All concentrations of the different samples with respect to the volume of the solvent are given in Table . A drop of the ink was applied onto the substrate and the ink was doctor bladed with a blade-distance between 300 and 500 µm. Ethylene glycol was then evaporated at 80°C on a precision hotplate (PZ2860-SR, Gestigkeit GmbH) to obtain the final films with thicknesses between 30 -100 µm. The thickness was determined with a digital micrometer or vertical scanning interferometry.