Mixed-halide vacancy-ordered double perovskite for photovoltaic and photocatalysis applications

Here, we report detailed first-principles calculations of the structural stability, optoelectronic properties, and interaction with water for a wide range of mixed-halide compositions of vacancy-ordered double perovskites ${\mathrm{Cs}}_{2}{\mathrm{Pt}(\mathrm{Cl}}_{x}{\mathrm{I}}_{1\ensuremath{-}x}{)}_{6}$. Our calculations reveal that lower halide dopant levels subdue phase segregation and enhance the stability. ${\mathrm{Cs}}_{2}{\mathrm{Pt}(\mathrm{Cl}}_{x}{\mathrm{I}}_{1\ensuremath{-}x}{)}_{6}$ demonstrate improved defect tolerance as compared to ${\mathrm{Cs}}_{2}{\mathrm{Pt}\mathrm{I}}_{6}$ due to the covalent nature of the Pt---X bond. The chloride-rich ${\mathrm{Cs}}_{2}\mathrm{Pt}$(${\mathrm{Cl}}_{x}{\mathrm{I}}_{1\ensuremath{-}x}$)${}_{6}$ exhibit notably improved stability against reaction with water, far surpassing ${\mathrm{Cs}}_{2}{\mathrm{Pt}\mathrm{I}}_{6}$ due to the enhanced Cs---Cl bond strength and lower charge transfer between adsorbed ${\mathrm{H}}_{2}\mathrm{O}$ and surface Cs atoms. The spectroscopic limited maximum photovoltaic efficiency for the optimal composition of ${\mathrm{Cs}}_{2}\mathrm{Pt}$(${\mathrm{Cl}}_{0.04}{\mathrm{I}}_{0.96}$)${}_{6}$ under 1 sun AM1.5G is determined to be $24$% for a 5-$\text{\ensuremath{\mu}}\mathrm{m}$-thick film. Our calculations also suggest that the valence-band edge of this material might be positioned more positive than the standard potential of the oxygen-evolution reaction. These two factors combined with the high stability against reaction with water indicate that ${\mathrm{Cs}}_{2}\mathrm{Pt}$(${\mathrm{Cl}}_{0.04}{\mathrm{I}}_{0.96}$)${}_{6}$ might be of considerable interest as a photovoltaic absorber, and possibly as a component of anodes for the photoelectrocatalytic water oxidation. Meanwhile, ${\mathrm{Cs}}_{2}\mathrm{Pt}$(${\mathrm{Cl}}_{0.96}{\mathrm{I}}_{0.04}$)${}_{6}$ traverses relevant reduction and oxidation redox potentials, affirming it as a promising candidate for the overall photo(electro)catalyst water-splitting reaction.