Thermal conductivity of benzothieno-benzothiophene derivatives at the nanoscale

MN Gueye and A Vercouter and R Jouclas and D Guerin and V Lemaur and G Schweicher and S Lenfant and A Antidormi and Y Geerts and C Melis and JM Cornil and D Vuillaume, NANOSCALE, 13, 3800-3807 (2021).

DOI: 10.1039/d0nr08619c

We study by scanning thermal microscopy the nanoscale thermal conductance of films (40-400 nm thick) of 1benzothieno3,2-b1benzothiophene (BTBT) and 2,7-dioctyl1benzothieno3,2-b1benzothiophene (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.

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