Organic light-emitting diodes (OLEDs) attract much attention being a promising, energy-efficient technology with high color quality for both display and lighting applications. Phosphorescence emitting materials prove to be inevitable because they can convert electrons into photons with an efficiency of one. Owing their comparably long exciton lifetime, non-linearities at high excitation levels reduce the emission efficiency (roll-off). Thus, especially the understanding and optimization of OLEDs at application-relevant brightness are of high interest. In the first part, triplet-triplet annihilation (TTA) – the process that dominates the efficiency roll-off – will be discussed in a highly efficient phosphorescent system. The results reveal that the nano-composition of the mixed film, where emitter molecules form aggregates, strongly decreases the high brightness efficiency. Concepts for phosphorescent OLEDs will be introduced that show improved high brightness performance.