Thermoelectric Figure Of Merit Research Articles

Optimizing thermoelectric power factor in magnetron sputtered Cu2Se thin films by varying substrate temperature and synergy of Cu/Se ratio

Copper selenide (Cu2Se) is recognized as a p-type thermoelectric (TE) semiconductor known for its eco-friendliness, abundance in the earth's crust, and cost-effectiveness. Current research on Cu2Se primarily centers around its exceptional TE capabilities in bulk materials. However, two-dimensional thin films hold distinct advantages in TE micro- and nano-device development and applications. In the present study, a series of Cu2Se thin films were deposited on a glass substrate by employing radio frequency magnetron sputtering deposition by varying the substrate temperatures (Ts) from 50 °C to 400 °C with an interval of 50 °C. The impact of Ts role on the structural, microstructural, and electrical transport characteristics of Cu2Se thin films was investigated using the Grazing Incidence X-ray diffraction, High-Resolution transmission electron microscopy, Field Emission Scanning Electron Microscopy, and Seebeck analysis. The change in the Ts increased grain growth as observed from microstructure studies and improved electrical and transport characteristics. Notably, films deposited at Ts 350 °C exhibited a significant power factor of 16.8 μW/cmK2 at 450 °C, making them highly promising for TE applications.

Theoretical Estimates of the Thermoelectric Power Factor of Graphene Encapsulated between 3D and 2D Semiconductor and Metal Slabs

Theoretical Estimates of the Thermoelectric Power Factor of Graphene Encapsulated between 3D and 2D Semiconductor and Metal Slabs

Optimized thermoelectric performance driven by A‐site deficient in SrxLa0.1TiO3−δ/TiO2−y composites

AbstractFor thermoelectric applications at high temperatures, perovskite SrTiO3 is a promising contender as an n‐type oxide thermoelectric (TE) material. In this work, SrxLa0.1TiO3−δ/TiO2−y composite ceramics were successfully synthesized by solid state reaction, using a combination of A‐site cation vacancy and composite fabrication. Through X‐ray diffractometer and scanning electron microscopy analysis, a complex microstructure composed of two kinds of titanium oxides was formed after reductive sintering and annealing. In addition, controlling the content of Sr vacancies in the SrxLa0.1TiO3−δ matrix increased the thermoelectric power factor (PF) by optimizing the weighted mobility in the composite, resulting in the highest PF of 425.8 μW m−1 K−2 and the maximum ZT of 0.16 for the Sr0.875La0.1TiO3−δ/TiO2−y composite through the synergistic optimization of electrical and thermal transport properties. The strategy proposed in this study of design A‐site‐deficient composites with nano‐sized metal paved a new way to fabricate high performance oxide thermoelectric materials.

Electrically Programmed Doping Gradients Optimize the Thermoelectric Power Factor of a Conjugated Polymer

AbstractFunctionally graded materials (FGMs) are widely explored in the context of inorganic thermoelectrics, but not yet in organic thermoelectrics. Here, the impact of doping gradients on the thermoelectric properties of a chemically doped conjugated polymer is studied. The in‐plane drift of counterions in moderate electric fields is used to create lateral doping gradients in films composed of a polythiophene with oligoether side chains, doped with 2,3,5,6‐tetrafluoro‐tetracyanoquinodimethane (F4TCNQ). Raman microscopy reveals that a bias voltage of as little as 5 V across a 50 µm wide channel is sufficient to trigger counterion drift, resulting in doping gradients. The effective electrical conductivity of the graded channel decreases with bias voltage, while an overall increase in Seebeck coefficient is observed, yielding an up to eight‐fold enhancement in power factor. Kinetic Monte Carlo simulations of graded films explain the increase in power factor in terms of a roll‐off of the Seebeck coefficient at high electrical conductivities in combination with a mobility decay due to increased Coulomb scattering at high dopant concentrations. Therefore, the FGM concept is found to be a way to improve the thermoelectric performance of not yet optimally doped organic semiconductors, which may ease the screening of new materials as well as the fabrication of devices.

A Thermoelectric Polymer Field-Effect Transistor via Iodine-Doped P3HT.

