Influence of charge balance and exciton distribution on efficiency and lifetime of phosphorescent organic light-emitting devices

We discuss the importance of appropriate charge carrier confinement and exciton management for the realization of highly efficient and stable organic light-emitting diodes (OLEDs). As an example, we choose red p-i-n-type OLEDs based on the iridium-based electrophosphorescent emitter Ir(MDQ)2(acac) doped in α-NPD as host material. We show how an appropriate choice of the hole blocking layer material allows external quantum efficiencies as high as 20% for this emitter. At the same time, the display-relevant brightness of 100 cd/m2 is reached at an operation voltage of only 2.4 V, which is close to the thermodynamic limit. As a result, a high total power efficiency of 37.5 lm/W at 100 cd/m2 is reached. In a further step, we study the influence of the blocker materials on device lifetime. We investigate the chemical reactions causing the degradation process by use of matrix assisted laser desorption time-of-flight mass spectrometry. It can be shown that discovered degradation reactions can be suppressed by an appropriate choice of the hole blocking material.