Thermal conductivity of benzothieno-benzothiophene derivatives at the nanoscale

• Published in: arXiv.org 2021-02
• Main Authors: Gueye, Magatte N, Vercouter, Alexandre, Jouclas, Rémy, Guérin, David, Lemaur, Vincent, Schweicher, Guillaume, Lenfant, Stéphane, Antidormi, Aleandro, Geerts, Yves, Melis, Claudio, Cornil, Jérôme, Vuillaume, Dominique
• Format: Article
• Language: English
• Subjects: Anisotropy; Benzothiophene; Charge transport; Crystal structure; Crystallinity; Heat conductivity; Heat transfer; Interlayers; Mathematical analysis; Molecular dynamics; Scanning thermal microscopy; Thermal conductivity; Thermal resistance; Thickness; Transport properties
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2102.05377

We study by scanning thermal microscopy the nanoscale thermal conductance of films (40 to 400 nm thick) of [1]benzothieno[3,2-b][1]benzothiophene (BTBT) and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8). We demonstrate that the out-of-plane thermal conductivity is significant along the interlayer direction, larger for BTBT (0.63 +/- 0.12 W m-1 K-1) compared to C8-BTBT-C8 (0.25 +/- 0.13 W m-1 K-1). These results are supported by molecular dynamics calculations (Approach to Equilibrium Molecular Dynamics method) performed on the corresponding molecular crystals. The calculations point to significant thermal conductivity (3D-like) values along the 3 crystalline directions, with anisotropy factors between the crystalline directions below 1.8 for BTBT and below 2.8 for C8-BTBT-C8, in deep contrast with the charge transport properties featuring a two-dimensional character for these materials. In agreement with the experiments, the calculations yield larger values in BTBT compared to C8-BTBT-C8 (0.6-1.3 W m-1 K-1 versus 0.3-0.7 W m-1 K-1, respectively). The weak thickness dependence of the nanoscale thermal resistance is in agreement with a simple analytical model.