Doping can alter certain electronics, including the thermoelectric properties of an organic semiconductor. These alterations may enable viable tunable devices that could be useful in temperature sensing for autonomous controls. Here, we demonstrate a dual-modulation organic field-effect transistor (OFET) where temperature can modulate the current-voltage characteristics of the OFET and gate voltage can modulate the thermoelectric properties of the active layer in the same device. Specifically, Poly(3-hexylthiophene-2,5-diyl) (P3HT) was utilized as the host p-type semiconducting polymer, and iodine was utilized as the thermoelectric minority dopant. The finished devices were characterized with a semiconductor analyzer system with temperature controlled using two thermoelectric cooling plates. The FETs with iodine doping levels in the range of 0.25% to 0.5% mole ratio with respect to the P3HT exhibit the greatest on/off ratios. This study also observed that P3HT thin film samples with an intermediate iodine doping concentration of 0.25% mole ratio exhibit an optimal thermoelectric power factor (PF).

Four-phonon scattering of so-As and improvement of the thermoelectric properties by increasing the buckling height.

In recent years, more and more thermoelectric (TE) materials have been discovered as the research boom of TE materials advances. However, due to the low conversion efficiency, most of the current TE materials cannot meet the commercial demand. The low-dimensional nanomaterials are promising to break the current status quo of low conversion efficiency of TE materials. Here, we predicted a stable two-dimensional TE material, namely so-As, based on density functional theory. The so-As has an ultra-low lattice thermal conductivity,κl= 1.829 W m-1K-1at 300 K, and when the temperature rises to 700 K theκlis only 0.788 W m-1K-1. This might be caused by the strong anharmonic interaction among the so-As phonon and the out-of-plane vibration of the low-frequency acoustic modes. Moreover, the maximumZTvalue of thep-type so-As is 0.18 at room temperature (0.45 at 700 K), while that of then-type can even reach 0.75 at 700 K. In addition, we have also studied the difference between the four- and three-phonon scattering rates. The increase of scattering channels leads to the ultra-lowκl, which is only 3.33 × 10-4W m-1K-1at room temperature, showing an almost adiabatic property. Finally, we adjust the TE properties of so-As by changing the buckling height. With the buckling height is increased by 2%, the scattering rate of so-As is extremely high. WhenTis 700 K, the maximumZTof then-type is 0.94 (p-type can also reach 0.7), which is 25% higher than the pristine one. Our work reveals the impact of buckling height on the TE figure of merit, which provides a direction for future search and regulation of the highZTTE materials.

Controlling Film Formation and Host-Guest Interactions to Enhance the Thermoelectric Properties of Nickel-Nitrogen-Based 2D Conjugated Coordination Polymers.

2D conjugated coordination polymers (cCPs) based on square-planar transition metal-complexes (such as MO4, M(NH)4, and MS4, M = metal) are an emerging class of (semi)conducting materials that are of great interest for applications in supercapacitors, catalysis, and thermoelectrics. Finding synthetic approaches to high-performance nickel-nitrogen (Ni-N) based cCP films is a long-standing challenge. Here, a general, dynamically controlled on-surface synthesis that produces highly conductive Ni-N-based cCP films is developed and the thermoelectric properties as a function of the molecular structure and their dependence on interactions with ambient atmosphere are studied. Among the four studied cCPs with different ligand sizes hexaminobenzene- and hexaaminotriphenylene-based films exhibit record electrical conductivity (100-200 S cm-1) in this Ni-N based cCP family, which is one order of magnitude higher than previous reports, and the highest thermoelectric power factors up to 10 µW m-1 K-2 among reported 2D cCPs. The transport physics of these films is studied and it is shown that depending on the host-guest interaction with oxygen/water the majority carrier type and the value of the Seebeck coefficient can be largely regulated. The high conductivity is likely reflecting good interconnectivity between (small) ordered domains and grain boundaries supporting disordered metallic transport.

Low Thermal Conductivity and High Thermoelectric Performance of Nb-Doped Quarternary Mixed Crystal Nb0.05W0.95-xMox(Se1-xSx)2.

