Proton Migration in Hybrid Lead Iodide Perovskites: From Classical Hopping to Deep Quantum Tunneling

The organic-inorganic halide perovskites (OIHPs) have shown enormous potential for solar cells, while problems like the current-voltage hysteresis and the long-term instability have seriously hindered their applications. Ion migrations are believed to be relevant. But the atomistic details still remain unclear. Here we study the migrations of ions in CH₃NH₃PbI₃ (MAPbI₃) at varying temperatures ( T's), using combined experimental and first-principle theoretical methods. Classical hopping of the iodide ions is the main migration mechanism at moderate T's. Below ∼270 K, the kinetic constant for ionic migration still shows an Arrenhius dependency, but the much lower activation energy is attributed to the migration of H⁺. A gradual classical-to-quantum transition takes place between ∼140 and ∼80 K. Below ∼80 K, the kinetic constant becomes T-independent, suggesting that deep quantum tunneling of H⁺ takes over. This study gives direct experimental evidence for the migrations of H⁺s in MAPbI₃ and confirms their quantum nature.