figureThe world’s first commercially available flexible and transparent display panel from LG Display, launched in June, is a 77-inch prototype that could be used for signage or TV displays on windows. [LG Display]

Imagine a car window in a self-driving car that is your phone interface, your map and your GPS—all voice-activated and transparent. Imagine eyeglasses that can lead you along a trail, identify a bird, and monitor your heart rate, hydration and blood oxygenation. Imagine a work surface or a wall that forms one big touch display, even around uneven edges and corners, and that can be retracted into a roll on one end with a gesture. These are the dreams of display makers today, shaping a future we would barely recognize.

This year marked the 30th anniversary of the unveiling of the first organic light-emitting diode (OLED) in 1987 by Kodak scientists Steven Van Slyke and Ching Tang, an advance that moved displays into the realm of vibrant-color thin films—a far cry from the thick, bulky boxes of that time, with their comparatively dim images. The invention of OLED technology launched dreams of transparent displays, wallpaper lighting and curved, flexible displays, all of which are now nearing commercial fruition. The story of how far displays have come in just the past few years is an incredible one. But even more mind-bending is the vision of how our world might be displayed in years to come.

What are OLEDs?

Organic light-emitting diode (OLED) devices are monolithic, solid-state structured arrays, typically constructed from layers of organic thin-film materials, including an emissive layer patterned in an RGB pixel structure that produces various colors of light. The organic films are sandwiched between two thin-film conductive layers (an anode and cathode) that, when driven by electric current, cause electrons to migrate into “holes” or empty charge carriers in the pixel layer, where they recombine to form excited electrons (excitons). When excitons relax to a lower energy state, they can emit photons or heat.

OLED-based displays manufactured in the last decade are typically active-matrix OLEDs (AMOLEDs), which use a thin-film transistor (TFT) array to control the amount of current flowing to the desired pixels, and therefore their brightness, to create an image. Some OLED designs produce white light in the emitting layer and use RGB color filters to create color images.

[Illustration by Phil Saunders / Adapted from Universal Display Corp.]

A shift to OLEDs

OLED technology, with its mass-market potential, is the biggest new trend to hit displays since liquid-crystal displays (LCDs) and plasma. In just the past few years, the display industry has shifted much of its focus from LCD to OLED displays for the next generation of mobile phones, tablets and TVs. According to a market research report by IDTechEx (Cambridge, U.K.), the market for all types of OLED displays could grow from $16 billion in 2016 to $57 billion in 2026. And a study by IHS Markit (London, U.K.) forecasts that flexible active-matrix OLED (AMOLED) displays, mostly supplied by Samsung Display, might account for more than 50 percent of display shipments starting in 2020.

OLEDs, in contrast to the ubiquitous LCD display, can be manufactured on very thin, flexible plastic substrates—an advantage that could someday make glass displays, with their penchant for shattering, a thing of the past. OLEDs also have a big edge over LCDs in contrast and color gamut, with a more vivid range of color reproduced more accurately, faster, and with no motion blur.

Another advantage of OLED technology is that because it’s based on a thin-film emitter, the display brightness and color can be independent of viewing angle, unlike LCDs, and the pixels can be arranged on a variety of substrates of any shape and contour—for example, up a curved vehicle dashboard or around a column. OLEDs eliminate the need for any backlight or filtering system, and they consume less power (and thus generate less heat) than LCDs. And the ability to manufacture the organic emitting layers via printing on large substrates could well scale cost-effectively to high-yield mass production of OLED TVs.

The two most common techniques for manufacturing full-color OLED displays are RGB sub-pixel patterning, in which RGB light is emitted from separate sub-pixels, and white-emitting OLEDs (WOLEDs) with RGB color filters. In the RGB sub-pixel approach, vacuum evaporation equipment deposits the active RGB films into the RGB sub-pixel array in the display’s emissive layer. Makers of most high-resolution smartphone screens and some small and mid-size displays use this approach.

