Researchers at RIKEN and the University of California San Diego, together with international collaborators, have discovered a way to considerably bring down the amount of energy needed by organic light emitting diodes (OLEDs).
OLEDs have gained attention as promising replacements for liquid crystal diodes, as they provide benefits such as being thin, flexible, and not needing backlighting.
The team achieved the enhancement, reported in Nature, by creating a new method to control the “excitons”—pairs of holes and electrons—that are vital to the transport of electrons within OLEDs. Typically, current passing through the device develops such pairs, and when they shift to a lower energy level, they release visible light in the process.
Usually, the excitons in OLEDs occur in two patterns, with the spins being either in the same or opposite direction. The ones with same spins—technically called triplet excitons—are three times more frequent. However, the singlets, which are developed together with the triplets, need more energy, and while they can be converted into triplets, it still implies that the device as a whole needs the energy to develop them in the first place.
In the present study, the team discovered a way to decrease the voltage so that only triplets are created. The research started with basic study to understand the fundamental physics behind the formation of excitons using precise single-molecule electroluminescence measurements with the help of a scanning tunneling microscope (STM) combined with an optical detection system. They prepared a model system on the basis of an isolated molecule of 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA), an organic semiconductor, adsorbed on a metal-supported ultrathin insulating film.
They used a unique method to impart a negative charge to the molecule. After that, they used the current from an STM (scanning tunneling microscope) to induce luminescence in the molecule and observed what type of exciton was developed based on the emission spectrum. The measurements indicated that at low voltage, only triplets were created. Theoretical calculations by Kuniyuki Miwa and Michael Galperin at UC San Diego validated the experimental outcomes and corroborated the mechanism.
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.
Kensuke Kimura, RIKEN Cluster for Pioneering Research
It was very exciting to discover this new mechanism. We believe that these findings could become a general working principle for novel OLEDs with low operating voltage.
Yousoo Kim, Leader, Surface and Interface Science Laboratory, RIKEN CPR
The research was conducted by scientists from RIKEN, the University of California San Diego, the University of Tokyo, and the Institute for Molecular Science.