Origin of a sharp spectral peak near the critical angle in the spectral power density profile of top-emitting organic light-emitting diodes

Abstract

We theoretically investigate the origin of the sharp spectral peak near the critical angle in the spectral power density profile of a top-emitting organic light-emitting diode (OLED). On the basis of the point dipole model, spectral power densities of the top-emitting OLED are calculated with respect to the thickness of the capping layer (CPL). Sharp peaks in the spectral power density profile are observed at certain CPL thicknesses near the spectral interface between the radiation- and waveguide-mode regions, which is determined by the critical angle. By calculating the waveguide-mode spatial profile, the sharp peak in the spectral power density profile is identified as the radiation-mode-like waveguide mode, which has a very large evanescent length near the cut-off condition of waveguide modes. By decomposing the total spectral power density into the out-coupled light and absorption loss components, we demonstrate that the sharp spectral peak near the critical angle can enhance the relative power contribution of the absorption loss.