Quantum dot LEDs get brighter, more efficient

Apr 20, 2012 by Lisa Zyga feature
Quantum dot LEDs get brighter, more efficient
Red, green, and blue QLEDs, with the applied voltages in the upper left corner. The green QLED has a luminance of 168,000 candelas per square meter, which is more than three times higher than the previous best QLED brightness. Image credit: Jeonghun Kwak, et al. ©2012 American Chemical Society

(Phys.org) -- While quantum dot-based light-emitting diodes (QLEDs) are not made of organic materials, they share many of the same advantages as organic LEDs (OLEDs). For instance, both QLEDs and OLEDs outshine semiconductor-based LEDs in terms of their greater flexibility, better color quality, and potential for lower cost since they can be fabricated using a simple process on a large-area substrate. But ever since the first QLEDs were demonstrated in the mid-'90s, about a decade after OLEDs, their performance has lagged behind OLEDs despite ongoing improvements. Now in a new study, a team of researchers from South Korea has designed and demonstrated QLEDs with an improved efficiency and unprecedented brightness that matches the brightness of today's best fluorescent OLEDs.

The research teams at Seoul National University, South Korea, led by Changhee Lee, Kookheon Char, and Seonghoon Lee, have published their study in a recent issue of Nano Letters.

As the researchers explain in their study, the key to improving the brightness and efficiency of the QLEDs is improving the injection of current-carrying and holes into the quantum dots. The more efficiently the electrodes can inject electrons and holes into the quantum dots, the more efficiently the device can emit light. Usually, the anode is made of , whose transparency allows light to escape. But here, the researchers inverted the device by making the indium tin oxide the cathode with the help of as an electron transport layer, which performed charge carrier injection much more efficiently than before.

“The most important cause of the low performance of QLEDs is the poor injection of holes into the quantum dots (QDs) from the anode and neighboring hole transport layer due to a huge potential energy barrier,” Changhee Lee told Phys.org. “Because of that, the electron-hole balance is not achieved, resulting in low quantum efficiency and low maximum brightness. Furthermore, the excess electrons or holes, which do not recombine in the QD layer and enter the neighboring organic hole-transport or layers (HTL or ETL), can cause leakage current and device degradation, resulting in poor efficiency and stability. Therefore, good carrier injection is a key factor for realizing high-performance QLEDs.”

By patterning different sized quantum dots on the layer of zinc oxide nanoparticles, the engineers could fabricate QLEDs of three different colors: red, green, and blue. Whereas previous QLED brightness levels were in the range of 10,000 candelas (cd) per m2, the new red QLED displayed a brightness of 23,000 cd/m2 and the green achieved a remarkable 218,000 cd/m2 – the highest ever for a QLED and comparable to the best OLEDs. The previous highest QLED brightness is 68,000 cd/m2, which was for a green QLED reported last year by Lei Qian, et al. The new blue QLED displayed a lower brightness of 2,000 cd/m2, but low blue performance has been one of the biggest disadvantages of both QLEDs and OLEDs.

In areas besides brightness, the QLEDs have also improved but still lag behind OLEDs. The new QLEDs' efficiencies (7.3%, 5.8%, and 1.7% for red, green, and blue devices, respectively) improve over previous QLEDs, although OLEDs can have efficiencies of up to 20%. Another challenge for both QLEDs (and OLEDs to a lesser extent) is lifetime. Since the early research of the '90s, QLED lifetimes have not improved past a few tens of hours, and they experience rapid deterioration within a few hours of operation. QLEDs with inverted structures, like those used here, can have half-lifetimes of up to 600 hours, compared with tens of thousands for OLEDs.

Although QLEDs don't match the performance of OLEDs, the engineers explain that QLEDs have a few potential advantages that make them worth investigating further.

