High performance solution-processed green phosphorescent organic light-emitting diodes with high current efficiency and long-term stability

In this study, we design and synthesize a new host and two new highly efficient green-emitting heteroleptic Ir(iii) complexes. These new materials are based on an amide-bridged, trifluromethyl-substituted, phenylpyridine skeleton with a longer alkyl chain as the main ligand, and on a phosphine oxide containing symmetrical dipyridinylphosphinate and asymmetrical phenyl(pyridin-2-yl)phosphinate as ancillary ligands. Their thermal, photophysical, electrochemical, and electroluminescent (EL) properties are fully investigated. The solution-processed green devices were fabricated using bis[5-ethylhexyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one](dipyridinylphosphinate)iridium(iii) as dopant, and (4′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-4-yl)di-o-tolylphosphine oxide (m-CBPPO1) and TPBi as hosts. The optimized devices containing a symmetrical-type ancillary ligand show excellent EL performance with a maximum current efficiency (CE_max) of 68.72 cd A^-1 and a maximum external quantum efficiency (EQE_max) of 20.82% without compromising the color purity. This is one of the best reported CE_max values with high EQE for solution-processed phosphorescent organic light-emitting diodes (PHOLEDs). To the best of our knowledge, this is the first report on green solution-processed PHOLEDs with EQE over 20% by using phosphine oxide functionalized symmetrical type ancillary ligand. Furthermore, these devices with symmetrical Ir(iii) complexes show better device stability than that of asymmetrical Ir(iii) complexes, which is attributed to the formation of undesirable isomers in asymmetrical complexes.