Title: A micrometer-thick oxide film with high thermoelectric performance at temperature ranging from 20-400 K,
Abstract: Thermoelectric (TE) materials achieve localised conversion between thermal
and electric energies, and the conversion efficiency is determined by a figure
of merit zT. Up to date, two-dimensional electron gas (2DEG) related TE
materials hold the records for zT near room-temperature. A sharp increase in zT
up to ~2.0 was observed previously for superlattice materials such as PbSeTe,
Bi2Te3/Sb2Te3 and SrNb0.2Ti0.8O3/SrTiO3, when the thicknesses of these TE
materials were spatially confine within sub-nanometre scale. The
two-dimensional confinement of carriers enlarges the density of states near the
Fermi energy3-6 and triggers electron phonon coupling. This overcomes the
conventional {\sigma}-S trade-off to more independently improve S, and thereby
further increases thermoelectric power factors (PF=S2{\sigma}). Nevertheless,
practical applications of the present 2DEG materials for high power energy
conversions are impeded by the prerequisite of spatial confinement, as the
amount of TE material is insufficient. Here, we report similar TE properties to
2DEGs but achieved in SrNb0.2Ti0.8O3 films with thickness within sub-micrometer
scale by regulating interfacial and lattice polarizations. High power factor
(up to 103 {\mu}Wcm-1K-2) and zT value (up to 1.6) were observed for the film
materials near room-temperature and below. Even reckon in the thickness of the
substrate, an integrated power factor of both film and substrate approaching to
be 102 {\mu}Wcm-1K-2 was achieved in a 2 {\mu}m-thick SrNb0.2Ti0.8O3 film grown
on a 100 {\mu}m-thick SrTiO3 substrate. The dependence of high TE performances
on size-confinement is reduced by ~103 compared to the conventional
2DEG-related TE materials. As-grown oxide films are less toxic and not
dependent on large amounts of heavy elements, potentially paving the way
towards applications in localised refrigeration and electric power generations.