The WOLED approach also uses vacuum evaporation, but deposits unpatterned red-, green- and blue-emitting OLED layers on top of each other. As a result, the entire pixel produces white light, and the three separate sub-pixel colors are generated by passing the white light through red, green or blue color filters. WOLED technology is used in most of today’s OLED TV displays. Although WOLED TVs are considered some of the highest-quality TVs on the market, RGB OLED TVs are expected to have even better picture quality, and to be less expensive to manufacture.

The story of how far displays have come in just the past few years is an incredible one. But even more mind-bending is the vision of how our world might be displayed in years to come.

Getting larger

Over the past decade, numerous manufacturers have launched smaller devices such as mobile phones and tablets featuring OLED displays, albeit mainly on flat screens covered in glass. Small devices will likely be the first that are glass-free. In contrast, glass-based displays may remain the norm for a number of years to come for larger screens—especially large-screen TVs, for which the main challenge is making large screens affordable.

In 2009, LG Electronics (Seoul, Korea) invested in Kodak’s OLED portfolio, which encompassed decades of development with WOLED technology. That investment enabled LG to launch, in 2013, the first commercially available OLED TV—a 55-inch flat-screen only 4 mm thick. Almost simultaneously, Samsung Electronics (also based in Seoul) unveiled a 55-inch OLED TV with a slightly curved screen using RGB subpixel architecture, a unit available only in Korea. While LG seemed ahead of the game in commercial availability, OLED TV technology remained costly, debuting at US$10,000. What followed was an arms race among display makers to decrease cost, increase display size, and finesse the novel features of OLEDs.

Fast-forward to 2017. Today, display manufacturers and device developers feature so many new OLED devices that it’s hard to keep track of what’s a prototype and what’s actually in production. The manufacturers are neck and neck with announcements of OLED advances. In June, LG Display unveiled the world’s first 77-inch flexible and transparent OLED display panels, a move that reinforced its stronghold in large-scale OLED technology. The prototype displays boast an ultra-high-definition (UHD), 3840×2160-px resolution with 40 percent transparency and an 80 mm radius of curvature (the radius of the rolled device if it could form a complete circle). The ultra-lightweight, rollable thin panels, which operate while under flexion, are designed for eventual use in TVs, signage or other large-scale applications.

This year was an especially interesting one for large-sized OLED TVs, as several manufacturers have joined LG in unveiling OLED product lineups (although LG Display is reportedly the manufacturer of most the OLED panels in those units). At the International Consumer Electronics Show (CES) 2017 in January, Panasonic announced its 4K UHD 55- and 65-inch flat-screen OLED TV, the EZ950/EZ952, commercially available in Europe. At a price point of £3,000 (approximately US$3,500) in the U.K. and €3,499 (approximately US$4,100) in other European countries, the unit is not cheap—but it’s still one of the most affordable OLED TVs available.

“Acoustic Surfaces” and quantum dots

figureExquisite in beauty and price: The SONY Bravia A1E is an OLED-based TV that is available in display sizes up to 77”; like other 4K UHDs it offers better motion clarity, color production and homogeneity over LEDs—but at launch it was priced at US$20,000. [SONY]

SONY launched an ultra-high-end line of 4K UHD OLED TVs, named the Bravia A1E, with “Acoustic Surface” technology that uses actuators behind the TV to vibrate the entire screen. The ultrathin Bravia A1E is the largest of the A1E family, measuring 77-inch diagonally. It features over 8 million OLED pixels and sells for US$20,000. As with those of Panasonic, the OLED panels in the expensive Bravia A1E TVs are manufactured by LG. By all accounts, the motion refresh rate, lifelike color, and real-black, high-contrast reproduction of 4K UHD OLED TVs are exquisite—whether they justify the steep price tag or not.

Samsung, for its part, has redirected. In spite of its strength in OLED technology for smartphone displays, the company announced in January an ultrathin TV (in curved or flat models) based on quantum-dot LED (QLED) technology, a departure from its competitor LG and from OLED TVs. Instead of organic layers, QLED features nanometer-scale quantum dots of inorganic semiconductor material, typically cadmium selenide—still in thin-film layers, but actually backlit as with LCD displays.

