Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - NanoGraphInk (Nano-Hybrid Graphene-Based Ink for Printable Flexible Transparent Applications)

Teaser

The stiff and brittle indium tin oxide (ITO) is by far the most widely used material for producing transparent conducting coatings for electronic devices. Nevertheless, as raw materials for ITO are becoming increasingly expensive and more importantly due to its rigidity, the...

Summary

The stiff and brittle indium tin oxide (ITO) is by far the most widely used material for producing transparent conducting coatings for electronic devices. Nevertheless, as raw materials for ITO are becoming increasingly expensive and more importantly due to its rigidity, the industry today faces the challenge of replacing the conventional brittle ITO with a flexible, yet robust, transparent but also conductive material. Graphene combines the aforementioned properties, therefore it is considered as the most promising candidate for substituting ITO. Graphene consists of a two-dimensional (2D) hexagonal lattice made of sp2 hybridized carbon atoms. The structure and hybridization of graphene result in its extraordinary mechanical properties and its high electrical and thermal conductivity, thanks to an extended conjugated system of π delocalized electrons. On the other hand, to date, the market of printed electronics is based on Ag inks/pastes. However, Ag can be expensive. Ag nanoparticles have poor adhesion, while other Ag forms, such as nanowires or flakes exhibit further processing difficulties and surface roughness. Printed Cu develops an insulating oxide layer, while other metals, carbon or polymers have average conductivity. Graphene has good conductivity and is emerging as a potential Ag substitute for printed electronics, to be used in wearable devices, e-paper, roll-up portable or/and transparent displays, etc.
The overall objective of NanoGraphInk was the development of fully applicable and viable conductive nano-hybrid graphene-based inks and pastes for printable applications and device fabrication. In addition, aiming at non-toxic final products, emphasis was placed in developing inks and pastes based on environmentally friendly solvents such as water, alcohols, or further non-toxic organic media. By controlling composition, rheological properties, possible compatibility issues, and other parameters, these materials can be enabled for use in diverse industrial applications. Thus, given the cost of ITO for electronics and of Ag for printed electronics, NanoGraphInk aimed at the production of low-cost substitutes suitable for easily implementable, direct industrial application.

Work performed

NanoGraphInk focused on two main methods, i.e. sonication and microfluidization, to develop stable graphene-based inks and pastes. Graphite was used as precursor, aiming at highly conductive inks by limiting non-conductive additives and stabilizers. Sonication was used to produce inks with polyhydroxylated fullerenes (fullerenols) as stabilizers, enabling over 6-month stability. These inks were used for inkjet printing and spray coating. Printed patterns were produced with sheet resistance down to ~15 Ω/sq for few hundred nm films or ~kΩ/sq for highly transparent, over 80% at 550 nm, films. Microfluidization was used to prepare pastes stable for at least 20 months (Figure 1). These were used to print highly conductive patterns by screen and flexo printing or blade coating, with sheet resistance values of as low as 2-3 Ω/sq for 20-25 μm films, at room temperature, while avoiding heat treatments. Moreover, the pastes were compatible with perovskites; this enabled their use in perovskite solar cells (Figure 2). The perovskite solar cells with graphene-based back electrodes exhibited efficiencies of at least 13%, stability for over 1000 h at room temperature with no degradation of performance, including stability at 85 °C. Thus, the main achievement of NanoGraphInk was the development of a varied-solvent series conductive graphene-based paste that can be applied on diverse materials and environments, while exhibiting high conductivity without annealing.

Final results

The advantages of the graphene paste include scalability of production, low-cost compared to Ag, ease of applicability on large areas, stability, high conductivity without annealing, environmental friendliness, and compatibility with perovskites. Their use in stable perovskite solar cells shows good efficiency and low-cost compared to Si-based solar cells. Such inks are ideally suited for exploitation. The commercialization potential of graphene-based pastes or devices can create new jobs, promote innovation and entrepreneurship and open new markets for printed electronics.

Website & more info

More info: http://users.uoi.gr/kdimos/Projects-Nanographink.html.