White Phosphorescent OLED Avoids Exciton-Blocking Layers

Daniel S. Burgess

A research team at Tsinghua University in Beijing has reported that exciton-blocking layers can be eliminated from a white phosphorescent organic LED (OLED) through the proper choice of different hosts for the red and blue emissive layers in a two-layer device, with potential applications in full-color OLED displays and in solid-state lighting. A device constructed using the approach displayed a maximum current efficiency and luminance of 10.5 cd/A and 22,000 cd/m², respectively.

The white phosphorescent organic LED employs the energy gap of different hosts for its blue and red emissive layers, rather than exciton-blocking layers, simplifying design and reducing driving voltage. Courtesy of Lian Duan, Tsinghua University.

Lian Duan, a researcher at the Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education at the university, explained that phosphorescent OLEDs convert both singlet and triplet excitons into photons, promising much higher external quantum efficiencies than fluorescent OLEDs. The most typical approach to fabricating a white phosphorescent OLED, however, involves the use of one host material for the two or three phosphorescent materials. This necessitates the incorporation of exciton-blocking layers to stabilize the device’s color, complicating the design and increasing the driving voltage.

“We found that, by choosing the right host materials for each color, exciton-blocking layers are not necessary,” Duan said. Instead, the energy gap of the hosts confines the diffusion of excitons.

In a demonstration of their approach, the investigators employed N,N'-dicarbazolyl-1,4-dimethene-benzene and iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C^2'](picolinato) as the host and guest materials, respectively, in the blue-emitting layer and 4,4'-N,N'-dicarbazole-biphenyl and iridium (III) bis(1-phenyl-isoquinoline)(acetylacetonate) as host and guest in the red layer.

They fabricated the OLEDs in a high-vacuum deposition chamber, sandwiching the emissive layers between ITO and Mg:Ag electrodes and organic hole- and electron-transport layers. Keeping the concentrations of the phosphorescent materials in the hosts constant, they optimized the output spectrum of the device by varying the thickness of the emissive layers, settling on a 20-nm-thick blue- and a 10-nm-thick red-emitting layer.

The researchers evaluated the performance of the device using a 4200-SCS semiconductor characterization system from Keithley Instruments Inc. of Cleveland and a PR-650 SpectraScan colorimeter from Photo Research Inc. of Chatsworth, Calif. The OLED displayed a turn-on voltage of approximately 5 V and a maximum luminance of 22,000 cd/m² at a bias voltage of 18.2 V.

CIE color coordinates were relatively stable as the bias was increased from 6 to 14 V, shifting from (0.28, 0.37) to (0.32, 0.30). The current efficiency gradually decreased at high current densities, which they noted is the result of triplet-triplet annihilation common to electrophosphorescent devices.

Duan said that white OLEDs are still in their infancy but show potential. He estimated that the devices will achieve luminous efficacies of more than 50 lm/W and lifetimes greater than 50,000 h at a brightness of 1000 cd/m² for lighting applications before 2010. Making this a reality will require high-triplet-energy host and hole-transporting materials as well as a means of enhancing outcoupling.

“We will continue to design and synthesize even better organic materials for OLEDs,” Duan said.

Applied Physics Letters, March 6, 2006, 103508.