Comprehensive Overview into Luminescent Iridium Complexes: Best Reported Light-Emitting Electrochemical Cells up to 2022

This chapter provides a comprehensive and systematic review of the current status of Ir-iTMC-based LEECs using the archetypal complex [Ir(ppy)2(bpy)]PF6 as a reference emitter. Cyclometallated iridium complexes possess fascinating electrochemical and photophysical properties that make them excellent candidates for various photonic and optoelectronic applications. In place of traditional organic light-emitting diodes, light-emitting electrochemical cells (LEECs) based on iridium-containing ionic transition-metal complexes (Ir-iTMCs) show promise. The advantages of LEECs based on Ir-iTMCs include a simpler device structure, solution processability, and reduced manufacturing costs compared with conventional light-emitting diodes. Here, the device fundamentals and important photophysical parameters are discussed, then key strategies for tuning the emission characteristics and device stability through LUMO and HOMO stabilization/destabilization are presented using numerous examples from the literature. It has concentrated on ligand modification with hydrophobic, electron-withdrawing, and electron-donating substituents, -stacking interactions, alternative ancillary ligand skeletons, and cyclometallated ligands. Comprehensive data tables summarizing the photophysical and LEEC properties of the various classes of iridium complexes reported to date are also provided. The presence of steric hindrance increases the intercomplex distance, thereby minimizing the non-radiative decay in the solid state. Thus, a low IL concentration in the emissive layer is sufficient for improving the device efficiency, as an excessive IL concentration could increase the intercomplex distance even further and lead to reduced electronic communication between the iridium complexes and lower LEEC efficiency. It is concluded that potential directions for future research by reporting the current champion iridium complexes for fabricating state-of-the-art LEECs and discussing the merits and limitations of existing approaches.