We present an integrated classical and quantum-mechanical theory of weak microcavity effects in layered media that treats both radiative and waveguided modes. The electromagnetic field of radiative modes is determined using classical field quantization, with the transition probability into each mode given by Fermi's ''golden rule.'' We apply this theory to model the dependence of the electroluminescence spectral intensity and polarization of organic light-emitting devices ͑OLED's͒ on emission angle, organic layer thickness, and applied voltage. Light propagation in the OLED layers and the substrate is described by both ray and wave optics. Theoretical predictions are compared to experimental observations on single heterostructure, and multiple layer stacked red-green-blue OLEDs. Analysis of the polarization, spectral shape, and intensity of the electroluminescence spectrum in the forward-scattered half plane accurately fits the experimental data. The theory predicts, and the experimental measurements confirm, that the in-plane emission from conventional OLED structures is strongly TM polarized, and can be redshifted by as much as 60 nm with respect to the peak emission in the normal direction. Measurements coupled to our analysis also indicate that the efficiency of generating singlet excitons in aluminum tris͑8-hydroxyquinoline͒ (Alq 3 )-based OLED's is 5Ϯ1 %, with a ϳ500-Å-thick Alq 3 layer corresponding to the highest external power efficiency. ͓S0163-1829͑98͒04532-9͔ PHYSICAL REVIEW B 15 AUGUST 1998-I VOLUME 58, NUMBER 7 PRB 58 0163-1829/98/58͑7͒/3730͑11͒/$15.00 3730

We study the internal and external quantum efficiencies of vacuum-deposited organic light-emitting devices (OLED's). The internal quantum efficiency of OLED's based on tris-(8-hydroxyquinoline) aluminum is calculated to be 5.7 times the observed external quantum efficiency ?(e), consistent with measurements. We demonstrate a shaped substrate that increases ?(e) by a factor of 1.9+/-0.2 over similar OLED's fabricated upon flat glass substrates and leads to a 100%-emissive aperture, i.e., the emitting area completely occupies the display area even in the presence of metal interconnects. We also discuss a substrate structure that increases ?(e) by an additional factor of 2. The high device efficiencies are promising for developing OLED-based displays with extremely low power consumption and increased operational lifetime.

We report the demonstration of transparent organic light emitting devices (OLEDs) which are ∼70% transparent throughout the visible spectrum when switched off, and emit light from both sides with a total external quantum efficiency of ∼0.1% when turned on. The devices are Alq3-based single heterostructure OLEDs grown on an ITO-coated glass substrate with a top electrode composed of a very thin layer of Mg–Ag and an overlaying ITO film. The top electrode is both electron injecting and transparent. The transparent OLEDs are expected to be useful in high-resolution full-color displays, as well as for helmet-mounted, windshield-mounted, or other ‘‘head-up’’ display applications.

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