Synthesis of phenanthroimidazole-based host materials with balanced hole–electron transport for highly efficient blue fluorescent OLEDs

Abstract

Three new U-shaped host materials, namely, DPPI-DBF, Cz-PPI-DBF and TPA-PPI-DBF, were designed and synthesized by employing a dibenzofuran bridge linker combining one phenanthroimidazole (PPI) unit with PPI/carbazole (Cz)/triphenylamine (TPA) units. Herein, dibenzofuran and Cz/TPA were expected to promote the transport of electrons and holes, respectively, while the PPI unit could boost utilization of triplet excitons. The U-shaped molecular structure could effectively inhibit intramolecular donor–acceptor interactions, resulting in high singlet and triplet energy (S₁ ≥ 3.19 eV/T₁ ≥ 2.59 eV). Moreover, the three compounds showed bipolar charge transport, and well-balanced hole and electron transport was achieved in TPA-PPI-DBF. Benefitting from employment of PPI, the three compounds exhibited enhanced utilization of triplet excitons, although they demonstrated moderate T₂–T₁ energy gaps and positive/negative S₁–T₂ energy gaps. When a commercial blue fluorescence emitter with a full width at half maximum (FWHM) of 23 nm was employed as the dopant, the device based on the TPA-PPI-DBF host provided a maximum EQE of 8.00% and current efficiency (CE) of 5.90 cd A⁻¹. When luminance was increased to 1000 cd m⁻², its EQE/CE could be maintained at 7.42%/5.45 cd A⁻¹. The combination of a hot-exciton host with a narrow FWHM blue emitter is a promising route that can pave a new path for achieving commercial blue OLEDs with high efficiency and stability.