OLED Technology Gives Displays New Flexibility

MICHAEL EISENSTEIN, SCIENCE WRITER

For generations, televisions were furniture — bulky appliances that were nearly impossible to ignore, even when turned off. But at this past January’s Consumer Electronics Show, LG Display pulled a dazzling disappearing act, with a 65-in. television that rolled itself up tightly when not in use, essentially vanishing from view.

A foldable transparent ‘origami’ prototype OLED display. Courtesy of Universal Display Corp.

The core technology behind such flexible screens — organic light-emitting diodes (OLEDs) — is now being explored by most of the world’s leading display manufacturers as a means for producing bright, high-contrast images on plastic screens that can readily be sculpted, bent, and rolled. “In five years, I think this notion of a flat display built on glass is going to seem obsolete, like legacy technology,” said Michael Hack, vice president of business development at Universal Display Corp. (UDC), a company that has played a major role in commercializing OLEDs over the past 25 years.

OLED displays can match or outperform the industry standard liquid crystal display (LCD) technology in many ways, but they remain considerably more costly. LG’s rollable TV could carry a whopping $60,000 price tag. But new manufacturing processes, including printing-based strategies, could make OLEDs more affordable and accessible and thus open up a wider range of applications for displays that can be compacted or custom shaped.

Organic alternative

OLED displays consist of layers of organic semiconductor material sandwiched between an anode and a cathode. The movement of electrons between electrodes through the semiconductor results in the release of photons, and the composition of this organic material affects both the wavelength of the emitted light and the efficiency of the photonic output. First-generation OLEDs were based on fluorescence, and they generally offered poor quantum yield, on the order of 25%. In contrast, most contemporary displays use phosphorescent OLEDs, a more efficient technology developed in the late 1990s by Mark Thompson of the University of Southern California and Stephen Forrest of Princeton University, with support from UDC. “We can convert almost 100% of the electrical energy into useful light,” Hack said.

A breakdown of the multilayered structure in a standard OLED display. Courtesy of Universal Display Corp.

According to Barry Young, CEO and president of the OLED Association industry group, contemporary OLEDs can deliver essentially comparable resolution to LCDs, which continue to dominate the display market overall. “What OLEDs offer that LCDs don’t is a much better contrast ratio,” Young said. “They also have a much faster response time, so you don’t get the ‘ghosting’ you might get with an LCD.” OLEDs can undergo a change in color or illumination state within microseconds, relative to the milliseconds required for LCD, resulting in a smoother viewing experience for fast-paced action such as sporting events or videogames.

But OLED TVs are also more expensive than LCD TVs and have captured only a relatively small portion of the market. According to Hack, roughly 1% to 1.5% of the TVs sold last year were OLEDs. However, this technology is flourishing in the mobile space, where OLEDs are used in high-end smartphone models including recent generations of Apple’s iPhone and Samsung’s Galaxy and Note devices. “In 2019, about 485 million out of 1.4 billion mobile units worldwide were OLEDs, almost a third of the market,” Hack said. Such displays have proven a particularly good match for mobile devices because they are thinner and lighter and can be readily shaped to include curved edges that maximize the phone’s active display area.

Flexible thinking

This display format is also ideally suited for producing truly flexible displays. While LCDs are generally thicker and more rigid, they can be produced in curved formats. Samsung and LG both launched curved LCD TVs in 2013, although these failed to take off. LCDs are not well suited for devices that can be actively folded by users. In contrast, OLEDs can readily be fabricated on flexible plastic substrates rather than glass. “This drops the thickness from around 0.5 mm to less than 0.1 mm,” Young said, adding that manufacturers have also developed ultrathin, scratch-resistant glass that provides robust protection without adding too much bulk. Hack noted that the OLED semiconductor materials themselves are generally flexible on their own, and that developing suitable substrates and coatings has been the biggest impediment to progress on this front.

But manufacturers can now achieve remarkable flexibility. “The industry is down to about 2 to 3 mm in terms of radius of curvature, and I think we’ll soon achieve 1 mm with the ability to withstand 100,000 flexes,” Hack said. There have been many demonstrations of the flexibility that can be achieved with such OLED screens, both as concept and as commercial product. Samsung hit a rocky patch with the initial commercial launch of its flexible OLED-based Galaxy Fold smartphone in 2019. This was partially because early-access users were unwittingly peeling off a critical protective layer from the screen, and also because of broader issues of device fragility. But subsequent launches from the company have proven far more successful, with heavy demand for the clamshell-style Galaxy Z Flip device that was released earlier this year. Computer makers are also exploring laptops that are essentially foldable tablets. For example, Lenovo plans to release its ThinkPad X1 Fold device this summer.

