Fraunhofer IPMS develops high-voltage CMOS backplane for brighter microdisplays...

22 Apr 2025

...while Lumileds and Eindhoven deploy metasurfaces to boost microLED output.

A common method to increase the brightness of OLEDs while maintaining a long lifespan is the use of multiple stacked OLEDs. Scientists at the Fraunhofer Institute for Photonic Microsystems (IPMS) have now developed a high-voltage CMOS backplane that enables exceptionally bright microdisplays.

These will be presented for the first time at the SID Display Week 2025, which will be held from May 13 to 15, 2025, in San José, USA (Booth No. 1135 at the German Pavilion).

Numerous applications benefit from the image quality and high brightness of OLED microdisplays. These displays are used in augmented reality glasses for vibrant and clearly visible content under varying lighting conditions, or in virtual reality headsets for realistic and bright images.

The IPMS announcement states, “OLEDs are considered limited at very high brightness in harsh environments. Therefore, microLEDs are often promoted as an alternative, claiming brightness levels even in the range of one million cd/m².”

MicroLEDs experience a significant efficiency loss at very high pixel densities, which are required in high-resolution microdisplays. This means they must be operated with more than 1A/cm². Furthermore, this technology is still not mature, especially for full color. In contrast, the current density for OLEDs during long-lifespan operation is typically below 100 mA/cm².

These limitations can be significantly improved by stacking OLED layers on top of each other. The current density of individual OLED layers is limited to ensure adequate lifespan and reliability. Stacking OLED layers increases the voltage drop and swing across the OLED stack. So the IPMS has now developed a high-voltage CMOS backplane for high-brightness OLED microdisplays.

Dr. Uwe Vogel, head of Microdisplays and Sensors at IPMS, commented, “We have developed an innovative pixel cell design that allows for a voltage swing of over 10 volts, enabling the operation of multiple stacked, top-emitting OLED layers. This approach enables full color maximum brightness of over 10,000 cd/m², while maintaining lifespan and reliability.”

By applying multiple stacked OLEDs on a high-voltage CMOS backplane, this brightness can now be extended to about 10,000 cd/m², significantly increasing market opportunities for very bright OLED microdisplays.

Lumileds and Eindhoven use metasurfaces to boost microLED output

A collaboration between Lumileds and Eindhoven University of Technology has produced a “technology breakthrough” that employs metasurfaces integrated at the chip level to improve microLED emission directionality and enhance efficiency.

The partners say that this technology is promising for a range of LED applications: “This advance offers the potential for greater microLED display luminance and significantly more effective optical coupling for applications such as augmented reality or data communication applications,” says the Lumileds announcement. The work is described in Nature Communications Engineering.

The researchers embedded a metasurface consisting of nano-sized disks within the p-contact layer of MicroLEDs and stimulated collective resonances with emitting dipoles in the active region. The enhanced light outcoupling and radiative recombination led to notable improvements in LED efficiency. Additionally, the design of the nanostructure lattice was tailored to manipulate the far-field emission pattern of the LED. The team successfully showed that on-axis candela can be doubled for comparable LED output.

“For various applications such as augmented reality and data communication, the typical Lambertian radiation pattern of LEDs is too broad to efficiently couple with secondary optics. It also limits luminance in high ambient light conditions for direct-view displays,” said Toni Lopez, Distinguished Scientist at Lumileds R&D. “To overcome these limitations, we turned to nanophotonics.”

Oleg Shchekin, CTO of Lumileds, added, “The practicality of this approach lies in integrating metasurfaces into common composite contact technology, which eliminates damage to the semiconductor while leveraging existing manufacturing technologies.

“Beyond MicroLEDs, this technology holds promise for a wide range of LED applications. The achievement of enhanced light extraction and radiative rate is a prelude to future LED products with higher efficiency than what is possible with the current state-of-the-art composite contact technologies.”

Lumileds is part of a multi-year partnership with Professor Jaime Gomez Rivas’ team at TU Eindhoven. This breakthrough demonstrates how narrowing the emission angle of LEDs can be achieved while also boosting efficiency and is just one of the beneficial results of this collaboration.