N-type Thermoelectric Materials Research Articles

Thermoelectric generator and temperature sensor based on polyamide doped n-type single-walled nanotubes toward self-powered wearable electronics

Due to its ability to convert body heat into electricity, organic thermoelectric material is considered a promising and smart maintenance-free power source to charge wearable electronics. However, developing flexible n-type organic thermoelectric materials and wearable p/n junction thermoelectric devices remains challenging. In this work, two insulated polyamides (PA6 and PA66) that have been widely used as fiber materials are employed as novel dopants for converting p-type single-walled carbon nanotubes (SWCNTs) to n-type thermoelectric materials. Because of the electron transferability of the amide group, polyamide-doped SWCNTs exhibit excellent thermopower values as large as −56.0 μV K−1 for PA66, and −54.5 μV K−1 for PA6. Thermoelectric devices with five p/n junctions connected in series are fabricated. The testing device produces a thermoelectric voltage of 43.1 mV and generates 1.85 μW thermoelectric power under temperature gradients of approximately 80 K. Furthermore, they display charming capability for temperature recognition and monitoring human activities as sensors. These promising results suggest that the flexible polyamide-doped SWCNT composites herein have high application potential as wearable thermoelectric electronics.

An n-Type Open-Shell Conjugated Polymer with High-Spin Ground-State and High Intrinsic Electrical Conductivity.

Open-shell conjugated polymers with a high intrinsic conductivity and high-spin ground state hold considerable promise for applications in organic electronics and spintronics. Herein, two novel acceptor-acceptor (A-A) conjugated polymers based on a highly electron-deficient quinoidal benzodifurandione unit have been developed, namely DPP-BFDO-Th and DPP-BFDO. The incorporation of the quinoidal moiety into the polymers backbones enables deeply aligned lower-lying lowest unoccupied molecular orbital (LUMO) levels of below -4.0 eV. Notably, DPP-BFDO exhibits an exceptionally low LUMO (-4.63 eV) and a high-spin ground state characterized by strong diradical characters. Moreover, a self-doping through intermolecular charge-transfer is observed for DPP-BFDO, as evidenced by X-ray photoelectron spectroscopy (XPS) studies. The high carrier concentration in combination with a planar and linear conjugated backbone yields a remarkable electrical conductivity (σ) of 1.04 S cm-1 in the "undoped" native form, ranking among the highest values reported for n-type radical-based conjugated polymers. When employed as an n-type thermoelectric material, DPP-BFDO achieves a power factor of 12.59 μW m-1 K-2. Furthermore, upon n-doping, the σ could be improved to 65.68 S cm-1. This study underscores the great potential of electron-deficient quinoidal units in constructing dopant-free n-type conductive polymers with a high-spin ground state and exceptional intrinsic conductivity.

Investigation into thermoelectric performance of a flexible n-type nanocomposite film from the AgxSe nanotube composited with PPy

Silver selenide (Ag2Se) has turn up as a promising n-type thermoelectric (TE) material. Especially, the Ag2Se nanotubes may be more attractive in view of their one-dimensional hollow tubular nanostructure. Regrettably, the TE application of Ag2Se nanotubes has not yet been reported so far, probably due to their hard accessibility and poor processability. In this work, we developed a simple and efficient template-free electrochemical deposition method for preparing AgxSe nanotubes, and thus investigated the TE application of the AgxSe-nanotube films electrochemically composited with polypyrrole (PPy). It was found that the incorporation of PPy into the n-type AgxSe-nanotube films not only settled the issues of their high brittleness and easy oxidation, but also significantly improved their TE performance through the synergistic effects. Consequently, the AgxSe nanotube/PPy composite film achieved a maximum power factor of 282.8 ± 16.5 μW m−1 K−2, which is nearly twice that of the pure AgxSe-nanotube film.

Developing Air-Stable n-Type SWCNT-Based Composites with High Thermoelectric and Robust Mechanical Properties for Wearable Electronics.

