Recent Advances in Multicomponent Organic Composite Thermoelectric Materials

Organic thermoelectric materials have the potential to be used as power supplies for wearable electronics, implantable medical devices, and sensors in Internet of Things. The past 10 years has witnessed the rapid development of the research on organic thermoelectric materials, different material systems and optimizing strategies are developed, and the ZT values of some organic materials have approached that of many inorganic thermoelectric material systems in the low‐temperature region. Among different material systems, organic composite materials, especially multicomponent organic composite materials show very promising thermoelectric properties and offer more tunability compared to single component thermoelectric materials. Multicomponent organic composite thermoelectric materials can not only further improve the ZT values, but also have unique advantages of combining the merits of different materials, thus rendering the composites with better processing, mechanical, or other properties on demand. This review summarizes the concepts, the design criteria, the research progress of organic–inorganic and all‐organic multicomponent thermoelectric materials which contain three or more different ingredients, as well as their applications. Furthermore, the challenges and prospects are also analyzed to provide guidelines for the development of multicomponent organic composite thermoelectric materials.


Introduction
With the development of industry and the consumption of resources, human beings are currently facing a serious problem of energy depletion. Traditional fossil fuels have the disadvantage of being nonrenewable, and the utilization of the development of high-performance organic thermoelectric materials. For example, general dopants or doping approaches applicable to different materials are still lacking, and high-concentration doping is difficult to achieve.
Various strategies have been proposed to improve thermoelectric properties of organic materials, such as backbone design, regulation doping, morphology control, applying external pressure, etc. [21][22][23][24][25][26][27][28][29][30][31] What is more, it is difficult to optimize all the relative parameters (σ, S, and κ) in single component simultaneously. Beyond, compositing of two or more materials offers another approach for improving the thermoelectric properties of organic materials. And in the composite system, the connection models of different components also have serious effects on the overall performance. Series and parallel connection models usually show various thermoelectric trends with changing component content. In the parallel connection model, simply introducing high conductivity components into high Seebeck materials may not improve the overall thermoelectric PF as expected. Understanding the connection models plays an important role in the designing and optimizing of the composition content and understanding the thermoelectric mechanism. [32][33][34] The proper compositing can not only synergistically combine the properties of different components, but also make full use of the exiting pool of organic thermoelectric materials. To date, numerous research results based on organic composite thermoelectric materials have been reported. At the same time, many reviews have also been published, but mainly focused on binary organic composites. [35][36][37][38][39] Adding the third or even more component into the binary composites form multicomponent composite materials, which have been extensively explored and achieved great results in organic solar cells and other fields. [40][41][42][43] The third or more component in multicomponent organic composite thermoelectric materials may play versatile functions including enhanced charge transport, optimized morphology, decreased thermal conductivity, better compatibility, etc., to improve the overall thermoelectric performance. Through careful selection of the three (or more) active components that form the multicomponent organic composite thermoelectric materials, all the three parameters can be simultaneously enhanced and some record ZT values have been made with this strategy. The research of multicomponent organic composite thermoelectric materials is attracting more and more attention; thus, it is necessary to give a timely summary of the advancements in this area. In this review, we present the research progress of organic composite thermoelectric materials with three or more components. According to the chemical nature of the active materials, we roughly classify the multicomponent organic composite thermoelectric materials into two categories, organic-inorganic type and all-organic type. Conducting polymers (poly (3,4-ethylenedioxythiophene) (PEDOT), polyaniline (PANI), polypyrrole (PPy), etc.), carbon nanomaterials (carbon nanotube (CNT), graphene, etc.), filmforming polymers (polyvinylidene fluoride (PVDF), poly(methyl methacrylate), etc.), and inorganic thermoelectric materials (Bi 2 Te 3 , Ag 2 Se, etc.) are the common components for the preparation of multicomponent organic composite thermoelectric materials. For better understanding, the chemical structures of organic and carbon-based thermoelectric materials involved in this review are summarized in Figure 1. It should be noted that any organic thermoelectric component can be mixed with each other or with other inorganic thermoelectric components. Moreover, we discuss the mechanism of the performance enhancement and applications of multicomponent organic composite thermoelectric materials. Finally, we provide some suggestions for future research on highly efficient multicomponent organic composite thermoelectric materials.

Organic-Inorganic Multicomponent Thermoelectric Materials
In this review, multicomponent organic composite thermoelectric materials are classified into two systems: organic-inorganic and all-organic. Take organic-inorganic multicomponent thermoelectric (TE) materials as example, high Seebeck coefficients inorganic fillers including Te, Bi 2 Te 3 -based alloys, oxides, and silicon compounds, are usually added to improve overall Seebeck coefficient; carbon materials with high electrical conductivity such as CNTs, graphene, fullerenes, etc., are included to improve the electrical conductivity of the composite material by forming a conductive network and other mechanisms. According to the chemical components of active materials, we can further classify these two systems into different categories. For simplicity, the conductive polymers are denoted by P, the carbon materials are denoted by C, the film-forming polymers are denoted by F, and the inorganic TE materials are denoted by I. Multicomponent composites with film-forming polymers are separately classified as one group, and all other components in this group such as inorganic TE materials, carbon materials, and conductive polymers will not be specified but are represented by O instead. For composites including components such as small-molecular semiconductors, cellulose, ionic liquid (IL), or polymer surfactant which cannot be denoted by simple combination of aforementioned symbols, we will put them all together as other type composites.

