Control of morphology in efficient photovoltaic diodes from discotic liquid crystals

We report on the role of morphology in photovoltaic diodes with blend active layers composed of perylene tetracarboxdiimide (EPPTC) and hexabenzocoronene (HBC) derivatives as electron and hole acceptors. Controlled annealing of HBC:EPPTC films while in conformal contact with a flat elastomeric stamp improves photovoltaic response, leading to an external quantum efficiency of 29.5% at 460 nm and an open-circuit voltage of 0.70 V. The improved performance is attributed in part to larger crystalline domains following annealing. The elastomeric stamp restricts the top surface of the thin film during annealing, leading to low surface roughness, while also allowing for greater vertical stratification of the components in the bulk. Blended HBC:EPPTC films also exhibit an unique optical absorption feature near 590 nm, which we attribute to an altered crystalline packing of EPPTC in the presence of HBC. The significance of the local structure at molecular heterojunctions in blended active layer photovoltaic diodes is discussed.