Deep blue high-efficiency solution-processed triplet-triplet annihilation organic light-emitting diodes using bis(8-carbazol-N-yl)fluorene- and benzonitrile-modified anthracene/chrysene fluorescent emitters

Triplet-triplet annihilation (TTA) emitters can effectively utilize non-radiative triplet excitons through the interaction of low triplet energy excitons to produce high energy singlet excitons, but they are mostly restricted by their multilayered device structure fabricated using layer-by-layer thermal vacuum evaporation. It is a great challenge to develop, for the first time, efficient solution-processed non-doped TTA organic light-emitting diodes (OLEDs). In this study, two solution-processable blue emissive TTA molecules (FAnCN and FCsCN) bearing (anthracen-9-yl)benzonitrile (AnCN) and (chrysen-6-yl)benzonitrile (CsCN) as TTA emissive cores modified with 9,9 '-bis(8-(carbazole-N-yl)octyl)fluorene (F) are designed and synthesized, respectively. The experimental and theoretical studies reveal that both molecules exhibit deep blue emissions, amorphous morphology with good thermal stability, high-quality solution-cast thin films, decent hole mobility, high-lying HOMO levels (similar to-5.45 eV), and suitable lowest singlet (S-1)/triplet (T-1) excited states (2T(1) > S-1) for TTA process. FAnCN and FCsCN are successfully employed as solution-processed non-doped emissive layers (EML) in simple structured TTA OLEDs. These devices show intense blue emissions, low turn-on voltages (similar to 3.6 V), excellent electroluminescent (EL) performances (EQE(max) = 5.47-6.84 % and LEmax = 5.66-5.83 cd/A), and TTA characteristics. Especially, FCsCN-based TTA OLED emits deep blue EL emission peaked at 435 nm with a high EQE(max) of 6.84 %. This work not only presents a new strategic design for the preparation of solution-processable TTA emitter, but also further ratifies that the TTA mechanism can also be applicable in solution-processed OLEDs.