Evidence of thermal transport anisotropy in stable glasses of vapour deposited organic molecules

TL;DR

This study reveals that molecular orientation in vapour-deposited organic glasses causes anisotropic thermal conductivity, influenced by deposition temperature, with implications for improving thermal management in organic electronic devices.

Contribution

It demonstrates the anisotropic thermal transport in vapour-deposited organic glasses and links it to molecular orientation and density, supported by molecular dynamics simulations.

Findings

Thermal conductivity varies with deposition temperature.

Heat flow prefers the molecular backbone direction.

Simulations confirm orientation-dependent vibrational transport.

Abstract

Vapour-deposited organic glasses are currently in use in many optoelectronic devices. Their operation temperature is limited by the glass transition temperature of the organic layers and thermal management strategies become increasingly important to improve the lifetime of the device. Here we report the unusual finding that molecular orientation heavily influences heat flow propagation in glassy films of small molecule organic semiconductors. The thermal conductivity of vapour-deposited thin-film semiconductor glasses is anisotropic and controlled by the deposition temperature. We compare our data with extensive molecular dynamics simulations to disentangle the role of density and molecular orientation on heat propagation. Simulations do support the view that thermal transport along the backbone of the organic molecule is strongly preferred with respect to the perpendicular direction. This is due to the anisotropy of the molecular interaction strength that limit the transport of atomic vibrations. This approach could be used in future developments to implement small molecule glassy films in thermoelectric or other organic electronic devices.