Thermal Conductivities and Figures of Merit of Tetracyanoquinodimethane-Based Thermoelectric Materials Consisting of Cations Exhibiting Order-Disorder Transitions

A reduction in thermal conductivity is a common challenge in the development of thermoelectric materials. The thermal conductivity of molecule-based crystals can be reduced by vibrating or disordered counter ions that scatter the heat-transporting phonons. In this work, the thermoelectric properties of five 1:2 salts of tetracyanoquinodimethane (TCNQ) were examined to study the effect of counter ions on the order-disorder transitions in thermal conductivity and on the thermoelectric figure of merit. The tetraethylammonium (TEA⁺) and dipropylammonium (DPA⁺) salts of TCNQ^0.5-, which undergo the order-disorder transitions above 200 K, exhibited significantly low thermal conductivities compared to the quinolinium (Q⁺) salt, which does not undergo any order-disorder transition. Methyltriphenylphosphonium (MTPP⁺) and methyltriphenylarsenium (MTPAs⁺) salts also showed lower thermal conductivities than the Q⁺salt, presumably because of the heavy P and As atoms. Despite the wide variation in thermal conductivities, the product of the phonon velocity v and mean free path l was minimized at similar temperatures, presumably because of the common vibronic property exhibited by the TCNQ^0.5-stacks. A comparison between the power factors P_maxand zT revealed the improvement of the conversion efficiency by the vibrating counter cations.