Today you can buy a smartphone with a foldable screen. Tomorrow we might have a stretching screen
Motorola unveiled the first portable mobile phone nearly half a century ago. It was about the size of a brick and weighed about half a brick. Ten years later, the first commercial mobile phone appeared on its basis. He, too, looked awkward, but allowed the owner to send and receive calls on the go, which was new at that time. Since then, mobile phones have acquired many other functions. Now they process text messages, surf the web, play music, take photos and videos, display them on the screen, show their location on a map - there is nothing to count. The possibilities of their application went beyond any dreams that existed at the time of their appearance.
But for all their versatility, smartphones still struggle with a fundamental flaw: their screens are too small. Yes, some phones do more to make the screen bigger. However, if the phone gets too big, it will no longer fit into your pocket, which will be undesirable for many.
The obvious solution is to make the display foldable like a wallet. For many years, we at Seoul National University have been developing suitable technology. The same has been done by smartphone manufacturers, who have only been able to bring this technology to market in the last couple of years.
Folding screens will no doubt spread faster soon. Some of your relatives or friends will have one, after which you ask yourself: how is it even possible for the screen to fold? We decided to explain how this technology works to prepare you for the moment you see a phone with a large, bright and flexible display put into your pocket. Not to mention, when the screens can stretch as well as flex, there will be many more radical electronic devices.
Researchers have been seriously dealing with flexible screens for about two decades. But for many years these projects remained in the research phase. In 2012, Bill Liu and several other Stanford alumni decided to bring flexible displays to market by founding Royole Corp.
Closed Book: Late 2018 Royole Corp. developed the first commercial foldable screen smartphone, FlexPai. It folds so that part of the screen remains visible from the outside.
At the end of 2018, Royole introduced the FlexPai device with a flexible display that unfolds into something like a tablet. The company demonstrated how a foldable display can withstand 200,000 folding cycles, and it is quite strong - with a radius of only 3 mm. However, this was not a commercial product, but only a prototype. For example, a review by The Verge dubbed it "charmingly awful."
Shortly thereafter, the two largest smartphone manufacturers Samsung and Huawei began offering foldable models of their own. Samsung Mobile officially announced the Galaxy Foldin February 2019. It has two foldable displays with a bend radius of just 1mm, which allows the phone to be folded so that the display stays inside. In the same month, Huawei announced its Mate the X . The Mate X is 11mm thick when folded and has a display on the outside (like the FlexPai) and has a fold radius of about 5mm. In February of this year, both companies showed their second foldable models: Samsung Galaxy Z Flip and Huawei Mate Xs / 5G.
Naturally, the most difficult thing in these phones was to make the displays themselves. It was necessary to reduce the thickness of the foldable display in order to minimize the load on it when folded. The smartphone industry has just figured out how to do this. Display vendors such as Samsung Displayand Beijing BOE Technology Group Co. are already producing foldable displays.
These are AMOLED displays (active matrix organic light-emitting diodes), like those used in conventional smartphones. However, instead of making screens as usual on a rigid glass substrate, companies use a thin flexible polymer. It houses the back of the display - a layer that contains many thin-film transistors that drive individual pixels. It has a built-in damping layer that prevents cracks when the screen is bent.
Although flexible displays of this design are increasingly found in phones and other consumer devices, the standards associated with them and the language that describes them are still in the making. At the very least, they can be described using the bend radius of curvature. A conformable display does not bend very much, a rollable display has a medium degree of flexibility, and a foldable display has a fairly small bend radius.
Since any material, be it a smartphone screen or a sheet of metal, experiences tension on the outside of the fold and compression on the inside, the display's electronic components must resist these stresses and deformations. The easiest way to minimize these forces is by bringing the outer and inner surfaces of the display closer together - in other words, making it as thin as possible.
To make the screen as thin as possible, developers are abandoning the protective film and polarizing film, which are usually glued to screens, and a layer of glue between them. This is not an ideal solution, but still a protective film and a polarizing anti-reflective layer are optional components for an AMOLED display. Such a display generates light from the inside, and does not change the light emitted by an LED backlight, as liquid crystal displays do.
Another difference between a flexible display and a conventional display is that transparent conductive electrodes run from both sides of the light-emitting organic materials, through which the pixels emit light. Typically, this role is played by indium tin oxide (ITO). However, ITO is fragile, so you shouldn't use it in flexible displays. Worse, ITO adheres poorly to flexible polymer substrates, warps and flakes when compressed.
Fighting this problem a decade ago, researchers have devised other strategies to improve the adhesion of ITOs to flexible substrates. One of them is to treat the substrate with oxygen plasma before gluing the ITO electrode. Another is to insert a thin layer of metal (such as silver) between the electrode and the substrate. Also helps to place the top of the underlay right in the middle of the display pie. Then the fragile interface in the ITO layer falls on the mechanically neutral plane of the display, which undergoes neither compression nor stretching when bent. So far, the leading electronic folding screen companies are using this strategy.
