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

Abstract

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) 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,C')acetylacetonate (PO-01). Dry-processed devices using a common host, 4,4'-bis(N-carbazolyl)-1,1'-biphenyl, and the fac-Ir(SFXpy) emitter at a doping concentration of 15 wt % exhibited a peak performance of 46.2 cd A, 36.3 lm W, 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)-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, 22.1 lm W, and 11.3%, respectively, while the EQE roll-off was as low as 1.7% at the luminance of 1000 cd m. 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.

Highly efficient yellow‐emitting iridium(III) complexes based on fluorinated 2‐(biphenyl‐4‐yl)‐2H‐indazole ligands: Syntheses, structures, properties, and density functional theory calculations

“…Upon cooling to 77 K, the emission major peaks of Ir1‐Ir2 undergo a weakly hypsochromic shift, while the shoulder peaks have a bathochromic shift (Figure , left). Note that the emission spectra exhibit a well‐resolved vibronic structure, indicating that the mixing between the 3 MLCT and 3 LC is so effective that a ligand‐centered emission can clearly be observed in the frozen matrix . In comparison to the solution state emission, there is a slightly blue shift by 2 nm in solid state emission at 298 K (Figure , right), as a result of the restriction of intramolecular rotation.…”

Incorporation of Fluorene and Its Heterocyclic Spiro Derivatives To Realize High-Performance and Stable Sky-Blue-Emitting Arylgold(III) Complexes

“…6,7 The distinct properties of fluorene and its heterocyclic spiro derivatives have been utilized to prepare efficient phosphorescent 8−10 and thermally activated delayed fluorescence (TADF) OLEDs. 11−16 Successful applications in phosphorescent emitters have been demonstrated in the platinum(II) 8 and iridium(III) 9 systems.…”

Tackling the Self‐Aggregation of IrIII Complexes: A Theoretical Study

“…In this regard, the modification of the complex structure by introducing sterically demanding groups is capable of preventing the dimerization and accumulation processes . Recently, multifunctional spiro ligands showed a great ability for suppressing aggregation of Ir III complexes, and thus, assisting charge‐carrier injection and transport . Another effective approach to restraining the intermolecular interactions of Ir III compounds was successfully demonstrated by the formation of host–guest complexes …”