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 u nderstand triplet energy transfer between molecules and also to ascertain t he relative importance under electrical injection of charge trapping and di rect exciton formation on phosphorescent guest molecules. As an example, we study the distribution of triplet excitons as they diffuse through amorpho us films of tris(g-hydroxyquinoline) aluminum (Alq(3)). We find that triple t transport in Alq(3) is dispersive, and for high concentrations of triplet s we find an average lifetime of tau =25+/-15 mus and a diffusion coefficie nt of D-tau= (8+/-5) x 10(-8) cm(2)/s. The understanding of the formation a nd transport of triplets in a host material is extended in the following pa per [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 elec trophosphorescent devices.