Grain-size-dependent Terahertz Conductivity of MAPbI3 Perovskite

Abstract

In recent years, hybrid organic–inorganic halide perovskites have attracted extensive attention due to their potential in low-cost fabrication across a broad spectrum of photovoltaic applications, particularly in perovskite-based solar cells. To increase the efficiency of the photovoltaic effect, grain size plays a pivotal role, and it must be reduced for the realization of a large charge diffusion length. In this study, we investigate the grain-size-dependent optical conductivity of CH3NH3PbI3 perovskite (MAPbI3) in the terahertz frequency range. The grain sizes examined are 100, 500, 1,000, and 2,000 nm. There are two distinct phonon features in the optical conductivity spectra, which show no discernible variations with respect to grain size. In contrast, spectral features associated with free carrier dynamics show a noticeable evolution as grain size is varied. We interpret these spectral changes arising from free-carrier dynamics as resulting from grain-size-dependent carrier trapping, which is consistently supported by pump-probe measurements. Our comprehensive experimental investigation indicates that increasing the grain size to approximately 1,000 nm is necessary to achieve a carrier scattering rate comparable to that of ideal MAPbI 3 crystals without grain boundaries.