Lithium fluoride insulating layer improves organic light-emitting diodes

Jan. 1, 1998

An insulating layer of lithium fluoride (LiF) between the aluminum (Al) cathode and the organic layer of organic light-emitting diodes (OLEDs) has allowed researchers at the Optical Sciences Center of the University of Arizona (OSC; Tucson, AZ) to increase the efficiency of the devices, while also improving color purity and device stability. Organic LEDs based on QAD-doped Alq3 and a hole transport layer TPD yielded in excess of 40,000 cd/m2 and about 3% external quantum efficiency.

The OSC researchers obtained pure blue light--with an emission peak at 470 nm--with DPVBi as the light-emitting layer in an Al/LiF/DPVBi/TPD/ITO device. Compared to devices without the LiF layer at 9 V, luminance was increased more than 100 times and external quantum efficiency was increased by more than 100%. The grou¥also obtained an efficient and bright red emission--with a peak at 612 nm--from Mg/LiF/PBD/PBD:Eu(TTFA)3/TPD/ITO. The OSC grou¥also introduced devices based on metal/LiF/organics/LiF/ITO and metal/organics/LiF/ITO structures, which also enhanced OLED performance. The enhancement of device performance by a LiF layer points to a general phenomenon--also seen in polymeric OLEDs--that is currently under investigation by OSC and the chemistry department, according to researchers Ghassan Jabbour and Bernard Kippelen (see related story on p. 38).