Tuesday, March 27, 2012

Balancing OLEDs

kw: analysis, oled, cleartype

I am a strong proponent of frugality with light, particularly with color image displays. Because of the inefficiency of converting an electron beam to light using phosphors, even high-efficiency rare-earth phosphors, a color cathode ray tube (CRT) about 25 inches in size consumes 200 watts. A liquid crystal display (LCD) of similar size, which uses polarized light and polarization-rotating materials, needs about 80-100 watts, a substantial savings. But the two polarizers in the screen each block 35-50% of the light, and the color filters each pass about one-third of the light, leading to a maximum efficiency of 13% (1/3 of the square of 62%). Almost 7/8 of the light created is filtered out.

By contrast, light-emitting diode (LED) emitters produce light of a specified color (or narrow range of wavelengths), leading to a display that consumes only about an eighth of the energy of an LCD display. There are production difficulties with producing a large LED panel, however, and another big factor is the scarcity of gallium. The LED's in flashlights, for example, use gallium arsenide or a gallium compound with indium and aluminum. Thus a number of companies are working hard to develop organic emitters that use no rare metals. Synthesis of light-emitting polymers is costly at present, but once appropriate materials are discovered, the costs should drop rapidly.

The problem is device lifetime. Red is the low-energy end of the visible spectrum, and red-emitting organic LED (OLED) materials have already been produced that are expected to last between one and two million hours. Green emitters have about half that expected lifetime. Blue, being the high-energy sort of light, is much harder to produce. The best materials, driven to comparable brightness as the red or green emitters, last only about 20,000 hours, at which point their brightness has dropped by half. That is only 2½ years if the display is left on.

Red light in the 640-680 nm range has an energy just under 2 eV/photon. Green in the 520-560 nm range has less than 2.5 eV/photon. But blue in the 420-460 nm range has an energy approaching 3 eV/photon, and it is quite a bit harder to manage without breaking bonds in the molecules. Thus, it is largely a voltage problem, but heating makes it worse, so if you don't drive them so hard in the first place, they can last a lot longer.

I know some people in the field, and have suggested a modified array similar to that shown on the right: leave more space for the blue emitter, and drive it only to half brightness. The red, and to some extent the green, can be given less space and driven harder.

There are other arrangements possible. This could lead to an OLED display that lasts 100,000 hours or more overall. That is almost twelve years, continuous. There is one significant problem, however. Messing with the arrangement this way makes ClearType calculations quite a bit harder. If more than one such modified arrangement is used, things will get messy fast!

This shows how ClearType looks on some italic type on my LCD screen. Note in particular the uprights of the "h" and "r". Sending color information to certain pixels rather than simply "on", "off" or "gray", simulates turning on or off, or dimming, the subpixels, and greatly increases the readability of the text.

Though this can be solved mathematically, it isn't a clean solution. Further chemical research will, I am sure, lead to the discovery of a blue emitter that lasts long enough to make OLED TV's and computer displays a reality. Imagine a 40 inch TV that needs only ten watts!

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