This year’s Mobile World Congress (MWC) was expected to be about the rise of 5G, but a very different technology – foldable phones – seemed to dominate the news cycle, with the Samsung Galaxy Fold and Huawei Mate X (among others) launching. Devices with flexible/foldable display technologies were a major fixation for the press at CES 2019 too, drawing more than five percent of all coverage on the first day (Source: Meltwater News).

And whether you think folding and curved displays are a gimmick or the start of a big trend, the bold design has always been coveted. At last year’s MWC, Carolina Milanesi, a consumer technology analyst at Creative Strategies, said, “Trying to get someone into a store for something that looks the same as last year is difficult, even if it has new things to offer... [I]f it looks different you get consumers’ attention.”

Flexible displays as a product differentiator

Flexible displays have been around for several years in different forms – for example, flexible displays have been adopted in some smaller screen consumer products such as the Apple Watch and the Samsung Edge. Yet, as this technology develops to address the limitations of conventional glass-based displays, we are only at the beginning of discovering how flexible displays can transform product design and bring about new use cases.

It’s important to note that flexible screen displays are not just for mobile phones. Displays are virtually the last remaining flat surface on most electronic devices. The initial focus has been on smartphones, but now emerging low-cost, long-life variants of flexible plastic displays are set to enable seamless integration of screens into designs and launch new device classes.

Here we summarize and compare the two major classes of flexible plastic displays.

What defines flexible?

Before we begin, it’s important to define flexible. In this article, we use it to describe displays manufactured on plastic substrates that are flexible during the manufacturing process. These displays can be laminated onto a rigid cover glass and integrated into a fixed product housing. For displays used in malleable devices, we use the term foldable.

OLED or OLCD?

Two major classes of organic display technologies exist: flexible organic light-emitting diode (OLED), which has been in production since 2012 and is used in the Samsung and Huawei phones announced at MWC 2019, and flexible organic liquid crystal display (OLCD), which has been in development since 2014 and will enter production next year.

Some applications work better with OLED technology, others better with OLCD technology, but both technologies exhibit several advantages over the current generation of glass:

  1. Display thickness can be reduced to <100µm (vs. 400-800µm for glass)1.
  2. The weight of displays can be cut using either OLED or OLCD by a factor of ~10.
  3. They replace glass with more durable, shatterproof plastic.
  4. They allow for the removal of the bezel by using a curved edge or folded border.
  5. They can be curved around surfaces, enabling a new degree of design freedom.

However, there are also significant differences between the two technologies that allow them to be applied to different sectors, notably cost, resolution, longevity, and foldability. Here we look at how these key variables suit particular applications.

OTFT backplane
Source: FlexEnable


Cost


OLED

At $2,000, the cost of foldable phones (and therefore flexible displays) is getting significant attention, but not all flexible display technologies are expensive. Samsung’s Galaxy Fold will retail at $1,980. Huawei has set its Mate X price tag at $2,6002. This is double the price of the iPhone XS Max (Apple’s most expensive phone, which starts at $1,099), and Samsung’s Galaxy S10+ (which starts at $999).

While the difference is not all in the screen – the fold requires additional design features that take into account both hardware and software – the flexible OLED screen does add a significant premium. When the iPhone X was launched, Apple was paying $110 for the display module. A comparable foldable OLED screen would cost roughly double that.

While it’s feasible to create a larger screen based on such technology, the cost is prohibitive to all but exceptionally niche markets. For example, LG’s 65-inch roll-up TV from CES is speculated to cost up to $60,000 if/when it goes to market. However, consumer electronics’ history would indicate that costs will go down over the coming years.

OLCD

OLCD display manufacturing uses organic rather than silicon-based thin film transistors (TFTs). Organic TFTs do not require as high a processing temperature as silicon-based TFTs, so the manufacturing process for OLCD displays has a significantly lower cost.

Silicon-based TFTs need temperatures of 350°C (or even higher), and this requires special substrate materials, e.g., Polyimide (PI) films and handling processes, which together affect cost and yield. Conversely, for OLCD the maximum manufacturing processing temperature is <100°C, which removes the need for PI films and allows low-cost triacetylcellulose (TAC) film to be used as a substrate. To put this in context, flexible OLCD displays can be created for just a third the cost of flexible active matrix organic light emitting diodes (AMOLED) displays, broadening flexible display use to a wider range of applications.

Organic LCD with a 10mm bend radius
Source: FlexEnable

Resolution

The required pixel density of a display changes with both application and screen size. A six-inch phone display needs a pixel density of 490ppi to deliver 2K (2560×1440) and 368ppi for HD. This changes to 188ppi and 141ppi for a 15.6-inch notebook, or 53ppi and 40ppi for a 55-inch TV.

OLED

The Samsung Galaxy Fold display is 350ppi (2152×1536), and flexible OLED is capable of delivering even higher pixel densities, for example, 450ppi in the iPhone X. This makes OLED well suited to smartphones and similar small devices that require very high pixel density to play very-high-definition video. However, as per the above, the current high cost of these displays will limit their use to flagship devices where the cost can be tolerated.

