Modulation of Charge Transport at Grain Boundaries in SrTiO₃: Toward a High Thermoelectric Power Factor at Room Temperature

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 La_0.08Sr_0.9TiO₃ (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 μW 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.