Sublimable cationic iridium(iii) complexes for red-emitting diodes with high colour purity

Abstract

Cationic iridium(III) complexes show great promise as phosphorescent materials due to their superior photophysical properties and electrochemical stabilities. However, the previously reported red-emitting cationic iridium(III) complexes suffered from low photoluminescence quantum yields (PLQYs) of less than 35% and colour coordinates deviating from (0.67, 0.33) of the standard red emission, due to which the resulting organic light-emitting diodes (OLEDs) exhibited low external quantum efficiencies (EQEs) with undesirable colour purity. Here we report a cationic iridium(III) complex with diphenylquinoxaline-type cyclometalated ligands, achieving pure red emission with PLQYs over 80%. We investigate the photophysical properties both experimentally and theoretically, and find that there is a high MLCT (metal-to-ligand charge transfer) percentage in the excited states, leading to strong spin–orbit coupling and configuration interaction. We fabricate OLEDs based on this complex, achieving a high EQE of 10.3% with small efficiency roll-off at high luminance, and pure red emission with colour coordinates of (0.67, 0.33), close to the National Television System Committee (NTSC)-recommended standard. Our study demonstrates a strategy to tune the nature of excited states to develop sublimable cationic iridium(III) complexes with MLCT predominant mixing states, thereby realizing OLEDs with high efficiency and colour purity, which sheds light on their application in lighting and displays.