Infrared Spectroscopy in the Ultra-High Vacuum

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Organic semiconductors, that is, conjugated molecules and polymers promise cost-effective large area processability into flexible structures for novel display technology, solid-state lightning, photovoltaics, intelligent textiles or printed electronics. In contrast to inorganic semiconductors such as silicon, however, our ability to precisely control the distribution of charge in complex structures by impurity doping is still limited. While essentially all successful applications in the field such as organic light emitting diode (OLED) based display technology employ doped materials, the doping efficiency of today's dopants is low and the underlying processes at work are not yet fully understood. This is mainly because dopants used for organic semiconductors are organic molecules themselves, where we have identified chemical interaction to significantly lower the degree of charge transfer and, therefore, the doping efficiency. From our previous work emerge design strategies for more efficient molecular hole dopants, which require to be bulky molecules of high electron affinity to reduce interaction through steric hindrance while promoting efficient charge transfer. We are currently synthesizing a library of novel doping agents based on this rationale, which are, however, highly sensitive to oxygen/water. While this is no issue for their practical application due to efficient encapsulation strategies in industry, their analysis via Fourier-transform infrared spectroscopy (FTIR) to assess chemical integrity and to quantify their efficiency requires to be done in-situ. In the same vein, we are exploring the mechanisms underlying the doping of organic semiconductors using Lewis acids as alternative dopants, where a water-assisted process inducing semiconductor protonation has recently been suggested as fundamental process. This is, however, at odds with recent data suggesting this process to be energetically unfavorable. Elucidating the process of Lewis acid doping therefore requires working in an entirely water-free environment which is only enabled by ultrahigh vacuum (UHV). Therefore, we request an FTIR spectrometer to be combined with our available interconnected UHV/glovebox system. This instrument enables in-situ characterization of organic semiconductor thin films doped with our novel hole dopants and Lewis acids without exposure to oxygen/water. Thin film samples are established via spin coating in the glovebox, transferred to the UHV system for doping via vacuum sublimation of the dopants, and analysed by FTIR in-situ without breaking the vacuum. The spectrometer's capabilities for grazing incidence reflectance allow to investigate ultrathin films down to the monolayer region to equally investigate bulk doping and interface effects. This enables pushing the limits of organic semiconductor doping and foster the understanding of its basic processes, which is key for the knowledge-based development of future doping strategies.