Thermoelectric Quality Factor Research Articles

Optimizing thermoelectric performance in molecular junction: The role of van der Waals interface coupling with boron-doped electrodes

The key prerequisite for fully rationalizing and optimizing thermoelectric performance of molecular devices is to effectively overcome the inherent mismatch between electrode molecular orbital energy and Fermi level (FL). Two structures were formed by stacking bis-phenylethynyl-anthrancene (BPA-molecule) with B-doped armchair/zigzag graphene nanoribbon (AGNR@B/ZGNR@B) electrodes. Optimal distance between the molecule and these two B-doped graphene nanoribbons was related to interfacial stacking mode (BPA-AGNR@B optimization distance is 3.387\\AA for the case of top-site stacking, and it is 3.472\\AA for malposed stacking of BPA-ZGNR@B). Thermal conductivities of both the two B-doped systems based on mass mismatch mechanism are greatly improved at FL. The role of mass matching effect on total thermal conductance of BPA-AGNR@B system is pivotal to result in significant enhancements, but it is not the case for BPA-ZGNR@B, because large layer spacing weakens mass matching effect. In addition, boron-doped molecular junctions (BPA-AGNR@B and BPA-ZGNR@B) show an excellent thermoelectric quality factor (ZT) of 3.5/3.3 near FL.

Synergistic Combination of Sb2Si2Te6 Additives for Enhanced Average ZT and Single-Leg Device Efficiency of Bi0.4Sb1.6Te3-based Composites.

Thermoelectric materials are highly promising for waste heat harvesting. Although thermoelectric materials research has expanded over the years, bismuth telluride-based alloys are still the best for near-room-temperature applications. In this work, a ≈38% enhancement of the average ZT (300-473K) to 1.21 is achieved by mixing Bi0.4Sb1.6Te3 with an emerging thermoelectric material Sb2Si2Te6, which is significantly higher than that of most BiySb2-yTe3-based composites. This enhancement is facilitated by the unique interface region between the Bi0.4Sb1.6Te3 matrix and Sb2Si2Te6-based precipitates with an orderly atomic arrangement, which promotes the transport of charge carriers with minimal scattering, overcoming a common factor that is limiting ZT enhancement in such composites. At the same time, high-density dislocations in the same region can effectively scatter the phonons, decoupling the electron-phonon transport. This results in a ≈56% enhancement of the thermoelectric quality factor at 373 K, from 0.41 for the pristine sample to 0.64 for the composite sample. A single-leg device is fabricated with a high efficiency of 5.4% at ΔT = 164 K further demonstrating the efficacy of the Sb2Si2Te6 compositing strategy and the importance of the precipitate-matrix interface microstructure in improving the performance of materials for relatively low-temperature applications.

Thermoelectric Transport Properties of Sb-doped SnSe<sub>2</sub> Polycrystalline Alloys

SnSe<sub>2</sub> alloys have been investigated in recent times as potential <i>n</i>-type thermoelectric materials. In this study, the thermoelectric transport properties of a series of Sb-doped SnSe<sub>2</sub>, Sn(Se<sub>1-x</sub>Sb<sub>x</sub>)<sub>2</sub> (<i>x</i> = 0, 0.015, 0.03, 0.045, 0.06) alloys are investigated. The electrical conductivity was generally enhanced with Sb doping owing to a large increase in electron concentration. However, the Seebeck coefficient largely decreased with doping. Consequently, the power factor was significantly lower at a low doping of <i>x</i> = 0.015, and then began rising as the doping was increased beyond <i>x</i> = 0.015. It was found that the density-of-states effective mass and weighted mobility decreased with Sb doping, implying that the electrical transport properties of SnSe<sub>2</sub> were degraded by Sb doping. The total and lattice thermal conductivities gradually decreased due to additional point defect scattering. Thus, the thermoelectric figure of merit declined significantly, from 0.30 of the pristine sample with a low doping of Sb (<i>x</i> = 0.015) at 750 K, to 0.18, and then for <i>x</i> = 0.06 it gradually recovered to the value of the undoped sample. The thermoelectric quality factor decreased as the Sb doping was increased, implying that Sb doping did not enhance the thermoelectric transport properties, despite the large increase in electron concentration.

