Ionic transition metal complexes (iTMCs) are receiving increased attention as materials capable of yielding efficient electroluminescent devices with air-stable electrodes. The operational characteristics of these devices are dominated by the presence of mobile ions that redistribute under an applied bias and assist in electronic charge injection. This article reviews recent efforts in the field of iTMC devices: i) to understand their physics, ii) to improve their efficiency, colour, turn-on time and lifetime, and iii) to expose their potential applications.

Transition metal complexes have emerged as promising candidates for applications in solid-state electroluminescent devices. These materials serve as multifunctional chromophores, into which electrons and holes can be injected, migrate and recombine to produce light emission. Their device characteristics are dominated by the presence of mobile ions that redistribute under an applied field and assist charge injection. As a result, an efficiency of 10 lm/W—among the highest efficiencies reported in a single layer electroluminescent device—was recently demonstrated. In this article we review the history of electroluminescence in transition metal complexes and discuss the issues that need to be addressed for these materials to succeed in display and lighting applications.