A combination of impedance spectroscopy, device characterization, and modeling is used to pinpoint key processes in the operation of polymer light-emitting electrochemical cells (LECs). At low applied voltage, electric double layers with a thickness of similar to 23 nm are shown to exist at the electrode interfaces. At voltages exceeding the bandgap potential of the conjugated polymer (V = 2.5 V for superyellow), a light-emitting pn junction forms in situ, with a steady-state structure that is found to depend strongly on the applied voltage. This is exemplified by that the effective pn junction thickness (dpn) for a device with an interelectrode gap of 90 nm decreases from similar to 23 nm at 2.5 V to similar to 6 nm at 3.9 V. The current increases with decreasing dpn in a concerted manner, while the brightness reaches its peak at V = 3.4 V when dpn similar to 10 nm. The existence of an optimum dpn for high brightness in LECs is attributed to an offset between an increase in the exciton formation rate with decreasing dpn, due to an increasing current, and a simultaneous decrease in the exciton radiative decay rate, when an increasing fraction of excitons diffuses away from the pn junction into the surrounding non-radiative doping regions.