By BizLED Bureau
October 27, 2015: Researchers in Japan have developed a new and highly efficient fluorescent material for blue OLED, that is organic LEDs. This new material is totally free of any metals. The researchers believe that the design approaches that they have used will help other researchers to develop other new and efficient LED materials.
To manufacture a white LED or a multi-colour LED, blue light is important. The 2014 Nobel prize for physics was awarded for developing blue gallium nitride LEDs.
OLEDs have several advantages over inorganic LEDs, which are very beneficial for flat panel displays and mobile phones because they are ultrathin, flexible and lightweight. Currently, the highly efficient OLEDs use organometallic complexes of metals such as iridium and platinum.
Researchers intended to develop OLEDs free of these metals. But, usually OLEDs without these materials become inefficient as three-quarters of the quantum created fail to emit photons.
For the last five years, Chihaya Adachi and his team at Kyushu University, Japan, have been working to find a process that is thermally activated delayed fluorescence, and which allows room temperature heat to convert into non-emissive states into emissive ones.
In 2012, Adachi and team developed a green LED using the same process. In 2014, the team developed a blue OLED with an external quantum efficiency, that is, the proportion of electrons passing through it that produce a photon, of 20%. This smashes a previous record of 11% for a metal free blue OLED. It is also getting very close to inorganic LEDs quantum efficiency. The team has developed a set of design methods using the molecules’ efficiencies, and demonstrated them with a series of emitters.
For thermally-activated delayed fluorescence to work efficiently, they showed that the emissive and non-emissive states have to be very close in energy. To achieve this, they calculate that both the states have to be delocalized, yet they have to be separated from each other. The team then followed density functional theory to design the molecules in which these conditions will get be satisfied. They tested a series of carbazole?triazine derivatives, as one which best combined a highly delocalised non-emissive state with a small separation between the energies of the two states, and was good emitter. The researchers now need to increase the efficiency of the devices by reducing the voltage needed to drive electrons through them.
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