Film Conductivity Affects the Electric Double Layer Formation in Iridium-based Light-emitting Electrochemical Cells

Abstract

Light-emitting electrochemical cells (LEECs) from ionic transition metal complexes (iTMCs) are promising candidates for low-cost lighting system because they are easy to make using solution-based technologies and air-stable materials with simple architecture. Typically, they are fabricated as single-layer devices whose active region acts as mixed conductor as well as the emissive layer. Once the LEEC device is applied with a bias, ions redistribute and accumulate near the electrodes to form electric double layers (EDLs). The EDLs facilitate facile charge carrier injections that lead to efficient operation of the devices. In this work, iridium(III)- based LEEC devices of various active layer thickness were fabricated via spin coating. We investigated the connection between the initial responses of sandwich LEECs and the width of EDL that formed in the devices. Electrochemical impedance spectroscopy (EIS) revealed that the film conductivity decreases with film thickness. Operating the devices with constant bias showed that the turn-on time increases and the rate of current growth decreases as the active layer thickness of the device is increased. By formulating the device operation into an equivalent circuit and then extracting the device parameters, it was revealed that the width of EDL follows with the film thickness, which was also confirmed by the ionic charge transport modeling studies. In addition, the device modeling studies also showed that the difference in the mobility between the anions and cations can contribute to the asymmetry of the EDL effective width that formed near each electrode. Overall, this work reports that the width of the EDL formed in iridium-based LEECs is affected by the film conductivity.