Enhanced Thermoelectric Properties in Cu-Doped c-Axis-Oriented Ca3 Co4 O9+δ Thin Films

Highly c-axis-oriented Ca3Co4−xCuxO9+δ (x = 0, 0.1, 0.2, and 0.3) thin films were prepared by chemical solution deposition on LaAlO3 (001) single-crystal substrates. X-ray diffraction, field-emission scanning electronic microscopy, X-ray photoelectron spectroscopy, and ultraviolet-visible absorption spectrums were used to characterize the derived thin films. The solubility limit of Cu was found to be less than 0.2, above which [Ca2(Co0.65Cu0.35)2O4]0.624CoO2 with quadruplicated rock-salt layers was observed. The electrical resistivity decreased monotonously with increasing Cu-doping content when x ≤ 0.2, and then slightly increased with further Cu doping. The Seebeck coefficient was enhanced from ~100 μV/K for the undoped thin film to ~120 μV/K for the Cu-doped thin films. The power factor was enhanced for about two times at room temperature by Cu doping, suggesting that Cu-doped Ca3Co4O9+δ thin films could be a promising candidate for thermoelectric applications.