Transition-metal dichalcogenide WSe2 has attracted increasing interest due to its large thermopower (S), low-cost, and environment-friendly constituents. However, its thermoelectric figure of merit, ZT, of WSe2 is limited due to its large lattice thermal conductivity (κL) and low electrical conductivity. In view of WSe2 and MoS2 having the same crystal structure, here we designed and prepared Nb-doped quarternary mixed crystal (MC) Nb0.05W0.95-xMox(Se1-xSx)2 (0 ≤ x ≤ 0.095). The results indicate that the κL of the MC can reach as low as 0.12 W m K-1 at 850 K, being 93% smaller than that of WSe2. Our analysis reveals that its low κL originates chiefly from intense scattering of both high-frequency phonons from point defects (mainly alloying elements) and mid/low-frequency phonons from MoS2 inclusions residual within MC. In addition, the alloying of WSe2 with MoS2 causes a 5-fold increase in cation vacancies (VW‴'), leading to a large increase in hole concentration and electrical conductivity, which gives rise to a ∼7.5 times increase in power factor (reaching 4.2 μ W cm-1 K-2 at 850 K). As a result, a record high ZTmax = 0.63 is achieved at 850 K for the MC sample with x = 0.076, which is 20 times larger than that of WSe2, demonstrating that MC Nb0.05W0.95-xMox(Se1-xSx)2 is a promising thermoelectric material.

High Thermoelectric Performance in Solution-Processed Semicrystalline PEDOT:PSS Films by Strong Acid-Base Treatment: Limitations and Potential.

Thermoelectric (TE) generation with solution-processable conducting polymers offers substantial potential in low-temperature energy harvesting based on high tunability in materials, processes, and form-factors. However, manipulating the TE and charge transport properties accompanies structural and energetic disorders, restricting the enhancement of thermoelectric power factor (PF). Here, solution-based strong acid-base treatment techniques are introduced to modulate the doping level of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films with preserving its molecular orientation, enabling to achieve a remarkably high PF of 534.5µWm-1 K-2 . Interestingly, theoretical modeling suggested that further de-doping can increase the PF beyond the experimental value. However, it is impossible to reach thisvalue experimentally, even without any degradation of PEDOT crystallinity. Uncovering the underlying reason for the limitation, an analysis of the relationship among the microstructure-thermoelectric performance-charge transport property revealed that inter-domain connectivity via tie-chains and the resultant percolation for transport are crucial factors in achieving high TE performance, as in charge transport. It is believed that the methods and fundamental understandings in this work would contribute to the exploitation of conducting polymer-based low-temperature energy harvesting.

Anomalous enhancement of thermoelectric power factor in multiple two-dimensional electron gas system.

Toward drastic enhancement of thermoelectric power factor, quantum confinement effect proposed by Hicks and Dresselhaus has intrigued a lot of researchers. There has been much effort to increase power factor using step-like density-of-states in two-dimensional electron gas (2DEG) system. Here, we pay attention to another effect caused by confining electrons spatially along one-dimensional direction: multiplied 2DEG effect, where multiple discrete subbands contribute to electrical conduction, resulting in high Seebeck coefficient. The power factor of multiple 2DEG in GaAs reaches the ultrahigh value of ~100 μWcm-1 K-2 at 300 K. We evaluate the enhancement rate defined as power factor of 2DEG divided by that of three-dimensional bulk. The experimental enhancement rate relative to the theoretical one of conventional 2DEG reaches anomalously high (~4) in multiple 2DEG compared with those in various conventional 2DEG systems (~1). This proposed methodology for power factor enhancement opens the next era of thermoelectric research.

