New mechanism reduces energy requirements for OLED displays

Scientists from RIKEN and the University of California at San Diego, working with international partners, have found a way to dramatically reduce the amount of energy required by organic light emitting diodes (OLEDs). OLEDs have attracted attention as potential substitutes for liquid crystal diodes because they offer advantages such that they are flexible, thin and do not require backlighting.

The group made the progress, published in Nature, developing a new way to manipulate the "excitons" – pairs of electrons and holes – which are the key to electron transport within OLEDs. The current flowing through the device creates such pairs, which then pass to a lower energy level and emit visible light during the process. Normally, excitons in OLEDs occur in two different ways: the spins are identical or opposite, and those with the same spins, known technically as excitons of triplets, are three times more common. However, singlets, which are created with triplets, require more energy and, although they can be converted into triplets, it does mean that the device as a whole needs the energy to create them in the first place.

In the work in progress, the group found a way to lower the voltage to form only triplets. The work began with a fundamental research to understand the physical basis of exciton creation using precise measurements of electroluminescence of a single molecule with the help of ## 147 ## ## # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 39, a tunneling microscope (STM) combined with an optical detection system. They prepared a model system based on an isolated molecule of 3, 4, 9, 10-perylenetetracarboxylicdianhydride (PTCDA), an organic semiconductor, adsorbed on an ultra-thin insulating film with a metal support. They used a special technique to impart a negative charge to the molecule. Then they used the current of a STM (Tunneling Microscope) to induce luminescence in the molecule and controlled the type of exciton created according to the emission spectrum. Measurements showed that at low voltage, only triplets were formed. The theoretical calculations made by Kuniyuki Miwa and Michael Galperin at UC San Diego confirmed the experimental results and corroborated the mechanism.

"We think," said Kensuke Kimura of the RIKEN Pioneering Research Cluster, "that we were able to do this through a previously unknown mechanism, in which the electrons are selectively removed from the charged molecule according to their spin state" .

"It was very exciting to discover this new mechanism," says Yousoo Kim, head of RIKEN CPR's scientific laboratory for surfaces and interfaces. "We believe these results could become a general operating principle for new low-voltage OLEDs operating."