The Fragility of Thermoelectric Power Factor in Cross-Plane Superlattices in the Presence of Nonidealities: A Quantum Transport Simulation Approach

Abstract

Original citation: Thesberg, M., Pourfath, M., Neophytou, Neophytos and Kosina, H.. (2016) The fragility of thermoelectric power factor in cross-plane superlattices in the presence of nonidealities : a quantum transport simulation approach. Journal of Electronic Materials, 45 (3). pp. 1584-1588. Permanent WRAP url:http://wrap.warwick.ac.uk/77432 Copyright and reuse:The Warwick Research Archive Portal (WRAP) makes this work by researchers of the University of Warwick available open access under the following conditions. Copyright © and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable the material made available in WRAP has been checked for eligibility before being made available.Copies of full items can be used for personal research or study, educational, or not-for profit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. Publisher's statement:"The final publication is available at Springer via http://dx.doi.org/10.1007/s11664-015-4124-7 ." A note on versions:The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher's version. Please see the 'permanent WRAP url' above for details on accessing the published version and note that access may require a subscription. AbstractEnergy filtering has been put forth as a promising method for achieving large thermoelectric power factors in thermoelectric materials through Seebeck coefficient improvements. Materials with embedded potential barriers, such as cross-plane superlattices provide energy filtering, in addition to low thermal conductivities, and could potentially achieve high figures of merit. Although there exist many theoretical works demonstrating Seebeck coefficient and power factor gains in idealized structures, experimental support has been scant. In most cases the electrical conductivity is drastically reduced due to the presence of barriers. In this work, using quantum mechanical simulations based on the Non-Equilibrium Green's Function method, we show that although power factor improvements can theoretically be observed in 1 optimized superlattices (something pointed out in previous studies), different types of deviations from the ideal potential profiles of the barriers degrade the performance. Some non-idealities being so significant as to negate all power factor gains. Specifically, the effect of tunneling due to thin barriers could be especially detrimental to the Seebeck coefficient and the power factor. Our results could partially explain why significant power factor improvements in superlattices and other energy filtering nanostructures mainly fail to be realized, despite theoretical predictions.

Thermoelectric power factor of nanocomposite materials from two-dimensional quantum transport simulations

“…An important effect that needs to be considered in evaluating the influence of nanoinclusions on the power factor of nanocomposites is quantummechanical tunneling. In prior works related to the effect of tunneling in superlattices, we have shown that tunneling is detrimental to the Seebeck coefficient as it makes potential barriers transparent and takes away any benefits that energy filtering could provide to the power factor [65,66]. We have shown that in the case of superlattices tunneling becomes important when the thicknesses of the barriers drop below 1-2 nm (for channels with effective mass m * = m 0 ).…”

“…The presence of impurity scattering as well as variation in the thermal conductivities of the different regions can also improve the Seebeck coefficient even further as explained in Refs. [21,58,65,66], which might allow for higher power factors compared to what we compute here.…”

“…The behavior in the presence of NIs is different, because the charge carriers flow not only above the NI barriers, but in between them as well. This means the NIs still allow a finite transmission of carriers across low energies, and thus, do not provide the energy filtering effect and large Seebeck coefficients that can be achieved in superlattices [58,65,66].…”

“…Another important variability case that is beneficial to the power factor is the variation in the lattice thermal conductivity between the different materials that form the nanocomposite. In superlattices, for example, a lower lattice thermal conductivity in the barrier regions which have a higher local Seebeck coefficient, results in a larger overall increase in the Seebeck coefficient [21,55,65]. We have not investigated this effect here, however it might be the case that such an effect might not be utilized strongly for NIs as their filtering capabilities are weaker.…”

“…In reality, in the vicinity of the heterojunction there will be potential variations that will affect the shape of the barrier, which as we show in Refs. [64] and [65], could be important in determining the PF. These potential variations are determined by the junction details, but also by the doping of the different regions and could only be captured accurately through self-consistent calculations, which we do not consider in this work.…”