Emission-layer-free OLED promises better efficiency

February 09, 2017 // By Julien Happich
Researchers from the South China University of Technology have devised 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.

Their paper "Realizing high-performance and low-cost fluorescent organic LEDs" published in the SPIE Newsroom describes a purely organic p–n junction directly used as the luminescent centre.

The authors explain that their 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.

"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", they write.

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/aluminum.

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 thus obtained 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.

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 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.