Thermoelectric Properties of Organic Charge-Transfer Compounds

We measured the thermoelectric (TE) properties of compressed pellets of various organic charge-transfer (CT) complexes, such as (TTF)(TCNQ), (BO)(TCNQ) and (ET)(2)(HCNAL), where TTF, TCNQ, BO, ET, and HCNAL represent tetrathiafulvalene, tetracyanoquinodimethane, bis(ethylenedioxy)-tetrathiafulvalene, bis(ethylenedithio)tetrathiafulvalene, and 2,5-dicyano- 3,6-dihydroxy-p-benzoquinone, respectively. The metallic (TTF)(TCNQ) and semiconducting (BO)(TCNQ) complexes showed Seebeck coefficients (S) of -18 mu V/K and -30 mu V/K at 300 K, respectively. On the contrary, the Mott insulator (ET)(2)(HCNAL) was found to show a rather high absolute S (-116 mu V/K at 300 K), the magnitude of which is comparable to those of the conventional inorganic TE materials. With increasing temperature (170 K to 300 K), the electrical conductivity was increased about two orders of magnitude while the S value was nearly constant. These results suggest that S values could be determined mainly by spin entropy (configurations) of carriers in the Mott insulator (ET)(2)(HCNAL). The magnitude of the observed S value was compared with that derived from a theoretical model (generalized Heikes formula).