Possibilities to improve the Seebeck coefficient S versus electrical conductance G trade-off of diffusive composite nano-structures are explored using an electro-thermal simulation framework based on the non-equilibrium Green’s function method for quantum electron transport and the lattice heat diffusion equation. We examine the role of the grain size d, potential barrier height UB, grain doping, and the lattice thermal conductivity jL using a one-dimensional model structure. For a uniform jL, simulation results show that the power factor of a composite structure may be improved over bulk with the optimum UB being about kBT, where kB and T are the Boltzmann constant and the temperature, respectively. An optimum UB occurs because the current flow near the Fermi level is not obstructed too much while S still improves due to barriers. The optimum grain size dopt is significantly longer than the momentum relaxation length kp so that G is not seriously degraded due to the barriers, and dopt is comparable to or somewhat larger than the energy relaxation length kE so that the carrier energy is not fully relaxed within the grain and jSj remains high. Simulation results also show that if jL in the barrier region is smaller than in the grain, S and power factor are further improved. In such cases, the optimum UB and dopt increase, and the power factor may improve even for UB (d) significantly higher (longer) than kBT (kE). We find that the results from this quantum mechanical approach are readily understood using a simple, semi-classical model.
Date of this Version
Computational study of energy filtering effects in one-dimensional composite nano-structures. Raseong Kim, and Mark S. Lundstrom. J. Appl. Phys. 111, 024508 (2012)