Enhanced thermoelectric performance of Ga-doped ZnO film by controlling crystal quality for transparent thermoelectric films

ZnO, a wide bandgap (3.3 eV) semiconductor has been expected to be a transparent thermoelectric material for the purpose of energy harvesting application because it is a low-cost and ubiquitous element material with a high optical transmittance and a high power factor. Bulk Ga-doped ZnO (GZO) is expected to have higher electrical conductivity and lower thermal conductivity than bulk Al-doped ZnO. However, because reports on their thermoelectric properties of GZO films have been scarce up to now, it has been unclear what film characters affect the thermoelectric properties effectively. In this work, GZO thin films with different characters (c-axis orientation, crystal domains and the domain interfaces, and carrier activation rate) were fabricated by two different methods, sol-gel method and pulsed laser deposition. All samples have optical transmittance over 80% in visible range. The highly-oriented GZO films exhibit the highest power factors up to 2.8 μWcm−1 K−2 in the reported GZO materials and low thermal conductivities of 8.4 Wm−1 K−1 (1/4 as high as that of bulk GZO). This enhanced thermoelectric performance is attributed to the high carrier activation rate, and the interfaces of highly-oriented crystal domains with the small carrier scattering effect.