Flexible organic thermoelectric generators are gaining prominence in wearable electronics, leveraging body heat as an energy source. Their advancement is hindered by the scarcity of air-stable n-type organic materials with robust mechanical properties. This study introduces two new polymers (HDCN4 and HDCN8), created through polycondensation of paraformaldehyde and diamine-terminated poly(ethylene glycol) (PEGDA) with molecular weights of 4000 and 8000 g/mol into single-walled carbon nanotubes (SWCNTs). The resulting HDCN4/SWCNT and HDCN8/SWCNT composites show impressive power factors of 225.9 and 108.2 μW m-1 K-2, respectively, and maintain over 90% in air for over four months without encapsulation. The HDCN4/SWCNT composite also demonstrates significant tensile strength (33.2 MPa) and flexibility (up to 10% strain), which is currently the best mechanically n-type thermoelectric material with such a high power factor reported in the literature. A thermoelectric device based on HDCN4/SWCNT generates 4.2 μW of power with a 50 K temperature difference. Additionally, when used in wearable temperature sensors, these devices exhibit high mechanical reliability and a temperature resolution of 0.1 K. This research presents a viable method to produce air-stable n-type thermoelectric materials with excellent performance and mechanical properties.

Identifying the promising n-type SmMg2Sb2-based Zintl phase thermoelectric material

The discovery of promising thermoelectric materials has often been challenged by the doping bottleneck. As an example, the n-type Zintl phases are promising thermoelectric materials but are challenging to synthesize due to intractable cation vacancies. In this work, by incorporating excess Mg and Te doping, n-type SmMg2Sb2 with high thermoelectric performance has been realized. Density functional theory calculations indicate that the conduction bands extrema of SmMg2Sb2 are located at the L and M points in the Brillouin zone, and the valley degeneracy is 6. The high valley degeneracy of conduction bands results in a much larger n-type density-of-state effective mass (∼2.6m0) compared to that of many p-type Zintl phases. With the optimization of the electron concentration, an n-type power factor ∼11.0 μW cm−1 K−2 is obtained in SmMg2+δSb1.97Te0.03 (δ = 0.3). Together with the effectively reduced lattice thermal conductivity by further alloying of SmMg2Bi2, a peak zT ∼1.0 at 873 K is achieved in n-type SmMg2+δSb1.72Bi0.25Te0.03 (δ = 0.3), demonstrating the great potential of n-type Zintl phase thermoelectric materials.

Reliability Investigation of Silicide-Based Thermoelectric Modules.

The reliability and failure mechanisms of silicide-based thermoelectric modules (n-type Mg2(Si,Sn)/p-type HMS) were investigated thanks to two types of thermal tests with either a fixed or a cycling thermal gradient, under different atmospheres. The hot interfaces of the thermoelectric modules were analyzed by scanning electron microscopy and X-ray diffraction after the reliability tests. The current thermoelectric modules do not exhibit any failure mechanism under ambient air for a hot side temperature of 250 °C for tests conducted either during 500 h at a fixed temperature gradient or after 1000 thermal cycles. However, when the temperature was increased to 350 °C, pesting phenomena were detected at the hot side of the n-type Mg2(Si,Sn) legs caused by the decomposition/oxidation of the material. These phenomena are strongly slowed down for thermoelectric modules tested under a primary vacuum, highlighting the predominant role of oxygen in the degradation mechanism. Interdiffusion phenomena are the most pronounced at the interface of the hot side of the n-type thermoelectric materials. The formation of a MgO layer, which is an electrical and thermal insulator, has decreased the thermoelectric modules' performances. For thermal cycling tests, cracks are observed on the hot side of the n-type legs. The presence of these cracks drastically increases the thermal and electrical resistances, leading to an overheating of the system and limiting its efficiency and failure by breaking electrical continuity. The interfaces at the hot side temperature of the p-type HMS legs remained intact whatever the test conditions were, indicating a high chemical stability and a good mechanical strength.

A Novel N-Type Molecular Dopant With a Closed-Shell Electronic Structure Applicable to the Vacuum-Deposition Process.

Rational design, synthesis, and characterization of a new efficient versatile n-type dopant with a closed-shell electronic structure are described. By employing the tetraphenyl-dipyranylidene (DP0) framework with two 7π-electron systems modified with N,N-dimethylamino groups as the strong electron-donating substituent, 2,2',6,6'-tetrakis[4-(dimethylamino)phenyl]-4,4'-dipyranylidene (DP7), a closed-shell molecule with an extremely high-lying energy level of the highest occupied molecular orbital, close to 4.0eV below the vacuum level, is successfully developed. Thanks to its thermal stability, DP7 is applicable to vacuum deposition, which allows utilization of DP7 in bulk doping for the development of n-type organic thermoelectric materials and contact doping for reducing contact resistance in n-type organic field-effect transistors. As vacuum-deposition processable n-type dopants are very limited, DP7 stands out as a useful n-type dopant, particularly for the latter purpose.

