High-Performance N-Type Carbon Nanotube Composites: Improved Power Factor by Optimizing the Acridine Scaffold and Tailoring the Side Chains

Enhanced Thermoelectric Properties of Stable n-Type Ferrocene Derivatives-Doped Polyethylenimine/Single-Walled Carbon Nanotube Composite Films

Preparing stable n-type flexible single-walled carbon nanotube (SWCNT)-based thermoelectric films with high thermoelectric (TE) performance is desirable for self-powering wearable electronics but remains a challenge. Here, the interface regulation and thermoelectric enhancement mechanism of ferrocene derivatives on polyethylenimine/single-walled carbon nanotube (PEI/SWCNT) composite films have been explored by doping ferrocene derivatives (f-Fc-OH) into PEI/SWCNT films. The results show that the introduction of f-Fc-OH leads to the formation of "thorn" structures on the surfaces of SWCNT bundles via hydrophilic and hydrophobic interactions, the generated energy-filtering effect improves the thermoelectric properties of the PEI/SWCNT film, and the f-Fc-OH-doped PEI/SWCNT (f-Fc-OH/PEI/SWCNT) achieves the highest room-temperature power factor of 182.22 ± 8.60 μW m-1 K-2 with a Seebeck coefficient of -64.28 ± 0.96 μV K-1 and the corresponding ZT value of 4.69 × 10-3. The Seebeck coefficient retention ratio of the f-Fc-OH/PEI/SWCNT nearly remained 68% after being exposed to air for 3672 h, while the PEI/SWCNT film changed from n-type to p-type after being exposed to air for about 432 h. In addition, the temperature-dependent thermoelectric properties show that the f-Fc-OH/PEI/SWCNT achieves a high power factor of 334.57 μW m-1 K-2 at 353 K. Finally, a flexible TE module consisting of seven pairs of p-n junctions is assembled using the optimum composite film, which produces an open-circuit voltage of 42 mV and a maximum output power of 4.32 μW at a temperature gradient of 60 K. Therefore, this work provides guidance for preparing stable n-type SWCNT-based composite films with enhanced thermoelectric properties, which have potential applications in flexible generators and wearable electronic devices.

High-Power-Density and Excellent-Flexibility Thermoelectric Generator Based on All-SWCNTs/PVP Composites

Flexible polymer/single-wall carbon nanotube (SWCNT) composites are a vital component for wearable/portable electronics, but the development of their n-type counterpart is laggard. Furthermore, little attention is paid to the interaction between SWCNT and polymers, especially the unconjugated polymers, as well as the conversion mechanism of conduction characteristics. Here, the n-type flexible SWCNTs/Polyvinyl Pyrrolidone (PVP) films are successfully fabricated, where the oxygen atoms in PVP interacted with SWCNT via hydrogen bonds, which can lower the energy barrier of electron tunneling, providing the pathway for the electron transfer. Furthermore, with the increasing synthesis temperature, the hydrogen bonds strengthened and the thermal activation energy further improved, both of which enhanced the electron-donating ability of PVP, resulting in a high-power-factor value of 260 µW m-1 K-2. Based on the optimized SWCNTs/PVP films, a thermoelectric module is assembled, which achieved a power density of 400 µW cm-2 at a temperature difference of 56 K, coupled with excellent flexibility, showing a less than 1% variation of resistance after 5000 bending cycles. It shows the highest output-performance and the best flexibility among the reported SWCNT-based thermoelectric modules. This work provides significant insights into the interaction mechanism and performance optimization of hybrid thermoelectric composites, based on SWCNTs/unconjugated polymers.

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.

n-Type PVP/SWCNT Composite Films with Improved Stability for Thermoelectric Power Generation