Enhanced Thermoelectric Transport Properties in Cu-added Bi2Se3 Polycrystalline Alloys

The addition of Cu to layered Bi2Te3-based thermoelectric alloys has been studied as an effective way to enhance thermoelectric transport properties. In this study, the influence of adding Cu to Bi2Se3 alloys, which have the same rhombohedral crystal structure as Bi2Te3, was investigated by synthesizing a series of CuxBi2Se3 (<i>x</i> = 0, 0.004, 0.008, 0.012, and 0.016) alloys. The power factors of all the Cu-added samples were enhanced compared with that of the pristine Bi2Se3 sample, primarily because of the increase in electrical conductivity. The power factor for the Cu0.016Bi2Se3 sample (<i>x</i> = 0.016) was 0.80 mW/mK<sup>2</sup>, a 35% increase compared to 0.59 mW/mK<sup>2</sup> for the pristine sample at 520 K. A decrease in the total and lattice thermal conductivity was observed for the Cu-added samples, caused by additional point defect scattering after doping. The lattice thermal conductivity of the Cu0.016Bi2Se3 sample (<i>x</i> = 0.016) was 0.56 W/mK, a 42% reduction. Consequently, the zT values of all the Cu-added samples were enhanced, and the maximum zT value was 0.38 for the Cu0.016Bi2Se3 sample (<i>x</i> = 0.016) at 520 K, a 48% increase compared to that of pristine Bi2Se3. The phenomenological transport parameters, including density of state, effective mass, weighted mobility, and thermoelectric quality factor, were calculated to analyze the enhanced electronic transport properties of the Cu-added Bi2Se3.

Thermoelectric Properties of Ba2–xEuxZnSb2, a Zintl Phase with One-Dimensional Covalent Chains

The compound Ba2ZnSb2 has been predicted to be a promising thermoelectric material, potentially achieving zT > 2 at 900 K due to its one-dimensional chains of edge-shared [ZnSb4/2]4- tetrahedra and interspersed Ba cations. However, the high air sensitivity of this material makes it difficult to measure its thermoelectric properties. In this work, isovalent substitution of Eu for Ba was carried out to make Ba2-xEuxZnSb2 in order to improve the stability of the material in air and to allow characterization of thermal and electronic properties of three different compositions (x = 0.2, 0.3, and 0.4). Polycrystalline samples were synthesized using binary precursors via ball milling and annealing, and their thermoelectric properties were measured. Samples showed low thermal conductivity (<0.8 W/m K), a high Seebeck coefficient (350-550 μV/K), and high charge carrier mobility (20-35 cm2/V) from 300 to 500 K, consistent with predictions of high thermoelectric efficiency. Evaluation of the thermoelectric quality factor suggests that a higher zT can be attained if the carrier concentration can be increased via doping.

Theoretical analysis of the thermoelectric properties of penta-PdX2 (X = Se, Te) monolayer.

Based on the successful fabrication of PdSe2 monolayers, the electronic and thermoelectric properties of pentagonal PdX2 (X = Se, Te) monolayers were investigated via first-principles calculations and the Boltzmann transport theory. The results showed that the PdX2 monolayer exhibits an indirect bandgap at the Perdew-Burke-Ernzerhof level, as well as electronic and thermoelectric anisotropy in the transmission directions. In the PdTe2 monolayer, P-doping owing to weak electron-phonon coupling is the main reason for the excellent electronic properties of the material. The low phonon velocity and short phonon lifetime decreased the thermal conductivity (κ l) of penta-PdTe2. In particular, the thermal conductivity of PdTe2 along the x and y transmission directions was 0.41 and 0.83Wm-1K-1, respectively. Owing to the anisotropy of κ l and electronic structures along the transmission direction of PdX2, an anisotropic thermoelectric quality factor ZT appeared in PdX2. The excellent electronic properties and low lattice thermal conductivity (κ l) achieved a high ZT of the penta-PdTe2 monolayer, whereas the maximum ZT of the p- and n-type PdTe2 reached 6.6 and 4.4, respectively. Thus, the results indicate PdTe2 as a promising thermoelectric candidate.

