Synergy between defects, charge neutrality and energy filtering in hyper-doped nanocrystalline materials for high thermoelectric efficiency.

Breaking the conventional decrease of the Seebeck coefficient with increasing conductivity would be a significant advancement towards large thermoelectric power factor enhancement and high thermoelectric efficiency. We report on a mechanism identified in hyper-doped nanocrystalline Si films that can lead to this task: a transition from dominant ionized impurity scattering to dominant phonon scattering upon thermal annealing at a high annealing temperature Ta that takes place to fulfill charge neutrality. We show that the synergy between charge neutrality and energy filtering activated by thermal annealing of the originally defective nanocrystalline sample leads to high mobility, simultaneous increase of the conductivity and the Seebeck coefficient and large enhancement of the thermoelectric power factor. This is demonstrated by means of advanced theoretical modeling and excellent quantitative agreement with the experiment. Our work provides interpretation of so far not understood observations in nanocrystalline Si and indicates a new route for engineering Si as well as other nanostructured materials for high thermoelectric efficiency.