OLCD

Flexible OLCD displays can achieve the same resolutions and pixel densities as amorphous silicon glass LCD displays (350ppi), allowing 2K (and higher) resolutions. 2K resolution meets the needs of in-vehicle displays, for example, while 4K matches those of 15.6-inch notebooks, monitors, TVs, and digital signage. For many other applications, such as smart home devices, lower resolutions (HD) are more than sufficient.

Brightness/display lifetime

The lifetime of an OLED display is closely linked to its brightness, with each doubling of brightness leading to a quartering of a lifetime, and exacerbating problems such as image burn-in.

OLED technology is ideal for cellphones, which have a global average upgrade cycle of approximately 28 months (IDC), and just 22.5 months in the U.S. (Canalys), and where the brightness can be lower. However, the technology is a poorer fit for applications such as automotive displays, which run on average for over 11 years and require a brightness level roughly 2.5× that of a cellphone.

OLCD technology uses a separate backlight, which allows high luminescence levels to be delivered without compromising the display lifetime; and unlike OLEDs, OLCDs aren’t prone to burn-in.

Level of flexibility

Do you need a foldable display? Or one that simply works with the curves and shapes of your design? Be it in automotive control panels or home appliances, displays are virtually the last remaining flat surface on a system, and both flexible OLED and OLCD technologies allow for the display to be merged into the design; rather than just being accommodated by the design.

Flexible OLED is currently the only technology to allow foldable displays. Samsung’s Infinity Flex OLED Display is used in the Samsung folding phone, which has a radius of curvature (ROC) of 1mm3. This panel uses a polymer that Samsung claims will keep its strength even when folded and unfolded “hundreds of thousands of times.” Comparatively, the ROC of an OLCD on a TAC panel currently stands at 10mm.

Conversely, OLCD panels are currently only flexible in the manufacturing process, enabling displays to be brought into the design of the system: being cut into non-square shapes during the production process and holes added to fit around the functional design of the system – for example, around switches or the gear stick in a car.


This concept of a car center console features a curved OLCD with apertures for the gear stick and controls. Source: FlexEnable

Applications

Flexible plastic displays are set for the mainstream, and both OLED and OLCD enable significant benefits over glass. However, each technology should be targeted at specific mass-market applications, which include the following four key segments.

Phones/smartwatches

Flexible OLED panels are already being adopted in high-end smartphones (e.g., Samsung Edge) and smart watches (e.g., Apple Watch). And the technology delivers high resolutions, with a tight ROC. However, the cost is higher than both glass LCD and plastic OLCD, and the brightness of the application shortens lifetime. OLED panels are therefore ideal for smartphones, smart watches, and similar small (sub 8-inch) consumer devices that have a shorter product life cycle and can command a high price point. However, as we said at the beginning, flexible displays are about more than phones.

Notebooks and tablets

Undoubtedly consumers welcome thinner, lighter, and shatterproof notebooks and tablets enabled by plastic displays. While both flexible OLED and flexible OLCD would allow thinner and lighter screens in this application, the cost of flexible OLED for notebook-sized displays is currently very high, although costs will fall over the coming years. The flexible OLCD cost structure is similar to glass LCDs – it uses many of the same low-cost components in its construction except the glass, making it at least 100g lighter and 0.5mm thinner for a notebook-sized display. Moreover, OLCD offers a route to bezel-less displays by folding the borders – enabling laptops and tablets to increase display size without increasing the weight.

Smart home devices

In addition to replacing glass and shrinking the bezel in notebooks and other existing applications, lower-cost flexible displays will be used to create new smart home device classes. For example, many smart speaker designs today are round or cylindrical, but glass displays are flat, which limits their incorporation in non-rectangular form factors. By using OLCD, wrap-around screens are possible – they provide a better user experience by enabling new audio visual use cases without compromising the product design and aesthetics.

This concept of a smart speaker features a wrap-around OLCD.
Source: FlexEnable

Automotive

One of our favorite examples of flexible displays comes from French automotive technology developer Novares, which in 2018 incorporated OLCD screens into its NovaCar #1. The company used the curved displays mounted on the dashboard by the A-pillars to replace side mirrors and thereby cut drag. And by using the same approach, the technology could also enable blind spots to be eliminated, wrapping the displays around A-pillars and displaying a live feed from a camera in the frame.

In 2018, Novares demonstrated side view mirror replacements using flexible OLCD technology.
Source: Novares

Thanks to flexible display technologies, novel designs and use cases will emerge where the use of screens has previously been unthinkable due to glass-based display limitations. Soon we will start seeing mainstream products featuring curved and shaped displays. And a day will come when all displays will be manufactured on plastic covering the surfaces around us and bringing them to life.

Paul Cain is Strategy Director at FlexEnable. He holds 25 patents relating to processes and architectures enabling the manufacture of flexible displays.

Notes

  1. Corning has said it is developing a foldable glass for smartphones, claiming it has a 75µm prototype that can survive “hundreds of thousands” of folds.
  2. Huawei is not able to sell to the U.S.; therefore, price is converted from European price of €2,299.
  3. Concerns have been raised over the Samsung Galaxy Fold’s inward fold, which gives a 1mm radius of curvature, and the stress this puts onto the display panel. Initial reports highlighted that it caused visible creases. Additionally, screens on journalist-review models have broken on the fold line after just a day.