Organic electroluminescent devices are currently under development for display and lighting applications due to their high efficiency, ease of fabrication, and facile color tunability. Within the class of organic electroluminescent materials, ionic transition metal complexes (iTMCs) are receiving increased attention as materials capable of yielding efficient light-emitting devices with air-stable electrodes. iTMCs can support all three processes of charge injection, charge transport, and emissive recombination, enabling the fabrication of efficient, single-layer devices. When paired with an appropriate counterion, iTMCs are mixed conductors, possessing significant ionic conductivity in addition to electronic conductivity. As a result, the operational characteristics of iTMC devices are dictated by the redistribution of ionic charge, which serves to assist electronic charge injection. This interplay between ionic and electronic conductivity in iTMC devices offers rich physics and unique device opportunities.
Our work with iTMC devices involves understanding the underlying physics, improving efficiency, extending the range of available colors, ascertaining modes of degradation, and inventing practical architectures for display and lighting applications. Concerning device physics, we have used electric force microscopy to measure the in situ electric field profile across an iTMC device and observed it to follow an electrodynamic model of operation. In demonstrating novel colors of electroluminescence in iTMCs, we have achieved efficient yellow and green devices with power efficiencies as high as 10 lumens per electrical Watt. By use of mass spectrometry and in situ Raman spectroscopy, we have observed that formation of oxo-bridged dimers during iTMC device operation coincides with device failure, understanding that could lead to improved device lifetime. We have demonstrated monolithic, scalable lighting panels from iTMC devices that show intrinsic fault tolerance and direct 120 volt, 60 hertz operation. We have also formed electrospun light-emitting nanofibers with nanoscale emission zones for high-resolution display and spectroscopy applications.