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.

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.

Conjugated backbones play a fundamental role in determining the electronic properties of organic semiconductors. On the basis of two solution-processable dihydropyrrolo[3,4-c]pyrrole-1,4-diylidenebis(thieno[3,2-b]thiophene) derivatives with aromatic and quinoid structures, we have carried out a systematic study of the relationship between the conjugated-backbone structure and the thermoelectric properties. In particular, a combination of UV-vis-NIR spectra, photoemission spectroscopy, and doping optimization are utilized to probe the interplay between energy levels, chemical doping, and thermoelectric performance. We found that a moderate change in the conjugated backbone leads to varied doping mechanisms and contributes to dramatic changes in the thermoelectric performance. Notably, the chemically doped A-DCV-DPPTT, a small molecule with aromatic structure, exhibits an electrical conductivity of 5.3 S cm and a high power factor (PF) up to 236 μW m K, which is 50 times higher than that of Q-DCM-DPPTT with a quinoid structure. More importantly, the low thermal conductivity enables A-DCV-DPPTT to possess a figure of merit (ZT) of 0.23 ± 0.03, which is the highest value reported to date for thermoelectric materials based on organic small molecules. These results demonstrate that the modulation of the conjugated backbone represents a powerful strategy for tuning the electronic structure and mobility of organic semiconductors toward a maximum thermoelectric performance.

Organic donor-acceptor (DA) complexes have attracted wide attention in recent decades, resulting in the rapid development of organic binary system electronics. The design and synthesis of organic DA complexes with a variety of component structures have mainly focused on metallicity (or even superconductivity), emission, or ferroelectricity studies. Further efforts have been made in high-performance electronic investigations. The chemical versatility of organic semiconductors provides DA complexes with a great number of possibilities for semiconducting applications. Organic DA complexes extend the semiconductor family and promote charge separation and transport in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). In OFETs, the organic complex serves as an active layer across extraordinary charge pathways, ensuring the efficient transport of induced charges. Although an increasing number of organic semiconductors have been reported to exhibit good p- or n-type properties (mobilities higher than 1 or even 10 cm V s), critical scientific challenges remain in utilizing the advantages of existing semiconductor materials for more and wider applications while maintaining less complicated synthetic or device fabrication processes. DA complex materials have revealed new insight: their unique molecular packing and structure-property relationships. The combination of donors and acceptors could offer practical advantages compared with their unimolecular materials. First, growing crystals of DA complexes with densely packed structures will reduce impurities and traps from the self-assembly process. Second, complexes based on the original structural components could form superior mixture stacking, which can facilitate charge transport depending on the driving force in the coassembly process. Third, the effective use of organic semiconductors can lead to tunable band structures, allowing the operation mode (p- or n-type) of the transistor to be systematically controlled by changing the components. Finally, theoretical calculations based on cocrystals with unique stacking could widen our understanding of structure-property relationships and in turn help us design high-performance semiconductors based on DA complexes. In this Account, we focus on discussing organic DA complexes as a new class of semiconducting materials, including their design, growth methods, packing modes, charge-transport properties, and structure-property relationships. We have also fabricated and investigated devices based on these binary crystals. This interdisciplinary work combines techniques from the fields of self-assembly, crystallography, condensed-matter physics, and theoretical chemistry. Researchers have designed new complex systems, including donor and acceptor compounds that self-assemble in feasible ways into highly ordered cocrystals. We demonstrate that using this crystallization method can easily realize ambipolar or unipolar transport. To further improve device performance, we propose several design strategies, such a...

Thermoelectric technology allows conversion between heat and electricity. Many good thermoelectric materials contain rare or toxic elements, so developing low-cost and high-performance thermoelectric materials is warranted. Here, we report the temperature-dependent interplay of three separate electronic bands in hole-doped tin sulfide (SnS) crystals. This behavior leads to synergistic optimization between effective mass (m*) and carrier mobility (μ) and can be boosted through introducing selenium (Se). This enhanced the power factor from ~30 to ~53 microwatts per centimeter per square kelvin (μW cm−1 K−2 at 300 K), while lowering the thermal conductivity after Se alloying. As a result, we obtained a maximum figure of merit ZT (ZTmax) of ~1.6 at 873 K and an average ZT (ZTave) of ~1.25 at 300 to 873 K in SnS0.91Se0.09 crystals. Our strategy for band manipulation offers a different route for optimizing thermoelectric performance. The high-performance SnS crystals represent an important step toward low-cost, Earth-abundant, and environmentally friendly thermoelectrics.

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.

A high-performance thermoelectric oxyselenide BiCuSeO ceramic with ZT > 1.1 at 823 K and higher average ZT value (ZTave ≈0.8) is obtained. The heavy doping element and nanostructures can effectively tune its electronic structure, hole concentration, and thermal conductivity, resulting in substantially enhanced mobility, power factor, and thus ZT value. This work provides a path to high-performance thermoelectric ceramics.

A significant enhancement of thermoelectric performance in layered oxyselenides BiCuSeO was achieved. The electrical conductivity and Seebeck coefficient of BiCu(1-x)SeO (x = 0-0.1) indicate that the carriers were introduced in the (Cu(2)Se(2))(2-) layer by Cu deficiencies. The maximum of electrical conductivity is 3 × 10(3) S m(-1) for Bicu(0.975)Seo at 650 °C, much larger than 470 S m(-1) for pristine BiCuSeO. Featured with very low thermal conductivity (∼0.5 W m(-1) K(-1)) and a large Seebeck coefficient (+273 μV K(-1)), ZT at 650 °C is significantly increased from 0.50 for pristine BiCuSeO to 0.81 for BiCu(0.975)SeO by introducing Cu deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.