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Emission-layer-free OLEDs are more efficient

Emission-layer-free OLEDs are more efficient

By BizLED Bureau

Feb 15, 2017: Researchers from the South China University of Technology have developed a high efficiency OLED, which is directly based on a p–n junction instead of sandwiching an emission layer between a p-type hole-transport layer, and an n-type electron-transport layer.

Their paper “Realizing high-performance and low-cost fluorescent organic LEDs” describes a purely organic p–n junction that is directly used as the luminescent centre.

p-type and n-type organic semiconductors

The researchers’ planar device consists of p-type and n-type organic semiconductors that are sandwiched vertically between an indium tin oxide anode and a lithium fluoride/aluminium cathode. It is simpler to manufacture than devices that are based on an emission layer. It also benefits from lower driving voltages unlike traditional OLEDs where the interfaces with the emission and transport layers induce two energy barriers to go through.

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“The light-emission behaviour of our device is a result of the synergetic energy release from both the p-type and n-type materials. This is in contrast to conventional OLEDs, where the light generation occurs from single-molecule emitters”, the researchers said.

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The researchers used 1,1-bis[4-[ N,N-di(p-tolyl)-amino]phen yl]cyclohexane (TAPC) as the p-type semiconductor and 2,4,6-tris(3-(pyridin-3-yl)phenyl)-1,3,5-triazine (TmPyTZ) as an in-house developed n-type semiconductor with a strong electron-withdrawing capacity and a good electron-transport ability. The pn-OLED that is obtained is exhibited a high peak external quantum efficiency (η ext) of up to 12%, which given the device’s light out-coupling efficiency of 20%, was translated to an internal quantum efficiency (η int) of 60%, surpassing the theoretical maximum efficiency of 25% for a conventional fluorescent emitter.

New material combination

The researchers changed the material combination that were used in the p–n junction, and were able to moderate the emission colour of the pn-OLEDs. They also demonstrated green pn-OLEDs with external quantum efficiencies of over 10% for a stack that used 4,4′,4”-tri(N-carbazolyl)triphenylamine (TCTA) as the p-type transport layer. The pn-OLEDs needed low operating voltages that make them suitable for low power portable devices.

The paper states that the concept of an organic active p–n junction can be extended to other applications like photodetectors, organic light-emitting transistors, electrically pumped organic semiconductor lasers, etc.

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