In this blog post, we will explore how flexible displays differ from conventional screens, what technologies they utilise, and why they are attracting attention as a technology of the future.

The potential of flexible displays

Laptop computers and mobile phones are among the most essential items in modern society. However, both devices require careful handling as their LCD screens are easily broken, and large screens can be inconvenient to carry around. To overcome these shortcomings, companies are now developing bendable displays and OLED flexible displays.
In the film Minority Report, set in 2054, the main character, John Anderton (Tom Cruise), hides in the subway to escape the authorities. Some of the passengers in the subway are reading newspapers, and if you look closely, you can see that the content of the newspapers is updated with information transmitted in real time and even videos appear. In the end, the protagonist’s identity is revealed through the newspaper, and he has no choice but to flee again. One of the technologies used in this newspaper is a flexible display, which is literally a display that can be bent. Many people who saw the film were impressed by the advanced future depicted in it. Let’s take a look at how this flexible display technology is possible.

Development stages of bendable displays

Bendable displays are divided into four stages. Stage 1 is impact resistance and light weight, stage 2 is curved moulding, or bending, stage 3 is elasticity, which allows the display to return to its original shape, and stage 4 is foldability and rollability. The technology currently available on the market is between stages 1 and 2. The ultimate goal of this technology is to incorporate flexible displays into wearable devices.

The secret of flexible displays

These magical displays, which can be folded and bent without breaking and rolled up for portability, have been made possible by the use of fundamentally different materials. Glass is used in conventional display substrates, but flexible displays use plastic substrates.

Differences from conventional displays

Now let’s see how they differ from conventional displays. In a typical LCD display, liquid crystal elements are sandwiched between two thin glass plates, and light from a backlight unit behind the display is selectively passed through the liquid crystal elements to produce colours from a colour filter. However, OLED displays use OLED materials that emit light on their own, eliminating not only the backlight unit, which is an essential part of LCD displays, but also the colour filter, thereby greatly reducing the thickness.

Technological advances in flexible displays

Flexible displays are evolving further, with efforts being made to make TFT substrates and front protective materials flexible while maintaining high durability and hardness. Glass was previously used as the substrate and protective material for panels, but plastic materials are used for flexible panels. A method of increasing the hardness of these plastic windows has been proposed, which involves mixing organic and inorganic materials and coating them on the plastic surface. The adhesive used to bond the plastic window and panel is supplemented with two films to maintain adhesion even when bent, and there is a view that the polarising plate should be thinner.

Improvements to the TFT structure

The structure of the TFT also needs to be changed. This is to increase the degree of display bending while minimising the stress on the TFT when bent. Lenovo Display has proposed a solution that involves leaving a certain amount of space between the dielectric film used for interlayer insulation of the TFT and the gate insulating film. According to Lenovo, this allows the display to bend more and reduces the tension on the TFT when bent to 1/20th of the previous level.

Features of plastic materials

The thick frames of horn-rimmed glasses, which are popular in fashion these days, can be easily bent because they are made of plastic. You can also see the characteristics of plastic, which does not break easily, in the plastic bowls used to serve food to young children instead of glass bowls. So why is plastic more flexible and less breakable than glass?

Molecular structure of plastic and glass

The photo below shows the molecular structure of polyacetylene, a type of plastic that conducts electricity. As you can see in the photo, the molecular structure is chain-like, which makes it easy to bend. This is the same principle as a single wire bending more easily than several wires bundled together, which will be explained in more detail later.
In contrast, glass is composed of two elements, silicon and oxygen, and its molecular structure is as shown in the photo below. As you can see in the photo, all the silicon atoms are surrounded by oxygen atoms, forming a huge three-dimensional molecular structure that does not bend easily. This is similar to the principle that a single wire can be easily bent with a small amount of force, but when several wires are twisted together to form a three-dimensional structure called a ‘barbed wire fence,’ it is difficult to bend even with a strong force. In addition, the proverb ‘unity is strength’ may help to understand this concept better.

The commercialisation potential of flexible displays

Ultimately, flexible displays have been made possible by using plastic instead of traditional glass materials. Although they are not yet widely commercialised, we believe that this technology will soon become deeply ingrained in our daily lives. Can you imagine a scene in the subway where people unfold rolled-up displays from their pockets every morning to read the newspaper? Or a display wrapped around your wrist like a watch that diagnoses your physical condition every moment?

Innovation in the encapsulation process

The encapsulation process, which involves applying multiple layers of thin film, also needs to be innovated. The encapsulation process is an essential technology for the mass production of flexible OLEDs. This is because organic EL, the main raw material of OLEDs, loses its function when exposed to oxygen or moisture. In other words, it is necessary to find a material that can bend the panel without being exposed to oxygen or moisture, unlike conventional displays. A representative example is Lenovo Display, which utilises thin film encapsulation (TFE) technology. TFE is a technology that protects organic EL from oxygen and moisture by depositing organic EL on a TFT substrate and then covering it with alternating layers of organic and inorganic materials. The key is to reduce the number of layers of organic and inorganic materials to increase productivity. Lenovo has announced that it can reduce the number of layers to five with its current technology.

Future prospects for displays

Flexible displays continue to evolve. With this progress, in the future, these displays will completely replace paper, and people will be able to move around inside newspapers like in J.K. Rowling’s Harry Potter series, or simply slip them into clear files in their bags.

The importance of materials engineering

As we have seen, even simply changing materials can bring about amazing technologies that can greatly change our lives. Materials engineering is a field that studies technologies that can bring about major changes in our lives by using familiar materials in the right places or developing new materials that did not previously exist. I wonder what other advanced technologies, such as bendable display technology, will take us to a world that was once only possible in our imagination.