Surface properties of lead-free halide double perovskites: Possible visible-light photo-catalysts for water splitting

Halide double perovskites based on combinations of monovalent and trivalent cations have been proposed as promising lead-free alternatives to lead halide perovskites. Among the newly synthesized compounds Cs₂BiAgCl₆, Cs₂BiAgBr₆, Cs₂SbAgCl₆, and Cs₂InAgCl₆, some exhibit bandgaps in the visible range and all have low carrier effective masses; therefore, these materials constitute potential candidates for various opto-electronic applications. Here, we use first-principles calculations to investigate the electronic properties of the surfaces of these four compounds and determine, for the first time, their ionization potential and electron affinity. We find that the double perovskites Cs₂BiAgCl₆ and Cs₂BiAgBr₆ are potentially promising materials for photo-catalytic water splitting, while Cs₂InAgCl₆ and Cs₂SbAgCl₆ would require controlling their surface termination to obtain energy levels appropriate for water splitting. The energy of the halogen p orbitals is found to control the conduction band level; therefore, we propose that mixed halides could be used to fine-tune the electronic affinity.