The annealed-nanograin phase: A route to simultaneous increase of the conductivity and the Seebeck coefficient and high thermoelectric performance

We introduce the annealed-nanograin (a-NG) phase effect and propose it as a new route to high thermoelectric performance. We support that in granular materials with small nanograins, the core of the grains (G-phase) and the grain boundaries (GB-phase) can be electrostatically coupled so that transport is dominated by a single phase, the a-NG phase. We show that concurrent increase in the mobility and the Seebeck coefficient can take place when originally defective nanograins are thermally annealed, because defect repair reduces scatterers in the core of the nanograins and concurrently stimulates more ionized impurities and higher energy barriers at the grain boundaries to fulfill charge neutrality. We compare the a-NG phase with the two phases of a composite grain (the G-phase and the GB-phase) and show that a transition takes place from dominant ionized impurity scattering to dominant phonon scattering. This transition is the signature of the formation of the a-NG phase and the thermoelectric power factor enhancement. Our model has been validated by interpretation of experimental observations in highly B-doped nanocrytalline films. Our findings can be used to engineer nanostructured materials with high thermoelectric performance.