Progress in the development of red OLEDs from ternary europium coordination complexes comprising anionic β-diketones and neutral imine/phosphine oxide ligands

The development of efficiently monochromatic red-light-emitting materials is intrinsically difficult due to the fundamental limitation posed by the energy gap law. The subsequent transformation of materials into devices, e.g., organic light-emitting diodes (OLEDs), is even more challenging due to the need for precise colour control. Eu(III) coordination complexes with β-diketone and auxiliary ligands, with their inherently easily recognisable fingerprint red emission with full width at half maximum (FWHM) <10 nm, CIE _x,y 0.67,0.33, along with their ease of synthesis, have proved instrumental in meeting these challenges. Given the importance of OLEDs, in this overview, we aim to provide readers with details of recent research developments in the design of red-light-emitting europium complexes and their subsequent use as the emitting layer (EML) in the manufacture of monochromatic red OLEDs (R-OLEDs). The review provides a brief introduction to the topic, followed by a concise description of progress in the synthesis and a brief account of recent trends, to establish the intricate energy-transfer mechanism, aided by ultrafast femtosecond spectroscopy used in tandem with theoretical calculations. The review then describes the application of the red-emitting Eu(III) complexes as EMLs, which are classified in two categories based on the ancillary ligands, namely, complexes with β-diketones and imine ligands and β-diketones and P=O ligands with EuO ₆N ₂/EuO ₆N ₃ and EuO ₆O ₂ coordination spheres, respectively, so that the readers can appreciate the advantage of each type of molecular design and device engineering. Finally, the limitations of the structural design and device fabrication are outlined, and future directions that could lead to high-performance monochromatic R-OLEDs are discussed.