Efficiency enhancement of top-emitting organic light-emitting diodes using conversion dyes

We report recent results on top-emitting organic light-emitting diodes (OLEDs) using color conversion layers (CCLs) embedded into the electron transport layer of the OLED structure. The method of color conversion provides the possibility to generate a color stable emission with operating lifetime. Due to a constant ratio between absorbed blue emission and converter emission, the spectral shape remains for all time. This guarantees constant color coordinates of the OLED, which is essential for lighting applications. It is shown that OLEDs using conversion layers reach external quantum efficiencies (EQE) which can be higher than the corresponding blue top-emitting OLED. The used conversion layer thickness is below 100nm, reaching Commission Internationale de l'Éclairage (CIE) coordinates of (0.23; 0.27) close to the Planckian locus at a maximum EQE of 3.16% using a blue fluorescent emitter system. Furthermore, we show that the excitation mechanism of the conversion layer is caused by absorption and no parasitic electrical excitation is taking place. Investigations on the emission color over the lifetime show color-stability over a period of up to 2200 h.