Defect Tolerance to Intolerance in the Vacancy-Ordered Double Perovskite Semiconductors Cs₂SnI₆ and Cs₂TeI₆

TLDR

Vacancy‑ordered double perovskites A₂BX₆ consist of isolated [BX₆] octahedra in a face‑centered lattice with A‑site cations in voids, yet the close‑packed iodide framework yields significant electronic dispersion that governs their electronic structure and defect tolerance, making Cs₂SnI₆ a candidate for photovoltaic applications. The study aims to elucidate structure‑property relationships by synthesizing the solid‑solution Cs₂Sn₁₋ₓTeₓI₆. The authors synthesized Cs₂Sn₁₋ₓTeₓI₆ solid solutions to probe how Te substitution affects electronic dispersion, carrier concentration, and defect chemistry. Te substitution in Cs₂Sn₁₋ₓTeₓI₆ increases electronic dispersion yet produces insulating behavior by reducing carrier concentration and mobility; density‑functional calculations reveal that iodine vacancies are shallow donors in Cs₂SnI₆, conferring defect tolerance, whereas stronger Te–I covalency suppresses vacancy formation and in Cs₂TeI₆ the defect level lies deep in the band gap, localizing carriers.

Abstract

Vacancy-ordered double perovskites of the general formula A2BX6 are a family of perovskite derivatives composed of a face-centered lattice of nearly isolated [BX6] units with A-site cations occupying the cuboctahedral voids. Despite the presence of isolated octahedral units, the close-packed iodide lattice provides significant electronic dispersion, such that Cs2SnI6 has recently been explored for applications in photovoltaic devices. To elucidate the structure-property relationships of these materials, we have synthesized solid-solution Cs2Sn1-xTexI6. However, even though tellurium substitution increases electronic dispersion via closer I-I contact distances, the substitution experimentally yields insulating behavior from a significant decrease in carrier concentration and mobility. Density functional calculations of native defects in Cs2SnI6 reveal that iodine vacancies exhibit a low enthalpy of formation, and that the defect energy level is a shallow donor to the conduction band rendering the material tolerant to these defect states. The increased covalency of Te-I bonding renders the formation of iodine vacancy states unfavorable and is responsible for the reduction in conductivity upon Te substitution. Additionally, Cs2TeI6 is intolerant to the formation of these defects, because the defect level occurs deep within the band gap and thus localizes potential mobile charge carriers. In these vacancy-ordered double perovskites, the close-packed lattice of iodine provides significant electronic dispersion, while the interaction of the B- and X-site ions dictates the properties as they pertain to electronic structure and defect tolerance. This simplified perspective based on extensive experimental and theoretical analysis provides a platform from which to understand structure-property relationships in functional perovskite halides.