Organometallic triplet emitters for OLED applications: controlling of emission properties by chemical variation

This contribution focuses on triplet emitters. They represent attractive OLED materials, since their efficiencies can in principle be by a factor of four higher than of (small) singlet emitter molecules. On the basis of introductory models, it is discussed how the exciton formation process can be visualized, how the emitter states are populated, and why the excitation energy is finally harvested in the lowest triplet state. Further, it is shown that essential photophysical properties of organometallic emitters depend systematically on the metal participation in the triplet states and on the effective spin-orbit coupling that control the amount of zero-field splitting (ZFS) of the triplet state into substates. Increase of ZFS corresponds to more metal character in the triplet state. High metal character reduces the energy difference between excited singlet and triplet states, enhances the singlet-triplet inter-system crossing rate, lowers the emission decay time, changes the vibrational satellite structure, decreases the excited state reorganization energy, etc.. These effects will be discussed by referring to well characterized compounds. Based on a new ordering scheme presented for triplet emitter materials a controlled development of compounds with pre-defined photophysical properties becomes possible