Yimeng Sun

Organic thermoelectric materials: emerging green energy materials converting heat to electricity directly and efficiently.

The abundance of solar thermal energy and the widespread demands for waste heat recovery make thermoelectric generators (TEGs) very attractive in harvesting low-cost energy resources. Meanwhile, thermoelectric refrigeration is promising for local cooling and niche applications. In this context there is currently a growing interest in developing organic thermoelectric materials which are flexible, cost-effective, eco-friendly and potentially energy-efficient. In particular, the past several years have witnessed remarkable progress in organic thermoelectric materials and devices. In this review, thermoelectric properties of conducting polymers and small molecules are summarized, with recent progresses in materials, measurements and devices highlighted. Prospects and suggestions for future research efforts are also presented. The organic thermoelectric materials are emerging candidates for green energy conversion.

Organic Thermoelectric Materials and Devices Based on p‐ and n‐Type Poly(metal 1,1,2,2‐ethenetetrathiolate)s

A series of metal coordination polymers containing 1,1,2,2-ethenetetrathiolate (ett) linking bridge (poly[Ax(M-ett)]) are synthesized. The Seebeck coefficients of these conducting materials are high, and vary according to the center metals and counter cations. The TE device fabricated demonstrates the great potentials of these materials for TE applications.

A two-dimensional π–d conjugated coordination polymer with extremely high electrical conductivity and ambipolar transport behaviour

Currently, studies on organic two-dimensional (2D) materials with special optic-electronic properties are attracting great research interest. However, 2D organic systems possessing promising electrical transport properties are still rare. Here a highly crystalline thin film of a copper coordination polymer, Cu-BHT (BHT=benzenehexathiol), is prepared via a liquid–liquid interface reaction between BHT/dichloromethane and copper(II) nitrate/H2O. The morphology and structure characterization reveal that this film is piled up by nanosheets of 2D lattice of [Cu3(C6S6)]n, which is further verified by quantum simulation. Four-probe measurements show that the room temperature conductivity of this material can reach up to 1,580 S cm−1, which is the highest value ever reported for coordination polymers. Meanwhile, it displays ambipolar charge transport behaviour and extremely high electron and hole mobilities (99 cm2 V−1 s−1 for holes and 116 cm2 V−1 s−1 for electrons) under field-effect modulation.

Thermoelectric energy from flexible P3HT films doped with a ferric salt of triflimide anions

Due to their low thermal conductivity, non-toxicity and low cost, conductive polymer materials are potential candidates for thermoelectric applications. Here, a detailed investigation into the thermoelectric properties of P3HT films is reported. A thermoelectric power factor over 20 μW m−1 K−2 at room temperature was obtained by employing a ferric salt of triflimide (TFSI−) anions as a dopant. Flexible films of P3HT-TFSI were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM), along with temperature-variant electrical measurements. Given the promising results obtained from ordinary P3HT films by a simple doping treatment, this work suggests the significance of the appropriate choice of dopants and/or counterions, as well as the polymers themselves.

Flexible n-Type High-Performance Thermoelectric Thin Films of Poly(nickel-ethylenetetrathiolate) Prepared by an Electrochemical Method.

Flexible thin films of poly(nickel-ethylenetetrathiolate) prepared by an electrochemical method display promising n-type thermoelectric properties with the highest ZT value up to 0.3 at room temperature. Coexistence of high electrical conductivity and high Seebeck coefficient in this coordination polymer is attributed to its degenerate narrow-bandgap semiconductor behavior.

Inkjet-printed flexible organic thin-film thermoelectric devices based on p- and n-type poly(metal 1,1,2,2-ethenetetrathiolate)s/polymer composites through ball-milling.

In this article, we put forward a simple method for the synthesis of thermoelectric (TE) composite materials. Both n- and p-type composites were obtained by ball-milling the insoluble and infusible metal coordination polymers with other polymer solutions. The particle size, film morphology and composition were characterized by dynamic light scattering, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. The TE properties of the drop-cast composite film were measured at different temperatures. An inkjet-printed flexible device was fabricated and the output voltage and short-circuit current at various hot-side temperatures (Thot) and temperature gradients (ΔT) were tested. The composite material not only highly maintained the TE properties of the pristine material but also greatly improved its processability. This method can be extended to other insoluble and infusible TE materials for solution-processed flexible TE devices.

n-Type thermoelectric materials based on CuTCNQ nanocrystals and CuTCNQ nanorod arrays