Single-walled carbon nanotubes (SWCNTs) are one of the promising thermoelectric materials in applications of powering wearable electronics. However, the electrical performance of n-type SWCNTs quickly decreases in air, showing a low stability, and low-cost and effective solutions to improving its stability are also lacking, all of which limit practical applications. In this study, we studied the stability of PVP/SWCNT composite films, where oxygen and moisture from air should be responsible for the decreased stability due to oxidation and hydration. In this case, we found that coating with a 0.20 g mL-1 PVP/0.002 g mL-1 PVDF layer on the surface of PVP/SWCNTs can prevent the penetration of oxygen and moisture from air, improving film stability, where there is almost no reduction in thermoelectric performance after they are exposed to air for 60 days. Based on the stable n-type PVP/SWCNT films, a thermoelectric generator was fabricated, where poly(dimethylsiloxane) (PDMS) was used to coat the surface of the thermoelectric leg to further improve its stability. This generator showed high output performance, which achieved an open-circuit voltage of 10.6 mV and a power density of 312.2 μW cm-2 at a temperature difference of 50 K. Particularly, it exhibited high stability, where the output performance kept almost unchanged after exposure to high-humidity air for 30 days. This coating technology is also applicable to other air-sensitive materials and promotes the development and application of thermoelectric materials and devices.

Molecular Engineering for Enhanced Thermoelectric Performance of Single-Walled Carbon Nanotubes/π-Conjugated Organic Small Molecule Hybrids

Hybridizing single-walled carbon nanotubes (SWCNTs) with π-conjugated organic small molecules (π-OSMs) offers a promising approach for producing high-performance thermoelectric (TE) materials through the facile optimization of the molecular geometry and energy levels of π-OSMs. Designing a twisted molecular structure for the π-OSM with the highest occupied molecular orbital energy level comparable to the valence band of SWCNTs enables effective energy filtering between the two materials. The SWCNTs/twisted π-OSM hybrid exhibits a high Seebeck coefficient of 110.4 ± 2.6 µV K-1 , leading to a significantly improved power factor of 2,136 µW m-1 K-2 , which is 2.6 times higher than that of SWCNTs. Moreover, a maximum figure of merit over 0.13 at room temperature is achieved via the efficient TE transport of the SWCNTs/twisted π-OSM hybrid. The study highlights the promising potential of optimizing molecular engineering of π-OSMs for hybridization with SWCNTs to create next-generation, efficient TE materials.

Ambient-Stable n-Type Carbon Nanotube/Organic Small-Molecule Thermoelectrics Enabled by Energy Level Control

The stability of n-type organic and hybrid thermoelectric materials is limited in terms of their practical application to p-n parallel thermoelectric devices. We demonstrate the ambient stability of an n-type single-walled carbon nanotube/organic small-molecule (SWNT/OSM) hybrid by deepening the lowest occupied molecular orbital energy level. This hybrid exhibited the best figure of merit (0.032) among n-type SWNT/OSM hybrid thermoelectrics and an enhanced power factor of 291.0 μW m-1 K-2. Furthermore, we observed that the n-type thermoelectric stability of a hybrid of SWNT and pip containing two N-ethylpiperidinyl groups on both sides of a naphthalenediimide core was retained at 87% over 7 months (220 days) under ambient conditions without encapsulation.

High-Performance MoS2/SWCNT Composite Films for a Flexible Thermoelectric Power Generator

Single-walled carbon nanotube (SWCNT)-based thermoelectric materials have been extensively studied in the field of flexible wearable devices due to their high flexibility and excellent electrical conductivity (σ). However, poor Seebeck coefficient (S) and high thermal conductivity limit their thermoelectric application. In this work, free-standing MoS₂/SWCNT composite films with improved thermoelectric performance were fabricated by doping SWCNTs with MoS₂ nanosheets. The results demonstrated that the energy filtering effect at the MoS₂/SWCNT interface increased the S of composites. In addition, the σ of composites was also improved due to the reason that S-π interaction between MoS₂ and SWCNTs made good contact between MoS₂ and SWCNTs and improved carrier transport. Finally, the obtained MoS₂/SWCNT showed a maximum power factor of 131.9 ± 4.5 μW m^-1 K^-2 at room temperature with a σ of 680 ± 6.7 S cm^-1 and an S of 44.0 ± 1.7 μV K^-1 at a MoS₂/SWCNT mass ratio of 15:100. As a demonstration, a thermoelectric device composed of three pairs of p-n junctions was prepared, which exhibited a maximum output power of 0.43 μW at a temperature gradient of 50 K. Therefore, this work offers a simple method of enhancing the thermoelectric properties of SWCNT-based materials.

