A detailed investigation of the diffusion of triplet excitons in a layer of $N,N^{\prime }$-di-1-naphthalenyl-$N,N^{\prime }$-diphenyl-[$1,1^{\prime }:4^{\prime },1^{″}:4^{″},1^{‴}$-quaterphenyl]-$4,4^{‴}$-diamine (4P-NPD) incorporated in organic light-emitting diodes is presented. An appropriate method to measure the triplet diffusion length in fluorescent host materials is the spatial separation of the site of exciton generation from the site of radiative triplet decay by inserting a host spacer layer of varying thickness. However, cavity effects, the quenching and blocking of excitons at the boundaries of the spacer layer, and direct charge-carrier recombination in the sensing layer need to be taken into account. We use a specially designed layer stack, which excludes the influence of cavity effects on the measurements and a strongly quenching sensing layer, which ensures well-defined boundary conditions. The quenching of excitons by the sensing layer, the generation zone, and direct charge-carrier recombination are investigated experimentally and their influence on the extracted diffusion length are discussed. The significance of triplet-triplet annihilation in this analysis is estimated by a current-dependent evaluation. An analytic model for the dependence of the sensing layer emission on the spacer thickness is presented, which includes the important effects. By this means, we find a triplet diffusion length of $11\pm 3\text{nm}$ in 4P-NPD.

• Received 1 March 2010

DOI:https://doi.org/10.1103/PhysRevB.81.245201