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Illuminating the Science Behind OLED Televisions


LCD and LED televisions are based on the same tech: A backlight is filtered and modulated using LCDs as molecular shutters. As the name suggests, in LED televisions the backlighting is provided by LED arrays rather than fluorescent lights. It's a really small step in terms of innovation.

OLED panels break entirely from this approach. You won't find liquid crystals in them at all, nor will you find a backlight. At its philosophical core, OLED technology is much closer akin to plasma technology, since both feature millions of pixels that actually generate their own light.

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Partners in philosophy: Both plasma (left) and OLED TVs (right) feature millions of light emitting pixels.

In the case of OLED displays, these light-generating pixels are made from organic molecules—they're the “O” in “OLED”. In chemistry terms, “organic” simply refers to molecules that contain carbon, which are different from the silicon-based light emitters found in run-of-the-mill LEDs. These molecular compounds are used because they naturally emit light when a current is passed through them, via mechanisms referred to as either fluorescence or phosphorescence.

(Left) Modern light bulbs take advantage the fluorescence of doped gases to produce light. (Right) Phosphorescent materials can store energy and release it over time as light.

Fluorescence and phosphorescence are very similar processes: Both involve exciting an electron in the molecule, kicking it into a higher-energy molecular orbital, and then relaxing back to the ground state, which results in the emission of a photon of light.

Phosphorescence's "triplet" state translates into higher energy efficiency.

The only difference between the two light-emitting processes is that in fluorescence the spin of the excited electron remains the same, while in phosphorescence the spin flips to access a “triplet” state. This results in slightly longer-lived excited state, and, at least in these molecules, a much higher quantum yield. That translates into higher energy efficiency.

The first generation of OLED displays relied on fluorescent compounds to generate all three colors of light. As a result, they left something to be desired in terms of maximum brightness and energy efficiency. Since then, the red and green compounds have been replaced by phosphorescent molecules, which has dramatically increased the display performance of late-generation mobile devices.

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Blue and purple plants and animals are rare, in part due to the inherent instability of blue organic dyes.

Phosphorescent blue compounds have been the last stumbling block for fully phosphorescent OLED displays. Making blue phosphorescent molecules is easy enough; the problem here is stability. To paraphrase comedian George Carlin: “Where's all the blue food?” Nature is suspiciously devoid of blue stuff, and as it turns out, it’s partially because of stability issues.

To illuminate a molecule capable of emitting blue light, you need to pump a relatively large amount of energy into it.

Deep blue light is positioned at the upper end of the visible light spectrum, meaning that it takes quite a bit of energy to make it. So, to illuminate a molecule capable of emitting blue light, you need to pump a relatively large amount of energy into it.

Combining lots of energy with even slight fragility can lead to lots of busted molecules. This is not much of a problem in fluorescence, where the energized state only exists for an instant, but in the case of phosphorescence the molecule stays in the energized excited state for several microseconds. That means it has ample opportunity to blow itself up.

If you're building a TV that's supposed to last for years, this is a serious a problem; the screen's ability to display blues will inevitably degrade over time. Most current devices still use fluorescent blue compounds, but labs around the world are working feverishly to be the first to bring a fully stable phosphorescent blue material to market.

OLED technology is poised to take the display industry by storm.

Once every few decades, a new technology comes along that's simply better than all others in its category. When it comes to displays, OLED is just that sort of technology. It requires less energy than LCD and LED, it's much more versatile, it provides better image quality, and it will cost a lot less to produce once all the manufacturing kinks are worked out.

In all likelihood, it will take about five years for OLED to become truly market-ready, but once it is, there will be no stopping it. For those of us who appreciate the finer points of TV viewing, such as contrast and black level, the future is bright—and soon, fully phosphorescent.

Photos: Flickr user "origami_potato" (CC-BY-SA-3.0),
Flickr user "jpaudit" (CC-BY-SA-3.0), Flickr user "hillsdalehouse" (CC-BY-SA-3.0)

TAGS: Television howitworks OLED LCD LED