Thermoelectric properties of Bi₂Sr₂Co₂O_y thin films and single crystals

Bi₂Sr₂Co₂O₉ exhibits a misfit-layered structure with good thermoelectric properties. We have investigated the thermoelectric properties of Bi₂Sr₂Co₂O_y 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 (S_ab²/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 S_ab is improved. Lastly, we discuss the thermoelectric performance of this material considering both oxygen concentration and phase purity.