Single-Step Triplet-Triplet Annihilation: An Intrinsic Limit for the High Brightness Efficiency of Phosphorescent Organic Light Emitting Diodes

We investigate triplet-triplet annihilation in molecular host-guest systems where triplets are localized on spatially separated guest molecules. Our results indicate that the dominant mechanism of annihilation is single-step long-range (Förster-type) energy transfer between two excited guests. This mechanism leads to a fundamental limit for the efficiency of phosphorescent organic light emitting diodes at high luminance. Our model is confirmed by photoluminescence decay experiments on 2,3,7,8,12,13,17,18-octaethylporphine platinum as guest in a host matrix of $4,4^{\prime }\mathrm{\text{−}}N,N^{\prime }$-dicarbazole-biphenyl.