Quantum dots unleash high-color-gamut performance in LED-backlit displays (MAGAZINE)

Oct. 21, 2011
Quantum-dot technology can be used in LCD BLUs to create an optimized spectrum that enables brighter, more-efficient displays with vibrant colors, writes JASON HARTLOVE.
LCD technology has made great progress in the past few years, with innovations such as high resolution and 3D, yet color performance continues to lag. Displays on popular tablets can only express about 20% of the color a human eye can see; for HDTVs this is still only 35%. Surprisingly, color performance in displays has actually gone backwards since the days of CRTs.

Still, LED-backlit LCDs have become the standard for the mobile-device and TV industries, due to their high resolution, low cost and thin form-factors.

While new technologies with better color capabilities have emerged in recent years, such as discrete RGB LED, YAG with red phosphor, and OLED, they face critical hurdles to mass adoption – primarily cost, scale and brightness. Until now, consumers have chosen cheaper, thinner and more efficient displays over a truly cinema-quality experience- but could they have it all?

What’s wrong with my current display?

To better understand the limitations faced by current TV and display makers, let’s take a look inside an LCD. For those who are not familiar, a typical LCD is made up of essentially two major parts: a light source, called the backlight unit (BLU) and a liquid-crystal module (LCM).

FIG. 1. Usually, when a display is operating, the BLU is on, providing a uniform, white sheet of light behind the LCM. The LCM contains millions of pixels, each of which is split into sub-pixels, typically with two green sub-pixels, one red and one blue. By controlling the amount of time each sub-pixel is open – i.e. allowing light to pass through it – and making use of the human eye’s persistence of vision, any color that can be rendered from a combination of red, green and blue can be displayed at each pixel location. Since the quality or fidelity of those colors is a direct function of the sub-pixel color quality, how good is the quality of red, green and blue light coming from each sub-pixel?

The color of each sub-pixel is a function of two things; the quality of the light in the BLU, and the color filter at the sub-pixel. The color filter will separate its component color from the white light of the BLU, for example, the red color filter on the red sub-pixels will cut off the green and blue light. However, to make a high quality color of red, either the filter function needs to be very narrow, which results in substantial attenuation and loss of brightness, or the red spectrum in the BLU white light should be narrow and well matched to the desired peak red color. The same is true for the green and blue sub-pixels as well.

Since making perfect color filters is not practical from either a cost or brightness perspective, why not make a better white light?

The problem is, the LED light source at the heart of the BLU is starving those filters of the colors that they really need to shine. Today, white LEDs are very good at producing some of the spectrum of light that we see as white – but not all. While there are a variety of approaches for making white light from LEDs, the conventional approaches all suffer drawbacks for LCD displays.

A YAG-based white LED (i.e. an yttrium-aluminum-garnet phosphor pumped by a GaN blue source) produces a spectrum rich in blue wavelengths and with a broad yellow component. This light has very weak green and red content, and the spectrum is widely distributed from aqua-marine through green, yellow, orange and red (Fig. 1). When this light is filtered into the component RGB colors by the sub-pixels, the result is not accurate enough to produce the quality of color we see when we look at the natural world as illuminated by daylight.

So, an ideal light source for an LED-based LCD BLU would therefore be something in between daylight and two-color white. For vibrant colors, it would need to generate lots of energy across all of the red, green and blue wavelengths used by the filters. But, for efficiency’s sake, it should also not spend energy producing light between R, G and B because we just won’t see that light after it’s passed through the filters.

Why quantum dots?

To solve the problems described above, what we need is a new class of material, not found naturally occurring anywhere on Earth, that can be tuned to emit light at just the right wavelengths for our displays and do so very efficiently. Fortunately, nanotechnology researchers have been working on designing just such a material for decades, building it literally one atom at a time, quantum-dot technology consists of tiny, nanocrystal phosphors that are a bit bigger than a water molecule but smaller than a virus in size.

FIG. 2. Unlike conventional phosphor technologies such as YAG that emit with a fixed spectrum, quantum dots can be fabricated to convert light to nearly any color in the visible spectrum. Pumped with a blue source, such as the GaN LED, they can be made to emit at any wavelength longer than the pump source wavelength with very high efficiency (over 90% quantum yield) and with very narrow spectral distribution of only 30-40 nm full width at half maximum (FWHM).

The real magic of quantum dots is in the ability to tune (at the fabrication stage) the color output of the dots, by carefully controlling the size of the crystals as they are synthesized so that their spectral peak output can be controlled within 2 nm to nearly any visible wavelength.

This capability makes quantum dots stand out against emerging iterations of YAG phosphor technology such as red-phosphor-doped YAG, which adds some red-emitting phosphor to the green-yellow-emitting YAG to boost color performance. This idea is similar to quantum-dot technology in that it attempts to engineer a spectrum of white light by combining materials with different emission spectra. However, these crystalline phosphor materials are still fundamentally limited by their atomic structure and therefore cannot be precisely tuned to match either existing color filters or manufacturers’ desired specifications. This leaves display manufacturers with a system that still results in light and efficiency losses due to the relatively-wide FWHM output of the phosphors and poor conversion efficiencies and stabilities of red phosphors.

With quantum-dot technology, display designers will have the ability to tune and match the backlight spectrum to the color filters (Fig. 2). This means displays that are brighter, more efficient, and that produce truly vibrant colors.

FIG. 3. How does it all come together?

Engineering the quantum dots to precise display-industry specifications isn’t enough on its own to revolutionize the way LCDs are experienced. The dots need to be easily integrated into current manufacturing operations with minimal impact on display-system design if they are to be widely adopted. To do this, Nanosys spent a lot of time working with major display manufacturers to get the packaging just right so that it would be a simple, drop-in product that did not require any line retooling or process changes. The end result is the quantum-dot enhancement film (QDEF) – see Fig. 3.

Designed as a replacement for an existing film in LCD backlights called the diffuser, QDEF combines red- and green-emitting quantum dots in a thin, optically-clear sheet that emits white light when stimulated by blue light. (Of course, some of that blue is allowed to pass through to make the B in RGB at the LCM). So manufacturers who’ve invested billions in plant and equipment for LCD production can simply slip this sheet into their process, change their white LEDs to blue (the same LEDs but without the phosphor) and start producing LCD panels with the colors and efficiencies of the best OLEDs, at a fraction of the cost.

Nanosys is currently shipping production samples to display manufacturers and is on track to begin producing at commercial volumes by the end of 2011.

What does it look like?

A QDEF-enabled display can express over 60% of the spectrum a human eye can detect, compared with 20% for today's LED-backlit LCDs. This means that browsing through photos on your tablet will be more like holding a stack of high-quality, professional prints in your hand and watching a movie on the big screen in your living room is more akin to attending a private screening at a Hollywood studio.

LED-backlit LCD TVs have established market dominance, and sales of tablet computers – which predominantly use LCDs–are expected to eclipse 100 million units over the next few years. Color is likely to be the next big differentiator in what is an increasingly cutthroat consumer-display market as more players enter the market and alternative technologies are further developed.

Displays with better color performance will allow developers and content creators to generate a stunning new visual experience for consumers. Display makers who can bring user experience closer to reality without sacrificing efficiency or cost will be able to establish a dominant market share.