Thermoelectric band engineering: The role of carrier scattering

Witkoske, Evan; Wang, Xufeng; Lundstrom, Mark; Askarpour, Vahid; Maassen, Jesse

doi:10.1063/1.4994696

Cited by 53 publications

(62 citation statements)

References 46 publications

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“…Our rigorous el-ph scattering calculations show that, in the case of GeTe, both the scattering rates and MFP are energy-dependent. An alternative simple model, that appears to work well, assumes the scattering rate is proportional to the electron DOS [49,50].…”

Section: Electron-phonon Scattering Rates and Mean-free-pathsmentioning

confidence: 99%

Unusual thermoelectric transport anisotropy in quasi-two-dimensional rhombohedral GeTe

Askarpour

Maassen

2019

Phys. Rev. B

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In this study, we calculate the T = 300 K scattering and thermoelectric transport properties of rhombohedral GeTe using first-principles modeling. The room-temperature phase of GeTe has a layered structure, with cross-plane and in-plane directions oriented parallel and perpendicular to [111], respectively. Based on rigorous electron-phonon scattering, our transport calculations reveal unusual anisotropic properties; n-type GeTe has a cross-plane electrical conductivity that is roughly 3× larger than in-plane. p-type GeTe, however, displays opposite anisotropy with inplane conducting roughly 2× more than cross-plane, as is expected in quasi-2D materials. The power factor shows the same anisotropy as the electrical conductivity, since the Seebeck coefficient is relatively isotropic. Interestingly, cross-plane n-GeTe shows the largest mobility and power factor approaching 500 cm 2 /V-s and 32 µW/cm-K 2 , respectively. The thermoelectric figure-of-merit, zT , is enhanced as a result of this unusual anisotropy in n-GeTe since the lattice thermal conductivity is minimized along cross-plane. This decouples the preferred transport directions of electrons and phonons, leading to a threefold increase in zT along cross-plane compared to in-plane. The n-type anisotropy results from high-velocity electron states formed by Ge p-orbitals that span across the interstitial region. This surprising behavior, that would allow the preferential conduction direction to be controlled by doping, could be observed in other quasi-2D materials and exploited to achieve higher-performance thermoelectrics. * Electronic address: jmaassen@dal.ca arXiv:1904.06439v2 [cond-mat.mtrl-sci]

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Section: Electron-phonon Scattering Rates and Mean-free-pathsmentioning

confidence: 99%

Unusual thermoelectric transport anisotropy in quasi-two-dimensional rhombohedral GeTe

Askarpour

Maassen

2019

Phys. Rev. B

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“…Simply, while aligning the bands can increase the number of carriers available for conduction, from simple Fermi's Golden Rule considerations, it can also increase the number of states that carriers scatter into, which hinders the carrier transport. Therefore, the energy dependence of the scattering mechanisms, as well as the specifics of intra-or inter-valley scattering considerations are important in identifying if a given bandstructure engineering approach leads to an improved power factor, or not [43][44][45] .…”

Section: Introductionmentioning

confidence: 99%

Band alignment and scattering considerations for enhancing the thermoelectric power factor of complex materials: The case of Co-based half-Heusler alloys

Kumarasinghe

Neophytou

2019

Phys. Rev. B

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Producing high band and valley degeneracy through aligning of conducting electronic bands is an effective strategy to improve the thermoelectric performance of complex bandstructure materials. Half-Heuslers, an emerging thermoelectric material group, has complex bandstructures with multiple bands that can be aligned through band engineering approaches, giving us an opportunity to improve their power factor. Theoretical calculations to identify the outcome of band engineering usually employ detailed density functional theory for bandstructure calculations, but the transport calculations are kept simplistic using the constant relaxation time approximation due to the complications involved with detailed scattering physics. In this work, going beyond the constant relaxation time approximation, we perform an investigation of the benefits of band alignment in improving the thermoelectric power factor under different density of states dependent scattering scenarios. As a test case we consider the Co-based p-type half-Heuslers TiCoSb, NbCoSn and ZrCoSb. First, using simplified effective mass models combined with Boltzmann transport, we investigate the conditions of band alignment that are beneficial to the thermoelectric power factor under three different carrier scattering scenarios: i) the usual constant relaxation time approximation, ii) intra-band scattering restricted to the current valley with the scattering rates proportional to the density of states as dictated by Fermi's Golden Rule, and iii) both intra-and inter-band scattering across all available valleys, with the rates determined by the total density of states at the relevant energies. We demonstrate that the band-alignment outcome differs significantly depending on the scattering details. Next, using the density functional theory calculated bandstructures of the half-Heuslers we study their power factor behavior under strain induced band alignment. We show that strain can improve the power factor of half-Heuslers, but the outcome heavily depends on the curvatures of the bands involved, the specifics of the carrier scattering mechanisms and the initial band separation. Importantly, we also demonstrate that band alignment is not always beneficial to the power factor. In addition, we show that the bandstructure itself can undergo changes as the bands are aligned in practice, which further affect the band alignment optimization. Our work illustrates the importance of going beyond the constant relaxation time approximation, as well as understanding how the bandstructure of each material behaves when considering band alignment. arXiv:1905.07951v1 [cond-mat.mtrl-sci]

