Double-layer formation in organic light-emitting electrochemical cells

We present a systematic analysis of the current transients accompanying the formation of the electrode–electrolyte double layers in organic light-emitting electrochemical cells. By using various room-temperature molten salts, conducting polymers, and electrodes, we show that the current I always decreases as a power law of time, I∝t−n. The current transients are formed of various time domains, each one being characterized by a power-law exponent n<1. Impedance measurements conducted from 5 Hz to 5 MHz demonstrate that these transients represent the time response of a simple combination of constant phase angle (CPA) impedances, Zn∝(jω)n, and of the electrolyte ionic conductivity. The physical origin of the CPA impedance is attributed to the roughness of the interface between the electrodes and the electrolyte, and to the phase separation within the salt–polymer blend.