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Modulation of the grain boundary properties in thermoelectric materials thathave thermally activated electrical conductivity is crucial in order to achievehigh performance at low temperatures. In this work, we show directly that themodulation of the potential barrier at the grain boundaries in perovskiteSrTiO3 changes the low-temperature dependency of the bulk material's electricalconductivity. By sintering samples in a reducing environment of increasingstrength, we produced La0.08Sr0.9TiO3 (LSTO) ceramics that gradually changetheir electrical conductivity behavior from thermally activated tosingle-crystal-like, with only minor variations in the Seebeck coefficient.Imaging of the surface potential by Kelvin probe force microscopy found lowerpotential barriers at the grain boundaries in the LSTO samples that had beenprocessed in the more reducing environments. A theoretical model using the bandoffset at the grain boundary to represent the potential barrier agreed wellwith the measured grain boundary potential dependency of conductivity. Thepresent work showed an order of magnitude enhancement in electricalconductivity (from 85 to 1287 S cm-1) and power factor (from 143 to 1745 {\mu}Wm-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 chargetransport in SrTiO3 provides an opportunity to achieve the ultimate 'phononglass electron crystal' by appropriate experimental design and processing.

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