Mechanism reduces energy requirements for OLEDs

Scientists have developed a new mechanism that could significantly reduce the amount of energy required by organic light emitting diodes (OLEDs).

RIKEN and the University of California San Diego scientists, in collaboration with international partners, have developed a way to manipulate the 'excitons' — pairs of electrons and holes — that are key to the transport of electrons within OLEDs.

Current passing through the device creates such pairs, which then change to a lower energy level and emit visible light in the process. Normally, the excitons in OLEDs arise in two patterns, with the spins being either the same (triplets) or opposite (singlets). The singlets require more energy, and although they can be converted into triplets, the device still requires the energy to create them in the first place. In the study, the researchers found a way to lower the voltage so that only triplets are formed.

They researched the physics behind the creation of excitons using precise single-molecule electroluminescence measurements using a scanning tunnelling microscope (STM) combined with an optical detection system.

The researchers prepared a model system based on an isolated molecule of organic semiconductor 3, 4, 9, 10-perylenetetracarboxylicdianhydride (PTCDA), adsorbed on a metal-supported, ultrathin insulating film. They imparted a negative charge to the molecule, used the current from an STM to induce luminescence in the molecule and monitored what type of exciton was created based on the emission spectrum.

The measurements showed that at low voltage, only triplets were formed. Theoretical calculations confirmed the results.

"We believe that we were able to do this thanks to a previously unknown mechanism, where electrons are selectively removed from the charged molecule depending on their spin state," said Kensuke Kimura of the RIKEN Cluster for Pioneering Research.

Yousoo Kim, Leader of the Surface and Interface Science Laboratory in the RIKEN CPR, said: "We believe that these findings could become a general working principle for novel OLEDs with low operating voltage."

The research was published in Nature.

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