Insight into luminescent iridium complexes: Their potential in light-emitting electrochemical cells

Cyclometallated iridium complexes possess fascinating electrochemical and photophysical properties that make them excellent candidates for a variety of photonic and optoelectronic applications. In particular, light-emitting electrochemical cells (LEECs) based on iridium-containing ionic transition-metal complexes (Ir-iTMCs) are a promising alternative to conventional organic light-emitting diodes with several advantages, including a simpler device structure, solution processability, and reduced manufacturing costs. This review aims to provide a comprehensive and systematic overview of the current status of Ir-iTMC-based LEECs using the archetypal complex [Ir(ppy)₂(bpy)]PF₆ as a reference emitter. After a discussion of the device fundamentals and important photophysical and device parameters, key strategies for tuning the emission characteristics and device stability through LUMO and HOMO stabilization/destabilization are presented using numerous examples from the literature, with a particular focus on ligand modification with hydrophobic, electron-withdrawing, and electron-donating substituents, π-stacking interactions, and alternative ancillary and cyclometalated ligand skeletons. Comprehensive data tables summarizing the photophysical and LEEC properties of the various classes of iridium complexes reported to date are also provided. Finally, in an effort to highlight promising directions for future research, the current champion iridium complexes for fabricating state-of-the-art LEECs are identified, and the merits and limitations of existing approaches are discussed.