In this paper, we discuss in detail a novel concept for white organic light-emitting diodes (OLEDs) comprising three different phosphorescent emitters. Its key feature, a matrix-emitter combination for the blue sub-emission layer combined with a specific arrangement of the green- and red-emitting layers, leads to very high luminous efficacies. We present the key parameters of this emission layer design that are essential for high efficiencies. Finally, we briefly discuss a second, highly efficient approach for white OLEDs: the triplet harvesting concept. It makes use of a fluorescent blue emitter having benefits for higher device stability and at the same time the potential for 100% internal quantum efficiency.