Direct p–n junction improves OLED efficiency

Article By : Julien Happich

Changing the material combination used in the p–n junction results to simpler manufacture, low operating voltages ideal for portable devices.

A team of researchers from the South China University of Technology (SCUT) has developed a high efficiency OLED based on a direct p–n junction rather than sandwiching an emission layer between a p-type hole-transport layer and an n-type electron-transport layer.

The study describes a purely organic p–n junction directly used as the luminescent centre, explaining that a planar device consisting of p-type and n-type organic semiconductors sandwiched vertically between an indium tin oxide anode and a lithium fluoride/aluminium cathode, is not only simpler to manufacture than devices based on an emission layer, but it also benefits from lower driving voltages than traditional OLEDs where the interfaces with the emission and transport layers typically induce two energy barriers to go through.

High peak external quantum efficiency

"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," according to the SCUT research team.

[SCUT modified OLED fig1 (cr)]
Figure 1: Schematic diagram of the organic p–n junction LED (pn-OLED) device configuration. EQE max: Maximum external quantum efficiency. ITO: Indium tin oxide. LiF/Al: Lithium fluoride/aluminium.

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 modified pn-OLED 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.

[SCUT modified OLED fig2 (cr)]
Figure 2: The molecular structures of the p-type and n-type organic semiconductor materials used. TAPC: 1,1-bis[4-[ N,N-di(p-tolyl)-amino]phen-yl]cyclohexane.TCTA:4,4',4”-tri(N-carbazolyl)triphenylamine. TmPyTZ: 2,4,6-tris(3-(pyridin-3-yl)phenyl)-1,3,5-triazine.

By changing the material combination used in the p–n junction, the researchers were able to modulate the emission colour of their pn-OLEDs, demonstrating green pn-OLEDs with external quantum efficiencies over 10% for a stack using 4,4',4”-tri(N-carbazolyl)triphenylamine (TCTA) as the p-type transport layer. The pn-OLEDs required low operating voltages, making them suitable for low power portable devices.

The paper concludes by extending the concept of an organic active p–n junction to other possible uses, including photodetectors, electrically pumped organic semiconductor lasers and organic light-emitting transistors.

First published by EE Times.

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