Achieving chemical stability in thermoelectric NaxCoO2 thin films

Abstract

Stability issues in thermoelectric NaxCoO2 thin films have been solved by the addition of an in situ amorphous AlOx capping layer, which prevents previously reported degradation when exposed to air. These chemically stable thin films enable detailed analysis of the intrinsic thermoelectric properties and form a significant progress towards applications. Single phase NaxCoO2 thin films with a low surface roughness are grown by pulsed laser deposition with either an epitaxial or textured crystal structure on Al2O3(001) or LaAlO3(001) single crystal substrates, respectively. For textured thin films a resistivity and thermopower of 0.99 mΩ cm and 69 μV K−1 are observed at room temperature, respectively. Based on an estimated thermal conductivity for thin films, the dimensionless figure of merit is very comparable to NaxCoO2 single crystals, demonstrating the effectiveness of the developed capping layer.