Optical Modeling of Organic Light-Emitting Diode Multilayer Device Structures
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Optical Modeling of Organic Light-Emitting Diode Multilayer Device Structures K. B. Kahen Research & Development, Eastman Kodak Company, Rochester, New York 14650-2216 ABSTRACT We present an exact classical solution to the problem of dipole emission in a planar multilayer organic light-emitting diode (OLED) device. The inputs to the model are the photoluminescence and quantum yield of the emitter material, and the device layer thicknesses and indices of refraction. The results of the model are applied to predicting the radiant intensity of OLEDs as a function of varying device layer thickness. It is shown that the calculated radiances are in excellent agreement with the data; this suggests that at constant current the variation in electroluminescence caused by modifications of the layer thicknesses can be completely accounted for by optical effects. Finally, we present results (for positions both internal and external to the diodes) for the Poynting power distribution from a randomly aligned dipole in the emission layer. INTRODUCTION Recently, there has been increasing interest in computing [1-4] the optical emission properties of OLEDs because of their application as lightweight, low-cost display sources. In order to optimize the radiative output of OLED devices, it is useful to have an optical model that can calculate both the color and intensity of the radiated emission as a function of variations in the device design. In addition, since a large fraction of the emitted light does not exit the device, it is also beneficial to have an optical model that can compute the Poynting power distribution inside of the device. Most of the optical models to date can be split up into three general schemes: (1) Employ a radiative transfer approach [5], which only allows one to compute the color accurately; (2) Use results from the model of Chance et al. [6], which enables one to compute accurately the intensity, but only for two-layer device structures; and (3) Use a quantum mechanical approach based on Fermi’s Golden Rule to compute both the radiative and internal field intensities [3]; however, that approach is limited to lossless devices. In this paper, we discuss a formalism that enables one to compute all three desired optical properties: color, radiated intensity, and the internal power distribution. The methodology is based on a Hertzian vector approach [7] to solving the inhomogenous vector wave equation, where we use Sommerfeld’s scheme [8] for computing the field distributions produced by a dipole in the emission layer. This approach was used by Chance et al. [6] to compute the dipole lifetime of dye molecules near metallic mirrors; however, their results were limited to two-layer structures (dielectric and metal). Because OLED devices, in general, are planar n-layer structures whose component layers can be either metals or lossy dielectrics, we extended the Sommerfeld formalism [8] to handle structures of this type. As has been discussed by Chance et al. [6], even though this approach is purely classical, one obtain
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