Tuning the emission characteristics of top-emitting organic light-emitting devices by means of a dielectric capping layer: An experimental and theoretical study

The emission characteristics of top-emitting organic light-emitting devices (OLEDs) have been studied experimentally and theoretically to derive a quantitative understanding of the effect of a dielectric capping layer. We demonstrated that the angular intensity distribution and the spectral characteristics can be tuned and the light outcoupling enhanced simply by varying the optical thickness of a dielectric layer deposited on top of a semitransparent metal electrode. With the capping-layer concept, the outcoupled light intensity in forward direction was increased by a factor of 1.7, and concomitantly a high color purity achieved. An optical model based on a classical approach was used to calculate the emission characteristics. The excellent agreement between measured and simulated data shows that the capping layer controls the interplay between different interference effects such as wide-angle and multiple-beam interference occurring in top-emitting OLEDs. The strength of the capping layer concept is in particular that the optical and the electrical device performance can be optimized separately.