Triplet emitters for OLED applications. Mechanisms of exciton trapping and control of emission properties

Triplet emitter materials present attractive possibilities for optimizations of organic/organometallic light emitting diodes (OLEDs). This is due to the significantly higher efficiencies obtainable with these compounds as compared to organic emitters. In this contribution, first a schematic introduction is given, how an OLED device is built-up and why multi-layer structures are preferred. Then a basic model is presented, how electron-hole recombination, i.e. the exciton formation process, can be visualized and how the singlet and triplet states of the (doped) emitter compounds are populated. This takes place by specific singlet and triplet paths. The occurrence of such paths is explained by taking into account that the dynamical process of exciton trapping involves dopant-to-matrix charge transfer states (1,3 DMCT states). It is also explained, why the excitation energy is harvested in the lowest triplet state of organo-transition-metal complexes. Due to spin statistics, one can in principle obtain an efficiency of a factor of four higher than using organic singlet emitter molecules. Simple comparisons suggest that electron-hole recombination should preferentially occur on the triplet emitter itself, rather than on matrix molecules with subsequent energy transfer to the emitter. Further, it is pointed out that essential photophysical properties of organometallic triplet emitters depend systematically on the metal participation in the triplet state and on the effective spin-orbit coupling. These factors control the amount of zero-field splitting (ZFS) of the triplet state into substates. Increase of ZFS corresponds to higher metal character in the triplet state. Higher metal character reduces the energy difference between excited singlet and triplet states, enhances the singlet-triplet intersystem crossing rate, lowers the emission decay time, changes the vibrational satellite structure, decreases the excited state reorganization energy, etc. These effects are 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.