Determining the Width of the Dynamic Emission Zone in Light-Emitting Electrochemical Cells

The light-emitting electrochemical cell (LEC) forms a p-n junction doping structure by bipolar electrochemical doping during its initial operation. The light emission originates from the p-n junction region through formation and radiative decay of excitons. The width of this emission zone (EZ) is important since it strongly affects the emission losses by exciton quenching, the outcoupling efficiency, and the drive voltage. The challenge is that it has proven very difficult to determine the width of the dynamic EZ in LECs. Here, we address this issue by presenting a method that fits simulated angle-resolved emission spectra to measured spectra, using the EZ width and position as the two free parameters. For improved accuracy, we employ a linear polarizer for the selective detection of s-polarized emission and a half-cylinder outcoupling structure for enhanced spectral output. We finally employ the method on a common conjugated-polymer LEC and establish that the EZ width decreases during the initial operation. We further find that the EZ width, during steady-state operation at a current density of 10 mA cm^-2, is equal to ~20 % of the active-material thickness, and that the steady-state EZ width appears to decrease with increasing current density.