Slow Electron–Hole Recombination in Lead Iodide Perovskites Does Not Require a Molecular Dipole

Hybrid organic/inorganic lead iodide perovskites of the formula APbI₃, where A is a molecular cation such as methylammonium, exhibit remarkably slow photoinduced charge carrier recombination rates, for reasons that remain uncertain. Prevalent hypotheses credit this behavior to the unique dipolar nature of the molecular cation. Herein, transient terahertz spectroscopy is applied to solution-processed, all-inorganic, perovskite-phase cesium lead iodide (CsPbI₃) thin films, which lack such a dipole. The recombination kinetics are studied as a function of the initial photoinduced carrier concentration and the wavelength of excitation. A kinetic model combining diffusion and recombination is fit to the data, from which the rate constants are determined, revealing a bimolecular recombination rate of 10^–10 cm³ s^–1, comparable to high-quality, single-crystal, direct-gap semiconductors. This rate, as well as a charge carrier mobility > 30 cm² V^–1 s^–1 measured herein for CsPbI₃, are similar to values reported for the hybrid perovskites, strongly suggesting that the organic cation does not confer a fundamental advantage