Low Efficiency Roll-Off Blue Phosphorescent OLEDs at High Brightness Based on [3+2+1] Coordinated Iridium (III) Complexes

A new series of blue phosphorescent cyclometalated [3+2+1] coordinated iridium (III) complexes containing 2-(2 ',4 '-difluoropyridyl) pyridine derivatives have been designed and synthesized to investigate their structure-property relationship on the nature of excited states and tune their emission wavelengths from saturated-blue to sky-blue region at 437-489 nm. These complexes exhibit high photoluminescent quantum yields (PLQYs) of up to 90% and short excited state lifetime in the range of 2-5 mu s in polymethylmethacrylate (PMMA) thin films. Sky-blue organic light-emitting diodes (OLEDs) based on such pyridine-containing [3+2+1] iridium (III) dopants have exhibited maximum external quantum efficiencies (EQE(max)) and current efficiency (CE) of 17.3% and 42.3 cd A(-1), respectively. To note, very small efficiency roll-off value from 4.3% to 19.7% is achieved at the luminance level of 1000 cd m(-2). The well-balanced electron and hole injection currents, the short lifetime of excited excitons, and wide recombination zone by leveraging the obtained iridium (III) dopants and exciplex co-host have greatly decreased the accumulation of triplet excitons at the high luminescence level, leading to dramatically reduced efficiency roll-off. It is anticipated that this work provides new insights into the design of saturated-blue-emitting iridium (III) complexes with reduced efficiency roll-off in their fabricated OLEDs.