Green‐Emitting Exciplex Organic Light Emitting Diodes Based on Heptazine Derivative with Low‐Efficiency Roll‐Off

Abstract In this work, the compound HzTFEX 2 , which features potential inverted singlet and triplet excited states, is leveraged to construct an exciplex system with the donor mCBP , taking advantage of the deep highest occupied molecular orbital (HOMO) level of HzTFEX 2 (−6.3 eV) for efficient charge transfer. Photophysical experiments are employed to investigate the exciplex formation and excited‐state dynamics. The mCBP :7 wt.% HzTFEX 2 exciplex exhibits a broad green emission band, with average prompt and delayed lifetimes of 104.6 ns and 1.59 µs, respectively, and a near‐unity photoluminescence quantum yield (PLQY). Vacuum‐deposited organic light emitting diodes (OLEDs) based on this exciplex achieve a maximum external quantum efficiency (EQE) of 17.2%, with a low efficiency roll‐off of 5.2% at a practical brightness of 1000 cd m −2 . Further exploration of exciplex‐based co‐host systems incorporating terminal emitters DBTN‐2 and BN3 yielded maximum EQEs of 19.9% (green) and 14.7% (yellow), respectively, demonstrating the versatility of this exciplex pair. It is anticipated that the findings will contribute valuable insights toward the design of high‐efficiency green OLEDs utilizing acceptors with thermal equilibrium singlet‐triplet energy gap (Δ E ST ), supporting the advancement of next‐generation optoelectronic devices.