Ionic Dopant-Induced Ordering Enhances the Thermoelectric Properties of a Polythiophene-Based Block Copolymer

Conjugated polymer-based block copolymers (CP-BCPs) are an unexplored class of materials for organic thermoelectrics. Herein, the authors report on the electronic conductivity (σ) and Seebeck coefficient (α) of a newly synthesized CP-BCP, poly(3-hexylthiophene)-block-poly (oligo-oxyethylene methacrylate) (P3HT-b-POEM), upon solution co-processing with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and subsequently vapor-doping with a molecular dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). It is found that the addition of the hydrophilic block POEM greatly enhances the processability of P3HT, enabling homogeneous solution-mixing with LiTFSI. Notably, interactions between P3HT-b-POEM with ionic species significantly improve molecular order and unexpectedly cause electrical oxidizing doping of P3HT block both in solution and solid-states, a phenomenon that has not been previously observed in Li-salt containing P3HT. Vapor doping of P3HT-b-POEM-LiTFSI thin films with F4TCNQ further enhances σ and yields a thermoelectric power factor PF = α²σ of 13.0  µ W m⁻¹ K⁻², which is more than 20 times higher than salt-free P3HT-b-POEM sample. Through modeling thermoelectric behaviors of P3HT-b-POEM with the Kang-Snyder transport model, the improvement in PF is attributed to higher electronic charge mobility originating from the enhanced molecular ordering of P3HT. The results demonstrate that solution co-processing CP-BCPs with a salt is a powerful method to control structure and performance of organic thermoelectric materials.     

Influence of electronic type of SWNTs on the thermoelectric properties of SWNTs/PANI composite films

Single-walled carbon nanotubes (SWNTs) have emerged as one of the leading additives for improving the thermoelectric properties of organic materials due to their unique structure and excellent electronic transport properties. However, since as-grown SWNTs possess different chirality, it is of high interest to determine the influence of electronic type of SWNTs on the thermoelectric properties of SWNTs/PANI composite films. Herein, we utilized metallic SWNTs (SWNT-M) and semiconducting SWNTs (SWNT-S) to prepare SWNTs/PANI composite films and studied their thermoelectric properties, respectively. Experimentally, the maximum thermoelectric power factor reached 51 μW m⁻¹ K⁻² for the 19 wt% SWNT-S/PANI composite films, while that value was only 16 μW m⁻¹ K⁻² for the 19 wt% SWNT-M/PANI composite films. The better power factor of SWNT-S/PANI composite films may be attributed to the more significantly enhanced Seebeck coefficient resulting from the effective energy filtering effect at the interfaces between SWNT-S and PANI. Our results reveal the influence of electronic type of SWNTs on the thermoelectric properties of composites, which will drive ongoing efforts to utilize SWNTs as fillers in nanocomposites for optimal thermoelectric properties.     

Enhanced Thermoelectric Performance in GeTe by Synergy of Midgap state and Band Convergence

The good co-existence of midgap state and valence band degeneracy is realized in Bi-alloyed GeTe through the In-Cd codoping to play different but complementary roles in the valence band structure modification. In doping induces midgap state and results in a considerably improved Seebeck coefficient near room temperature, while Cd doping significantly increases the Seebeck coefficient in the mid-high temperature region by promoting the valence band convergence. The synergistic effects obviously increase the density of state effective mass from 1.39 to 2.65 m₀, and the corresponding carrier mobility still reaches 34.3 cm² V⁻¹ s⁻¹ at room temperature. Moreover, the Bi-In-Cd co-alloying introduces various phonon scattering centers including nanoprecipitates and strain field fluctuations and suppresses the lattice thermal conductivity to a rather low value of 0.56 W m⁻¹ K⁻¹ at 600 K. As a result, the Ge_0.89Bi_0.06In_0.01Cd_0.04Te sample obtains excellent thermoelectric properties of zT_max ≈2.12 at 650 K and zT_avg ≈1.43 between 300 and 773 K. This study illustrates that the thermoelectric performance of GeTe can be optimized in a wide temperature range through the synergy of midgap state and valence band convergence.     

