Molecular Semiconductors in Organic Photovoltaic Cells

Molecular semiconductors (MSCs) are progressing rapidly, as is the field of organic photovoltaics (OPV). The major use of MSCs at present is in nonsemiconducting applications such as pigments and dyes. One reason for choosing these particular compounds from the many known perylene diimides and phthalocyanines is that they form highly photoactive polycrystalline phases in which much of the oscillator strength resides in a low energy charge transfer absorption band. The intermolecular forces between MSC molecules in a solid are highly directional and spatially complex. Many planar aromatic MSCs form cofacial π-stacks in which the degree of in-phase electronic overlap between adjacent molecules determines the strength of the intermolecular electronic coupling. The bandwidths of some MSCs vary substantially as the molecules in the crystal lattice shift just slightly relative to their neighbors, causing the wave functions of the adjacent molecules to move in and out of phase with each other.