Thermoelectrical properties of spray pyrolyzed indium oxide thin films doped by tin

Abstract The search for materials with thermoelectric parameters capable of operating at high temperatures continues to be of great interest; n-type metal oxides are promising candidates. Here, two series of thin (~ 100 nm) indium oxide films doped by tin (from 0 to 50 at.%) were deposited by spray pyrolysis at 350 °C and 450 °C. Characterization of the films was performed using X-ray diffraction, scanning electron microscopy and atomic force microscopy. Thermoelectric properties, i.e., the conductivity and the Seebeck coefficient, were then studied over a temperature range of 20–450 °C. It was shown that these parameters as well as their nanostructure were strongly dependent on the Sn content and deposition temperature. Specifically, the conductivity had maxima near 5% and 20% for films deposited at 350 °C and 450 °C, respectively. The power factor (PF) as a function of Sn content also demonstrated non-monotonous behavior with two maxima; for films deposited at 350 °C these maxima were again observed near 5% and 20% of Sn content. The maximal PF value equaled to 4.7 mW/(m·K2) at a temperature of 450 °C was observed at 5 at.% Sn. This result is one of the best ever obtained for metal oxides in a given temperature range. The optimal films were characterized by a cubic-like crystallite nanostructure with {400} surface faceting. A model explaining such high parameters was subsequently proposed. We also determined the effect of ambient humidity on the thermoelectric properties of nanostructured In2O3:Sn films at an operating temperature range below 400 °C, which is caused by the change of surface conductivity under the influence of water vapor.