High Conductivity and Electron‐Transfer Validation in an n‐Type Fluoride‐Anion‐Doped Polymer for Thermoelectrics in Air

Air‐stable and soluble tetrabutylammonium fluoride (TBAF) is demonstrated as an efficient n‐type dopant for the conjugated polymer ClBDPPV. Electron transfer from F^? anions to the π‐electron‐deficient ClBDPPV through anion–π electronic interactions is strongly corroborated by the combined results of electron spin resonance, UV–vis–NIR, and ultraviolet photoelectron spectroscopy. Doping of ClBDPPV with 25 mol% TBAF boosts electrical conductivity to up to 0.62 S cm^?1, among the highest conductivities that have been reported for solution‐processed n‐type conjugated polymers, with a thermoelectric power factor of 0.63 µW m^?1 K^?2 in air. Importantly, the Seebeck coefficient agrees with recently published correlations to conductivity. Moreover, the F^?‐doped ClBDPPV shows significant air stability, maintaining the conductivity of over 0.1 S cm^?1 in a thick film after exposure to air for one week, to the best of our knowledge the first report of an air‐stable solution‐processable n‐doped conductive polymer with this level of conductivity. The result shows that using solution‐processable small‐anion salts such as TBAF as an n‐dopant of organic conjugated polymers possessing lower LUMO (lowest unoccupied molecular orbital), less than ?4.2 eV) can open new opportunities toward high‐performance air‐stable solution‐processable n‐type thermoelectric (TE) conjugated polymers.     

A Chemically Doped Naphthalenediimide‐Bithiazole Polymer for n‐Type Organic Thermoelectrics

The synthesis of a novel naphthalenediimide (NDI)‐bithiazole (Tz2)‐based polymer [P(NDI2OD‐Tz2)] is reported, and structural, thin‐film morphological, as well as charge transport and thermoelectric properties are compared to the parent and widely investigated NDI‐bithiophene (T2) polymer [P(NDI2OD‐T2)]. Since the steric repulsions in Tz2 are far lower than in T2, P(NDI2OD‐Tz2) exhibits a more planar and rigid backbone, enhancing π–π chain stacking and intermolecular interactions. In addition, the electron‐deficient nature of Tz2 enhances the polymer electron affinity, thus reducing the polymer donor–acceptor character. When n‐doped with amines, P(NDI2OD‐Tz2) achieves electrical conductivity (≈0.1 S cm^?1) and a power factor (1.5 µW m^?1 K^?2) far greater than those of P(NDI2OD‐T2) (0.003 S cm^?1 and 0.012 µW m^?1 K^?2, respectively). These results demonstrate that planarized NDI‐based polymers with reduced donor–acceptor character can achieve substantial electrical conductivity and thermoelectric response.     

Synergistic Impacts of Electrolyte Adsorption on the Thermoelectric Properties of Single‐Walled Carbon Nanotubes

Single‐walled carbon nanotubes are promising candidates for light‐weight and flexible energy materials. Recently, the thermoelectric properties of single‐walled carbon nanotubes have been dramatically improved by ionic liquid addition; however, controlling factors remain unsolved. Here the thermoelectric properties of single‐walled carbon nanotubes enhanced by electrolytes are investigated. Complementary characterization with absorption, Raman, and X‐ray photoelectron spectroscopy reveals that shallow hole doping plays a partial role in the enhanced electrical conductivity. The molecular factors controlling the thermoelectric properties of carbon nanotubes are systematically investigated in terms of the ionic functionalities of ionic liquids. It is revealed that appropriate ionic liquids show a synergistic enhancement in conductivity and the Seebeck coefficient. The discovery of significantly precise doping enables the generation of thermoelectric power factor exceeding 460 µW m^–1 K^–2.     

