Low current density driving leads to efficient, bright and stable green electroluminescence

Abstract

Electroluminescent devices have the potential to reshape lighting and display technologies by providing low-energy consuming solutions with great aesthetic features, such as flexibility and transparency. In particular, light-emitting electrochemical cells (LECs) are among the simplest electroluminescent devices. The device operates with air-stable materials and the active layer can be resumed to an ionic phosphorescent emitter. As a consequence, LECs can be assembled using solution-process technologies, which could allow for low-cost and large-area lighting applications in the future. High efficiencies have been reported at rather low luminances (<50 cd m -2) and at very low current densities. Moreover, these efficiencies could be sustained for a brief moment only during operation time. Here, we demonstrate that a pulsed driving mode at low current densities leads to unequalled overall performances with excellent efficiencies throughout the lifetime of the device. The lifetime of the LECs is defined as the time it takes to reach 50% of the peak luminance. Upon optimization of various parameters (frequency, duty cycle and average current density), the green LEC reaches efficacies and power efficiencies of 28.2 cd A-1 and 17.1 lm W -1, respectively, at a luminance above 750 cd m-2 and 98 hours lifetime. The present work also rationalizes why high efficiencies have been obtained only at low current densities so far. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.