Materials Matter
Developments in materials and materials processing are the foundation on which flashier advances in downstream technologies are built. That has certainly been true over the entire history of display development, and it continued at SID’s Display Week 2014, held last June in San Diego.
by Ken Werner
THREE OF THE FOUR most important driving categories at Display Week this year (OLEDs, quantum dots, and backplane technologies) have been highly visible for several years, but the devil is in the details and in the progress developers have made. At the show, there were lots of devilish – and angelic – details to capture the imagination. The fourth important driver, technologies for replacing ITO, has been less in evidence but is rapidly gaining momentum, with ITO-replacement technologies finding their initial high-volume application in touch panels.
Quantum Dots
To briefly recap, quantum dots are the elegant realization in an engineered material of the quantum wells that physics majors have learned about for decades. Unlike phosphors, it is the size of quantum dots – not their chemical composition – that is the main determinant of their behavior. And the behavior that is of primary interest in LCD backlight design is that incoming light from blue LEDs can be converted by quantum dots of the appropriate diameters into green and red light with very narrow spectral distributions. The result is an RGB spectrum that looks much like that produced by an OLED display and gives the LCD similar properties of high contrast, saturated colors, and wide color gamut.
It is worth noting that although an RGB spectrum is what is now being used for all commercial quantum-dot applications, it is possible to produce quantum dots that emit at virtually any wavelength that is longer than the blue used to excite the dot. So, Sharp, for example, could add quantum dots to the mix that were sized to produce yellow, thus enabling a quantum-dot-enhanced version of Sharp’s Quattron (RGBY) display.
The two major players in quantum-dot materials at the moment are Nanosys, which supplies the materials to 3M for its Quantum Dot Enhancement Film (QDEF), and QD Vision, which supplies the dots for its own glass-encapsulated Color IQ optical element.
At Display Week, 3M and Nanosys showed new ASUS notebook PCs with 15.6-in. 4K × 2K quantum-dot-enhanced screens with a visually striking color gamut (Fig. 1). This is the first high-volume notebook product using 3M QDEF, and 3M sees the product’s introduction as a milestone. We are not used to seeing high-resolution wide-gamut images such as those on the ASUS at a short distance, and the effect is so striking that it is hard to look away from the screen. This is a display that could easily sell the product of which it is a part, and it is a fine example of quantum dots’ ability to provide dramatic increases in display impact at a very small increase in cost. 3M also showed a 65-in. QDEF-enhanced panel that produced 85% of the new Rec. 2020 gamut. The company’s models predict it is possible to get to 92% of the UHD gamut.
Fig. 1: This 4K QDEF-enhanced display appears in a new ASUS notebook PC and was shown at the 3M booth at Display Week. The high resolution and wide color gamut of this image cannot be appreciated in the printed version of this image or in a version seen on a conventional display. Photos courtesy Ken Werner.
Until this year’s Display Week, most industry analysts would not have thought of quantum dots as an enabler for high-dynamic-range (HDR) consumer television. Indeed, a year ago, HDR was not on anybody’s stated feature list for advanced-generation television. Now, all major TV makers are at least mentioning it. In his Display Week keynote address, Kazumasa Nomoto (Senior GM, Sony Display Device Development Division) specifically mentioned HDR as part of the envelope of future TVs, and at CES this past January, Sharp showed an HDR technology demonstrator using Dolby technology.
And that brings us to the combination of quantum dots and HDR that was demonstrated by Nanosys and Dolby in Nanosys’s booth (which won a Best-in-Show award in the Small Exhibit Category). Nanosys showed two TV sets side by side. One was a conventional LCD TV; the second incorporated both QDEF and Dolby’s HDR technology, and that image was compelling. But what does QDEF have to do with HDR?
