The Fraunhofer Institute for Photonic Microsystems IPMS has made a breakthrough in increasing the transparency of OLED microdisplays.
While transparent electronics are already utilized in various applications, such as ultra-thin touch display layers and transparent films with printed antennas for mobile communications, transparent OLED microdisplays were previously unavailable.
Thanks to the HOT project (“High-performance transparent and flexible micro-electronics for photonic and optical applications,” funding number MAVO 840092) funded by the Fraunhofer Society, OLED microdisplays with 20% transparency were successfully developed. Building on this success, the latest advancement has achieved an impressive 45% transparency in a CMOS OLED microdisplay.
The innovative technology of OLED-on-silicon incorporates a silicon backplane housing the complete active matrix drive electronics for the pixels, enhancing efficiency and performance.
The organic frontplane is seamlessly integrated on the topmost metallization layer, serving as the drive contact for the organic light-emitting diode. A semi-transparent top electrode connects the OLED’s second connection, benefitting all pixels.
Utilizing silicon CMOS technology, the pixel circuitry necessitates multiple metal layers to link the embedded transistors, mainly composed of aluminum or copper. Additionally, a highly reflective bottom electrode is crucial for optimal upward optical efficiency, although the resulting pixels themselves are not 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,” explains Philipp Wartenberg, group leader of IC and system design at Fraunhofer IPMS, “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.”
There are two essential methods for achieving semi-transparency in optical systems: The pixel approach creates transparent areas between individual pixels, while the cluster approach groups several pixels into a larger, non-transparent cluster, creating larger transparent areas between these clusters. Both approaches have practical applications. The pixel approach is suitable for image overlay within a complex optical system, where the image is inserted between other image planes.
The cluster approach offers a perfect solution for augmented reality (AR) applications, especially in data glasses. Pixel clusters are combined into a seamless virtual image using micro-optics over each cluster. This technology allows for transparent areas between the clusters, providing a clear view of the real environment.
A new AR optic was showcased at IMID, demonstrating the cluster approach using transparent microdisplays. The optical combination of pixel clusters into a uniform virtual image is achieved through a microlens array designed to be positioned close to the eye, similar to regular corrective glasses.
