Utilization of novel phenanthrene–imidazole-based ionic small molecules for blue light-emitting electrochemical cells

Abstract

Light-emitting electrochemical cells (LECs), which represent solid-state lighting (SSL) systems, are next generation displays to replace organic light-emitting diodes. LECs have a single active layer of organic luminescent material between two air-stable electrodes and have a unique operating mechanism. In this study, we synthesized two luminescent small molecules, PhIm and PyIm, using phenanthrene–imidazole derivatives for blue emission and demonstrated their thermal, photophysical, and electrochemical properties. In addition, we fabricated small molecule LECs (SM-LECs) to evaluate their electroluminescence performances. Both compounds showed high thermal stabilities, which means that these luminescent materials are suitable for LEC emission layers. PyIm is less influenced by the polarity of solvents than PhIm when comparing the maximum emission wavelengths of the two compounds dissolved in various solvents. The photoluminescence (PL) spectra of PhIm and PyIm showed a blue spectral region, where the maximum emission wavelengths were 461 and 454 nm in solution, respectively. The electroluminescence spectra of the two compounds, which were red-shifted with respect to the PL spectra, showed peaks at 467 and 459 nm, and their Commission Internationale de l’Eclairage coordinates were (0.24, 0.25) and (0.21, 0.22) for PhIm and PyIm, respectively. In addition, in terms of device performances, PhIm and PyIm showed maximum current efficiencies of 0.87 and 1.85 cd A⁻¹, maximum power efficiencies of 0.30 and 0.73 lm W⁻¹ and maximum external quantum efficiencies of 1.53 and 1.70%, respectively. Both compounds successfully emitted blue light and PyIm showed better performance than PhIm.