  Transition metal oxides are promising candidates for thermoelectric
applications, because they are stable at high temperature and because strong
electronic correlations can generate large Seebeck coefficients, but their
thermoelectric power factors are limited by the low electrical conductivity. We
report transport measurements on Ca3Co4O9 films on various perovskite
substrates and show that reversible incorporation of oxygen into SrTiO3 and
LaAlO3 substrates activates a parallel conduction channel for p-type carriers,
greatly enhancing the thermoelectric performance of the film-substrate system
at temperatures above 450 {\deg}C. Thin-film structures that take advantage of
both electronic correlations and the high oxygen mobility of transition metal
oxides thus open up new perspectives for thermopower generation at high
temperature.