Mechanical and dynamical stability, electronic, magnetic, and thermoelectric properties of RbBaX (X=Si and Ge) half-Heusler compounds

This research paper explores the structural, electronic, magnetic, elastic, dynamic, and thermoelectric properties of sp-based half-Heusler (HH) RbBaX ([Formula: see text] and Ge) compounds using density functional theory (DFT) and the full-potential linearized augmented plane wave (FP-LAPW) method within the WIEN2k software package. We evaluate the exchange correlation potential using two different approaches: The generalized gradient approximation (GGA) and the modified Becke–Johnson approach. We observe that RbBaX ([Formula: see text] and Ge) adopts a ferromagnetic configuration based on the analysis of total energy against volume. We establish that these alloys possess a genuine half-metallic character because of the full spin polarization observed at the Fermi level in their electronic spectrum. Both compounds under investigation have a total magnetic moment of 1.00[Formula: see text][Formula: see text]B, in agreement with the Slater–Pauling principle. The Born stability criteria for cubic structures and phonon dispersion patterns confirm the mechanical and dynamic stability of RbBaX ([Formula: see text] and Ge) alloys. The thermoelectric properties reveal a p-type semiconductor nature, characterized by significant positive Seebeck values. The thermoelectric figure of merit (ZT) calculated for both alloys at 300[Formula: see text]K approaches unity, highlighting their remarkable thermoelectric efficiency.

Grand herringbone architecture securing the high thermoelectric performance of GeTe

This work demonstrates a unique two-stage cooling procedure for synthesizing high-performance thermoelectric (TE) GeTe alloys with a maximum zT value of 2.0 (720 K) without assistance of chemical doping. The efficiency breakthrough stems from the grand herringbone structure resulted from the annealing step intentionally inserted near the phase transition temperature Tc of GeTe when cooling. Compared with the samples prepared by conventional methods, the two-stage cooled GeTe processes not only lower carrier concentrations p (4.7–5.3 × 1020 cm−3) but also higher mobility (∼125 cm2 V−1 s−1) that prevent electrical conductivity and TE power factor from declining as p decreases. The twinned domains in the two-stage cooled samples are further analyzed and classified by the compatibility theory. Our calculations show that the twinning structures in the two-stage cooled samples fit well with the compatibility criterion for twin variant selections and are thermodynamically in a stable state. These results not only attest to the efficacy of using the two-stage cooling procedure to manipulate the microstructures of GeTe but also provides a new paradigm for optimizing the performance of GeTe-based TE materials.

Investigation of thermoelectric and magnetotransport properties of single crystalline Bi2Se3 topological insulator

The realization of remarkable thermoelectric (TE) properties in a novel single-crystalline quantum material is a topic of prime interest in the field of thermoelectricity. It necessitates a proper understanding of transport properties under magnetic field and magnetic properties at low field. We report polarized Raman spectroscopic study, TE properties, and magneto-resistance (MR) along with magnetic characterization of single-crystalline Bi2Se3. Polarized Raman spectrum confirms the strong polarization effect of A1g1 and A1g2 phonon modes, which verifies the anisotropic nature of the Bi2Se3 single crystal. Magnetization measurement along the in-plane direction of single crystal divulges a cusp-like paramagnetic response in susceptibility plot, indicating the presence of topological surface states (TSSs) in the material. In-depth MR studies performed in different configurations also confirm the presence of anisotropy in the single-crystalline Bi2Se3 sample. A sharp rise in MR value near zero magnetic field and low-temperature regime manifests a weak anti-localization (WAL) effect, depicting the quantum origin of the conductivity behavior at low temperature. Moreover, in-plane magneto-conductivity data at low-temperature (up to 5 K) and low-field region (≤15 kOe) confirm the dominance of the WAL effect (due to TSS) with a negligible bulk contribution. Quantum oscillation (SdH) in magneto-transport data also exhibits the signature of TSS. Additionally, an exceptional TE power factor of ∼950 μW m−1 K−2 at 300 K is achieved, which is one of the highest values reported for pristine Bi2Se3. Our findings pave the way for designing single crystals, which give dual advantages of being a good TE material along with a topological insulator bearing potential application.

Developing a Multiband Electronic Band Structure Model and Predictive Maps for Bismuth-Rich Mg3(Sb1-xBix)2 Thermoelectric Materials.