In Situ Oxidation Synthesis of p‐Type Composite with Narrow‐Bandgap Small Organic Molecule Coating on Single‐Walled Carbon Nanotube: Flexible Film and Thermoelectric Performance

Although composites of organic polymers or n‐type small molecule/carbon nanotube (CNT) have achieved significant advances in thermoelectric (TE) applications, p‐type TE composites of small organic molecules as thick surface coating layers on the surfaces of inorganic nanoparticles still remain a great challenge. Taking advantage of in situ oxidation reaction of thieno[3,4‐b]pyrazine (TP) into TP di‐N‐oxide (TPNO) on single‐walled CNT (SWCNT) surface, a novel synthesis strategy is proposed to achieve flexible films of TE composites with narrow‐bandgap (1.19 eV) small molecule coating on SWCNT surface. The TE performance can be effectively enhanced and conveniently tuned by poly(sodium‐p‐styrenesulfonate) content, TPNO/SWCNT mass ratio, and posttreatment by various polar solvents. The maximum of the composite power factor at room temperature is 29.4 ± 1.0 µW m^?1 K^?2. The work presents a way to achieve flexible films of p‐type small organic molecule/inorganic composites with clear surface coating morphology for TE application.     

Ultrahigh Power Factor and Ultralow Thermal Conductivity at Room Temperature in PbSe/SnSe Superlattice: Role of Quantum-Well Effect

Reduced dimension is one of the effective strategies to modulate thermoelectric properties. In this work, n-type PbSe/SnSe superlattices with quantum-well (QW) structure are fabricated by pulsed laser deposition. Here, it is demonstrated that the PbSe/SnSe multiple QW (MQW) shows a high power factor of ≈25.7 µW cm^?1 K^?2 at 300 K, four times larger than that of PbSe single layers. In addition, thermal conductivity falls below 0.32 ± 0.06 W m^?1 K^?1 due to the phonon scattering at interface when the PbSe well thickness is confined within the scale of phonon mean free path (1.8 nm). Featured with ultrahigh power factor and ultralow thermal conductivity, ZT at room temperature is significantly increased from 0.14 for PbSe single layer to 1.6 for PbSe/SnSe MQW.     

2D Chalcogenide Nanoplate Assemblies for Thermoelectric Applications

Engineered atomic dislocations have been used to create a novel, Sb₂Te₃ nanoplate‐like architecture that exhibits a unique antisymmetric chirality. High‐resolution transmission electron microscopy (HRTEM) coupled with atomic force microscopy and X‐ray photoelectron spectroscopy reveals the architectures to be extremely well ordered with little residual strain. Surface modification of these topologically complex macrostructures (≈3 µm) has been achieved by direct growth of metallic Ag nanoparticles onto the edge sites of the Sb₂Te₃. Again, HRTEM shows this nanoparticle decoration to be atomically sharp at the boundaries and regularly spaced along the selvedge of the nanostructure. Transport experiments of densified films of these assemblies exhibit marked increases in carrier density after nanoengineering, yielding 3.5 × 10⁴ S m^?1 in electrical conductivity. An increased Seebeck coefficient by 20% in parallel with electrical conductivity is also observed. This gives a thermoelectric power factor of 371 µW m^?1 K^?2, which is the highest value for a flexible, freestanding film to date. These results suggest an entirely new direction in the search for wearable power harvesters based on topologically complex, low‐dimensional nanoassemblies.     

High Thermoelectric Performance in n-Type Perylene Bisimide Induced by the Soret Effect

Low-cost, non-toxic, abundant organic thermoelectric materials are currently under investigation for use as potential alternatives for the production of electricity from waste heat. While organic conductors reach electrical conductivities as high as their inorganic counterparts, they suffer from an overall low thermoelectric figure of merit (ZT) due to their small Seebeck coefficient. Moreover, the lack of efficient n-type organic materials still represents a major challenge when trying to fabricate efficient organic thermoelectric modules. Here, a novel strategy is proposed both to increase the Seebeck coefficient and achieve the highest thermoelectric efficiency for n-type organic thermoelectrics to date. An organic mixed ion–electron n-type conductor based on highly crystalline and reduced perylene bisimide is developed. Quasi-frozen ionic carriers yield a large ionic Seebeck coefficient of −3021 μV K⁻¹, while the electronic carriers dominate the electrical conductivity which is as high as 0.18 S cm⁻¹ at 60% relative humidity. The overall power factor is remarkably high (165 μW m⁻¹ K⁻²), with a ZT = 0.23 at room temperature. The resulting single leg thermoelectric generators display a high quasi-constant power output. This work paves the way for the design and development of efficient organic thermoelectrics by the rational control of the mobility of the electronic and ionic carriers.     

