Faster organic phosphorescence for better display tech

Curiously, the material also seemed to break a quantum rule.

OLED devices currently on the market include heavy metal components like iridium and platinum, which improve the efficiency, brightness and color range of the screen. But they come with drawbacks -- significantly higher cost, a shorter device lifetime and increased health and environmental hazards.

In OLEDs, light emission through the more energy-efficient phosphorescence is preferred over fluorescence, but phosphorescence happens more slowly, taking milliseconds or longer without the heavy metal component. Speeding up phosphorescence to happen in microseconds is necessary to keep up with modern displays, which operate at 120 frames per second, without producing a lingering "ghost" image. This is a key role of the heavy metals.

"We found a way to make a phosphorescent organic molecule that can emit light on the microsecond scale, without including heavy metals in the molecular framework," said Jinsang Kim, U-M professor of materials science and engineering and co-corresponding author of the study published in Nature Communications.

Dong Hyuk Park, professor of chemical and biomedical science and engineering at Inha University, and Sunkook Kim, professor of advanced materials science and engineering at Sungkyunkwan University, both in the Republic of Korea, are also co-corresponding authors.

The speed difference between fluorescence and phosphorescence is driven by what happens after electrons from the electrical current running through the OLED material slide into the high energy level within the molecule's available electron orbitals, known as an excited state -- sort of like jumping onto a rung of a ladder. In fluorescence, they can immediately release the energy as light, jumping back down to the ground state. But in phosphorescence, they have to make a conversion first.

The conversion has to do with the electron's spin. Each electron has a partner in its ground state, and a quantum mechanical rule -- Pauli Exclusion Principle -- demands that they spin in opposite directions. But when an electron slides into that higher rung, it can end up spinning in either direction because each electron is now alone in its orbital. It only remains opposite its partner a quarter of the time, and this is the case that results in fluorescence.

Phosphorescence is three times more efficient because it harnesses the other 75% of excited electrons too, but it requires the electron to flip its spin before it can come back down. In conventional phosphorescent materials, the large atomic nucleus of the heavy metal generates a magnetic field that forces the same spin direction excited electron to turn quickly, resulting in faster light emission as it returns to its ground state.

The new material positions a 2D layer of molybdenum and sulfur near a similarly thin layer of the organic light emitting material, achieving the same effect by physical proximity without any chemical bonding. This hybrid construction sped up light emission by 1,000 times, achieving speeds fast enough for modern displays.

Light emission happens entirely within the organic material without having the weak metal-organic ligand bonding, helping the material last longer. Phosphorescent OLEDs that rely on heavy metals also use the metals to help produce the color, and the weaker chemical bonds between the metal and organic material can break apart when two excited electrons come into contact, dimming out the pixel.

Pixel burnout is a particular problem for high energy blue light that has yet to be solved, but the research team hopes their new design approach can help work towards stable, blue phosphorescent pixels. Current OLEDs use phosphorescent red and green pixels and fluorescent blue pixels, avoiding blue pixel burnout at the expense of lower energy efficiency.

Beyond the potential applications, analysis of this molecular hybrid system measured something once thought to be impossible -- paired electrons sharing an orbital seemed to have a combined spin under dark conditions, suggesting a forbidden 'triplet' state when instead their spins should cancel one another out.

"We don't yet fully understand what causes this triplet character in the ground state because this violates the Pauli Exclusion Principle. That is very impossible, but looking at the measurement data, yes, that seems to be the case," Kim said. "That's why we have a lot of questions about what really makes that happen."

The research team will explore how the material achieves triplet character ground states while also pursuing potential spintronics device applications.

The team has applied for patent protection with the assistance of U-M Innovation Partnerships and is seeking partners to create devices using this new type of material.

This work was supported by the National Research Foundation of Korea grant funded by the Korea government and START grant from the U-M College of Engineering.

Collaborators from the University of California, Berkeley and Dongguk University contributed to the study. Jinsang Kim is also a director of academic programs for macromolecular science and engineering and a professor of chemistry.