Oligoyne molecular junctions for efficient room temperature thermoelectric power generation

TL;DR

This study demonstrates that oligoyne molecular junctions exhibit low phonon thermal conductance and high thermoelectric efficiency at room temperature, making them promising for waste heat to electricity conversion.

Contribution

It reveals that oligoynes have lower phonon conductance and higher thermoelectric performance than alkanes, due to phonon filtering effects and molecular rigidity.

Findings

Oligoynes have lower phonon thermal conductance than alkanes.

Maximum thermoelectric figure of merit ZT = 1.4 achieved with oligoynes.

Phonon filtering by electrodes influences thermal conductance behavior.

Abstract

Understanding phonon transport at a molecular scale is fundamental to the development of high-performance thermoelectric materials for the conversion of waste heat into electricity. We have studied phonon and electron transport in alkane and oligoyne chains of various lengths and find that due to the more rigid nature of the latter, the phonon thermal conductances of oligoynes are counter intuitively lower than that of the corresponding alkanes. The thermal conductance of oligoynes decreases monotonically with increasing length, whereas the thermal conductance of alkanes initially increases with length and then decreases. This difference in behaviour arises from phonon filtering by the gold electrodes and disappears when higher-Debye-frequency electrodes are used. Consequently a molecule that better transmits higher-frequency phonon modes, combined with a low-Debye-frequency electrode that filters high-energy phonons is a viable strategy for suppressing phonon transmission through the molecular junctions. The low thermal conductance of oligoynes, combined with their higher thermopower and higher electrical conductance lead to yield a maximum thermoelectric figure of merit of ZT = 1.4, which is several orders of magnitude higher than for alkanes.