Simultaneous Boost of Power Factor and Figure‐of‐Merit in In–Cu Codoped SnTe

Significantly enhanced thermoelectric performance is achieved for eco‐friendly SnTe by a coorperative effect between a dopant resonant energy level and interstitial defects. By manipulating the band structure through indium doping, the Seebeck coefficient is remarkably improved, leading to an enhanced power factor, with a high level of ≈29 µW cm^?1 K^?2 at 873 K. Lattice thermal conductivity is sharply reduced, approaching the amorphous limit, through the strong phonon scattering induced by multiple scales of Cu₂Te nanoprecipitates, as well as Cu interstitials, leading to a high ZT value of ≈1.55 at 873 K.     

Tailoring the Thermal and Mechanical Properties of Graphene Film by Structural Engineering

Due to substantial phonon scattering induced by various structural defects, the in‐plane thermal conductivity (K) of graphene films (GFs) is still inferior to the commercial pyrolytic graphite sheet (PGS). Here, the problem is solved by engineering the structures of GFs in the aspects of grain size, film alignment, and thickness, and interlayer binding energy. The maximum K of GFs reaches to 3200 W m^?1 K^?1 and outperforms PGS by 60%. The superior K of GFs is strongly related to its large and intact grains, which are over four times larger than the best PGS. The large smooth features about 11 µm and good layer alignment of GFs also benefit on reducing phonon scattering induced by wrinkles/defects. In addition, the presence of substantial turbostratic‐stacking graphene is found up to 37% in thin GFs. The lacking of order in turbostratic‐stacking graphene leads to very weak interlayer binding energy, which can significantly decrease the phonon interfacial scattering. The GFs also demonstrate excellent flexibility and high tensile strength, which is about three times higher than PGS. Therefore, GFs with optimized structures and properties show great potentials in thermal management of form‐factor‐driven electronics and other high‐power‐driven systems.     

Quantum-Confinement-Enhanced Thermoelectric Properties in Modulation-Doped GaAs–AlGaAs Core–Shell Nanowires

Nanowires (NWs) hold great potential in advanced thermoelectrics due to their reduced dimensions and low-dimensional electronic character. However, unfavorable links between electrical and thermal conductivity in state-of-the-art unpassivated NWs have, so far, prevented the full exploitation of their distinct advantages. A promising model system for a surface-passivated one-dimensional (1D)-quantum confined NW thermoelectric is developed that enables simultaneously the observation of enhanced thermopower via quantum oscillations in the thermoelectric transport and a strong reduction in thermal conductivity induced by the core–shell heterostructure. High-mobility modulation-doped GaAs/AlGaAs core–shell NWs with thin (sub-40 nm) GaAs NW core channel are employed, where the electrical and thermoelectric transport is characterized on the same exact 1D-channel. 1D-sub-band transport at low temperature is verified by a discrete stepwise increase in the conductance, which coincided with strong oscillations in the corresponding Seebeck voltage that decay with increasing sub-band number. Peak Seebeck coefficients as high as ≈65–85 µV K⁻¹ are observed for the lowest sub-bands, resulting in equivalent thermopower of S²σ ≈ 60 µW m⁻¹ K⁻² and S²G ≈ 0.06 pW K⁻² within a single sub-band. Remarkably, these core–shell NW heterostructures also exhibit thermal conductivities as low as ≈3 W m⁻¹ K⁻¹, about one order of magnitude lower than state-of-the-art unpassivated GaAs NWs.     