The thermo-electromotive force of an N-type thermoelectric material: Assessment of the contribution due to the presence of minority carriers

The thermo-electromotive force of an N-type thermoelectric material: Assessment of the contribution due to the presence of minority carriers

Layered PrZnOX (X = P, As) compounds: Promising n-type thermoelectric materials with low lattice thermal conductivity

The heteronanionic materials exhibit low lattice thermal conductivities and excellent electronic transport properties on account of their unique ZrSiCuAs-type crystal structures. In this study, the thermoelectric (TE) properties of the layered PrZnOX (X = P, As) compounds are theoretically elaborated in combination with the first-principles calculations and Boltzmann transport theory. The layered PrZnOP and PrZnOAs compounds are direct semiconductors with wide bandgaps of 1.96 eV and 1.88 eV within HSE06 hybrid functional, respectively. Through a comprehensive evaluation of the crystal structures and mechanical properties, the PrZnOX (X = P, As) compounds are thermodynamically and mechanically stable. The lattice thermal conductivities are 3.60 W/mK and 3.26 W/mK for the PrZnOP and PrZnOAs compounds at 300 K, respectively. The n-type layered PrZnOX (X = P, As) compounds exhibit large power factors on account of the high electrical conductivities and large Seebeck coefficients compared to the p-type situations. Consequently, the high figure of merits (ZTs) of 1.88 and 1.78 at 1000 K are discovered for the n-type PrZnOX (X = P, As) compounds. Our current work not only reveals the underlying mechanisms responsible for the excellent TE properties of PrZnOX (X = P, As) materials, but also elaborates the potential TE application of heteroanionic material at high temperature.

Computational approach to enhance thermoelectric performance of Ag2Se by S and Te substitutions.

Designing an n-type thermoelectric material with a high thermoelectric figure of merit at near room temperature is extremely challenging. Generally, pristine Ag2Se reveals unusually low thermal conductivity along with a high electrical conductivity and Seebeck coefficient, which leads to high thermoelectric performance (n-type) at room temperature. Herein, we report a pseudo-ternary phase (Ag2Se0.5Te0.25S0.25) that exhibits significantly high thermoelectric performance (zT ∼ 2.1) even at 400 K. First-principles calculation reveals that the Rashba type of spin-dependent band spitting, which originates due to sulfur and tellurium substitution, helps to improve the thermopower magnitude. We also show that the intrinsic carrier mobility is not only controlled by the carrier effective mass but is substantially limited by longitudinal acoustic and optical phonon modes, which is an extension of the deformation potential theory. Locally off-center sulfur atoms, together with the increase in configurational entropy via substitution of Te and S atoms in Ag2Se, lead to a drastic reduction in the lattice thermal conductivity (klat ∼ 0.34 W m-1 K-1 at 400 K). The Rashba effect coupled with the configurational entropy synergistically results in a high thermoelectric figure of merit in the n-type thermoelectric material working in the near-room-temperature regime.

Synergistic carrier and phonon transport advance Ag dynamically-doped n-type PbTe thermoelectrics via Mn alloying

Optimizing n-type PbTe thermoelectric materials to match their better-performing p-type counterparts is critical for realizing their practical applications.

Doping-induced grain refinement contributes to enhanced thermoelectric performance of n-type PbSe at room temperature

The grain refinement leads to an ultra-low lattice thermal conductivity and a relatively high room-temperature zT value in n-type PbSe thermoelectric materials.

A new thermoelectric Ag8SiSe6 argyrodite for room temperature application: sensitivity of thermoelectric performance to cooling conditions

Ag8SiSe6 is a promising n-type thermoelectric material for near-room temperature applications. We reveal the pronounced relationship between the cooling conditions (including quenching parameters) and thermoelectric properties of Ag8SiSe6 compounds.

