Polymer Thermoelectric Materials Research Articles

Isomers of n-Type Poly(thiophene-alt-co-thiazole) for Organic Thermoelectrics.

n-Type polythiophene represents a promising category of n-type polymer thermoelectric materials known for their straightforward structure and scalable synthesis. However, n-type polythiophene often suffers from a twisted backbone and poor stacking property when introducing high-density electron-withdrawing groups for a lower lowest unoccupied molecular orbital (LUMO) level, which is considered to be beneficial for n-doping efficiency. Herein, we developed two isomers of polythiophene derivatives, PTTz1 and PTTz2, by inserting thiazole units into the polythiophene backbone composed of thieno[3,4-c]pyrrole-4,6-dione (TPD) and thiophene-3,4-dicarbonitrile (2CNT). Although PTTz1 and PTTz2 share a similar polymer skeleton, they differ in thiazole configuration, with the nitrogen atoms of the thiazole units oriented toward TPD and 2CNT, respectively. The insertion of thiazole units significantly planarizes the polythiophene backbone while largely preserving low LUMO levels. Notably, PTTz2 exhibits a more coplanar backbone and closer π-stacking compared to PTTz1, resulting in a greatly enhanced electron mobility. Both PTTz1 and PTTz2 can be easily n-doped due to their deep LUMO levels. PTTz2 demonstrates superior thermoelectric performance, with an electrical conductivity of 50.3 S cm-1 and a power factor of 23.8 μW m-1 K-2, which is approximately double that of PTTz1. This study highlights the impact of the thiazole unit on n-type polythiophene derivatives and provides valuable guidelines for the design of high-performance n-type polymer thermoelectric materials.

Thermoelectric Performance in Triplet−Ground−State Polymer Intrinsically Boosted by Enhanced Proquinoidal Characteristic

Polymer thermoelectric materials featuring flexibility, lightness, and bio−friendliness have been paid increasing attention for waste heat recovery and energy generation. The dominant approach to optimizing the thermoelectric performance of most organic materials is chemical doping, which, however, is not always ideal for practical applications due to its tendency to involve intricate processing procedure and trigger material and device instability. The pursuit of single−component neutral thermoelectric materials without exogenous doping presents a compelling alternative. In this work, we designed and synthesized a high−spin polymer material, PBBT−TT, by simultaneously employing thieno[3,4−b]thiophene (TbT) and benzo[1,2−c:4,5−c′]bis[1,2,5]thiadiazole (BBT) units with pronounced proquinoidal characteristics, and its analogue, PBBT−T. Because of the enhanced quinoidal resonance, increased spin density, and strong intermolecular antiferromagnetic coupling, PBBT−TT exhibits high intrinsic electrical conductivity and Seebeck coefficient, which showcases an outstanding power factor of 26.1 μW m−1 K−2 without doping, surpassing other neutral organic conjugated polymer and radical conductors. Notably, PBBT−TT exhibits remarkable ambient stability, retaining its initial thermoelectric performance over a 120−day period. Our finding demonstrates that modulating the intermolecular spin interactions in open‐shell polymers through the introduction of strong proquinoidal units is an effective strategy for the development of doping−free, intrinsically high−performance polymer thermoelectric materials.

Thermoelectric Performance in Triplet-Ground-State Polymer Intrinsically Boosted by Enhanced Proquinoidal Characteristic.

Polymer thermoelectric materials featuring flexibility, lightness, and bio-friendliness have been paid increasing attention for waste heat recovery and energy generation. The dominant approach to optimizing the thermoelectric performance of most organic materials is chemical doping, which, however, is not always ideal for practical applications due to its tendency to involve intricate processing procedure and trigger material and device instability. The pursuit of single-component neutral thermoelectric materials without exogenous doping presents a compelling alternative. In this work,we designed and synthesized a high-spin polymer material, PBBT-TT, by simultaneously employingthieno[3,4-b]thiophene(TbT) and benzo[1,2-c:4,5-c']bis[1,2,5]thiadiazole (BBT)units with pronounced proquinoidal characteristics, and its analogue, PBBT-T. Because of the enhanced quinoidal resonance, increased spin density, and strong intermolecular antiferromagnetic coupling, PBBT-TT exhibits high intrinsic electrical conductivity and Seebeck coefficient, whichshowcases an outstanding power factor of 26.1 μW m-1K-2without doping, surpassingother neutral organic conjugated polymer and radical conductors. Notably, PBBT-TT exhibitsremarkable ambient stability, retaining its initial thermoelectric performance over a 120-day period.Our finding demonstratesthat modulating theintermolecularspin interactions inopen-shell polymers through the introduction of strong proquinoidal units is an effective strategy forthe development ofdoping-free, intrinsically high-performance polymer thermoelectric materials.

