According to foreign media reports, an international research group led by Kensuke Kimura, a scientist from the Surface and Interface Science Laboratory of RIKEN, Japan, is developing a technology that can reduce the energy consumption of OLED displays.
The team recently published an article entitled Formation of Selective Triple Excitons in Single Molecules in the Journal Nature. With regard to this technology, the following background needs to be understood.
The current passing through the OLED material results in the formation of excitons, which are paired electrons and holes. More precisely, the current injected into OLED materials forms spin-singlet excitons (with opposite spins) and spin-triplet excitons (with the same spin direction) in a ratio of 1 to 3. Singletons are higher energy states that can be converted to triplets. Light is produced when singlet or triplet states are converted to lower energy states.
Phosphorescence (phenomena) in current generation OLED materials is usually based on triplet decay. Based on this fact, it is not efficient to use energy to produce singletons with higher energy. Materials/processes that generate triplet states only and emit light through triplet decay should have lower OLED operating voltage, thus improving the energy efficiency of the overall OLED display. However, up to now, no effective method has been developed to enhance the direct formation of triplet states. Therefore, the research team carried out to solve this problem.
The team's recent research aims to understand the basic physics behind exciton generation. To this end, the team developed a model system based on organic semiconductors called 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). The semiconductor is adsorbed on an ultra-thin insulating film consisting of three single layers of NaCl, which is supported by a metal film consisting of Ag (111). It should be pointed out here that (111) refers to the crystal structure of silver. They charge the molecule negatively in a specific way. In the next step, current from the scanning tunneling microscope (STM) can induce the molecule to emit light. The exciton type generated by this process is determined by an optical detection system for analyzing emission spectra. The team's recent research aims to understand the basic physics behind exciton generation. To this end, the team developed a model system based on organic semiconductors called 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). The semiconductor is adsorbed on an ultra-thin insulating film consisting of three single layers of NaCl, which is supported by a metal film consisting of Ag (111). It should be pointed out here that (111) refers to the crystal structure of silver. They charge the molecule negatively in a specific way. In the next step, current from the scanning tunneling microscope (STM) can induce the molecule to emit light. The exciton type generated by this process is determined by an optical detection system for analyzing emission spectra.
When high voltage is applied, both phosphorescent and fluorescent signals appear. However, when low voltage is applied, only phosphorescence appears. These results indicate that the selective formation of triplet excitons does not produce singlet excitons. The theoretical calculation confirms the experimental results and the mechanism.
The following figure (a) illustrates the configuration of the measurement; (b) the scanning tunneling microscope image of PTCDA adsorbed on NaCl grown on Ag (111) and (c) the luminescence spectrum produced by the material.
Describing their results, the team said: "We believe that we are able to do this mainly because of previously unknown mechanisms in which electrons can be selectively removed from charged molecules based on their spin states. Subsequently, we demonstrated a new method of manipulating excitons, which is very important for electron transport in OLED. By manipulating the electron spin inside the molecule, the electron transport process accompanied by the formation of excitons in OLED can be controlled.
In their research paper, they said they expected to "design a device that takes into account switching interaction to achieve OLED with lower operating voltage".
