Articles
Frank Nüesch, head of laboratory, Swiss Federal Laboratories for materials science and technology (EMPA), Dübendorf
In November 2012, we started a project aiming to develop technologies that would dramatically reduce the production costs of organic electronic devices such as solar cells and LED lighting panels.
The TREASORES (Transparent Electrodes for Large Area Large Scale Production of Organic Optoelectronic Devices) project, which was funded with €9 million from the European Commission and €6 million from partners in the consortium involved, has developed and scaled up production processes for several new transparent electrode and barrier materials for use in the next generation of flexible optoelectronics. Three of these electrodes-on-flexible substrates that use carbon nanotubes, metal fibres or thin silver are already being produced commercially, or expected to be this year.
The new electrodes have been tested with several types of optoelectronic devices using rolls of more than 100m in length, and found to be especially suitable for next-generation light sources and solar cells. The roll of organic light-emitting diode (OLED) light sources with the project logo was made using roll-to-roll techniques at the Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology (Fraunhofer FEP) on a thin silver electrode developed within the project by Rowo Coating.
The advantage of roll-to-roll fabrication is a high throughput. Therefore, the cost of depreciation for the production machine is reduced accordingly. The use of lighting technology already has a long history with organic optoelectronics. OLED displays occupy about 10% of the market and will certainly progress. It is important to protect these devices from water vapour and from oxygen – a requirement that is being solved by using glass sheets. For roll-to-roll fabrication, flexible barrier layers have to be developed that are compatible with the production process.
The most important development concerns flexible electrode substrates that do not use scarce elements such as indium, and that can be produced in a continuous process on the kilometre scale. During the project, electrode roughness was minimised, making it possible to deposit optoelectronic devices such as LEDs and organic photovoltaic cells. Rolls of such electrode materials will become available on a large enough scale for partners that would like to implement this technology industrially. In the course of the project, such electrodes were also produced directly on barrier substrates, providing a combined electrode-barrier substrate – this saves using an additional polyethylene terephthalate substrate, since both barrier and electrode are combined, making the devices cheaper and more flexible.
Such processing techniques promise to make light sources and solar cells much cheaper in future, but require flexible and transparent electrodes and water-impermeable barriers – which we have also developed during the project.
The electrodes from the project are technically at least as good as those now being used – made from indium tin oxide – but will be cheaper to manufacture and do not rely on the import of indium. Because of the new electrodes, the OLED light source was homogeneous over a large area, achieving an efficiency of 25 lumens per watt – as good as the much slower sheet-to-sheet production process for equivalent devices. In the course of the project, new test methods were developed by the National Physical Laboratory in the UK to make sure that the electrodes would still work after being repeatedly bent – a test that may become a standard in the field.
One key challenge was to preserve the necessary flatness of the electrodes, even when handling and shipping samples. Even a few dust particles are detrimental to device efficiency and lifetime, so foils had to be sealed during transport. Transport was often necessary during the project, since the competences of the consortium partners were complementary. Rolls could be shipped more safely because a roll is self-protecting.
The goal was to produce materials that are roll-to-roll compatible – from barrier substrates, electrodes, active materials to the lamination of the final encapsulation form. Roll-to-roll manufacturing is the cheapest large-scale method to date.
We expect that the technologies developed in the project will be adopted almost immediately for manufacture by our partners. Several patents have been filed, and some issued, that involve these technologies.