ac impedance of polymer light-emitting electrochemical cells and light-emitting diodes: a comparative study

The alternating current (ac) impedance of light-emitting electrochemical cells (LEC) is studied and compared with that of light-emitting diodes (LED) in the frequency range from 100 Hz to 5 MHz. In the LED, the capacitance is independent of voltage; in the LEC, the capacitance increases at voltages sufficient to induce electrochemical redox and doping near the electrodes. However, the capacitance of the LEC increases only by approximately 20% when the p–i–n junction is created in situ. Thus, in the LEC under operating conditions, the crossover region from p- to n-type doping occupies most of the film thickness.