Effect of emitter disorder on the recombination zone and the quantum yield of organic electroluminescent diodes

The electroluminescence (EL) quantum yield (QY) of double- and triple-layer organic electroluminescent diodes based on a N,N^′-diphenyl-N,N^′bis(3-methylphenyl)-1,1^′-biphenyl4,4^′-diamine /tris (8-hydroxyquinolinato) aluminum III $({\mathrm{Alq}}_3)$ junction has been measured as a function of the electric field and the emitting guest (6,13-diphenylpentacene) concentration in the host ${\mathrm{Alq}}_3.$ The well-resolved maxima of the QY plots versus applied field for neat and low dopant concentration emitter layers (EMLs) shift strongly toward high fields and disappear at high dopant concentrations. Based on the EL QY data and the measured absolute photoluminescence quantum efficiency of the emitter, the recombination zone width is determined and shown to be a decreasing function of electric field for all of the diodes. The dopant reduces the width of the recombination zone at low dopant concentrations and increases at high dopant concentrations (>0.5 mol %). The results are discussed in terms of a two-step recombination mechanism, assuming disorder-controlled charge carrier mobilities. The dopant concentration effect on the recombination zone width and EL QY can be explained using the disorder formalism that predicts low dopant concentrations create a high degree of positional (off-diagonal) disorder whereas energetic (diagonal) disorder dominates at higher doping levels in the EMLs. The latter makes the recombination zone width as well as EL QY practically field independent.