Production of (Fe,Co)Si2 and (Fe.Mn)Si2 Thermoelectric Materials by Spark Plasma Sintering

In this study, FeSi2 bulk specimens were prepared by mechanical alloying, spark plasma sintering, and subsequent annealing. The annealed FeSi2 bulk specimens consisted of the β-FeSi2 phase and exhibited high Seebeck coefficient values. The maximum Seebeck coefficient of 356 μVK−1 was achieved in the FeSi2 bulk specimen annealed at 1173 K for 6 h. However, the power factor of the FeSi2 bulk specimen was quite small due to its high electrical resistivity, and a drastic improvement is required. Therefore, Mn- and Co-substituted specimens, Fe1−xMnxSi2 (x = 0.2–0.8) and Fe1−xCoxSi2 (x = 0.2–0.8), were produced, and their thermoelectric properties were evaluated. The Mn- and Co-substituted specimens exhibited lower electrical resistivity and a higher power factor than the FeSi2 bulk specimen. The Fe1−xMnxSi2 (x = 0.2–0.8) bulk specimens were p-type thermoelectric materials, and a Seebeck coefficient of 262 μVK−1 and a power factor of 339 μWm−1K−2 were achieved in the Fe0.94Mn0.06Si2 bulk specimen. On the other hand, the Fe1−xCoxSi2 (x = 0.2–0.8) bulk specimens were n-type thermoelectric materials, and a Seebeck coefficient of −180 μVK−1 and a power factor of 667 μWm−1K−2 were achieved in the Fe0.96Co0.04Si2 bulk specimen.

Boosting room-temperature thermoelectric performance of Mg3Sb1.5Bi0.5 material through breaking the contradiction between carrier concentration and carrier mobility

Recently, low-cost Mg3Sb2-yBiy-based materials with high performance near room temperatures have been reported, which are considered to be promising candidates for replacing commercial Bi2Te3-based materials. Herein, we report a high power factor of ∼3000 μW m−1 K−1 and a ZT value of 0.82 at room temperature in Ti0.05Mg3.15Sb1.5Bi0.49Te0.01. Ti dopant entering into the Mg1 sublattice slightly increases the carrier concentration by the introduction of impurity states and enhances the carrier mobility by the promotion of grain size growth. The two-donor doping strategy breaks the contradiction between carrier concentration and carrier mobility, thus significantly improving the electrical transport properties near room temperatures. In addition, the introduction of defects by Ti dopant contributes to the decreased lattice thermal conductivity. Consequently, a ZTavg value of 1.19 is obtained at 50–250 ℃, ranking the top among reported n-type thermoelectric materials. Moreover, the as-fabricated single-leg device shows an high engineering efficiency considering the radiation calibration, i.e., 7.2 % and 11.8 % at temperature differences of 250 ℃ and 500 ℃ with Tc = 0 ℃, respectively, demonstrating the great potential of substituting the commercial Bi2Te3-family.

Optimization of the thermoelectric properties of Nb-doped Bi2S3 via high pressure and high temperature

Bi2S3 is considered a promising n-type thermoelectric (TE) material due to its inherent extremely low thermal conductivity and earth-abundant elements. Herein, we have successfully synthesized a series of Nb-doped Bi2S3 (Bi2-xNbxS3, x = 0, 0.01, 0.02, 0.03) bulk materials by combining mechanical alloying (MA) method and high pressure and high temperature (HPHT) technology. The enhanced electrical transport properties of the Nb-doped Bi2S3 samples is derived from the collaborative optimization of carrier concentration and effective mass, which indicated that the Nb dopants acted as donors. In addition, differences in the direction of the lattice fringes were observed in the samples, and a large number of lattice distortions and dislocation defects occurred, which significantly reduced the lattice thermal conductivity of the Bi2S3 material. Consequently, a peak zT of 0.22 was achieved for the Bi1.98Nb0.02S3 sample at 543 K, which is 120% higher than that of pure Bi2S3.

High-Performance and Ecofriendly Organic Thermoelectrics Enabled by N-Type Polythiophene Derivatives with Doping-Induced Molecular Order.

The ability of n-type polymer thermoelectric materials to tolerate high doping loading limits further development of n-type polymer conductivity. Herein, two alcohol-soluble n-type polythiophene derivatives that are n-PT3 and n-PT4 are reported. Due to the ability of two polymers to tolerate doping loading more significantly than 100mol%, both achieve electrical conductivity >100Scm-1 . Moreover, the conductivity of both polythiophenes remains almost constant at high doping concentrations with excellent doping tunability, which may be related to their ability to overcome charging-induced backbone torsion and morphology change caused by saturated doping. The characterizations reveal that n-PT4 has a high doping level and carrier concentration (>3.10×1020 cm-3 ), and the carrier concentration continues to increase as the doping concentration increases. In addition, doping leads to improved crystal structure of n-PT4, and the crystallinity does not decrease significantly with increasing doping concentration; even the carrier mobility increases with it. The synergistic effect of these two leads to both n-PT3 and n-PT4 achieving a breakthrough of 100 in conductivity and power factor. The DMlmC-doped n-PT4 achieves a power factor of over 150µWm-1 K-2 . These values are among the highest for n-type organic thermoelectric materials.