Enhanced thermoelectric performance in the zintl antimonides (Ca,RE)9Cd4Sb9 (RE = rare-earth metal). Synergy between increased structural complexity and drive towards optimized chemical bonding

The interplay of structural complexity, high carrier mobility, and high density of states effective mass can play a pivotal role in achieving enhanced thermoelectric (TE) performance in candidate materials. In this regard, the Zintl phases represent a class of compounds that are susceptible to harboring these key ingredients. This, in addition to their amenability to various forms of chemical substitution mechanisms makes them a good choice of systems to explore systematically. Here we demonstrate the role-play of these ingredients in achieving excellent TE properties on single-crystals of Ca9–xREyCd4+δSb9 (RE = Ce, Pr, Nd, Sm, Gd, Tb; x ≈ 0.5–0.8, y ≈ 0.5–0.7, δ ≈ 0.25). These phases represent a new addition to the “9–4–9” family with intricate chemical bonding arising from both a purposely introduced disorder on Ca sites and the inherent presence of interstitial Cd positions. Many of the newly synthesized and characterized phases show moderate values of the Seebeck coefficient, lying in the range of 71–116 μV/K at 600 K and evolving as degenerate semiconductors. Simultaneously, the electrical resistivity ρ(T) of the measured samples can be as low as 0.18 mΩ cm at this temperature. As a result, the observed TE power factors in the Ce-, Nd-, and Sm-samples are in the range 6–46 μW/cm.K2. For Ca9–xCexCd4+δSb9, the estimated thermoelectric quality factor B > 0.4 at 300 K, which corresponds to a figure of merit zT ≥ 1. Calculations based on the single parabolic band (SPB) model show that the optimum region for thermoelectric performance requires carrier concentration n = 2–6 × 1019 cm−3 thus providing for an open window to further tune the TE properties.

Thermal conductivity of solid solutions of Bi2(Te1-xSex )3 (x = 0-0.07)

Relevance. Solid solutions Bi2 (Te1-xSex )3 are effective materials for an n-leg of thermoelectric cooling devices. Recently, concentration anomalies of properties with a low impurity content were detected in these solid solutions (x ~0.01). It is important to establish whether similar anomalies would be observed on the dependence of thermal conductivity λ from the composition of the solid solution, since λ is one of the parameters that determine the thermoelectric quality factor of the material. Purpose. The purpose of this study was to investigate the concentration dependences of the thermal conductivity of the solid solution of Bi2 (Te1-xSex )3 in the interval x = (0-0.07) to identify concentration anomalies and their possible impact on thermoelectric (TE) efficiency. Methodology. Investigation of temperature dependences of thermal conductivity λ was performed by a dynamic calorimeter. Isotherms of λ for different temperatures in the range T = 175-400 K were built on their basis. Results. On isotherms of λ, there is an interval of abnormal growth λ, which becomes more pronounced when the temperature decreases. The presence of this interval is associated with critical phenomena that accompany the transition of the percolation type from dilute to concentrated solid solutions. Conclusions. The estimation of the thermoelectric figure of merit Z of materials is given. It is shown that the detected growth of λ leads to a decrease in Z in the ranges near the critical (x = 0.01), which should be considered in the practical use of solid solutions of Bi2 (Te1-xSex )3

Contrasting role of bismuth doping on the thermoelectric performance of VFeSb half-Heusler

Isoelectronic heavy elemental substitutions are generally preferred for enhancing the phonon scattering in a material without deteriorating its electrical transport. In this work, we demonstrate the efficacy and contrasting role of bismuth, an isoelectronic substitute of antimony, for enhancing the thermoelectric transport in structurally ordered cubic VFeSb half-Heusler. Despite its limited solubility, Bi-substitution at Sb-site, was found to be effective in enhancing the electrical power factor near room temperature. Alongside, synergistic lowering of lattice thermal conductivity was observed due to point defect scattering of phonons by Bi-induced mass and strain fluctuations. The electronic transport properties of nominal compositions and effects of Bi-doping induced disorder were evaluated using the Korringa–Kohn–Rostoker method with a coherent potential approximation (KKR-CPA). A highest thermoelectric figure-of-merit ( zT ) ~ 0.22 at 478 K was attained with improved weighted mobility and quality factor for optimally doped VFeSb 0.98 Bi 0.02 half-Heusler alloy making them prospective compositions for near room temperature thermoelectric applications. • Contrasting role of bismuth as prospective substitute for isovalent antimony in Sb-based half-Heusler. • High-power factor > 3.0 × 10 −3 Wm −1 K −2 at room temperature. • Lattice thermal conductivity reduction by point defect scattering in Bi-substituted VFeSb alloys. • Enhanced weighted mobility, thermoelectric quality factor and zT in Bi-substituted VFeSb alloys.

