Completely dry transfer printing of graphene for organic electronic devices

Abstract

Despite extensive research on using graphene as transparent electrodes, its application to organic electronic devices has been rather limited. In the most commonly used fabrication technique, graphene-polymer stack floating on water is transferred onto a target substrate, and the polymer layer is subsequently removed by an organic solvent. As a result, in most previous applications, graphene has been deposited on a substrate before organic layer depositions, and it has been used mostly as a bottom electrode. Furthermore, when one works with wet chemicals that are known not to damage a given organic material, patterning of the graphene on the organic layer is likely to involve chemical and/or physical processes that may adversely affect the electrical and/or optical properties of the underlying organic layer. Here, we describe a completely dry technique capable of transfer printing a graphene monolayer onto an organic layer. In this process, a CVD-grown graphene monolayer is first transferred onto a PDMS stamp by scooping a graphene-support layer stack floating on a liquid with the PDMS stamp, following the conventional wet-transfer process except two important differences. First, we decrease the surface tension of the liquid by mixing ethanol and water. This enhances the wetting of the hydrophobic PDMS surface by the liquid, which is crucial to obtain a graphene monolayer with a low defect density on the PDMS stamp. Second, as a support layer deposited on graphene on a copper foil, we use a thin film of gold as opposed to commonly used polymers, such as PMMA and polystyrene. This allows us to use an aqueous chemical to remove the support layer, and hence prevents the swelling of PDMS, which would severely degrade the quality of the graphene monolayer on the PDMS stamp. We discuss the effects of the surface tension of the liquid on the quality of a graphene monolayer transferred on an organic material commonly used in organic electronic devices, such as PEDOT:PSS, PVP and ZnPc.