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-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(+).

Automated summary

The reduction in thermal conductivity and improvement in conversion efficiency of thermoelectric materials can be achieved by using vibrating or disordered counter ions. In this study, the effect of counter ions on the order-disorder transitions in thermal conductivity and thermoelectric figure of merit was investigated. The results showed that salts with order-disorder transitions displayed significantly lower thermal conductivities compared to salts without any transitions. Additionally, vibrating counter cations were found to improve the conversion efficiency.