More clarity Transparent electronics optimize 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 it works.

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

This is how to improve the transparency of OLED displays

The "OLED-on-silicon" process uses a silicon backplane that contains the entire active-matrix control electronics for the pixels, as the researchers continue to explain. The organic front plane is monolithically integrated on the top metallization level, serving as a control contact for the organic light emitting diode (OLED) at the same time. The second connection of the OLED is formed by a semi-transparent upper electrode that all pixels share. The pixel circuitry is based on silicon CMOS technology and requires several metal layers to connect the transistors embedded in the substrate. These metal connections consist of aluminum or copper.

In addition, the optical structure of the OLED requires a highly reflective lower electrode to achieve high optical efficiency upwards. These two aspects lead to the pixels themselves not being transparent. However, a transparent microdisplay can be constructed through a spatially distributed design of this basic pixel structure, creating transparent areas between the pixels and minimized column and row wiring. Also, 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. In this, transparent areas are created between the pixels. It is suitable, for example, for overlaying images within a complex optical system, with the image inserted between other image planes, as the experts explain. On the other hand, there is the cluster approach, in which several pixels are combined into larger, opaque clusters, with larger transparent areas created between the clusters. It is ideal for applications in Augmented Reality (AR), such as data glasses, in which a micro-optic is implemented above each cluster. The micro-optics combine the cluster images into a virtual image. The transparent areas remain unaffected. Thus, you can still see the real environment. By the way, with the new development of IPMS, they want to support both options.