Copper(I) Complex as Sensitizer Enables High‐Performance Organic Light‐Emitting Diodes with Very Low Efficiency Roll‐Off

Abstract Organic light‐emitting diodes (OLEDs) utilizing purely organic thermally activated delayed fluorescence (TADF) sensitizers have recently achieved high efficiencies and narrow‐band emissions. However, these devices still face intractable challenges of severe efficiency roll‐off at practical luminance and finite operational lifetime. Herein, a carbene‐Cu(I)‐amide complex, (MAC*)Cu(Cz), is demonstrated as a TADF sensitizer for both fluorescent and TADF OLEDs. The (MAC*)Cu(Cz)‐sensitized fluorescent OLED not only achieves a high external quantum efficiency (EQE) of 14.6% with an extremely low efficiency roll‐off of 12% at the high luminance of 10 000 nits, but also delivers a 15 times longer operational lifetime than that of the non‐sensitized reference device. More importantly, utilizing the (MAC*)Cu(Cz) sensitizer in the multi‐resonance (MR) TADF OLED results in a record‐high EQE of 26.5% together with a full‐width at half maximum of 46 nm and an emission peak at 566 nm. This value is the state‐of‐the‐art efficiency for yellow‐emitting MR‐TADF OLEDs. The photophysical analysis proved that the fast reverse intersystem crossing process of (MAC*)Cu(Cz) is the key factor to suppress triplet exciton involved quenching at high luminance. This finding firstly demonstrates the use of Cu(I) complex as an efficient TADF sensitizer and paves the way for practical applications of TADF sensitized OLEDs.