Computing the Lattice Thermal Conductivity of Small‐Molecule Organic Semiconductors: A Systematic Comparison of Molecular Dynamics Based Methods

Abstract

While the Green‐Kubo and non‐equilibrium molecular dynamics methods have been compared quite extensively to calculate the thermal conductivity in inorganic compounds, there is currently a lack of comparison of these algorithms with the more recently developed approach‐to‐equilibrium molecular dynamics (AEMD) method for other types of materials such as organic semiconductors. To fill this gap, this article reports a theoretical description of thermal transport in single crystals made of terthiophene as prototypical system based on the three most popular molecular dynamics approaches. A systematic comparison of the computed values of thermal conductivity and its anisotropy is carried out and the strengths and weaknesses associated with each method are discussed. Although the three algorithms give essentially the same trends, this study points to the “AEMD” approach as the most suitable compromise between accuracy and computing cost. On the material aspects, the theoretical modeling yields an anisotropic character of the thermal transport in crystals whose out‐of‐plane thermal conductivity component is approximately twice larger than the in‐plane components. The AEMD approach is further used to investigate the influence of temperature on thermal transport in terthiophene. The trends are utterly rationalized by relying on the concepts of phonon mean free paths and phonon group velocities.

Anisotropic Phonon Bands in H-Bonded Molecular Crystals: The Instructive Case of α-Quinacridone

“…Phonon properties of the entire 1st Brillouin zone, however, play a major role for quantities like the thermodynamic stability, the heat capacity, and all transport properties. Phonons (from within the entire 1st Brillouin zone) are the main carriers of heat in molecular crystals , and, therefore, dictate the thermal conductivities of OSCs, which typically differ between directions within and perpendicular to the planes in which the OSC molecules are arranged. − Additionally, in OSCs, the coupling between charge carriers and phonons is particularly strong, thus significantly inhibiting charge carrier transport, − where in recent studies, the dominant role of specific phonon modes has been identified. − In the context of charge transport, it has also been shown that a proper description of electron–phonon couplings in OSCs requires the consideration of phonons from the entire 1st Brillouin zone . Measuring the phonon band structures of molecular crystals is, however, quite challenging, as it typically requires large single crystals of fully deuterated materials .…”

“…Phonons (from within the entire 1st Brillouin zone) are the main carriers of heat in molecular crystals 16 , 17 and, therefore, dictate the thermal conductivities of OSCs, which typically differ between directions within and perpendicular to the planes in which the OSC molecules are arranged. 18 − 21 Additionally, in OSCs, the coupling between charge carriers and phonons is particularly strong, thus significantly inhibiting charge carrier transport, 22 − 24 where in recent studies, the dominant role of specific phonon modes has been identified. 25 − 28 In the context of charge transport, it has also been shown that a proper description of electron–phonon couplings in OSCs requires the consideration of phonons from the entire 1st Brillouin zone.…”