Poly(3-hexylthiophene-2,5-diyl) (P3HT) is one of the most prominent semiconducting, conjugated polymers in the fields of organic electronics and photovoltaics. Besides its high electric conductivity, it shows thermoelectric properties when doped with metal chlorides or nanoparticles, e.g. gold (Au). To optimize the charge transport and hence the thermoelectric Seebeck coefficient, it is advantageous to form small gold domains inside the polymer films. This is usually achieved by small dopant concentrations on the surface of the film. Although there is almost always an intermixing layer of gold and P3HT near the surface, higher thermoelectric coefficients can be achieved by implanting the dopant deep inside the organic films. In addition, poly(3-hexylthiophene-2,5-diyl)-block-poly(methyl methacrylate) (P3HT-b-PMMA) diblock derivates show multiple morphological variations ranging from nanometer-sized lamellae or cylindrical structures, which will be interesting pathways towards future applications with tunable in-plane electric or thermoelectric properties. We choose both spin-casted and spray-deposited thin films since they both are used in industry and spray-coating offers excellent scalability in future industrial applications. From our previous studies, the fabrication routes influence the surface morphology and penetration of Au into a subsurface layer. Grazing-incidence small-angle X-ray scattering (GISAXS) allowed for the determination of the structural properties, e.g. size, morphology, and distance distribution, of the formed gold clusters on the surfaces and the interface region as well as the respective polymer domains. These results were cross-checked with subsequent atomic force microscopy of the resulting film surfaces. Using both neutron (NR) and X-ray reflectivity (XRR), the dopant distribution throughout the films was tracked and correlated with crystallinity changes observed by grazing-incidence wide-angle X-ray scattering (GIWAXS). This information will be crucial for improving the thermoelectric properties of thin films and providing a scalable approach for future industrial manufacturing.

Effect of substrate morphology and doping technique on the structure and thermoelectric properties of P3HT-based thin films

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