Enhanced power factor of poly (3,4-ethyldioxythiophene):poly (styrene sulfonate) (PEDOT:PSS)/RTCVD graphene hybrid films

The thermoelectric generator has been an attractive alternative power source to operate a wireless sensor node. Usually, inorganic compounds are most often used in thermoelectric devices, and hence, are extensively studied due to their superior thermoelectric performance. We have investigated a novel interfacial technique to fabricate a hybrid film of highly conductive PEDOT:PSS (poly 3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) and graphene. Organic materials PEDOT doped with PSS exhibits outstanding electrical properties due to its high conductivity, low bandgap, and energy migration. Furthermore, we utilized graphene fabricated by rapid thermal chemical vapor deposition (RTCVD) as a thermoelectric material. Our results show that the interfacial technique between substrate and hybrid film could be clearly improved due to the UV plasma treatment. The thermoelectric hybrid film of PEDOT:PSS and RTCVD graphene (P/RTG) exhibited an enhanced power factor of 56.28 μW m⁻¹ K⁻² with a Seebeck coefficient of 54.0 μV K⁻¹.     

Theoretical and experimental investigations of the thermoelectric properties of Al-, Bi- and Sn-doped ZnO

In this paper, the thermoelectric properties of ZnO doped with Al, Bi and Sn were investigated by combining experimental and theoretical methods. The average Seebeck coefficient of Bi doped ZnO over the measured temperature range is improved from −90 to −497 μV/K. However, segregation of Bi₂O₃ in ZnO:Bi sample, confirmed by FESEM, lead to enormous grain growth and low electrical conductivity, which makes Bi is not a good dopant to improve ZT value of ZnO. As a 4+ valence cation, Sn doping actually show an increase in carrier concentration to 10²⁰ cm⁻³, further enhancing the electrical conductivity. Unfortunately, the Seebeck coefficient of ZnO:Sn samples is even lower than pure ZnO sample, which lead to a low ZT value. As for ZnO:Al sample, with nearly no change in lattice thermal conductivity, electrical conductivity and Seebeck coefficient were both enhanced. Threefold enhancement in ZT value has been achieved in ZnO:Al sample at 760 °C compared with pure ZnO.     

Synthesis and Characterization of <Emphasis Type="Italic">p</Emphasis>-Type Pb<Subscript>0.5</Subscript>Sn<Subscript>0.5</Subscript>Te Thermoelectric Power Generation Elements by Mechanical Alloying

A mechanical alloying (MA) process to transform elemental powders into solid Pb_0.5Sn_0.5Te with thermoelectric functionality comparable to melt-alloyed material is described. The room-temperature doping level and mobility as well as temperature-dependent electrical conductivity, Seebeck coefficient, and thermal conductivity are reported. Estimated values of lattice thermal conductivity (0.7 W m⁻¹ K⁻¹) are lower than some reports of functional melt-alloyed PbSnTe-based material, providing evidence that MA can engender the combination of properties resulting in highly functional thermoelectric material. Though doping level and Sn composition have not been optimized, this material exhibits a ZT value >0.5 at 550 K.     

Influence of Substrate Temperature on Structural and Thermoelectric Properties of Antimony Telluride Thin Films Fabricated by RF and DC Cosputtering

Antimony and tellurium were deposited on BK7 glass using direct-current magnetron and radiofrequency magnetron cosputtering. Antimony telluride thermoelectric thin films were synthesized with a heated substrate. The effects of substrate temperature on the structure, surface morphology, and thermoelectric properties of the thin films were investigated. X-ray diffraction patterns revealed that the thin films were well crystallized. c-Axis preferred orientation was observed in thin films deposited above 250°C. Scanning electron microscopy images showed hexagonal crystallites and crystal grains of around 500 nm in thin film fabricated at 250°C. Energy-dispersive spectroscopy indicated that a temperature of 250°C resulted in stoichiometric Sb₂Te₃. Sb₂Te₃ thin film deposited at room temperature exhibited the maximum Seebeck coefficient of 190 μV/K and the lowest power factor (PF), S ² σ, of 8.75 × 10⁻⁵ W/mK². When the substrate temperature was 250°C, the PF increased to its highest value of 3.26 × 10⁻³ W/mK². The electrical conductivity and Seebeck coefficient of the thin film were 2.66 × 10⁵ S/m and 113 μV/K, respectively.     

Enhancement of the field-effect mobility of poly(3-hexylthiophene)/functionalized carbon nanotube hybrid transistors

With the aim of enhancing the field-effect mobility of poly(3-hexylthiophene) (P3HT) field-effect transistors (FETs), we added functionalized multiwalled carbon nanotubes (CNTs) to the P3HT solution prior to film formation. The nanotubes were found to be homogeneously dispersed in the P3HT films because of their functional groups. We found that at the appropriate CNT concentration (up to 10 wt% CNT), the P3HT FETs have a high field-effect mobility of 0.04 cm² V⁻¹ s⁻¹, which is an improvement by a factor of more than 10. This remarkable increase in the field-effect mobility over that of the pristine P3HT film is due to the high conductivity of the CNTs which act as conducting bridges between the crystalline regions of the P3HT film, and the reduction in the hole-injection barrier due to the low work function of CNTs, which results in more efficient carrier injection.     

