First-Principles Exploration of Thermodynamically Stable Cs-O Compounds

Cesium oxides, well-established low work function materials, have been used as surface coating and interfacial layer substances to improve the performance of photocathodes, thermionic energy converters, light-emitting devices, and solar cells. Cesium oxide compounds (Cs_xO_y) exist in many different stoichiometries with high air sensitivity, thus it remains a challenge to understand from experimental data their structural properties and resultant electronic properties. We performed global structure searches of Cs–O compounds with different stoichiometries (CsO, Cs₂O, CsO₂, and Cs₃O) by using a particle swarm optimization algorithm coupled with first-principles total energies. For each composition, stable structures of 5 ~ 8 different space groups were identified in the energy range about 10 meV/atom from the lowest energy configurations, which reflects the complicated phases of Cs–O beyond those in the literature. Our first-principles calculations unravel the correlation between the structures’ stoichiometries and symmetries and the resulting electronic structures, such as band gaps and work functions. Our results are expected to provide a route to improving the functional properties of cesium oxides for various device applications.