TRIANGULATION OF THE Cu-Sn-Se SYSTEM

Copper-containing compounds exhibit a wide range of properties, including thermoelectric, photoelectric, optical magnetic, superionic, superconducting, etc., which determines the areas of their practical use. In recent years, studies of complex copper selenides as promising thermoelectric (TE) materials have been actively carried out due to their advantages over traditional TE materials. Like binary Cu2Se, ternary selenides have low phonon thermal conductivity and high electrical conductivity and thermoelectric quality factor. Typically, copper-containing compounds belong to the p-type conductors and crystallize in four main structural types, among which phases with a diamond-like structure should be distinguished. Data on the nature of physicochemical interaction in the Cu – Sn – Se system are limited and contradictory. In view of this, it is important to carry out the triangulation of the ternary system Cu–Sn–Se, which is the first stage of the study of multicomponent systems.&#x0D; The investigated alloys of the Cu – Sn – Se system were obtained by fusing elementary components of high purity in vacuum quartz ampoules. The obtained alloys were investigated using X-ray powder diffraction (XRD) and differential thermal (DTA) analyzes. At the temperature of homogenizing annealing (170 ° С) there are seven binary Cu2Se, CuSe, CuSe2, Cu6Sn5, Cu3Sn, SnSe, SnSe2 and one ternary phase Cu2SnSe3 stable in the Cu – Sn – Se ternary system. The existence of the ternary phase of Cu2SnSe4 has not been confirmed, because the alloy corresponding to its stoichiometric composition is a mixture of Cu2SnSe3 and Se. To establish quasibinary sections of the Cu – Sn – Se system were performed the synthesis and phase analysis of only the significant points in the most informative areas. This ensures the establishment of the nature of the maximum number of quasibinary sections with a minimum number of syntheses. According to the results of phase analysis in combination with the literature data the triangulation of the Cu – Sn – Se system was carried out at 170 ° С. The quasibinarity of the Cu2Se – SnSe, Cu2Se – SnSe2, Cu2SnSe3 – Se, Cu2SnSe3 – SnSe, Cu6Sn5 – SnSe, Cu3Sn – SnSe, and Cu3Sn – Cu2Se sections was confirmed, and the quasibinarity of the Cu3Sn – Cu2Se was established at first.&#x0D; Keywords: triangulation; quasibinary section; phase analysis.

Manipulating the phonon transport towards reducing thermal conductivity via replacement of Cu by Mn in Cu2SnSe3 thermoelectric system

The substitution of Cu by Mn in the Cu2-xMnxSnSe3 (0 ​≤ ​x ​≤ ​0.20) system is presented with an objective to optimize the thermal transport and analyse thermoelectric behaviour in the low and near room temperature regime (10–350 ​K). The existence of hole-like small polarons as thermally activated carriers, mediating the p-type electrical transport at high temperatures (>80 ​K), is experimentally validated. Temperature dependence of Seebeck coefficient and electrical transport at low temperatures reveals that the variable range hopping (VRH) mechanism is responsible for conduction for temperatures (<80 ​K). Mn doping resulted in the improvement of the Seebeck coefficient, attaining the highest value of 228.3 ​μV/K at 350 ​K for the x ​= ​0.20 sample. A reduction in thermal conductivity is achieved in all the Mn-doped samples, presumably due to strong point defect scattering of high-frequency phonons. The x ​= ​0.20 sample has the lowest thermal conductivity of 1.68 ​W/mK at 350 ​K. Even though the ZT value is observed to decrease with Mn doping, enhancement in thermoelectric quality factor is seen for the sample with x ​= ​0.05, which is attributed to the reduction in lattice thermal conductivity.

Enhancing Thermoelectrics for Small-Bandwidth n-Type PbTe-MnTe Alloys via Balancing Compromise.

Small-bandwidth n-type PbTe-MnTe alloys effectively modify the valley shape, while it also inevitably aggravates the deterioration of carrier mobility as nonpolar phonons dominate the scattering. It is found that a trace amount of Cu doping can alleviate the compromises among thermoelectric parameters, thereby significantly optimizing the electrical-transport performance near room temperature of n-type PbTe-MnTe alloys. The single-Kane model reveals that the physical origin of performance improvement lies in the carrier mobility enhancement and self-optimization of carrier concentration. The Debye-Callaway model further quantifies the contribution of copper defect scattering to the lattice thermal conductivity. Notably, the high thermoelectric quality factor obtained in this work rationalizes their superior properties and reveals immense potential for achieving higher zT. Herein, an extremely high zT of ∼0.52 at room temperature and a maximum zTmax of ∼1.2 at 823 K are achieved in 0.3% Cu-intercalated n-type PbTe-MnTe. The mechanism in balancing compromise elaborated in principle contributes to an improvement of thermoelectric properties of the n-type PbTe alloys in a broad temperature range.