Photo stability enhancement of Poly(3-hexylthiophene)-PCBM nanocomposites by addition of multi walled carbon nanotubes under ambient conditions

Poly(3-hexylthiophene)-Phenyl-C61-butyric acid methyl ester (P3HT–PCBM) composites find wide application in optoelectronic devices, especially bulk-hetero junction (BHJ) solar cells. These composites, even though could give efficient polymer solar cells with ∼4–5% power conversion efficiencies (PCE), a major problem of photo stability is associated with it and remains unsolved. P3HT–PCBM composite was found to be degrading on irradiation with ultraviolet radiation or a solar simulator providing AM1.5G illumination (1000 W m^–2, 72 ± 2 °C or 330 W m⁻², 25 °C), in presence of oxygen and moisture. Here, we have studied the photo stability of P3HT–PCBM under ambient conditions and showed that a new ternary composite, P3HT–PCBM–MWCNT (multi walled carbon nanotube) has superior photo stability even on extended UV–Vis exposure. A total of 7% (w/w) PCBM and 3% (w/w) MWCNT with respect to P3HT resulted in optimum stability. UV–Visible and fluorescence spectral analysis have been used to study the photo stability, both in solution state and solid/film state. Transmission electron micrograph (TEM) along with selected area electron diffraction (SAED) pattern and Field Emission Scanning Electron Microscopy (FE-SEM) micrographs have been used to show the well coating of MWCNT on P3HT–PCBM composite. Since MWCNT is one of the very important carbon based nanomaterial with several supreme characteristics, this new ternary composite has great importance for optoelectronic applications.     

Processing and doping of thick polymer active layers for flexible organic thermoelectric modules

While the majority of research on organic thermoelectric generators has focused on individual devices with organic films having thicknesses of several hundred nanometers (nano-films), films with micrometer-scale thicknesses (micro-films) provide a longer thermal conduction path that results in a larger temperature gradient and higher thermoelectric voltages in modules. In this study, the properties of solution-processed nano- and micro-films of the p-type semiconductor P3HT doped with two different dopants, F₄-TCNQ and Fe³⁺-tos₃·6H₂O, were investigated. While doping with F₄-TCNQ resulted in high electrical conductivity only in nano-films, doping with Fe³⁺-tos₃·6H₂O from a 25 mM solution yielded power factors of up to ∼30 μWm⁻¹ K⁻² with a conductivity of 55.4 Scm⁻¹ in micro-films. Changes in the molecular packing were compared based on X-ray diffraction, and the best operational stability in air was found for the Fe³⁺-tos₃·6H₂O-doped micro-films. Using Fe³⁺-tos₃·6H₂O as dopant, flexible thermoelectric modules with solution-processed micro-films patterned by a photo-etching technique that does not require alignment and assembly of individual devices were demonstrated, exhibiting a maximum power output of 1.94 nWK⁻² for a uni-leg module with 48 elements. Analysis of the flexible module performance showed that the performance is limited by the contact resistance, which must be taken into consideration when optimizing module structure.     

Increase in thermoelectric power factor of carbon-nanotube films after addition of polystyrene

We report the enhancement of the thermoelectric power factors of single-walled carbon-nanotube (SWCNT) films caused by the addition of polystyrene to the films as a binder. The Seebeck coefficient of the SWCNT films was increased by addition of polystyrene. On the other hand, the electrical conductivity gradually decreased with increasing amount of polystyrene. The power factor was maximum for a polystyrene concentration of 20 wt%; it was approximately 1.7 times higher than that of a pure SWCNT film. These results indicate that polystyrene is a superior binder polymer for synthesizing CNT/polymer thermoelectric composites.     

Highly improved thermoelectric performances of PEDOT:PSS/SWCNT composites by solvent treatment

Composites of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) and single-wall carbon nanotube (SWCNT) were prepared by mixing aqueous dispersions of PEDOT:PSS and SWCNT at different weight ratios. By being soaked with DMSO for 2 min at room temperature, the PEDOT:PSS/SWCNT composite with an optimized SWCNT weight ratio of 74 wt% exhibited a high electric conductivity of 3800 S cm⁻¹ and a reasonable Seebeck coefficient of 28 μV K⁻¹, leading to a promising power factor of 300 μW m⁻¹ K⁻² and a hopeful ZT value of 0.13. Possible reasons for the highly improved properties are carefully discussed.     