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.

Popular strategies for constructing polymer gel thermoelectric materials

AbstractPolymeric thermoelectric materials utilizing the Soret effect have the advantages of high Seebeck coefficient, non‐toxicity, light weight, and excellent mechanical properties that make them shine in the field of thermoelectrics. However, ionic thermoelectric systems based on polymer gels are still in their infancy and research is relatively limited, and there is an increasing interest in how to prepare high performance polymer thermoelectric materials. In this paper, we review some of the existing polymer TE materials based on the Soret effect and discuss the different approaches to achieve high Seebeck coefficient and enhanced thermoelectric properties from the perspective of material design strategies. Moreover, we highlight the popular strategies of the construction of thermoelectric materials and provide a brief outlook on the development of the polymer thermoelectric field for its future research.

A High‐Performance n‐Type Thermoelectric Polymer from C−H/C−H Oxidative Direct Arylation Polycondensation

Abstractn‐Type conjugated polymers (CPs) are crucial in the applications of organic electronics. Direct coupling of electron‐deficient C−H monomer via selective C−H activation, namely C−H/C−H oxidative direct arylation polycondensation (Oxi‐DArP), is an ideal approach toward such CPs. Herein, Oxi‐DArP is firstly adopted to synthesize a high‐performance n‐type CP using a newly developed monomer, i.e., 3,6‐di(thiazol‐5‐yl)‐diketopyrrolopyrrole (Tz‐5‐DPP). Tz‐5‐DPP based homopolymer PTz‐5‐DPP with a molecular weight of 22 kDa has been synthesized via Oxi‐DArP. After n‐doping, PTz‐5‐DPP films exhibited electric conductivity values up to 8 S cm−1 and power factors (PFs) up to 106 μW m−1 K−2. Notably, this PF value is the highest for n‐type polymer thermoelectric materials to date. The Oxi‐DArP synthesis and the excellent n‐type performance of the polymer make this work an important step toward the straightforward and sustainable preparation of high‐performance n‐type polymer semiconductors.

A High-Performance n-Type Thermoelectric Polymer from C-H/C-H Oxidative Direct Arylation Polycondensation.

n-Type conjugated polymers (CPs) are crucial in the applications of organic electronics. Direct coupling of electron-deficient C-H monomer via selective C-H activation, namely C-H/C-H oxidative direct arylation polycondensation (Oxi-DArP), is an ideal approach toward such CPs. Herein, Oxi-DArP is firstly adopted to synthesize a high-performance n-type CP using a newly developed monomer, i.e., 3,6-di(thiazol-5-yl)-diketopyrrolopyrrole (Tz-5-DPP). Tz-5-DPP based homopolymer PTz-5-DPP with a molecular weight of 22 kDa has been synthesized via Oxi-DArP. After n-doping, PTz-5-DPP films exhibited electric conductivity values up to 8 S cm-1 and power factors (PFs) up to 106 μW m-1 K-2 . Notably, this PF value is the highest for n-type polymer thermoelectric materials to date. The Oxi-DArP synthesis and the excellent n-type performance of the polymer make this work an important step toward the straightforward and sustainable preparation of high-performance n-type polymer semiconductors.

