A New Frontier for Quantum Dots in Displays
Quantum-dot enhancement film (QDEF) is giving LCD technology an edge as it battles new TV display entrants such as white organic light-emitting diodes (WOLEDs). Recent advances in quantum-dot properties are also enabling new and innovative implementations. This includes their evolving use in LCDs, their possible deployment in OLED and microLED displays, and their potential as emitter materials for future printable electroluminescent displays.
by Ernest Lee, Chunming (Kevin) Wang, Jeff Yurek, and Ruiqing Ma
THE Kindle Fire HDX tablet, released in 2013, introduced the display industry to quantum-dot enhancement film (QDEF), which was one of the first commercial uses of quantum dots in displays.1 Two years later, in 2015, Samsung brought the first display using a
cadmium-free quantum-dot film to the premium TV market.
2 Since that time, quantum-dot technology has moved steadily into the mainstream market, with dozens of devices available today from most of the world’s top display makers. As a result, consumers can now buy an LCD TV enhanced with quantum-dot technology for roughly half the cost of a comparably sized OLED set.3 In addition to TVs, monitors are now being made with quantum-dot film, including a number of models from Samsung, Acer, and Asus that are aimed at the gaming and creative professional markets.4,5 Now that the display industry has embraced the use of QDEF, particularly in mainstream televisions and monitors, new implementations of quantum dots are poised to further improve the performance and quality of displays.
Quantum dots are tiny semiconductor particles that emit light with a narrow spectral shape and at a wavelength dependent on their size. These two properties make quantum dots an ideal material for use in displays. Together they enable quantum dots to provide a larger range of pure colors compared with other light-generating technologies for displays. This in turn results in displays capable of reproducing larger color gamuts.
For example, UltraHD content that is recorded and mastered for Blu-ray and streaming relies on the BT.2020 color-gamut standard.6 This new color specification is designed to capture over 99 percent of the colors found in nature for a truly lifelike image. Most displays currently employ phosphor-based white LEDs and rely on color filters to create the red, green, and blue primary colors for the RGB subpixels. However, these phosphors have limited wavelength tunability and relatively wide spectral distributions, even after filtering. As a result, many displays advertised as having wide color gamut achieve less than 80 percent coverage of BT.2020.7 Despite better color purity compared with conventional LCDs, OLED displays limit themselves to smaller color gamuts such as DCI-P3 because the emission spectra of OLED materials are still too broad for high BT.2020 gamut coverage. Quantum dots, on the other hand, have both the unique wavelength tunability and color purity that enable them to deliver over 90 percent of the BT.2020 UltraHD color standard (see Fig. 1).8
Fig. 1: This color-gamut coverage chart for current display technologies compares WOLED, QDEF, and BT.2020.
QDEF, Cadmium, and Cadmium-Free QDs
Quantum dots would be a mere laboratory curiosity without other properties such as efficiency, stability, and manufacturing scalability. In addition, quantum 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 worked closely with major display manufacturers to develop the aforementioned QDEF, an optical film with a thin quantum-dot layer that is a simple, “drop-in” product that does not require any line retooling or manufacturing process changes.9
Designed as a replacement for an existing diffuser film in LCD backlights, QDEF combines red and green quantum dots in a thin, semitransparent sheet. When excited by light from blue LEDs, the quantum dots emit light at the desired green and red wavelengths. This green and red light combines with a portion of the blue LED light to provide a white light composed of highly saturated red, green, and blue light.
The first quantum dots to be incorporated into consumer displays were based on the element cadmium. At the time, these were the only quantum dots with the necessary efficiency and stability. Since cadmium is a regulated substance under the Restriction of Hazardous Substances (RoHS) Directive,10 many manufacturers were hesitant about using quantum dots. This led to extensive work in improving the properties of cadmium-free quantum dots. However, the emission spectra for cadmium-free quantum dots are wider than those for cadmium-based dots, so these displays only managed to cover the smaller DCI-P3 color gamut.
Recently the gap in performance has narrowed considerably, as shown in Table 1. The high quantum yield for both cadmium and cadmium-free quantum dots enables high optical efficiency. In addition, the narrow emission spectra of Nanosys’ cadmium-free quantum dots already provide greater BT.2020 color gamut coverage compared with other phosphor or OLED technologies.
Continuing improvements to stability enable QDEF to be used in even higher luminance displays. This vastly improves the appearance of the high-dynamic-range (HDR) content shown on QDEF-enhanced LCDs. Furthermore, these stability improvements can also be leveraged into reducing the cost of quantum-dot implementation by reducing the need for additional environmental protection.
With proven performance levels acceptable for commercial displays, quantum-dot manufacturers such as Nanosys and Hansol Chemical have scaled up capacity to produce enough quantum dots to supply millions of square meters of display area. Depending on the needs of the display manufacturer, the precise peak-emission wavelength can also be targeted to single-nanometer precision over a wide range of wavelengths for both green and red.