We report the synthesis and thermoelectric (TE) performance of organometallic coordination polymers, including copper 7,7,8,8-tetracyano-p-quinodimethane nanocrystals (NC-CuTCNQ) and thin films of CuTCNQ nanorod arrays (NrA-CuTCNQ). The characterization of NC-CuTCNQ was carried out with the compressed samples. For NrA-CuTCNQ films, the TE properties were investigated with a hybrid Au/Cu/CuTCNQ/Au architecture along the direction either vertical or parallel to the film surface and obviously anisotropic behaviors were observed. We found that CuTCNQ can be a potential n-type material for future application in thermoelectric devices with a power factor of 1.5 μW m−1 K−2, accompanied by a high Seebeck coefficient of −632 μV K−1 at 370 K. In order to optimize its TE performance, a cousin molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) was mixed with TCNQ reacted with CuI. We found that the CuTCNQ blend possessed the highest power factor of 2.5 μW m−1 K−2 with a 1 mol% blend ratio of F4TCNQ (related to TCNQ) at 370 K.

Optimization of the thermoelectric properties of poly(nickel-ethylenetetrathiolate) synthesized via potentiostatic deposition

The coordination polymer poly(nickel-ethylenetetrathiolate) (poly(Ni-ett)), formed by nickel(II) and 1,1,2,2-ethenetetrathiolate (ett), is the most promising N-type organic thermoelectric material ever reported; it is synthesized via potentiostatic deposition, and the effect of different applied potentials on the optimal performance of the polymers is investigated. The optimal thermoelectric property of poly(Ni-ett) synthesized at 0.6 V is remarkably greater than that of the polymers synthesized at 1 and 1.6 V, exhibiting a maximum power factor of up to 131.6 μW/mK2 at 360 K. Furthermore, the structure-property correlation of poly(Ni-ett) is also extensively investigated. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the larger size of crystalline domains and the higher oxidation state of poly(Ni-ett) synthesized at 0.6 V possibly results in the higher bulk mobility and carrier concentration in the polymer chains, respectively, accounting for the enhanced power factor.

Two soluble polymers with lower ionization potentials: doping and thermoelectric properties

Doping and thermoelectric properties of two solution-processable conjugated polymers with low ionization potentials (IPs) have been studied and compared. An optimized thermoelectric power factor (PF) approaching 40 μW m−1 K−2 at 390 K was achieved in films of polymer PDTPT-C12, by performing doping treatment with LiTFSI solution in air, while an optimized thermoelectric power factor around 12 μW m−1 K−2 at 390 K was observed when CuTFSI2 solution was used instead of LiTFSI. In contrast, such effects on thermoelectric performance as a result of dopant species were not observed in the other studied polymer PTVT2T-C12 with comparable IP. Based on the results of thermoelectric measurements and optical spectroscopy as well as photoelectron spectroscopy, the role of Li+ in the resultant thermoelectric performance was proposed. Moreover, owing to the relatively low IPs of the two studied polymers, the as-doped polymer films are reasonably stable under ambient conditions. Therefore, N-alkyl dithieno[3,2b:2′,3′-d] pyrroles (DTPs) as exemplified in the case of PDTPT-C12 are suggested to be promising building-blocks and the incorporation of small cations like Li+ may be an alternative to increase the thermopower in solid state devices.

Highly Conducting Polythiophene Thin Films with Less Ordered Microstructure Displaying Excellent Thermoelectric Performance

Polythiophene (PTh) with highly regular molecular structure is synthesized as nearly amorphous thin films by electrochemical methods in a BFEE/DTBP mixed medium (BFEE = boron fluoride ethyl ether; DTBP = 2,6-di-tert-butypyridine). The doping level and film morphology of PTh are modulated through adjusting the current density applied during the polymerization process. A combined analysis with solid-state NMR, FT-IR, and Raman spectra reveals the molecular structural regularity of the resulted PTh films, which leads to the highest electrical conductivity up to 700 S cm-1 for films obtained under an optimized current density of 1 mA cm-2 . By applying the self-heating 3ω-method, thermal conductivities are measured along the in-plane direction. A highly reduced Lorenz number of 6.49 × 10-9 W Ω K-2 and low lattice thermal conductivity of 0.21 W m-1 K-1 were extracted based on the analyses of the electrical and thermal conductivities according to the Wiedemann-Franz Law; the former is about one-third of the Sommerfeld value. Finally, the maximized ZT value can reach up to 0.10 under room temperature, which shows that the highly conducting polymers with less ordered structure is the practical direction for developing organic thermoelectric materials.