Modulating LUMO extension of Spiro-junction TADF emitters for efficient OLEDs with relieved efficiency Roll-Off

Thermally activated delayed fluorescent (TADF) emitters have shown their unique advantages in attaining efficient organic light emitting diodes (OLEDs). However, the large efficiency roll-off still stands in the way of their further application. In this study, we design and synthesize a series of TADF emitters based on spiro-junction via a LUMO extension strategy to optimize the roll-off and comprehend the structure–property relationship. Spiro-junction ensures all emitters with decent TADF behaviors. SIR-B with proper phenyl conjugation exhibits the fastest radiative decay and efficient reversed intersystem crossing (RISC) process which ensures the fast conversion of both singlet and triplet excitons. Therefore, SIR-B based device achieves the best performance with the maximum external quantum efficiency of 20.4%. Notably, the EQE of 17.6% is maintained at a practical luminance of 1000 cd/m−2, demonstrating a much-relieved roll-off. As comparison, SIR-A and SIR-C separately feature slow radiative decay and RISC resulting in their serious roll-offs. This contribution could provide a guidance to develop efficient OLEDs with small efficiency roll-off.