PCI Type Composites
Although the addition of inorganic fillers to conductive polymer materials is a good way to improve the properties of composites, the thermoelectric properties of the composites formed by inorganic materials/conductive polymers are still weaker than those of inorganic thermoelectric materials. There are many studies trying to improve the thermoelectric properties using composites with carbon materials. Due to their remarkable electrical conductivity, high carrier transport properties, narrow bandgap energy, and mechanical flexibility with environmental stability, carbon materials have been used as effective additives to improve the thermoelectric behavior of conjugated polymers. The formation of a network of carbon materials in the polymer facilitates charge transport, resulting in high electrical conductivity.
The special structure of inorganic materials and carbon materials allowed them to form special electron transport networks, such as Zhang et al. designed and built a hierarchical conductive network structure of silver nanowires (AgNWs)/ single-walled carbon nanotubes (SWCNTs) to significantly enhance the thermoelectric properties of nanocomposites. AgNWs with high aspect ratio acted as backbone networks to accelerate electron transport, while smaller-scale SWCNTs facilitated charge transport through nanowire spaces within the AgNWs network. After post-treatment with dimethyl sulfoxide (DMSO) and sodium hydroxide (NaOH) to de-dope the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) chains, the thermoelectric properties of the multicomponent materials were significantly improved, and the PF reached 670 µW m −1 K −2 . [44] The thermoelectric performance of the PEDOT:PSS/SWCNTs/Se NWs (Se nanowires) ternary material prepared by Feng et al. was significantly better than that of the binary PEDOT:PSS/Se NWs material. It was observed by scanning electron microscopy (SEM) and highresolution transmission electron microscopy (HRTEM) that PEDOT:PSS was uniformly adsorbed on both sides of Se NWs and SWCNTs, forming a tightly interwoven and interconnected 3D network. Moreover, because the work functions of the three materials were very close, the formed interface energy barrier could hinder the carriers with lower energy, thereby improving the thermoelectric properties of the ternary material, so that the maximum PF at 434 K was 863.8 µW m −1 K −2 . [45] The thermoelectric properties of multicomponent films were closely related to the weight ratio between the components. Liang et al. prepared PEDOT-tosylate (Tos)/SWCNTs/Te multicomponent thin films whose properties were highly dependent on the dispersion of SWCNTs and the weight ratio of the components. First, binary PEDOT-Tos/Te with different composition ratios were prepared to explore the optimal PEDOT-Tos content. When the PEDOT-Tos content was 20 wt%, the binary material could obtain the best PF value. The optimal thermoelectric properties of the ternary nanocomposite films were further explored by compounding with SWCNTs at this ratio. With the increase of SWCNTs content, the electrical conductivity of the multicomponent material increased significantly, and the Seebeck coefficient remained basically unchanged (Figure 2a,c). The PF of the multicomponent material was greatly enhanced, and the best PF was achieved when the mass ratio of SWCNTs to PEDOT-Tos/Te was 3:5, which was 120 times higher than that of pristine PEDOT-Tos (Figure 2b,d). [46] Another possibility for the formation of the conductive network was due to the fact that the components contained a large number of π bonds, and the special π-π conjugated interaction between different components formed a carrier transport channel that facilitated the movement of carriers. For example, there was π-π stacking between PANI and multi-walled carbon nanotubes (MWCNTs) materials, and the addition of Ag 2 Se may intercalate between PANI and MWCNTs to form a tightly interwoven network. [47] To explore the role of SWCNTs in the network structure, PEDOT:PSS/SWCNTs/Te and PEDOT:PSS/graphene nanoparticles (GNPs)/Te materials were prepared. The PF of PEDOT:PSS/Te with SWCNTs was four times that without SWCNTs, and the PF of PEDOT:PSS/Te with GNPs was twice that without GNPs. The main reason was that the addition of carbon materials improved the electrical conductivity. The reason for the difference in the effect of improving the PF of the two carbon materials was the difference in nanostructure. The PEDOT:PSS/SWCNTs/Te composite film had a dense structure, and the nanostructure on the cross-section was slightly fractured during the preparation process, and a microopen nanocrack was formed by cracking, but the SWCNTs were still connected to each other like a bridge. The interconnection network formed a continuous pathway through which charge carriers could tunnel and hop along the interconnection network of SWCNTs for transport through a conductive mechanism (Figure 2e,f). [48] The combination of semiconductors with carbon materials and conducting polymers facilitated the formation of additional heterojunctions with energy barriers at the interface that helped filter low-energy carriers and allowed high-energy carriers to cross energy barrier, thereby improving TE performance. For example, the ternary hybrid paper composed of PEDOT:PSS/ reduced graphene oxide (rGO)/tellurium nanowires (TeNWs) had two heterojunctions to induce carrier filtering, and the two interfaces acted as energy filters to scatter low-energy carriers. The PF was 143 µW m −1 K −2 , which was one to two orders of magnitude higher than that of single-or binary-component materials. [49] Likewise, in PANI/amine-functionalized SWCNTs, double-walled carbon nanotubes (DWCNTs) (a-CNT)/TiO 2 ternary composites, the addition of TiO 2 reduced the carrier density and the energy filtering of low-energy carriers at the a-CNT/ TiO 2 and PANI/TiO 2 interface. [50] In recent years, great achievements have been made