Transient analysis of organic electrophosphorescence. II. Transient analysis of triplet-triplet annihilation

In the preceding paper, Paper I [Phys. Rev. B 62, 10 958 (2000)], we studie d the formation and diffusion of excitons in several phosphorescent guest-h ost molecular organic systems. In this paper, we demonstrate that the obser ved decrease in electrophosphorescent intensity in organic light-emitting d evices at high current densities [M. A. Baldo et al., Nature 395, 151 (1998 )] is principally due to triplet-triplet annihilation. Using parameters ext racted from transient phosphorescent decays, we model the quantum efficienc y versus current characteristics of electrophosphorescent devices. It is fo und that the increase in luminance observed for phosphors with short excite d-state lifetimes is due primarily to reduced triplet-triplet annihilation. We also derive an expression for a limiting current density (J(0)) above w hich triplet-triplet annihilation dominates. The expression for J(0) allows us to establish the criteria for identifying useful phosphors and to assis t in the optimized design of electrophosphorescent molecules and device str uctures.