Nanosys’ Jeff Yurek told me that QDEF turns out to be a critical tool in reducing the cost and power consumption of HDR, factors which have until now limited the technology to very expensive professional monitors. Dolby’s current technology uses 2300 red, green, and blue LEDs in a full-matrix backlight array. Controlling the LEDs in clusters, known as local-area dimming, is the key to HDR, and Dolby is currently controlling the LEDs individually. Some of the LEDs are very inefficient, with lots of heat generated and very high power consumption.
However, if designers employ only efficient blue LEDs and use QDEF to convert the blue light to red and green, the power consumption would be reduced. For consumer television, it is likely that the number of LEDs would be sharply reduced as well.
Two other quantum-dot companies are not yet in commercial production for displays but deserve watching. Nanoco Group gave a presentation at the SID/Cowen and Company Investors Conference at Display Week and discussed the company’s cadmium-free technology. (Nanosys and QD Vision’s dots have cadmium selenide cores. Some people have objected to the production of quantum dots on the basis of the well-known biological hazards of bulk cadmium, but it is not clear that encapsulated CdSe in a quantum dot poses any biological hazard.)
In early 2013, Nanoco announced that Dow Electronic Materials would have exclusive worldwide rights for the sale, marketing, and manufacturing of Nanoco’s dots for electronic- display applications.
At the time of Display Week, Quantum Materials Corp. (QMC) announced that it had received its higher-capacity equipment and would be launching volume production of its tetrapod quantum dots (TQDs) several months ahead of schedule. The announcement read that “production-phase equipment is capable of producing 250 kg of quantum dots per annum.” Early in August, the company followed up with an announcement that it had achieved a 95% quantum yield with dots produced in an automated production system.
QMC claims greater efficiency and consistency of application with tetrapodal (as opposed to spherical) dots. Although the dots’ tetrapod shape could be exploited to produce different intensity or color of emission in different directions, isotropic emission is the goal for display applications.
To sum up, many new quantum-dot-enhanced products to be introduced this year will have Nanosys and QD Vision quantum dots inside them, but do not forget to keep an eye on Nanoco and QMC.
OLEDs
The 30,000-ft. view of the OLED situation had not changed much for the last couple of years. Smallish OLEDs for smartphones, made mostly by Samsung Display and mostly for Samsung phones, were (and are) a success. Large OLEDs for television were a niche product due to expensive manufacturing, low yields, and consequent high prices. Tablet-sized OLEDs were in an unprofitable no-man’s land.
But things are changing. The Samsung technology that produced what Ray Soneira of DisplayMate called the best display he had ever seen in a smartphone has now been introduced in commercially available tablets.
One would think that Samsung’s long-held position as king of the small-OLED hill would have invited some serious competition by now, but the modest scale of activity in China and Taiwan has not been enough to make an appreciable dent in Samsung’s dominance. Now, however, a significant Chinese player is making a major investment in AMOLED manufacturing.
At Display Week, Tianma was showing several good-looking AMOLED prototypes, including a 4.3-in. display with 480 × 800 pixels, on-cell touch, 250 nits, and a claimed contrast ratio of over 10,000 (Fig. 2) and a 5.5-in. display with 720 × 1280 pixels, 4-lane MIPI interface, 200 nits, and that same claimed a contrast ratio of over 10,000.
Fig. 2: Tianma’s cell-phone-sized AMOLED prototype was on display at Display Week in San Diego. Photo courtesy Ken Werner.
Tianma’s Stephen Liu told me that the company is now building a Gen 5.5 AMOLED fab in China and expects to be ramping up commercial production of smartphone and tablet sizes in mid-2015.
It’s worth saying that Tianma’s exhibit area at Display Week was decidedly major league and fully competitive in design and presentation with those of its first-tier competitors. Tianma combined its China-based Tianma and U.S.-based Tianma-NLT teams to design the booth and select the products and technologies to be presented. Tianma and NLT personnel said they worked hard and long on the exhibit and were very pleased with the result.
On the larger side, LG Display has announced substantially improved manufacturing yields for TV panels, and Tier 1 OLED-TV makers have announced prices that are so sharply reduced that a larger percentage of consumers in a (slowly) improving economy are likely to be tempted.
