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Ultra-thin TV set made from OLED panel.

OLEDs (Organic LEDs) and LEPs (light-emitting polymers)

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by Chris Woodford. Last updated: November 1, 2017.

Do you remember old-style TVs powered by cathode-ray tubes (CRTs)? The biggest ones were about 30–60cm (1–2ft) deep and almost too heavy to lift by yourself. If you think that's bad, you should have seen what TVs were like in the 1940s. The CRTs inside were so long that they had to stand upright firing their picture toward the ceiling, with a little mirror at the top to bend it sideways into the room. Watching TV in those days was a bit like staring down the periscope of a submarine! Thank goodness for progress. Now most of us have computers and TVs with LCD screens, which are thin enough to mount on a wall, and displays light enough to build into portable gadgets like cellphones. If you think that's good, wait till you see the next generation of displays made using OLED (organic light-emitting diode) technology. They're super-light, almost paper-thin, theoretically flexible enough to print onto clothing, and they produce a brighter and more colorful picture. What are they and how do they work? Let's take a closer look!

Photo: OLED technology promises thinner, brighter, more colorful TV sets. Photo courtesy of LG Electronics published on Flickr in 2009 under a Creative Commons Licence.

What is an LED?

Bank of LEDs on an instrument panel

Photo: LEDs on an electronic instrument panel. They make light by the controlled movement of electrons, not by heating up a wire filament. That's why LEDs use much less energy than conventional lamps.

LEDs (light-emitting diodes) are the tiny, colored, indicator lights you see on electronic instrument panels. They're much smaller, more energy-efficient, and more reliable than old-style incandescent lamps. Instead of making light by heating a wire filament till it glows white hot (which is how a normal lamp works), they give off light when electrons zap through the specially treated ("doped") solid materials from which they're made.

An OLED is simply an LED where the light is produced ("emitted") by organic molecules. When people talk about organic things these days, they're usually referring to food and clothing produced in an environmentally friendly way without the use of pesticides. But when it comes to the chemistry of how molecules are made, the word has a completely different meaning. Organic molecules are simply ones based around lines or rings of carbon atoms, including such common things as sugar, gasoline, alcohol, wood, and plastics.

How does an ordinary LED work?

Before you can understand an OLED, it helps if you understand how a conventional LED works—so here's a quick recap. Take two slabs of semiconductor material (something like silicon or germanium), one slightly rich in electrons (called n-type) and one slightly poor in electrons (if you prefer, that's the same as saying it's rich in "holes" where electrons should be, which is called p-type). Join the n-type and p-type slabs together and, where they meet, you get a kind of neutral, no-man's land forming at the junction where surplus electrons and holes cross over and cancel one another out. Now connect electrical contacts to the two slabs and switch on the power. If you wire the contacts one way, electrons flow across the junction from the rich side to the poor, while holes flow the other way, and a current flows across the junction and through your circuit. Wire the contacts the other way and the electrons and holes won't cross over; no current flows at all. What you've made here is called a junction diode: an electronic one-way-street that allows current to flow in one direction only. We explain all this more clearly and in much more detail in our main article on diodes.

pn junction diode in forward-bias mode

Photo: A junction diode allows current to flow when electrons (black dots) and holes (white dots) move across the boundary between n-type (red) and p-type (blue) semiconductor material.

An LED is a junction diode with an added feature: it makes light. Every time electrons cross the junction, they nip into holes on the other side, release surplus energy, and give off a quick flash of light. All those flashes produce the dull, continuous glow for which LEDs are famous.

How does an OLED work?

Layers in a typical OLED

Artwork: The arrangement of layers in a simple OLED.

OLEDs work in a similar way to conventional diodes and LEDs, but instead of using layers of n-type and p-type semiconductors, they use organic molecules to produce their electrons and holes. A simple OLED is made up of six different layers. On the top and bottom there are layers of protective glass or plastic. The top layer is called the seal and the bottom layer the substrate. In between those layers, there's a negative terminal (sometimes called the cathode) and a positive terminal (called the anode). Finally, in between the anode and cathode are two layers made from organic molecules called the emissive layer (where the light is produced, which is next to the cathode) and the conductive layer (next to the anode).

How an OLED emits light

How does this sandwich of layers make light?

A simple animation showing how an OLED organic light emitting diode emits light.

  1. To make an OLED light up, we simply attach a voltage (potential difference) across the anode and cathode.
  2. As the electricity starts to flow, the cathode receives electrons from the power source and the anode loses them (or it "receives holes," if you prefer to look at it that way).
  3. Now we have a situation where the added electrons are making the emissive layer negatively charged (similar to the n-type layer in a junction diode), while the conductive layer is becoming positively charged (similar to p-type material).
  4. Positive holes are much more mobile than negative electrons so they jump across the boundary from the conductive layer to the emissive layer. When a hole (a lack of electron) meets an electron, the two things cancel out and release a brief burst of energy in the form of a particle of light—a photon, in other words. This process is called recombination, and because it's happening many times a second the OLED produces continuous light for as long as the current keeps flowing.