Boosting the Thermoelectric Performance of the Doped DPP-EDOT Conjugated Polymer by Incorporating an Ionic Additive

The thermoelectric (TE) performance of organic materials is limited by the coupling of Seebeck coefficient and electrical conductivity. Herein a new strategy is reported to boost the Seebeck coefficient of conjugated polymer without significantly reducing the electrical conductivity by incorporation of an ionic additive DPPNMe₃Br . The doped polymer PDPP - EDOT thin film exhibits high electrical conductivity up to 1377 ± 109 S cm⁻¹ but low Seebeck coefficient below 30 µV K⁻¹ and a maximum power factor of 59 ± 10 µW m⁻¹ K⁻². Interestingly, incorporation of small amount (at a molar ratio of 1:30) of DPPNMe₃Br into PDPP - EDOT results in the significant enhancement of Seebeck coefficient along with the slight decrease of electrical conductivity after doping. Consequently, the power factor (PF) is boosted to 571 ± 38 µW m⁻¹ K⁻² and ZT reaches 0.28 ± 0.02 at 130 °C, which is among the highest for the reported organic TE materials. Based on the theoretical calculation, it is assumed that the enhancement of TE performance for the doped PDPP - EDOT by DPPNMe₃Br is mainly attributed to the increase of energetic disorder for PDPP - EDOT .     

Revealing the Enhanced Thermoelectric Properties of Controllably Doped Donor-Acceptor Copolymer: The Impact of Regioregularity

Albeit considerable attention to the fast-developing organic thermoelectric (OTE) materials due to their flexibility and non-toxic features, it is still challenging to design an OTE polymer with superior thermoelectric properties. In this work, two “isomorphic” donor–acceptor (D–A) conjugated polymers are studied as the semiconductor in OTE devices, revealing for the first time the internal mechanism of regioregularity on thermoelectric performances in D–A type polymers. A higher molecular structure regularity can lead to higher crystalline order and mobility, higher doping efficiency, order of energy state, and thermoelectric (TE) performance. As a result, the regioregular P2F exhibits a maximum power factor (PF) of up to 113.27 µW m⁻¹ K⁻², more than three times that of the regiorandom PRF (35.35 µW m⁻¹ K⁻²). However, the regular backbone also implies lower miscibility with a dopant, negatively affecting TE performance. Therefore, the trade-off between doping efficiency and miscibility plays a vital role in OTE materials, and this work sheds light on the molecular design strategy of OTE polymers with state-of-the-art performances.     

Vertically Aligned Niobium Nanowire Arrays for Fast‐Charging Micro‐Supercapacitors

Planar micro‐supercapacitors are attractive for system on chip technologies and surface mount devices due to their large areal capacitance and energy/power density compared to the traditional oxide‐based capacitors. In the present work, a novel material, niobium nanowires, in form of vertically aligned electrodes for application in high performance planar micro‐supercapacitors is introduced. Specific capacitance of up to 1 kF m^?2 (100 mF cm^?2) with peak energy and power density of 2 kJ m^?2 (6.2 MJ m^?3 or 1.7 mWh cm^?3) and 150 kW m^?2 (480 MW m^?3 or 480 W cm^?3), respectively, is achieved. This remarkable power density, originating from the extremely low equivalent series resistance value of 0.27 Ω (2.49 µΩ m² or 24.9 mΩ cm²) and large specific capacitance, is among the highest for planar micro‐supercapacitors electrodes made of nanomaterials.     

