  Measurements of the diffusion length L for triplet excitons in small
molecular-weight organic semiconductors are commonly carried out using a
technique in which a phosphorescent-doped probe layer is set in the vicinity of
a supposed exciton generation zone. However, analyses commonly used to retrieve
$L$ ignore microcavity effects that may induce a strong modulation of the
emitted light as the position of the exciton probe is shifted. The present
paper investigates in detail how this technique may be improved to obtain more
accurate 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 3
mA/cm2) is inferred from experiments. The influence of triplet-triplet
annihilation, responsible for an apparent decrease of L at high current
densities, is theoretically investigated, as well as the 'invasiveness' of the
thin probe layer on the exciton distribution. The interplay of microcavity
effects and direct recombinations is demonstrated experimentally with the
archetypal 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 is
shown that in this device holes do cross the NPB/CBP junction, without the
assistance of electrons and despite the high energetic barrier imposed by the
shift between the HOMO levels. The use of the variable-thickness doped layer
technique in this case is then discussed. Finally, some guidelines are given
for improving the measure of the diffusion length of triplet excitons in
operational OLEDs, applicable to virtually any small molecular-weight material.
