Quantum dots are the hottest thing to hit display technology in a very long time. These nano-scale particles emit light at very specific wavelengths in response to various stimuli. They can be used in several ways within video-display devices, some of which are now available to consumers, while other applications are still under development.
All current displays that incorporate quantum dots are based on their photoluminescent capability—that is, their ability to emit light when they are exposed to light. But the Holy Grail is the development of electroluminescent quantum dots (EL QDs, also called QD-LEDs), which emit light when they are exposed to an electric current, much like OLED material. If a display could be made with EL QDs, it would be much brighter than OLED panels as well as thinner and simpler to manufacture than current LCD panels—and in either case, it would be able to reproduce a wider color gamut.
Among the companies at the forefront of this effort is Nanosys. Its photoluminescent quantum dots (PL QDs) are used in LCD TVs from Hisense, JVC, Philips, Samsung, TCL, and Vizio as well as computer monitors from Acer, ASUS, HP, and Philips. But the company knows that EL QDs are the ultimate future of QD-based displays.
In collaboration with LG Display—LG’s panel-manufacturing and R&D subsidiary, not to be confused with LG Electronics, which markets finished TVs—Nanosys will present a major paper on the subject of EL QDs at DisplayWeek 2019, the annual confab of SID (Society for Information Display). The paper is entitled “High Efficiency Heavy Metal Free QD-LEDs for Next Generation Displays,” and it will be presented along with several other papers in a session called Cadmium-Free Materials & Devices.
Before I get into the meat of the paper, let’s review what quantum dots are and why they’re important for display devices.
What the Heck are Quantum Dots?
Quantum dots are tiny particles of crystalline semiconductor material, typically measuring 2-10 nanometers in diameter. That’s the size of a large molecule, though a bit smaller than DNA. They typically consist of hundreds to thousands of atoms in a rigid crystalline lattice.
The structure of a quantum dot is a bit more complicated than a simple speck of homogeneous semiconductor material (see Fig. 1). A core is coated with a shell of similar material and crystalline configuration, which improves the quantum dot’s optical properties and insulates the core from the surrounding environment. In addition, small molecules called ligands are attached to the shell to further insulate the quantum dot from the surrounding environment and to aid in fabrication and processing by keeping the dot suspended in a chemical solution.
Fig. 1: Most quantum dots consist of three parts—core, shell, and ligands. The precise composition of each part as well as the shell thickness affect the behavior of the quantum dot.
If a quantum dot absorbs a certain amount of energy—say, from an impinging photon—an electron within the dot jumps to a higher energy state. After a very brief time, typically 10-40 nanoseconds, the excited electron “falls” back to its previous lower energy state, emitting a photon of visible light in the process.
When a quantum dot is exposed to a continuous stream of energy, electrons jump to the higher energy state and fall back to the lower state over and over, emitting a continuous flow of visible photons—i.e., light. Remarkably, the wavelengths of these photons occur within a very narrow range, resulting in nearly monochromatic light. This is one of the quantum effects that give quantum dots their name.
Equally remarkable is that the size of a quantum dot determines the peak wavelength of its narrow emission spectrum. To explain exactly how that works is beyond the scope of this article. For now, suffice to say that, as quantum dots get smaller, it takes more energy to kick electrons into the higher state. However, there is also more energy available for emitted photons when the excited electrons fall back to the lower state, resulting in more energetic—i.e., shorter—emission wavelengths.
This size dependency is crucial for “tuning” quantum dots to emit specific wavelengths. Fortunately, the fabrication process allows very precise control of the size of the dots, providing equally precise control of the wavelengths they emit—to within ±1 nm of the target.
Narrow Emission Spectra = Wide Color Gamut
The width of a quantum dot’s emission spectrum is quantified with a metric called “full width at half maximum” (FWHM). As depicted in Fig. 2, the emission spectrum from a quantum dot forms a tall, narrow bell curve. To calculate the FWHM, find the point on either side of the peak that is half of that maximum value and determine the difference in wavelength between those two points.
Fig. 2: FWHM is defined as the difference in wavelength between two points on a quantum dot’s spectrum that are half of its maximum value. The FWHM of CdSe red and green quantum dots is narrower than that of InP quantum dots, which is explained a bit later in this article.
