Thermoelectric properties of Bi2Sr2Co2Oy thin films and single crystals

Bi2Sr2Co2O9 exhibits a misfit-layered structure with good thermoelectric properties. We have investigated the thermoelectric properties of Bi2Sr2Co2Oy in both thin-film and single-crystal forms. Among thin films grown at different temperatures, we find that both the in-plane thermoelectric power (Sab) and electrical resistivity (ρab) vary in an opposite trend, i.e., Sab is high when ρab is small. This results in large power factor (Sab2/ρab~5.5 μW/K2 cm for the film grown at 700 °C), comparable to that for whiskers. For single crystals, the electrical resistivity shows metallic behavior in a large temperature range, but has higher magnitude than that of films grown at 675 °C and 700 °C. The annealing of single crystals under Ar atmosphere leads to even higher resistivity while Sab is improved. We discuss the thermoelectric performance of this material considering both oxygen concentration and phase purity.