Solution Processing of OLEDs
August 14, 2012, Emerging Display Technologies Conference, Santa Clara, CA—David Flattery from DuPont described the research and investigations needed to support solution processing of OLEDs. The chemicals for the displays are a work in progress.
The materials and processing for those materials is challenged by the need to quickly scale to volume manufacturing. As the materials change as the display and lighting vendors learn about the chemistries, the formulations change. The challenge is to integrate the development process to adjust for changes in material properties and the processes themselves.
The materials must meet the requirements for coating, containment, and the mixing of layers. The solutions need to be easily and completely cleaned, especially for the high-speed printing processes that are performed in normal atmospheric conditions. The solutions have to be a suspension with active small molecules. The compounds for the hole injection layers need to be at a ph between 3 and 7, and the characteristics are tuned by the ph.
The materials need to be optimized for a slot die coat process, must have medium-term stability, and also meet the environmental requirements for vapors and effluents. The chemicals have to be stable under their normal operating conditions, which is a challenge for the blue colors. The next generation materials will have better application efficiency and longer lifetimes. The expected time to a light reduction of 5 percent is now being projected to be over 2 thousand hours..
They are working to reduce the cost of manufacturing by developing new materials and application methods with the equipment suppliers and the panel manufacturers. The move to solution processing eliminates the need for a vacuum and the attendant time to pump down the processing chambers. The guiding principles for product development are to maximize the layers using solution processing, minimize the pattern layers, and make it all work with nozzle printers.
They are working to use existing standard processing equipment to the extent possible. When standard equipment in not available for a specific task, they help to develop specialty equipment. Some of the special needs for solution processing are in getting a better way to apply the metals. The current sputtering methods have issues with evaporation and need a fine metal mask.
A process that depends upon self-alignment is possible with a modified primer layer that has the ability to self mask. After the active devices are placed, the processes use a plasma etch to access the contacts. Metal is evaporated on to the substrate and the LEDs are encapsulated. The printing equipment is based on nozzle printers. With 15 heads, a large printer can process about 5 meters per second.
One of the hardest problems is luminance stitching, where illumination varies with position. The target is for less than 1 percent variation across the display. This is measured in a short range of 8×8 pixels, and current products are in the 90 percent uniformity range. They need to work on better layer uniformity and higher performance for the materials. Their metrology compares the differences between printed and empty cells. At the same time, they can check for flatness in the wells.
The barriers to TV are many. The material performance and application efficiency need to be improved to approach semiconductor-levels of uniformity and performance. The manufacturing equipment has to be scaled up to a gen 8 size for better costs and manufacturing throughput. The resolution is getting close to the dispersion of the glass, so high pixel counts have to be over 250 dpi. Unfortunately, there may not be an easy fix for higher resolutions in small form factors that can use solution printing for the electronics.
