Simultaneous Observation of the Electrical and Luminous Characteristics of Light-Emitting Electrochemical Cells by Using a Displacement Current Measurement Technique

An extended displacement current measurement (DCM) is performed to analyze the operation mechanisms of a poly(p-phenylenevinylene) derivative (Super Yellow)-based light-emitting electrochemical cell (LEC). The characteristics of the actual current, displacement current, and electroluminescence (EL) intensity during the relaxation processes of electrochemical doping are simultaneously measured. The transient characteristics indicate correlations between the conductance, capacitance, and EL efficiency. The conductance and capacitance decrease with proceeding doping relaxation, whereas the EL efficiency increases. The EL efficiency is dominated by the electrochemical doping state, rather than the actual current density. Moreover, the EL efficiency deteriorates with increasing direct current (DC) voltage applied for electrochemical doping. Since the DCM reveals that the electrical properties of the device obey the trap charge limited current regime in the intrinsic region and are almost independent of the applied DC voltage, efficiency deterioration is not responsible for the intrinsic region but for the doped region. The results suggest that the self-absorption in the doped region causes efficiency loss, otherwise the emitting zone is formed close to the doped region. The extended DCM enables us to comprehensively analyze the operation mechanisms of LECs on the basis of the simultaneous observations of electrical and luminous characteristics, including their transient changes.