Blending ionic liquids to Ir complexes for applications in light-emitting electrochemical cells (LEECs).

摘要

At present, the search for novel efficient and low cost solid state light sources is experiencing impressive progress.;Light emitting electrochemical cells (LEECs) are two-electrode light sources made of ionic transition metal complexes (iTMCs) or organic polymers as the light emitting material, in some cases blended with electrolytes. Electrolytes are intended for lowering the operating voltage of the device by improving the efficiency of the charge carrier injection process. The advantages of polymer semiconductors include low cost, easy processing in solution for use in large area and flexible devices, different colors of light emission tailorable by molecular synthesis. Nevertheless, LEECs based on iTMCs show higher efficiency due to light emission via phosphorescence in addition to fluorescence processes.;One of the main challenges in the field of LEECs is their long turn on time, due to ionic transport in the cell. The turn on time can be decreased by using as the electrolyte an ionic liquid (IL). Compared to conventional salts, ionic liquids exhibit higher ion mobility leading to lower turn on times. However, the presence of ILs in the film is unfortunately accompanied by a decrease in the stability of the device. Fundamental studies are needed to better understand the LEEC working mechanism and improve device performance. For this purpose, the ideal architecture for LEECs is a planar one since it enables the direct measurement of the intensity of the emitted light by optical probes as well as the imaging of the emission. In addition to that, planar device fabrication is much simpler than vertical.;LEECs with a planar configuration based on Ir complexes have not been reported. In this work, thin films of Ir(ppy)2(bpy) +PF6- and blends of Ir(ppy)2(bpy) +PF6-/BMIm+PF6-- were spin coated on different substrates, including Au-patterned SiO 2, Au-patterned glass, ITO, and PEDOT:PSS covered-ITO. The effect on the film morphology of the nature of the substrate and the film processing conditions was investigated, to shed light in to the LEEC working mechanism.;Film formation was studied using fluorescence hyperspectral imaging and Atomic Force Microscopy (AFM). We found that the films are constituted of particles whose density, shape,