Effect of impurity elements on the structural stability and electronic state in the tin iodide perovskite, which is useful for tuning carrier concentration to improve the thermoelectric performance, have been investigated using first-principles calculations. MASnI3 (MA = CH3NH3+) and CsSnI3 are considered as mother materials, and an impurity injection from three types of oxides—Al2O3, TiO2, and Y2O3— acting as scaffolds are considered. Three cations, Al3+, Ti3+, and Y3+, occupy the A or B sites, and one anion O2− occupies an I site. From the estimated formation energy Eform, Al and Ti impurities cause structural instability. In contrast, a Y impurity placed at a B site decrease Eform to −0.05 eV/atom and renders the perovskite structure more stable, while a Y impurity at an A site causes instability in the structure. For the calculated structural stability, the bonding analysis clearly explains the difference between the aforementioned sites. The Y–I covalent bonding around E = −4 eV is relatively strong when a Y is placed at a B site. Covalent bonding formed by the impurity elements has a finite contribution to the structural stability, even in the ionic crystal like an iodide perovskite.
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