Oligomeric Derivatives Yield Efficient Blue OLED

Daniel S. Burgess

In the pursuit of better blue-emitting materials for organic LEDs (OLED), scientists in China have fabricated emitters based on derivatives of oligo(phenylenevinylene) that display CIE color coordinates of (0.154, 0.255) and a luminous efficiency of 12.2 cd/A. The investigators are affiliated with the State Key Laboratory of Integrated Optoelectronics and the Key Lab for Supramolecular Structure and Materials of the Ministry of Education, both located at Jilin University in Changchun.

Although they currently require the use of more complex vacuum thermal deposition processes to yield OLEDs, oligomers offer an advantage over their polymeric counterparts in that their lower molecular weight makes it possible to obtain higher-purity materials that offer higher device efficiencies and stabilities, said Shiyong Liu of the State Key Laboratory of Integrated Optoelectronics. They also can be used to fabricate multilayer structures to boost efficiencies further.

The researchers investigated the performance of devices incorporating an active region of 2,5,2',5'-tetrastyrylbiphenyl doped with 1,4-di (4'-N,N-diphenylaminostyryl) benzene at 1, 2, 4 or 8 percent by weight. They discovered that the luminous efficiency fell at dopant concentrations of less than 1 percent and at those of more than 4 percent, which they attributed to inefficient energy transfer from host to guest and to fluorescence quenching, respectively.

The best results were obtained from devices doped at 2 percent by weight (see figure), yielding an external quantum efficiency of 6.2 percent and a maximum power efficiency of 6.39 lm/W.

Atomic force microscope images reveal 2,5,2',5'-tetrastyrylbiphenyl doped with 1,4-di (4'-N,N-diphenylaminostyryl)benzene at 0 percent (a), 2 percent (b), 10 percent (c) and 50 percent (d) by weight. The guest material is dispersed uniformly in the host at a dopant concentration of 2 percent by weight. Courtesy of Shiyong Liu.

Liu said that the lifetimes of blue OLED materials continue to be a concern for display and lighting applications and that the team is working to characterize the lifetime of the new material. It will continue to explore the potential of other derivatives of oligo(phenylenevinylene), already using one in a white OLED to obtain a maximum brightness of 13,290 cd/m² and luminous efficiency of 14.97 cd/A, and others to offer a higher photoluminescence quantum yield and deep-blue emission.

Applied Physics Letters, May 29, 2006, 223508.