For the current generation of quantum dots, the FWHM is in the range of 30-35 nm. By contrast, blue LEDs exhibit a typical FWHM of about 20 nm, while laser diodes have a typical FWHM of about 2 nm.
The FWHM of quantum dots is critical for wide color-gamut displays. With a narrow FWHM, quantum dots can be tuned to the red, green, and blue primaries that correspond to 100% of the DCI/P3 gamut and even up to 90% of the BT.2020 gamut (see Fig. 3).
Fig. 3: As the FWHM for red, green, and blue in a display get narrower, the color gamut that it can reproduce gets wider.
By contrast, the spectrum of white LEDs in the backlight of LCD TVs has a narrow blue peak and a broad spectrum throughout the green and red wavelengths (see Fig. 4). Red, green, and blue color filters within each pixel isolate the three primary colors, and those filters can be tuned to various color gamuts, but they block a lot of light energy in the process.
Fig. 4: Quantum dots produce narrow emission spectra, whereas phosphor-based white LEDs within today’s LCD TVs produce a narrow blue spectrum with a wide spectrum throughout green and red. The spectra in this graph do not take the color filters into account.
Thus, quantum dots offer a far more efficient light source by concentrating much more of the light energy into narrow ranges to begin with. That means they can be much brighter than conventional technology, which is critical for high dynamic-range displays. Also, this increases the size of the color volume that quantum dots can reproduce, providing more saturated colors even at high brightness levels (see Fig. 5). The end result is a more realistic, lifelike image on the screen.
Fig. 5: Color volume represents the colors that a display can reproduce at different brightness level. In this example, the bright orange lava is within the color gamut at a certain brightness, but not within the BT.709 (standard dynamic range) color volume.
Early quantum dots were made of a semiconductor material called cadmium selenide (CdSe), which is highly toxic when released into the environment. But CdSe makes very efficient quantum dots with narrow FWHM, so that material has been widely used.
Nanosys was among the first companies to produce CdSe-based quantum dots for commercial displays in 2013 under a regulatory exemption granted by the European Union Commission’s RoHS (Restriction on Hazardous Substances). RoHS establishes limits on the use of hazardous materials in commercial products sold in Europe, but that doesn’t mean those limits aren’t important elsewhere in the world.
Without an extension, that exemption is due to expire in October 2019, after which there will be a grace period until May 2021. At that point, there must be less than 100 parts per million (ppm) of cadmium in any cadmium-containing component in a display device. That is a very tough requirement to meet, so the search for cadmium-free quantum dots has become increasingly important. (As of this writing, the RoHS exemption is likely to be extended, but the problem of toxicity in the environment remains.)
Samsung released the first cadmium-free QD-based display in 2015, with quantum dots made of indium phosphide (InP), a material that hasn’t yet achieved a level of performance similar to CdSe. For example, the typical FWHM for green CdSe quantum dots is 23 nm, while it’s 38 nm for InP; the FWHM for red CdSe quantum dots is 20 nm, while it’s 39 nm for InP (see Fig. 2).
As a result, current QD-based TVs with standard color filters can reproduce about 85-90% of the BT.2020 color gamut using CdSe quantum dots and about 80% of BT.2020 using cadmium-free QDs. The green and red color points are close to their BT.2020 targets, but blue is way off because of light leakage through the blue color filter.
Things improve dramatically using color filters that are optimized for wide color gamut. In this case, displays using CdSe quantum dots can theoretically reach up to 97% of the BT.2020 color gamut, while those using Cd-free quantum dots can reach as much as 92% of BT.2020. The blue point is still farthest from its target, and brightness is somewhat reduced compared with standard color filters.
Continued development will reduce these differences, which is crucial in the long term. Meanwhile, Nanosys and some of its partners have started to transition to low-cadmium or cadmium-free quantum dots for commercial display devices.
As I mentioned at the outset, all currently available QD-based displays use photoluminescent quantum dots within LCD panels. Photons from a blue-LED backlight pass through a film—called a “quantum-dot enhancement film” (QDEF)—embedded with billions of randomly scattered red and green quantum dots. Some of the blue photons are absorbed by the dots, which then emit red or green photons depending on their size. Those red and green photons combine with the backlight’s blue photons that are not absorbed, forming white light that passes through the LCD pixels and their associated red, green, and blue color filters (see Fig. 6).
