Heat transport in crystalline organic semiconductors: coexistence of phonon propagation and tunneling

TL;DR

This paper investigates heat transport in crystalline organic semiconductors, revealing that both phonon propagation and tunneling mechanisms contribute, with tunneling explaining experimental anomalies and varying with temperature and molecular length.

Contribution

It introduces a combined phonon and tunneling model using the Wigner Transport Equation to accurately describe heat conduction in organic semiconductors.

Findings

Tunneling significantly contributes to heat transport, especially at higher temperatures.

Traditional phonon-based models fail to explain experimental results.

Thermal conductivity shows unusual temperature invariance and length dependence.

Abstract

Understanding heat transport in organic semiconductors is of fundamental and practical relevance. Therefore, we study the lattice thermal conductivities of a series of (oligo)acenes, where an increasing number of rings per molecule leads to a systematic increase of the crystals' complexity. Temperature-dependent thermal conductivity experiments in these systems disagree with predictions based on the traditional Peierls-Boltzmann framework, which describes heat transport in terms of particle-like phonon propagation. We demonstrate that accounting for additional phonon-tunneling conduction mechanisms through the Wigner Transport Equation resolves this disagreement and quantitatively rationalizes experiments. The pronounced increase of tunneling transport with temperature explains several unusual experimental observations, such as a weak temperature dependence in naphthalene's conductivity and an essentially temperature-invariant conductivity in pentacene. While the anisotropic conductivities within the acene planes are essentially material-independent, the tunneling contributions (and hence the total conductivities) significantly increase with molecular length in the molecular backbone direction, which for pentacene results in a surprising minimum of the thermal conductivity at 300K.