Transparent conductive films (TCF) are thin materials used in a range of electronic devices including liquid-crystal and organic light-emitting displays and touchscreens. They are typically made of indium tin oxide (ITO), which, while offering a good transparent conductor, is also fragile and costly. A promising alternative is a conductive grid based on silver nanoparticles covered with a protective zinc oxide nanolayer. But widespread adoption requires the development of large-scale and high-speed industrial ink and inkjet printing. To offer such a solution, an inkjet printing manufacturer and a conductive nanoinks producer, KELENN Technology (KT) and GenesInk (GNK) respectively, collaborated on the EU-funded project CLEARSILVER. GNK developed silver (Ag), zinc oxide (ZnO) and aluminium doped zinc oxide (AZO) nanomaterials and nanoinks. While KT developed a reel-to-reel (R2R) printing machine for TCF manufacturing. With organic light-emitting diode (OLED) and organic photovoltaic (OPV) devices realised, CLEARSILVER demonstrated an economically and technically viable alternative to ITO. All-printed solutions for TCFs While indispensable to our smart world, used in everything from sensors to displays, the main problems with TCFs are firstly its reliance on indium, which is scarce and thus expensive. Secondly, ITO, which is deposited by evaporation methods onto rigid glass or polymer substrates, cannot be directly patterned. This means that a whole surface has to be covered with ITO, increasing the costs of TCFs per square metre. CLEARSILVER’s all-printed solution for TCFs is based on silver nanoparticle grids covered with zinc oxide which offers high conductivity, while ensuring smooth prepared films of high homogeneity. Crucially, compared to ITO, the silver nanoparticles can be printed and patterned, meaning that lower quantities of material need to be deposited, making it more cost-effective. To create the nanoinks, first the vehicle (solvents, binders and additives) was prepared and mixed in the formulation reactor; silver nanoparticles concentrate was then added. Tests were performed for rheology (flow of solid and liquid materials), surface tension, printability, electrical performance and sheet resistance. The KT R2R inkjet printer, which was also developed, ultimately achieved a high throughput printing performance with a high speed of 60 metres per second. To highlight one result, the Ink Smart Jet I can manufacture ink at an industrial scale of 25 litres a batch. CLEARSILVER succeeded in manufacturing OLED and OPV demonstrators, based on TCFs with high transparencies (70-85 %), in partnership with project partner Fraunhofer FEP. Over 70 % of the OLED devices functioned well, achieving large light-emitting areas in tests realised at a luminance of 1 000 candelas per square metre. All manufactured OPV devices worked at a power conversion efficiency (PCE) equivalent to 70 % of ITO-based TCF performance, even before process optimisation. “The performance of the printed CLEARSILVER demonstrators was equivalent to ITO-based TCF demonstrators prepared by classical techniques,” says Dr Rita Faddoul, R&D Project Manager at GenesInk. “As the life cycle assessment prepared by the Leitat institute as part of the project indicates, CLEARSILVER offers the EU market an eco-friendly and cost-effective TCF solution, which promises to replace ITO.” Today, GNK has four commercialised inkjet printing inks adapted for OLED and OPV applications available in the market. The team is currently working to further optimise the process (printing, deposition, curing) for the enhancement of final device performance.
CLEARSILVER, Photovoltaic, organic light-emitting diode, nanosilver, nano zinc-oxide, transparent conductive film, touchscreen, Indium Tin Oxide, liquid crystal display, nanoinks, printing