Interfacial States, Energetics, and Atmospheric Stability of Large-Grain Antifluorite Cs2TiBr6

Abstract

Recent reports post conflicting results on the atmospheric stability of Cs2TiBr6, a nontoxic, Earth-abundant solar energy conversion material. Here, a high-temperature melt of CsBr and TiBr4 yielded large-grain samples with >1 mm2 facets as verified by optical microscopy and scanning electron microscopy (SEM). With pristine-material properties of particular interest, we investigated a series of physicochemical surface treatments including rinsing, abrasion, and cleaving in ultrahigh vacuum (UHV). For each surface treatment, X-ray photoelectron spectroscopy (XPS) quantified surface chemical species, while ultraviolet photoelectron spectroscopy (UPS) established valence-band structure as a function of surface treatment. Amorphous titanium oxide with crystalline cesium bromide dominates the surfaces of nascent Cs2TiBr6 material. UHV cleaving yielded oxide-free surfaces with excellent alignment between valence-band structure and a density functional theory (DFT)-calculated density of states, a 3.92 eV work function, and 1.42 eV Fermi energy vs the valence band maximum. Band energetics are commensurate with moderate n-type doping for this melt-synthesized large-grain Cs2TiBr6. Titanium oxide once again dominates UHV-cleaved samples following a 10 min exposure to an air ambient. We discuss the implications of these surface chemical and electronic results for photovoltaics.