in thermoelectric generators (TEGs) based on organic/inorganic nanocomposites. The TEG of PEDOT:PSS/SWCNTs/Te fabricated by inkjet printing technique had an output power of 126 nW at a temperature difference of 20 °C, and 28 thermoelectric legs were printed on a flexible polyarylate substrate, arranged in two rows ( Figure 2g). This TEG could be easily adapted to foldable and bendable shapes to generate small amounts of electricity from human body heat. The voltage generated by the flexible TEG was also measured by applying human body heat in the ambient atmosphere (Figure 2h). The printed flexible TEG produced a stable output voltage exceeding 7.8 mV when both sides of the array were grasped by human hands. [48] PEDOT-Tos/SWCNTs/Te material was used to design a 3D TEG with helical structure, which could achieve device-level flexibility, stretchability, and compressibility. At a temperature difference of 80 K, the TEG models with 10 p-type legs and five pairs of p-n couples could obtain large output powers of 7.04 and 9.59 µW, respectively. The TEG composed of five p-n pairs could provide stable energy output during multiple stretching and compression cycles. This TEG could not only harvest human body heat on the wrist (open circuit voltage ≈1.8 mV), but also generate electricity from the temperature difference induced by hot water or liquid nitrogen (open circuit voltage ≈9 and 32.5 mV). [46] Adv. Electron. Mater. 2023, 9,2201310 [46] Copyright 2021, Wiley-VCH. e,f) Illustration of PEDOT:PSS/SWCNTs/Te of TEM cross-section and magnification of the cracked part. g) Bendable TEG image. h) Photo of generating voltage by holding both sides of a thermoelectric array at room temperature. Reproduced with permission. [48] Copyright 2017, Royal Society of Chemistry.

PII Type Composites
Conductive polymers could also be polymerized with two different inorganic materials to achieve high thermoelectric performance of the composites through the template effect on the organic-inorganic interface and the de-doping effect of the conductive polymer chains. When selecting inorganic materials, two materials with close work functions were usually selected, which could form a small energy barrier, scattered low-energy carriers while maintaining high carrier concentration and electrical conductivity, and improved the thermoelectric properties of composite materials. For example, the Seebeck coefficient of PEDOT:PSS/Cu 1.75 Te/Te reached 220 µV K −1 , and the PF was increased by 22%. [51,52] In addition to the above effects that could improve the thermoelectric properties of composites, solvent treatment was also a facile method. The Cu-coated Bi 0.5 Sb 1.5 Te 3 was prepared by a simple electroless plating method. The polymer was pretreated with DMSO to obtain crystallized PEDOT:PSS, and then posttreatment with concentrated H 2 SO 4 was used to selectively remove the insulating PSS and increase the electrical conductivity. Finally, the TE ink was drop-cast on the precleaned glass substrate to prepare a composite film with good flexibility. A home-made flexible thermoelectric device was fabricated using the composite film, demonstrating efficient power generation using the human wrist as a heat source (Figure 3a). Compared with the carrier blocking effect caused by the original Bi 0.5 Sb 1.5 Te 3 filler, the highly conductive Cu layer could make the carriers pass through the Bi 0.5 Sb 1.5 Te 3 filler instead of being dispersed. In this way, the carrier mobility was increased, making the conductivity of the composite film reached 2270 S cm −1 . [53] It should be noted that the postprocessing may introduce ions to generate ionic conduction or change the contribution ration between ions and electrons, which will affect the measurement parameters. [54][55][56] Inorganic materials produced defects such as lattices, grain boundaries, and voids, which played an important role in improving the electronic structure, carrier, and surface properties of metal oxides. The fabrication of heteronanostructured materials with defects was expected to create large-density interfaces capable of phonon scattering without affecting the carrier concentration and carrier mobility. Therefore, they reduced the mean free path of electron phonons, and the resulting heteronanostructures may reduce thermal conductivity. Lu et al. prepared the optimized PEDOT/CuAgSe/Ag 2 Se composite film with hierarchical microstructural defects had a PF of about 1696 µW m −1 K −2 at 324 K. Due to the intrinsic low thermal conductivity of PEDOT and structural defects of the films, such as irregularly shaped pores, stacking faults, grain boundaries, and heterointerfaces, these defects could significantly scatter phonons across the entire wavelength. [58] A large number of grain boundaries were generated in PEDOT/TiO 2 / ZnO, which generated more phonon scattering centers. Meanwhile, the nanostructure of TiO 2 /ZnO (ZTO) embedded in the PEDOT matrix effectively reduced the hopping barrier of grain size and increased the number of grain boundaries for longwave phonon scattering. [59] Inorganic materials could also form 3D conductive networks without the addition of carbon materials. Te nanorods with single-crystal structure and Cu 7 Te 4 rods with polycrystalline structure were used to form composite films with PEDOT:PSS. The straight Te nanorods had thin amorphous PEDOT:PSS layers, and the wound and kinked Cu 7 Te 4 nanorods were coated with PEDOT:PSS, which could form a special interconnected 3D network. [60] The nanobarbell structure formed by Te/Bi 2 Te 3 was wrapped by a thin layer of PEDOT:PSS, which could help the Te/Bi 2 Te 3 nanowire heterostructure to be uniformly dispersed in water ( Figure 3b). The nanoplates mainly containing Bi 2 Te 3 and the insulating PSS were partially removed by acid treatment, and the amount of Bi in the PEDOT:PSS/Te/Bi 2 Te 3 hybrid was reduced relative to that of Te, which improved the electrical conductivity of the hybrid material, thereby optimizing the PF. [57] Hybrid materials composed of inorganic materials and conducting polymers had made great progress in flexible devices.  