Copper Raises OLED Efficiency to 100%

By Tom Allen

Scientists at the Karlsruhe Institute of Technology (KIT), Cynora, and the University of St Andrews have made contributions to the advancement of OLED power efficiency, using copper. Specifically, the researchers measured the underlying quantum mechanics phenomenon of intersystem crossing (ISC) in a copper complex.

Copper, used as a fluorescent material, enables the manufacture of inexpensive and more environmentally-friendly OLEDs, it was said. Thermally-activated delayed fluorescence (TADF) ensures high light yield.

OLEDs are built using thin layers of organic materials, which serve as an emitter; these are located between two electrodes. When voltage is applied, electrons move from the cathode and combine with holes from the anode, where they form pairs, called excitons. The decay back into their original state produces energy.

Excitons can assume two different states: singlet and triplet. Singlet excitons decay immediately and emit the energy in the form of light; triplet excitons release energy as heat. The ratio between the two in an OLED is usually 25% singlet and 75% triplet. Triplet excitons must be used to generate light in order to increase OLED’s energy efficiency. Heavy metals such as iridium are added to traditional OLEDs for this purpose, although these are expensive and require complex OLED production methods.

Using copper instead of heavy metals is cheaper and friendlier to the environment. TADF, in which triplet excitons are transformed into a singlet state, ensures high light yields and, thus, efficiency. TADF is based on the above quantum mechanics phenomenon of ISC: a transition from one electronic excitation state to another one of changed multiplicity.

In organic molecules, ISC is determined by spin-orbit coupling: the interaction of the orbital angular momentum of an electron in an atom with the electron’s spin. In this way, all excitons can be used to generate light. With TADF, say the researchers, copper luminescent material reaches an efficiency of 100%.

The researchers measured the speed of ISC in a luminescent, TADF copper complex in the solid state. They determined a time constant of intersystem crossing, from singlet to triplet, of 27 picoseconds. The reverse process, from triplet to singlet, is slower and leads to a TADF lasting, on average, 11.5 microseconds.

These measurements improve the understanding of mechanisms leading to TADF. They could also lead to the specific development of TADF materials for energy-efficient OLEDs.

The research was published in the journal Science Advances, in January.