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Measurements of the diffusion length L for triplet excitons in smallmolecular-weight organic semiconductors are commonly carried out using atechnique in which a phosphorescent-doped probe layer is set in the vicinity ofa supposed exciton generation zone. However, analyses commonly used to retrieve$L$ ignore microcavity effects that may induce a strong modulation of theemitted light as the position of the exciton probe is shifted. The presentpaper investigates in detail how this technique may be improved to obtain moreaccurate results for L. The example of 4,4'-bis(carbazol-9-yl)1,1'-biphenyl(CBP) is taken, for which a triplet diffusion length of L=16 +/- 4 nm (at 3mA/cm2) is inferred from experiments. The influence of triplet-tripletannihilation, responsible for an apparent decrease of L at high currentdensities, is theoretically investigated, as well as the 'invasiveness' of thethin probe layer on the exciton distribution. The interplay of microcavityeffects and direct recombinations is demonstrated experimentally with thearchetypal trilayer structure[N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)]-4,4'-diaminobiphenyl (NPB)/CBP/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (named bathocuproine, BCP). It isshown that in this device holes do cross the NPB/CBP junction, without theassistance of electrons and despite the high energetic barrier imposed by theshift between the HOMO levels. The use of the variable-thickness doped layertechnique in this case is then discussed. Finally, some guidelines are givenfor improving the measure of the diffusion length of triplet excitons inoperational OLEDs, applicable to virtually any small molecular-weight material.

Diffusion of triplet excitons in an operational Organic Light Emitting Diode