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

Abstract

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 TCNQ0.5–, 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 TCNQ0.5– stacks. A comparison between the power factors Pmax and zT revealed the improvement of the conversion efficiency by the vibrating counter cations. The Pmax value for the DPA+ salt was approximately 23 times smaller than that for Q+; however, the thermal conductivity of the DPA+ salt in the disordered phase was approximately a quarter that of Q+, and the zT value for DPA+ remained 7 times smaller than that for Q+