Full color displays such as the above Kodak-Sanyo prototype can be prepared using various methods. In the first and most direct method, all three essential colors (Red, Green, Blue) are emitted separately by three different electrolumophores. Alq3 can be doped with dyes that shift the emission color (undoped emits green light). Other electrolumophores are then used for other colors. Stripes of different emitters can be deposited through shadowmasks to create pixels with red, green, and blue subpixels. This approach has been used where the pixels are relatively large, but it becomes somewhat impractical for microdisplays, where the subpixels can only be a few microns in size.
A second approach is to dope the emitter with more than one dye, so that it emits white light. Red, Green and Blue passive color filters (PF), such as the ones used in liquid crystal displays, can then be used to create the red, green, and blue subpixels. The filters can be patterned on a separate plate, using photolithography, then overlaid onto the white emitter array. This is the easiest way to make a color OLED display, although it does waste light in the filters, since from each subpixel only about one third of the white light passes through the color filter.
A better method for achieving color is the use of fluorescent, or color-changing, media in place of filters. In this case a blue emitter is used, and the blue light is converted to green or red by the color-changing media (CCM). Less light is wasted in this case relative to the use of filters, provided the CCM materials have high conversion efficiency. As with the filters, the CCM materials can be patterned by photolithography, and some materials have efficiencies of 50-80%. Color purity with CCMs is generally good, and if filters are used in conjunction with the CCMs, the color gamut can be excellent.
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