They also attached one side of the device to the arm and exposed the other side to air, using bubble wrap as insulation ( Figure 4e). The TE module produced an output voltage of 6.7 mV at a temperature difference of 5.3 K, indicating that the composite film could convert body heat into electricity for sustainable application in the driving of wearable electronic devices. [58] A flexible thermoelectric device composed of a PEDOT:PSS/Cu/Bi 0.5 Sb 1.5 Te 3 film was fabricated using the structure shown in Figure 4f, and the thermoelectric voltage could be efficiently generated when the device was attached to the human wrist and chest. Taking the human wrist as the heat source, an open-circuit thermal voltage of about 7.7 mV was generated (Figure 4g,h). [53] The flexible TEG composed of six legs fabricated by PPy/Se/Ag 2 Se could output a voltage of 21.2 mV and a maximum power of 4.04 µW under a temperature difference of 34.1 K. [61] A flexible TEG with six legs fabricated by inkjet printing of a PEDOT:PSS/Te/Bi 2 Te 3 hybrid solution generated an open-circuit voltage of 1.54 mV at a temperature difference of 10 °C. [57]

FOO Type Composites
Insulating polymers could improve flexibility, but due to their electrical insulating properties, the overall conductivity of the composite films was negatively affected, hindering the significant improvement in TE performance. In order to obtain flexi ble TE materials with excellent properties, various methods had been tried. Among them, the formation of inorganic TE films on insulating polymer substrates was an effective method to obtain both high TE performance and good flexibility. Insulating polymers such as poly(ethylene terephthalate), polyamide, nylon, polycarbonate, PVDF, and copy paper had been used as substrates to develop flexible TE films. The addition of polyvinylpyrrolidone (PVP) enhanced the flexibility of the hybrid film, and the highly conductive silver nanoparticles were dispersed in the insulating PVP layer, which could form a conductive channel and make the conductivity of the composite material reach 360.9 S cm −1 . A flexible TEG composed of five legs fabricated from PVP/Ag/Ag 2 Te achieved a maxi mum output power of 469 nW under a temperature gradient of 39.6 K, corresponding to a power density of 341 µW cm −2 . [62] The composite film composed of Ta 4 SiTe 4 and graphdiyne (GDY) with a similar conduction band level and PVDF was used, and the film had good flexibility and the ZT value reached 0.2. [63] Similarly, Dey et al. investigated composites of graphene and titanium dioxide nanocomposites (GTNC) or iron oxide nanocomposites (GINC) with polyvinyl acetate (PVAc). Among them, nanotitanium dioxide particles or nanoiron oxide particles were decorated on the 2D graphene sheets, which reduced the stacking properties of graphene and helped to destroy the thermal conduction network, but allowed the electrical network to remain intact. An important aspect of simultaneously reducing the thermal conductivity of the matrix in the thermoelectric field could be achieved by the insulating properties of PVAc. [64,65] Using Bi 2 Te 3 -based alloy (BTBA) with high Seebeck coefficient and electrical conductivity, carbon black (CB) with high electrical conductivity as filler, and polylactic acid (PLA) with low thermal conductivity as matrix. As the mass ratio of BTBA in the composite increased, more interfaces (BTBA-BTBA, BTBA-PLA, CB-CB, and BTBA-CB interfaces) were formed between BTBA, CB, and PLA ( Figure 5). This results in increased scattering of charge carriers (holes) in the composite. In addition, since more interfaces were formed in the composite and there were more nanoscale barriers on the interface, low-energy holes could not pass through the interface, while high-energy holes could pass through (i.e., enhancing the energy filtering effect), thereby increasing the thermoelectric performance. [66] The thermoelectric properties of organic-inorganic multicomponent composites are shown in Table 1.

All-Organic Multicomponent Thermoelectric Materials
Organic semiconducting polymers have attracted much attention due to their low cost, environmental friendliness, and good mechanical flexibility. In addition, the intrinsic low thermal   [58] Copyright 2020, Elsevier. f) Schematic diagram of a flexible thermoelectric device. g) Photo of flexible thermoelectric devices attached to human arms and chest to generate thermoelectric voltages. h) Thermal voltage measured with heating plate, human wrist and chest as heat sources. Reproduced with permission. [53] Copyright 2020, Elsevier. conductivity helps to further improve their thermoelectric properties, thus conducting polymers are considered to be promising thermoelectric materials, especially in the application of smart and wearable electronic products. To date, the most widely studied polymer thermoelectric materials include PEDOT, PANI, PPy, poly(3-hexylthiophene-2,5-diyl) (P3HT), etc. Compared with traditional inorganics, their common disadvantage is lower electrical transport properties, and there have been some recent reports that high PFs can be obtained through various doping strategies and multicomponent composites, especially with carbon materials. Besides semiconducting polymers, organic donor-acceptor charge transfer complexes, as a unique class of small molecular organic electronic materials, have received more and more attention in organic composite thermoelectric materials. Interestingly, in organic donoracceptor complexes, charge transfer not only controls the static electronic structure, but also has a significant impact on charge dynamics and TE properties. [88][89][90] This section will introduce all-organic multicomponent thermoelectric materials from different composition types, as well as their applications.

