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Indium-Free Transparent Conductive Oxides for Glass and Plastic Substrates

Periodic Reporting for period 1 - INFINITY (Indium-Free Transparent Conductive Oxides for Glass and Plastic Substrates)

Reporting period: 2014-12-01 to 2016-05-31

The INFINITY project, “Indium-free transparent conductive oxides for glass and plastic substrates”, is focused on the development of novel inks for transparent conductive thin coatings that are used in a variety of optoelectronic devices including flat panel displays, photovoltaic cells, etc. Nowadays, 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 is to develop alternative indium-free oxide coatings with similar electrical conductivity and high optical transmission as ITO coatings, to deposit these coatings and patterns by a direct, cost-effective printing process.

Specifically, the INFINITY project will develop inks with nanoparticles of two novel chemical compositions: doped zinc oxide and doped titanium dioxide. The nanoparticles will be synthesised by an innovative sol-gel method, employing specifically formulated precursors. Then, the novel inks will be 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 will be used for low-temperature sintering of the printed conductive coatings, thereby allowing for not only glass but also plastic substrates to be used. 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 project is a 3-year research project funded by the EU Framework Programme for Research and Innovation: Horizon 2020, and it brings together European experts from all aspects of the fabrication supply chain: EpiValence, Lurederra, TWI, INM, University of Hull, Belectric OPV and FlexEnable.
The FSP (flame spray pyrolysis) process has been tuned to produce aluminium-doped zinc oxide (AZO), silica-doped zinc-oxide (ZnO:Si) and niobium-doped titanium oxide (TiO2:Nb) nanopowders at a lab scale. The AZO and ZnO:Si powders have also been produced at a pilot scale (by FSP). These nanopowders are currently being further characterised and formulated in indium-free inks to study their behaviour, optimise their dispersion and assess the conductivity of the deposited cured coating layers.

ZnO:Si and TiO2:Nb precursors have been designed and synthesised for further use by the partners in the FSP process and for the indium-free matrix development.

The indium-free matrix development based on the precursors prepared for the project is currently on-going. Films of TiO2:Nb were successfully deposited by spin coating; their characterisation is currently on-going. The effects of sintering on the conductivity of the coatings will also be studied as well as the incorporation of TiO2:Nb nanoparticles in the system.

Tin-doped indium oxide (ITO) inks were 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. The conductivity of these ITO films was optimised by reducing the coffee ring effect (observed in the inkjet process) and by defining a post-deposition and post-curing treatment.

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

A model has been developed to predict the bulk temperature of the coatings and of the substrate surface when lasers process the coating layer. The model and experimental results are starting to converge and further optimisation is currently being considered. This model enables selection of the optimum laser parameters in order to sinter the coating layer within a temperature range that does not damage the substrate. A combination of different post-deposition treatments needs to be considered in order to obtain conductive materials.

The project is therefore on-track and has already overcome some technical challenges. More results will be produced in the second half of the project.

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The INFINITY project focuses on two main indium-free TCO’s to provide a low-cost and sustainable manufacturing process to make highly conductive and transparent, conductive films on flexible plastic substrates and traditional glass substrates, which can have an additional impact of providing a sustainable solution for the future.

The TCO’s will be based on easily accessible elements rather than critical raw materials. TCO nanoparticles (produced by flame spray pyrolysis), a sol-gel matrix, a printing deposition of the coating and the low-temperature laser process make this a novel approach to obtaining TCOs in a sustainable manner, leading to reduced waste and is an overall more efficient process than the currently used magnetic sputtering process and thus will have reduced environmental impact.

The project objectives outline an innovative and ground-breaking approach to TCCs, with optical transparency >90% at 400nm and electrical resistivity <20Ω/square and a projected cost of €7/m2.

The project will deliver novel enabling technologies on a number of fronts:
• New cost effective indium-free sol-gel derived materials, with well-defined functionality (electrical, colour/transparency) which are capable of being deposited onto not only glass but also flexible polymeric substrates.
• Ink formulations that allow for a low-temperature application on glass and plastic substrates, with good adhesion and uniform thickness.
• The development of an easily printable TCC that replaces cost-intensive and wasteful vacuum-based techniques like magnetic sputtering.
• The use of lasers to provide a novel methodology for achieving conductive coatings on a wide variety of substrates without damaging the underlying substrate.
• The development of TCCs on plastic substrates, allowing for commercialisation of flexible displays and smart windows.

Printed TCC films with the above characteristics will be deposited on glass and plastic substrates with the bulk substrate temperature maintained below 150°C. This is an ambitious target since annealing of ITO layers is typically carried out in the 380-500°C range, but the use of lasers allows for a unique and innovative approach to achieving crystallisation of the conductive oxide layer without affecting the base substrate.

The use of laser curing methods provides a ground-breaking new method for providing high energy density without thermally affecting the underlying substrate. This should open up the industry to a radically new way of delivering transparent conductive coatings which does not depend on traditional physical vapour deposition (PVD) methods like magnetic sputtering. The combination of sol-gel chemistry and laser curing offers a drastically new ways for the deposition and crystallisation of TCO on glass and plastic substrates.
Gravure plate and patterned coating on a flexible substrate obtained by gravure printing.