New Implementations for Quantum Dots
Although the use of quantum dots through QDEF realizes the improved color performance capability of QDs, these displays still rely on conventional LCD modules, which are inherently inefficient. All LCD backlights generate white light that is then filtered to create the red, green, and blue subpixels. Quantum dots help to optimize this system, reducing waste by generating only the red, green, and blue light needed by the display in the backlight. Still, color filters block about two thirds of the light.
To avoid this inefficiency, an entirely new implementation of quantum dots in displays can be used: a quantum-dot color-conversion (QDCC) layer.11 Since quantum dots are so small, a dense, thin layer of quantum dots can replace the color filters in a conventional LCD module and generate light right at the plane where the image is reproduced.
In such a device, the backlight provides only blue light as opposed to the white light of a conventional LCD. The blue subpixel can simply pass the blue light with minimal losses. The green and red subpixels, each with a layer of quantum dots instead of an absorbing color filter, absorb the blue light and downconvert it into green and red light, respectively. Not only does each green and red subpixel solely emit the desired color (and thus provide a saturated color primary for the display), the light throughput of each subpixel can in principle be much higher than in a conventional LCD. In addition to a significant efficiency improvement, an LCD with QDCC layers can have a wider viewing angle, since the light generated by the QDCC layer is at the front of the display.
Although the benefits are significant, incorporating QDCC layers into LCDs introduces several complications. Since the light emitted from a QDCC layer is both unpolarized and isotropic, this requires a change in the structure of the LCD module. The second polarizer in a conventional LCD module is located after the color filters, but with a QDCC layer it must be moved “in-cell.” In addition, the blue light passing through the blue subpixel would require some form of scattering; otherwise, there would be angular color shifting. Finally, any blue light that leaks through in the green or red subpixel desaturates the color point, so optimally the QDCC layer would absorb 100 percent of the blue excitation light.
QDCC layers are not limited to LCDs. They can also be used to create green and red sub-pixels on a single-color blue OLED or microLED array (Fig. 2). Such a display with a QDCC layer provides the advantages of individual pixel control while needing only a single-color emitter layer, which vastly simplifies the manufacturing process. This new type of hybrid display would combine the benefits of electroluminescence (perfect blacks and wide viewing angles) with inorganic emitters for high luminance, saturated color, stability, and low-cost, solution-based manufacturing.
Fig. 2: The QDCC layer can be incorporated into LCDs, as shown in the top row, and into an OLED or microLED display, as shown in the bottom row.
For QDCC layers to become a viable display technology, they must possess several additional optical and physical properties. As a color filter replacement in LCD panels, the QDCC layer must be thin (on the order of 6 to 10 microns) to be compatible with current LCD technology. Although using QDCC with OLED or microLED arrays could support thicker layers, keeping the QDCC thin is preferred. As we shall see in a following section, the requirement of a thin layer has profound implications for the necessary optical properties of the quantum dots.
In all formats, the QDCC layer must be patterned. This can be done through a photolithography process or through inkjet printing. Each method has its advantages and disadvantages. Photolithography can produce much smaller features than inkjet printing (5 microns vs. 50 microns). On the other hand, inkjet printing is much more efficient in terms of material utilization. For both cases, the quantum dots will need to be compatible with the process. This means that the quantum dots must be formulated into photoresist or an ink solution. These materials must be stable in air to leverage existing manufacturing equipment. They must also be stable during the various thermal and chemical processing steps that are not used for manufacturing QDEF. This imposes even more stringent stability requirements on the quantum dots. Finally, in order to comply with the RoHS limitation requiring that any homogenous layer of the system contain fewer than 100 parts per million of cadmium, the QDCC layer must be made from completely cadmium-free quantum dots.
In terms of optical properties, the QDCC layer must produce an efficient, highly saturated light output. Since quantum dots self-absorb a portion of the light they emit, the re-absorption losses in a highly concentrated QDCC layer reduce the optical efficiency of the entire layer. Thus, near-unity intrinsic quantum yield for the quantum dots is critical.
One additional requirement for achieving pure colors is for the QDCC layer to absorb all the excitation light. With optimized peak emission wavelengths, a display with QDCC layers can achieve greater than 95 percent BT.2020 color-gamut coverage if all the blue excitation light is absorbed. However, with 1 percent of the blue light leaking through each conversion layer, the BT.2020 color-gamut coverage would be about 86 percent (see Fig. 3). This desaturation of the color point is especially pronounced for the red color-conversion layer. Although the green color-conversion layer can tolerate more blue light leakage while maintaining high color saturation, the blue-light leakage still must be under 1 percent for a display using QDCC layers to achieve color-gamut coverages at least as high as conventional LCDs with QDEF.
Fig. 3: Blue-light leakage has a significant effect on color gamut coverage.