PCP Type Composites
In recent years, some studies usually combine carbon materials with conductive polymers to achieve the purpose of improving the overall conductivity of the material, but whether carbon materials could be uniformly dispersed in the material is a problem that people have been facing. Many efforts have been made to improve the dispersion of carbon materials through experimental methods, and there are four commonly used experimental methods. One of the simplest methods is the drop-cast method, in which a multicomponent film is obtained by alternately drop-casting a polymer dispersed in a solution onto a substrate (Figure 6a). This method could easily control the thickness of each layer and could effectively improve TE. Electrochemical polymerization could well control the degree of oxidation and thickness of conducting polymers, since the deposition potential, deposition time, and electrolyte ion concentration could be easily tuned. Electrochemical deposition of conducting polymer nanostructures on CNTs was carried out on a three-electrode structure in which flexible carbon nanotube papers (CNTPs) were used as working electrodes. CNTPs are first coated with one conducting polymer, followed by a second electrochemical coating of another conducting polymer (Figure 6b). In situ polymerization is one of the most common methods for synthesizing conductive polymer materials. This method is simple, inexpensive, and could be combined with a variety of methods. During ultrasonic dispersion, the SWCNTs bundles were well dispersed in the aqueous solution in the presence of the surfactant sodium dodecyl benzene sulfonate (SDBS). The hydrophobic aniline monomer was automatically located at the center of the SDBS micelle. Subsequently, these micelles were adsorbed on the SWCNTs surface due to Adv. Electron. Mater. 2023, 9, 2201310 Figure 5. Schematic illustration of PLA/CB/BTBA thermoelectric composites with different BTBA contents, and phonon and carrier transport in the interface. Reproduced with permission. [66] Copyright 2020, Elsevier. electrostatic attraction and π-π stacking. When the oxidant ammonium persulfate was added to the suspension, the monomers polymerized along the surface of SWCNTs, so that PANI nanoparticles were successfully prepared and adsorbed on the surface of SWCNTs. After the pyrrole monomer was added to the binary composite, the PPy layer was tightly wrapped on the surface of the PANI/SWCNT binary composite to form a ternary composite (Figure 6c). In the preparation of multicomponent composite thermoelectric materials, layer-by-layer (LBL) assembly is an efficient technology which alternately forms films of different components independently. With the help of strong interactions (such as covalent bonds) or weak interactions between the molecules of each layer (such as electrostatic attraction, hydrogen bonds, coordination bonds, etc.), the spontaneous association between layers will form molecular aggregates of ordered structures. LBL assembly technology shows unique advantages due to its simplicity, low cost, abundant film-forming substances, high order, and controllable thickness. At the same time, the 3D carrier transport network formed between different layers could also effectively improve the transport performance of the system (Figure 6d). Carbon materials, as thermoelectric materials with high electrical conductivity, are often used in multicomponent composite thermoelectric materials. The dispersion of CNTs is often a problem that multicomponent thermoelectric materials need to face. Usually, one component of multicomponent materials, such as PSS, poly(nickel 1,1,2,2-ethenetetrathiolate) (PETT), meso-tetra(4-carboxyphenyl) porphine (TCPP), etc., can stabilize and disperse the components in solution by interacting with CNTs. The CNTs are uniformly dispersed, and the carrier transport between the materials is enhanced, thereby improving the electrical conductivity. Tonga used PSS as a dispersant and dopant for CNTs, which could effectively cover nanotubes, and could also interconnect interfaces between layers to form ordered carrier transport pipes. In addition to the uniform dispersion of materials, the special structure and conductive network formed between polymer materials could also promote charge transfer and transport at the interface and improve the electrical conductivity of composite materials. [91] The SEM morphology of pristine CNTPs showed typical CNTs network structure, PPy coating on CNTPs, and dispersed aggregated spheres that originate from the uneven electric field distribution during deposition. It should be noted that many of these spheres were located at the junctions of the nanotubes, as indicated by the yellow arrows. Subsequent PANI deposition further coated the PPy/CNTPs with an additional polyaniline layer, and the spheres at the junction were well preserved (Figure 7a). The PPy spheres at the CNT junctions were in direct contact with the CNT and PANI layers, which facilitated the transport of carriers. After IL treatment, the molecular chains were ordered and the π-electron conjugation was further enhanced, thereby promoting electron transport within the molecular chains. The PF after secondary processing was 534.5 µW m −1 K −2 . [92] The combination of different organic components could also achieve the effect of improving the Seebeck coefficient through the template effect and the carrier energy filtering effect. The PEDOT:PSS/SWCNTs/PEDOT NWs composites prepared by the solution mixing method produced a carrier energy filtering effect between the PEDOT NW/PEDOT:PSS interfaces. Low-energy carriers could be selectively filtered at the PEDOT NW/PEDOT:PSS interface, while high-energy carriers could be allowed to pass through, which was indicated by the carrier Adv. Electron. Mater. 