A hybrid material made from organic chromophores and transition metal dichalcogenides (TMDs) can produce stable, fast, phosphorescent light emission for OLED displays. The new hybrid, developed by a team co-led by the University of Michigan (U-M), could replace the heavy metal components currently used to improve efficiency, brightness, and color range in OLED devices. Organic materials with room-temperature phosphorescence are an appealing alternative to heavy metals because of their tunable luminescent properties, large design window, environmentally-friendly components, and economical production cost. Phosphorescence is 3 times more energy-efficient than fluorescence, but happens more slowly. To keep pace with modern displays, which operate at 120 frames per second, phosphorescence must occur in microseconds. The metals used in OLEDs, like iridium and platinum, enable phosphorescence to take place in microseconds instead of milliseconds. The large atomic nucleus of the heavy metal generates a magnetic field that causes the excited electrons to emit light faster as they go from the excited to the ground state. A new organic molecule with fast phosphorescence is compared to conventional phosphorescence. Screens for TVs, smartphones, and other displays could be made with a hydrid material developed by an international team, co-led by University of Michigan engineers. The material maintains sharp color and contrast while replacing the heavy metals used in existing OLEDs. Courtesy of the University of Michigan. The researchers developed an alternative strategy for developing emitters for phosphorescence by creating heterostructures of organic chromophores and TMDs. The heterostructures were made of diethyl 2,5-dihydroxy terephthalate (DDT), an organic fluorophore, using various TMDs. A 2D layer of molybdenum (MoS2) and sulfur is positioned near a similarly thin layer of the organic light-emitting material, achieving physical proximity without any chemical bonding. Light emission occurs entirely within the organic material, without the need for weak, metal-organic ligand bonding. The team observed the TMD-induced photophysical variations in the DDT and found that the DDT on the TMDs emitted microsecond phosphorescence at room temperature. It further found that spin-orbit couplings of the DDT were enhanced by the through-space, spin-orbit proximity effect of the TMDs in the heterostructures. The hybrid construction increased light emission by 1000 times, achieving speeds fast enough for modern displays. “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,” professor Jinsang Kim said. Phosphorescent OLEDs that rely on heavy metals also use the metals to help produce color. The weak chemical bonds between the metal and the organic material can break apart when two excited electrons come into contact, dimming the pixel. Pixel burnout in high-energy blue light has yet to be resolved, but the researchers hope their new design approach will contribute to stable, blue phosphorescent pixels. Currently, OLEDs use phosphorescent red and green pixels and fluorescent blue pixels, avoiding blue pixel burnout at the expense of lowering energy efficiency. When the researchers analyzed the molecular hybrid system, they made an unusual discovery — the system appeared to break a rule of quantum mechanics. Paired electrons sharing an orbital seemed to have a combined spin under dark conditions, suggesting a “forbidden” triplet state, when instead their spins should have cancelled one another out. According to a principle of quantum mechanics, the Pauli Exclusion Principle, an electron and its partner in the ground state must spin in opposite directions. “We don’t yet fully understand what causes this triplet character in the ground state because this violates the Pauli Exclusion Principle,” Kim said. “That’s why we have a lot of questions about what really makes that happen.” The researchers plan to explore how the hybrid 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 the hybrid material. In addition to the team from U-M, researchers from Inha University; Sungkyunkwan University; the University of California, Berkeley; and Dongguk University contributed to the study. The research was published in Nature Communications (www.doi.org/10.1038/s41467-024-51501-8).