High-Performance Paper-Based Thermoelectric Generator from Cu2SnS3 Nanocubes and Bulk-Traced Bismuth.

Flexible paper-based thermoelectric generators (PTEGs) have drawn significant interest in recent years due to their various advantages such as flexibility, adaptability, environment friendliness, low cost, and easy fabrication process. However, the reported PTEG's output performance still lags behind the performance of other flexible devices as it is not so easy to obtain a compact film on a paper-based substrate with desirable power output with the standard thermoelectric (TE) materials that have been previously utilized. In this direction, Cu2SnS3 (CTS), an earth-abundant, ternary sulfide, can be a good choice p-type semiconductor, when paired with a suitable n-type TE material. In this article, CTS nanocubes are synthesized via a simple hot injection method and a thick film device on emery paper was prepared and optimized. Furthermore, a flexible, 20-pair PTEG is fabricated with p-type CTS legs and traced and pressed n-type bismuth legs assembled using Kapton tape that produced a significantly high output power of 2.18 μW (output power density ∼0.85 nW cm-2 K-1) for a temperature gradient of ΔT = 80 K. The TE properties are also supported by finite element simulation. The bending test conducted for the PTEG suggests device stability for up to 800 cycles with <0.05% change in the internal resistance. A proof-of-concept field-based demonstration for energy harvesting from waste heat of a motorbike exhaust is shown recovering an output power of ∼42 nW for ΔT = 20 K, corroborating the experimental and theoretical results.

Advances in n-type Bi2O2Se thermoelectric materials: Progress and perspective

Thermoelectric technology has been an alternative for waste heat recovery due to its ability to realize direct conversion between heat energy and electricity. As a promising n-type thermoelectric material, layer-structured Bi2O2Se has been widely investigated in the past decades. Bi2O2Se has excellent merits including extremely high carrier mobility, large Seebeck coefficient, low lattice thermal conductivity, low cost, as well as good thermal and chemical stability. Up to now, reviews especially regarding Bi2O2Se-based thermoelectric material are rare. Hence, in this review, the pristine features of Bi2O2Se as a thermoelectric material were systematically summarized from crystal structure and microstructure, electronic band structure, as well as phonon spectra. Then, the recent advances in optimization strategies for Bi2O2Se were updated and discussed, which were categorized into effective doping, texturing by shear exfoliation, and compositing with other materials. In the end, the current challenges and possible perspectives were highlighted toward further developments of Bi2O2Se.

High-Performance thermoelectric generator based on n-Type flexible composite and its application in Self-Powered temperature sensor

Thermoelectric generators (TEGs) present a promising alternative to conventional power generation technologies for powering low-power wearable electronics. However, there are few n-type organic materials with high thermoelectric and mechanical performance. In this study, a polymer (PEI-PFA) via low temperature polycondensation between polyethyleneimine (PEI) and paraformaldehyde (PFA) is synthesized, and this is the first reported polymer which can be not only used as a novel n-type dopant of single-walled carbon nanotubes (SWCNTs) due to the PEI moieties, but also endow the composites with good flexibility due to the synthesized hemiaminal dynamic covalent networks. Subsequent addition of sodium hydroxide (NaOH) can further enhance the electron conductivity. The resulting SWCNT/PEI-PFA/NaOH composite film exhibits an excellent electrical conductivity of 1641.22 S cm−1, a power factor of 115.77 μW m−1 K−2, a tensile strength of 36.4 MPa, and flexibility up to 21 % strain. In addition, a self-powered sensor is assembled, which can efficiently monitor temperature changes and strain deformations. Moreover, a thermoelectric device containing five p-n junctions based on the pristine SWCNTs and SWCNT/PEI-PFA/NaOH composite is prepared, and a high output power of 5.33 μW at the temperature difference of 75.2 K is achieved. This work presents a promising approach to developing flexible n-type thermoelectric materials for self-powered wearable electronics.