How does a transparent TV work? You can make one yourself

Recently, the news about transparent display screens appearing on the windows of trains in Shenzhen and Beijing subways has attracted a lot of attention: the windows of Shenzhen Metro Line 6 and Line 10, and Beijing Metro Line 6 have all appeared with this “intelligent train passenger service system” [1][2]. These transparent display screens integrated into the windows can not only display weather and station information, but also surf the Internet, watch videos, and browse online stores.

In addition, Xiaomi launched a transparent TV priced at 50,000 yuan at its 10th anniversary conference not long ago, which further heated up the popularity of "transparent display" technology. From some reports in domestic media and information from the supply chain, it is basically clear that these transparent display panels come from LG Display. There are not many manufacturers supplying transparent display panels nowadays. It is said that Samsung decided to stop producing such panels in 2016 [3]. To this day, such products are far from popular.

In fact, the development of transparent display technology has been going on for at least 10 years. It seems that every once in a while, we can always see transparent TVs or displays at exhibitions. They come in various forms, such as transparent TVs, transparent cabinet doors, and transparent mobile phones.

In 2009, when Sony Ericsson was still in existence, there was a transparent screen mobile phone called Xperia Pureness X5 that attracted special attention because its transparent screen was so cool at that time - even though this phone was not mass-produced and was just a feature phone.

Xperia Pureness phone, source: Pocket-lint

The last time a transparent display screen caught people’s attention was at the 2016 CES, when Panasonic displayed a transparent display screen in the form of a cabinet door, which it called an “invisible TV”. When it is not turned on, it looks like an ordinary transparent glass cabinet door. But when it is turned on, it is a TV that can display dynamic images. [4] Last year, Panasonic once again displayed its transparent OLED TV, but at that time it was still a concept product.

This type of product is so popular probably because of its futuristic sense of technology. After all, in science fiction movies, we can always see various high-end transparent displays. This is very common in Marvel's superhero movies in recent years. In this article, we try to briefly talk about the basic principles of transparent displays and how far it is from entering our daily lives.

Transparent display? You can DIY
transparent display technology in various directions, such as LCD and OLED can be made transparent. Here we will not talk about some unconventional transparent display technologies, such as transparent display technologies like Google Glass and Microsoft HoloLens: these augmented reality AR glasses sometimes have 3D display properties.

Source: Geng, Jason. "Three-dimensional display technologies." Advances in optics and photonics 5.4 (2013): 456-535

For example, the AR glasses shown above use the partial reflection characteristics of the lenses to achieve a 3D image perception for the left and right eyes, while also ensuring that the real world outside the glasses can be seen. This is a type of see-through display solution, but it is not the focus of this article.

In addition, MIT has been working on developing a passive transparent display system using nanoparticle technology - this solution uses a projector as an external light source to project images onto a transparent medium (this transparent medium is embedded with a nanoparticle that can partially display the projected image). There are also some transparent display technologies that use similar projection schemes. These solutions are cheaper, but they are not the mainstream of transparent displays, at least not yet.

In the past two years, we have seen a lot of transparent displays, usually LCD or OLED transparent displays. It feels like we are back to the time when we discussed flexible screens not long ago, divided into two directions: OLED flexible screens and LCD flexible screens. So how do these two panels achieve transparency?

I won't spend too much time talking about the structure of LCD and OLED screens here. The basic concept is that no matter what kind of panel it is, they are all multi-layer stacked structures. The display screen is divided into many different layers, each layer plays a different role - stacking these many layers together constitutes the panel and display screen. The difference between LCD and OLED is that the hierarchical structure that constitutes the two types of panels is very different.

As we mentioned in the introduction article of flexible screens, to achieve flexibility of the display screen, it is essentially required that each layer must be bendable; the same is true for transparent screens. To achieve transparency of the display screen, each layer must be transparent (or have a certain light transmittance).

Source: The Backoffice, YouTube[6]

This is a semi-natural property of display panels. For example, there is a backlight layer at the bottom of an LCD screen, which emits light to illuminate the various layers on top - this at least shows that most layers of traditional LCD screens are originally translucent. There are many DIY videos on YouTube that show how to turn your long-unused screen into a transparent display [7] - they all use traditional LCD screens, remove the backlight layer including the light guide plate, and then use natural light or other light sources as backlight to illuminate the screen, so that the screen naturally becomes translucent, as shown in the figure above.

It can be seen that LCD screens originally have the basis for being made into transparent screens. As for OLED screens, the simplest example to prove their transparency is that many smartphones now support under-screen fingerprint recognition (some even have under-screen cameras). These fingerprint recognition solutions are usually optical fingerprint recognition, which means that the screen itself must be transparent in order to achieve the purpose of fingerprint recognition under the screen. These screens are generally OLED screens, which shows that OLED screens naturally have this "transparent" property. (For example, transparent electrode ITO indium tin oxide material; some layers are translucent because they are thin enough)

OLED transparent screens will probably become the future
. Obviously, since you can DIY a transparent display at home, screen transparency is not a black technology. However, the research on transparent screens has been ongoing in recent years, and the core issue should be how to increase the transparency or light transmittance of the screen. After all, DIY transparent displays are still far behind the current Xiaomi transparent TV in terms of transparency.

In theory, this is also an important issue that affects the two transparent routes of LCD and OLED. As mentioned above, LCD screens require backlights, which creates obstacles for the complete form of LCD screens to achieve transparency. In the early years, some LCD transparent screens removed the backlight system and used external light sources: whether natural light or artificial external backlights to achieve screen visualization. At the same time, LCD also has two polarizers (polarizers), which are also components that affect light transmittance.

JDI's transparent LCD screen

In 2017, JDI (Japan Display Inc.) introduced its own transparent display screen at the SID 2017 conference[8]. This transparent screen is still an LCD in general, but JDI has made improvements to the panel light-emitting structure and technology to avoid the problems mentioned above. First, the backlight is placed on the side of the display screen (hidden in the frame), and the specific light guide scheme is unknown. Foreign media materials mentioned that "internal reflection of the front and rear walls of the liquid crystal unit is used to achieve LED light diffusion", which should be considered a side-entry backlight solution. In addition, due to improvements in backlight, materials ("fast response liquid crystal materials" that can support high-speed switching between transparent and scattered states) and working characteristics, this LCD transparent panel also removes the polarizer and CF (color filter) to further improve the transmittance.

In the past few years, suppliers including Samsung, LG, and MMT have all promoted transparent LCD displays, and have made some improvements to the structure of LCD to increase the transparency of the screen. When it comes to achieving transparency, the structure of OLED does not naturally have the obstacles mentioned above. Even ProDisplay, which is promoting both transparent LCD and transparent OLED products, specifically mentioned that "general transparent LCD screens require backlights to achieve visible images, while transparent OLED screens are made up of millions of self-luminous pixels. This creates a whole new field of innovation...[9]" Moreover, because OLED has a simple hierarchical structure, it is easier to make thinner.

We have discussed the structure of OLED screens more than once before. Apart from the TFT backplane, the main hierarchical structure of a general AMOLED display screen includes substrate, cathode layer, organic molecule layer (including emission layer, conductive layer), and anode layer, as shown in the figure above. This is a structure that emits light when powered on and does not require additional backlight.

As mentioned above, these different layers have transparent properties. Most contemporary panel manufacturers have the technology to make different layers transparent. We don’t know the specific technical details of this part. But the Xiaomi transparent TV and subway window transparent screen mentioned above are both OLED panels, so it is not difficult to imagine which direction the mainstream of transparent display screens is heading.