Nanoporous Si as an Efficient Thermoelectric Material

Abstract

Room-temperature thermoelectric properties of n-type crystalline Si with periodically arranged nanometer-sized pores are computed using a combination of classical molecular dynamics for lattice thermal conductivity and ab initio density functional theory for electrical conductivity, Seebeck coefficient, and electronic contribution to the thermal conductivity. The electrical conductivity is found to decrease by a factor of 2-4, depending on doping levels, compared to that of bulk due to confinement. The Seebeck coefficient S yields a 2-fold increase for carrier concentrations less than 2 x 10(19) cm(-3), above which S remains closer to the bulk value. Combining these results with our calculations of lattice thermal conductivity, we predict the figure of merit ZT to increase by 2 orders of magnitude over that of bulk. This enhancement is due to the combination of the nanometer size of pores which greatly reduces the thermal conductivity and the ordered arrangement of pores which allows for only a moderate reduction in the power factor. We find that while alignment of pores is necessary to preserve power factor values comparable to those of bulk Si, a symmetric arrangement is not required. These findings indicate that nanoporous semiconductors with aligned pores may be highly attractive materials for thermoelectric applications.

Combination of Electrodeposition and Transfer Processes for Flexible Thin-Film Thermoelectric Generators

“…In addition, thin-film thermoelectric materials possess favorable features not exhibited by bulk materials. The presence of nanostructured materials, including superlattices [20][21][22], nanocrystals [23][24][25], nanoporous structures [26][27][28], and inducing stresses [29][30][31], enhances thermoelectric performance. Thermoelectric performance is defined as the figure of merit, ZT = S 2 σT/κ, where S is the Seebeck coefficient, σ is electrical conductivity, T is the absolute temperature, and κ is thermal conductivity.…”

Thermal transport in 2- and 3-dimensional periodic “holey” nanostructures

“…Clearly, at a small enough feature size, the distribution function for phonons transporting heat in the structure would depend on the specific geometry of the structure and would not be the same as in the bulk. The most detailed consideration of transport in periodic structures till date involves modeling phonons through either atomistic simulations [23][24][25] or tracking their trajectories through Monte Carlo simulations. 9,10 We review basic phonon scattering physics here and discuss specific calculations later in Sec.…”

“…The main conclusion from these studies was that sub-continuum transport is indeed an important factor to consider in evaluating thermal conductivity of these structures. Atomistic simulations [23][24][25] provided more insight into the role of various phonon scattering mechanisms, particularly surface scattering, and FIG. 3.…”

“…Further, porosity also plays an important role in reducing the effective thermal conductivity, which can be explained by various effective medium theories mentioned in previous section. Theoretical approaches to model phonon transport in porous silicon include, for example, solution of the BTE by the discrete ordinate method, 33 3D Monte-Carlo simulations, 58 molecular dynamic simulation 23,24 and a combination of analytical and phonon-tracking methods. 59 It remains a challenge to clearly understand thermal transport in a non-periodic porous structure.…”

Role of interlayer coupling for the power factor ofCuSbS2andCuSbSe2

“…Only few materials exist with ZT 1 before nanostructuring and/or optimization of the carrier concentration to increase S 2 σ [11,12]. Nanostructuring is an effective tool to boost the thermoelectric performance, because κ l can be reduced by enhancing the phonon scattering [13,14]. For the same reason, two-dimensional semiconductors are promising materials for thermoelectric devices [15][16][17].…”