  Organic thin film materials with molecular ordering are gaining attention as
they exhibit semiconductor characteristics. When using them for electronics,
the thermal management becomes important, where heat dissipation is directional
owning to the anisotropic thermal conductivity arising from the molecular
ordering. However, it is difficult to evaluate the anisotropy by simultaneously
measuring in-plane and cross-plane thermal conductivities of the film on a
substrate, because the film is typically as thin as tens to hundreds of
nanometers and its in-plane thermal conductivity is low. Here, we develop a
novel bidirectional 3{\omega} system that measures the anisotropic thermal
conductivity of thin films by patterning two metal wires with different widths
and preparing the films on top, and extracting the in-plane and cross-plane
thermal conductivities using the difference in their sensitivities to the
metal-wire width. Using the developed system, the thermal conductivity of
spin-coated poly(3,4-ethylenedioxythiophene) olystyrene sulfonate (PEDOT:PSS)
with thickness of 70 nm was successfully measured. The measured in-plane
thermal conductivity of PEDOT:PSS film was as high as 2.9 W m-1 K-1 presumably
due to the high structural ordering, giving anisotropy of 10. The calculations
of measurement sensitivity to the film thickness and thermal conductivities
suggest that the device can be applied to much thinner films by utilizing metal
wires with smaller width.
