Decouple Electronic and Phononic Transport in Nanotwinned Structure: A New Strategy for Enhancing the Figure-of-merit of Thermoelectrics
Thermoelectrics (TE) materials manifest themselves in direct conversion of temperature differences to electric power and vice versa. Despite remarkable advances have been achieved in the past decades for various TE systems, the energy conversion efficiencies of TE devices, which is characterized by a dimensionless figure-of-merit (ZT ), remain a generally poor factor that severely limits their competitiveness and range of employment. The bottleneck for substantially boosting ZT coefficient lies in the strong interdependence of the physical parameters involved in electronic and phononic transport. Here, we propose a new strategy of incorporating nanotwinned structures to decouple the electronic and phononic transport. Combining the new concept of nanotwin with the previously widely used nanocrystalline approach, the power factor of the Si nanotwin-nanocrystalline heterostructures is enhanced by 120% compared to bulk crystalline Si, while the lattice thermal conductivity is reduced to a level well below the amorphous limit, yielding a theoretical limit of 0.43 for ZT coefficient at room temperature. This value is almost two orders of magnitude larger than that for bulk Si and twice of the polycrystalline Si. Even for the experimentally existing nanotwin-nanocrystalline heterostructures (e.g. grain size of 5 nm), the ZT coefficient can be as high as 0.2 at room temperature, which is the highest ZT value among all the Si based bulk nanostructures so far. Such substantial improvement stems from two aspects: (1) the improvement of the power factor is caused by the increase of Seebeck coefficient (degeneracy of the band valley) and the enhancement of electrical conductivity (the reduction of the effective band mass); (2) the significant reduction of the lattice thermal conductivity is mainly caused by the extremely strong phonon-grain boundary and phonon-twin boundary scattering.