Temperature dependence of the work function in barium fluoride ultra-thin films on semiconductors for hybrid thermionic converters studied by ultraviolet photoelectron spectroscopy

Thin films based on barium fluoride (BaFx) have demonstrated the capability to reduce the work function of photovoltaic (PV) anodes in hybrid thermionic devices for the conversion of thermal-to-electrical energy. Because PV anodes integrated in thermionic converters can operate at temperatures higher than room temperature, in this study, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were used to investigate the temperature dependence up to 300 degrees C of the chemical and physical properties of BaFx thin-films, deposited on gallium arsenide and silicon, that represent possible candidates for PV anodes. A correlation between chemical composition and work function reduction by 0.5 +/- 0.1 eV for the thin films on both GaAs and Si has been found as a function of temperature from room temperature to 300 degrees C. According to XPS analyses, this effect can be ascribable to the thermal desorption of the first contaminated layers of BaFx coatings.