In recent years, bismuth-rich Mg3(Sb1-xBix)2 (x = 0.5-0.8) compositions have generated significant interest due to their excellent thermoelectric (TE) performance near room temperature, making them potential applicants for recovery of low-grade waste heat. The superior performance in these materials is due to its complex electronic band structure (EBS) with presence of multiple near degenerate bands close to the conduction band edge. The position and curvature of these bands strongly depend on the alloy composition, doping amount as well as temperature. Thus, identifying optimal material compositions to get the best TE performance depends on an understanding of the temperature dynamics of EBS and forms the objective of this work. Mg3Sb0.6Bi1.4 (x = 0.7) is chosen for this study due to its reported high near room temperature performance, and compositions with varying doping concentrations (Te used as dopant) have been synthesized. EBS parameters like effective mass and deformation potential of bands, interband separation and band gap values have been estimated using a recently developed refinement approach. Refinement results indicate that the interband separation between conduction bands to be a function of both temperature and doping concentration. Further, thermal conductivity (κ) was estimated for all of the compositions. Utilizing the EBS and κ information, predictive 3D maps indicating the variation in zT (TE figure of merit) with doping concentration and temperature have been generated. The 3D maps reveal an interesting surface topography with a broad peak zT region. This observation explains why these materials have high TE performance and are less sensitive to doping inhomogeneities. Our results provide detailed EBS information and fundamental insights on the TE properties of Mg3Sb0.6Bi1.4. Further, the proposed technique can be utilized to probe other Mg3(Sb1-xBix)2 compositions and TE materials.

Non-destructive tuning of thermoelectric power factor of ZnO by surface-confined optical gating

We have studied the complex relationship between the thermoelectric properties of Zinc Oxide (ZnO) thin films and their interaction with various atmospheric conditions and ultraviolet (UV) illumination. We focused on the dynamic changes in the Seebeck coefficient (SC) and conductivity of ZnO under varied environments, which are crucial for tuning the thermoelectric power factor (PF). ZnO films showed pronounced variations in their carrier concentration and mobility when exposed to UV light in air and argon atmospheres, which can be attributed to the interplay of oxygen adsorption and desorption on the ZnO surface. The profound effect of the oxygen interaction, evidenced in a depletion layer wider than the ZnO film itself, significantly changed the electron concentration, and affected the surface conductivity and thus, the thermoelectric PF of ZnO. The experiments revealed that the SC decreased with increasing carrier concentration and mobility under UV illumination, while the thermoelectric PF increased in both atmospheres, providing pivotal insights into the nondestructive optimization of the thermoelectric properties of ZnO through controlled surface-sensitive reactions and optical gating.

Odd‐Stoichiometry and Secondary Phase Relations in Higher Manganese Silicides

AbstractHigher manganese silicides (denoted as MnSiγ) are a prominent class of intermetallic compound for thermoelectric applications that crystallizes into a Nowotny Chimney Ladder (NCL) phase and constitutes a higher concentration of silicon with odd stoichiometry, typically represented by an irrational number. In this work, the genesis of Si odd stoichiometry and secondary phase relations in silicon‐rich and silicon‐deficient MnSix is presented to clarify the significance of labile (i.e., easily alterable) Si‐subsystem in inheriting the modulated lattice structure with characteristic anharmonicity. The transport properties show an interrelation to the presence of a secondary phase, which is driven by the odd stoichiometry, where the thermoelectric figure of merit (zT) is found to be optimal for MnSi1.74. The structural characterization of rapidly solidified melt‐spun ribbons and spark plasma sintered bulk specimens reveal a characteristic presence of MnSi and Si as secondary phases in Si‐deficient (x ≈ 1.5) and Si‐excess (x ≈ 2) compositions of nominal MnSix, respectively, that coexists with the major MnSiγ based NCL phase. This study provides the fundamental basis of phase evolution and the relevance of odd stoichiometry in silicon‐rich manganese silicides which exhibit fascinating phonon dynamics in optimal MnSi1.74 stoichiometry.

Tripling magnetite's thermoelectric figure of merit with rare earth doping

Using density functional theory (DFT) and machine-learning force fields, we calculated the thermoelectric properties of magnetite doped with four rare-earth elements: lanthanum, cerium, praseodymium, and neodymium.