Fig. 6: QDEF LCD TVs use a blue-LED backlight that sends blue photons through a QD enhancement film embedded with red and green quantum dots. The red and green photons from the quantum dots combine with unabsorbed blue photons from the backlight to create white light, which then passes through the LCD and color-filter layers.
A related approach deposits red and green PL QDs directly onto the glass light-guide plate within the display, eliminating the need for a separate QDEF (see Fig. 7). This application is called “quantum dots on glass” (QDOG), and it’s available today in some computer monitors.
Fig. 7: In a QDOG display, the quantum dots are deposited directly on the glass light-guide plate.
One future application of photoluminescent quantum dots is using them to replace the red and green color filters within a display. In this case, the backlight’s blue photons are virtually all absorbed by the red and green quantum dots that replace the corresponding color filters, while the blue color filter is replaced with a transparent window that allows the backlight’s blue photons to pass through (see Fig. 8a and 8b).
This approach is called “quantum-dot color conversion” (QDCC), because quantum dots are used to convert the blue color of the backlight directly to red or green. The result is a much more efficient and brighter display that is simpler to manufacture. However, the density of quantum dots in the color-conversion layer must be much higher than it is in QDEF to prevent any blue photons from passing through without being absorbed.
Fig. 8a: In a QDCC-based LCD display, the red and green color filters for each pixel are replaced with high-density PL QDs. The blue color filter is replaced with a transparent window that lets the backlight’s blue photons pass through it to form the blue part of the image.
Fig. 8b: QDCC can also be applied to OLED and even microLED displays, which only need to produce blue light behind the QDCC layer.
The ultimate display technology using quantum dots relies on electroluminescence, in which quantum dots emit photons when stimulated by an electric current rather than light. A display with pixels based on electroluminescent quantum dots (a technology often called QDEL) would emit red, green, and blue photons in response to electrical signals, much like OLED displays (see Fig. 9). This is the next generation of direct-emissive displays—no backlight, no color filters, no color conversion, wide color gamut, and very high brightness.
Fig.9: QDEL requires the fewest layers of any QD-based display technology.
We’ve long heard that QDEL displays are years away from commercialization. However, the paper that Nanosys and LG Display are presenting at DisplayWeek 2019 indicates that products based on QDEL might be closer than previously thought.
In that paper, the companies discuss their recent work on cadmium-free EL QDs. This is critical because the density of quantum dots in a QDEL configuration is very high, so the dots must be cadmium-free to meet the RoHS requirement and protect the environment.
Along with FWHM, you need to understand two other metrics. One is “quantum yield” (QY), which is simply the ratio of the number of emitted photons to the number of electrons that are kicked up to the higher energy level—in other words, the native efficiency with which quantum dots convert incoming energy to emitted photons. For display devices, QY should be over 90%.
The other important metric is “external quantum efficiency” (EQE). This is a measure of how efficiently the entire display device—including other optical layers within the display—converts energy into light. EQE is generally much lower than QY, but it’s more directly relevant to a display’s performance.
Recently, electroluminescent quantum dots based on cadmium have achieved a reported QY of 83% for red and 85% for green and EQE around 20% for both colors. By contrast, cadmium-free EL QDs have had lower QY and EQE values so far. Table 1 lists the maximum EQE of various cadmium-free EL QDs, including those made from zinc selenide (ZnSe) and indium phosphide (InP).
Table 1: As reported in the literature to date, the maximum EQE of violet, green, yellow, and red cadmium-free EL QDs is less than 10%—but the DisplayWeek paper reports much higher EQEs, as we shall see.
The lower QY of InP-based EL QDs is often attributed to surface defects caused by incomplete shell coverage or interfacial strain—that is, strain between the core and shell—due to a lattice mismatch between the InP core and the conventional zinc-sulfide (ZnS) shell. Various research efforts report improvements using other shell materials, such as zinc selenide (ZnSe), zinc selenide sulfide (ZnSeS), or gallium phosphide (GaP), which allow thicker shells by reducing the strain within the crystal lattice. Also, reducing oxidation at the surface of the InP core results in fewer interfacial defects and thus better luminescent efficiency and narrower emission spectra.
