Two blue iridium complexes for efficient electroluminescence with low efficiency roll-off

Abstract

Two bis-cyclometalated iridium complexes ((dfpypy)₂Ir(tpip) and (dfpypy)₂Ir(Ftpip)) with fluorinated substituted bipyridine (2′,6′-difluoro-2,3′-bipyridine, dfpypy) as the main ligand and tetraphenylimidodiphosphinate derivatives (tpip and Ftpip) as the ancillary ligands were prepared, and their X-ray crystallographic, photoluminescence and electrochemical properties were investigated. The (dfpypy)₂Ir(tpip) and (dfpypy)₂Ir(Ftpip) complexes showed blue emission at 457 nm with quantum efficiency yields of 7.0% and 7.1%, respectively. Organic light emitting diodes (OLEDs) with the structure of ITO/TAPC (1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane, 40 nm)/mCP (1,3-bis(9H-carbazol-9-yl)benzene, 10 nm)/(dfpypy)₂Ir(tpip) or (dfpypy)₂Ir(Ftpip) (8 wt%): PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole, 25 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 50 nm)/LiF (1 nm)/Al (100 nm) (B2 and B3) exhibit performances with the maximum current efficiency (η_c) values of 22.83 and 20.79 cd A⁻¹, respectively, with low efficiency roll-off. For example, at 100 cd m⁻² display brightness, the current efficiencies of devices B2 and B3 are 19.78, 13.74 cd A⁻¹, respectively. At 1000 cd m⁻² light brightness, these values are still 20.39 and 20.75 cd A⁻¹, respectively. Even at the high luminance of 5000 cd m⁻², these data also remained at 19.95 and 20.08 cd A⁻¹, respectively.