Archetype Cationic Iridium Complexes and Their Use in Solid-State Light-Emitting Electrochemical Cells

• Published in: Advanced functional materials 2009-11, Vol.19 (21), p.3456-3463
• Main Authors: Costa, Rubén D., Ortí, Enrique, Bolink, Henk J., Graber, Stefan, Schaffner, Silvia, Neuburger, Markus, Housecroft, Catherine E., Constable, Edwin C.
• Format: Article
• Language: English
• Subjects: electrochemical cells; electroluminescence; ionic liquids; iridium complexes; transition metals
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200900911

The archetype ionic transition‐metal complexes (iTMCs) [Ir(ppy)2(bpy)][PF6] and [Ir(ppy)2(phen)][PF6], where Hppy = 2‐phenylpyridine, bpy = 2,2′‐bipyridine, and phen = 1,10‐phenanthroline, are used as the primary active components in light‐emitting electrochemical cells (LECs). Solution and solid‐state photophysical properties are reported for both complexes and are interpreted with the help of density functional theory calculations. LEC devices based on these archetype complexes exhibit long turn‐on times (70 and 160 h, respectively) and low external quantum efficiencies (∼2%) when the complex is used as a pure film. The long turn‐on times are attributed to the low mobility of the counterions. The performance of the devices dramatically improves when small amounts of ionic liquids (ILs) are added to the Ir‐iTMC: the turn‐on time improves drastically (from hours to minutes) and the device current and power efficiency increase by almost one order of magnitude. However, the improvement of the turn‐on time is unfortunately accompanied by a decrease in the stability of the device from 700 h to a few hours. After a careful study of the Ir‐iTMC:IL molar ratios, an optimum between turn‐on time and stability is found at a ratio of 4:1. The performance of the optimized devices using these rather simple complexes is among the best reported to date. This holds great promise for devices that use specially‐designed iTMCs and demonstrates the prospect for LECs as low‐cost light sources. Simple but efficient materials: Two archetype cationic iridium complexes are extensively characterized and used as the primary active component in simple electroluminescent devices. The addition of small amounts of ionic liquid to the active layer significantly improves the device performance. This works shows that even with rather simple starting compounds promising electroluminescent devices can be prepared with thorough device optimization.