We can make an OLED produce colored light by adding a colored filter into our plastic sandwich just beneath the glass or plastic top or bottom layer. If we put thousands of red, green, and blue OLEDs next to one another and switch them on and off independently, they work like the pixels in a conventional LCD screen, so we can produce complex, hi-resolution colored pictures.

Types of OLEDs

There are two different types of OLED. Traditional OLEDs use small organic molecules deposited on glass to produce light. The other type of OLED uses large plastic molecules called polymers. Those OLEDs are called light-emitting polymers (LEPs) or, sometimes, polymer LEDs (PLEDs). Since they're printed onto plastic (often using a modified, high-precision version of an inkjet printer) rather than on glass, they are thinner and more flexible.

Polymer solar cells

Photo: In OLEDs, thin polymers turn electricity into light. Polymers can also work in the opposite way to convert light into electricity, as in polymer solar cells like these. Photo by Jack Dempsey courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

OLED displays can be built in various different ways. In some designs, light is designed to emerge from the glass seal at the top; others send their light through the substrate at the bottom. Large displays also differ in the way pixels are built up from individual OLED elements. In some, the red, green, and blue pixels are arranged side by side; in others, the pixels are stacked on top of one another so you get more pixels packed into each square centimeter/inch of display and higher resolution (though the display is correspondingly thicker).

Advantages and disadvantages of OLEDs

Ultra-thin TV set made from OLED panel.

Photo: TVs and computer monitors will be much thinner once OLED technology becomes widespread. Photo courtesy of LG Electronics published on Flickr in 2009 under a Creative Commons Licence.

OLEDs are superior to LCDs in many ways. Their biggest advantage is that they're much thinner (around 0.2–0.3mm or about 8 thousandths of an inch, compared to LCDs, which are typically at least 10 times thicker) and consequently lighter and much more flexible. They're brighter and need no backlight, so they consume much less energy than LCDs (that translates into longer battery life in portable devices such as cellphones and MP3 players). Where LCDs are relatively slow to refresh (often a problem when it comes to fast-moving pictures such as sports on TV or computer games), OLEDs respond up to 200 times faster. They produce truer colors (and a true black) through a much bigger viewing angle (unlike LCDs, where the colors darken and disappear if you look to one side). Being much simpler, OLEDs should eventually be cheaper to make than LCDs (though being newer and less well-adopted, the technology is currently much more expensive).

As for drawbacks, one widely cited problem is that OLED displays don't last as long: degradation of the organic molecules meant that early versions of OLEDs tended to wear out around four times faster than conventional LCDs or LED displays. Manufacturers have been working hard to address this and it's much less of a problem than it used to be. Another difficulty is that organic molecules in OLEDs are very sensitive to water. Though that shouldn't be a problem for domestic products such as TV sets and home computers, it might present more of a challenge in portable products such as cellphones.

What are OLEDs used for?

OLED technology is still relatively new and little used compared to similar, long-established technologies such as LCD. Broadly speaking, you can use OLED displays wherever you can use LCDs, in such things as TV and computer screens and MP3 and cellphone displays. Their thinness, greater brightness, and better color reproduction suggests they'll find many other exciting applications in future. They might be used to make inexpensive, animated billboards, for example. Or super-thin pages for electronic books and magazines. How about paintings on your wall you can update from your computer? Tablet computers with folding displays that neatly transform into pocket-sized smartphones? Or even clothes with constantly changing colors and patterns wired to visualizer software running from your iPod!

Samsung started using OLED technology in its TVs back in 2013, and in its Galaxy smartphones the following year. Apple, originally dominant in the smartphone market, has lagged badly behind in OLED technology until quite recently. In 2015, after months of rumors, the hotly anticipated Apple Watch was released with an OLED display. Since it was bonded to high-strength glass, Apple was presumably less interested in the fact that OLEDs are flexible than that they're thinner (allowing room for other components) and consume less power than LCDs, offering significantly longer battery life. In 2017, the iPhone X became the first Apple smartphone with an OLED display.

Who invented OLEDs?

Organic semiconductors were discovered in the mid-1970s by Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa, who shared the Nobel Prize in Chemistry in 2000 for their work. The first efficient OLED—described as "a novel electroluminescent device... constructed using organic materials as the emitting elements"—was developed by Ching Tang and Steven VanSlyke, then working in the research labs at Eastman Kodak, in 1987. Their work, though novel, built on earlier research into electroluminescence, which was first reported in organic molecules by a French physicist named André Bernanose in the 1950s. By 1970, Digby Williams and Martin Schadt had managed to create what they called "a simple organic electroluminescent diode" using anthracene, but it wasn't until Tang and VanSlyke's work, in the 1980s, that OLED technology became truly practical.

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Text copyright © Chris Woodford 2009, 2017. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2009/2017) OLEDs (Organic LEDs) and LEPS (light-emitting polymers). Retrieved from http://www.explainthatstuff.com/how-oleds-and-leps-work.html. [Accessed (Insert date here)]

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