Why Don't Foldable Phones Break Even When Folded? [Im Juhyung's Tech Talk]

'Flexible Display' Used in Bendable Phones
Unlike Traditional Displays, Uses Plastic-Based Materials
Ensures Not Only Light Transmission but Also Flexibility to Bend or Curve

Samsung Electronics' foldable smartphone 'Galaxy Z Fold2' / Photo by Yonhap News

[Asia Economy Reporter Lim Juhyung] Last year, Samsung Electronics' 'Galaxy Fold' and 'Flip', along with Huawei's 'MateX', so-called 'foldable smartphones', were released, capturing worldwide attention. They left a deep impression by demonstrating durability that withstands folding and unfolding hundreds of thousands of times without a single crack.

Most of the smartphones we use today do not have the property of bending or folding. However, foldable phones secured flexibility by introducing a new display called the 'flexible display'. Unlike conventional displays that use glass-based materials, flexible displays are a technology that implements bendability using plastic materials.

Displays have undergone continuous development over the past several decades. In the 20th century, cathode ray tube (CRT) displays were mainly used. Since they displayed images by shooting electron beams, televisions made at that time were large and bulky.

The displays mainly used today are flat panel displays (FPD) that make the screen flat using organic light-emitting diode (OLED) technology and others. The development of FPDs has greatly contributed to making laptops, digital cameras, and other devices very thin, as well as opening the era of smartphones with displays mounted on the front.

Flexible displays are classified as the third generation of displays, following CRT and FPD. However, mass production is still difficult, so only a few IT companies such as Samsung and Huawei have introduced them in some products. But as the technology matures, it is expected to be installed in more electronic devices.

So, what makes flexible displays different from previous generation displays? Existing displays cannot be bent or folded because both the 'encapsulation' that covers the organic light-emitting layer of the display to protect it from external substances, and the 'substrate' underneath the display are made of specially manufactured thin glass.

A flexible display prototype unveiled by Samsung Electronics at CES 2013 in the United States. Unlike traditional displays, the flexible display secured flexibility by using plastic-based materials instead of glass. / Photo by Yonhap News

Glass is mainly used as a material for making flat panel displays such as liquid crystal displays (LCD) and OLEDs, so it is relatively inexpensive. However, it is very hard and does not have the flexibility to bend or fold.

However, flexible displays replaced glass by developing different materials for both encapsulation and substrate. Instead of conventional glass encapsulation, a very thin film called thin film encapsulation (TFE) was used, and polyimide (PI), a type of plastic, was used for the substrate.

Thin film encapsulation is made by stacking thin layers made of organic and inorganic materials alternately. It uses organic films that are flexible but weak at blocking external substances, and inorganic films that have no weaknesses but have uneven surfaces, layered alternately.

Meanwhile, PI used as the substrate material is a plastic material with high heat resistance. When a very thin film is made from PI, it becomes transparent by transmitting light like glass, while also having the flexibility to bend or fold.

Flexible displays produced through this process are currently integrated into various forms of electronic devices. For example, 'curved TVs' and 'curved monitors' with rounded curved surfaces are representative examples. Samsung Electronics' 'Edge Display', introduced in the 'Galaxy Note Edge' unveiled in September 2014, is also a type of flexible display. It featured a display with both ends curved.

Moreover, research is underway on flexible displays that go beyond simply folding or bending parts, to displays that can be rolled up like paper. These flexible displays are known as 'rollable displays'.

The Chinese company 'Oppo' unveiled the world's first rollable smartphone prototype, 'Oppo X 2021,' on the 17th of last month (local time). When a specific button is pressed, the display, which was rolled up inside the device, unfolds, causing the smartphone to extend. / Photo by Oppo Twitter Capture

Earlier, the Chinese IT company 'Oppo' unveiled the world's first rollable smartphone concept device, the 'Oppo X 2021', at 'Oppo Inno Day 2020' held on the 17th of last month (local time) in Shenzhen, Guangdong Province, China. This new concept smartphone allows users to extend or retract the device sideways by using a rollable display that rolls part of the phone screen inside the device and unfolds it when needed.

Such rollable phones are also being actively researched by Korean companies such as LG Electronics.

Previously, LG showcased the silhouette of a device presumed to be a rollable smartphone during the 'LG Wing' launch event in September. Additionally, LG confirmed that it registered the trademarks 'LG Rollable' and 'LG Slide' with the patent office earlier last month.

Although flexible display technologies such as foldable and rollable are still in their early stages, they are expected to experience rapid growth in the future. According to a report published in September by the market research firm 'Display Supply Chain Consulting', the flexible display smartphone market size recorded $1 billion (approximately 1.1 trillion KRW) last year. It is also forecasted to achieve an average annual growth rate of 80% until 2025.