Quantum efficiency enhancement in top-emitting organic light-emitting diodes as a result of enhanced intrinsic quantum yield

The authors compare red, green, and blue top- and conventional bottom-emitting organic light-emitting diodes (OLEDs) based on iridium complexes. Surprisingly, red top-emitting OLEDs are much more efficient than their bottom-emitting counterpart, while for green and blue devices, top and bottom emitters are comparable. The authors explain these results by an improved internal quantum efficiency only in the red top-emitting OLED: Due to strong microcavity effect, spontaneous emission of emitters in all top-emitting devices is enhanced. However, only the internal quantum yield of the red emitters is then improved due to its comparable radiative and nonradiative decay rates. They also show that the efficiency roll-off at higher current densities is reduced in top-emitting OLEDs. (c) 2006 American Institute of Physics.