High thermoelectric performance of spray-coated Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) films enabled by two-step post-treatment process

Herein, poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) films with improved thermoelectric properties are fabricated via spray coating followed by a sequential, two-step post-treatment process with ethylene glycol (EG) and a methylammonium iodide (MAI) solution. The EG treatment greatly increases the electrical conductivity of the PEDOT-PSS film up to 1752.1 S cm−1, with an unchanged Seebeck coefficient of 15–17 μV K−1, while the optimal use of 0.05 M MAI in DMSO/DI water for the second step provides a high power factor of 122.3 μW m−1 K−2, along with an increased conductivity of 2226.8 S cm−1 and Seebeck coefficient of 22.8 μV K−1. Notably, the obtained power factor is among the highest reported for spray-coated polymer-based thermoelectric devices. The performance enhancement is attributed to phase separation of the non-conductive PSS from the PEDOT, the change in chain conformation, the preferential orientation of the PEDOT crystallites, and the manipulation of energy levels. High thermoelectric performance of the as-fabricated PEDOT:PSS on a plastic substrate is established by using a facile proof-of-concept thermoelectric generator to generate a maximum power density of 12.1 nW cm−2. The fabrication approach developed herein provides an essential paradigm for the production of polymer thermoelectric materials with great potential for application in wearable thermoelectric devices.

A Distannylated Monomer of a Strong Electron‐Accepting Organoboron Building Block: Enabling Acceptor–Acceptor‐Type Conjugated Polymers for n‐Type Thermoelectric Applications

AbstractAcceptor–acceptor (A‐A) copolymerization is an effective strategy to develop high‐performance n‐type conjugated polymers. However, the development of A‐A type conjugated polymers is challenging due to the synthetic difficulty. Herein, a distannylated monomer of strong electron‐deficient double B←N bridged bipyridine (BNBP) unit is readily synthesized and used to develop A‐A type conjugated polymers by Stille polycondensation. The resulting polymers show ultralow LUMO energy levels of −4.4 eV, which is among the lowest value reported for organoboron polymers. After n‐doping, the resulting polymers exhibit electric conductivity of 7.8 S cm−1 and power factor of 24.8 μW m−1 K−2. This performance is among the best for n‐type polymer thermoelectric materials. These results demonstrate the great potential of A‐A type organoboron polymers for high‐performance n‐type thermoelectrics.

A Distannylated Monomer of a Strong Electron‐Accepting Organoboron Building Block: Enabling Acceptor–Acceptor‐Type Conjugated Polymers for n‐Type Thermoelectric Applications

Acceptor-acceptor (A-A) copolymerization is an effective strategy to develop high-performance n-type conjugated polymers. However, the development of A-A type conjugated polymers is challenging due to the synthetic difficulty. Herein, a distannylated monomer of strong electron-deficient double B←N bridged bipyridine (BNBP) unit is readily synthesized and used to develop A-A type conjugated polymers by Stille polycondensation. The resulting polymers show ultralow LUMO energy levels of -4.4 eV, which is among the lowest value reported for organoboron polymers. After n-doping, the resulting polymers exhibit electric conductivity of 7.8 S cm-1 and power factor of 24.8 μW m-1 K-2 . This performance is among the best for n-type polymer thermoelectric materials. These results demonstrate the great potential of A-A type organoboron polymers for high-performance n-type thermoelectrics.

High thermal conductivity states and enhanced figure of merit in aligned polymer thermoelectric materials

Control of morphology and chain alignment in polymer thermoelectric materials is critical to their performance. A x25 improvement in zT is reported for chain aligned films.

Recent development of n-type thermoelectric materials based on conjugated polymers

Abstract Thermoelectric (TE) materials based on conjugated polymers have received much attention due to their great advantages of solution processibility, light weight, flexibility, and low thermal conductivity. These advantages make them potential candidates for large-area, low-cost and low-power TE applications. Both efficient p-type and n-type conjugated polymers with high and comparable thermoelectric performance are required for practical TE applications. However, due to the inefficient n-doping efficiency and unstable electron transport of most n-type conjugated polymers, the TE performance of n-type polymers is much poorer than that of their p-type counterparts, impeding the development of polymer TE materials. Great efforts have been made to address the low n-doping efficiency and TE performance of n-type polymers, including the chemical modification of traditional n-type polymers, the design of new n-type conjugated polymers, and the development of more efficient n-dopants, as well as doping engineering. Nowadays, the TE performance of n-type polymers has been greatly improved, indicating a bright future for polymer TE materials. In this review, we summarize the recent progress made on n-type polymer TE materials, mainly focusing on the structure-performance relationships based on promising n-type polymers for TE applications. This review aims to provide some guidelines for future material design.

