Non-parabolic Band Effect of ZnO Enhanced by Hybridization with Complex Point Defect Donors for Boosting Thermoelectric Conversion

The simultaneous increase of Seebeck coefficient and electrical conductivity is vital for high thermoelectric performance. With Fermi level upshift in the non-parabolic band, the effective mass increases, bringing relatively high Seebeck coefficient in addition to higher electrical conductivity. However, it is difficult to introduce many carriers for a drastic Fermi level upshift because of the solubility limit of dopants. Here, we introduce many native donor-type point defects in ZnO films by growth orientation control, influencing strain and crystallinity. This introduction not only raises the Fermi level but also enhances band non-parabolicity via hybridization with defect levels, resulting in a higher effective mass. This led to a simultaneous realization of high electrical conductivity and Seebeck coefficient, which is a non-parabolic band effect. The ZnO films with this effect exhibited a thermoelectric power factor of 11.8 μW cm-1 K-2 at 300 K, which is the highest among all the transparent thermoelectric thin film materials ever reported. This method provides a new approach to increase the thermoelectric power factor.