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The emitter 9,9′,9′'‐(5‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)benzene‐1,2,3‐triyl)tris(9H‐carbazole) (3CzTRZ) seems an ideal candidate for efficient single‐layer blue organic light‐emitting diodes (OLEDs). It combines trap‐free electron and hole transport with thermally activated delayed fluorescence (TADF) for triplet harvesting. However, in spite of the absence of charge trapping defects, neat films of 3CzTRZ suffer from a low photoluminescence quantum yield (PLQY) of only 40%. Time‐resolved photoluminescence measurements reveal that the low PLQY results from the fact that next to fast intersystem crossing the rate for reverse intersystem crossing is very slow and nearly similar to the rate of non‐radiative recombination of triplet excitons to the ground state. This loss process of non‐radiative triplet decay is even more pronounced in OLEDs due to the 75% direct triplet‐exciton formation, resulting in a low external quantum efficiency (EQE) of 4.5%. By diluting 3CzTRZ in a large bandgap host, the triplet lifetime is increased by at least an order of magnitude, resulting in an enhanced PLQY of 70% and a more than three‐fold enhancement of the EQE to 15% in a single‐layer blue OLED. Device simulations confirm that the increased EQE can be ascribed to an elongated triplet lifetime upon dilution of the emitter in a host matrix. [ABSTRACT FROM AUTHOR]

For single‐layer OLEDs to achieve efficiencies approaching those of multilayer devices, balanced charge transport is a prerequisite. We demonstrate a single‐layer blue‐emitting OLED with unity internal quantum efficiency and an external quantum efficiency of 27.7% with minor roll‐off at high brightness. These findings lay the foundations for efficient solution‐processed OLEDs. [ABSTRACT FROM AUTHOR]