Polymer based thermoelectric nanocomposite materials and devices: Fabrication and characteristics

Organic thermoelectric (TE) materials capitalize on advantages such as low thermal conductivity, low-cost, eco-friendly, versatile processability, light-weight, mechanical flexibility, roll-to-roll production, which are advantageous for the development of portable and wearable self-powered electronics. On the negative side, the figure-of-merit of polymer TE materials is low, mainly owing to typical low electrical conductivity. Various efforts have been made to enhance the TE performance of organic TE materials, i.e. chemical or electrochemical doping, modification of molecular structure, and fabrication of polymer-based composites or blends, which is the simplest and most cost-effective method for modifying polymer properties. Solution-processed polymer/inorganic or organic hybrids pave the way for formulation of functional TE inks/paste which can be used to fabricate large scale cost-effective manufacturing of TE generators. In this review, we briefly summarize the TE properties of conjugated polymers, and focus on recent developments in polymer/carbon nanofillers (e.g. CNT, graphene, GO) composites and polymer/inorganic TE nanoparticles composites along with preparation methods and thermoelectric performance. Finally, we cover recent advances in the development of functional TE inks, processing techniques, and their usage in device fabrication using various printing techniques and review device output characteristics. • Covers hot topic of thermoelectric materials & devices, suitable for energy harvesting to power IoT sensors and devices. • Review & analysis on advances on enhanced thermoelectric properties of polymer based nanocomposites (CNT, graphene, NP, NW). • Analyzing effect of composite processing mechanisms, and also interchain and cross-domain transport on properties. • Recent progress in the fabrication methods of a variety of functional nanocomposite inks/pastes for different printers. • Analysis of recent nanocomposites TE devices and their output characteristics in dual-mode or single-mode operation.

Improved Thermoelectric Properties and Environmental Stability of Conducting PEDOT:PSS Films Post-treated With Imidazolium Ionic Liquids.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most popular conducting polymers and widely used as polymer thermoelectric materials, and its thermoelectric performance could be improved by a variety of post-treatment processes. This paper reported two series of post-treatment methods to enhance the thermoelectric performance. The first series method included pre-treatment of PEDOT:PSS film with formamide, followed by imidazolium-based ionic liquids. The second series method included pre-treatment of PEDOT:PSS film with formamide, followed by sodium formaldehyde sulfoxylate, and finally imidazolium-based ionic liquids. Two series of post-treatment methods significantly improved the power factor of PEDOT:PSS when compared to that of PEDOT:PSS treated with formamide only. For example, using the first series post-treatment method with 40 vol.% ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide, the Seebeck coefficient of the PEDOT:PSS film increased from 14.9 to 28.5 μV/K although the electrical conductivity reduced from 2,873 to 1,701 S/cm, resulting in a substantial improvement in the overall power factor from 63.6 to 137.8 μW/K2m. The electrical conductivity enhancement in the formamide-treatment process was in part ascribed to the removal of the insulating PSS component. Further treatment of PEDOT:PSS film with ionic liquid caused dedoping of PEDOT and hence increased in Seebeck coefficient. In contrast, second series post-treatment method led to the reduction in electrical conductivity from 2,873 to 641 S/cm but a big improvement in the Seebeck coefficient from 14.9 to 61.1 μV/K and thus the overall power factor reached up to ~239.2 μW/K2m. Apart from the improvement in electrical conductivity, the increase in Seebeck coefficient is on account of the substantial dedoping of PEDOT polymer to its neutral form and thus leads to the big improvement of its Seebeck coefficient. The environmental stability of ionic liquid-treated PEDOT:PSS films were examined. It was found that the ionic liquid treated PEDOT:PSS retained more than 70% Seebeck coefficient and electrical conductivity at 75% RH humidity and 70°C for 480 h. The improved long-term TE stability is attributed to the strong ionic interaction between sulfonate anions and bulky imidazolium cations that effectively block the penetration of water and lessen the tendency to take up water from the air.