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“…and the transport distribution in the diffusive limit written in the Landauer form 48 , 49 for a short derivation of (2e) and 37 for a longer discussion). In (2e), the mean--free--path for backscattering is defined as 37 λ E…”

Section: A Landauer Transport Methodsmentioning

confidence: 99%

The use of strain to tailor electronic thermoelectric transport properties: A first principles study of 2H-phase CuAlO₂

Witkoske

Guzmán

Feng

et al. 2019

Journal of Applied Physics

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Using first principles calculations, the use of strain to adjust electronic transport and the resultant thermoelectric (TE) properties is discussed using 2H phase CuAlO2 as a test case. Transparent oxide materials, such as CuAlO2, a p--type transparent conducting oxide (TCO), have recently been studied for high temperature thermoelectric power generators and coolers for waste heat. Given TCO materials with relative ease of fabrication, low cost of materials, and non-toxicity, the ability to tailor them to specific temperature ranges, power needs, and size requirements, through the use of strain opens an interesting avenue. We find that strain can have a significant effect on these properties, in some cases detrimental and in others beneficial, including the potential for n--type power factors larger than the highest p--type case. The physical reasons for this behavior are explained in the terms of the thermoelectric transport distribution and the Landauer distribution of modes. I. IntroductionThermoelectric (TE) devices and materials hold great promise for broad use in solid--state energy generation and solid--state cooling. However, as robust and reliable as these devices are, they have been limited by low conversion efficiencies since their inception 1-5 . The past three decades have witnessed the thermoelectric material figure of merit, zT, improved from under one to over two 5 . These gains have been primarily driven by a reduction in the lattice thermal conductivity of materials and devices through the use of nano--structuring 6-12 and the development of novel materials that have an inherently low thermal conductivity due to large discrepancies in the masses of their constituent elements. These advances, however, have not translated into working devices 13 . As we approach the lower limit of the lattice thermal conductivity for common and even complex TE materials at room temperature and above, the variety of avenues capable of moving the field of thermoelectrics forward are being narrowed, therefore ideas that have the potential to advance the field need to be explored carefully. In this paper we look at an alternate route forward, given materials with relative ease of fabrication, low cost, and non--toxicity, the ability to tailor them to specific

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Thermoelectric band engineering: The role of carrier scattering

Cited by 53 publications

References 46 publications

Unusual thermoelectric transport anisotropy in quasi-two-dimensional rhombohedral GeTe

Unusual thermoelectric transport anisotropy in quasi-two-dimensional rhombohedral GeTe

Band alignment and scattering considerations for enhancing the thermoelectric power factor of complex materials: The case of Co-based half-Heusler alloys

The use of strain to tailor electronic thermoelectric transport properties: A first principles study of 2H-phase CuAlO₂

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Thermoelectric band engineering: The role of carrier scattering

Cited by 53 publications

References 46 publications

Unusual thermoelectric transport anisotropy in quasi-two-dimensional rhombohedral GeTe

Unusual thermoelectric transport anisotropy in quasi-two-dimensional rhombohedral GeTe

Band alignment and scattering considerations for enhancing the thermoelectric power factor of complex materials: The case of Co-based half-Heusler alloys

The use of strain to tailor electronic thermoelectric transport properties: A first principles study of 2H-phase CuAlO2

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The use of strain to tailor electronic thermoelectric transport properties: A first principles study of 2H-phase CuAlO₂