Organic LED materials enable faster organic phosphorescence and improve display technology

Screens in televisions, smartphones and other displays could be made with a new type of organic LED material developed by an international team co-led by University of Michigan engineers. This material maintains vivid colors and contrast while replacing heavy metals with a new hybrid material.

Strangely enough, this material also appeared to violate quantum laws.

OLED devices currently on the market contain heavy metal components such as iridium and platinum, which improve the screen’s efficiency, brightness, and color range. However, they have the disadvantages of significantly higher costs, shorter device lifetimes, and increased health and environmental risks.

In OLEDs, energy-efficient phosphorescence is preferred over fluorescence, but phosphorescence is slow to occur in the absence of heavy metal components, taking milliseconds or more. To accommodate modern displays operating at 120 frames per second without producing lingering “ghost” images, phosphorescence must be accelerated by microseconds. This is an important role of heavy metals.

“We found a way to make phosphorescent organic molecules that can emit light on the microsecond scale without heavy metals in their molecular backbones,” said Professor of Materials Science and Engineering and co-author of the published study. One Jin-sang Kim said. At Nature Communications.

South Korea’s Dong Hyuk Park, professor of chemical and biomedical engineering at Inha University, and Sunkook Kim, professor of advanced materials science and engineering at Sungkyunkwan University, are also co-corresponding authors.

The speed difference between fluorescence and phosphorescence depends on what happens after the electrons from the electrical current flowing through the OLED material slip into higher energy levels within the molecules’ available electron orbitals, known as excited states. In a sense, this is like jumping onto a stair rung. Ladder. Fluorescence allows energy to be instantly released as light and jump back to the ground state. However, with phosphorescence, a conversion must first occur.

The transformation involves the spin of the electron. Each electron has a partner in the ground state, and the Pauli exclusion principle, a law of quantum mechanics, requires the electrons to rotate in opposite directions. But when the electrons slide into that higher step, each electron is alone in its orbit, so it can end up rotating in either direction. It remains on the opposite side of its partner only a quarter of the time, which is when fluorescence occurs.

Phosphorescence also utilizes the remaining 75% of the excited electrons, making it three times more efficient, but the electrons must flip their spins before returning to normal. In traditional phosphorescent materials, large nuclei of heavy metals generate a magnetic field that causes excited electrons to rotate rapidly in the same spin direction, resulting in faster light emission as they return to the ground state.

This new material achieves the same effect by placing a 2D layer of molybdenum and sulfur in close physical proximity to a similarly thin layer of organic light-emitting material without chemical bonding. This hybrid structure allows light to emit 1,000 times faster, making it fast enough for modern displays.

The light emission occurs entirely within the organic material without the weak bonds of organometallic and organic ligands, thus increasing the lifetime of the material. Phosphorescent OLEDs that rely on heavy metals also use metals to generate color, and when two excited electrons come into contact, the weak chemical bond between the metal and the organic material breaks, darkening the pixel.

Pixel burn-in is a problem specific to high-energy blue light and has not yet been solved, but the researchers hope their new design approach will help them achieve stable blue phosphorescent pixels. Current OLEDs use phosphorescent red and green pixels and fluorescent blue pixels, avoiding burn-in of the blue pixels at the expense of reduced energy efficiency.

Beyond its potential applications, analysis of this molecular hybrid system has allowed measurements that were once thought impossible. A pair of electrons that share an orbital appears to have coupled spins under dark conditions, suggesting a forbidden “triplet” state in which their spins should instead cancel each other out. .

“We still don’t fully understand what causes this triplet nature in the ground state, as this violates the Pauli exclusion principle. It’s highly unlikely, but looking at the measured data, yes, it seems that way. ” said Kim. “So we have a lot of questions about what actually makes that happen.”

The research team will explore how the material achieves a ground state with triplet properties, while also pursuing potential applications in spintronic devices.

Collaborators at the University of California, Berkeley and Dongguk University contributed to this research. Jinsang Kim is director of the polymer science and engineering academic program and professor of chemistry.