coating thickness analysis (case 16)
Can you determine a coating thickness?
We were asked to provide a non-destructive method to quantify the coating thickness of sub-micron coatings of conductive, organic PEDOT:PSS formulations on a coated plastic substrate (PET).
Conventional metal optics methods (such as microscopic measurement or profilometric measurement of scratch depth) were not applicable due to the ambiguity of the mechanical reference height. Control of microtome cross-sections via TEM analysis was too expensive.
Thickness measurement based on optical properties
We applied the method of thin layer interference analysis for thickness measurement. The analysis is based on an optical model and needs the optical properties (n and k) of both the organic conductive material and the PET film. These are derived from reflection and transmission spectra of coated and non-coated films.
We decided to extend the optical analysis to the UV/VIS/NIR range (300-2500 nm) in order to include the effect of free charge carriers on the optical properties of the conductive material. The model was validated with the analysis of test coatings with systematic variation of coating thickness in comparison to analysis via TEM cross-section.
Left: measured UV/VIS/NIR reflection and transmission spectra of a PEDOT:PSS coating (red curve) and fit with optical model (blue curve). Right: corresponding optical properties n and k.
Model for metal optics predictions
The metal optics thickness analysis gives access to the sub-micron coating thickness of PEDOT:PSS layers on coated PET foils. In addition, the optical model approach provides the intrinsic electric properties of the coating, such as charge carrier density and their damping resistance. These are related to the sheet resistance of the conductive film.
The model can also be used to predict the optical performance (reflection/transmission) of PEDOT:PSS coatings in specific layer stacks with other materials and allows to benchmark the optical and intrinsic electric performance of PEDOT:PSS with other conductive materials, such as ITO.
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