Quantum dots for ultra-high color gamuts in LCDs
The color gamut of LCDs is currently limited by the backlight. In today's LCDs, the dominant backlight technology is based on white LEDs, which are composed of blue LED chips combined with cerium-doped yttrium aluminum garnet,1 a broadband yellow phosphor with low spectral weights at green and red wavelengths. To fabricate displays with a high color gamut using these white LEDs requires color filters with very narrow transmission bandwidths. As a result, the transmissivity of the liquid crystal panels is much reduced, leading to poor power efficiency.2
In practice, LCDs based on white LEDs can deliver a HDTV (high-definition TV) broadcast standard (Rec. 709)3 color gamut. Discrete RGB (red, green, blue) LEDs have been implemented for higher color gamut standards (e.g., Adobe RGB) but, due to their low power efficiency (green LEDs have much lower efficiency than blue and red LEDs), LCDs based on RGB LEDs have very limited market share and suffer high system costs. Although new RG phosphors that are capable of achieving a high color gamut have recently been introduced in some mobile displays, they are phosphorescent. As a result, they are incapable of matching the high refresh rates required for the local dimming in LCD TVs, which enables the luminosity of some zones of the screen to be altered, creating deeper blacks. Furthermore, these RG phosphors still have lifetime issues, such as white-point instability.
Quantum dots (QDs) represent a new phosphor material with high efficiency and narrow emission linewidths (in the low 30nm).4 Narrow linewidths are desirable in backlit display applications because they deliver increased color purity and improve efficiency by eliminating light waste at the color filters. Over the past two years, QDs have seen market adoption in tablets, notebook computers, and TVs. In the case of tablets, the use of QDs enables an accurate Rec. 709 display and increases the power efficiency of the backlight by 20%. In notebook and TV displays, QDs deliver broad color gamuts covering both Adobe RGB and DCI-P3 color standards.5
The ultra-high color gamut standard Rec. 2020, in which the color primaries are on the color locus of the CIE (Commission Internationale de l'Eclairage) 1931 diagram, was originally defined for laser-based projectors. Because of its deeply saturated color coordinates, Rec. 2020 is beyond the capabilities of today's cutting-edge organic LEDs. We have explored the possibility of using QDs in consumer-level LCDs to reach the Rec. 2020 color gamut, typically only achievable using laser-based projection systems.6
To demonstrate the color performance of LCDs using QDs, we retrofitted two off-the-shelf displays: a 65in direct-lit 4K LED TV and a 13in edge-lit 1080p ultrabook.7 The original panels, both standard Rec. 709 displays, use white LEDs. In both demos, we made only two changes to each display: replacing the white LEDs with blue LEDs and replacing the bottom diffuser with a QD enhancement film (QDEF). Compared to the QDEF sheets used in existing LCD products on the market, our QDEF sheets have QDs with different green and red wavelengths.8 The green and red peak wavelengths are 525 and 642nm, respectively, compared to the typically used wavelengths, which are closer to 530 and 630nm, respectively. The power density spectra of the RGB primaries measured from the 13in display are shown in Figure 1. The spectra of the 65in TV are very similar. Note that the red spectrum has a markedly higher intensity than the blue and green spectra. This disparity is necessary in displays due to the low photopic response of the human eye to long (red) wavelengths.
We achieved a color gamut of 114% NTSC (measured in CIE 1931) in both retrofitted demos, covering 90% of Rec. 2020 (see Figure 2). In particular, the red and green primaries are very close to those of the Rec. 2020 color standards, lying within 0.02 in both u′ and v′ chromaticity coordinates. The blue primary, off by less than 0.03, lies furthest from the color standard. The reason for this deviation can be understood from the power density spectra shown in Figure 1. The blue spectrum in particular has its primary peak at a blue LED wavelength of close to 445nm and a satellite peak at close to 520nm. This is a result of the green peak of the backlight leaking through the blue color filter on the liquid crystal panel, limiting the coverage of Rec. 2020 to 90%. The blue color filters used today are mostly designed for Rec. 709 and white LEDs, in which the yellow peak lies at around 560nm. The leakage of this yellow peak through the blue color filter is minimal enough to be insignificant. For Rec. 2020, however, the green peak must lie at much shorter wavelengths to hit the green primary target. In this case, the green leakage through the blue color filter is more pronounced, especially for a deeper blue primary target. If this leakage can be minimized by using an optimized blue color filter, a Rec. 2020 coverage of 95+% could be achieved in LCDs.
In summary, we have demonstrated displays with ultra-high color gamuts by combining QDEF sheets with today's standard LCD panels. In both of the demos that we built—a 65in direct-lit 4K TV and a 13in edge-lit ultrabook—we achieved 90% coverage of the Rec. 2020 color standard. If the color filters (particularly for blue wavelengths) are optimized, a 95+% Rec. 2020 coverage is feasible. QDEF therefore offers a practical and ready-to-implement solution for ultra-high-definition LCD TVs for drastic expansion of the color gamut, to bring true-to-life viewing experiences to end users. We are current focusing on developing an innovative roadmap of materials and display integration technologies with the aim of making displays brighter, more colorful, and more power-efficient.
Jian Chen is chief technology officer of research and development at Nanosys, where he is leading the development of display technology based on QDs. Since joining Nanosys in 2002, he has directed research efforts across a variety of applications.
Steve Gensler is the director of engineering at Nanosys, where he leads systems integration to develop products that incorporate QD technology. He received his BSc in chemistry from the University of California, Berkeley, and his PhD in physics from the University of Chicago, IL. He has focused on display applications for nanotechnology since joining Nanosys in 2009 and worked in semiconductor technology before that.
Jeff Yurek is product marketing manager at Nanosys. He writes a blog on emerging display technologies, about which he notes: “My aim is to provide a useful resource on the science, software, engineering, and art that go into making the displays we use every day so amazing.” He has advanced degrees from the Sawyer School of Management, Suffolk University, MA, and the Berklee College of Music, MA.
