Enhanced high temperature thermoelectric properties of Bi-doped c -axis oriented Ca₃ Co₄ O₉ thin films by pulsed laser deposition

Ca_3-xBi_xCo₄ O₉ (x=0-0.4) thin films were deposited on single-crystal sapphire (0001) substrates by pulsed laser deposition. Structural characterizations indicated that these thin films exhibited perfect c -axis orientation and were well crystallized. Surface chemical states analysis confirmed Bi-substitution for Ca in the thin films with x<0.4. For the thin film with x=0.4, excessive Bi were found isolated within the film. Due to their perfect orientation, in-plane electrical properties of these thin films measured from 300 to 740 K were found to be comparable to those of the single crystals. Furthermore, Bi-substitution was noted for the reduced electrical resistivity and enhanced Seebeck coefficient. The above superior properties resulted in a high power factor of 0.81 mW m-1 K-2 at 740 K for thin film Ca2.7 Bi0.3 Co₄ O₉, which was about 29% improvement as compared to that of pure Ca₃ Co₄ O₉ thin film. The results suggested that Bi-doped Ca₃ Co₄ O ₉ thin films could be a promising candidate for thermoelectric applications at elevated temperatures.