Efficient red phosphorescent organic light emitting diodes with double emission layers

We demonstrate efficient red phosphorescent organic light emitting diodes with a bipolar emission structure (D-EML) formed by two different layers doped with a red phosphorescent dye. Due to its self-balancing character, the recombination zone is shifted far from the emission/carrier-blocking-layer interfaces. This prevents the accumulation of carriers at the interfaces and reduces the triplet-triplet annihilation, resulting in an improved efficiency of the D-EML device compared with the standard single-EML architecture. However, a current efficiency of 8.4 cd A at 10 mA cm is achieved in the D-EML device compared with 3.7 cd A in the single-EML device. © 2008 IOP Publishing Ltd.