Strained Endotaxial PbS Nanoprecipitates Boosting Ultrahigh Thermoelectric Quality Factor in n-Type PbTe As-Cast Ingots.

Lead telluride (PbTe) has long been regarded as an excellent thermoelectric material at intermediate temperature range (573-873 K); however, n-type PbTe's performance is always relatively inferior to its p-type counterpart mainly due to their different electronic band structures. In this work, an ultrahigh thermoelectric quality factor (µ/κL ≈ 1.36 × 105 cm3 KJ-1 V-1 ) is reported in extra 0.3% Cu doped n-type (PbTe)0.9 (PbS)0.1 as-cast ingots. Transmission electron microscopy (TEM) characterization reveals that excess PbS exists in PbTe matrix as strained endotaxial nanoprecipitates, which affect electrical and thermal conduction discriminately: (1) coherent PbTe/PbS lattice minimizes the interface scattering of charge carriers; (2) periodic strain centers at PbTe/PbS interface exhibit intensive strain contrast, which can strongly scatter heat-carrying phonons. Electron backscattered diffraction (EBSD) characterization illustrates very large PbTe grains (≈1mm) in these as-cast ingots, ensuring an extremely low grain boundary scattering rate thus a very high charge carrier mobility. Eventually, a remarkably high ZTmax ≈ 1.5 at 773 K and an outstanding ZTavg ≈ 1.0 between 323 and 773 K are simultaneously achieved in the (PbTe)0.9 (PbS)0.1 +0.3%Cu sample; these values are highly competitive with reported state-of-art n-type PbTe materials.

Enhancing thermoelectric performance of Sb2Te3 through swapped bilayer defects

Lattice defects are typically used to tailor the thermoelectric properties of materials. It is desirable that such defects improve the electrical conductivity, while, at the same time, reduce the thermal conductivity, for an overall improvement on the thermoelectric properties of materials. Here, we report on an extended defect in Sb2Te3 consisting of swapped bilayers with chemical intermixing of Sb and Te atoms, which can be generated and effectively manipulated in polycrystalline samples through synthetic methods and thermal treatments. The swapped bilayers bridge the spatial gaps between the Sb2Te3 quintuple-layer blocks, enhancing the charge carrier mobility and thus the electrical conductivity. These defects also result in a reduced lattice thermal conductivity through suppressing phonon transport. These synergistic effects contribute together to an improved thermoelectric quality factor and an enhanced figure of merit (zT) value in Sb2Te3.

Achieving high thermoelectric quality factor toward high figure of merit in GeTe

In recent years, GeTe has received tremendous attention from the research community owing to its favorable electronic and thermal properties which make it one of the best performing thermoelectric compounds. In many reports, high performance has often been achieved via various doping/alloying methods, which typically involve more than one type of dopants. In contrast to the widely used codoping strategies, this work only uses a minute amount of 1% doping, giving rise to one of the highest quality factor (1.30) at 673 K amongst GeTe, with a corresponding zT of 1.5. The high performance is attributed to simultaneously improved electronic properties via carrier concentration optimization, as well as reduced thermal conductivity via additional phonon scattering brought about by In-mass fluctuations. More importantly, we elucidate on the importance of preserving the high quality factor via choosing the right dopants to optimize the carrier concentration. Furthermore, we showed that the strategy of evaluating the quality factor can be applied to other material systems, serving as a general guideline for thermoelectric materials design. The quality factor of GeTe in this work is superior to most other high-performing chalcogenides such as PbTe, SnTe, and SnS, revealing the large space for further enhancing its zT.

Thermoelectric transport enhancement of Te-rich bismuth antimony telluride (Bi0.5Sb1.5Te3+x) through controlled porosity

Alloys of Bi2Te3 and Sb2Te3 are the best performing p-type thermoelectrics near room temperature and have been the subject of extensive engineering efforts. Dramatic improvement is achieved by introducing defects that effectively scatter phonons and reduce thermal conductivity. Unfortunately, outstanding results are often difficult to reproduce as the process variables involved are difficult to control or possibly unknown. Here, a reproducible and controllable method of fabricating porous Bi0.5Sb1.5Te3+x is presented. While effective medium theory (EMT) predicts no benefit, improvements in the thermoelectric quality factor, B (which determines the maximum zT of a materials), were as high as 45% parallel to the pressing direction for a sample of roughly 20% porosity. The study of microstructural evolution with increasing porosity is facilitated by a combination of Scanning/Transmission Electron Microscopy (S/TEM) and Electron Backscattered Diffraction (EBSD). This study reveals a statistically significant shift in the distribution of grain boundaries favoring lower energy twins, which coincides with an increase in the presence of stepped twin boundaries. This work demonstrates the potential benefits of careful grain boundary engineering and the need for further detailed studies of the dependence of thermal and electrical transport on grain boundary structure and orientation in these alloys.