Zero‐Field Nernst Effect in a Ferromagnetic Kagome‐Lattice Weyl‐Semimetal Co3Sn2S2

The discovery of magnetic topological semimetals has recently attracted significant attention in the field of topology and thermoelectrics. In a thermoelectric device based on the Nernst geometry, an external magnet is required as an integral part. Reported is a zero‐field Nernst effect in a newly discovered hard‐ferromagnetic kagome‐lattice Weyl‐semimetal Co₃Sn₂S₂. A maximum Nernst thermopower of ≈3 µV K^?1 at 80 K in zero field is achieved in this magnetic Weyl‐semimetal. The results demonstrate the possibility of application of topological hard magnetic semimetals for low‐power thermoelectric devices based on the Nernst effect and are thus valuable for the comprehensive understanding of transport properties in this class of materials.     

Design of Domain Structure and Realization of Ultralow Thermal Conductivity for Record‐High Thermoelectric Performance in Chalcopyrite

Chalcopyrite compound CuGaTe₂ is the focus of much research interest due to its high power factor. However, its high intrinsic lattice thermal conductivity seriously impedes the promotion of its thermoelectric performance. Here, it is shown that through alloying of isoelectronic elements In and Ag in CuGaTe₂, a quinary alloy compound system Cu_1?xAg_xGa_0.4In_0.6Te₂ (0 ≤ x ≤ 0.4) with complex nanosized strain domain structure is prepared. Due to strong phonon scattering mainly by this domain structure, thermal conductivity (at 300 K) drops from 6.1 W m^?1 K^?1 for the host compound to 1.5 W m^?1 K^?1 for the sample with x = 0.4. As a result, the optimized chalcopyrite sample Cu_0.7Ag_0.3Ga_0.4In_0.6Te₂ presents an outstanding performance, with record‐high figure of merit (ZT) reaching 1.64 (at 873 K) and average ZT reaching 0.73 (between ≈300 and 873 K), which are ≈37 and ≈35% larger than the corresponding values for pristine CuGaTe₂, respectively, demonstrating that such domain structure arising from isoelectronic multielement alloying in chalcopyrite compound can effectively suppress its thermal conductivity and elevate its thermoelectric performance remarkably.     

Revisiting some chalcogenides for thermoelectricity

Abstract

Thermoelectric materials that are efficient well above ambient temperature are needed to convert waste-heat into electricity. Many thermoelectric oxides were investigated for this purpose, but their power factor (PF) values were too small (～10^?4 W m^?1 K^?2) to yield a satisfactory figure of merit zT. Changing the anions from O^2? to S^2? and then to Se^2? is a way to increase the covalency. In this review, some examples of sulfides (binary Cr–S or derived from layered TiS₂) and an example of selenides, AgCrSe₂, have been selected to illustrate the characteristic features of their physical properties. The comparison of the only two semiconducting binary chromium sulfides and of a layered AgCrSe₂ selenide shows that the PF values are also in the same order of magnitude as those of transition metal oxides. In contrast, the PF values of the layered sulfides TiS₂ and Cu_0.1TiS₂ are higher, reaching ～10^?3 W m^?1 K^?2. Apparently the magnetism related to the Cr–S network is detrimental for the PF when compared to the d⁰ character of the Ti⁴⁺ based sulfides. Finally, the very low PF in AgCrSe₂ (PF = 2.25 × 10^?4 W m¹ K^?2 at 700 K) is compensated by a very low thermal conductivity (κ = 0.2 W m^?1 K^?1 from the measured C_p) leading to the highest zT value among the reviewed compounds (zT_700K = 0.8). The existence of a glassy-like state for the Ag⁺ cations above 475 K is believed to be responsible for this result. This result demonstrates that the phonon engineering in open frameworks is a very interesting way to generate efficient thermoelectric materials.     

