Re: What\'s the latest on the higher powered LEDs?
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by hotfoot:
Now bringing this topic to our next great hope - Organic LEDs!
White is supposed to be intrinsically possible with OLEDs, so the efficiencies are far improved over current phosphor-doped white LEDs.
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The basic principle of polymers that emit light (OLEDS) can be explained as follows. A thin-film semiconducting polymer is sandwiched between two electrodes. Electrons and holes are injected from the electrodes and recombination of these charge carriers leads to luminescence. The bandgap, the energy difference between valence band and conduction band of the semiconducting polymer, determines the wavelength (color) of the emitted light. Sooo, the physics of operation is similar to "normal" monochromatic LEDS, but OLEDS are simpler and cheaper to make. Thus they can be made large. One can't economically make an 8x10 inch GaN LED!!! They probably can make a white OLED, but it would involve phosphors just like a white LED, or mixing red, green, and blue OLEDS in a sigle device. A state of the art OLED has two different polymer layers (one for injecting electrons and one for injecting holes). The light is produced where these electrons and holes recombine (sort of annihilate each other). The energy of the recombination determines the color of the light - which is pretty closely monochromatic. They can cause the recombination to take place in or near a phosphor layer to make a different light color.
As far as efficiencies, efficacies approaching 10 lm/W have been achieved for some laboratory OLEDS in certain colors. Nichia whites are 15-20 lm/W and you can get some green LEDs as high as 50+ lm/W. So OLEDs have a way to go for efficiency.
The big stumbling block for OLEDs is limited lifetime. Right now they are not as good as standard LEDs, but I'm sure they are working on it. I heard they are up to 30,000 hours lifetime in some LAB OLEDs.
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Craig also mentioned something about newer formulation LEDS (the normal ones we're used to, that is) being able to natively generate white light. Any other info here, like their output ratings and comparative efficiencies against other monochromatic LEDs?
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I defer to Craig if he has new info on this, but the mechanism used to generate light in normal LEDs would would not support producing white light from a single semiconductor chip with no phosphor coating. I've heard of white light LASERS, but these are large hot powerful and expensive gas lasers. It would be possible to combine several monochromatic LEDs in one case to make a white LED, but this is frought with problems. The different colors age and respond to heat differently. These effects would have to be compensated for in the device to assure that the "color" of the white light remained "white" from hour 1 to hour 100,000, from freezing temps to hot temps, and from low current to high current.