In 1995, Pei et al. demonstrated a device in which ion incorporation in a polymer light-emitting diode architecture leads to dynamic, symmetric light emission in a device with reduced thickness dependence and reduced dependence on electrode work function.1 The so-called polymer light-emitting electrochemical cell (LEC) introduced a new approach for the operation of a solution-processed optoelectronic device. In essence, the LEC was first described as a dynamic, self-assembled polymer p-i-n junction in which p-and n-type doping occur via electrochemical oxidation and reduction, respectively.2 In an LEC, the electric field and light emission occur over a thin intrinsic (non-doped) layer often positioned close to the cathode (Figure 6.1). Several models have been described to explain the mechanism of operation of these devices.1-6 It is generally agreed, however, that in an LEC, the cations and anions dissociate under an applied electric field and accumulate along opposite electrode/polymer interfaces. Device characteristics can be impacted both through induced electrochemical doping and through the buildup of uncompensated charges at the electrodes. The increased conductivity of the doped material reduces barriers to charge injection and allows the potential in the device to drop over a relatively thin undoped region. Ions not ultimately compensated by doped polymer contribute to these effects by introducing a dipole layer, which results in a potential drop at the electrode interface. The history, mechanism, and progress of LEC operation and current progress in the field are the subjects of Chapter 5.