Highly efficient cuprous complexes with thermally activated delayed fluorescence and simplified solution process OLEDs using the ligand as host

Abstract

Two strongly luminescent complexes, [Cu(czpzpy)(PPh₃)]BF₄ (1) and [Cu(czpzpy)(POP)]BF₄ (2), bearing a new carbazole-based diimine ligand (czpzpy = 2-(9H-carbazolyl)-6-(1H-pyrazolyl)pyridine, PPh₃ = triphenylphosphine, and POP = bis[2-(diphenylphosphine)phenyl]ether) were synthesized and characterized. Complex 2 emits intense thermally activated delayed fluorescence (TADF) with high photoluminescence quantum yields (PLQYs) of up to 0.98 at room temperature in the solid state. A highly efficient solution-processed OLED, with an emissive layer was spin-coated using a mixture of the diimine ligand czpzpy and the starting material, [Cu(CH₃CN)₂)(POP)]BF₄, was fabricated with peak efficiencies of 6.36% (external quantum efficiency, EQE) and 17.53 cd A⁻¹ (current efficiency, CE) and a maximum brightness of 3251 cd m⁻². These performances are nearly identical to those of the device whose emissive layer was comprised of complex 2 and czpzpy, indicating that this simplified process, which omits the preparation and purification of the emissive complex, is practicable in preparing efficient solution-processed OLEDs. In this approach, czpzpy, which bears a carbazole unit, was used both as a ligand to form the emissive cuprous complex and as a host matrix for the formed emitter.