  Modulation of the grain boundary properties in thermoelectric materials that
have thermally activated electrical conductivity is crucial in order to achieve
high performance at low temperatures. In this work, we show directly that the
modulation of the potential barrier at the grain boundaries in perovskite
SrTiO3 changes the low-temperature dependency of the bulk material's electrical
conductivity. By sintering samples in a reducing environment of increasing
strength, we produced La0.08Sr0.9TiO3 (LSTO) ceramics that gradually change
their electrical conductivity behavior from thermally activated to
single-crystal-like, with only minor variations in the Seebeck coefficient.
Imaging of the surface potential by Kelvin probe force microscopy found lower
potential barriers at the grain boundaries in the LSTO samples that had been
processed in the more reducing environments. A theoretical model using the band
offset at the grain boundary to represent the potential barrier agreed well
with the measured grain boundary potential dependency of conductivity. The
present work showed an order of magnitude enhancement in electrical
conductivity (from 85 to 1287 S cm-1) and power factor (from 143 to 1745 {\mu}W
m-1 K-2) at 330 K by this modulation of charge transport at grain boundaries.
This significant reduction in the impact of grain boundaries on charge
transport in SrTiO3 provides an opportunity to achieve the ultimate 'phonon
glass electron crystal' by appropriate experimental design and processing.
