Role of phase separation in nanocomposite indium-tin-oxide films for transparent thermoelectric applications

We report that oxygen vacancies have a profound impact on phase separation and thermoelectric properties of ITO films grown at room temperature. Oxygen vacancies in non-stoichiometric In1.8Sn0.2O2.5 films aided the formation of In-rich metallic clusters. It yields a high electrical conductivity σ = 1540 Scm−1 and Seebeck coefficient |α| = 27.2 μVK−1, which resulted in the highest power factor (α2σ = 113.8 μW m−1K−2) but low optical transmission (Top ∼ 25%). An increase in oxygen partial pressure resulted in stochiometric In1.8Sn0.2O3 films which improved the optical transparency by 300% (Top ∼75.4%), but power factor was reduced by ∼85% due to a decrease in α and σ. A decrease in α was due to the lack of energy filtering of charge carriers in the stoichiometric ITO film which did not have In-rich metallic clusters. XPS results showed that the valence band energy shifts with a change in oxygen partial pressure due to a decrease in carrier density, which implied a change in Fermi energy due to the reverse Moss-Burstein effect. Our results showed that phase separation can be obtained in nanocomposite ITO films by tuning their stoichiometry simply by varying the oxygen partial pressure during deposition of thermoelectric materials at low temperatures.