Engineering semiconductor devices requires an understanding of charge carrier mobility. Typically, mobilities are estimated using Hall effect and electrical resistivity meausrements, which are are ...routinely performed at room temperature and below, in materials with mobilities greater than 1 cm2 V‐1 s‐1. With the availability of combined Seebeck coefficient and electrical resistivity measurement systems, it is now easy to measure the weighted mobility (electron mobility weighted by the density of electronic states). A simple method to calculate the weighted mobility from Seebeck coefficient and electrical resistivity measurements is introduced, which gives good results at room temperature and above, and for mobilities as low as 10−3 cm2 V‐1 s‐1,
μw=331cm2Vs(mΩ cmρ) (T300 K)−3/2 exp |S|kB/e−21+exp−5(|S|kB/e−1) +3π2|S|kB/e1+exp5(|S|kB/e−1) Here, μw is the weighted mobility, ρ is the electrical resistivity measured in mΩ cm, T is the absolute temperature in K, S is the Seebeck coefficient, and kB/e = 86.3 µV K–1. Weighted mobility analysis can elucidate the electronic structure and scattering mechanisms in materials and is particularly helpful in understanding and optimizing thermoelectric systems.
The weighted mobility, easily computed from measurements of the Seebeck coefficient and electrical resistivity, is an accurate measure of the charge carrier mobility and effective mass. It is even more sensitive than measurements of the Hall effect for revealing electron transport mechanisms in complex materials ranging from metals, semiconductors, and conducting polymers.
While the thermoelectric materials figure of merit is a well defined metric to evaluate thermoelectric materials, it can be a poor metric for maximum thermoelectric device efficiency because of the ...temperature dependence of the Seebeck coefficient S , the electrical resistivity ρ , and the thermal conductivity κ where T is the absolute temperature. Historically the field has used a thermoelectric device figure of merit ZT to characterize a device operating between a hot side temperature T h and cold side temperature T c . While there are many approximate methods to calculate ZT from temperature dependent materials properties, an exact method is given here that uses a simple algorithm that can be performed on a spreadsheet calculator. The figure of merit is defined for a thermoelectric generator using the maximum efficiency of the thermoelectric device η calculated from the exact method.
The growing technological importance of conducting polymers makes the fundamental understanding of their charge transport extremely important for materials and process design. Various hopping and ...mobility edge transport mechanisms have been proposed, but their experimental verification is limited to poor conductors. Now that advanced organic and polymer semiconductors have shown high conductivity approaching that of metals, the transport mechanism should be discernible by modelling the transport like a semiconductor with a transport edge and a transport parameter s. Here we analyse the electrical conductivity and Seebeck coefficient together and determine that most polymers (except possibly PEDOT:tosylate) have s = 3 and thermally activated conductivity, whereas s = 1 and itinerant conductivity is typically found in crystalline semiconductors and metals. The different transport in polymers may result from the percolation of charge carriers from conducting ordered regions through poorly conducting disordered regions, consistent with what has been expected from structural studies.
A figure of merit for flexibility Peng, Jun; Snyder, G Jeffrey
Science (American Association for the Advancement of Science),
2019-Nov-08, 2019-11-08, 20191108, Letnik:
366, Številka:
6466
Journal Article
Recenzirano
A thickness-dependent metric allows for comparison of materials for device applications
Flexible materials are widely used in health care, robotics, and other industries, but flexible electronic ...devices require that normally brittle electronic materials become flexible. Stiff materials can become flexible if they are sufficiently thin. Future applications, such as energy systems for the internet-of-things, will likely require new materials where trade-offs in performance and flexibility must be weighed with a metric of flexibility. The yield strain ϵ
y
—that is, how much a material can stretch elastically (still recover its shape) before deforming plastically (stretching it out of shape)—for a given thickness of a material can serve as a figure of merit (FOM) for flexibility (
f
FoM
).
The coupled transport properties required to create an efficient thermoelectric material necessitates a thorough understanding of the relationship between the chemistry and physics in a solid. We ...approach thermoelectric material design using the chemical intuition provided by molecular orbital diagrams, tight binding theory, and a classic understanding of bond strength. Concepts such as electronegativity, band width, orbital overlap, bond energy, and bond length are used to explain trends in electronic properties such as the magnitude and temperature dependence of band gap, carrier effective mass, and band degeneracy and convergence. The lattice thermal conductivity is discussed in relation to the crystal structure and bond strength, with emphasis on the importance of bond length. We provide an overview of how symmetry and bonding strength affect electron and phonon transport in solids, and how altering these properties may be used in strategies to improve thermoelectric performance.
