Management of Host–Guest Triplet Exciton Distribution for Stable, High‐Efficiency, Low Roll‐Off Solution‐Processed Blue Organic Light‐Emitting Diodes by Employing Triplet‐Energy‐Mediated Hosts.

High‐quality hosts are indispensable for simultaneously realizing stable, high efficiency, and low roll‐off blue solution‐processed organic light‐emitting diodes (OLEDs). Herein, three solution processable bipolar hosts with successively reduced triplet energies approaching the T1 state of thermally activated delayed fluorescence (TADF) emitter are developed and evaluated for high‐performance blue OLED devices. The smaller T1 energy gap between host and guest allows the quenching of long‐lived triplet excitons to reduce exciton concentration inside the device, and thus suppresses singlet‐triplet and triplet‐triplet annihilations. Triplet‐energy‐mediated hosts with high enough T1 and better charge balance in device facilitate high exciton utilization efficiency and uniform triplet exciton distribution among host and TADF guest. Benefited from these synergetic factors, a high maximum external quantum efficiency (EQEmax) of 20.8%, long operational lifetime (T50 of 398.3 h @ 500 cd m−2), and negligible efficiency roll‐off (EQE of 20.1% @ 1000 cd m−2) are achieved for bluish‐green TADF OLEDs. Additionally introducing a narrowband emission multiple‐resonance TADF material as terminal emitter to accelerate exciton dynamic and improve exciton utilization, a higher EQEmax of 23.1%, suppressed roll‐off and extended lifetime of 456.3 h are achieved for the sky‐blue sensitized OLEDs at the same brightness. [ABSTRACT FROM AUTHOR]