Highly Efficient and Stable Blue Organic Light-Emitting Diodes based on Thermally Activated Delayed Fluorophor with Donor-Void-Acceptor Motif

Thermally activated delayed fluorophores (TADF) with donor-acceptor (D-A) structures always face strong conjugation between donor and acceptor segments, rendering delocalized new molecular orbitals that go against blue emission. Developing TADF emitters with blue colors, high efficiencies, and long lifetimes simultaneously is therefore challenging. Here, a D-void-A structure with D and A moieties connected at the void-position where the frontier orbital from donor and acceptor cannot be distributed, resulting in nonoverlap of the orbitals is proposed. A proof-of-the-concept TADF emitter with 3,6-diphenyl-9H-carbazole (D) connected at the 3'3-positions of 9H-xanthen-9-one (A), the void carbon-atom with no distribution of the highest occupied molecular orbital (HOMO) of A-segment, realizes more efficient and blue-shifted emission compared with the contrast D-A isomers. The deeper HOMO-2 of A is found to participate into conjugation rather than HOMO, providing a wider-energy-gap. The corresponding blue device exhibits a y color coordinate (CIEy) of 0.252 and a maximum external quantum efficiency of 27.5%. The stability of this compound is further evaluated as a sensitizer for a multiple resonance fluorophore, realizing a long lifetime of approximate to 650 h at an initial luminance of 100 cd m(-2) with a CIEy of 0.195 and a narrowband emission with a full-width-at-half-maxima of 21 nm.

Automated summary

This study proposes a novel D-void-A structure for TADF emitters, which achieves high efficiency and blue-shifted emission by connecting D and A moieties at the void position between donor and acceptor segments. Compared to the contrast D-A isomers, the TADF emitter in this study exhibits more efficient and blue-shifted emission. Additionally, the involvement of the lower HOMO-2 of the acceptor segment in conjugation provides a wider energy gap.