2023, 9, 2201310  [91] Copyright 2021, Informa UK Limited. b) Electrochemical deposition. Reproduced with permission. [92] Copyright 2021, Royal Society of Chemistry. c) In situ method. Reproduced with permission. [93] Copyright 2020, Royal Society of Chemistry. d) LBL assembly. Reproduced with permission. [94] Copyright 2016, Wiley-VCH. mobility enhancement observed by Hall effect measurements. [95] The doping efficiency of PPy chains is low during the in situ preparation of composites, PPy would aggregate at the interface of SWCNTs and the junction between SWCNTs and PANI as an energy barrier, resulting in an energy filtering effect. Post-treatment strategies could enhance the thermoelectric properties of the composites by controlling the oxidation level of conducting polymers, enhancing the alignment of conducting polymer chains, and/or removing insulating materials from the film surface. The treatment of PANI/SWCNTs/PPy with ferric chloride (FeCl 3 ) resulted in an improved conductivity from 70.3 to 245.3 S cm −1 . [93] The layered structure of PEDOT:PSS provided a template effect for the attachment of PPy/MWCNTs during the mixing process, and the strong π-π conjugation interaction between the components and the interfacial energy filtering effect led to an increase in the Seebeck coefficient of the composite. The Seebeck coefficient of the ternary component was nearly four times higher than that of the binary component. [96] PPy had a 1D NW morphology, and the PPy/PEDOT:PSS binary composite exhibited a morphology similar to that of 1D NWs, with the main difference being the rough surface caused by the PEDOT:PSS coating. Unexpectedly, the PPy/SWCNTs/ PEDOT:PSS ternary composite exhibited a layered structure consisting of parallel SWCNT nanosheets covered by PPy/ PEDOT:PSS NWs (Figure 7b). The electrical conductivity of pure PPy NWs and PEDOT:PSS was very low, 2.2 and 0.5 S cm −1 , respectively, and the electrical conductivity was as high as 907.17 S cm −1 after forming a ternary material with SWCNTs. [97] Cho et al. fabricated the PANI/graphene/DWCNTs/PEDOT:PSS all-organic multicomponent thermoelectric thin film by LBL assembly technique. The π-π interaction between the interfaces and the synergy effect between the components made the film structure more ordered, and the sequential self-assembly of each component produced a continuous 3D interconnected network. Among them, PEDOT:PSS played the role of stabilizing carbon materials. The sheet resistance of the films decreased with the number of cycles, indicating that the conductivity of these films increased with the number of layers (Figure 7c). The conductivity of both three-component thin-film systems increased gradually with the number of deposition cycles, indicating that the film had a more interconnected electron transport network. PANI/ graphene/PEDOT:PSS was difficult to see in the figure due to its relatively low conductivity. The conductivity started to level off beyond 60 cycles and reached 1885 S cm −1 at 80 cycles (Figure 7d). The Seebeck coefficients of the multicomponent films also increased with the number of cycles, and the Seebeck coefficients of the three-component films composed of graphene or DWCNTs reached 58 and 92 µV K −1, respectively (Figure 7e). For these multilayer films, the PF increased in a similar manner to the conductivity. Compared with the other two, the PF of the four components was improved by about 20 000 times and 2 times, respectively (Figure 7f). [94] The TEG assembled from five optimized PEDOT:PSS/ SWCNTs/PEDOT NW composite films produced a maximum output power of 414 nW at a temperature difference of 40 K. The TEG containing 15 legs was wrapped on a glass bottle, and the as-prepared TEG could fit to the curved surface due to the good flexibility of the film. Pouring hot water at 70 °C into a bottle produced an output voltage of about 5 mV, indicating the potential for practical thermal energy harvesting. [95]

PCC Type Composites
Multicomponent thermoelectric films composed of conductive polymers and carbon materials could improve the thermoelectric properties of composites through interconnected networks, special morphologies, and synergistic effects. Cho et al. deposited PANI, graphene, and DWCNTs sequentially using the LBL assembly technique. There was a strong synergistic effect between the three components, and the strong π-π interactions along the DWCNTs direction of PANI in the multicomponent composite led to structural rearrangement. This in turn increased the effective degree of electron delocalization, from randomness in solution (ring twisting) to extended chain conformations on the surface of DWCNTs in the deposited films.  This extended-chain conformation allowed better electron flow in the DWCNTs network, and in addition, the large surface area of graphene acted as a conductive bridge connecting the PANI-DWCNTs conductive domains, improving percolation and reducing the energy of electronic transitions. Therefore, in the 3D PANI/graphene/DWCNT network, the charge delocalization of the carriers along the main chain became higher, creating a PANI-based extended conjugated system. Although not clearly observed, the terminal fragments of the PANI-covered DWCNTs were likely to bridge adjacent graphene nanosheets along their surfaces to form a conductive network. This composite network could synergistically improve conductivity (four orders of magnitude). As the number of layers increased, an extended network of conductive graphene pathways was created, with carriers passing through the PANI-covered DWCNTs interface and graphene sheets. Low-energy carriers were strongly scattered, while high-energy carriers were not affected, so the average carrier energy and Seebeck coefficient would increase (approximately five times). [98] A composite material was prepared of in situ polymerized PEDOT:PSS, 1D MWCNTs, and 2D graphene. There were strong interactions among the multicomponent materials, with good electrical bridging and electronic coupling between PEDOT and carbon materials, resulting in a synergistic effect of multidimensional electrical bridges. The MWCNT bundles had a high aspect ratio, providing an additional network of long-range bridges between the conducting polymer domains. Graphene sheets had unique electronic properties with strong electronic interactions with PEDOT:PSS. Graphene sheets covered with conductive PEDOT combined with long MWCNTs could form an electrical grid. The conductivity of pristine PEDOT:PSS was 548 S cm −1 and the Seebeck coefficient was 18.0 µV K −1 , while the film composited with carbon material showed more excellent performance. Although the three-component resulted in higher thermal conductivity due to the addition of carbon materials, the calculated ZT value was higher than that of the single-component or two-component composite films (Figure 8). [99] Adv. Electron. Mater. 2023, 9,2201310 [99] Copyright 2015, Royal Society of Chemistry.

