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Investigation of self-assembled monolayer formation using infrared-reflection-absorption-spectroscopy

Charge injection barriers caused by a misalignment of energy levels are of major concern in organic semiconductor devices. One possibility to improve charge carrier injection is the application of an additional layer at the interface between the contact and the organic semiconductor. Self-assembled monolayers (SAMs) have been proven to form stable and well defined layers on various contact materials. Depending on their molecular dipole they can lower or raise the work function of a material and are therefore very well suited as injection layers. Since SAMs can be processed from solution they form a relevant material for printed organic electronics. The orientation of the SAM and thus important interface properties like the interface dipole and the work-function shift are influenced by various parameters such as concentration of the molecule in solution, immersion time and cleanliness of the solution and of the substrate. Infrared-reflection-absorption-spectroscopy (IRRAS) is a very sensitive tool to measure changes in the orientation of SAMs on metal substrates. We performed IRRAS measurements on SAMs consisting of perfluorinated decanethiol (PFDT) on evaporated gold films in order to probe the orientation, ordering and quality of the SAMs. By systematic variation of immersion time and concentration, we were able to conclude on the process steps of layer formation. Taking into account realistic printing circumstances, we also investigated the impact of oxygen in the solvent and the gold substrate on the layer formation process.