But these devices will remain out of reach of many users until prices fall further. For example, the Z Flip is among the most expensive smartphones on the market, priced at $1,400. “The deposition and encapsulation process for OLEDs is probably double what the capital costs for LCDs are, and obviously it’s a complex process,” Young said, although he noted that the material costs for manufacturing may be slightly less for OLEDs.

First-generation printed OLED displays. Courtesy of JOLED Inc.

Manufacturing is typically performed via the deposition of evaporated OLED materials onto the target substrate within a vacuum chamber. For smaller displays, manufacturers typically rely on metal masks to achieve sequential patterning of red, green, and blue OLEDs at each pixel. This is relatively inefficient, however, resulting in the loss of two-thirds of the OLED material during each round of deposition. This approach is also impractical for large displays, such as TVs. Here, the entire substrate is instead blanketed entirely with layers of red, green, and blue OLEDs to produce an overall white OLED output. Filters are then used to convert the white into the appropriate color at each pixel, a process that can result in some loss of brightness.

OLED displays on thin plastic substrates can be remarkably lightweight and flexible. Courtesy of JOLED Inc.

Targeted printing strategies could offer a cost-effective solution for achieving targeted OLED patterning in a less wasteful fashion. In 2013, Panasonic was first to demonstrate proof of concept for an inkjet-printed 55-in. 4K-resolution OLED television. In the ensuing years, Panasonic has joined forces with Sony and Japan Display to form a business venture called JOLED, with the aim of accelerating commercial development of printed displays. Rather than the powdered OLED materials used in standard fabrication, JOLED uses liquid OLED preparations developed by Sumitomo Chemical, which are then applied with a printer technology developed at Panasonic.

OLED TVs are more expensive than LCD TVs and have captured only a relatively small portion of the market.

JOLED has been producing small batches of printed monitors for medical devices from a pilot plant since 2016 but recently scaled up its efforts with the launch of the world’s first mass-production line for inkjet-printed OLEDs in November 2019. “We are looking to start mass production within this year,” spokesperson Megumi Sakuma said. “At maximum capacity, we can produce 20,000 glass substrates per month.” These will in turn be used to produce monitors ranging in size from 21 to 32 in.

Fit to print

Sakuma noted that this process is still relatively expensive, but has the potential to make OLEDs far more affordable in the long run. “Some research suggests the cost of the whole process will be reduced by about 20% to 30%,” she said. “This is because we don’t use a huge vacuum chamber, and the fabrication process is much simpler than evaporation.” Other companies are likewise exploring the potential of this approach. In April 2019, LG reportedly spent $175 million to acquire a portfolio of printable soluble OLED technology from DuPont. And in the booming Chinese display market, which has pushed past its Japanese and Korean competitors, companies such as BOE Technology and TCL Corp. have recently showcased demonstration models of inkjet-printed televisions.

Glass substrates being prepared for printing. Courtesy of JOLED Inc.

Inkjet still falls short of evaporation-based production in some ways. For example, this method generally achieves lower resolution in terms of pixels per inch. “We can produce 200 PPI, but RGB evaporation for smartphone displays can produce 400 to 450 PPI,” Sakuma said.

According to Young, this constraint arises because the OLED droplets must be sufficiently spaced out so that the solvent can evaporate while leaving behind appropriately distanced OLED spots. Higher resolution will therefore require smaller droplets of soluble OLEDs than can currently be produced. As an alternative, UDC is exploring another printing approach — organic vapor-jet printing. “We print from the gaseous phase, and we can use exactly the same materials that we use today in our regular evaporation-based system, but with no masks,” said UDC’s Hack. The company is still refining its approach with a pilot manufacturing line that can produce small-scale 15- × 15-cm printed substrates.

Flexible OLED screens, such as this pen concept, could roll up into far smaller forms than a typical phone. Courtesy of Universal Display Corp.

Flexible, roll-up televisions may wow attendees at electronics shows, but Young believes that it could be hard for OLEDs to make major headway in the TV market in the near term. “The display industry is totally saturated. We have more capacity than we have demand,” he said. But ready access to affordable, form-fitting, high-definition screens could also provide stimulus for creative new ideas and applications. These could include lightweight and comfortable displays for augmented reality, tablet-size screens that fold to pocket-size proportions, or environmental displays that replace windows in the interior of commercial aircraft. “I think it’s really exciting,” Hack said. “Over the next few years, I think we’re going to see innovative products that we didn’t even think of five years ago.”