  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.