Strategies To Enhance the Conductivity of n-Type Polymer Thermoelectric Materials

In the past several decades, conducting polymers have achieved remarkable progress and have been widely applied as the active materials for optoelectronics. So far, p-type conducting polymers exhibit high conductivities over 1000 S cm–1 and thermoelectric performance comparable to that of inorganic materials; however, only a few n-type conducting polymers showed conductivities over 1 S cm–1 after doping. The low conductivity of n-type conducting polymers is considered as the major barrier for further enhancing their thermoelectric performances. In this perspective, we highlight the scientific and engineering challenges to enhance the conductivity of n-type polymer thermoelectric materials, including n-doping efficiency in n-type polymers, factors influencing charge carrier mobilities after doping, and stability of n-type conducting polymers. Recent development and strategies to address these issues and enhance the conductivity of n-type conjugated polymers are summarized and discussed, providing materials...

Bringing Conducting Polymers to High Order: Toward Conductivities beyond 105 S cm−1 and Thermoelectric Power Factors of 2 mW m−1 K−2

AbstractHere, an effective design strategy of polymer thermoelectric materials based on structural control in doped polymer semiconductors is presented. The strategy is illustrated for two archetypical polythiophenes, e.g., poly(2,5‐bis(3‐dodecyl‐2‐thienyl)thieno[3,2‐b]thiophene) (C12‐PBTTT) and regioregular poly(3‐hexylthiophene) (P3HT). FeCl3 doping of aligned films results in charge conductivities up to 2 × 105 S cm−1 and metallic‐like thermopowers similar to iodine‐doped polyacetylene. The films are almost optically transparent and show strongly polarized near‐infrared polaronic bands (dichroic ratio >10). The comparative study of structure–property correlations in P3HT and C12‐PBTTT identifies three conditions to obtain conductivities beyond 105 S cm−1: i) achieve high in‐plane orientation of conjugated polymers with high persistence length; ii) ensure uniform chain oxidation of the polymer backbones by regular intercalation of dopant molecules in the polymer structure without disrupting alignment of π‐stacked layers; and iii) maintain a percolating nanomorphology along the chain direction. The highly anisotropic conducting polymer films are ideal model systems to investigate the correlations between thermopower S and charge conductivity σ. A scaling law S ∝ σ−1/4 prevails along the chain direction, but a different S ∝ −ln(σ) relation is observed perpendicular to the chains, suggesting different charge transport mechanisms. The simultaneous increase of charge conductivity and thermopower along the chain direction results in a substantial improvement of thermoelectric power factors up to 2 mW m−1 K−2 in C12‐PBTTT.

Optimizing the Thermoelectric Performance of Poly(3-hexylthiophene) through Molecular-Weight Engineering.

Poly(3-hexylthiophene) (P3HT) films with various molecular weights (MWs) were successfully prepared, and both their molecular structures and thermoelectric (TE) properties were investigated. It was found that the molecular weight of P3HT had an important effect on the carrier-transport properties by affecting the molecular structure and, as a result, also had an effect on the TE performance. The electrical conductivity of the P3HT films first increased upon increasing the molecular weight and then decreased at high molecular weights, whereas the Seebeck coefficient remained at the same level. As a result, the P3HT-M50k (MW≈50 000 g mol-1 ) film reached an electrical conductivity of (103.8±1.2) S cm, which is more than 20 times higher than the electrical conductivity of P3HT-M10k (MW≈10 000 g mol-1 ) and almost 30 % higher than the electrical conductivity of P3HT-M100k (MW≈100 000 g mol-1 ). Consequently, the maximum TE power factor of P3HT-M50k at room temperature was as high as (22.6±0.6) μW mK-2 , which is much higher than that of either P3HT-M10k or P3HT-M100k . Microstructure analysis combined with C-AFM suggested that carrier transport between most of the ordered and amorphous regions was unconnected in films with low molecular weights, and this resulted in a high migration barrier and poor carrier mobility. Upon increasing the molecular weight, the long molecular chain provided enough connectivity for the charge to move through the ordered regions, which decreased the carrier barrier and increased carrier mobility. Therefore, both the conductivity and Seebeck coefficient were significantly improved. However, a too-high molecular weight could cause more folding of the polymer chain, which would deteriorate the electrical-transport properties. The experimental results not only reveal the intrinsic effect of molecular weight on the electric transporting properties of conducting polymers but also suggest that molecular-weight engineering is an effective way to design and screen high-performance polymer TE materials.