FOO Type Composites
Higher flexibility, mechanical stability, and sometimes even self-healing properties are increasingly required to cope with the frequent mechanical motions inevitable in practical applications and to fully adapt to the rather uneven surfaces of flexible electronics. Therefore, it has also become a trend to introduce insulating polymers as a matrix to tune the rheological and mechanical properties of organic thermoelectric composites. The self-healing membrane could fully restore its appearance after incision and maintain 75% of the initial PF value. Benefiting from the synergistic effect of PE and PEDOT, the maximum elongation at break of the ternary composite film was 12.23%. After dropping the poly(ethyleneimine) (PEI) doping solution into PE/PEDOT/SWCNTs, the prepared composites could be easily converted into n-type composites with a Seebeck coefficient of −36.41 µV K −1 . Therefore, a simple p-n junction module with seven pairs of legs produces an output voltage of 5.75 mV at a temperature difference of 15 K between human skin and the environment. [100] PETT was used to disperse CNTs in hybrid materials which acted as a charge transport dopant to enhance carrier transport between CNTs and improved the conductivity of hybrid films. Compared with pristine n-PETT, the electrical conductivity of the three-component hybrid film was improved by four orders of magnitude. [101] Polydopamine (PDA) had been widely used in multifunctional surface coatings in different fields due to its excellent adhesive properties. In the multicomponent material, the PDA layer acted as a building block for covalent cross-linking of carbon materials and PANI, forming a conductivity channel that enhanced short-range order. From another perspective, there were a large number of nanointerfaces between the three components, which could act as phonon scattering centers, so there was long-range disorder (Figure 9a). This results in higher electrical conductivity and Seebeck coefficient of all ternary composites than binary composites, among which the maximum Seebeck coefficient of PDA/graphene nanosheets (GNS)/PANI could reach 43.02 µV K −1 (Figure 9b,c). [102] The five-leg thermoelectric device with a single-leg structure fabricated by PVC/n-PETT/CNT is shown in Figure 9d. When a temperature difference of about 100 K was applied across the device in the in-plane direction, the open-circuit voltage was 19.9 mV and the maximum power was 3.88 µW (Figure 9e). [101] A flexible TEG with 24 thermoelectric units was formed by mixing n-type MWCNT dispersion and pyrolysis treatment CNT (c-CNT)/PDA powder to obtain n-type composite film, and mixing PEDOT:PSS solution with PDA/GNS/PANI powder as p-type film, the output voltage was 52.8 mV at a temperature difference of 60 °C. [102]

Other Type Composites
Organic semiconducting materials generally have low carrier mobility due to the hopping transport behavior of carriers. Generally, the conductivity of conductive polymers can be improved Adv. Electron. Mater. 2023, 9, 2201310   Figure 9. a) Schematic illustration of short-range order and long-range disorder structures in PDA/GNS/PANI ternary composites. b) Electrical conductivity and c) Seebeck coefficient of binary composites and ternary composites at 363 K. Reproduced with permission. [102] Copyright 2021, American Chemical Society. d) Photo of flexible thin-film thermoelectric devices composed of three-component hybrid films. e) Current as a function of voltage and power. Reproduced with permission. [101] Copyright 2015, Wiley-VCH.
