Halide Perovskites Breathe Too: The Iodide–Iodine Equilibrium and Self-Doping in Cs2SnI6

The response of an oxide crystal to the atmosphere can be personified as breathing─a dynamic equilibrium between O₂ gas and O^2– anions in the solid. We characterize the analogous defect reaction in an iodide double-perovskite semiconductor, Cs₂SnI₆. Here, I₂ gas is released from the crystal at room temperature, forming iodine vacancies. The iodine vacancy defect is a shallow electron donor and is therefore ionized at room temperature; thus, the loss of I₂ is accompanied by spontaneous n-type self-doping. Conversely, at high I₂ pressures, I₂ gas is resorbed by the perovskite, consuming excess electrons as I₂ is converted to 2I^–. Halide mobility and irreversible halide loss or exchange reactions have been studied extensively in halide perovskites. However, the reversible exchange equilibrium between iodide and iodine [2I^–_(s) ↔ I_2(g) + 2e^–] described here has often been overlooked in prior studies, though it is likely general to halide perovskites and operative near room temperature, even in the dark. An analysis of the 2I^–_(s)/I_2(g) equilibrium thermodynamics and related transport kinetics in single crystals of Cs₂SnI₆ therefore provides insight toward achieving stable composition and electronic properties in the large family of iodide perovskite semiconductors.

We study the reversible exchange between I₂ gas and iodide ions in a single crystal of the double perovskite Cs₂SnI₆. Measurements of bulk ion diffusion, electron transport properties, and thermodynamics of the 2I⁻_(s)/I_2(g) equilibrium indicate that I₂ off-gassing is spontaneous near room temperature and dopes the perovskite with excess electrons. Mitigating this reaction is critical to achieving stable electronic properties from perovskite semiconductors.