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.
Mendes, J. L., Gao, W., Martin, J. L., Carl, A. D., Deskins, N. A., Granados-Focil, S., & Grimm, R. L. (2020). Interfacial States, Energetics, and Atmospheric Stability of Large-Grain Antifluorite Cs2TiBr6. The Journal of Physical Chemistry C. https://doi.org/10.1021/acs.jpcc.0c08719.
*denotes a WPI undergraduate student author