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: Emerging Green Energy Materials Converting Heat to Electricity Directly and Efficiently

The bifunctional salt [Co(terpy)2](TCNQ)3·CH3CN (terpy = 2,2′;6′,2′′-terpyridine, TCNQ = 7,7,8,8-tetracyanoquino-dimethane) exhibits a high room temperature conductivity of 0.13 S cm−1 and an anomaly in conductivity at ∼190 K as evidenced by variable temperature structural, magnetic and conductivity studies. The anomaly in the conductivity at 190 K has been correlated with the temperature dependent structural breathing and Jahn–Teller distortion of the low spin state of the SCO units, as well as the charge fluctuations and supramolecular π-stacking interactions of partially charged TCNQ radicals. The modular synthetic approach leads to an accessible source of partially charged TCNQ radicals for the facile preparation of bifunctional molecular materials with high electrical conductivity.

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A cobalt(II) spin-crossover compound with partially charged TCNQ radicals and an anomalous conducting behavior

A complete report of experimental results on the dc electrical conductivity of TEA (TCNQ)2 is presented. In order to reconcile these results with the known structural and magnetic properties, it is suggested that the electron interactions play a fundamental role in the low lying magnetic excitations but have only a small influence on the much more energetic excitations to the conducting states. Conduction thus appears to resemble that of a band gap semiconductor modified by band tailing as a result of the cation disorder.



Nous presentons un compte rendu complet de nos resultats experimentaux sur la conductivite electrique du TEA (TCNQ)2 en courant continu. La comparaison de ces resultats avec les proprietes structurales et magnetiques connues par ailleurs, suggere une interpretation dans laquelle les interactions electroniques jouent un role primordial vis a vis des excitations magnetiques, de basse energie, mais influent peu sur les excitations beaucoup plus energetiques qui conduisent aux etats conducteurs. La conduction apparaǐt ainsi proche de celle d'un semiconducteur de bandes classique, perturbee par la presence en bord de bandes de “queues” d'energie resultant du desordre des cations.

Further Aspects of the Electrical Conduction in the Highly Anisotropic Organic Semiconductor TEA (TCNQ)2

We studied the crystallization processes of the complex ion-radical salt of 7,7,8,8-tetracyanoquinodimethan (TCNQJ and the N-methylquinolinium cation. A variety of solvents, rates of crystallization, and concentrations of the solution gave three groups of crystals. The crystals in the first group, which were crystallized as black rods, occluded the solvent molecules in the crystals, and their resistivities were in the range of 2.8–26 Ωcm. Those in the second group were crystallized into the form of fine needles or fine powders. Although the TCNQ-to-cation ratios in the salts varied from 1.6 to 2.2, their electrical resistivities were almost all the same (about 1.00 Ωcm), and these crystals were thought to have a metastable crystal structure. The crystals in the third group were crystallized in the form of triclinic prisms, which belonged to a stable crystal of a high resistivity (1100 Ωcm). The appearance of multiple forms is interpreted on the basis of Ostwald’s “Law of Stages” of crystallization and als...

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Studies of the Crystallization Processes of N-Methylquinolinium TCNQ Ion-radical Salts

This chapter deals with salts (or adducts) of 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) with metal ions or metal-containing compounds or ions Reflecting the state of knowledge of such compounds, the greater portion of this review will focus on structural and physical properties of alkali salts of TCNQ For reasons of analogy, ammonium (NH+ 4) salts will be considered, too, but not salts of substituted ammonium cations, ie, (R n NH4-n )+

Salts of 7,7,8,8- Tetracyano-p-quinodimethane with Simple and Complex Metal Cations

Electrical conductivity, activation energy for conduction, thermoelectric power and Hall mobility have been measured in compactions of a series of simple and complex TCNQ salts. The results indicate nearly intrinsic semi-conduction in extremely narrow bands, with an average drift mobility of about 10–2 cm2/Vs.

A. R. Blythe

Papers

Measurement of Seebeck and Hall effects in compactions of TCNQ salts