Energy Level Modulation of Small Molecules Enhances Thermoelectric Performances of Carbon Nanotube-Based Organic Hybrid Materials

Although numerous thermoelectric materials based on single-walled carbon nanotubes (SWNTs) and organic semiconductors have been reported during the past decade, the correlation between energy levels of organic semiconductors and thermoelectric performances of their hybrids is still ambiguous. In this study, we demonstrate that simultaneous modulation of the bandgap and highest occupied molecular orbital levels in organic small molecules (OSMs) largely improves the Seebeck coefficient and thus maximizes the figure of merit (ZT) of SWNT/OSM hybrids. SWNT/CzS with an enlarged bandgap and reduced barrier energy exhibited a synergistic increment in the Seebeck coefficient (108.7 μV K^-1) and power factor (337.2 μW m^-1 K^-2), with the best ZT of 0.058 at room temperature among dopant-free carbon nanotube-hybridized thermoelectrics. The efficient charge carrier transport and reduced thermal conductivity of SWNT/CzS provided enhanced thermoelectric performance. Our strategy based on energy level modulation could be broadly applied for performance enhancement of organic and hybrid thermoelectric materials.

Enhancement Effect of the C60 Derivative on the Thermoelectric Properties of n-Type Single-Walled Carbon Nanotube-Based Films

Obtaining air-stable and high-performance flexible n-type single-walled carbon nanotube (SWCNT)-based thermoelectric films used in wearable electronic devices is a challenge. In this work, the microstructure and thermoelectric properties of n-type SWCNT-based films have been optimized via doping C₆₀ and its derivative into polyethylenimine/single-walled carbon nanotube (PEI/SWCNT) films. The result demonstrated that the dispersity of triethylene glycol-modified C₆₀ (TEG-C₆₀) was better in PEI/SWCNT films than that of pure C₆₀. Among the prepared composite films, TEG-C₆₀-doped PEI/SWCNT (TEG-C₆₀/PEI/SWCNT) films exhibited the highest TE performance, achieving a peak electrical conductivity of 923 S cm^-1 with a Seebeck coefficient of -42 μV K^-1 at a TEG-C₆₀/SWCNT mass ratio of 1:100. Compared to that of PEI/SWCNT, the power factor was increased significantly from 40 to 162 μW m^-1 K^-2 after the addition of TEG-C₆₀, which was higher than that of films after the addition of C₆₀. In addition, the n-type doped SWCNT films had good air stability at high temperatures, and the Seebeck coefficients of C₆₀/PEI/SWCNT and TEG-C₆₀/PEI/SWCNT at 120 °C were still negative and remained at 92% and 85%, respectively, after 20 days. Furthermore, a flexible TE device consisting of five pairs of p-n junctions was assembled using the optimum hybrid film, which generated a maximum output power of 3.6 μW at a temperature gradient of 50.2 K. Therefore, this study provides a facile way to enhance the thermoelectric properties of n-type carbon nanotube-based materials, which have potential application in flexible power generators.

Unconventional Thermoelectric Materials for Energy Harvesting and Sensing Applications

Heat is an abundant but often wasted source of energy. Thus, harvesting just a portion of this tremendous amount of energy holds significant promise for a more sustainable society. While traditional solid-state inorganic semiconductors have dominated the research stage on thermal-to-electrical energy conversion, carbon-based semiconductors have recently attracted a great deal of attention as potential thermoelectric materials for low-temperature energy harvesting, primarily driven by the high abundance of their atomic elements, ease of processing/manufacturing, and intrinsically low thermal conductivity. This quest for new materials has resulted in the discovery of several new kinds of thermoelectric materials and concepts capable of converting a heat flux into an electrical current by means of various types of particles transporting the electric charge: (i) electrons, (ii) ions, and (iii) redox molecules. This has contributed to expanding the applications envisaged for thermoelectric materials far beyond simple conversion of heat into electricity. This is the motivation behind this review. This work is divided in three sections. In the first section, we present the basic principle of the thermoelectric effects when the particles transporting the electric charge are electrons, ions, and redox molecules and describe the conceptual differences between the three thermodiffusion phenomena. In the second section, we review the efforts made on developing devices exploiting these three effects and give a thorough understanding of what limits their performance. In the third section, we review the state-of-the-art thermoelectric materials investigated so far and provide a comprehensive understanding of what limits charge and energy transport in each of these classes of materials.