QLEDs have the potential to be twice as power efficient as OLEDs, with similar color reproduction and ultrathin screens. And they offer up to 40 percent more luminance efficiency than OLEDs, emitting over 1500 cd/m2 with no color distortion, compared with around 1000 cd/m2 for other top-end 4K UHK TVs. The downside is that the backlight design suffers in deep-black accuracy and contrast ratio, some of the same limitations that plague conventional LCD displays. A 75-inch TV in the QLED series is priced at less than US$3,000. Remote devices are connected to the display by a single streamlined cable, so the curved QLED TV can be positioned as a work of art, either mounted cleanly to the wall or sitting on an available easel stand.

Whether the display is large or small, device makers are fully embracing OLED as the next-gen must-have technology. LG Display announced plans in late July to invest a total of KRW15 trillion (US$13.4 billion) in OLED production capacity through 2020, with KRW1.8 trillion (US$1.6 billion) of that earmarked for establishing OLED production in China. This move will be the first time that any Korean company has established an OLED manufacturing plant in China. The company reported an Q2 2017 operating profit of KRW804 billion (US$711 million)—a year-over-year increase of 1,712 percent!

figure(Left) The QLED TV from Samsung is thin enough to resemble artwork, and designed to mount on the wall or on an easel like a painting. (Top, right) Samsung’s prototype stretchable display can displace 12 mm to the front and back without losing much image resolution. (Bottom, right) The Samsung Galaxy S8 is the company’s third-generation smartphone with a curved-edge AMOLED display. Samsung

Mastering the manufacturing

Many quality issues surrounding OLEDs have largely been resolved in recent years. OLED displays don’t really suffer the dreaded “burn in” anymore, and life spans are competitive with those of LEDs. OLED devices remain very sensitive, however, to contaminants, particles and the slightest traces of oxygen and moisture. The processing environment can now keep these contaminants at levels below 1 ppm to ensure performance and high yield. But RGB pixel printing for large-screen formats remains elusive.

The prevailing technique for producing OLEDs, vacuum evaporation deposition, is a complex process that uses an evaporator to form the OLED device’s active films. Unlike many manufacturing processes, vacuum evaporation deposition does not use toxic solvents or chemicals; however, it still requires a TFT encapsulation (TFE) process to hermetically seal the sensitive active OLED emitting layers in smaller devices from oxygen and moisture. According to Mike Hack, vice president of business development at OLED technology developer Universal Display Corp. (UDC; Newark, Calif., USA), improving current OLED manufacturing processes to handle very-large-area TVs will depend on mastering manufacturing processes that use direct printing of RGB organic films on the sub-pixels.

Others concur. “TV display manufacturers see inkjet printing as the most viable approach for enabling pixel patterning for RGB OLED TVs, because it is simpler, more efficient, and most importantly, it is inherently scalable to the large glass sizes required for mass production,” said Jeff Hebb, vice president of global marketing at front-end OLED capital equipment provider Kateeva (Newark, Calif., USA).

In the late 1990s, researchers led by Stephen R. Forrest at Princeton University and Mark E. Thompson at the University of Southern California, USA, developed the first phosphorescent OLED, or PHOLED. Phosphorescence enables OLEDs to emit all of their energy as light rather than heat. The development of PHOLEDs eventually led to widespread use of OLED technology in today’s smartphones and TVs. The Princeton-USC team started with single dots that put out red and green light. Blue PHOLEDs followed in 2005. Forrest also invented a next-generation manufacturing technology to allow maskless printing for large-area TVs, a technology called organic vapor inkjet printing (OVJP) that’s currently being developed and commercialized by UDC.

To more efficiently make large-screen TVs, the OVJP process directly prints the RGB pixels side by side on large-area substrates with multiple side-by-side TVs, a process that potentially eliminates the need for an expensive fine metal mask. The OLED materials in the OVJP process are kept in controllable vapor form for precise application of high-resolution lines with segmented, independently actuated print heads. The technology has the potential, according to UDC, to transfer to many other applications beyond TVs.

