Photo-Seebeck effect of ZnO single crystals and thin films

We have investigated the thermoelectric properties of ZnO single crystals and thin films under illumination. In both samples, the electrical conductivity and the Seebeck coefficient are varied under ultraviolet light ($h\nu$ = 3.4 eV) while a negligible change is observed under visible green light ($h\nu$ = 2.4 eV), indicating a carrier excitation across the band gap of ZnO ($E_g\sim$ 3.3 eV) by the ultraviolet illumination. This phenomenon thus can be ascribed to a photo doping effect into thermoelectric materials [1]. The carrier concentration doped by illumination is estimated to be about 10$^{19}$ cm$^{-3}$, which is close to the optimal value for conventional thermoelectrics, suggesting a possible optical control of the thermoelectric efficiency. We also investigate the sample thickness dependence of the photo-Seebeck effect in ZnO thin films, whose thickness is comparable to the absorption length of ultraviolet light. These results are compared with the bulk sample results in terms of a parallel-circuit model consisting of photo-excited metallic and unexcited insulating layers. \\[4pt] [1] R. Okazaki {\it et al}., J. Phys. Soc. Jpn. {\bf 81}, 114722 (2012).