Interfacial States, Energetics, and Passivation of Large-Grain and Thin-Film Antifluorite Cesium Titanium Bromide

Mendes, Jocelyn L.

Student Work

Interfacial States, Energetics, and Passivation of Large-Grain and Thin-Film Antifluorite Cesium Titanium Bromide

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We studied the surface chemistry, atmospheric stability, and passivation of solar-relevant Cs2TiBr6. In particular, we studied the synthesis of both large- grain and thin film Cs2TiBr6. A high-temperature melt successfully produced high quality large-grain samples with >1 mm2 facets as verified by optical microscopy and scanning electron microscopy (SEM). We confirmed synthesis using X-ray diffraction (XRD). X-ray photoelectron spectroscopy characterized the resulting surface chemistries. Following initial characterization, we established the surface basicity of interfacial bromine species on both oxide- rich and oxide-free surfaces. In an attempt to further understand metal oxide species on the Cs2TiBr6 surface and 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 sur- faces 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. TiO2 is well aligned with the band edges of Cs2TiBr6 for facile electron transport, and we hypothesized that it would act as an ideal passivation material. These findings motivated us to fabricate a Schlenk-line integrated atomic layer deposition (ALD) instrument for air-free passivation of Cs2TiBr6 with TiO2 and present preliminary ALD studies on silicon and Cs2TiBr6 surfaces. We discuss the implications of these surface chemical and electronic results for photovoltaics.

This report represents the work of one or more WPI undergraduate students submitted to the faculty as evidence of completion of a degree requirement. WPI routinely publishes these reports on its website without editorial or peer review.

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