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LEDs and OLEDs
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What we have here is light emitting diodes, LEDs and what they are is a semiconductor, the diode only conducts electricity in one direction
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and it stops it going the other way, if you feed it with alternating current it will change it into direct current
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and if you take a diode and ad various chemical compounds to it, it will emit light
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This is an LED demo board, typically they make white light with LEDs
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you have a very efficient blue LED with a yellow phosphor coated device
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you end up with a nice warm white emission, or
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you have white emitting LEDs which are a combination of RGB, red green blue emitters inside the LED package.
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It's another way to make white LEDs
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Ok, what we are looking at some samples of OLED panels, these are organic light emitting diodes
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The difference between an LED and an OLED is that with an LED you have to use 3 5 substrates.
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because of the fact that, when you use something like silicon which is a fourth group element
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you have an indirect band gap, so that indirect band gap means that the electron and the hole
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the hole being the absence of an electron, when they come together to form the photon
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they have to travel in an x-y direction so you are traveling off of
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a y direction to get to the electron or hole, and as a result you get the release of heat as well as light
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you get more heat than you do light because of that phenomena, you don't have as much light because
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you have a lot of other phenomena going on, so that is why they use a lot of 3-5 materials like
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gallium arsenide to get that light output, and you are limited to those materials to get that effect
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so when you use OLEDs the difference is you can use any substrate
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and you use organic materials, which is where the light emission process actually occurs
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and as a result it allows for a lot more flexibility in terms of the materials and devices you can use.
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typically we form an indium tin oxide layer, which is a transparent conductive material
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very commonly used in optoelectronic devices
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that is sputtered onto the glass substrate, and on top of that ITO film we form a stack of very thin organic material
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coatings, anywhere from about 5 nanometers thick to about 200 nanometers
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depending on your device construction, there can be anywhere from 5-10 coatings that we stack on top of each other
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to make a white device typically we make 3 emission layers, red green and blue, and the combined emission from those layers forms white light
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through the emission layers there is typically an electron transport layer, and the cathode acts to inject electrons into the device
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it's also a reflective material which is very important, if its not a reflective metal a lot of the photos will be absorbed at that interface
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in the off state it is a mirror, what you are seeing is a reflection off of the aluminum cathode
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the OLED materials are very efficient, and they can efficiently generate photons
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we can create an OLED device which is 30-40 lumens per watt without using any extraction technology
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but the potential is there to get well over 100 lumens per watt if we solve some of the problems of light extraction.