What’s Next In Mobile Displays

Creating screens that can be folded and rolled isn’t so simple. Here’s why.

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The next wave of smartphones and wearables is invading the market. These systems will feature a new class of high-resolution displays, and in the near future displays will become foldable and rollable, although there are still some challenges with this technology.

To be sure, mobile display technology is advancing on several fronts. On one front, for example, Apple and other systems vendors continue to push the limits for traditional LCD display technology for mobiles. In fact, Sony has taken LCDs to new heights by recently introducing the world’s first smartphone with 4K, or ultra-high definition (UHD), resolution.

Generally, though, mobile OEMs are moving toward a rival display technology called organic light-emitting diodes (OLEDs). For this, Samsung is leading the OLED charge, while China’s OEMs are also embracing the technology. And Apple, which uses an OLED display for the Apple Watch, plans to move from LCDs to OLEDs for the iPhone by 2017 and 2018, according to analysts.

OLEDs use a series of thin, light-emitting films, which enable brighter displays than LCDs. And unlike LCDs, OLEDs are flexible. In fact, in the next two to three years, OEMs hope to offer smartphones with OLED-based mobile displays that are actually foldable, rollable and stretchable.

For example, a futuristic OLED display could be folded like paper and easily carried. “You can unfold this type of display. Then, all of a sudden, your phone can become a tablet,” said James Xiao, vice president and general manager of the CVD & EPG Division at Applied Materials’ AKT Display Business Group. “That will create a lot of new applications and market segments for displays.”

Still, OEMs and display makers face some challenges with current and future OLEDs. Compared to LCDs, OLED displays are expensive and difficult to make. In fact, it’s difficult to build large-screen OLED TVs and sell them at a competitive cost. On the other hand, OLED manufacturing costs are dropping for small-sized displays, making them ideal for automotive and mobile applications.

The problem? There is a shortage of OLED fab capacity. Today, Samsung, which makes OLED displays for its own mobile products, also devotes 30% of its OLED fab capacity to other OEMs. Another company, LG Display, has a similar strategy.

In addition, a growing number of display makers from China, Japan, Taiwan and the U.S. are also developing OLED displays. But beyond LG and Samsung, it will take time before other vendors ramp up their OLEDs with good yields.

So today’s OLED fabs can support the current demand in the market, but it’s nowhere near enough to handle the enormous requirements for Apple. “Currently, the OLED capacity is not enough for Apple’s iPhone now,” said Jennifer Colegrove, chief executive of Touch Display Research, a market research firm. As a result, Apple won’t offer an iPhone with an OLED display until the 2017/2018 time frame, Colegrove said.

Why OLEDs
Display makers are scrambling to enter the OLED market, and for good reason. LCD technology still dominates the display landscape, but the competition is fierce and LCD vendors are making little or no money in the business.

In contrast, OLEDs are higher-margin products that are growing faster than the overall display market. The OLED display market, including both mobile and TVs, is expected to reach $13.7 billion in 2016, up from $12.9 billion in 2015, according to IHS.

Besides TVs and mobile, OLEDs are moving into other areas. “We will strive to secure new growth engines by reinforcing technical development of new applications (for OLEDs), such as transparent (TVs), gyros, head mounts and automotive displays,” said Chang Hoon Lee, vice president at Samsung Display.

LCDs and OLEDs are different technologies. An LCD consists of a backplane. That backplane consists of thin-film transistors (TFTs), which determine the resolution of the display.
The transistors sit on a glass substrate. On the bottom of the substrate, there is a backlight. On top of the transistor backplane, there is a liquid crystal layer, followed by a red-green-blue (RGB) color filter and a polarizer.

In operation, a voltage is applied to the liquid crystals. This impacts the transmittance of the panel. It also changes the quantity of light that passes from the backlight to the front display, according to Japan Display Inc. (JDI).

A big problem with high-resolution mobile LCDs is power consumption, which has increased two-fold over the last three years, said Akira Sakaigawa, general manager of the Advanced Technology R&D Department at JDI. JDI was formed in 2012, when Hitachi, Sony and Toshiba combined their small-sized display units.

To help solve the power problems, JDI has devised a technology that introduces an extra white sub-pixel into the RGB pixels in the LCD. This boosts the transmittance and reduces backlight power consumption by as much as 40%.

JDI and other Japanese companies also are developing OLEDs. “OLEDs are very difficult to make,” Sakaigawa said. “OLEDs need two or three transistors per pixel. LCDs need just one. Mass production capabilities are also limited for OLEDs.”

Unlike LCDs, an OLED is a series of organic thin films between two conductors. On a transistor backplane, the OLED consists of the following layers in order from bottom to top—an anode; organic layers; a conducting layer; an RGB emissive layer; and a cathode.

