Effect of dendrimer surface groups on the properties of phosphorescent emissive films

Fac-tris[2-phenylpyridinato-C2,N]iridium(III) [Ir(ppy)3] is a well-known phosphorescent material for organic light-emitting diodes (OLEDs) and while uniform thin films can be formed using evaporation under high vacuum, it is not sufficiently soluble to enable it to be processed from solution. Ir(ppy)3-cored dendrimers with solubilizing surface groups can be processed from solution to form good quality neat or guest:host blends, even when the host has relatively poor solubility. In this manuscript, we report the effect of adding different solubilizing surface groups, namely 2-ethylhexyloxy, n-propyl or t-butyl to first generation dendrons attached to the Ir(ppy)3 core on the optoelectronic properties of neat and blend films with tris(4-carbazoyl-9-ylphenyl)amine (TCTA). The different dendrons were found to have minimal effect on the photoluminescence spectra and efficiency of energy transfer from the host to the guest in the blend films. The hole mobility of neat films of the solution-processed Ir(ppy)3-cored dendrimer films was around 10−6 cm2 V−1 s−1, which was an order of magnitude less than Ir(ppy)3 films formed by evaporation. Blending the dendrimers with TCTA at a concentration of 11 mol% (the concentration of the highest film photoluminescence quantum yield) led to an order of magnitude decrease in the hole mobility compared to the neat films. However, despite the relatively low mobilities, simple two-layer OLEDs composed of a blend light-emitting layer and an electron transporting layer were found to reach a maximum EQE of 11.1% at a luminance of 1000 cd m−2 for a film with a PLQY of 60%.