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Defect Tolerance to Intolerance in the Vacancy-Ordered Double Perovskite Semiconductors Cs2SnI6 and Cs2TeI6
journal contribution
posted on 2016-06-10, 00:00 authored by Annalise
E. Maughan, Alex M. Ganose, Mitchell M. Bordelon, Elisa M. Miller, David O. Scanlon, James R. NeilsonVacancy-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
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band gapperovskite derivativesCs 2 TeI 6conduction banddefect tolerancelatticesite cationstellurium substitution increasesBX 6charge carrierssite ions dictatesformation2 BX 6Defect Toleranceoctahedral unitsdispersionCs 2 SnI 6defect energy leveldefect levelcarrier concentrationiodine vacancies exhibitphotovoltaic devicesdefect statesTe substitutioniodine vacancy states
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