Thermal conductivity of benzothieno-benzothiophene derivatives at the nanoscale

TL;DR

This study investigates the nanoscale thermal conductance of specific organic semiconductor films using scanning thermal microscopy and molecular dynamics, revealing significant out-of-plane thermal conductivity and anisotropy, with implications for thermal management.

Contribution

It provides the first nanoscale measurements of out-of-plane thermal conductivity in BTBT derivatives and supports findings with molecular dynamics simulations showing 3D-like thermal transport.

Findings

BTBT has higher out-of-plane thermal conductivity than C8-BTBT-C8.

Thermal conductivity values are anisotropic and comparable to molecular dynamics predictions.

Thickness has minimal effect on nanoscale thermal resistance.

Abstract

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.