One of the most promising display technologies is quantum dots. At CES 2018, I got a quantum-dot update from Nanosys, one of the main suppliers of quantum dots to the display industry.
Quantum dots (QDs) are microscopic spheres of semiconductor material that absorb blue light and emit light at a longer wavelength, depending on their size. Such QDs are called photo-emissive. In current-generation QD-based LCD TVs, the backlight consists of blue LEDs and a plastic film impregnated with randomly distributed QDs of two different sizes—one size emits green light and the other emits red light. The red and green light combines with unabsorbed blue light from the LEDs, forming white light that passes through the LCD layer and red, green, and blue color filters to form the final image.
When I entered the Nanosys suite at the Westgate hotel, I immediately noticed a chart on the wall illustrating various advanced display architectures, including how quantum dots would be used in the future. Check it out:
The current architecture—which is called quantum-dot enhancement film (QDEF)—is at the top of the chart. It can achieve over 90% of the BT.2020 color gamut and over 2000 nits of peak luminance.
OLED is also on the chart for comparison. Of course, this is an emissive display technology that exhibits deeper black levels and a wider viewing angle than LCD. Consumer-oriented OLED TVs use blue and yellow OLED emitters to create white light that passes through red, green, blue, and clear filters to form a full-color image. (The clear filter increases the overall brightness.) However, even with the clear filter, it can achieve less than 1000 nits of peak luminance, and the blue emitter has a relatively short lifespan. Also, OLED can suffer from image retention and burn-in.
The next generation of quantum-dot architecture is called quantum dot on glass (QDOG). In this design, the quantum dots are deposited on a very thin sheet of glass instead of plastic. The glass is rigid, so it needs no support structure, and it can serve as its own light-guide plate, resulting in even thinner TVs. Nanosys says we should see QDOG TVs—most likely edgelit—in the second half of 2018, but there are no specific product announcements at this time.
Even more exciting is the possibility of replacing the red and green color filters in LCD and even OLED TVs with red and green quantum dots. This idea is called quantum-dot color filter (QDCF). The blue color filter would be replaced with a clear scattering layer that allows the blue light from the LED backlight to pass through while scattering it to emulate the emission pattern of the red and green QD subpixels. According to Nanosys, this could yield up to three times more peak luminance and greatly improved viewing angles.
However, the QDs must be very tightly packed so that no blue light passes through the red and green subpixels. This patterning can be achieved using photolithography or an inkjet-printing process. (For more on inkjet printing QDs, click here.) The photo at the top of this article shows large-scale inkjet-printed red and green QDCFs along with a blue-scattering area. Using this technique, Nanosys has created a 49″ panel with 4K/UHD resolution, which corresponds to a pixel size of about 300 microns, and sizes down to 20 microns are certainly possible.
According to Nanosys, we should see QDCF displays in the second half of 2018. I was told that FALD (full-array local-dimming) prototypes were being shown in private demos at CES, but I was unable to find them.
Quantum dots can also be used in conjunction with microLEDs by coating some of the blue LEDs on a wafer with red or green QDs. Conventional red, green, and blue LEDs differ in terms of drive voltages and degradation profiles, but QD LEDs eliminate these problems, since they are all blue LEDs at their heart. This approach can be applied to microLEDs as small as 3 microns, which would allow smaller ultra-high-resolution displays and—perhaps most importantly—VR headsets.
In addition to being photo-emissive, quantum dots can also be electro-emissive (aka electro-luminescent)—that is, they can emit light when subjected to an electric voltage. A display based on quantum-dot electro-luminescence (QDEL) is fully emissive like OLED, with no backlight, LCD layer, or color filters. That means perfect blacks, wide color gamut, and large viewing angles as well as super-high peak luminance.
This is a photo of Nanosys’ first demonstration of electro-emissive red, green, and blue quantum dots. The camera could not capture the true color of red being emitted; it was much redder than the photo depicts.
QDEL displays are a bit further down the road. Nanosys thinks we might see actual products in the 2021-2023 timeframe.
Clearly, video-display technology is not languishing. There are many fascinating technologies on the horizon that promise ever-better images. It’s a great time to be a video geek!