The Thermoelectric Properties of n-Type Bismuth Telluride: Bismuth Selenide Alloys Bi2Te3-x Se x.

Alloying bismuth telluride with antimony telluride and bismuth selenide for p- and n-type materials, respectively, improves the thermoelectric quality factor for use in room temperature modules. As the electronic and thermal transports can vary substantially, the alloy composition is a key engineering parameter. The n-type Bi2Te3-xSex alloy lags its p-type counterpart in thermoelectric performance and does not lend itself as readily to simple transport modeling which complicates engineering. Combining literature data with recent results across the entire alloy composition range, the complex electronic structure dynamics and trends in lattice thermal conductivity are explored. Spin-orbit interaction plays a critical role in determining the position and degeneracy of the various conduction band minima. This behavior is incorporated into a two-band effective mass model to estimate the transport parameters in each band. An alloy scattering model is utilized to demonstrate how phonon scattering behaves differently on either side of the intermediate ordered compound Bi2Te2Se due to chalcogen site occupancy preference. The parametrization of the electronic and thermal transports presented can be used in future optimization efforts.

Measurement of Thermoelectric Parameters of Thin-Film Semiconductor Materials Using the Harman Method

For the analysis of the measurement of thermoelectric parameters of semiconductors, the Harman pulsed method was used. The authors propose a new approach to determine the thermoelectric quality factor of thin semiconductor films in the temperature interval (300 ÷ 500) K by directly measuring a series of electric circuit parameters. The theory of the method is described in detail and its application in the measurement methodology. The dependences of electrical quantities on the time, namely voltage – V(t), are investigated at different values of current pulses for thin films PbTe&lt;Tl&gt; grown by the pulsed laser deposition.

The Thermoelectric Properties of Bismuth Telluride

AbstractBismuth telluride is the working material for most Peltier cooling devices and thermoelectric generators. This is because Bi2Te3 (or more precisely its alloys with Sb2Te3 for p‐type and Bi2Se3 for n‐type material) has the highest thermoelectric figure of merit, zT, of any material around room temperature. Since thermoelectric technology will be greatly enhanced by improving Bi2Te3 or finding a superior material, this review aims to identify and quantify the key material properties that make Bi2Te3 such a good thermoelectric. The large zT can be traced to the high band degeneracy, low effective mass, high carrier mobility, and relatively low lattice thermal conductivity, which all contribute to its remarkably high thermoelectric quality factor. Using literature data augmented with newer results, these material parameters are quantified, giving clear insight into the tailoring of the electronic band structure of Bi2Te3 by alloying, or reducing thermal conductivity by nanostructuring. For example, this analysis clearly shows that the minority carrier excitation across the small bandgap significantly limits the thermoelectric performance of Bi2Te3, even at room temperature, showing that larger bandgap alloys are needed for higher temperature operation. Such effective material parameters can also be used for benchmarking future improvements in Bi2Te3 or new replacement materials.

Universal behavior of the thermoelectric figure of merit, zT, vs. quality factor

To increase the performance of thermoelectric materials, the electronic parameters in the figure of merit must be improved. In this article, we use full, numerical band structures and solve the Boltzmann equation in the relaxation time approximation using energy-dependent scattering times informed by first principles simulations. By varying the strength of the electron–phonon coupling or the lattice thermal conductivity, we compute the thermoelectric figure of merit, zT, vs. a generalized thermoelectric quality factor. More than a dozen of different complex electronic structures are examined. Surprisingly, we find that at a given quality factor, none provides a better figure of merit than that of a material with a simple, parabolic band and acoustic deformation potential scattering. A qualitative argument for this unexpected finding is presented. This apparent universal behavior suggests that even for complex electronic band structures, the thermoelectric figure of merit depends solely on the ratio of electrical to thermal conductivity; the Seebeck coefficient and Lorenz number need not be considered. This observation should simplify the search for promising new materials, but if exceptions to this behavior can be identified, new paths for increasing thermoelectric material performance will open up.