The blue-light absorption of the conversion layer is directly related to the number of quantum dots in the layer. This is determined by both the thickness of the layer and the concentration of quantum dots in the layer. As noted earlier, the thickness of the layer is limited by manufacturing requirements.
The maximum concentration of quantum dots is limited by several factors. At very high concentrations, the quantum dots may aggregate, which reduces the quantum efficiency. For QD ink formulations, the concentration of quantum dots has a strong impact on the viscosity of the ink. This will affect compatibility with existing inkjet nozzles, printing equipment, and processing techniques. Since quantum dots strongly absorb ultraviolet light, the curing properties of UV-curable ink are also affected at high quantum-dot concentrations. Quantum-dot photoresists also have the same UV-curing constraints. In addition, high quantum-dot concentrations can adversely affect other steps in the photolithography process such as development and patternability.
With these thickness and concentration limitations, conventional Cd-free quantum dots have difficulty achieving the necessary level of blue-light absorption. One solution is to apply a simple, single-color blue-absorbing filter on top of the quantum-dot conversion layers. Although this can be done without significant loss in optical efficiency, it does introduce an additional processing step and therefore manufacturing complexity. To avoid having to use an additional filtering layer, the preferred solution is to increase the intrinsic absorption of the quantum dots themselves. This not only improves the color-gamut performance but also has the potential to reduce the cost of the display by requiring fewer quantum dots.
The intrinsic absorption is a function of the number of energy states between the excitation and the emission wavelengths. The energy spacing is only 300 meV (milli-electron volts) between a 450-nm excitation and a 525-nm emission compared with an energy spacing of over 700 meV between a 450-nm excitation and a 628-nm emission. It is therefore especially challenging to increase intrinsic absorption for green dots due to the smaller number of available states in the conduction band. Techniques such as modifying the core-shell structure of the quantum dot or using an alternate material system based on other elements have shown promise in increasing the blue absorption of a fixed-thickness QDCC layer, as shown in Table 2.
With these additional improvements to cadmium-free quantum dots, Nanosys and collaborators have demonstrated patterned QDCC layers using both photoresist and inkjet printing.12 Figure 4 shows an RGB-printed array with 280 µm × 80 µm subpixels demonstrated by Nanosys and ink maker DIC.13 The green and red subpixels contain thermal-cure quantum-dot ink, while the blue subpixel contains a scattering media to better match the angular distribution of the emissions.
Fig. 4: Inkjet-printed QDCC layers are incorporated into a patterned RGB array.
The Move to Commercialization
As critical as it is, the quantum-dot color-conversion layer is just one component in the final display. Complementary technologies need to be developed to realize the full potential of this new display platform. One significant challenge in applying QDCC layers to LCDs is related to the configuration of the polarizers. In a conventional LCD, the liquid-crystal layer and the color-filter (CF) layer are both sandwiched between crossed polarizers. In this configuration, the polarizers can be easily laminated to both sides of the LC glass cell, which encloses both the LC and CF layers. This CF layer cannot be simply replaced by a QDCC layer because the light emission from quantum dots is unpolarized and thus would interfere with the LC switching. In this case, the QDCC layer must be relocated outside of the crossed polarizers. One of the fundamental requirements of display operation is to place the switching component as close as possible to the CF or QDCC layer to minimize optical crosstalk. As a result, to utilize QDCC layers in an LCD, a thin in-cell polarizer is required. Efficient in-cell polarizers are currently being developed,14 but have not yet reached commercial release.
For OLED-based displays utilizing QDCC technology, the polarizer is not an issue. However, the design demands that all the light is initially generated by a blue OLED emitter. Currently, blue OLED emitters have the lowest efficiency and shortest lifetime among all colors.14 Although their performance level is high enough for some applications involving QDCC layers, additional improvement in both efficiency and lifetime for blue OLED emitters is necessary for this combination to become widespread in use for general displays.
Compared to OLED blue emitters, inorganic LED sources are highly efficient and significantly more stable. Thus, the combination of QDCC layers and a single-color blue microLED array could be a powerful combination for display applications. Using QDCC layers eliminates the need for separate red and green LEDs to make full-color displays, which is one of the major technical challenges for microLED displays.15 Other challenges exist for even single-color microLED arrays, but the use of QDCC layers has the potential to accelerate the development of commercial microLED displays.
In summary, quantum-dot color-conversion layers promise high efficiency, better color, and low-cost implementation for LCD, OLED, and microLED displays, as well as additional benefits for each specific technology. Improvements in both the optical properties and the stability of cadmium-free quantum dots, along with advancements in process technology, have brought QDCC layers very close to commercialization. Additional advancements in complementary technologies by panel makers have placed this new implementation of quantum dots in displays on the cusp of commercial realization.
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All the authors are with Nanosys, Inc. Ernest Lee is a senior engineer. He can be reached at firstname.lastname@example.org. Dr. Chunming Wang is senior director of R&D; Jeff Yurek is the director of marketing; and Dr. Ruiqing Ma is director of device development.