The relation of the thermoelectric figure of merit and the nanocomposite morphology is studied for thermoelectric thin films consisting of poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) with different amounts of silicon nanoparticles (Si‐NPs). An increase in the figure of merit of up to 150% is found for an Si‐NP concentration of 0.5 wt% as compared to pristine PEDOT:PSS films. The improvement originates from a disruption in the molecular ordering and therefore reduced electrical conductivity, which leads to an increased Seebeck coefficient, while also reducing thermal conductivity for higher concentrations through phonon scattering. The thermal conductivity is measured with steady‐state IR thermography on free‐standing PEDOT:PSS/Si‐NP composite films, enabling a full determination of the figure of merit. The morphology is investigated with grazing incidence resonant tender X‐ray scattering (GIR‐TeXS) around the sulfur K‐absorption edge. Without need for extrinsic labeling, GIR‐TeXS measurements have varying scattering contrast conditions for the components of the ternary system. By comparing the scattered intensities at different photon energies with the corresponding scattering contrast, the Si‐NPs are found to be preferentially dispersed in the large and medium‐sized PEDOT‐rich domains. The changes in size for the PEDOT‐rich domains as function of Si‐NP concentration cause improvement of the thermoelectric properties of the films.

We review the Raman shift method as a non-destructive optical tool to investigate the thermal conductivity and demonstrate the possibility to map this quantity with a micrometer resolution by studying thin film and bulk materials for thermoelectric applications. In this method, a focused laser beam both thermally excites a sample and undergoes Raman scattering at the excitation spot. The temperature dependence of the phonon energies measured is used as a local thermometer. We discuss that the temperature measured is an effective one and describe how the thermal conductivity is deduced from single temperature measurements to full temperature maps, with the help of analytical or numerical treatments of heat diffusion. We validate the method and its analysis on 3-and 2-dimensional single crystalline samples before applying it to more complex Si-based materials. A suspended thin mesoporous film of phosphorus-doped laser-sintered Si 78 Ge 22 nanoparticles is investigated to extract the in-plane thermal conductivity from the effective temperatures, measured as a function of the distance to the heat sink. Using an iterative multigrid Gauss-Seidel algorithm the experimental data can be modelled yielding a thermal conductivity of 0.1 W/m K after normalizing by the porosity. As a second application we map the surface of a phosphorus-doped 3-dimensional bulknanocrystalline Si sample which exhibits anisotropic and oxygen-rich precipitates. Thermal conductivities as low as 11 W/m K are found in the regions of the precipitates, significantly lower than the 17 W/m K in the surrounding matrix. The present work serves as a basis to more routinely use the Raman shift method as a versatile tool for thermal conductivity investigations, both for samples with high and low thermal conductivity and in a variety of geometries.

We demonstrate a simple and quick method for the measurement of the in-plane thermal conductance of thin films via steady-state IR thermography. The films are suspended above a hole in an opaque substrate and heated by a homogeneous visible light source. The temperature distribution of the thin films is captured via infrared microscopy and fitted to the analytical expression obtained for the specific hole geometry in order to obtain the in-plane thermal conductivity. For thin films of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate post-treated with ethylene glycol and of polyimide, we find conductivities of 1.0 W m K and 0.4 W m K at room temperature, respectively. These results are in very good agreement with literature values, validating the method developed.