Unusual defect properties in multivalent perovskite Cs2Au2I6 : A first-principles study

Double halide perovskites have attracted increasing interest due to their great potential to eliminate the toxicity and instability issues in lead halide perovskites. Unfortunately, octahedral cation antisites easily form in common double halide perovskites and induce deep defect levels. Here, using first-principles calculation methods, we report unusual defect properties in the double halide perovskite ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{I}}_{6}$, including the coexistence of shallow and deep defect transition energy levels of iodine vacancies, absent of octahedral cation antisites, and different dominant donor defects such as interstitials $({\mathrm{Au}}_{\mathrm{i}})$ and I substituting Au $({\mathrm{I}}_{\mathrm{Au}})$. We attribute these unusual defect properties to the multivalence effect of Au and strong covalent bonding between Au and I. Based on the defect properties, we propose two possible applications of ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{I}}_{6}$. On one hand, ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{I}}_{6}$ could be a useful candidate for photodetectors under I-rich growth conditions due to its relatively low carrier densities because the Fermi level is pinned near the middle of the band gap due to the strong compensation of the dominant donors ${\mathrm{Au}}_{\mathrm{i}}$ and acceptor ${\mathrm{V}}_{\mathrm{Cs}}$. On the other hand, ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{I}}_{6}$ can be made $n$-type if it is grown under I-poor conditions, which can be further enhanced by growing the sample at 600 K and then quenching to 300 K. Consequently, ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{I}}_{6}$ has potential as an $n$-type photovoltaic absorber. Our studies not only enrich the defect physics in multivalent double halide perovskites but also provide useful information for advancing ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{I}}_{6}$ technologies.