Tailoring Interfacial Charge Transfer for Optimizing Thermoelectric Performances of MnTe-Sb2Te3 Superlattice-Like Films

Interfacial charge transfer has a vital role in tailoring the thermoelectric performance of superlattices (SLs), which, however, is rarely clarified by experiments. Herein, based on epitaxially grown p-type (MnTe)_x(Sb₂Te₃)_y superlattice-like films, synergistically optimized thermoelectric parameters of carrier density, carrier mobility, and Seebeck coefficient are achieved by introducing interfacial charge transfer, in which effects of hole injection, modulation doping, and energy filtering are involved. Carrier transport measurements and angle-resolved photoemission spectroscopy (ARPES) characterizations reveal a strong hole injection from the MnTe layer to the Sb₂Te₃ layer in the SLs, originating from the work function difference between MnTe and Sb₂Te₃. By reducing the thickness of MnTe less than one monolayer, all electronic transport parameters are synergistically optimized in the quantum-dots (MnTe)_x(Sb₂Te₃)₁₂ superlattice-like films, leading to much improved thermoelectric power factors (PFs). The (MnTe)_0.1(Sb₂Te₃)₁₂ obtains the highest room-temperature PF of 2.50 mWm⁻¹K⁻², while the (MnTe)_0.25(Sb₂Te₃)₁₂ possesses the highest PF of 2.79 mWm⁻¹K⁻² at 381 K, remarkably superior to the values acquired in binary MnTe and Sb₂Te₃ films. This research provides valuable guidance on understanding and rationally tailoring the interfacial charge transfer of thermoelectric SLs to further enhance thermoelectric performances.     

Thin-Film Thermoelectric Module for Power Generator Applications Using a Screen-Printing Method

A new process for fabricating a low-cost thermoelectric module using a screen-printing method has been developed. Thermoelectric properties of screen-printed ZnSb films were investigated in an effort to develop a thermoelectric module with low cost per watt. The screen-printed Zn _x Sb_1−x films showed a low carrier concentration and high Seebeck coefficient when x was in the range of 0.5 to 0.57 and the annealing temperature was kept below 550°C. When the annealing temperature was higher than 550°C, the carrier concentration of the Zn _x Sb_1−x films reached that of a metal, leading to a decrease of the Seebeck coefficient. In the present experiment, the optimized carrier concentration of screen-printed ZnSb was 7 × 10¹⁸/cm³. The output voltage and power density of the ZnSb film were 10 mV and 0.17 mW/cm², respectively, at ΔT = 50 K. A thermoelectric module was produced using the proposed screen-printing approach with ZnSb and CoSb₃ as p-type and n-type thermoelectric materials, respectively, and copper as the pad metal.     

Enhancing the Thermoelectric Properties via Modulation of Defects in P-Type MNiSn-Based (M = Hf,Zr, Ti) Half-Heusler Materials

The thermoelectric figure-of-merit (zT) of p-type MNiSn (M = Ti, Zr, or Hf) half-Heusler compounds is lower than their n-type counterparts due to the presence of a donor in-gap state caused by Ni occupying tetrahedral interstitials. While ZrNiSn and TiNiSn, have been extensively studied, HfNiSn remains unexplored. Herein, this study reports an improved thermoelectric property in p-type HfNi_1−xCo_xSn. By doping 5 at% Co at the Ni sites, the Seebeck coefficient becomes reaching a peak value exceeding 200 µV K⁻¹ that breaks the record of previous reports. A maximum power factor of ≈2.2 mW m⁻¹ K⁻² at 973 K is achieved by optimizing the carrier concentration. The enhanced p-type transport is ascribed to the reduced content of Ni defects, supported by first principle calculations and diffraction pattern refinement. Concomitantly, Co doping also softens the lattice and scatters phonons, resulting in a minimum lattice thermal conductivity of ≈1.8 W m⁻¹ K⁻¹. This leads to a peak zT of 0.55 at 973 K is realized, surpassing the best performing p-type MNiSn by 100%. This approach offers a new method to manipulate the intrinsic atomic disorder in half-Heusler materials, facilitating further optimization of their electronic and thermal properties.     

