Copper Complex's Quantum Properties Increase OLED Efficiency

A thermally activated fluorescent copper complex has been used to manufacture inexpensive, highly efficient organic LEDs (OLEDs).

Researchers from Karlsruhe Institute of Technology and the University of St. Andrews in Scotland prototyped the novel light sources and also measured the underlying quantum mechanics to explain their higher output.

OLEDs comprise ultrathin layers of organic materials, which serve as emitters and are located between two electrodes. When voltage is applied, electrons from the cathode and holes (positive charges) from the anode are injected into the emitter, where they form electron-hole pairs, called excitons. When excitons decay into their initial state again, they release energy.

A copper complex can be used to increase the efficiency of organic LEDs thanks to its electrical properties at the quantum level. Courtesy of Karlsruhe Institute of Technology.

Excitons may assume two different states: Singlet excitons decay immediately and emit light, whereas triplet excitons release their energy in the form of heat. Usually, the researchers said, 25 percent singlets and 75 percent triplets are encountered in OLEDs.

Triplet excitons can also be made to generate light. In conventional LEDs, heavy metals such as iridium and platinum are added for this purpose. However, these materials are expensive, have a limited availability, and require complex OLED production methods.

To address this challenge, the researchers examined the quantum mechanical phenomenon of intersystem crossing in their copper complex, which is classified as a thermally activated delayed fluorescence (TADF) material.

TADF is an emerging technology that enables the transformation of triplets into light-producing singlets. In organic molecules, this process is determined by spin-orbit coupling, or the interaction of the orbital angular momentum of an electron in an atom with the spin of the electron.

With TADF, the copper luminescent material reached an efficiency of 100 percent, the researchers said. They also determined a time constant of intersystem crossing from singlet to triplet of 27 ps. The reverse process from triplet to singlet is slower and leads to a TADF lasting for an average of 11.5 μs.

The research was published in Science Advances (doi: 10.1126/sciadv.1500889).