Very bright emissive microdisplays through multiple stacked OLED

Researchers at the Fraunhofer Institute for Photonic Microsystems IPMS have developed novel OLED stacks that allow exceptionally bright microdisplays.

Users of AR glasses require particularly bright displays in daylight to clearly recognise content. Therefore, high brightness and low power consumption are crucial development goals, as optical systems – such as AR glasses – exhibit high brightness losses and wearable devices are limited by battery storage. The institute introduces a highly efficient, monochrome OLED microdisplay with a brightness of over 70,000 Nits. The OLED stack used even reaches over 200,000 Nits on reference substrates.

Johannes Zeltner, a PhD student at Fraunhofer IPMS, explains: "By stacking OLED layers, this outstanding brightness was achieved. The individual OLED units are 'series connected', which increases the brightness with each additional unit without raising the current density in the component. This can be utilised to either achieve extremely high brightness or, at a given brightness, to significantly reduce the current density that determines lifetime. Measurements have shown that when comparing a 1-unit and a 2-unit OLED, the lifetime LT95, i.e., the drop in brightness by 5%, at 50,000 Nits can be significantly improved from 900 to 1300 hours."

The current efficiency and brightness of one, two, and three-fold stacked OLEDs were initially evaluated on passive test substrates and successfully transferred to 0.62" CMOS backplanes with SXGA resolution. New challenges for further research have emerged: While conventional OLED displays often have several tens of micrometres between subpixels, in microdisplays, it is only a few hundred nanometers. This can lead to crosstalk between adjacent pixels in thicker layer stacks and multiple stacked OLEDs in microdisplays. Approaches to lower this crosstalk are being prepared.

Also, the work has shown that multiple stacking permits for narrowband emission with high brightness. The spectral emission can be specifically adjusted, allowing the use of optical concepts with special requirements, such as waveguides or holographic elements.

The researchers are convinced that the ongoing development towards ever higher brightness and improved lifetimes of OLED technology secures a firm place in the field of AR applications. Nevertheless, research is continuously needed, e.g., on optical crosstalk, improved OLED materials, and novel backplane architectures.

The researchers offer the results attained and research services to interested partners worldwide to elevate the OLED microdisplay technology to the next level and integrate it into marketable products.

These results are presented for the first time at SPIE AR VR MR 2025, from January 28 to 29, 2025, in San Francisco (Booth No. 6202).