Organic thin film photovoltaic devices from discotic materials

Since the first demonstration of organic photovoltaic devices much progress has been made. Organic solar cells reach now power conversion efficiencies of up to 3% over the solar spectrum. The morphology of the active film is very important for efficient devices. Films spin-coated from blend solutions phase separate. The scale of the phase separation depends on the solvent, solubility of the materials and parameters of the spin-coating process such as speed, temperature, etc. If the morphology could be controlled on a molecular scale the efficiency of charge separation and transport could be expected to be substantially higher. The use of discotic liquid crystalline materials might help to reach this goal, because of their capacity to self-organise into columnar stacks. In this work we describe photovoltaic devices made with discotic liquid crystalline hexabenzocoronene and perylene dye molecules. Thin films have been produced by spin coating blends directly from solution. Devices with an external quantum efficiency (incident photon to current efficiency) of up to 34% at monochromatic illumination at 490 nm have been achieved with a blend of hexaphenyl-substituted hexabenzocoronene (HBC-PhC₁₂) and a perylene diimide. Photovoltaic devices with other hexabenzocoronene derivatives as hole conductor show lower efficiencies. We attribute the lower device performance of the latter to the different film morphology occurring from spin coating of these materials.