[18] D. Temple, C. A. Ball, W. D. Palmer, L. N. Yadon, D. Vellenga, J. Mancusi, G. E. Mcguire, H. F. Gray, J. Vac. Sci. Technol. B 1995, 13, 150. [19] C. R. Martin, Science 1994, 266, 1961. [20] G. Che, B. B. Lakshmi, E. R. Fisher, C. R. Martin, Nature 1998, 393, 346. [21] S. A. Sapp, B. B. Lakshmi, C. M. Martin, Adv. Mater. 1999, 11, 402. [22] D. Xu, Y. Xu, D. Chen, G. Guo, L. Gui, Y. Tang, Adv. Mater. 2000, 12, 520. [23] G. A. Bootsma, H. J. Gassen, J. Cryst. Growth 1971, 10, 223. [24] D. Crouse, Y. Lo, A. E. Miller, M. Crouse, Appl. Phys. Lett. 2000, 76, 49. [25] H. Masuda, K. Fukuda, Science 1995, 268, 1466. [26] H. Masuda, F. Hasegwa, J. Electrochem. Soc. 1997, 441, 1127. [27] D. G. W. Goad, M. Moskovits, J. Appl. Phys. 1978, 49, 2929. Rare Earth Complex as a High-Efficiency Emitter in an Electroluminescent Device** By Ziruo Hong, Chunjun Liang, Ruigang Li, Wenlian Li,* Dan Zhao, Di Fan, Dongyue Wang, Bei Chu, Faxin Zang, L.-S. Hong, and Shuit-Tong Lee Electroluminescence (EL) of organic materials has at- tracted great interest since efficient bilayered EL devices based on tris(8-hydroxyquinoline) aluminum (Alq 3 ) were demonstrated by Tang and Vanslyke. [1] However, EL efficien- cies of fluorescent materials are limited to 1/4 of their photo- luminescence (PL) efficiencies, because about 3/4 of the excited states formed by electron±hole recombination in the EL process are triplet states, which decay non-radiatively. Therefore, further efficiency improvement requires that both singlet and triplet states contribute to luminescence. Efficient energy transfer from both singlet and triplet states in Alq 3 film to doped phosphorescent dye leads to high-efficiency EL and an external quantum efficiency of 4 %. [2] Cao et al. re- ported a 50 % ratio of EL quantum efficiency to PL emission in a polymer and weak exciton binding energy was proposed to explain this high value. [3] A general solution to overcome the 25 % upper limit to EL efficiency is to use rare-earth (RE) complexes as emitters. Earlier PL studies on RE complexes showed that emission of RE ions originates from excitation of ligands. [4,5] Under UV excitation, excited singlet states of the ligand are formed at first. Then relaxation from singlet states to triplet states occurs through intersystem crossing, and finally the central ion is excited by intramolecular energy transfer from the low- est excited triplet state (T 1 ) of the ligands. Therefore, in EL both singlet and triplet excitons formed by electron±hole recombination should be conducive to emission from RE ions. Theoretically, EL efficiencies of RE complexes are not lim- ited to 1/4 of PL efficiencies. Even assuming that the intersys- tem crossing process is inefficient, the EL efficiency of RE complexes could at least exceed their PL efficiency. Sharp-band EL emission of red, green, blue, and white col- ors from complexes of Eu 3+ , Tb 3+ , Tm 3+ , and Dy 3+ has been reported by Kido et al. [6] and by our group. [7±10] Moreover, there is increasing interest in producing an electrically pumped laser from RE complexes, such as Nd and Er com- plexes, [11±13] because many RE ions have suitable energy structures for laser output. Recently, Adachi et al. reported highly efficient EL devices based on an Eu complex doped into a wide energy-gap host, showing an external EL quantum efficiency of 1.4 % at 0.4 mA/cm 2 . This is the highest effi- ciency of an EL device based on RE complexes so far, although the red-light emission of Eu 3+ was accompanied by blue broad-band luminescence from the host material at high current densities. [14] However, EL devices based on RE com- plexes showed relatively lower EL efficiencies than their PL efficiencies, and no attempt has been made to improve the stability of such EL devices. This severely impedes further progress from the point of view of practical application. In this communication, an Eu-complex, europium(diben- zoylmethanato) 3 (bathophenanthroline) (Eu(DBM) 3 bath), was used to fabricate a red-light emitting EL device. Holes injected into the RE complex film were found to be quenchers of lumi- nescence and sources leading to poor stability. Based on this finding, significant improvements in efficiency and stability have been achieved. An external quantum efficiency of 4.6%, was obtained at low current density for our optimized EL device, which is higher than 1/4 of its PL efficiency and thus is experimental evidence for the contribution of triplet states. Figure 1 shows the molecular structure and PL spectrum of Eu(DBM) 3 bath. The sharp spectral bands are characteristic emissions of the Eu 3+ ion. The main emission peak at 612 nm corresponds to the 5 D 0 ® 7 F 2 transition of Eu 3+ . Eu(DBM) 3 - bath was verified to be an excellent electron-transport materi- Adv. Mater. 2001, 13, No. 16, August 16 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,2001 0935-9648/01/1608-1241 $ 17.50+.50/0 1241 COMMUNICATIONS ± [*] Prof. W. L. Li, Dr. Z. R. Hong, Dr. C. J. Liang, Dr. D. Zhao, Dr. D. Fan, Dr. D. Y. Wang,Dr. B. Chu, Dr. F. X. Zang Laboratory of Excited States Processes Chinese Academy of Sciences Changchun, 130021 (P. R. China) E-mail: pjblwl@public.cc.jl.cn Prof. W. L. Li, Dr. R. G. Li, Dr. B. Chu Changchun Institute of Optics, Fine Mechanics and Physics Chinese Academy of Sciences Changchun, 130021 (P. R. China) Dr. L.-S. Hong, Prof. S.-T. Lee Department of Physics and Material Sciences City University of Hong Kong Hong Kong (P. R. China) [**] The authors are grateful to Dr. Xingyuan Liu for valuable discussions and suggestions. This study is supported by the National Science Foundation of China, Project 863. Fig. 1. Molecular structure and PL spectrum of Eu(DBM) 3 bath.