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Indium-Free Transparent Conductive Oxides for Glass and Plastic Substrates

Periodic Reporting for period 2 - INFINITY (Indium-Free Transparent Conductive Oxides for Glass and Plastic Substrates)

Reporting period: 2016-06-01 to 2018-04-30

The INFINITY project, “Indium-free transparent conductive oxides for glass and plastic substrates”, focused on the development of novel inks for transparent conductive coatings (TCCs) that are used in a variety of optoelectronic devices including flat panel displays, photovoltaic cells, etc. Currently, indium tin oxide (ITO) is the most commonly used material for these applications; however indium is a scarce and expensive element. Therefore the main objective of the INFINITY project was to develop alternative indium-free oxide coatings with similar electrical conductivity and high optical transmission as ITO coatings, to deposit these coatings by a direct, cost-effective printing process.
Specifically, the INFINITY project has developed UV curable inks containing doped metal oxide (zinc and titanium oxides) nanoparticles. These novel inks have been modified and adapted to printing techniques such as gravure and ink-jet printing to enable direct writing of multi-layers and patterns, avoiding the waste associated with existing etch patterning processes. A novel laser-based approach was used for low-temperature sintering of the printed conductive coatings, thereby allowing for not only glass but also flexible substrates to be used.
The project has progressed TCC technology by allowing for ink formulations that facilitate the application of sol-gel based coatings at low temperatures, providing a conduit for the application of conductive coatings onto plastic without causing damage. This will allow for a wider variety of end-user applications, ranging from displays to solar panels to organic photovoltaics and energy harvesting by smart windows.

INFINITY was a 3-year research project funded by the EU Framework Programme for Research and Innovation: Horizon 2020, bringing together European experts from all aspects of the fabrication supply chain: EpiValence, Lurederra, TWI, INM, University of Hull, Belectric OPV and FlexEnable.  
Tin-doped indium oxide (ITO) inks have been prepared from commercially available ITO nanoparticles and a UV-curable binder. These inks were deposited by spin coating and printed by gravure and inkjet printing to be used as a benchmark for the products developed in the INFINITY project.

Screening of surface modifiers, solvent mixtures, binders and mixing methodologies has been carried out to establish the film forming matric for formulation that will ensure stability, homogeneity and overall electrical conductivity. An electrically conductive matrix has been sought but has proven elusive. A UV curable passive matrix has been adopted and enhanced.
Novel alkoxide prescursors have been produced and validated for cation-doped metal oxide systems and have been assessed suitable for scaled manufacture. The solution stability, thermal properties and batch-to-batch reproducibility have been established for Nb-doped TiO2 and Si-doped ZnO.
Flame spray pyrolysis (FSP) methods have been developed and optimised to transform the precursors into nanopowders. These nanopowders have been characterised and evaluated. Novel conductivity measurement techniques and materials processing methods have had to be developed within the project in order to enhance the inherent conductivity of the nanopowders. Comparison with ITO powders show that resistance of the indium-free materials is still much higher but considerable progress has been made to improve the conductivity.

ZnO:Al (AZO), ZnO:Si (SZO) and TiO2:Nb (NTO) nanoparticles have been designed and synthesised by FSP. Thermal treatment of the powders before the fabrication of the ink gave improved conductivity.

AZO and ZnO:Si nanoparticles were incorporated in the UV-curable binder and formulated as inks. These inks were spin coated on glass and flexible substrates and were printed by inkjet printing and gravure printing and the properties of the obtained cured films were investigated, including their adherence and the transparency among others.

Pulsed (picosecond) white light laser irradiation of 1.75 m has been used to sinter the deposited coatings. This irradiation couples with the conductive filler but it transmitted through PET substrates and so a sheet resistance of 80Ω/sq can be achieved without damaging the PET.
Silver (Ag) grids in combination with the indium-free TCO coatings were investigated as a way of improving coatings conductivity. The lowest resistivity of 0.005 Ωcm with a .transmission of 70.6% was obtained with a ZnO:Si coating. This resistivity is only a factor of 12 higher than that obtained with the combination Ag grid/ In2O3:Sn coating although the resistivity of the ZnO:Si coating without Ag grid is about a factor 1000 to 10000 higher than that of In2O3:Sn (ITO) coatings.

In summary, the key achievements made in the project are:

• The development of printable zinc oxide inks with a passive (UV curable) binder.
• Printing of UV curable indium-free inks onto flexible substrates.
• Establishment of laser processing method to achieve preliminary demonstrator specification.
• Establishment of transparent titania based coatings.

The project has a website that has a public access area, which includes an infographic, links to peer review papers, conference presentations and posters. Dissemination activities have been carried out through out the course of the project, such as attendance at the TCO2017 conference in Leipzig and the continuous updating of the INFINITY website. In addition, during the last year there has also been a poster presentation at Heriot Watt University, an oral presentation at the TCO2017 conference and a section in a ‘Focus on Research’ article for the Association of Industrial Laser Users magazine.
The INFINITY project focused on the simultaneous developments of printable TCCs, indium free coating compositions and novel laser processing to provide a low-cost and sustainable manufacturing process.
The approach adopted uses oxide nanoparticles incorporated into UV curable coating formulations. Printing of these coatings and low-temperature laser processing make this a novel approach to obtaining TCCs in a sustainable manner.
The project objectives outline an innovative approach to TCCs, with optical transparency >90% at 400nm and electrical resistivity <20Ω/square and a projected cost of €7/m2.
The specific achievements of the project are:
• Printable coatings with an ITO fillers have been developed with optical transparency of >93% and sheet resistance of ~200Ω/square.
• Printable coatings with indium-free AZO filler have been developed with optical transparency of >92% and sheet resistance of ~600kΩ/square.
• A printed silver grid system that reduced the sheet resistance of the printed ITO based coating to <1Ω/square and the indium-free coating to <2Ω/square. The use of the silver grid reduces transparency to 65-70%
• Material costs of the indium-free coating have been project at ~€3-12/m2 after scale-up.
The project has delivered novel enabling technologies on a number of fronts:
• TCCs capable of selective deposition using printing to reduce or eliminate indium use that is an alternative to the industrial standard of cost-intensive and wasteful vacuum-based techniques.
• Ink formulations that allow for a low-temperature application on glass and plastic substrates, with good adhesion and uniform thickness.
• The use of lasers to provide a novel methodology for achieving conductive coatings without damaging sensitive substrates.