Optimizing Electronic Quality Factor toward High‐Performance Ge1−x−yTaxSbyTe Thermoelectrics: The Role of Transition Metal Doping

Abstract

Owing to high intrinsic figure-of-merit implemented by multi-band valleytronics, GeTe-based thermoelectric materials are promising for medium-temperature applications. Transition metals are widely used as dopants for developing high-performance GeTe thermoelectric materials. Herein, relevant work is critically reviewed to establish a correlation among transition metal doping, electronic quality factor, and figure-of-merit of GeTe. From first-principle calculations, it is found that Ta, as an undiscovered dopant in GeTe, can effectively converge energy offset between light and heavy conduction band extrema to enhance effective mass at high temperature. Such manipulation is verified by the increased Seebeck coefficient of synthesized Ge1−x−yTaxSbyTe samples from 160 to 180 µV K⁻¹ at 775 K upon doping Ta, then to 220 µV K⁻¹ with further alloying Sb. Characterization using electron microscopy also reveals the unique herringbone structure associated with multi-scale lattice defects induced by Ta doping, which greatly hinder phonon propagation to decrease thermal conductivity. As a result, a figure-of-merit of ≈2.0 is attained in the Ge0.88Ta0.02Sb0.10Te sample, reflecting a maximum heat-to-electricity efficiency up to 17.7% under a temperature gradient of 400 K. The rationalized beneficial effects stemming from Ta doping is an important observation that will stimulate new exploration toward high-performance GeTe-based thermoelectric materials.

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2102575 (1 of 8) © 2021 Wiley-VCH GmbH
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ReseaRch aRticle
Optimizing Electronic Quality Factor toward High-
Performance Ge1−x−yTaxSbyTe Thermoelectrics: The Role
of Transition Metal Doping
Meng Li, Qiang Sun, Sheng-Duo Xu, Min Hong, Wan-Yu Lyu, Ji-Xing Liu, Yuan Wang,
Matthew Dargusch, Jin Zou,* and Zhi-Gang Chen*
M. Li, Dr. Q. Sun, S.-D. Xu, Dr. Y. Wang, Prof. M. Dargusch,
Prof. J. Zou, Prof. Z.-G. Chen
School of Mechanical and Mining Engineering
The University of Queensland
Brisbane, Queensland 4072, Australia
E-mail: j.zou@uq.edu.au; zhigang.chen@uq.edu.au
Dr. M. Hong, W.-Y. Lyu, Prof. Z.-G. Chen
Centre for Future Materials
University of Southern Queensland
Springfield Central, Queensland 4300, Australia
E-mail: zhigang.chen@usq.edu.au
Dr. J.-X. Liu
Superconducting Materials Research Centre
Northwest Institute for Nonferrous Metal Research
Xi’an 710016, China
Prof. J. Zou
Centre for Microscopy and Microanalysis
The University of Queensland
Brisbane, Queensland 4072, Australia
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202102575.
DOI: 10.1002/adma.202102575
1. Introduction
Thermoelectrics, enabling direct conver-
sion from ambient waste heat to elec-
tricity, have attracted increasing interest
from both academia and industry as an
important part of green energy tech-
nology.[1] The working mechanism of a
thermoelectric device is based on an irre-
versible thermodynamic process where
electrical potential dierence is created by
a temperature gradient, thus outputting
power to a connected circuit.[2] Funda-
mentally, it is the distortion of the Fermi
surface of constituent materials under dif-
ferent temperature.[3] Therefore, the heat-
to-electricity eciency can be estimated by
the dimensionless figure-of-merit (ZT) of
thermoelectric materials, defined as:
2
ZT
ST
σ
κ
= (1)
with S, σ, and κ being the Seebeck coef-
ficient, electrical conductivity, and thermal
conductivity (sum of electron κe and lat-
tice κl components), respectively. Obvi-
ously, good thermoelectric materials favor a combined high
power factor (PF = S2σ) and low κ, corresponding to transport
of electrons and phonons.[4] However, several trade-os among
these terminologies restrict an infinitely enhanced ZT, thus
there is a need to compromise.[5] For instance, an increase in
σ usually degrades S due to competition on carrier concentra-
tion (n, mediated by electron ne or hole nh), while decreasing κl
by nanostructuring may also intensify carrier scattering to sup-
press carrier mobility (µ), and thus reduce σ.[6]
Rock-salt chalcogenides, such as SnTe[7] and PbTe,[8] are the
most investigated and applied mid-temperature thermoelectric
materials. However, high toxicity or poor abundance of these
materials are conflicting with the benchmark of green energy
promotion, making GeTe an alternative candidate.[9] As shown in
Figure1a, pristine GeTe undergoes a reversible transition from
the high-temperature cubic phase (C-, 3
Fm
m) to low-temper-
ature rhombohedral phase (R-, R3m) at around 700 K, featured
by an elongated diagonal axis and ferroelectric displacement of
the central site. Reflected in reciprocal space, C-GeTe is in typical
rock-salt coordinates and shares many similarities with analogous
Owing to high intrinsic figure-of-merit implemented by multi-band valley-
tronics, GeTe-based thermoelectric materials are promising for medium-
temperature applications. Transition metals are widely used as dopants for
developing high-performance GeTe thermoelectric materials. Herein, relevant
work is critically reviewed to establish a correlation among transition metal
doping, electronic quality factor, and figure-of-merit of GeTe. From first-prin-
ciple calculations, it is found that Ta, as an undiscovered dopant in GeTe, can
eectively converge energy oset between light and heavy conduction band
extrema to enhance eective mass at high temperature. Such manipulation
is verified by the increased Seebeck coecient of synthesized Ge1−x−yTaxSbyTe
samples from 160 to 180 µV K−1 at 775 K upon doping Ta, then to 220 µV K−1
with further alloying Sb. Characterization using electron microscopy also
reveals the unique herringbone structure associated with multi-scale lattice
defects induced by Ta doping, which greatly hinder phonon propagation to
decrease thermal conductivity. As a result, a figure-of-merit of ≈2.0 is attained
in the Ge0.88Ta0.02Sb0.10Te sample, reflecting a maximum heat-to-electricity
eciency up to 17.7% under a temperature gradient of 400 K. The rationalized
beneficial eects stemming from Ta doping is an important observation that
will stimulate new exploration toward high-performance GeTe-based thermo-
electric materials.
Adv. Mater. 2021, 33, 2102575