Transient analysis of triplet exciton dynamics in amorphous organic semiconductor thin films

We study triplet exciton diffusion in the archetype organic material 4,4'-bis(N-carbazolyl)biphenyl (CBP) commonly used as a conductive host in the emissive zone of organic light emitting devices. Using time-resolved spectral decay ensuing from the diffusion of an initially localized triplet population to a spatially separated phosphor doped region, we model the delayed fluorescence and phosphorescence decays based on non-dispersive triplet transport. Fits to the model yield a diffusion coefficient of D = (1.4 +/- 0.3) x 10(-8) cm(2)/s, and a triplet-triplet bimolecular quenching rate constant of K-TT = (1.6 +/- 0.4) x 10(-14) cm(3)/s. The results are extended by doping a wide energy-gap molecule into CBP that serves to frustrate triplet transport, lowering both the diffusion coefficient and annihilation rate. These results are used to model a recently demonstrated white organic light emitting device that depends on triplet diffusion in CBP to excite spatially separate fluorescent and phosphorescent doped regions of the emissive layer. We determine the extent to which diffusion contributes to light emission in this structure, and predict its performance based on ideal lumophores with unity quantum yield. (C) 2006 Elsevier B.V. All rights reserved.