Direct observation and evolution of electronic coupling between organic semiconductors

Abstract

The electronic wave functions of an atom or molecule are affected by its interactions with its environment. These interactions dictate electronic and optical processes at interfaces, and is especially relevant in the case of thin film optoelectronic devices such as organic solar cells. In these devices, charge transport and interfaces between multiple layers occur along the thickness or vertical direction, and thus such electronic interactions between different molecules---same or different---are crucial in determining the device properties. Here, we introduce an in situ spectroscopic ellipsometry data analysis method called differential analysis in real time (DART) with the ability to directly probe electronic coupling due to intermolecular interactions along the thickness direction using vacuum-deposited organic semiconductor thin films as a model system. The analysis, which does not require any model fitting, reveals direct observations of electronic coupling between frontier orbitals under optical excitations leading to delocalization of the corresponding electronic wave functions with thickness or, equivalently, number of molecules away from the interface in C60 and MeO-TPD deposited on an insulating substrate $(\mathrm{Si}{\mathrm{O}}_{2})$. Applying the same methodology for C60 deposited on phthalocyanine thin films, the analyses shows strong, anomalous features---in comparison to C60 deposited on $\mathrm{Si}{\mathrm{O}}_{2}$---of the electronic wave functions corresponding to specific excitation energies in C60 and phthalocyanines. Translation of such interactions in terms of dielectric constants reveals plasmonic type resonance absorptions resulting from oscillations of the excited state wave functions between the two materials across the interface. Finally, reproducibility, angstrom-level sensitivity, and simplicity of the method are highlighted showcasing its applicability for studying electronic coupling between any vapor-deposited material systems where real-time measurements during thin film growth are possible.