Improved performance of solution-processable OLEDs by silyl substitution to phosphorescent iridium complexes

Development of new materials for solution-processable organic light-emitting diodes (OLEDs) has been gaining much attention recently. Although many results have been reported about new OLED materials, most of them are aiming for vacuum-deposition. The OLED materials developed for vacuum-deposition cannot be easily applied to solution processing due to their limited solubility and processability. We therefore designed and synthesized two kinds of iridium complexes, Ir(Si-bppy)2(acac) and Ir(bppy)2(acac), with similar chemical structures but with or without silyl substitution. From the optical and electrochemical characterization, Ir(Si-bppy)2(acac) showed an emission maximum at 537 nm (ET1 = 2.33 eV) and HOMO energy level of −5.13 eV, while Ir(bppy)2(acac) showed an emission maximum at 531 nm (ET1 = 2.31 eV) and HOMO energy level of −5.14 eV. While the optical and electrochemical properties are almost the same, we found out that OLED device performance can differ pretty fair. The emitting layer of our OLEDs consists of PVK, TPD, PBD and the iridium complexes. With the configuration of [ITO/PEDOT:PSS/emitting layer/CsF/Al], the maximum current efficiency of Ir(Si-bppy)2(acac) reached 24.5 cd/A, while that of Ir(bppy)2(acac) was 18.0 cd/A. This enhancement is attributed to the existence of the trimethylsilyl substituents at the pyridine ring of the ligand, which increased solubility and processability of the compounds, and reduced intermolecular interaction.