The Use of Substituted Iridium Complexes in Doped Polymer Electrophosphorescent Devices: The Influence of Triplet Transfer and Other Factors on Enhancing Device Performance

The problem of phosphorescence quenching by the host polymer of a dopant in a polyfluorene‐based electrophosphorescent device has been extensively studied. This paper concentrates on reduction of the rate of triplet‐energy transfer from the dopant to the host by making inert t‐butyl substitutions to the ligands of the well‐understood fac‐trisphenylpyridine iridium phosphorescent dopant. These substitutions introduce steric bulk to the dopant that approximately halves the rate of energy transfer compared to the unsubstituted dopant, and a concomitant increase in device performance is observed. This is attributed to the strong distance dependence of the Dexter‐type energy transfer involved, the steric bulk of the t‐butyl groups effectively preventing the energy transfer from emissive dopant to the host. In addition, through the use of specific substitutions on either the pyridyl or phenyl ring, the pathway of the energy transfer has been identified as being through the pyridyl ring of the ligand. Employing this technique of steric prevention of the triplet‐energy transfer to the host reduces the need for development of hosts with a high triplet level for electrophosphorescent devices. The effect of increasing the steric bulk of Ir(ppy)3 (ppy: phenylpyridine) on the efficiency of dopant emission in a polyfluorene electrophosphorescent device (see figure) has been investigated. In particular, it is found that the rate of triplet host quenching on the dopant can be controlled. A three to six times improvement in device performance is achieved when two tert‐butyl substitutions, instead of one, are made on the dopant.