“The luminous efficiency of the best OLEDs (phosphorescent OLEDs) and inorganic LEDs are comparable, up to ~100 lm/W for white emission,” Changhee Lee said. “However, the efficiency of QLEDs is still way behind, about 10 times lower. The efficiency of red and green QLEDs reported in our paper is comparable to the efficiency of the best 'fluorescent' OLEDs, which use fluorescent organic dyes as emitters. Of course, the lifetime of QLEDs is much lower than OLEDs and inorganic LEDs at this time. The potential advantages of QLEDs are: (1) much narrower emission bandwidth (full width at half maximum ~30 nm compared with 60-80 nm of OLEDs), which means that QLEDs have more saturated and purer color than OLEDs; (2) easier tunability of emission colors in the entire visible range by simply controlling the particle size and shape with the same chemical composition for the QD; (3) and therefore the cost of emitters are much lower for QLEDs while organic phosphorescent emitters used for best OLEDs are very expensive.”

Overall, the brightness, efficiency, lifetime, and low turn-on voltage of the new QLEDs suggest that the quantum dot devices could have promising applications as TV, computer, and phone displays as well as lighting devices. Since can be printed as ink, these displays and devices could also benefit from low-cost production methods.

“Our future plan is to further improve the efficiency and reliability of QLEDs, in particular, blue QLEDs,” Changhee Lee said. “In parallel, we will make a full-color active matrix QLED display using our improved RGB QLED technology. We will also work on developing Cd-free QLEDs because of environmental and safety concerns related with Cd. We recently reported InP QLEDs in Chemistry of Materials, but their efficiency is very low. Therefore, we will work on developing new precursors for InP QDs and improving the performance of Cd-free OLEDs.”

Explore further: Spider's web weaves way to advanced networks and displays

More information: Jeonghun Kwak, et al. “Bright and Efficient Full-Color Colloidal Quantum Dot Light-Emitting Diodes Using an Inverted Device Structure.” Nano Letters. DOI: 10.1021/nl3003254

Journal reference: Chemistry of Materials search and more info website Nano Letters search and more info website

4.6 /5 (16 votes)

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antialias_physorg
not rated yet Apr 20, 2012
When can I have QLED wallpaper? Sriously - I'm currenntly trying to find new lighting fixtures and I would love to just forego the traditional ones and just put up luminescent wallpaper.
Eikka
2.3 / 5 (6) Apr 20, 2012
Narrow emission bandwidth is actually a bad thing for general lighting applications, because it means your light consists of sharply monochromatic components, which makes for a poor color rendering index.

If you have only exactly red, green and blue in your light, what would a banana look like under it? That's the basic problem, which is solved by adding the yellow light, but then, what would and orange look like? So you need to add orange as well; fill in all the gaps in the spectrum to make it look right.

Monochromaticity is great for display devices which can employ less selective filters to achieve the same color saturation and therefore more brightness and better contrast, but that's really the difference between additive and subtractive color. They don't work the same way.

However, the ability to tune the nanodots means that it's possible to spread the emission spectrum by varying the spot size. How exactly they would work in a single chip is another question though.
Eikka
2.3 / 5 (6) Apr 20, 2012
And since someone is bound to pull out the old fruit basket comparison with the laser light where people compare a fruit basket illuminated by a laser light source consisting of exactly red, green, blue and yellow - that's a flawed comparison for a simple reason:

We know yellow bananas are yellow and green apples are green, so making the comparison with a fruit basket creates an illusion where the brain automatically interprets the banana as yellow even though the actual color is off by miles, so of course people say they see no difference, because they percieve no difference.

Picking familiar objects instead of colored squares for example partially masks the fact that the light is different because the colors are interpreted differently. The brain assumes that it's seeing wrong and "corrects" it. You may then argue that this CFL or LED or laser bulb is just as good in practice as say, a halogen bulb, but it isn't. You just get used to the difference.

italba
2.3 / 5 (3) Apr 20, 2012
Your eyes cannot see "continuous spectrum" colors. Your eyes can only distinguish red, green or blue colors, and their relative luminosity. When a light stimulates equally your red and green receptors, you see "yellow", but you cannot see any difference between a single, pure yellow light and a green/red mix.
Eikka
4.2 / 5 (5) Apr 20, 2012
I would love to just forego the traditional ones and just put up luminescent wallpaper.