Superior Thermoelectric Performance of Textured Ca3Co4−xO9+δ Ceramic Nanoribbons

Calcium cobaltite Ca₃Co_4−xO_9+δ (CCO) is a promising p-type thermoelectric (TE) material for high-temperature applications in air. The grains of the material exhibit strong anisotropic properties, making texturing and nanostructuring mostly favored to improve thermoelectric performance. On the one hand multitude of interfaces are needed within the bulk material to create reflecting surfaces that can lower the thermal conductivity. On the other hand, low residual porosity is needed to improve the contact between grains and raise the electrical conductivity. In this study, CCO fibers with 100% flat cross sections in a stacked, compact form are electrospun. Then the grains within the nanoribbons in the plane of the fibers are grown. Finally, the nanoribbons are electrospun into a textured ceramic that features simultaneously a high electrical conductivity of 177 S cm⁻¹ and an immensely enhanced Seebeck coefficient of 200 µV K⁻¹ at 1073 K are assembled. The power factor of 4.68 µW cm⁻¹ K⁻² at 1073 K in air surpasses all previous CCO TE performances of nanofiber ceramics by a factor of two. Given the relatively high power factor combined with low thermal conductivity, a relatively large figure-of-merit of 0.3 at 873 K in the air for the textured nanoribbon ceramic is obtained.     

A facile chemical reduction approach for effectively tuning thermoelectric properties of PEDOT films

Poly(3,4-ethylenedioxythiophene)–tosylate–polyethylene glycol–polypropylene glycol–polyethylene glycol (PEDOT–Tos–PPP) films were prepared via a vapor phase polymerization (VPP) method. The films possess good electrical conductivity (1550 S cm⁻¹), low Seebeck coefficient (14.9 μV K⁻¹) and thermal conductivity (0.501 W m⁻¹ K⁻¹), and ZT ∼ 0.02 at room temperature (RT, 295 K). Then, the films were treated with NaBH₄/DMSO solutions of different NaBH₄ concentrations to adjust the redox level. After the NaBH₄/DMSO treatment (dedoping), the electrical conductivity of the films continuously decreased from 1550 to 5.7 S cm⁻¹, whereas the Seebeck coefficient steeply increased from 14.9 to 143.5 μV K⁻¹. A maximum power factor of 98.1 μW m⁻¹ K⁻² has been achieved at an optimum redox level. In addition, the thermal conductivity of the PEDOT–Tos–PPP films decrease from 0.501 to 0.451 W m⁻¹ K⁻¹ after treated with 0.04% NaBH₄/DMSO solution. A maximum ZT value of 0.064 has been achieved at RT. The electrical conductivity and thermal conductivity (Seebeck coefficient) of the untreated and 0.04% NaBH₄/DMSO treated PEDOT–Tos–PPP films decrease (increases) with increasing temperature from 295 to 385 K. And the power factor of the films monotonically increases with temperature. The ZT at 385 K of the 0.04% NaBH₄/DMSO treated film is 0.155.     

Planar and three-dimensional thick-film thermoelectric microgenerators

This paper describes manufacturing process as well as thermoelectric properties and long-term stability of planar and three-dimensional (3-D) thermoelectric structures made in thick-film/LTCC technology. Screen-printed thick-film thermocouples based on PdAg, Ag and Ni inks were manufactured and investigated. Seebeck coefficient and electrical output power were measured with the help of custom built automatic measuring system. Achieved results were compared with literature data and earlier authors’ results. Seebeck coefficient greater than 20 μV/K and about 5–8 μW/junction output power were measured for temperature difference of about 100 K for Ag–Ni thick-film planar thermocouple made on both used substrates. This combination of materials was chosen to create three-dimensional thick-film thermocouples (reported in the literature for the first time). We received for them Seebeck coefficient greater than 15 μV/K and the output power on level of 1 μW/junction for temperature gradient of about 60 K.     

Analysis of Thermoelectric Properties of Scaled Silicon Nanowires Using an Atomistic Tight-Binding Model

Low-dimensional materials provide the possibility of improved thermoelectric performance due to the additional length scale degree of freedom for engineering their electronic and thermal properties. As a result of suppressed phonon conduction, large improvements in the thermoelectric figure of merit, ZT, have recently been reported in nanostructures, compared to the raw materials. In addition, low dimensionality can improve a device’s power factor, offering an additional enhancement in ZT. In this work the atomistic sp³d⁵s* spin-orbit-coupled tight-binding model is used to calculate the electronic structure of silicon nanowires (NWs). The Landauer formalism is applied to calculate an upper limit for the electrical conductivity, the Seebeck coefficient, and the power factor. We examine n-type and p-type nanowires with diameters from 3 nm to 12 nm, in [100], [110], and [111] transport orientations, at different doping concentrations. Using experimental values for the lattice thermal conductivity in nanowires, an upper limit for ZT is computed. We find that at room temperature, scaling the diameter below 7 nm can at most double the power factor and enhance ZT. In some cases, however, scaling does not enhance the performance at all. Orientations, geometries, and subband engineering techniques for optimized designs are discussed.     

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

Functionally 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.