The Role of Electron-Phonon Interaction in Heavily Doped Fine-Grained Bulk Silicons as Thermoelectric Materials

High thermal conductivity of silicon limits its application prospect in thermoelectric technology for direct thermal to electrical energy conversion. Nanostructuring has been demonstrated to be an effective approach for significantly reducing lattice thermal conductivity of silicon and hence improving thermoelectric figure of merit zT due to the enhanced phonon scattering at boundaries. Here, it is shown that in fine‐grained (≈800 nm) heavily doped bulk silicon with optimized carrier concentration, electron–phonon scattering also plays an important role in the phonon transport in silicon above room temperature, and contributes with a ≈36% reduction in lattice thermal conductivity of heavily doped Si0.94P0.06 at room temperature. Benefiting from the sharp decline of the lattice thermal conductivity, the zT value of the samples increases by a factor of ≈3 compared with the single‐crystal silicon. The results can also be extended to other high efficiency thermoelectric materials with high optimal carrier concentration for understanding and optimizing phonon transport and thermoelectric performance. It is experimentally demonstrated that electron–phonon scattering, in addition to boundary scattering and point defect scattering, plays an important role in phonon transport even above room temperature in fine‐grained heavily doped polycrystalline silicons. This deepens the understanding of the origin of reduced thermal conductivity in heavily doped fine‐grained Si.