One main challenge associated with solid-state thermoelectric materials is to combine electron-crystal electrical conductivity and phonon-glass thermal properties that are difficult to conciliate. A markedly different principle of thermoelectric conversion is based on the micro-thermionic generator that exploits electron injection and heat rejection across two electrodes, hot and cold, immersed in vacuum [1], a promising pathway to new electronic devices. To properly operate thermionic emission converters near room temperature, highly efficient emitting electrodes must be integrated. This naturally motivates the development and characterization of coatings that feature a very low work function, lower than 1eV. Even better than alkalis like K and Cs, their oxides have been reported to produce work functions as low as 0.4 and 0.9eV [2], respectively. In this paper, to promote material integration, the optimal synthesis conditions of Cs2O are first determined from the calculated volatility diagram and appear to be less restrictive than those of K2O2. In a second part, we present experimental results on the composition and electrical properties of cesium oxide thin film synthesized in ultra high vacuum. To finalize the study, it is shown that a consistent work function value is extracted based on three independent techniques, namely, thermionic emission, photoemission and Kelvin probe measurements. [1] T. Zeng, App. Phys. Lett. 88, 153104 (2006) [2] J.X. Wu et al, Appl. Surf. Sci. 173, 8 (2001)