cel Mare av. 168, Chisinau, Rep. of Moldova A more complete physical model for nanostructured crystals of tetrathiotetracene-iodide that takes into account the interaction of carriers with the neighboring one-dimensional (1D) conductive chains and also the scattering on impurities and defects is presented. For simplicity the 2D approximation is applied. It is shown that this model describes very well the temperature dependencies of electrical conductivity in the temperature interval between 180 and 300 K and of Seebeck coefficient between 50 and 300 K, the highest temperature for which the measurements were reported. For lower temperatures it is needed to also consider the fluctuations of dielectric phase which appear before the metal-dielectric transition. It is found that the predictions made in 1D approximation are valid, if the crystal purity is not very high, the electrical conductivity is limited up to ~ 3.5106  -1 m -1 and the thermoelectric figure of merit up to ZT ~ 4.

Previously, it has been predicted that nanostructured crystals of tetrathiotetracene iodide are very promising candidates for thermoelectric applications. However, these predictions are based on a strictly one-dimensional (1D) model. In order to verify these conclusions, a two-dimensional (2D) model is elaborated which explicitly takes into account the weak interaction of carriers with the nearest conductive chains. It is shown that for crystals with a rather low degree of purity this interaction does not affect significantly the results obtained by the 1D approximation, but for ultrapure crystals this interaction can no longer be neglected.

Thermoelectric properties of existing crystals of tetrathiotetracene-iodide, TTT 2 I 3 , are modeled in the most complete 3D physical model. The expected values of the thermoelectric power factor and of the thermoelectric figure of merit are determined for crystals with different degrees of perfection and carrier concentration. Optimal values of electrical conductivity, Seebeck coefficient, and electronic thermal conductivity in order to achieve the predicted values of the thermoelectric figure of merit are calculated.

cel Mare av. 168, Chisinau, Rep. of Moldova Thermoelectric properties of quasi-one-dimensional TTT(TCNQ) 2 organic crystals are investigated in order to appreciate the prospect of using this compound as n -type thermoelectric material. A more complete three-dimensional (3D) physical model is elaborated. It takes into account two the most important interactions of conduction electrons with longitudinal acoustic phonons, the electrons' scattering on neighbor molecular chains, as well as the scattering by impurities and defects. The electrical conductivity, thermopower, the power factor, electronic thermal conductivity and the thermoelectric figure of merit in the direction along conductive molecular chains are calculated numerically for different degrees of crystal purity. It is shown that in stoichiometric compounds the thermoelectric figure of merit ZT remains small even after the increase of crystal perfection degree. The thermoelectric properties may be significantly enhanced by simultaneous increase of crystal perfection and of electron concentration. The latter can be achieved by additional doping with donors. For less pure crystals, the interaction with impurities predominates over the weak interchain interaction and the simpler one-dimensional (1D) physical model is applicable. When the impurity scattering is reduced, the interchain interaction begins to limit the carrier mobility and the application of the 3D physical model is required. The optimal parameters permitting to predict ZT ~ 1 are determined.

The charge and energy transport in highly conducting quasi-one-dimensional organic crystals of p-type tetrathiotetracene-iodide, TTT2I3, and of n-type tetrathiotetracene-tetracyanoquinodimethane, TTT(TCNQ)2, is studied. Two electron-phonon interactions are considered simultaneously. One interaction is of the acoustic deformation potential type and the other one is of polaronic character. Charge transport along the conducting molecular chains is bandlike, whereas in the transversal directions, it is of the hopping type. It is shown that due to a partial compensation of these interactions for a narrow interval of states in the one-dimensional conduction band, the relaxation time is of Lorentzian shape and shows a distinct dependence on carrier energy with a pronounced maximum. The scattering of charge carriers on adjacent molecular chains and by impurities and structural defects limits the height of this maximum. However, rather high relaxation times might be anticipated in the case of perfect single crystals. As the carriers in these states show an enhanced mobility, this will lead to a simultaneous increase of electrical conductivity and Seebeck coefficient. It is proposed that, if the above-mentioned crystals could be accomplished by means of sufficient purification and by optimization of the carrier concentration, so that the Fermi level is close to energetic states for which the relaxation time has a maximum, one might achieve values for the thermoelectric figure of merit ZT∼5 in crystals of tetrathiotetracene-iodide and ZT∼1.5 in those of tetrathiotetracene-tetracyanoquinodimethane.