“OLED is a completely different device,” said Max McDaniel, director and chief marketing officer for the Display Business Group at Applied Materials. “You get rid of the backlight with OLEDs. And you have a red, green and blue OLED material that is deposited on the glass. When you put a current through it, it lights up all by itself.”

In LCDs, the manufacturing process is mature and inexpensive. In contrast, the OLED manufacturing flow is complex. And the OLED materials are expensive, particularly the emitter layer. “Another challenge is how to make very long life times for the phosphors, especially the blue color OLED material,” Applied’s Xiao said.

In the basic OLED manufacturing flow, the transistors are manufactured on a substrate. Then the substrate is moved into a separate manufacturing flow in an OLED fab. Basically, the OLED flow involves three main steps—deposition, patterning and encapsulation.

One common method to make OLEDs is to use a vacuum evaporation process, sometimes called vapor deposition, with a shadow mask. Basically, a mask or template is placed on the substrate of the OLED. Then, the RGB emitter materials are heated in a chamber and condensed on the substrate. The materials appear on the substrate not covered by the mask.

“The patterning is done by evaporating the OLED materials through a fine metal mask,” Applied’s McDaniel said. “This is a 1:1 physical mask. Then you have to overlay that. So you do red with a physical mask. When you want to do green, it has to be shifted (to another location). You line up the green with the pixel right next to it. And when you do the blue, it’s a separate step.”

This process is sometimes inefficient, however. Some of the critical materials are wasted. Patterning the emitting materials in a precise manner is difficult.

For this reason, display makers are also using an assortment of competitive processes, such as small mask scanning, inkjet and nozzle printing. Each of those has their advantages and disadvantages.

Thenthe OLED undergoes arguably the most critical step—thin-film encapsulation (TFE). The entire OLED structure must be encapsulated. This is because OLEDs can’t be exposed to water or oxygen; they are also vulnerable to contamination and temperature.

There are several ways to encapsulate an OLED, such as ALD, PECVD, inkjet and others. For this, the idea is to form multiple barrier layers with silicon nitride films.

What’s next?
Meanwhile, the first OLED displays were based on a rigid glass substrate. While rigid-based OLEDs are still being shipped, the new mobile devices from Samsung and others are based on plastic substrates.

Plastic substrates are more flexible than glass. They allow the display to curve and bend. “There were significant technical challenges in developing plastic substrates,” said Mike Hack, vice president of business development at Universal Display Corp., a supplier of OLED materials. “That relates to encapsulation. Developing flexible barrier layers is a challenge. That challenge has been met.”

The next big thing is to make OLED displays that are actually foldable, bendable and rollable. For this, the challenge is to devise a new class of technologies, especially barrier films that are even more flexible. “The next thing is to get those barrier films to bend in a fairly tight radius of curvature,” Hack said. “It’s not just the OLEDs that have to do that. It’s also the module. That’s an OLED with a touch function and circular polarizers. Every layer in the stack has to bend and fold. And then it has to be protected. So, getting this whole stack, with a mechanical barrier that protects it and is foldable, is not trivial.”

There are other requirements. For consumers, a foldable OLED display must withstand at least 100,000 fold–unfold cycles without breaking or degrading the mechanism, according to Cheng-Chung Lee, a researcher from Taiwan’s Industrial Technology Research Institute (ITRI).

As a result of those and other technical challenges, true foldable OLED displays are not expected to ship for at least another two to three years.

Still, a number of entities are working on the technology. According to the Semiconductor Energy Laboratory (SEL), there are two basic processes to make a true foldable OLED display—direct and indirect.

One type of direct process is based on a roll-to-roll technology. Fraunhofer, for one, has developed a roll-to-roll system that can make foldable OLEDs within a single vacuum-based unit. A key to the system is the encapsulation process within the flow. Using a reactive dual magnetron sputtering technology, the system can deposit multi-layer encapsulation barriers, based on zinc-tin-oxide layers and hybrid polymer interlayers.

Meanwhile, Japan’s SEL has developed an indirect manufacturing process. In the lab, the company devised several OLEDs, including an 8.67-inch foldable display with an in-cell touch sensor, according to Kazunori Watanabe, a researcher at SEL.

SEL’s indirect process flow starts with two separate glass substrates. The TFTs and the OLED are on one substrate. The sensor and color filters are on the other. In the flow, the two substrates are attached. Then, the upper and lower glass sections are removed. Finally, flexible substrates are attached on both the bottom and top portions of the structure.

There are other approaches. For example, ITRI has developed what it calls the Flexible Universal Plane (FlexUP) technology. Using a debonding technology, ITRI developed a six-inch foldable OLED integrated with a flexible touch sensor.

To be sure, the industry should keep a close eye on these and other developments. Using the same manufacturing techniques, the industry is also developing flexible solar cells and bendable LED lighting products. Then, of course, there are large-screen OLED TVs, which may one day take off from the runway.