The origins of OLED technology
If you’ve ever glanced at an OLED screen you’ll know just how impressive they are. They’re famed for their thin design, high efficiency and sensational images, but they didn’t appear from nowhere — so allow us to explain how your TV screen made its way from science lab to living room.
The story kicks off in the 1950s when André Bernanose, a Frenchman from Nancy-Université with a passion for high voltages, began applying electricity to materials such as the acridine orange dye. With the compound deposited on thin films of cellophane, he noticed that high enough voltages — we’re talking thousands of volts — caused the material to emit light.
A few years later New York University researchers noticed a similar effect in a handful of other materials. They didn’t realise it at the time, but this was special. When they passed current through the materials they were experimenting with, now called organic semiconductors, electrons were finding homes in holes — regions of the material currently devoid of the charged particles — to create excitons.
As the name suggests, these little guys are particularly lively, and spit out some of their excess energy in the form of photons – individual particles of light. The more electricity is pumped through, the brighter the light emitted. Neat, but power supplies running at thousands of volts don’t feature in most homes.
Little changed until 1987, when Ching Tang and Steven Van Slyke at Kodak had the bright idea of sandwiching two layers of organic semiconductor together. With one made from material great at sucking in electrons and another great at turning them into light, they were on to a winner. “That was the first real milestone in the history of OLEDs,” explains Professor Andy Monkman from the University of Durham. “They managed to create an organic LED that worked using a few volts. Suddenly, these things became plausible for use in screens, because you could power them with a mains supply or even a battery.”
Then, in 1990, came a real game-changer. Instead of using semiconductors made of small molecules, which had to be meticulously deposited inside a vacuum, scientists at the University of Cambridge were able to achieve the same results using polymers. That may sound trivial, but polymers could be laid down using technology similar to an inkjet printer, making it easier to produce larger — and cheaper — surfaces covered in OLEDs.
Once that was possible, companies clamoured to make screens. Using thin films of transistors — essentially a matrix of tiny electronic switches — they could control a huge grid of OLEDs. Combining red, green and blue at each point created a pixel, each of which could be individually controlled to make a display. If that sounds complex, that’s because it is. “What the big screen manufacturers do is amazing,” admits Monkman. “In the lab, we can make small displays a few inches square, but the big manufacturers can now create huge displays, metres across. It’s incredibly impressive.”
The images they produce are equally as impressive, too. “Whenever anybody sees an OLED for the first time, they’re blown away,” explains Monkman. “They’re so bright and rich with colour.” You see, unlike Liquid Crystal Displays (LCDs), OLEDs don’t need a backlight because each pixel emits the light itself. The result: deeper blacks, higher contrast, and far, far better images. That’s why, along with their thin construction, OLEDs have cemented themselves as an industry favourite in TVs.
But while they’ve long looked beautiful, they’ve been power-hungry, too. “In the past, only a quarter of the excitons were converted into photons,” explains Dr Stuart Kitney from the University of Hull. “But newer phosphorescent OLEDs contain heavy metals that allow all the excitons to be converted into light.”
It was scientists from Princeton University and the University of Southern California who were able to increase the quantum efficiency — the number of photons spat out as a percentage of the electrons sent in — right the way up to 100 per cent by adding a sprinkle of heavy metal to the mix. With every spark of electricity made to count, OLEDs are now everybody’s favorite.
The story of OLEDs isn’t through yet. In the early noughties, researchers began showing off flexible OLED displays, built on bendable plastic instead of glass; now, we’re finally beginning to see TVs with curved screens, and it shouldn’t be too long before flexible devices follow in their footsteps.
Elsewhere, researchers are trying to make OLED screens transparent. That’s not easy, because it requires making the electronics so tiny that they don’t obscure the light they create. Small clear screens are already available, so it might not be long before your windows can double as TVs. Not bad for a technology that started life with a Frenchman electrocuting cellophane at his desk.
Find out more about LG’s ground-breaking curved OLED TVs here