by the rearrangement of polymer chains and the transformation of molecular conformation to form an ordered molecular arrangement structure and more efficient charge transport between layers, thereby improving the conductivity of materials. By adding bacterial cellulose (BC) and SWCNTs to improve the order degree of PEDOT molecular structure, PEDOT uniformly coated the surface of BC nanofibers and SWCNTs, forming multiple paths to facilitate electron transport, increased the conductivity by about 50 times. Due to the porous structure, the abundant junctions in the composite films reduced lowemissivity heat and increased phonon scattering, thereby reducing the thermal conductivity of the material (Figure 10a). The thermal conductivity was 0.13 W m −1 K −1 , lower than that of traditional PEDOT/CNT composites (0.3-0.5 W m −1 K −1 ). The eight single-leg thermoelectric prototype device connected with copper wires obtained a maximum open-circuit voltage of 6.9 mV and a maximum output power of 169 nW at a temperature difference of 65.6 K. [103] Donor-acceptor complexes are also promising components for the construction of multicomponent composite thermoelectrics. Complex of 2-decyl-7-phenyl [1] benzothieno [3,2- [4,5]thieno [2,3-d]thiophene (C8BTBT) could form an n-type C8BTBT-F 4 TCNQ/ SWCNTs composite film with moderately high PF of 105.1 µW m −1 K −2 . [106] Self-healable and stretchable thermoelectric material composed of DMSO mixed with PEDOT:PSS and polymeric surfactants exhibited a thermal conductivity of 0.27 W m −1 K −1 . The samples without surfactant showed obvious cracks in the film with a width of about 40 µm after scratching (Figure 10b). In contrast, the composite films exhibited thinner cracks with a width of ≈5 µm (Figure 10c). Due to the low stiffness and high flexibility of the viscoelastic composite film, the surface of the composite film could deform flexibly under the action of cutting force (Figure 10d). In the absence of any external stimuli or pressure, the reconnection between fractured areas took the form of plaques. According to the viscoelastic properties of the composite material layer, the composite material layer was ruptured, separated, and then elastically moved back close to the original position while applying cutting force and pressure. Through the reconstruction of intermolecular interactions, the electrical connectivity and thermoelectric properties of the composite membranes could be successfully restored (Figure 10f). After the film was completely broken by cutting or overstretching, it could automatically and rapidly heal based on the induced reassembly of surfactants (Figure 10g), and the self-healed film still maintained its mechanical stretchability. After the film was self-healed after cutting, the electrical conductivity recovered by 70%, and the Seebeck coefficient remained unchanged before and after cutting. The prepared Adv. Electron. Mater. 2023, 9, 2201310 Figure 10. a) Schematic illustration of electron transport and thermal transport along pore walls or pores in BC/PEDOT/SWCNTs films. Reproduced with permission. [103] Copyright 2019, American Chemical Society. SEM images of b) the PEDOT:PSS film without surfactant and c) the composite film on glass support substrate. d) Magnified SEM image of the cut on the composite film. Reproduced with permission. [104] Copyright 2019, Wiley-VCH. e) Schematic diagram of the morphology changes of PEDOT after adding IL. Reproduced with permission. [105] Copyright 2019, American Chemical Society. f) Schematic illustration for the self-healing process of the composite film. g) Photo of the composite film after cutting, reconnecting, and stretching. Reproduced with permission. [104] Copyright 2019, Wiley-VCH. 3D-printed TEG could heal itself and quickly, retaining more than 85% of its initial power output even after damage caused by repeated cutting. At the same time, in order to prove the practical application of the TEG in the environment with human body heat, the TEG on the flexible plastic substrate produced a stable output voltage of 0.6 mV under a temperature difference of 7 K. [104] The reduction of the oxidation level and morphology of PEDOT:PSS by IL resulted in a nanofibrous network (Figure 10e). More importantly, in composite membranes, ILs could act as soft domains based on their liquid properties. After adding ILs, the tensile strength of PEDOT:PSS was reduced from 35 to 11−13 MPa, the elongation fracture point was increased by a factor of 3, and the Young's modulus was greatly reduced from 479 to 55−58 MPa. [105] The thermoelectric properties of typical all-organic multicomponent composites are shown in Table 2.

Conclusion and Outlook
In this review, we revisited the development of organic-inorganic and all-organic multicomponent thermoelectric materials. The main materials used in the composites are carbon materials, PEDOT:PSS, PANI, PPy, etc. This review classifies the composition of multicomponent materials, and introduces organic-inorganic-carbon composites, organic-inorganicinorganic composites, composites containing film-forming substances, organic-carbon-organic composites, organiccarbon-carbon composite material, etc. Thermoelectric composites combine the advantages of high electrical conductivity, high Seebeck coefficient of fillers, and low thermal conductivity of polymer matrices. At the same time, their overall thermoelectric properties are optimized through energy filtering effect, network structure, interface phonon scattering effect, etc. Although significant progress has been made in multicomponent organic composite thermoelectric materials, its development is still at an early stage, and more efforts need to be done to advance the performance. First, the mechanism at the molecular level is not very clear. It is often necessary to draw on initial conceptual reports adopted in inorganic thermoelectric materials, such as the carrier energy filtering effect, interfacial phonon scattering, and the formation of ordered structures at the polymer/inorganic interface. New concepts and theories suitable for organic thermoelectric materials are needed. Second, most of the current multicomponent composites are p-type, so there is a need to develop more novel n-type thermoelectric composites with long lifetime and air stability. Third, it remains a great challenge to significantly improve the thermoelectric performance of the organic composites to be comparable to that of the inorganic thermoelectric materials. Understanding the limitations of binary systems and searching of universal models for multicomponent composites will have significant implications for the design of multicomponent blends and devices. Machine learning will also accelerate the design and discovery process from massive combinations. What is more, for composites with ionic conducting species, it is necessary to differentiate the contribution of ionic and electronic part when measure thermoelectric parameters, especially Seebeck coefficient under high humidity. Finally, the introduction of additional components also leads to more complex morphologies than the binary composites, and the characterization and understanding of the morphology is crucial to further improve their performance.