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Unformatted text preview: Confinement of triplet energy on phosphorescent molecules for highly-efficient organic blue-light-emitting devices Shizuo Tokito a) NHK Science and Technical Research Laboratories, 1-10-11 Kinuta, Setagaya-ku, Tokyo 157-8510, Japan Toshiki Iijima Tokyo University of Science, 1 3 Kagurazaka, Tokyo 162-8610, Japan Yoshiyuki Suzuri and Hiroshi Kita Konica Corporation, No.1 Sakuramachi, Hino-shi, Tokyo 191-8511, Japan Toshimitsu Tsuzuki and Fumio Sato NHK Science and Technical Research Laboratories, 1-10-11 Kinuta, Setagaya-ku, Tokyo 157-8510, Japan ~ Received 31 March 2003; accepted 21 May 2003 ! We have significantly improved the emission efficiency in an organic light-emitting device ~ OLED ! based on iridium ~ III ! bis @~ 4,6-di-fluoropheny !-pyridinato-N,C 2 8 # picolinate ~ FIrpic ! . To improve the efficiency, 4,4 8-bis ~ 9-carbazolyl !-2,2 8-dimethyl-biphenyl, which has a high triplet energy, was used as the carrier-transporting host for the emissive layer. The FIrpic-based OLED exhibited a maximum external quantum efficiency of 10.4%, corresponding to a current efficiency of 20.4 cd/A, and a maximum power efficiency of 10.5 lm/W. The efficiency was drastically improved compared to that of a previously reported FIrpic-based OLED. This result indicates that triplet energy is efficiently confined on FIrpic molecules, resulting in the high efficiency. 2003 American Institute of Physics. @ DOI: 10.1063/1.1594834 # Blue emission in organic light-emitting devices ~ OLEDs ! is important for application to full-color flat-panel displays because blue is one of the three primary colors. 1 The blue light can also generate other low energy emissions such as green and red by using a color-change medium. 2 To ac- celerate the commercialization of OLEDs, though, improved blue-emission efficiency is needed. However, blue-emission efficiency is limited in previously developed OLEDs where organic fluorescence materials are used for the emissive layer; only the singlet states of the organic molecules con- tribute to the light emission, leading to an upper limit on the external quantum efficiency of 5%. A Princeton University group has demonstrated a way to break through the efficiency limitation by using certain or- ganic materials which emit phosphorescence from triplet states. 3,4 The phosphorescent materials used were iridium or platinum complexes with organic ligands, and they were doped as an emissive guest into a host material of the emis- sive layer. The phosphorescence can be understood in terms of the energy transfer from both the singlet and triplet states of the host molecules to triplet states of the phosphorescent guest molecules and/or by direct excitation of the phospho- rescent guest molecules followed by charge trapping. The devices employing phosphorescent materials showed a high external quantum efficiency of over 5% in green, red, and blue....
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