ITO-free lighting panel
20 April 2010
CDT has announced production of an ITO-free P-OLED lighting device using a fine copper mesh.
This work was enabled by CDT’s (Cambridge Display Technology) work with Conductive Inkjet Technology and their joint ‘NOMAD’ project funded by the Government-backed Technology Strategy Board.
Project NOMAD started in 2007 with the aim of developing technology for the next generation of low-cost OLED devices by combining advanced manufacturing methods with state-of-the-art polymer OLED materials (P-OLEDs) to enable significant reductions in manufacturing costs for devices such as P-OLED lighting panels.
This demonstration has shown the potential for patterned metal tracking using electroless metal deposition as a replacement for Indium Tin Oxide (ITO) and traditional sputtered tracking. ITO is widely used as a transparent conductor in the displays, lighting and photovoltaics industries, but is in short supply and expensive. Furthermore, ITO is very brittle and can crack during processing, limiting its attractiveness for the next generation of flexible electronic devices.
ITO has a high resistivity, which creates problems for large area lighting panels due to the large voltage drops encountered towards the centre of the device, giving rise to an unacceptable drop-off in light intensity.
The manufacturing method demonstrated in the NOMAD project reduces costs by eliminating ITO along with significantly reducing the capital and processing costs for patterning metal bus bars which distribute current and ensure the uniformity of light emission. CIT’s process eliminates the need for traditional vacuum sputtering equipment and etching tanks.
CIT’s photoimageable materials have been used to produce sub 10micron copper tracks on glass substrates, resulting in a highly transparent, highly conductive surface without the voltage drops of ITO based technologies. By applying a conductive polymer to these grids, a true ITO replacement has been demonstrated and this has allowed CDT to process its layers of white emitting P-OLED materials. This development is extremely encouraging and when device optimisation has been completed, it is expected to lead to a significant increase in device efficiency.
The project is being part-funded by the UK Government’s Technology Strategy Board. Their Lead Technologist in Electronics, Photonics and Electrical Systems, Mike Biddle, said: “We are extremely happy with the progress made in this project, and to see that this new approach may soon be ready for commercial exploitation in OLED lighting. This is yet another great example of world class businesses coming together in the UK to develop innovative technology with global market potential.”
The term 'nanotechnology' is widely used today, and P-OLED technology can certainly be thought of as an example. The total thickness of all layers in a P-OLED display device can be less than 500nm, so that in effect, the thickness of a display is similar to the thickness of the substrates (usually glass) that form the top and bottom of the device.
The structure of a basic P-OLED display device can be extremely simple, consisting of a sandwich containing:
• A transparent conducting electrode with a large work function (Anode). Indium tin oxide (ITO) is commonly used, coated on a substrate
• A conducting polymer layer which transports and injects holes into the active layers (hole injection / transport layer)
• A thin organic interlayer material sometimes referred to ‘primer layer’ developed by CDT to improve efficiency and lifetime
• A thin light emitting polymer (LEP) layer less than 100nm thick (emissive layer)
• A metallic electrode with a low work function, such as a barium/aluminium bi-layer (cathode)
Compared to competing technologies such as LCDs, the structure of a P-OLED device is extremely simple. The ability to dissolve the active materials (hole injection/transport layer, interlayer/primer layer and Light Emitting Polymer) in a solvent to form an ‘ink’ and deposit by a range of printing techniques on a wide variety of substrates at low temperatures provides a number of manufacturing advantages over small molecule OLED technology. The simplicity and solution processability of P-OLED materials together make P-OLEDs an exciting prospect for future display applications from small mobile displays though to large screen TVs and large area panels for lighting.
In operation, voltage is applied across the contacts, creating an electric field and injecting charges into the polymer where they recombine and emit light. P-OLEDs offer the ultra-fast switching speeds typical of LEDs (and around a thousand times faster than LCDs).
Since the first devices were fabricated, very rapid progress has been made in improving the quantum efficiencies of P-OLED devices. Initially, internal quantum efficiencies of only 0.01% were achieved (defined as the number of photons generated in the polymer film relative to the number of carriers injected into the polymer). Now, figures three times that are possible.
These improvements have been developed through a combination of new materials development, device engineering and process optimisation, and CDT has extensive activities in all these areas. The Interlayer/Primer Layer technology introduced by CDT enabled substantial improvements in efficiency and stability to be achieved. The thin solution processed layer, inserted between the LEP and Hole Injection/Transport Layer assists with balancing hole and injection and transport, improving the efficiency of radiative recombination that leads to light emission and controlling where the light is emitted in the device.
Cambridge Display Technology (CDT), is a Sumitomo Chemical Group Company with headquarters near Cambridge. Conductive Inkjet Technology (CIT), is a wholly-owned subsidiary of Carclo plc, and has developed a revolutionary family of catalytic materials and associated equipment to address the needs of the rapidly emerging printed electronics market. CIT's catalytic inks can be used with inkjet and flexographic printers, and a variation of that ink can be photoimaged to support the higher resolution requirements of touch screens and displays. CIT has focussed on roll to roll manufacturing methods and has developed a full suite of equipment to support processing across a variety of markets.
Jim Veninger, General Manager at CDT commented: “I am impressed by the quality of the fine mesh generated by the CIT process and the resulting emission uniformity of our lighting devices without the need for ITO. While further development is required, I can see CIT’s technology supporting low-cost processes for OLED lighting in the near future.”
Chris Malley, CEO at CIT stated: “It is extremely encouraging to see functioning device being manufactured that, not only offer the potential for high volume solution processable manufacture, but also offer lower cost and higher performance.”
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