Transient Behavior of Light-Emitting Electrochemical Cells

Recent prototypes of the individual identification friend or foe IIFF patch use a light-emitting electrochemical cell LEC as the emitter. This research characterizes the transient behavior of LECs by measuring transient capacitance. The transient capacitance data are important to improve understanding of the underlying physics describing the operation of the LEC. The research goal was to make the first transient measurements of an LECs capacitance as a function of temperature and bias, while simultaneously measuring the transient light output and current, to monitor in-situ junction formation inside an LEC. Capacitance changes varying from 5-30 nF are measured, depending on applied voltage and device temperature. Strong temperature dependence of the rate of change of capacitance suggests Arrhenius-type behavior associated with ion motion with an activation energy of 1.27 eV. The initial rate of change of capacitance is faster than the rate of change of light and current, suggesting that modification of the field near the contacts plays a key role in controlling free carrier injection. Initially capacitance increases monotonically upon application of bias, however, at longer times decreasing and even oscillating capacitance has been observed. This behavior provides new information on the dynamics of ion motion and carrier injection in LECs.