Triplet Tank Layer Activates Triplet‐Triplet Fusion to Fluorescence in Blue‐Emissive Organic Light‐Emitting Diode

ABSTRACT Magneto‐electroluminescence (MEL) measurements are employed to investigate the role of the triplet tank layer (TTL) in activating triplet‐triplet fusion (TTF) within blue Hyper TTF organic light‐emitting diodes (OLEDs). The Hyper TTF OLEDs structure incorporates a TTL adjacent to the TTF‐emitting layer (TTF‐EML), effectively suppressing triplet‐polaron quenching (TPQ) and facilitating efficient TTF processes. The TTL confines carrier recombination and generates triplet excitons, which are predominantly injected into the TTF‐EML through the Dexter energy transfer process, subsequently forming singlet excitons. This mechanism enhances the formation of additional singlet excitons, characterized by delayed fluorescence in transient electroluminescence measurements and the TTF process‐dominated fingerprint curve in MEL responses. Owing to the enhanced efficiency of the TTF process, the external quantum efficiency (EQE) of the Hyper TTF OLED increased to 6.03%, significantly higher than the 2.47% achieved by a single TTF‐EML device. Additionally, adding a triplet blocking layer or modifying the TTL thickness influences the TTF process's intensity and the device's performance. Upon Hyper TTF OLED optimization, a maximum EQE of 13.24% is achieved. MEL measurements verify that the role of integrating the TTL into a single TTF‐EML device effectively suppresses TPQ and enhances TTF processes, ultimately achieving high‐performance blue OLED.