Fully flexible and transparent graphene-based OLED device produced EU-funded scientists have succeeded in developing fully flexible, transparent organic light-emitting diode (OLED) electrodes from graphene. The hyped wonder material was used to create OLED light panels, but applications also span many other industries. Industrial Technologies © Foto-Ruhrgebiet, Shutterstock Consisting of bonded atoms formed into sheets that measure just one atom thick, graphene is the thinnest material known and possibly the strongest. The material’s high strength- to- weight ratio makes it ideal for a great variety of applications ranging from bendable electronics and thin lighting sources to improved solar cells and product packaging. Thanks to its outstanding optical, electrical and mechanical properties, graphene is also a very promising candidate for a new generation of transparent electrodes. Until now, indium tin oxide (ITO) has been the market leader for making transparent electrode materials for OLED. However, ITO electrodes are brittle, and scarce supplies and increasing demand make their use economically challenging for the industry. Within the EU-funded project GLADIATOR (Graphene layers: Production, characterization and integration), scientists successfully developed a functional flexible OLED lighting panel based on graphene electrodes. The new production technologies used in the project facilitate industrial-scale production of less expensive, higher-quality and larger-area graphene sheets, thereby making graphene an important alternative to ITO for transparent electrodes with much less resistance. Mass producing high-quality graphene Large-scale roll-to-roll production of graphene based on chemical vapour deposition (CVD) was first demonstrated in 2010. The project team used this promising route to grow high-quality graphene sheets on a large scale. ‘In a controlled environment, scientists supplied a mixture of methane and hydrogen to a copper wafer plate. This initiates a chemical reaction that causes methane to dissolve in the copper and as the copper surface is saturated, graphene starts to grow and spread over. After cooling down, the graphene is transferred to the desired substrate by a polymer carrier. This two-layer structure consisting of the polymer support and graphene is then stripped from the copper wafer and placed on the target substrate. After removing this polymer carrier, the graphene electrode area can then be structured to allow fabrication of electronic devices,’ explains project coordinator Beatrice Beyer. Although CVD is one of the best ways to produce graphene, reliable large-volume production of high-quality graphene is still problematic. GLADIATOR scientists succeeded though in optimising catalysts and increasing catalyst coverage of the substrate. ‘A novel technique based on the use of a water-soluble transfer polymer ((poly)vinylalcohol) enabled the project team to transfer graphene of 300 mm in length in A4-sized barrier foils, demonstrating reuse of the expensive copper catalysts five times with no effect on graphene quality,’ continues Dr Beyer. Scientists also made important progress increasing the conductivity of graphene by external doping. The dopants chloroauric acid and iodine gave the best doping efficiency of the materials tested, and special methods were developed to improve their stability. Functional transfer of graphene Transferring large-area graphene from its growth surface to the device substrate is simultaneously one of the biggest challenges and amongst the main bottlenecks towards its commercialisation. The project team investigated different transfer techniques, including etching transfer, electrochemical transfer based on hydrogen evolution, and a novel electrochemical transfer exploiting controlled oxidation and reduction of copper at the copper-graphene interface. Focusing mainly on the last technique, scientists lowered the average delamination time for large sheet sizes by an order of magnitude and investigated the potential of removing smaller tiles and reconnecting them to form a larger continuous-area of graphene ‘patchwork’. Hazard assessment For the first time, scientists investigated potential health risks that might be associated with future high-volume graphene production in a production environment. Results demonstrated that there was a negligible concentration of graphene particles in the air. In vivo experiments showed that graphene does cause cell toxicity and shows genotoxic effects. Large graphene sizes are essential for the successful commercialisation of graphene. The 16-partner GLADIATOR consortium achieved the critical mass to develop technology that improves production of large-area graphene. The project produced a patent and several high-profile publications. Potential socioeconomic impact could be tremendous. Keywords Graphene, OLED light panels, transparent electrodes, GLADIATOR, chemical vapour deposition
