Organic displays achieve bright colors and rich contrasts in high-end TVs. The self-illuminating OLEDs have a big advantage over LCDs: They do not need a backlight and they simply switch off the pixels on black image content. In TVs, notebooks, tablets and smartphones, this ensures rich black levels. Since organic displays are also paper-thin, they are recommended as a translucent picture surface – for example for the free-standing television or the video wall on the living room window.

There is, however, a catch: While the luminance of the display in bright areas of the image exceeds that of the surroundings, the background shines through in the black or very dark areas of the image. Thus, a transparent OLED would only deliver acceptable black values ​​in the evening or with the shutters down.

Before the end of this year, Panasonic plans to offer transparent OLED modules with a switchable rear wall.

(Image: Panasonic)

Panasonic has found a way out of the dilemma: The Japanese TV manufacturer provides the transparent OLED with a digital back wall. This consists of a liquid crystal layer that changes from the transparent to the opaque state at the push of a button – i.e. when a voltage is applied. Panasonic uses liquid crystals dissolved in plastic (Polymer-Dispersed Liquid Crystal, PDLC), as offered by film specialist Toppan, among others. The Darmstadt-based company Merck has already demonstrated at trade fairs how glass conference rooms can be transformed into rooms with opaque walls with such PDLC films.

For switchable PDLC films, the transparent plastic layer with liquid crystal molecules is coated on both sides with electrically conductive membranes. If light falls from behind on the plastic LC sandwich, it is reflected by the irregularly distributed liquid crystals and therefore hardly or not at all on the surface of the sandwich. If you apply an electric field to the conductive membrane, the liquid crystals align themselves along the field lines and thus form a kind of guidance system for the incident light – it penetrates the surface. Toppan also offers foils that are transparent when de-energized and become opaque when a voltage is applied (with LCD this is called normally white).

If the PDLCs align in the electric field, the light can pass through the film and you can see objects behind the OLED.

(Image: Toppan)

The liquid crystals (PDLC) dissolved in plastic scatter the incident light and thus shield the OLED background.

(Image: Toppan)

Digital back wall Panasonic uses PDLC technology in the transparent OLED TP – 55 ZT 110. It measures 1, 40 meters in the diagonal (55 inches) and shows 1920 × 1080 pixels. It should be the same OLED panel that LG uses in its industrial monitor 55 EW5G . The PDLC layer transforms the transparent display into an almost normal TV module at the push of a button – almost because the PDLC does not block the light 100 percent shields. Four modules of the panel, which is frameless on three sides, can be seamlessly integrated into one 16: 9-format Join the video wall. The timing controller board (TCON) for the panel control sits behind the lower module surround.

The PDLC-OLED -Module is just 7.6 millimeters thick, according to the manufacturer, the transparent version without PDLC film (TP – 55 ZT 100) only adds 3.8 millimeters; both come with an external power supply. Panasonic connected all module layers to one another in a vacuum in order to minimize reflections at the boundary layers and to optimize the module’s transparency. The display will initially be offered in Japan, Taiwan, Singapore, Australia and New Zealand, Panasonic has not yet given a price.

Advertising and access control LG has found an application that does not need to shield the display back: The Korean company is cooperating with the Swedish door manufacturer Assa Abloy, who apply transparent screens to automatic glass doors would like to. These could serve as advertising space or enable personalized access controls. LG has been working on transparent organic displays for some time and has already shown OLED prototypes that were transparent and flexible. The company already has the 55 – inch transparent industrial monitor 55 EW5G.

Assa Abloy wants to coat glass automatic doors with a transparent OLED in order to use them as advertising space or To use the access control system.

(Image: LG)

At trade fairs, the display specialist has also been presenting a refrigerator with a glass door on which image content can be displayed for years. This is also of interest for commercial applications, for example for price information on the objects visible behind the door or for advertising videos. The glass front is a transparent LC display. Since LCDs, unlike OLEDs, do not light up themselves, they always need a backlight. That is why there is a very bright, energy-hungry Edge-LED backlight in the refrigerator door, which switches off as soon as the refrigerator door is opened. 2 / 2021 In c’t 2 / 2021 take a look inside the crystal ball and shed light on the IT trends of the coming years. You will learn how you can surf privately and securely, and which browser supports this particularly well. In the IT salary report, we examine whether the pandemic is a good job engine. We test multifunctional printers for the home office, show you how you can protect your NAS against hacker attacks, and introduce a universal Windows boot stick. You can read this and much more in c’t 2 / 2021. The issue is available from January 1st 16 in the Heise shop and at the well-stocked magazine kiosk.

(uk)