cells, light-emitting diodes, lasers, and photodetectors [1,2]. As for these perovskite-based devices, deep understanding of transport properties of excitons and charge carriers in perovskite layer is essentially needed in order to achieve a better performance. Although the recombination dynamics in MAPbX_3 materials have been investigated extensively [3,4], there are limited works devoted to elucidating the scattering processes of excitons and charge carriers with phonons and impurities. Thus far, the spectral width extracted from conventional photoluminescence (PL) spectroscopy has been used to examine the electron- and exciton-phonon interactions in perovskite polycrystalline thin films [5]. However, polycrystalline thin films consist of grains with various sizes and compositions; these lead to a broad inhomogeneous PL spectral width, and therefore, prevent an appropriate evaluation of the scattering processes. In addition, PL spectrum at low temperatures is composed of multiple emission bands [4,5] and usually dominated by the emission band from the bound excitons due to their giant oscillator strength. A precise extraction of the spectral width of the free-exciton emission band could not be obtained qualitatively based on only PL measurement at low temperatures. Here, we report a study of the free-exciton properties and the exciton-phonon interaction in orthorhombic-phase MAPbI_3 single crystals by means of simultaneous temperature-dependent photocurrent (PC) and PL measurements.