ELECTRONIC TRANSPORT PROPERTIES OF NI-DOPED ZNO FOR THERMOELECTRIC APPLICATIONS: COMPUTATIONAL AND EXPERIMENTAL APPROACHES

Zinc oxide is an important thermoelectric (TE) compound for waste heat harvesting at high temperatures. Electrical transport properties of Ni-doped ZnO compounds are studied applying the density functional theory (DFT). The electrical conductivity, σ, Seebeck coefficient, S, and electronic thermal conductivity, κ_e, were calculated in the framework of the Boltzmann transport theory applying the relaxation time (τ) approximation; all magnitudes were measured experimentally, as well. Comparison between both datasets provides us with both temperature- and concentration- dependences of τ. It is shown that nickel doping causes formation of d-electron resonance peak in the vicinity of Fermi level (E_F), which results in increase of σ and decrease of |S| with increasing Ni concentration, in full accordance with experimental results. It is also shown that for low temperatures τ values correlate with Ni concentration, whereas above 500 K τ is dominated by thermal processes. The change of Ni concentration from 2.08 to 6.25 at. % results in three times increase of τ. Simultaneously, the difference between low temperature and high temperature τ values range up to two orders of magnitude for all compounds in the series. The temperature dependence of the Lorenz number, L, is evaluated from calculated κ_e(T) and σ(T) dependences.

Our results reveal that the commonly used constant relaxation time approximation does not always provide adequate depiction of the electronic transport properties in the presence of resonance impurity levels, and develop an alternative approach.