Intermolecular interactions govern the optical and electronic properties of organic semiconductor thin films. Optimizing the function of molecular devices such as organic solar cells and light emitting diodes relies on understanding structural effects in a heterogeneous film environment. Because organic thin films are typically more disordered than inorganic crystalline semiconductors, advanced techniques are often needed to sufficiently characterize these materials. However, polarized infrared reflectance measurements are a benchtop method that can provide well-resolved vibrational spectra with specificity in their molecular orientation. By using infrared reflectance-absorbance spectroscopy (IRRAS) we seek to further relate the effects of packing on thin film vibrational spectra. We analyze a suite of prototypical organic semiconductor molecules with differing sidechain substitutions which adopt distinct crystal packing arrangements in thin films. We then connect spectral changes such as Davydov splitting to intermolecular interactions that vary as a function of the stacking geometry, with implications for characterizing donor-donor, acceptor-acceptor, and donor-acceptor interactions within crystalline domains and at heterojunction interfaces using IR spectroscopy.

Exploring the orientation and intermolecular coupling in organic semiconductor thin films via infrared reflectance-absorbance spectroscopy

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