A Yellow-Emitting Homoleptic Iridium(III) Complex Constructed from a Multifunctional Spiro Ligand for Highly Efficient Phosphorescent Organic Light-Emitting Diodes

To suppress concentration quenching and to improve charge-carrier injection/transport in the emission layer (EML) of phosphorescent organic light-emitting diodes (PhOLEDs), a facial homoleptic iridium­(III) complex emitter with amorphous characteristics was designed and prepared in one step from a multifunctional spiro ligand containing spiro­[fluorene-9,9′-xanthene] (SFX) unit. Single-crystal X-ray analysis of the resulting fac-Ir­(SFXpy)₃ complex revealed an enlarged Ir···Ir distance and negligible intermolecular π–π interactions between the spiro ligands. The emitter exhibits yellow emission and almost equal energy levels compared to the commercial phosphor iridium­(III) bis­(4-phenylthieno­[3,2-c]­pyridinato-N,C²′)­acetylacetonate (PO-01). Dry-processed devices using a common host, 4,4′-bis­(N-carbazolyl)-1,1′-biphenyl, and the fac-Ir­(SFXpy)₃ emitter at a doping concentration of 15 wt % exhibited a peak performance of 46.2 cd A^–1, 36.3 lm W^–1, and 12.1% for the current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE), respectively. Compared to control devices using PO-01 as the dopant, the fac-Ir­(SFXpy)₃-based devices remained superior in the doping range between 8 and 15 wt %. The current densities went up with increasing doping concentration at the same driving voltage, while the roll-offs remain relatively low even at high doping levels. The superior performance of the new emitter-based devices was ascribed to key roles of the spiro ligand for suppressing aggregation and assisting charge-carrier injection/transport. Benefiting from the amorphous stability of the emitter, the wet-processed device also exhibited respectful CE, PE, and EQE of 32.2 cd A^–1, 22.1 lm W^–1, and 11.3%, respectively, while the EQE roll-off was as low as 1.7% at the luminance of 1000 cd m^–2. The three-dimensional geometry and binary-conjugation features render SFX the ideal multifunctional module for suppressing concentration quenching, facilitating charge-carrier injection/transport, and improving the amorphous stability of iridium­(III)-based phosphorescent emitters.