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

Abstract 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 µs 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.