2019 Volume 77 Issue 1 Pages 26-39
Organic light-emitting diodes (OLED) have attracted much attention from the viewpoint of application to next-generation flat panel displays and illumination devices. Phosphorescent iridium(III) and platinum(II) complexes are frequently used as the emitting materials because they are superior to fluorescent ones in terms of exciton generation. Here we report the development of phosphorescent organometallic dendrimers bearing charge carrier-transporting dendrons, especially focusing on fabrication of non-doped multilayer OLEDs. We chose our blue phosphorescent bis- and tris-cyclometalated iridium(III) complexes as the phosphorescent core. Attaching liposoluble tert-butyl groups on the periphery of the dendrons, the iridium(III) dendrimers showed good solubility in cyclohexane and insolubility in lower alcohols. The unique properties allowed us to fabricate multi-stacked, spin-coated thin films of hole-transporting poly(vinylcarbazole)/phosphorescent iridium(III) dendrimer/alcohol-soluble electron-transporting material by using an orthogonal solvent system of toluene-cyclohexane-alcohol. Thus, non-doped multilayer OLEDs were successfully obtained by solution processing. We also developed phosphorescent platinum(II) dendrimers bearing hole-transporting dendrons, where peripheral tert-butyl groups provided similar solubility to the iridium(III) dendrimers. Upon fabrication of non-doped multilayer OLEDs with the platinum(II) dendrimers, the excimer emission was tuned by steric hindrance of the ligands, and the optimized balance of blue monomer and reddish orange excimer emissions afforded white emission.
