Ambipolar organic semiconductor blends, i.e. mixtures of electron and hole conducting materials, attain growing interest due to their utilization in quasi-complementary organic field-effect transistors and organic photovoltaic cells. Many investigations in the latter field have reported an increase of the solar cell efficiency by optimizing the balance between charge carrier transport in phase-separated structures and exciton dissociation at the interface between these phases. Here we show the implications of blending molecular materials for structural, optical, and electrical properties in two model systems for organic photovoltaic cells. We have investigated blends and neat films of the hole transporting material Cu-phthalocyanine (CuPc) together with fullerene C60 and Cu-hexadecafluorophthalocyanine (F16CuPc) as electron transporting materials, respectively. On the one hand, the difference in molecular structure of the spherical C60 and the planar molecule CuPc leads to nanophase separation in a blend of both of them, causing charge carrier transport being limited by the successful formation of percolation paths. On the other hand, blends of the similar shaped CuPc and F16CuPc molecules entail mixed crystalline films, as can be clearly seen by X-ray scattering measurements. We discuss differences of both systems with respect to their microstructure as well as their electrical transport properties in diodes and field-effect transistors. Furthermore, we compare the photovoltaic properties of planar- and bulk-heterojunction devices under white light illumination to relate the different morphologies of both material systems to their performance in solar cells.
Sketches of different molecular arrangements in blended systems. The formation of phase-separated (left) or molecularly mixed crystalline films (right) can occur, depending on the geometry of the involved molecules.