Effects of emitting layer host composition profile on the recombination zone of blue phosphorescent organic light emitting diodes

The spatial distribution of charge recombination in blue phosphorescent organic light-emitting diodes with linearly graded, step-graded and uniformly mixed host architectures was identified by selective doping of the emissive layer. Using TCTA and UGH-3 for hole and electron transporting hosts in the emitting layer, the recombination zones were found to be near the interface with electron transport layer due to the relatively high hole mobility of TCTA. For linearly graded host concentration profiles, however, the recombination region extends much further into the emissive layer. Expansion of the recombination region increases device efficiency by reducing quenching at the interface with the electron transport layer or by reducing triplet diffusion into the electron transport layer.