Recent progress in thermoelectric layered cobalt oxide thin films

Oxide-based thermoelectric materials that show a high figure of merit are promising because of their good chemical and thermal stabilities and their relative harmlessness compared with chalcogenide-based state-of-the-art thermoelectric materials. Although several high-ZT thermoelectric oxides (ZT > 1) have been reported thus far, their reliability levels are low due to the lack of careful observations of their stabilities at elevated temperatures. Herein, we review the epitaxial film growth and thermoelectric properties of representative p-type layered cobalt oxides: Na_3/4CoO₂, Ca_1/3CoO₂, Sr_1/3CoO₂, Ba_1/3CoO₂, and Ca₃Co₄O₉. Among these specimens, Ba_1/3CoO₂ and Ca₃Co₄O₉ are stable in air at elevated temperatures (~600 °C). The ZT of Ba_1/3CoO₂ reaches ~ 0.55 at 600 °C in air, which is reliable and the highest among thermoelectric oxides. Moreover, this value is comparable to those of p-type PbTe and p-type SiGe.