I'd have a luminescent ceiling.

But there might be a problem with the uniformity of the lighting, because a room without shadows is actually quite distracting. Discerning shape is difficult if the object is evenly illuminated on all sides because you can't make out the surface texture etc.

Your eyes cannot see "continuous spectrum" colors. Your eyes can only distinguish red, green or blue colors, and their relative luminosity.


False. The cones and the rods are sensitive to a wide range of wavelenghts. They are most sensitive to specific wavelenghts, but they are not very selective. The combination of each does span the entire visible spectrum - otherwise we couldn't see some colors at all, like violet.

It looks like violet because blue light actually excites the green cones a little bit as well, and violet light doesn't.
antialias_physorg
3 / 5 (2) Apr 20, 2012
Narrow emission bandwidth is actually a bad thing for general lighting applications, because it means your light consists of sharply monochromatic components, which makes for a poor color rendering index.


Well, since the article mentions this:
easier tunability of emission colors in the entire visible range by simply controlling the particle size and shape with the same chemical composition for the QD

There is really no reason why one cannot have a substrate with nanodots off all kinds of sizes to get a good, customized light spectrum
Eikka
1 / 5 (1) Apr 20, 2012

There is really no reason why one cannot have a substrate with nanodots off all kinds of sizes to get a good, customized light spectrum


Well, that's the interesting bit, because what happens to the treshold voltage and current requirement of the LED when you mix different sizes of nanodots in the same chip? Can you optimize them all to work together, or do you have to separate them and create some sort of bayer pattern before it'll work right? If you have to separate them, then that gives further problems like the need for diffusers which eat up some of the light.

But that's probably a topic for the next paper.

PPihkala
not rated yet Apr 20, 2012
I think that color mixing at one substrate does not work. Just connect red and green led in parallel and see what happens: The red one has lower turn on voltage, so it will eat all current and green one will not turn on at all. I assume the same will also happen with QLEDs. So probably no color mixing at the same substrate.
italba
1 / 5 (1) Apr 21, 2012
@ Eikka: Rods are sensitive to all colors. You can only see black and white with rods. I never said cones are narrow-band receptors, I said that color perception is the differential stimulation of your different cone receptors. As you wrote, short wave cone stimulation (violet) and medium wave (green) appears as blue. But you can have the very same sensation with a single bright blue light or two lower violet and green lights. In the same way you can have any color you like in your pc monitor that works with 3 different colors subpixels.
Eikka
1 / 5 (1) Apr 23, 2012
In the same way you can have any color you like in your pc monitor that works with 3 different colors subpixels.


Yes indeed, but that is again the difference between additive and subtractive color.

The pc monitor works differently from an apple in your hand because the apple does not make its own light. If the apple's color is a result of it reflecting light at 550 nm and there is no 550 nm light around, then what does the apple look like?

For subtractive color to work, there must be all wavelenghts of light to subtract from. If the spectrum isn't full, objects start to fade off into grey, and in sharply monochromatic light things just appear black if they happen to absorb that particular wavelenght.
italba
1 / 5 (1) Apr 23, 2012
You can be right only if you look at strongly monochromatic objects (quantum dots, some precious stones, some butterfly's wings), objects that reflects light only for a given frequency. Surely NOT for apples! About every object we see reflects light in a wide spectrum of frequencies, with a peak for its "color". If an apple reflects 100% of a 550 nm red light, it will reflects 99% for 540 or 560 lights, and we still will see it red. Anyway, if you look at the emission spectrum of tungsten (the wire of incandescent lights) it is NOT a wide spectrum light, it is a bunch of monochromatic lights!

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