Understanding molecular fragmentation in blue phosphorescent organic light-emitting devices

We investigate molecular fragmentation in the long lived archetype blue PHOLED guest/host system based on iridium (III) tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f] phenanthridine] (Ir(dmp)(3)) and 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (mCBP) as a function of luminance loss using laser desorption ionization time-offlight mass spectrometry and X-ray photoelectron spectroscopy (XPS). The blue phosphor experiences significant ligand loss due to its labile Ir-ligand bonds, whereas the mCBP host is relatively stable as the luminance decays to 40% of its initial value at 3000 cd/m(2). Density functional theory calculations indicate that fragments from mCBP are less likely to cause luminance loss via exciton quenching compared to those of the Ir-complex. Phosphor fragments are observed by XPS only in aged samples, and the concentration of fragments is found to increase with aging time. These findings suggest that the lifetime of blue PHOLEDs can be improved by using phosphors without labile Ir-ligand and intra-ligand bonds, and a host material that does not form large mass fragments or complexes with the degraded phosphor molecules. Insights into the origin of the loss in luminance due to trap formation from molecular fragmentation of this guest/host system during use are discussed.