N‐Type Organic Thermoelectrics: Improved Power Factor by Tailoring Host–Dopant Miscibility

In this contribution, for the first time, the polarity of fullerene derivatives is tailored to enhance the miscibility between the host and dopant molecules. A fullerene derivative with a hydrophilic triethylene glycol type side chain (PTEG‐1) is used as the host and (4‐(1,3‐dimethyl‐2,3‐dihydro‐1H‐benzoimidazol‐2‐yl)phenyl)dimethylamine n ‐DMBI) as the dopant. Thereby, the doping efficiency can be greatly improved to around 18% (<1% for a nonpolar reference sample) with optimized electrical conductivity of 2.05 S cm^?1, which represents the best result for solution‐processed fullerene derivatives. An in‐depth microstructural study indicates that the PTEG‐1 molecules readily form layered structures parallel to the substrate after solution processing. The fullerene cage plane is alternated by the triethylene glycol side chain plane; the n ‐DMBI dopants are mainly incorporated in the side chain plane without disturbing the π–π packing of PTEG‐1. This new microstructure, which is rarely observed for codeposited thin films from solution, formed by PTEG‐1 and n ‐DMBI molecules explains the increased miscibility of the host/dopant system at a nanoscale level and the high electrical conductivity. Finally, a power factor of 16.7 µW m^?1 K^?2 is achieved at 40% dopant concentration. This work introduces a new strategy for improving the conductivity of solution‐processed n‐type organic thermoelectrics.     

Thickness-dependent thermoelectric power factor of polymer-functionalized semiconducting carbon nanotube thin films

The effects of polymer structures on the thermoelectric properties of polymer-wrapped semiconducting carbon nanotubes have yet to be clarified for elucidating intrinsic transport properties. We systematically investigate thickness dependence of thermoelectric transport in thin films containing networks of conjugated polymer-wrapped semiconducting carbon nanotubes. Well-controlled doping experiments suggest that the doping homogeneity and then in-plane electrical conductivity significantly depend on film thickness and polymer species. This understanding leads to achieving thermoelectric power factors as high as 412 μW m^?1 K^?2 in thin carbon nanotube films. This work presents a standard platform for investigating the thermoelectric properties of nanotubes.     

Microstrucutre and thermoelectric properties of rapidly prepared Sn<Subscript>1−x</Subscript>Mn<Subscript>x</Subscript>Te alloys

Sn_1?xMn_xTe (x?=?0, 0.09, 0.15, 0.20) bulk materials were prepared by melt spinning combined with spark plasma sintering process. Nanoscale grains were obtained, and the solid solubility of Mn was much enhanced by the ultrafast-cooling synthesis technique. The maximum of Seebeck coefficient and power factor are 242 µVK^?1 and 19.97 µW cm^?1K^?2 at 873 K with the doping concentration of 15 at% Mn. A large amount of grain boundaries and doped atoms improve the scattering of heat-carrying phonons in a wide range of frequencies, and the scattering mechanisms are also explained by theoretical calculation. As a result, the minimum of lattice thermal conductivity is 0.66 µVK^?1 at 873 K, the corresponding figure of merit is 1.26 for Sn_0.85Mn_0.15Te sample. This value is improved by 35% comparing with previously reported result. Our work indicates that melt spinning process is effective to develop SnTe related thermoelectric materials with excellent thermoelectric properties, which has the widespread commercial value and the prospects for development.     

Ultrahigh Thermal Conductive yet Superflexible Graphene Films

Electrical devices generate heat at work. The heat should be transferred away immediately by a thermal manager to keep proper functions, especially for high‐frequency apparatuses. Besides high thermal conductivity (K ), the thermal manager material requires good foldability for the next generation flexible electronics. Unfortunately, metals have satisfactory ductility but inferior K (≤429 W m^?1 K^?1), and highly thermal‐conductive nonmetallic materials are generally brittle. Therefore, fabricating a foldable macroscopic material with a prominent K is still under challenge. This study solves the problem by folding atomic thin graphene into microfolds. The debris‐free giant graphene sheets endow graphene film (GF) with a high K of 1940 ± 113 W m^?1 K^?1. Simultaneously, the microfolds render GF superflexible with a high fracture elongation up to 16%, enabling it more than 6000 cycles of ultimate folding. The large‐area multifunctional GFs can be easily integrated into high‐power flexible devices for highly efficient thermal management.