  We examine how the photo-induced carriers contribute the thermoelectric
transport, i.e. the nature of the photo-Seebeck effect, in the wide-gap oxide
semiconductor ZnO for the first time. We measure the electrical conductivity
and the Seebeck coefficient with illuminating light. The light illumination
considerably changes the Seebeck coefficient as well as the conductivity, which
is sensitive to the photon energy of the illuminated light. By using a simple
parallel-circuit model, we evaluate the contributions of the photo-induced
carriers to the conductivity and the Seebeck coefficient, whose relationship
shows a remarkable resemblance to that in doped semiconductors. Our results
also demonstrate that the light illumination increases both the carrier
concentration and the mobility, which can be compared with impurity-doping case
for ZnO. Future prospects for thermoelectrics using light are discussed.
