Rational Quantification of Recombination Pathways of Phosphorescent Organic Light-Emitting Diodes

One of the most challenging aspects in the fine design of organic light-emitting diodes (OLEDs) adopting a host-dopant structure in the emitting layer is the quantitative understanding of the Langevin recombination (LR) and trap-assisted recombination (TAR). Here as fundamental research, the quantitative contributions of LR and TAR of the exciplex-forming cohost-based phosphorescent OLEDs to electroluminescence are explored by time-dependent behaviors of current and luminance with numerical modeling. The LR and TAR coefficients are extracted from experimental measurements rather than calculations using theoretical formulas. The analysis of the TAR coefficient allows for determination of the trap generation efficiency according to the doping concentration, indicating the possibility of dopant aggregation and percolation of trap states. The portions of LR and TAR are successfully analyzed by drift-diffusion modeling using these two coefficients, revealing that the TAR ratio increased from 15% to 44% with increasing doping concentration from 0.5 to 3 wt%. A novel strategy for enhancing the LR portion is presented based on the prediction map of the recombination mechanisms.