All told, improvements in core composition, shell composition, core/shell interface, fabrication, and purification have led to much higher QY and EQE in red and green EL QDs. Table 2 summarizes those results.
Table 2: Nanosys has developed green and red InP quantum dots with tunable peak wavelengths (PWL), narrow FWHM, and QY over 95%.
Then there’s blue. The target wavelength of 450-460 nm for a BT.2020 blue emitter is much more difficult to achieve with cadmium-free materials. InP-based QD-LEDs have been reported with an emission peak longer than 460 nm, which is in the cyan region, and a QY of only 40%. The highest QY is typically achieved with ZnSe-based EL QDs in the 420-440 nm range, which is in the violet region.
Nanosys has developed a new, proprietary material for blue EL QDs that greatly improves on those stats. The new material can be tuned to a peak wavelength in the 440-460 nm range with high QY and narrow FWHM. Table 3 lists these properties for violet ZnSe-based quantum dots and blue EL QDs based on the new material.
Table 3: ZnSe-based quantum dots have a peak wavelength that is too low for a good blue emitter. The new material developed by Nanosys is much better in that regard, though its QY is a bit lower.
Figure 10 shows the emission spectra of ZnSe-based violet and the new blue EL QDs:
Fig. 10: The new QD material puts the peak wavelength right where it should be for a blue emitter.
Thick as a Brick
In addition to a new material for blue EL QDs, Nanosys and LG Display studied how the thickness of the shell affects the performance of red photoluminescent quantum dots. The researchers synthesized red quantum dots with varying shell thickness; they labeled the different samples QD1 (the thinnest shell) to QD7 (the thickest shell). The QY as measured when the quantum dots were suspended in a liquid solution (the “solution QY”) and the QY as measured when the quantum dots were deposited onto a glass substrate in a solid film (the “film QY”) are listed in Table 4.
Table 4: This table lists the QY of red quantum dots with varying shell thicknesses when they are suspended in solution and in a solid film. The solution QY remains fairly constant from QD1 to QD5, falling a bit for QD6 and QD7. However, the film QY increases with shell thickness to a maximum of 57% for QD5. The film QY of QD6 and QD7 was not measured because of the lower solution QY with those shell thicknesses.
The EQE of quantum dots with different shell thicknesses follows a similar pattern to the corresponding film QY values (see Fig. 11).
Fig. 11: As you can see in this graph, the EQE increases with shell thickness up to QD5, after which it drops as the shell thickness continues to increase.
This research clearly demonstrates that there is an optimum shell thickness for quantum dots. As the thickness of the shell increases, the quantum dot has better charge confinement and reduced energy transfer among the dots. However, when the thickness exceeds a certain threshold, the strain caused by lattice mismatch between the core and shell reduces EQE and QY.
The Future Looks Bright
The DisplayWeek paper by Nanosys and LG Display concludes with the current state of the art in EL QDs. By improving the core/shell/ligand structure, fabrication process, and device structure, the companies have achieved the results listed in Table 5.
Table 5: These are the best EQE values for red, green, and blue cadmium-free and cadmium-based EL QDs as measured by Nanosys and LG Display for their DisplayWeek paper. The cadmium-free values are still not as good as those for Cd-based quantum dots, but they are getting better all the time.
So, how soon will we see QDEL flat-panel TVs? Not for a few years yet. But with the research outlined in the DisplayWeek paper and the work that will surely follow, I believe it will be fewer years than most people have expected up to now.
At DisplayWeek, TCL is expected to demonstrate a 31″ monitor with red and green cadmium-based EL QDs and blue OLED material in a hybrid design. Meanwhile, Apple seems to be going in the opposite direction, using blue EL QDs with red and green OLED material. In addition, we should see some technology demos at CES 2020, with commercial products coming a couple of years after that.
That’s still a long time to wait for what will certainly be a major disruption in the TV marketplace. But I bet it will be worth the wait.
If you’d like to read the paper itself—which is much more technical than this article—you can download it here. Unfortunately, it’s not free.