Transient analysis of organic electrophosphorescence: I. Transient analysis of triplet energy transfer

We examine triplet-exciton dynamics in several phosphorescent organic guest-host systems. In this first of two papers, transient studies are used to understand triplet energy transfer between molecules and also to ascertain the relative importance under electrical injection of charge trapping and direct exciton formation on phosphorescent guest molecules. As an example, we study the distribution of triplet excitons as they diffuse through amorphous films of tris(8-hydroxyquinoline) aluminum $\left({\mathrm{Alq}}_3\right).\mathrm{}$ We find that triplet transport in ${\mathrm{Alq}}_3$ is dispersive, and for high concentrations of triplets we find an average lifetime of $\tau =25\mathrm{}\pm 15\mu \mathrm{s}$ and a diffusion coefficient of $D_T=(8\mathrm{}\pm 5)\times {10}^{-8}{\mathrm{cm}}^2/\mathrm{s}.$ The understanding of the formation and transport of triplets in a host material is extended in the following paper [Phys. Rev. B 62, 10 967 (2000)] to the study of nonlinearities in the electroluminescent decay of phosphorescent organic guest materials. Finally, we summarize the principle determinants of the efficiency of organic electrophosphorescent devices.