Multiple‐Resonance‐Type TADF Emitter as Sensitizer Improving the Performance of Blue Fluorescent Organic Light‐Emitting Diodes

Abstract Due to the limitation of donor and acceptor group selection, the efficient thermally activated delayed fluorescence (TADF) type sensitizer used for blue organic light‐emitting diodes (OLEDs) is rare. Multiple resonance (MR) type TADF emitters can easily achieve efficient blue emission. And the compounds exhibit small Stokes shift and lower absorption energy under the same emission color compared with traditional TADF, mitigating the damage of high‐energy absorption of sensitizer on material stability. However, their characteristics as sensitizers have not been explored. In this work, a deep‐blue MR‐TADF compound (3tPAB) is selected as a sensitizer for both blue traditional fluorescence and MR‐TADF OLED. Given the improved photoluminescence quantum yield and the utilization of triplet excitons, the TADF‐sensitized fluorescent device using 3tPAB as sensitizer presents a maximum external quantum efficiency (EQE max ) of 14.4%, showing ≈2.4 times increase compared with the device without sensitizer. More impressively, TADF‐sensitized TADF (TST) device using 3tPAB as sensitizer and MR‐TADF compound PhDMAC‐BN as emitter exhibits EQE max of 33.9%. And in TST device, efficient Förster energy transfer is demonstrated, thus, the device can maintain high color purity. The work first demonstrates the feasibility of MR‐TADF as a sensitizer and provides a new strategy for developing high‐performance blue OLED with high color purity.