Highly Efficient Blue Fluorescent OLEDs Based on Upper Level Triplet–Singlet Intersystem Crossing

Abstract

Purely organic electroluminescent materials, such as thermally activated delayed fluorescent (TADF) and triplet-triplet annihilation (TTA) materials, basically harness triplet excitons from the lowest triplet excited state (T₁ ) to realize high efficiency. Here, a fluorescent material that can convert triplet excitons into singlet excitons from the high-lying excited state (T₂ ), referred to here as a "hot exciton" path, is reported. The energy levels of this compound are determined from the sensitization and nanosecond transient absorption spectroscopy measurements, i.e., small splitting energy between S₁ and T₂ and rather large T₂ -T₁ energy gap, which are expected to impede the internal conversion (IC) from T₂ to T₁ and facilitate the reverse intersystem crossing from the high-lying triplet state (hRISC). Through sensitizing the T₂ state with ketones, the existence of the hRISC process with an ns-scale delayed lifetime is confirmed. Benefiting from this fast triplet-singlet conversion, the nondoped device based on this "hot exciton" material reaches a maximum external quantum efficiency exceeding 10%, with a small efficiency roll-off and CIE coordinates of (0.15, 0.13). These results reveal that the "hot exciton" path is a promising way to exploit high efficient, stable fluorescent emitters, especially for the pure-blue and deep-blue fluorescent organic light-emitting devices.