LG Display’s View of OLEDs and Backplanes
At Display Week, Changho Oh (SVP for LG Display’s OLED TV Development Division 1) told me that the company’s Fab 1 was producing panels for LG’s 55-in. OLED TV at a 70–80% yield. This is a remarkable improvement from what was widely estimated to be a 10% yield in the middle of last year and 50% early this year.
LG will be offering new 55-, 65-, and 77-in. curved OLED TV models this year. All new models for 2014 will be curved, although some holdover models will be flat. The panel yield will be different for each size, Oh said.
On the question of IGZO stability, Oh said very frankly that the oxide-TFT process has very narrow process margins and was difficult in the development stage. It is necessary, he said, to understand all of the characteristics and to be able to control them precisely. The situation with the OLED frontplane, he said, “…is not so difficult because we use WOLED,” referring to the white OLED process LG uses for its TV panels. He confirmed that most of the yield issues were related to the oxide-TFT process and the “very complicated backplane,” which uses three or four transistors per pixel (LG uses four). As a result, an extra power line must be designed into the backplane. Farther down the line, LG might consider using a different oxide. For now, the company has made its investment and is enjoying the fruits of its labors.
Oh agreed that the blue OLED lifetime remains on the short side. LG’s spec is a D6500 white point that does not vary by more than 500°C over 20,000 hours, which represents about 7 years of viewing for the typical consumer. Oh said this is a tough spec, but it is being met. He also said that an oxygen/ moisture barrier for TV-sized panels is not the problem that I had thought it was. LG uses a 0.1-mm metal sheet and tests the seal by bending the panel 20,000 times without any problems.
I asked about solution processing, which is widely regarded as the way to make OLED manufacturing costs competitive with that of LCDs. Oh confirmed that LG has a large research program in this area, and also that equipment and development are expensive. LG’s goal is to have solution-based panels available in 2018.
To date, LG has had the most experience and the most success with IGZO backplanes for OLED displays, but Sharp has taken the lead with IGZO for LCDs. Sharp is not stopping with LCDs, though. For some time, the company has been working with the Qualcomm subsidiary Pixtronix on using IGZO TFTs to drive Pixtronix’s in-plane MEMS shutter display. At Display Week, Sharp showed a very-late-state prototype of a 7-in. Pixtronix display with IGZO backplane (Fig. 3). The display will initially be marketed for industrial applications, said Sharp Senior Product Marketing Manager Dave Hagan, and it is likely we will see commercially available displays before the end of the year.
Fig. 3: Sharp used its IGZO backplane to drive this 7-in. Pixtronix in-plane MEMS display, which was exhibited in Sharp’s booth at Display Week. The entire display will be manufactured by Sharp, initially for industrial applications, and should be available this year. Photo courtesy Ken Werner.
ITO Replacements
New transparent conductors to replace the industry standard but unloved ITO were intended to be a featured topic at Display Week. But the only regular session devoted to touch sensor materials contained a mere three papers, two devoted to metal mesh and one to Canatu Oy’s Carbon NanoBud material.
The Exhibitors Forum session entitled “Transparent Conducting Films” contained six presentations: Carestream’s FLEXX silver nanowire, NanoWeb’s submicron metal mesh, Cima NanoTech’s self-assembling nanoparticle technology, Canatu’s Carbon NanoBud (again), Daido’s sputtering targets for barrier layers and copper interconnects, and Oxford Advanced Conductors’ silicon zinc oxide as an ecologically sensitive substrate for ITO. Not mentioned, as far as I could determine, were graphene or organic semiconductors, which are getting lots of attention elsewhere.
For the moment, self-assembling silver nanowires and silver metal mesh are the ITO replacement technologies that have made an impact in commercial products.
Materials Make the Difference
It is clear that developments in display materials and materials processing are being energetically pursued, and we have not even discussed developments in OLED and LCD materials themselves. As long as those developments continue, we can look forward to significant improvements in display panels and display-centric products. •