WEAK MICROCAVITY EFFECTS IN ORGANIC LIGHT-EMITTING DEVICES

We present an integrated classical and quantum-mechanical theory of we ak microcavity effects in layered media that treats both radiative and waveguided modes. The electromagnetic field of radiative modes is det ermined using classical field quantization, with the transition probab ility into each mode given by Fermi's ''golden rule.'' We apply this t heory to model the dependence of the electroluminescence spectral inte nsity and polarization of organic light-emitting devices (OLED's) on e mission angle, organic layer thickness, and applied voltage. Light pro pagation 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 re d-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 emissio n 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 indic ate that the efficiency of generating singlet excitons in aluminum tri s(8-hydroxyquinoline) (Alq(3))-based OLED's is 5 +/- 1 %, with a simil ar to 500-Angstrom-thick Alq(3) layer corresponding to the highest ext ernal power efficiency.