Beyond Hyperfluorescence: Leveraging a Host‐Guest Exciplex to Significantly Suppress the Efficiency Roll‐Off and Improve the Operational Stability in Narrowband Emissive Devices

ABSTRACT Narrowband emissive devices using multiple resonance (MR) thermally activated delayed fluorescence (TADF) emitters show strong efficiency roll‐offs and low operational stability, largely due to the low reverse intersystem crossing (RISC) rates ( k RISC ) of the emitters. TADF/phosphorescence‐sensitized fluorescence (i.e., hyperfluorescence) effectively solves this issue but requires a suitable sensitizer. Here, a “host‐guest exciplex promoted RISC” strategy effectively solves the issue beyond the hyperfluorescence mechanism. Three electron‐transporting hosts with a 2,4,6‐triphenyl‐1,3,5‐triazine motif, namely SF3‐TRZ , SF3‐TRZ‐DBF , and SF3‐TRZ‐DBSe , are developed for a typical MR‐TADF guest (DtBuCzB). The exciplex formed between the host and DtBuCzB effectively promotes the RISC of DtBuCzB, especially for selenium‐embedded SF3‐TRZ‐DBSe . The host‐guest films show narrowband emission from DtBuCzB, with the k RISC of the [ SF3‐TRZ‐DBSe : DtBuCzB] film being boosted by nearly 60‐fold to 4.7 × 10 5 s −1 . A narrowband blue‐green organic light‐emitting diode (OLED) based on a [ SF3‐TRZ‐DBSe : DtBuCzB] emissive layer shows maximum external quantum efficiency (EQE max )/EQE at 1000 cd m −2 (EQE 1000 ) of 30.1%/28.1%, with the EQE 1000 being the highest and the efficiency roll‐off being the lowest among narrowband OLEDs (including hyperfluorescence ones) based on DtBuCzB reported so far. The device also shows markedly improved operational stability compared to counterpart devices using the other two hosts or counterpart hyperfluorescence devices.