“`html

Current commercial devices often compromise with less effective blue OLEDs; however, an array of design advancements has produced a blue variant that is as resilient as efficient green OLEDs.

Research from the University of Michigan has shown that blue phosphorescent OLEDs can now match the lifespan of green phosphorescent OLEDs already utilized in devices, setting the stage for enhanced energy efficiency in OLED screens.

“This advancement elevates the blues to a level comparable to that of green lifetimes,” remarked Stephen Forrest, the Peter A. Franken Distinguished University Professor of Electrical Engineering and lead author of the research published in Nature Photonics.

“I can’t assert that the issue is entirely resolved—obviously it’s not resolved until it’s utilized in your display—but I believe we’ve illustrated the route to a genuine solution that has eluded the field for two decades.”

OLED displays are prevalent in flagship smartphones and high-end televisions, delivering exceptional contrast and energy efficiency, as brightness variations are managed by the light emitters rather than relying on a liquid crystal layer. Nevertheless, not all OLEDs exhibit the same energy efficiency.

In existing displays, red and green OLEDs generate light through the highly efficient phosphorescent mechanism, while blue OLEDs still depend on fluorescence. This indicates that while red and green OLEDs achieve a theoretical maximum of one photon per electron flowing through the device, blue OLEDs reach a significantly lower efficiency ceiling.

The challenge arises from the fact that blue light carries the highest energy that an RGB device must generate: The molecules in blue phosphorescent OLEDs (PHOLEDs) must manage higher energies compared to their red and green equivalents. Although most energy dissipates as blue light, if trapped, it can instead decompose the color-producing molecules.

Previously, Forrest’s team discovered a method to expel trapped energy more swiftly by incorporating a coating on the negative electrode that assists in converting energy into blue light. Haonan Zhao, a recent Ph.D. graduate in physics, likened it to creating a fast lane.

“On a roadway lacking sufficient lanes, impatient drivers may collide with one another, blocking traffic—analogous to two excitons colliding and generating excessive energy that damages the molecule,” explained Zhao, the first author of both that study and the current one. “The plasmon exciton polariton serves as our optical design for an exciton fast lane.”

The principles are rooted in quantum mechanics. When an electron enters through the negative electrode, it creates an “excited state” in one of the molecules responsible for producing blue light. That state consists of a negatively charged electron elevated to a higher energy level and a positively charged “hole” left behind—the combination forms an exciton.

Ideally, the electron would promptly revert to its original state and emit a blue photon, yet excitons that leverage the phosphorescent pathway tend to linger. Simply returning to their original state would contravene a law of quantum mechanics. However, excitons close to the electrode are quicker in producing photons because the shiny surface supports another quantum quasiparticle—surface plasmons—similar to ripples on the surface of a pond of electrons in the metal.

If the exciton in the light-emitting substance is positioned sufficiently near the electrode, it benefits from a bit of assistance in converting to blue light because it can transfer its energy into a surface plasmon—a phenomenon known as the Purcell effect. This occurs as the exciton oscillates like a broadcasting antenna, generating waves in the electrons located on the electrode’s surface. However, this is not inherently advantageous, as not all surface plasmons yield photons. For the photon to materialize, the exciton must attach to the surface plasmon, forming a plasmon exciton polariton.

Forrest’s team facilitated this process by layering a thin film of a carbon-based semiconductor onto the reflective electrode, promoting energy transfer from the exciton and allowing for resonation in the appropriate manner. This also extends the effect further into the light-emitting material, enabling excitons situated farther from the electrode to reap the benefits.

The team reported on this achievement last year, and they have since been integrating this effect with various approaches to ultimately create a blue PHOLED that can endure as long and illuminate as brightly as a green one. Here are the noteworthy aspects of the design:

• Two light-emitting layers (a tandem OLED): This halves the light-emitting load on each layer, decreasing the likelihood that two excitons will merge.
• Incorporating a layer that aids the excitons in resonating with surface plasmons near both electrodes, ensuring that both emitting layers can access the fast lane.
• The entire assembly functions as an optical cavity where blue light resonates between the two mirror-like electrodes. This shifts the color of the photons deeper into the blue spectrum.

This research received partial support from the Department of Energy and Universal Display Corporation.

Claire Arneson, a Ph.D. candidate in physics at U-M, also participated in this research.

The device was constructed at the Lurie Nanofabrication Facility and analyzed at the Michigan Center for Materials Characterization.

The team has secured a patent for the technology with help from U-M Innovation Partnerships and has licensed it to Universal Display Corp. Forrest, along with the University of Michigan, holds a financial stake in Universal Display Corp.

Forrest also serves as the Paul G. Goebel Professor of Engineering and is a professor of electrical computer engineering, materials science and engineering, physics, and applied physics.

“`