Lower power OLED displays could emerge from the latest work from an international collaboration between researchers at RIKEN, the University of California San Diego, the University of Tokyo, and the Institute for Molecular Science.
Organic light emitting diodes (OLEDs) have been with us for many years now. They offer many advantages over other types of display based on inorganic LEDs and liquid crystal displays. They can be flexible, thin, and preclude the need for a power-draining backlight.
Writing in the journal Nature, the team explains how they have found a new way to manipulate the "excitons"- the electron-hole pairs that are key for charge transport within an OLED. The team points out that a current passing through an OLED device creates exciton pairs and when these drop down to a lower energy level, they emit visible light in a quantum process. OLED excitons arise in one of two patterns - the spins are either in the same direction or opposing. The former, so-called triplet excitons, are three times more common than the latter, the singlet excitons. Singlet excitons are formed only at higher energy and they can ultimately convert into triplets, but it would cut energy costs in a device if singlets were not formed at all.
The team has now demonstrated that lowering the voltage so that only triplets are formed is possible where it was always assumed this would not work. They used precise single-molecule electroluminescence measurements with a scanning tunneling microscope (STM) and an optical detection system to observe the formation of excitons. A model system with a single, isolated molecule of the organic semiconductor 3, 4, 9, 10-perylenetetracarboxylicdianhydride (PTCDA) on metal-supported ultrathin insulating film was key to their understanding. By imparting a negative charge to this supported molecule, they could use the STM tip to induce luminescence in the molecule, and monitored record the emission spectrum of the resulting excitons. Critically, at low voltage, only triplets form. Theoretical calculations support the suggested 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," explains RIKEN's Kensuke Kimura. Team member Yousoo Kim adds that "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."