To the same end, Kateeva’s approach to “RGB pixel printing” is based on its recent success with a TFE process in smartphones and other devices using their YIELDjet printing platform. This platform, which was designed by combining best practices from the semiconductor, flat-panel display and inkjet-printing industries, is also used for the RGB pixel printing system. It uses ultrafast drop metrology and “smart mixing” printing technology to accurately print the RGB active layers into the millions of sub-pixels on the substrate. The mass-production tools will be gigantic (collectively they’ll occupy a space as large as a football field), incorporating the pure-nitrogen processing that is required to prevent oxygen, moisture and particle contamination that could degrade the OLED device. Kateeva has pilot RGB OLED production tools installed at customer sites now.

figureThe next stage of OLEDs is mastering printing of RGB pixels. The YIELDjet inkjet printing platform from Kateeva has been involved in mass production of OLED devices since 2013, and will be the basis of a pilot-stage, next-gen RGB printing platform. [Kateeva]

Going foldable

Despite its switch from OLED TVs to QLED, Samsung still dominates in manufacturing small-screen OLEDs. In April, Samsung launched the Galaxy S8 smartphone family with a curved-edge (but not dynamically flexing) OLED display, its third-generation dual-edge screen. Meanwhile, Apple announced that the iPhone X, scheduled for pre-order beginning on 27 October, will feature a 5.8-in. edge-to-edge OLED display (made by Samung), Apple’s first in an iPhone. In May, LG announced a switch from LCD to OLED technology for its next premium smartphone, the V30.

However, while many smartphone OLED displays are now being manufactured on plastic substrates, all still have a glass cover. Everything commercially available as of 2017, whether flat or modestly curved, is still breakable. The holy grail at present is to be the first manufacturer to release a commercially available phone or tablet featuring a foldable, bendable, flexible OLED (FOLED) display, with a substrate and other parts that will bend together—in other words, an all-plastic display.

“The next wave of inflection in smartphone technology will be foldable phones, something the size of a tablet that you can fold in half or in thirds so it’s the size of a phone,” says Kateeva’s Hebb. “Every major display maker is in an intense race to see who can release it first. It’s only a matter of time before the first foldable phone hits the market.”

figure(Left) The flexible roll-out OLED display of the Universal Communication Device concept, from Universal Display Corporation. (Right) The transparent light origami prototype demonstrates how thin transparent OLED panels, when overlapping, combine colors in an additive way, and can be folded and bent into an endless variety of shapes. The panels remain clear when turned off. [Universal Display Corp.]

For years, key manufacturers have hinted at plans for a foldable tablet/smartphone hybrid, which folds along a single line or hinge. Market research firm Strategy Analytics (Newton, Mass., USA) predicts that foldable hybrids could be unveiled as early as 2019. But for futuristic concept displays like the “roll-out pen-sized tablet” or transparent origami lighting panels, the challenge is more than just the OLED panel itself, which is inherently bendable and foldable. The manufacturers also must design bendable, foldable backplanes, batteries and PCBs.

In May, at the Society of Information Display 2017 show, Samsung showcased the “world’s first” stretchable OLED display, a prototype 23 cm in diameter (the size of a tablet) that stretches as much as 12 mm forward (convex) and backward (concave) without losing much resolution. The company presented the stretchable display for its potential in Internet of Things devices, wearables, phones and cars. But stretchable OLED remains in its infancy, a long way from practical reality.

Indeed, mass production of foldable, rollable and stretchable OLEDs still has many challenges to overcome before our world resembles those of Blade Runner or Minority Report. Not only must the rest of the display materials bend or fold along with the OLED panel, but companies must ace complicated high-volume manufacturing to bring down cost.

“OLEDs are still in the early days,” said Hack of UDC. “But I think over the next ten years we’ll see a huge shift in how and where displays are used. We’re going to look back and say, Wow, those old rigid, breakable glass displays look like museum pieces.”

“This is truly,” sums up Kateeva’s Hebb, “the age of OLEDs.”

Valerie C. Coffey is a freelance science and technology writer and editor based in Palm Desert, Calif., USA.

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