Enhancing the n-Type Conductivity and Thermoelectric Performance of Donor-Acceptor Copolymers through Donor Engineering.

Conjugated polymers with high thermoelectric performance enable the fabrication of low-cost, large-area, low-toxicity, and highly flexible thermoelectric devices. However, compared to their p-type counterparts, n-type polymer thermoelectric materials show much lower performance, which is largely due to inefficient doping and a much lower conductivity. Herein, it is reported that the development of a donor-acceptor (D-A) polymer with enhanced n-doping efficiency through donor engineering of the polymer backbone. Both a high n-type electrical conductivity of 1.30 S cm-1 and an excellent power factor (PF) of 4.65 µW mK-2 are obtained, which are the highest reported values among D-A polymers. The results of multiple characterization techniques indicate that electron-withdrawing modification of the donor units enhances the electron affinity of the polymer and changes the polymer packing orientation, leading to substantially improved miscibility and n-doping efficiency. Unlike previous studies in which improving the polymer-dopant miscibility typically resulted in lower mobilities, the strategy maintains the mobility of the polymer. All these factors lead to prominent enhancement of three orders magnitude in both the electrical conductivity and the PF compared to those of the non-engineered polymer. The results demonstrate that proper donor engineering can enhance the n-doping efficiency, electrical conductivity, and thermoelectric performance of D-A copolymers.

Enhancing polymer thermoelectric performance using radical dopants

Abstract Organic thermoelectric materials provide a means to reclaim, in part, potentially lost energy through a solid-state process that converts low-value thermal gradients (in the form of heat) to electricity; however, a new avenue towards improving the performance of these emerging thermoelectric materials must be brought to light before their widespread implementation becomes warranted. Here, we develop a blend of open-shell small molecules and closed-shell, conjugated polymers in order to evaluate how the chemical composition of the distinct open-shell, charge-neutral molecular dopants impacts the thermoelectric performance of a common hole-transporting (p-type) polymer semiconductor, poly(3-hexylthiophene) (P3HT). In doing so, we are able to increase the electrical conductivity of the P3HT composite both with and without affecting the oxidation state of the polymer. Specifically, the electrical conductivity of the conjugated polymer increases without changing the oxidation state of the P3HT when the preferentially-oxidized species 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) radical is incorporated into the hole-conducting thin film. Moreover, the inclusion of the preferentially-reduced galvinoxyl (GAV) radical also increases the electrical conductivity of the thin film; however, this small molecule dopant changes the oxidation state of the P3HT moiety. The ability to achieve the gains in the electrical conductivity of P3HT with the TEMPO radical in a manner that is independent of its oxidation state was confirmed using optical spectroscopy, cyclic voltammetry, and x-ray diffraction techniques. Importantly, this ability to enhance the electrical conductivity of P3HT in a manner that is independent of the oxidation state of the polymer provides a means to circumvent the oft-observed inverse relationship between the electrical conductivity and thermopower of the semiconducting polymer, and this combined effect allows for an ∼170-fold increase for the TEMPO-containing composites compared to a ∼70-fold increase for the GAV-containing composites. Thus, the described phenomena for charge-neutral radical dopants provides a set of critical design parameters for future polymer thermoelectric materials and composites, and it opens a new pathway towards high-performance organic thermoelectric devices.

Polymer composites-based thermoelectric materials and devices

Polymer thermoelectric materials have attracted increasing attention and exhibited great potential in green energy conversion due to their inexpensive to process, lightweight, mechanical flexibility, and intrinsic low thermal conductivity in the past decades. Integration of polymers with nanofillers has been one main approach to tailor the thermoelectric properties of polymers, which not only combines the properties of each component but also displays interesting transport phenomenon induced by the interfaces between polymers and nanofillers. In this review, we discussed the thermoelectric properties of polymer composites, especially for the conducting polymer based composites, including polymer/inorganic, polymer/carbon nanofillers, polymer/polymer hybrids. The fabrication and structure-dependent thermoelectric properties were discussed. In addition, polymer composites based thermoelectric devices were also systematically addressed, and potential issues in the devices fabrication were also analyzed. Regarding the future research opportunities, some visionary discussion was also provided.