# Thermal conductance and thermoelectric figure of merit of C$_{60}$-based single-molecule junctions: electrons, phonons, and photons

## TL;DR

This study combines ab initio methods to analyze how electrons, phonons, and photons influence the thermal conductance and thermoelectric efficiency of C60-based molecular junctions, revealing the dominant role of phonons in dimers and the impact of photon tunneling.

## Contribution

It provides a detailed ab initio analysis of phonon and photon contributions to thermoelectric performance in C60 junctions, highlighting the importance of these factors in device efficiency.

## Key findings

- Phonons have a minor role in monomer junctions' thermal conductance.
- Phonons dominate thermal conductance in dimer junctions, reducing thermoelectric efficiency.
- Photon tunneling can further decrease thermoelectric performance.

## Abstract

Motivated by recent experiments, we present here an ab initio study of the impact of the phonon transport on the thermal conductance and thermoelectric figure of merit of C$_{60}$-based single-molecule junctions. To be precise, we combine density functional theory with nonequilibrium Green's function techniques to compute these two quantities in junctions with either a C$_{60}$ monomer or a C$_{60}$ dimer connected to gold electrodes, taking into account the contributions of both electrons and phonons. Our results show that for C$_{60}$ monomer junctions phonon transport plays a minor role in the thermal conductance and, in turn, in the figure of merit, which can reach relatively high values on the order of 0.1, depending on the contact geometry. At the contrary, phonons completely dominate the thermal conductance in C$_{60}$ dimer junctions and strongly reduce the figure of merit as compared to monomer junctions. Thus, claims that by stacking C$_{60}$ molecules one could achieve high thermoelectric performance, which have been made without considering the phonon contribution, are not justified. Moreover, we analyze the relevance of near-field thermal radiation for the figure of merit of these junctions within the framework of fluctuational electrodynamics. We conclude that photon tunneling can be another detrimental factor for the thermoelectric performance, which has been overlooked so far in the field of molecular electronics. Our study illustrates the crucial roles that phonon transport and photon tunneling can play when critically assessing the performance of molecular junctions as potential nanoscale thermoelectric devices.