Efficient light emission was obtained in a silole-based organic light-emitting diode. A high luminous current efficiency of 20 cd/A, corresponding to an external quantum efficiency of 8%, was achieved. The apparent violation of the upper theoretical limit of 5.5% for the external quantum efficiency of a singlet emitter is discussed. With a suitably designed cathode, a high power efficiency of ∼14 lm/W was obtained. A strong dependence of the power efficiency on the thickness of Alq3 layer is also observed and explained.

We have directly observed the Anderson localized wave functions in three dimensions in a new class of photonic band gap systems. Such systems are networks made of one-dimensional waveguides. By adopting a simple scattering geometry in a unit cell, we are able to obtain large photonic band gaps. In the presence of defects or randomness, we have systematically studied the structures of transmission and the localized wave functions inside a gap. The effects due to absorption are investigated. Excellent quantitative agreements between theory and experiments have been obtained. [S0031-9007(98) PACS numbers: 41.20.Jb In the past decade, the localization of classical waves in random media has been under intensive studies [1]. Unlike electrons, the localization of classical waves is purely a result of multiple scatterings in a random environment and free from the complications arising from interaction effects. However, due to the Rayleigh scattering at low frequencies, it is more difficult to localize classical waves than to localize electrons. In 3D, waves can be localized only in certain windows in the intermediate frequency range with a minimum dielectric contrast [2]. It has been suggested that waves are more easily localized inside a gap or pseudogap of a photonic band gap (PBG) material [3]. The PBG material in its own right is of great interest and has important implications in both fundamental science and technological applications [4]. The localization of electromagnetic waves has been observed in 1D and 2D PBG materials [5,6]. For 3D systems, efforts have been focused only on wave localization in random media. The effects arising from wave localization have been reported in microwave experiments [7]. Nevertheless, a direct interpretation of localization was complicated by the presence of large absorption. Very recently, direct evidence of light localization has been reported in strong scattering media of semiconductor powders based on the size dependence of the transmission coefficient [8]. These important developments lead us to question if one can directly observe Anderson localized wave functions in 3D. For this purpose, like earlier investigations in 2D [6], we need to investigate the strongly localized states inside the gap of a PBG system, where the localization length is short. In addition, a direct measurement of the 3D wave function should be allowed in such systems.To meet these two requirements, we propose a new class of PBG systems here. Such systems are networks connected by segments of 1D waveguides [9]. There are two important advantages in such systems. First, strong scattering can be easily introduced in a unit cell to produce large full gaps in any dimension. Thus, unlike usual PBG systems, our systems do not require a material with a large dielectric constant. Second, the wave function at each node is physically accessible so that 3D localized wave functions can be probed.In our study, the coaxial cable was adopted as the 1D waveguide. The 3D network considered was in a diamond structure...

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