Balancing the Concentrations of Redox Species to Improve Electrochemiluminescence by Tailoring the Symmetry of the AC Voltage

We propose an effective methodology to improve the brightness of Ru(bpy)₃ ²⁺-based electrochemiluminescence (ECL) devices driven by the annihilation path, based on an understanding of the great importance of concentration balance between oxidized Ru(bpy)₃ ^.3+ and reduced Ru(bpy)₃ ^.1+ species. From cyclic voltammetry experiments, we verify that Ru(bpy)₃ ^•3+ is more stable than Ru(bpy)₃ ^.1+, which is a fundamental physical parameter for balanced electrochemical annihilation reaction. Accordingly, we adjust the symmetry (i.e. the duty ratio) of the input AC voltage to control the concentrations of redox species. The frequency and peak-to-peak voltage (V_pp) are fixed at 60 Hz and 7.5 V_pp, respectively. When the duty ratio is 40 %, a luminance of approximately 174 Cd/m² is detected, which is brighter than the 148 Cd/m² achieved under a symmetric voltage wave. We investigate the feasibility of further enhancing device luminance by modulating the frequency of applied voltage at the fixed duty ratio of 40 %, achieving a maximum luminance of approximately 207 Cd/m² at 150 Hz. Overall, this work elucidates the critical role of electrochemical stability of redox species that participate in the light-emitting annihilation reaction and suggests a simple but effective route (e.g. adjusting symmetry of AC input) to improve luminance of ECL devices.