Experiment and Modeling of Conversion of Substrate-Waveguided Modes to Surface-Emitted Light by Substrate Patterning

To predict the optical power that could be harvested from light emission that is waveguided in the substrate of organic light emitting devices (OLEDs), a quantitative quantum mechanical model of the light emitted into the waveguided modes has been developed. The model was used to compute the exact distribution of energy in external, substrate and ITO/organic modes as a function of the distance of the emission zone from the cathode. The results are compared to the classical ray optics model and to experiments in two-layer OLED devices. Classical ray optics is found to substantially over-predict the light in waveguided modes. INTRODUCTION OLEDs have received enormous interest because of their promise for cheaper and more efficient flat panel displays. A large amount of light is trapped in the substrate due to total internal reflection; therefore, substrate patterning can be used to increase external coupling efficiency [1-3]. The exact distribution of optical energy in all of the waveguide modes has not been calculated previously, except for by classical ray optics. The goal of this paper is to develop such a quantitatively accurate model of such waveguided light and verify it with experiments. The radiative modes can be classified into external, substrate and ITO/organic modes (Figure 1a). External modes are those with angle to the surface normal in the organic layer less than the critical angle between air and Alq3, θc1 = sin nair/nalq; substrate modes are those with angle between θc1 and the critical angle between glass and Alq3, θc2 = sin nglass/nalq; and