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 heattransporting 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 max and zT revealed the improvement of the conversion efficiency by the vibrating counter cations. The P max 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 + .

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