An Intermediate Model For Fitting Triplet-Triplet Annihilation In Phosphorescent Organic Light Emitting Diode Materials

Triplet-triplet annihilation (TTA) is one of the primary contributors to efficiency roll-off and permanent device degradation in phosphorescent organic light-emitting diodes (OLEDs). The two limiting case models used to quantify this quenching mechanism are multi-step Dexter and single-step Förster, which respectively assume ideal Fickian diffusion or perfect trapping of triplet excitons. For device relevant guest doping levels (typically 5-12vol%), significant diffusion and trapping of excitons exist, so neither model fits experimental data well. We introduce an intermediate TTA model, which is a weighed average of the limiting cases of pure radiative decay and multi-step Dexter TTA. Kinetic Monte-Carlo simulations and time-resolved photoluminescence measurements of an archetype host-guest system demonstrate that our intermediate model provides significantly improved fits with more realistic physical values, is more robust to variations in experimental conditions, and provides additional insight into the TTA processes.