More transparency Transparent electronics optimizes OLED microdisplays

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Researchers at the Fraunhofer Institute for Photonic Microsystems (IPMS) have significantly increased the transparency of OLED microdisplays. Here they explain how this works ...

Transparent electronics are already providing reliable services in some applications, say the IPMS researchers. They can be found as ultra-thin layers on touch displays or as transparent foils with printed antennas for mobile communications. OLED microdisplays have not been transparent until now. However, as part of the HOT project (High-performance transparent and flexible microelectronics for photonic and optical applications—sponsorship number MAVO 840092) supported by the Fraunhofer Society, OLED microdisplays with a transparency of 20 percent have already been developed. Now the whole thing has been further advanced and a transparency of 45 percent has been achieved for the first time with a CMOS-OLED microdisplay.

How to improve the transparency of OLED displays

The "OLED-on-silicon" process uses a silicon backplane that contains all the active-matrix drive electronics for the pixels, as the researchers further explain. The organic front plane is monolithically integrated on the top metallization level, where it also serves as the control contact for the organic light emitting diode (OLED). The second connection of the OLED is formed by a semi-transparent upper electrode that all pixels use in common, as it continues. The pixel circuit is based on silicon CMOS technology and requires several layers of metal to connect the transistors embedded in the substrate. These metal connections are made of aluminium or copper.

In addition, the optical structure of the OLED requires a highly reflective lower electrode to achieve a high optical efficiency upwards. These two aspects mean that the pixels themselves are not transparent. However, a transparent microdisplay can be built by a spatially distributed design of this basic pixel structure, creating transparent areas between the pixels and minimized column and row wiring. Furthermore, avoiding OLED layers in the transparent areas, introducing anti-reflective coatings and redesigning the wiring help to further increase transparency.

Basically, there is the pixel approach and the cluster approach

To make optical systems semi-transparent, there is the so-called pixel approach. This creates transparent areas between the pixels. It is suitable, for example, for image overlay within a complex optical system, where the image is inserted between other image levels, as the experts explain. On the other hand, there is the cluster approach, where several pixels are grouped into larger, opaque clusters, creating larger transparent areas between the clusters. This is ideal for applications in Augmented Reality (AR), such as data glasses, where a micro-optics is implemented above each cluster. The micro-optics combine the cluster images into a virtual image. The transparent areas remain untouched. This way one can still see the real environment. By the way, the new development of IPMS aims to support both possibilities.