Study of ion-paired iridium complexes (soft salts) and their application in organic light emitting diodes.

Three Ir-based materials were synthesized through metathesis reaction between halide and alkali metal salts of two cationic and three anionic Ir complexes, respectively. The resulting "soft salt" complexes are composed of an organometallic cation and an organometallic anion. The electrochemical and photophysical characterization of these compounds is reported. The redox potentials of the soft salts are shown to be determined by the lowest energy potentials of the two ions. Energy transfer between the ions in solution is observed, and found to take place at diffusion controlled rates. Organic LEDs were prepared with each of the three soft salts, using the simple structure of anode/PVK/soft salt/BCP/cathode. The soft salts yielded maximal external quantum efficiencies (EQE) ranging from 0.2% to 4.7%. The study suggests that the internal energy alignment between two ions in the soft salts is responsible for the widely disparate results. To achieve a high EQE, it is critical to have the HOMO and LUMO values of one of the ions fall between those of the other ion, that is, one ion has both the lowest oxidation potential and the least negative reduction potential.