Anionic iridium complexes for solid state light-emitting electrochemical cells

Abstract

Two anionic iridium complexes Na[Ir(2-(p-tolyl)pyridine)2(CN)2] (A1) and Na[Ir(2-phenylquinoline)2(CN)2] (A2) were synthesized and characterized for use in light-emitting electrochemical cells (LECs). The photophysical and electrochemical studies show that the emission wavelengths and the LUMO energy levels of these complexes are governed by the conjugation length of the cyclometalated ligands. Single-layer LEC devices incorporating anionic complexes were fabricated and tested. Suffering from poor solubility of the anionic complexes, the device performances were rather limited due to the lack of film uniformity. The solubility issue was circumvented by using 18-crown-6 as an additive, efficiently eliminating the cluster structure formed by ion-dipole (sodium ion–lone pair of the nitrile ligand) interaction. The maximum brightness and peak external quantum efficiency achieved by crown-mediated LEC were up to 69 cd m−2 and 1.38%, respectively. The device efficiencies exhibit a great dependence with the LUMO level of the complex, suggesting substantial importance of the electron injection process.