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

Abstract 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 the 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, this issue is addressed through the presentation of 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, a linear polarizer is employed for the selective detection of s‐polarized emission and a half‐cylinder outcoupling structure for enhanced spectral output. The method is finally employed on a common conjugated‐polymer LEC, and it is derived that its EZ width decreases during the initial operation, that the steady‐state EZ width is equal to ≈20% of the active‐material thickness at a current density of 10 mA cm⁻², and that the steady‐state EZ width appears to decrease with increasing current density.