At the International Display Workshop trade show this week in Sendai, Japan, quantum-dot leader Nanosys and printing-ink giant DIC are announcing a breakthrough that will soon spawn the next generation of QD-based displays. A new process will allow quantum-dot materials to be inkjet printed in finely detailed patterns, leading to QD color-filter replacement (CFR) in LCD and even WRGB OLED TVs.
In today’s quantum dot-based TVs, light from blue LEDs hits a polymer film embedded with QDs that absorb some of the blue light and emit red or green light. When the red and green light from the QDs combines with the unabsorbed blue light from the LEDs, the result is white light, which then proceeds through the LCD layer and subpixel color filters to form the final image.
In a “conventional” QD backlight, blue light from LEDs stimulates the quantum dots in a film to emit red and green light, which combines with unabsorbed blue light to form white light.
Using QDs in the backlight of LCD TVs allows a wider color gamut than white LEDs can manage, and the spectrum exhibits less overlap between the peaks of red, green, and blue. However, the wavelengths of the QDs and the blue LED must match the color filters very closely to avoid a significant drop in light output.
At CES last January, Nanosys demonstrated that, instead of having red and green quantum dots randomly distributed in a polymer film, they could be segregated into microscopic patterns—specifically, the same pattern as the subpixel color filters found in LCD and WRGB OLED TVs. The point is to replace the red and green color filters with tiny QD cells. The blue filters would be replaced by transparent cells that allow light from the blue LED backlight to pass through unimpeded. This would avoid any interference between the QDs and color filters, greatly increasing the efficiency of each color.
In the photo-emissive color-conversion process, the red and green subpixel color filters are replaced by patterned QDs; the blue color filters are replaced by a transparent cell that simply passes light from the blue LED backlight.
Back in January 2017, the company was talking about such quantum-dot patterning using photolithography. But in collaboration with DIC, it can now be accomplished using an inkjet-printing process, which is much less expensive. However, this required technical breakthroughs in several key areas. For example, Nanosys has designed its latest cadmium-free quantum dots to be compatible with DIC’s materials and inkjet-printing processes. Also, DIC has created new inks that can be used in a wide range of print heads and tuned for UV and thermal curing, giving display makers flexibility to deploy the materials in different manufacturing lines.
Another challenge is the density of the quantum dots. Large QD films in backlights must allow some of the LEDs’ blue light to pass through the film so it combines with the red and green light from the quantum dots. However, in a color filter-replacement scenario, the QD subpixels must absorb virtually all the blue light so their color is very pure with no blue component, which means the density of QDs must be much higher.
The image at the top of this article is a photomicrograph of an inkjet-printed CFR array with complete pixels measuring about 300×300 microns, which is roughly equivalent to the pixel density of a 50″ UHD TV. The blue subpixels are essentially clear, allowing the blue light from the LED backlight to pass through. They do have some scattering materials so the blue light scatters in much the same way as the red and green light from the QDs.
Color-filter replacement (CFR), also known as color conversion, offers many benefits. Among them is greater power efficiency, which translates to as much as 300% higher brightness. Other benefits include a wider color gamut and 180-degree viewing angle.
Inkjet-printed quantum dots will also hasten the development of electro-emissive QD displays, in which quantum-dot subpixels emit light directly under electrical stimulation—no backlight needed. This is still a few years away from commercialization, but the Nanosys/DIC announcement paves the way for this exciting development.