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.