Economics is a key factor for application of organic light-emitting diodes (OLEDs) in general lighting relative to OLED flat panel displays that can handle high-cost materials such as indium tin oxide (ITO) or indium zinc oxide (IZO) as the transparent conducting oxide (TCO) on display glass. For OLED lighting to penetrate into general illumination, economics and sustainable materials are critical. The issues with ITO are exacerbated by export controls from China, one of the major sources of elemental indium. There fore, ITO is not sustainable because of fluctuating costs and the United States' dependency on other nations such as China.
Numerous alternatives to ITO/IZO are being evaluated, such as silver (Ag) nanoparticles/ nanowires, carbon nanotubes, graphene, and other metal oxides. Of these other metal oxides, doped zinc oxide (ZnO) has attracted much attention over the last ten years. The volume of zinc mined is a factor of 80,000 greater than indium and the U.S. has significant volumes of zinc mined domestically. The costs of elemental zinc is also over two orders of magnitude less than indium, reflecting the relative abundance and availability of the elements.
The genesis of this project was to determine if doped zinc oxide technology can be taken from the commodity-based window market and translate the technology to OLED lighting. This project had a clear focus on economics and the work plan focused both on a doped ZnO process and OLED device structure that would be consistent with a new TCO. Six-inch OLEDs were successfully made with a serial construction. More process development is required to optimize commercial OLED structures. Feasibility was demonstrated on two different light extraction technologies: 1⁄4 lambda refractive index matching and high-low-high band pass filter. Process development was also completed on the key precursors for the TCO, which are ready for pilot-plant scale-up.
The overall outcome of the project was the demonstration that doped zinc oxide can be used for OLED devices without a drop-off in performance, while gaining the economic and sustainable benefits of a more readily available TCO. The broad impact of this project is the facilitation of OLED lighting market penetration into general illumination, resulting in significant energy savings, decreased greenhouse emissions, and no environmental impact issues such as mercury found in fluorescent technology.
This work was done by Gary S. Silverman, Martin Bluhm, James Coffey, Roman Korotkov, Craig Polsz, Alexandre Salemi, Robert Smith, Ryan Smith, Jeff Stricker, Chen Xu, Jasmine Shirazi, George Papakonstantopulous, and Steve Carson of Arkema Inc., with colleagues from Philips Lighting GmbH, Pacific Northwest National Laboratories, and National Renewable Research Laboratories.