Comparison of structural and electric properties of ZnO-based n-type thin films with different dopants for thermoelectric applications

This paper presents a study on ZnO-based n-type thin films doped with Al, Ga, and Sn deposited on the glass substrates, using the solution derived by sol–gel reactions. The basic material properties associated with using the alternative chemical substances in synthesis process and different dopants to generate n-type conductivity in ZnO-based thin films which are transparent oxides, are investigated and discussed. Namely, the crystal structure and surface morphology of the obtained films were examined by means of XRD analysis and field-effect scanning electron microscope (FESEM). The electric properties of those films were characterized by Hall-effect measurements and temperature dependence of electrical conductivity, as well as Seebeck coefficients. The remarkable advantages corresponding to the certain characterization of each material composition were compared together. The aim is to fully understand the performance limitation of known materials and to set the scene for a suitable synthesis condition to get optimized materials for thermoelectric applications. Highlights ZnO thin films doped with Al, Ga and Sn in the range from 1 to 3 mol % were fabricated by dip coating technique using the solutions derived by sol–gel reactions. The grain sizes and shapes together with the grain boundary were affected by doping contents and different size of doping ions. Namely, the grain size of AZO and GZO samples is smaller than that of ZnO and SZO ones, meanwhile it is scarcely changed with increasing Sn concentration for SZO films. Among samples, Ga doped ZnO demonstrate most improved thermoelectric properties as compared to the Al- and Sn-doped ZnO. Introduction of Sn into ZnO lattice also remarkably increases the electrical parameters at room tempertature and the transport property throughout its temperature-dependent electrical conductivity and Seebeck coefficient.