Optical, electrical and morphological study of PEDOT: PSS single layers spiral-bar coated with various secondary doping solvents

•PEDOT:PSS layers having large electric conductivity and high transparency were prepared using spiral bar coater with various secondary dopants.•The electric conductivity of the layers raised from 0.3 (no solvent) up to 78.3S/cm applying N-methyl methanamide as secondary dopant.•The optical properties of layers were analyzed over the UV, visible, near infrared and mid infrared spectral regions to assess layer transparency and to discuss their electronic and vibrational transitions.•The isotropic and anisotropic model were considered to describe the optical properties of secondary doped PEDOT:PSS layers concluding that isotropic model is more appropriate.•Thicknesses and surface roughness of the layers obtained by spectroscopic ellipsometry were found comparable with values determined by standard mechanical measurement methods (profilometry and AFM). The electrical, morphological and optical properties of a series of spiral-bar coated single layers of PEDOT:PSS influenced by the addition of 10 different secondary dopants have been studied. The optical properties of these samples have been analyzed over a broad spectral range from 190nm to 30μm using spectroscopic ellipsometry and transmittance. The isotropic model fits the ellipsometric data quite well. No substantial differences in the optical constants were obtained, despite a difference being expected from the significant change of specific electrical conductivity (by 3 orders of magnitude). In the infrared part of spectra, the multiple Lorentz oscillators’ model was used instead of the frequently used Drude model by applying narrow oscillators for molecular vibrations together with the broad oscillators describing electronic transitions in the mid-gap states. The geometrical parameters obtained from ellipsometry evaluation have been found to be in good agreement with standard mechanical characterization probes (profilometry and AFM). The highest value of the specific electric conductivity, 78.3S/cm, was achieved by using n-methyl methanamide as a secondary dopant. The research results confirm that spectroscopic ellipsometry is a valuable tool for characterization of the functional layers used in printed electronics.