Bonding interactions in thermoelectrics: Chemical bonding concepts and molecular orbital theory are used to understand electronic structures and the electronic and thermal transport in semiconductors. Emphasis is placed on the influence of local bonding interactions, such as bond length and orbital overlap, coordination environment, and the expression of lone‐pairs.
The past two decades have witnessed the rapid growth of thermoelectric (TE) research. Novel concepts and paradigms are described here that have emerged, targeting superior TE materials and higher TE ...performance. These superior aspects include band convergence, “phonon‐glass electron‐crystal”, multiscale phonon scattering, resonant states, anharmonicity, etc. Based on these concepts, some new TE materials with distinct features have been identified, including solids with high band degeneracy, with cages in which atoms rattle, with nanostructures at various length scales, etc. In addition, the performance of classical materials has been improved remarkably. However, the figure of merit zT of most TE materials is still lower than 2.0, generally around 1.0, due to interrelated TE properties. In order to realize an “overall zT > 2.0,” it is imperative that the interrelated properties are decoupled more thoroughly, or new degrees of freedom are added to the overall optimization problem. The electrical and thermal transport must be synergistically optimized. Here, a detailed discussion about the commonly adopted strategies to optimize individual TE properties is presented. Then, four main compromises between the TE properties are elaborated from the point of view of the underlying mechanisms and decoupling strategies. Finally, some representative systems of synergistic optimization are also presented, which can serve as references for other TE materials. In conclusion, some of the newest ideas for the future are discussed.
Enhancing the figure of merit zT is the central theme of thermoelectric research. The various recently proposed concepts and paradigms for optimizing thermoelectric properties are reviewed. Contradiction and compromise between the thermoelectric parameters and decoupling mechanisms are elaborated. Examples of synergistic optimization are given, highlighting typical efforts toward high‐performance thermoelectric materials.
A model for the thermal conductivity of bulk solids is proposed in the limit of diffusive transport mediated by diffusons as opposed to phonons. This diffusive thermal conductivity,
κ
diff
, is ...determined by the average energy of the vibrational density of states,
ω
avg
, and the number density of atoms,
n
. Furthermore,
κ
diff
is suggested as an appropriate estimate of the minimum thermal conductivity for complex materials, such that (at high temperatures):
. A heuristic finding of this study is that the experimental
ω
avg
is highly correlated with the Debye temperature, allowing
κ
diff
to be estimated from the longitudinal and transverse speeds of sound:
. Using this equation to estimate
κ
min
gives values 37% lower than the widely-used Cahill result and 18% lower than the Clarke model for
κ
min
, on average. This model of diffuson-mediated thermal conductivity may thus help explain experimental results of ultralow thermal conductivity.
Diffuson-mediated thermal transport suggests a lower minimum thermal conductivity than phonon models.
Bi2Te3 alloys have been the most widely used n-type material for low temperature thermoelectric power generation for over 50 years, thanks to the highest efficiency in the 300–500 K temperature range ...relevant for low-grade waste-heat recovery. Here we show that n-type Mg3Sb0.6Bi1.4, with a thermoelectric figure-of-merit zT of 1.0–1.2 at 400–500 K, finally surpasses n-type Bi2Te3. This exceptional performance is achieved by tuning the alloy composition of Mg3(Sb1−xBix)2. The two primary mechanisms of the improvement are the band effective-mass reduction and grain size enhancement as the Mg3Bi2 content increases. The benefit of the effective-mass reduction is only effective up to the optimum composition Mg3Sb0.6Bi1.4, after which a different band dominates charge transport. The larger grains are important for minimizing grain-boundary electrical resistance. Considering the limited choice for low temperature n-type thermoelectric materials, the development of Mg3Sb0.6Bi1.4 is a significant advancement towards sustainable heat recovery technology.
A new type of high performance thermoelectric material Cu2‐xS composed of non‐toxic and earth‐abundant elements Cu and S is reported. Cu2‐xS surprisingly has lower thermal conductivity and more ...strikingly reduced specific heat compared to the heavier Cu2Se, leading to an increased zT to 1.7.
Thermoelectric technology, harvesting electric power directly from heat, is a promising environmentally friendly means of energy savings and power generation. The thermoelectric efficiency is ...determined by the device dimensionless figure of merit ZTdev, and optimizing this efficiency requires maximizing ZT values over a broad temperature range. Here, we report a record high ZTdev ~1.34, with ZT ranging from 0.7 to 2.0 at 300 to 773 kelvin, realized in hole-doped tin selenide (SnSe) crystals. The exceptional performance arises from the ultrahigh power factor, which comes from a high electrical conductivity and a strongly enhanced Seebeck coefficient enabled by the contribution of multiple electronic valence bands present in SnSe. SnSe is a robust thermoelectric candidate for energy conversion applications in the low and moderate temperature range.
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