Crystalline Domain Formation to Enable High-Performance Polymer Thermoelectrics Inspired by Thermocleavable Materials

Improving the electrical conductivity is an important role in realizing high thermoelectric performance of solution-processable polymers. Herein, a simple and robust approach to boost the mobility and doping efficiency of a diketopyrrolopyrrole-based copolymer with the introduction of thermocleavable side chains (PDPPS-X, where X is the molar ratio of the thermocleavable side chains and alkyl chains) is first provided. Notably, the incorporated thermocleavable groups can be effectively removed after thermal treatment and therefore contribute to the crystalline domain formation via hydrogen-bonded networks, which is critical for conductivity enhancements. Grazing incidence wide-angle X-ray scattering (GIWAXS) patterns give a clear indication that the thermal treatment of PDPPS-5 can greatly improve the structural arrangement, resulting in a significantly enhanced hole mobility (5.4 times that of PDPPS-0 without thermocleavable chains). Compared to PDPPS-0, a larger Fermi level shift is observed after doping PDPPS-5 with FeCl₃, reflecting a better doping efficiency. Consequently, remarkably improved conductivity and power factor are achieved by PDPPS-5 after doping with 0.03 M FeCl₃ at room temperature, which are about 2.2 and 3.5 times higher than that of PDPPS-0 at the same testing condition, respectively. Moreover, PDPPS-5 achieved a maximum power factor of 57.5 μW m^-1 K^-2 at 404 K.

Oxygen-Rich Polymer Polyethylene Glycol-Functionalized Single-Walled Carbon Nanotubes Toward Air-Stable n-Type Thermoelectric Materials

It is crucial for thermoelectric (TE) devices to obtain both p-type and n-type materials and control charge carrier density. However, n-type thermoelectric materials are quite deficient and have lower thermoelectric properties. We report one oxygen-rich polymer named polyethylene glycol (PEG) for converting p-type single-walled carbon nanotubes (SWCNTs) to air-stable n-type thermoelectric materials. When pristine SWCNTs were doped with 2 mg·mL^-1 PEG in an ethanol solution, the optimal Seebeck coefficient of PEG/SWCNT composites reached -50.8 μV·K^-1. The result of ultraviolet photoelectron spectroscopy demonstrated that the lone pair of oxygen atoms in the PEG chain has electron transferability to SWCNTs. According to the hard and soft acid and base theory, sodium hydroxide (NaOH) was further introduced to improve air stability and thermoelectric performance of doped SWCNTs. As a result, PEG/NaOH/SWCNT composites achieved the highest power factor of 173.8 μW·m^-1·K^-2 at 300 K. Meanwhile, their final changes in electrical conductivity and the Seebeck coefficient are less than 8% in the investigation of air stability over two months. Inspired by this finding, we fabricated the TE generator composed of the pristine p-type SWCNTs and n-type PEG/NaOH/SWCNT composites. The maximum output power of this robust TE device reached 5.3 μW at a temperature gradient of 76 K, which is superior to many reported TE devices. Moreover, the experimental procedure is attractive as a sustainable process for materials preparation. Our study has indicated that the oxygen-rich polymer-functionalized SWCNTs have huge potential for developing air-stable n-type carbon-based thermoelectric materials.

Novel butterfly-shaped organic semiconductor and single-walled carbon nanotube composites for high performance thermoelectric generators

Herein, a series of novel butterfly-shaped small-molecule organic semiconductors (OSCs) have been designed, synthesized and complexed with single-walled carbon nanotubes (SWCNTs) as p-type thermoelectric materials. The butterfly-shaped molecules exhibit curved molecular structures, which tune their frontier molecular orbitals and increase their interactions with SWCNTs. A systematic study shows that the composites based on butterfly-shaped OSCs exhibit significantly improved thermoelectric performances compared with that of the composite based on the analoguous planar OSC. The enhanced thermoelectric performances are attributable to the higher activation energy, improved doping level and charge transport process between the organic molecules and SWCNTs. The butterfly-shaped OSC and SWCNT composite opens up a new avenue for the design of thermoelectric materials and devices.

