However, the idea of using inkjet printing for quantum dot displays makes a lot of sense. Quantum dots are solution-processed semiconductors that can be sputtered. Remarkable work has been done in the area of quantum dot printing, but commercialization is not there yet.
This is changing, however. Nanosys, the leading manufacturer of quantum dots, reports success in the early stages of printing QD displays. Jeff Yurek, vice president of marketing for Nanosys, says quantum dot printing is closer than people think.
“It will be sooner than you imagine,” Yurek noted. “We’ve gone beyond the first lab prototypes and are starting to see some super cool work that looks a lot more like a real product. At the annual SID Display Week show last May, a few demonstrations of printed quantum dot display prototypes were shown on the floor and in private suites. Nanosys partner BOE showed off a 55-inch 4K emissive quantum dot TV made using inkjet printing.
There has always been the idea that printing screens would be a significant benefit to screen manufacturers.
“Display manufacturers have had a long interest in printing,” Yurek said. “There were efforts to try this many times over the years, but printing never took off as a mass production process. There were several technical challenges – printing was difficult to do both with precision and with the throughput necessary for mass production on the massive scale of the display industry.
“So why now? First, printing technology has improved dramatically, but I think, more importantly, quantum dots have provided the industry with a way to create a near-perfect display. This is an exciting reason to review the print,” he added.
Yurek observed that quantum dot (or QDCC) color conversion on blue OLED, sometimes referred to as “QD-OLED”, is the first printed application of quantum dots to be commercialized for displays.
“QD-OLED brings together two incredible technologies and takes the benefits of both – perfect contrast for OLED and lifelike colors for quantum dots – to new heights,” said Yurek. “It’s a very different visual experience.
“When I first saw a QD-OLED late last year, I knew a new bar had been set. It’s unlike anything that’s come before” , he added. “We’ve seen screens with perfect contrast, accurate colors and lifelike brightness, wider viewing angles, etc. But we’ve never really seen all of these attributes in one. only display product.
“It’s exciting to see the first QD-OLED displays already hitting the market this year with products from Samsung, Sony, Dell Alienware and MSI. This technology is a big step into the future of low cost printed emissive displays that we are all heading towards.
Inkjet printing is used to print QD-OLED. Yurek pointed out that in a QD-OLED display, a layer of color-converting active red and green quantum dots are printed on top of a blue-emitting OLED layer. Red and green quantum dots actively convert this blue light to create the precise RGB color at each pixel. This efficient color conversion, combined with the color purity of the light emitted by the quantum dots, results in a brighter, more colorful display that consumes less power.
“The first step in producing a QD-OLED TV is to create a blue-emitting OLED ‘backlight’ on top of a backplane that controls the brightness of each pixel,” Yurek noted. “This OLED layer is produced using standard OLED evaporation processing. It covers the entire screen with blue light emitters.
“This is where quantum dot printing comes in. An industrial scale printer precisely prints quantum dot inks in tiny sub-pixels onto another glass substrate which will then be bonded to the blue emitters.”
Several challenges must be overcome to do this well.
“First of all, the printing process has to be incredibly precise,” Yurek said. “A standard 4K TV, regardless of size, has nearly 8.3 million pixels. Each of these pixels is made up of three sub-pixels, one for red, green and blue, for a total of 24.9 million light-emitting dots.
“The printer must also be fast. Today, QD-OLED is produced in “Gen 8” scale, which in display industry lingo means huge 2.2 x 2 “mother of glass” pieces .5 meters which will eventually be cut into 65″ and 55″ televisions. Typical display factories produce tens of thousands of parent glass sheets per month. You start to do the math and you realize that the printheads have to move really fast to achieve that kind of throughput, even if you have multiple lines running.
Still, the benefits of screen printing using quantum dots make the process ideal for inkjet printing.
“One of the unique things about quantum dots is how they’re made,” Yurek said. “Quantum dots are solution-processed semiconductors. This makes them incredibly versatile. They can be formulated with solvents, monomers, polymer films, inks, photoresists, etc. They are therefore ideal for printing applications.
“Unlike traditional semiconductors, which are typically grown in the vapor phase and on atomically flat wafers, quantum dots can be produced at scale in chemical processing equipment,” he added. “This allows us to manufacture precise optical transmitters at an extremely low cost compared to traditional semiconductor fabs. For comparison, if you were to create quantum dots on wafers, as traditional semiconductors are made, it would take over 50 square meters of wafer area to make just 1 gram of quantum dots.
Yurek noted that some breakthroughs in quantum dot development were also needed to make QD-OLED a reality.
“We have worked closely with ink manufacturing partners to make our quantum dots compatible with printing inks,” he said. “Quantum dots are incredibly small. Unlike all other phosphorescent materials, the smallest of which has a particle size on the order of microns, QDs are molecular in size, which means they can easily flow through jet nozzles. ink as part of the media “ink” that is printed.
“QDs are literally attached to the ink of the medium, and they in turn provide incredible color conversion capability,” Yurek said. “We are able to make the QDs compatible with the carrier ink so that the ink molecules attach to the QDs and the whole group comes out beautifully from the IJ nozzle, so the dispense volume and accuracy QDs in this drop-to-drop volume, etc. is very precise.
Yurek noted that quantum dot printing offers compelling advantages over lithography techniques used in traditional display fabrication.
“Basically, if you only drop materials where you need them, you can use fewer materials, simplify the production process, and reduce your BOM,” he said. “If you look at the process of making color filters for today’s LCD and WOLED TVs, both use photolithography to pattern the red, green and blue filters on the glass. Photolithography requires multiple steps, essentially covering the entire screen surface with each color, etching the part you don’t need, and starting over with the next color. Really, you’re wasting two-thirds of your hardware in this process.
When it comes to the next steps in terms of using inkjet printing for displays, the use of quantum dots will be key.
“QD-OLED has already been commercialized and the color conversion story is going to continue to be interesting as QDs are integrated into other types of displays in the short term,” Yurek said. “We are seeing tremendous activity and interest in quantum dot patterning on top of microLEDs as well as OLEDs. In microLEDs, printed QDs can help solve some of the manufacturing challenges of selecting and placing millions of flawless R, G and B pixels.
“Beyond color conversion, there is a true emissive quantum dot display,” Yurek concluded. “We call this technology NanoLED and it’s sort of the holy grail of display technologies, with incredible performance from inorganic quantum dot emitting materials in terms of brightness, color and contrast. Most importantly, it offers extremely low cost since displays can be made entirely using solution processing. We can get rid of the expensive evaporation steps and large vacuum chambers used in display manufacturing today. So it’s incredibly exciting and it’s not too far. We’re still a few years away from full commercialization, but we’re making rapid progress, so stay tuned. »