Harvesting Hot Excitons for High‐Efficiency OLEDs with Extremely Low‐Efficiency Roll‐Off via Utilizing the Cascade Exciton Energy Transfer from Host and Sensitizer to Emitter.

Although high‐efficiency host‐sensitizer‐TBRb‐based organic light‐emitting diodes (HST‐OLEDs) are widely reported, the reasons for their high performances are still vague because the important role of hot exciton in TBRb emitter (T2, TBRb) has not been recognized previously. Herein, a maximum external quantum efficiency (EQE) of 24.9% is obtained from a HST‐OLED by co‐doping the Ir(ppy)3 sensitizer and TBRb emitter into the CBP host. Such a remarkable efficiency is mainly attributed to the efficient utilization of the T2, TBRb via the cascade Dexter energy transfer (DET) channels from triplet excitons of host and sensitizer to T2, TBRb (T1, CBP→DET$\mathop \to \limits^{{\mathrm{DET}}} $ T1, Ir(ppy)3→DET$\mathop \to \limits^{{\mathrm{DET}}} $ T2, TBRb) followed by the hot‐exciton reverse intersystem crossing (RISC, T2, TBRb→RISC$\mathop \to \limits^{{\mathrm{RISC}}} $ S1, TBRb) and triplet fusion (TF, T2, TBRb + T2, TBRb→TF$\mathop \to \limits^{{\mathrm{TF}}} $ Sm, TBRb→IC$\mathop \to \limits^{{\mathrm{IC}}} $ S1, TBRb) processes for radiative decays. These cascade DET channels are demonstrated by spectroscopic analyses and magneto‐electroluminescence studies. More importantly, this HST‐OLED presents an extremely low‐efficiency roll‐off, that is, the EQEs separately exhibit 24.34% and 19.73% at 1000 and 10 000 cd m−2 compared to the maximum EQE of 24.9%. Obviously, this work paves a promising pathway for fabricating further high‐efficiency HST‐OLEDs for applications in illumination and flat‐panel display. [ABSTRACT FROM AUTHOR]