Enhanced Thermoelectric Performance of Poly(3,4-ethylenedioxythiophene) and Graphene/Poly(3,4-ethylenedioxythiophene) Composites via Vapor Phase Polymerization

Organic thermoelectric (TE) materials are interesting candidates for wearable energy harvesting systems because they are flexible and convert thermoelectric energy. Thermally conductive flexible thermoelectric generators (TEGs) built of these materials produce power from body and environmental temperature differences, enabling devices to run without recharging. The exceptional electrical conductivity, mechanical flexibility, and environmental robustness of organic transition metal (TE) materials like PEDOT and graphene composites are becoming more known. Due to their synergistic link, PEDOT and graphene layers are appealing thermoelectric possibilities because they increase See beck coefficient and electrical conductivity. Vapor phase polymerization (VPP) may replace chemical or electrochemical polymerization for PEDOT manufacture. This method allows one to accurately regulate PEDOT layer electrical characteristics by altering thickness and structure. These PEDOT and graphene/PEDOT composites were made using iron (III) to sylate as the VPP oxidizing agent. VPP technique produces homogeneous films with better electrical conductivity than existing approaches. We tested the electrical resistance and Linseis LSR-3/800 Seebeck coefficient of PEDOT and graphene/PEDOT composites to determine their thermoelectric characteristics. VPP’s graphene/PEDOT composites have better electrical conductivity and See beck coefficient than conventional PEDOT, improving their thermoelectric power factor. These results show that VPP may be utilized to develop organic thermoelectric materials for lightweight energy collectors.

Enhanced thermoelectric performance of a wide-bandgap twisted heterostructure of graphene and boron nitride.

The manipulation of the relative twist angle between consecutive layers in two-dimensional (2D) materials dramatically modulates their electronic characteristics. Twisted bilayer graphene (tblg) and twisted boron nitride (tBN) exhibit Moiré patterns that have the potential to revolutionize various fields, from electronics to quantum materials. Here, the electronic and thermoelectric properties of 21.8° tblg and 21.8° tBN and a 21.8° twisted graphene/boron nitride (Gr/BN) heterostructure were investigated using density functional theory and Boltzmann transport theory. The twisted Gr/BN heterostructure possesses a wide band gap of 1.95 eV, which overcomes the limitations of the absence of a band gap of graphene and boron nitride's extremely wide band gap. A significant increase in thermoelectric power factor was obtained for the heterostructure compared to its parent materials, 21.8° tblg and 21.8° tBN. It has a thermal conductivity of 5.88 W m-1 K-1 at 300 K, which is much lower than those of 21.8° tblg and 21.8° tBN. It is observed that graphene plays an important role in electron transport or power factor enhancement, whereas BN helps in reducing the thermal conductivity in twisted Gr/BN systems. A strong role of boundary scattering in thermal transport compared to electrical transport was observed. A high figure of merit (ZT) of 1.28 for the twisted Gr/BN heterostructure at a ribbon width of L = 10 nm and T = 900 K was obtained. This suggests its suitability as an effective material for thermoelectric applications.

Controlling conjugated polymer morphology by precise oxygen position in single-ether side chains.

Recently, polar side chains have emerged as a functional tool to enhance conjugated polymer doping properties by improving the polymer miscibility with polar chemical dopants and facilitate solvated ion uptake. In this work, we design and investigate a novel family of side chains containing a single ether function, enabling the modulation of the oxygen atom position along the side chain. A meticulous investigation of this new polymer series by differential scanning calorimetry, fast scanning chip calorimetry and X-ray scattering shows that polymers bearing single-ether side chains can show high degree of crystallinity under proper conditions. Importantly, due to a gauche effect allowing the side chain to bend at the oxygen atom, the degree of crystallinity of polymers can be controlled by the position of the oxygen atom along the side chain. The further the oxygen atom is from the conjugated backbone, the more crystalline the polymer becomes. In addition, for all new polymers, high thermomechanical properties are demonstrated, leading to remarkable electrical conductivities and thermoelectric power factors in rub-aligned and sequentially doped thin films. This work confirms the potential of single-ether side chains to be used as polar solubilizing side chains for the design of a next generation of p- and n-type semiconducting polymers with increased affinity to polar dopants while maintaining high molecular order.