Among the highest efficiency dopants are the iridium organometallic complexes. They have a short triplet lifetime of 1-100 ps, which means that radiative recombination is assisted since the normally forbidden radiation from the triplet exciton is somewhat allowed due to spin-orbital interaction in the molecule. This relaxes the requirement that spin is invariant during the transition and triplet excitons become allowed radiative transitions. High-efficiency phosphorescence results. Iridium, a transition metal with an unfilled inner shell and a net angular momentum, provides the needed spin-orbit interaction.
Examples of red, green and blue iridium-based emitters are shown in Figure 6.27, although numerous phosphorescent emitters are well studied and some have been commercialized in OLEDs for battery-powered devices that require high efficiency.
