New organic LED technology, developed by a global team led in part by engineers from the University of Michigan, could revolutionize screens for TVs, smartphones, and other devices. This innovative material ensures vibrant colors and sharp contrasts while replacing heavy metals with a hybrid composition.
Interestingly, this new material appears to defy a quantum rule.
Currently, OLED devices contain heavy metals such as iridium and platinum, which enhance brightness, efficiency, and color range. However, these metals come with significant drawbacks, including higher costs, reduced device lifespan, and increased health and environmental risks.
While light emission from OLEDs is ideally achieved through phosphorescence for better energy efficiency, the process is inherently slower, often taking milliseconds or longer without heavy metals. To meet the demands of modern screens operating at 120 frames per second, phosphorescence must occur in microseconds, avoiding unwelcome lingering “ghost” images—a vital function of the heavy metals.
“We have developed a phosphorescent organic molecule capable of emitting light on a microsecond timescale without the need for heavy metals in its structure,” explained Jinsang Kim, a professor of materials science and engineering at U-M and co-corresponding author of the study published in Nature Communications.
Co-corresponding authors include Dong Hyuk Park, a professor at Inha University, and Sunkook Kim, a professor at Sungkyunkwan University, both in South Korea.
The difference in speed between fluorescence and phosphorescence stems from the behavior of electrons when an electrical current excites them to a higher energy level within the molecule’s electron orbitals, termed the excited state—similar to jumping to a higher rung on a ladder. In fluorescence, electrons can swiftly release energy as light by returning to the ground state. Conversely, phosphorescence requires a conversion process first.
This conversion is linked to the electron’s spin. In their ground state, each electron is paired, adhering to the Pauli Exclusion Principle, which states they must spin in opposite directions. When one electron transitions to a higher energy level, it may spin in either direction because it’s no longer paired, leading to a situation where it spins oppositely with only a 25% chance—this scenario leads to fluorescence.
Phosphorescence utilizes the remaining 75% of excited electrons, making it three times more efficient, but requires the electron to change its spin before it can return to its original state. In traditional phosphorescent materials, the heavy metal’s large atomic nucleus generates a magnetic field that prompts this spin flip, facilitating quicker light emission as the electron descends to the ground state.
The novel material layers molybdenum and sulfur in a 2D configuration next to a similarly thin layer of the organic light-emitting substance, achieving the desired effect simply through proximity, without any chemical bonds. This hybrid setup accelerates light emission by 1,000 times, which is rapid enough for current display technology.
Light emission occurs entirely within the organic layer without relying on weak metal-organic ligand bonding, enhancing the material’s durability. Traditional phosphorescent OLEDs depend on heavy metals for color generation; the breakage of weaker bonds when excited electrons meet can lead to pixel dimming.
Pixel burnout, especially prevalent with high-energy blue light, remains a challenge, but the research team is optimistic that their new design could contribute to stable blue phosphorescent pixels. Current OLED technology uses phosphorescent red and green pixels but relies on fluorescent blue pixels to prevent burnout, which comes at the cost of lower energy efficiency.
Beyond practical applications, studies of this molecular hybrid system revealed something previously deemed impossible: paired electrons in the same orbital exhibited a combined spin in dark conditions, implying a forbidden ‘triplet’ state that conflicts with the expectation that their spins should cancel each other out.
“We still don’t have a complete grasp of why this triplet characteristic exists in the ground state since it contradicts the Pauli Exclusion Principle. This scenario seems improbable, but the data indicates this may indeed be the case,” Kim continued. “We have many questions about the underlying mechanisms at play.”
The research team will further investigate how this material achieves triplet character ground states while also exploring potential applications in spintronics devices.
They have sought patent protection with support from U-M Innovation Partnerships and are looking for collaborators to develop devices using this groundbreaking material.
Funding for this research came from the National Research Foundation of Korea and a START grant from the U-M College of Engineering.
Contributors to the study included researchers from the University of California, Berkeley, and Dongguk University. Jinsang Kim also serves as the director of academic programs for macromolecular science and engineering and is a professor of chemistry.
