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.
But displays made with OLED (organic light-emitting
diode) technology are even better. 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—even with
curved screens. Photo of a curved, Samsung UHD OLED TV by courtesy of Kārlis Dambrāns
published on Flickr
under a Creative Commons (CC BY 2.0) Licence.
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.
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.
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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.
Artwork: 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?
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).
Artwork: The arrangement of layers in a simple OLED.
How an OLED emits light
How does this sandwich of layers make light?
To make an OLED light up, we simply attach a voltage (potential difference) across the anode and cathode.
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).
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).
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.
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 NREL
(image id #6322357).
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
Photo: TVs, computer monitors, and mobile devices (laptops and tablets) are gradually becoming thinner thanks to OLED technology. Photo courtesy of LG Electronics published on Flickr 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?
Photo: TVs and phones are still the most familiar application of OLEDs—but expect many more
things to follow as prices become increasingly competitive with older technologies such as LCD.
Photo of a curved LG OLED TV by courtesy of Kārlis Dambrāns
published on Flickr
under a Creative Commons (CC BY 2.0) Licence.
OLED technology is still relatively new compared to similar, long-established
technologies such as LEDs and LCDs (both of which were invented in 1962).
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.
Despite the hype, consumers were originally less enthusiastic about mobiles and TVs with OLED screens, largely because LCDs were much cheaper and a tried and trusted technology.
That's no longer true, certainly not of TVs: prices of OLED kit have fallen dramatically, with some OLED TVs on sale in 2020/2021 going for about half the price that they were just a year or two earlier and predictions that the technology would become truly affordable by 2023.
Where phones are concerned, the advantages of OLEDs—(arguably) better display quality, improved battery life, lighter weight, and thinness/flexibility—often
outweigh any simple cost difference. In a telling 2020 analysis, Ross Young of Display Supply Chain Consultants noted a steady shift from LCD as
Asian manufacturers switch production to OLEDs and new technologies such as 5G wireless
become increasingly important. Young forecasts that OLEDs will account for just over a half
(54.5 percent) of the smartphone display market by 2025, compared to just under a quarter
(23.9 percent) in 2016.
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 1955. He and his
colleagues applied high-voltage AC (alternating current) electric fields to thin films of cellulose and cellophane "doped" with acridine orange (a fluorescent, organic dye) and carbazole. 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.
Milestones in the development of OLEDs since then have included the first commercial OLED (Pioneer, 1997), the first full-sized OLED display (Sony, 2001), the first OLED mobile phone display (Samsung, 2007), commercial OLED lighting systems (Lumiotec, 2013), and large-screen commercial OLED TVs (by Samsung, LG, Panasonic, Sony, and others in 2012 and 2013).
[1]
In 2020, Chinese manufacturer TCL announced it would invest almost $7 billion in a new method of making OLEDs using a
technology similar to inkjet printing, with the promise of producing cheaper OLED products by 2023.
OLED TVs could plummet in price by 2023 by Henry St Leger, Tech Radar, April 16, 2021. TCL's inkjet technology could finally make OLED more affordable.
by Charles Q. Choi, IEEE Spectrum, 23 October 2020. A look at new HD OLED displays developed by researchers from Samsung and Stanford University.
OLED TV Arrives by Tekla S. Perry. IEEE Spectrum, January 2013. OLED TVs are going to be "big, bright, fast, and thin", but more expensive than conventional LCDs.
Audi Visions, OLED Technology Revealed: TechnologicVehicles, March 25, 2012. YouTube video showing how Audi is experimenting with OLEDs as a replacement for conventional car lights and displays.
Natural light 'to reinvent bulbs': BBC News, 13 April 2006. How OLEDs are set to replace conventional lamps, producing light closer to daylight and saving energy.
Technical papers
Organic electroluminescent diodes by C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. 51, 913 (1987). Announcing the development of a practical and efficient OLED.
Conductive Polymers by Alan Heeger, Alan MacDiarmid, Hideki Shirakawa. An advanced-level summary of the work that won the Nobel Prize in Chemistry 2000.
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