Bi 2 Sr 2 Co 2 O 9 exhibits a misfit-layered structure with good thermoelectric properties. We have investigated the thermoelectric properties of Bi 2 Sr 2 Co 2 O 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 (S ab ) and electrical resistivity ( ab ) vary in an opposite trend, i.e., S ab is high when  ab is small. This results in large power factor (S ab 2 / ab ~ 5.5 W/K 2 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.We discuss the thermoelectric performance of this material considering both oxygen concentration and phase purity.

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