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

Julian A Vigil; Nathan R Wolf; Adam H Slavney; Roc Matheu; Abraham Saldivar Valdes; Aaron Breidenbach; Young S Lee; Hemamala I Karunadasa

doi:10.1021/acscentsci.4c00056

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

ACS Cent Sci. 2024 Apr 2;10(4):907-919. doi: 10.1021/acscentsci.4c00056. eCollection 2024 Apr 24.

Authors

Julian A Vigil^{1

2}, Nathan R Wolf¹, Adam H Slavney¹, Roc Matheu¹, Abraham Saldivar Valdes¹, Aaron Breidenbach^{3

4}, Young S Lee^{5

4}, Hemamala I Karunadasa^{1

4}

Affiliations

¹ Department of Chemistry, Stanford University, Stanford, California 94305, United States.
² Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States.
³ Department of Physics, Stanford University, Stanford, California 94305, United States.
⁴ Stanford Institute for Materials and Energy Sciences, SLAC National Laboratory, Menlo Park, California 94025, United States.
⁵ Department of Applied Physics, Stanford University, Stanford, California 94305, United States.

Abstract

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.