You can do it even easier, and completely get rid of the ITO electrodes. This has not yet been done in commercial devices, but the strategy seems to be beneficial regardless of the flexibility of the screens. The fact is that indium is toxic and expensive, so ideally it is better not to use it. Fortunately, over the years of research, scientists, including the two of us, have picked up other materials that can work as transparent electrodes in flexible displays.
The most promising candidate is a flexible film with silver nanowires. The mesh of these tiny wires conducts electricity while remaining almost completely transparent. It can be created inexpensively by adding a solution containing silver nanowires to a substrate - just like when printing with ink on paper.
In 2019, Huawei unveiled a line of flexible display phones. The photo shows the Mate Xs phone.
Much of the research on silver nanowires has focused on reducing resistance at the intersection of individual wires. This can be done, for example, by adding other substances to them. Or you can physically process the layer of nanowires - heating it, or applying such a current so that the intersections are soldered to each other. Or you can stamp it hot, treat it with plasma, or irradiate it. Which method works best depends primarily on the substrate on which the layer is applied. The polymer substrate deforms too much when heated. This is, for example, a polymer such as polyethylene terephthalatefrom which transparent food containers are made. Polyimide is not so sensitive to heat, but its yellowish tint disturbs the transparency of the layer.
Metal nanowires are not the only ITO replacement option for making transparent electrodes. There is also graphene, a special form of carbon in which atoms are arranged in two-dimensional honeycombs. Graphene falls short of the conductivity and transparency of ITO, but it resists bending better than any other flexible display material considered today. And graphene's meager conductivity can be improved by combining it with a conductive polymer, or adding nitric acid or gold chloride to it.
Another possibility is the use of conductive polymers. The main example is poly (3,4-ethylenedioxythiophene) with the addition of polystyrene sulfonic acid. This complex name is usually replaced by the abbreviation PEDOT: PSS . Such polymers dissolve in water, so that transparent and thin electrodes can be printed or centrifuged. Suitable chemical additives can significantly improve the flexibility of such a conductive polymer and even make it extensible. Careful selection of additives also improves light per unit current - the display can be brighter than those produced with ITO.
So far, OLED displays used in mobile phones, monitors and televisions are manufactured in the following sequence. The substrate is placed in a vacuum environment, the organic material to be added is vaporized and metal masks are used to control the deposition of the material. It turns out something like high-tech screen printing. But these thinly patterned metal masks are difficult to manufacture, and much of the material is wasted, making large displays expensive to manufacture.
An interesting alternative to the production process of such displays has emerged: inkjet printing. The applied organic material dissolves in the liquid and then is applied to the substrate where necessary. It forms pixels, after which it is heated to evaporate the remaining solution. This tactic is being tested by DuPont, Merck, Nissan Chemical Corp. and Sumitomo, although the efficiency and reliability of the resulting devices are still far from desired. But if they succeed, the cost of manufacturing displays will drop significantly.
Samsung also introduced its line of flexible display phones in 2019. In the photo - Galaxy Fold.
Manufacturers of small displays for smartphones have an even higher priority than keeping costs down: reducing power consumption. OLEDs turn out to be less and less power hungry over time, but the further, the more difficult it is to reduce power consumption from the current level of 6 mW per square centimeter. This is especially frustrating for foldable phones, which are much larger than normal screens. Therefore, we can safely assume that foldable phones will have quite voluminous batteries in the near future.
How is the fate of flexible displays going to unfold after they make our smartphones foldable? Considering how much time people spend on smartphones today, you can imagine that in the not too distant future people will start wearing displays that are attached directly to the skin. Initially, it will be biometric data visualization, but other applications are coming soon. Perhaps such wearable displays will one day become part of the high-tech fashion.
Naturally, for the production of such displays, sufficiently soft materials will be used that do not cause inconvenience to the skin. In addition, they will need to be able to stretch. Developing tensile conductors and semiconductors is incredibly difficult. For several years now, researchers have been studying something similar but simpler: geometrically stretchable displays. They contain small, rigid components attached to an expandable cover. They are connected by conductive tracks that carry deformation under tension.
Recently, however, there has been progress in the development of expandable displays - those that stretch the conductors, semiconductors, and the substrate. They, of course, need new materials, but the main obstacle so far remains the question of how to develop a protective coating for such stretchable devices that protects them from the destructive effects of moisture and oxygen. Our team recently made good progress on this issue by developing air-stable, stretchable devices that emit light and do not require an elastic protective cover. They can be stretched almost twice without disruption.
Rough prototypes of extensible displays are being produced today, with a coarse grid of luminous elements. But the industry is showing tremendous interest in expandable displays. In June, South Korea's Ministry of Trade, Industry and Energy tasked LG Display to manage a consortium of industrial and scientific researchers developing expandable displays.
It's easy to imagine what awaits us next: athletes, hung with biometric displays placed on their arms and legs. Smartphones that can be carried in the palm of your hand. Displays stretching over various uneven surfaces. Developers of such displays of the future will certainly be able to take advantage of the years of research gained from the research that has enabled today's flexible smartphone screens. Without a doubt, the era of not only bending, but also stretching electronics will soon come.