Origin of n type properties in single wall carbon nanotube films with anionic surfactants investigated by experimental and theoretical analyses

We investigated the origin of n-type thermoelectric properties in single-wall carbon nanotube (SWCNT) films with anionic surfactants via experimental analyses and first-principles calculations. Several types of anionic surfactants were employed to fabricate SWCNT films via drop-casting, followed by heat treatment at various temperatures. In particular, SWCNT films with sodium dodecylbenzene sulfonate (SDBS) surfactant heated to 350 °C exhibited a longer retention period, wherein the n-type Seebeck coefficient lasted for a maximum of 35 days. In x-ray photoelectron spectroscopy, SWCNT films with SDBS surfactant exhibited a larger amount of sodium than oxygen on the SWCNT surface. The electronic band structure and density of states of SWCNTs with oxygen atoms, oxygen molecules, water molecules, sulfur atoms, and sodium atoms were analyzed using first-principles calculations. The calculations showed that sodium atoms and oxygen molecules moved the Fermi level closer to the conduction and valence bands, respectively. The water molecules, oxygen, and sulfur atoms did not affect the Fermi level. Therefore, SWCNT films exhibited n-type thermoelectric properties when the interaction between the sodium atoms and the SWCNTs was larger than that between the oxygen molecules and the SWCNTs.

Pseudo in situ construction of high-performance thermoelectric composites with a dioxothiopyrone-based D-A polymer coating on SWCNTs

Organic polymer/inorganic particle composites with thermoelectric (TE) properties have witnessed rapid progress in recent years. Nevertheless, both development of novel polymers and optimization of compositing methods remain highly desirable. In this study, we first demonstrated a simulated in situ coagulation strategy for construction of high-performance thermoelectric materials by utilizing single-walled carbon nanotubes (SWCNTs) and a new D-A polymer TPO-TTP12 that was synthesized via incorporating dioxothiopyrone subunit into a polymeric chain. It was proven that the preparation methods have a significant influence on thermoelectric properties of the TPO-TTP12/SWCNT composites. The in situ prepared composite films tend to achieve much better thermoelectric performances than those prepared by simply mixing the corresponding polymer with SWCNTs. As a result, the in situ compositing obtains the highest Seebeck coefficient of 66.10 ± 0.05 μV K-1 at the TPO-TTP12-to-SWCNT mass ratio of 1/2, and the best electrical conductivity of up to 500.5 ± 53.3 S cm-1 at the polymer/SWCNT mass ratio of 1/20, respectively; moreover, the power factor for the in situ prepared composites reaches a maximum value of 141.94 ± 1.47 μW m-1 K-2, far higher than that of 104.68 ± 0.86 μW m-1 K-2 for the by-mixing produced composites. This indicates that the dioxothiopyrone moiety is a promising building block for constructing thermoelectric polymers, and the simulated in situ compositing strategy is a promising way to improve TE properties of composite materials.

Supramolecular Functionalization for Improving Thermoelectric Properties of Single-Walled Carbon Nanotubes-Small Organic Molecule Hybrids

Single-walled carbon nanotube (SWCNTs-P)-small organic molecule hybrid materials are promising candidates for achieving high thermoelectric (TE) performance. In this study, we synthesized rod-coil amphiphilic molecules, that is, tri(ethylene oxide) chain-attached bis(bithiophenyl)-terphenyl derivatives (1 and 2). Supramolecular functionalization of SWCNTs-P with 1 or 2 induced charge-transfer interactions between them. Improved TE properties of the supramolecular hybrids (SWCNTs-1 and SWCNTs-2) are attributed to increased charge-carrier concentration (electrical conductivity), interfacial phonon scattering (thermal conductivity), and energy difference between the transport and Fermi levels (E_Tr - E_F; Seebeck coefficient). SWCNTs-2 exhibited a ZT of 0.42 × 10^-2 at 300 K, which is 350% larger than that of SWCNTs-P. Furthermore, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ)-doped SWCNTs-2 showed the highest ZT value of 1.96 × 10^-2 at 300 K among SWCNTs-P/small organic molecule hybrids known until now. These results demonstrated that the supramolecular functionalization of SWCNTs-P with small organic molecules could be useful for enhancement of TE performance and applications in wearable/flexible thermoelectrics.