Researchers at the University of Michigan in the United States have recently made a major breakthrough in the field of blue phosphorescent organic light-emitting diodes (PHOLEDs), and have successfully developed blue light-emitting devices with a lifetime comparable to those of today's widely used green phosphorescent OLEDs. The results, published in the journal Nature Photonics, open up a new path to improve the overall energy efficiency of OLED screens.
The study was led by Stephen Forrest, Distinguished Professor of Electronic Engineering at the university. "Today, the lifespan of blue light devices has reached the level of green light, which means that we can finally hope to achieve more efficient and energy-efficient OLED displays," he said. "While this problem hasn't been fully solved – and we can't say that until it actually makes it into a commercial product – we've found a viable technology path that the industry hasn't been able to achieve in the last two decades."
OLED (organic light-emitting diode) technology has become the standard configuration of high-end smartphones and flagship TV screens due to its advantages of high contrast, wide color gamut and low power consumption. However, there are significant differences in the energy efficiency performance of different colored OLEDs. Today's commercially available red and green OLEDs use a highly efficient phosphorescent light-emitting mechanism with a theoretical maximum efficiency of nearly 100%, i.e., one photon per electron can be generated. However, due to material limitations, blue OLEDs still mainly rely on the fluorescence mechanism, resulting in their theoretical energy efficiency upper limit being much lower than that of red and green devices.
The core challenge for blue OLEDs is that they require more energy to emit light. Among the three primary colors of RGB, blue light has the shortest wavelength and the highest energy, so the requirements for light-emitting materials are more stringent. Traditional blue phosphorescent OLEDs (PHOLED) tend to accumulate a large amount of energy during operation, and if these energies are not released in time, they will accelerate the aging of materials and reduce the life of the device.
Fig:OLED Screen Performance Upgrade the Road to a Breakthrough in the Lifetime of Blue PHOLED
Previously, the Forrest team had proposed a way to accelerate energy release and improve luminous efficiency by adding a carbon-based semiconductor coating to the surface of the anode. This method is equivalent to constructing a "fast channel" for excitons, thereby reducing the energy loss and material destruction caused by exciton accumulation. Ph.D. graduate Haonan Zhao vividly said: "Just like when there are not enough lanes on the highway, drivers are prone to collisions and cause traffic paralysis, two excitons colliding with each other will produce high heat energy, which will destroy the molecular structure." We designed the 'exciton fast lane' to circumvent this problem.”
From a quantum physics perspective, when electrons are injected from the cathode and combine with holes to form excitons, they need to quickly transition back to the ground state to emit blue light. However, in the traditional structure, the phosphorescent exciton has a long lifetime and is prone to initiate a non-radiative recombination process. By introducing surface plasma near the metal electrode, the rate of exciton-to-photon conversion can be enhanced, which is known as the "Purcell effect". However, not all plasmas can be efficiently converted into photons, and efficient luminescence can only be achieved when excitons are coupled with plasma to form "plasma-exciton-polarons".
On this basis, the research team further optimized the structure of the device, adopted a tandem design of dual light-emitting layers, and introduced auxiliary resonance layers near the poles, so that both light-emitting layers can benefit from the "exciton fast lane". The entire structure forms an optical resonator that helps blue light resonate between two mirror electrodes, thereby increasing luminous efficiency and stabilizing the blue light wavelength.
In addition, the research was supported by the U.S. Department of Energy and the Universal Display Corporation (UDC), a world-renowned OLED materials company. Equipment fabrication and testing are done at the University of Michigan's Lurie Nanofabrication Laboratory and Materials Characterization Center, respectively.
Also involved in the study was Claire Arneson, a PhD student in the Department of Physics at the University of Michigan. Professor Forrest is also the Paul G. Goebel Professor of Engineering, where he is a professor in electrical and computer engineering, materials science and engineering, physics, and applied physics.
This major breakthrough in the lifetime of blue PHOLED not only improves the overall energy efficiency potential of OLED, but also provides a new direction for the development of future display technology. As the technology moves towards industrialization, consumers can expect to use next-generation OLED displays that are more energy-efficient, have more realistic colors, and last longer.
