Discotic mesophase materials: Their use in organic field-effect transistors and organic photovoltaics

New discotic mesophase materials, especially those based on phthalocyanines, have become increasingly interesting as charge transport and photoactive materials for new molecular electronic technologies (organic field-effect transistors (OFETs) and organic photovoltaics (OPVs)). Side chains added to the periphery of these molecules render them solution processable and enhance the coherence in their rod-like aggregates. It has been hypothesized that these columnar aggregates will provide for charge mobilities competitive with amorphous silicon (ca. 1 cm ²/volt-sec), when the coherence length exceeds ca. 100 nm, and the processing conditions lead to thin films with low charge trap densities, and good electrical contacts (source and drain electrodes in OFETs; anodes/cathodes In OPVs). Our work with octa-substituted phthalocyanines, where the alkoxy- or thio-ether-linked side chains are terminated with benzyl groups, has shown that it is possible to achieve the desired coherence and electrical properties on sub-micron length scales (measured by conductive-tip AFM), but that achieving the desired device properties still requires optimization to achieve i) low contact resistances, and ii) the proper column orientation for both OFET and OPVs.