Researchers Boost Transparency in Microdisplays

Researchers from the Fraunhofer Institute for Photonic Microsystems IPMS have significantly increased the transparency of OLED microdisplays. Transparent electronics have found use in ultrathin layers for touch display and as transparent films with printed antennas for mobile communications. As of yet, OLED displays have not been used for this purpose.

As part of the Fraunhofer Society’s HOT project (High-performance transparent and flexible micro-electronics for photonic and optical applications), researchers succeeded in the development of OLED microdisplays with 20% transparency. Recent work has pushed that further, enabling 45% transparency in a CMOS OLED microdisplay.

There are two fundamental methods to achieve semitransparency in optical systems, the first being the pixel approach, which involves creating transparent areas between individual pixels. The second method is the cluster approach, in which several pixels are grouped into a larger, nontransparent cluster in order to produce larger transparent areas in between these clusters.

Fraunhofer will present a transparent OLED microdisplay device at the 2024 International Meeting on Information Display in South Korea. Courtesy of Fraunhofer IPMS.

Both approaches are relevant for different applications in practice. The pixel approach is suitable, for example, for image overlay within a complex optical system, where the image is inserted between other image planes.

The cluster approach is particularly suitable for AR applications, such as in data glasses, where the pixel clusters are combined into a uniform virtual image using a micro-optic over each cluster. The transparent areas between the clusters remain unaffected by the optics, allowing a clear view of the real environment.

The technology for transparent microdisplays was developed to support both techniques.

The OLED-on-silicon technology uses a silicon backplane that contains the entire active-matrix drive electronics for the pixels. The organic frontplane is monolithically integrated on the topmost metallization layer, which simultaneously serves as the drive contact for the OLED. The second connection of the OLED is formed by a semitransparent top electrode shared by all pixels.

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 are made of aluminum or copper. Additionally, the optical structure of the OLED requires a highly reflective bottom electrode to ensure high optical efficiency upwards. These two aspects result in the pixels themselves not being transparent.

“A transparent microdisplay, however, can be realized through a spatially distributed design of this basic pixel structure, creating transparent areas between the pixels and minimizing column and row wiring. Further optimization of the OLED layers, for example, by avoiding OLED layers in the transparent areas, introducing anti-reflective coatings, and redesigning the wiring also contributes to increasing transparency,” said Philipp Wartenberg, group leader of Integrated circuit and system design at Fraunhofer IPMS.

The researchers will present a microdisplay at the International Meeting on Information Display 2024 showcasing the cluster approach with a new AR optic based on a microlens array. The optics were designed to enable a setup close to the eye with a similar distance to the eye as regular corrective glasses.