In organic small-molecule solar cells, highly conjugated and planar molecular units stack through strong π-π interaction into supramolecular structures, with varying degrees of order in thin films. Intermolecular coupling on the nanoscale determines the efficiency of exciton transport and charge migration. Therefore, optimizing these materials for device applications necessitates an understanding of how donor-acceptor interactions are affected by molecular level coupling. In our lab, we selected several molecular semiconductors inspired by structural subunits found in champion organic solar cell materials, in order to identify combinations that result in some degree of ground state charge transfer (CT) in the solid state. These then operate as model systems to understand the effects of intermolecular coupling and orientation on CT interactions. Of the eight studied, we discovered four CT pairs in thin films. We present results from analyzing these thin films using polarization-resolved infrared reflectance spectroscopy, optical spectroscopy, and other methods, alongside single crystal growth to obtain co-crystals for X-ray analysis.

Structural effects on ground state charge transfer in planar organic semiconductor thin films

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