Impedance characteristics of ITO/Alq3/Al organic light-emitting diodes depending on temperature

Temperature-dependent impedance characteristics of ITO/Alq3/Al organic light-emitting diodes were studied in the frequency range from 40 to 108 Hz, and the temperature was varied from 10 to 300 K. At each temperature, the frequency-dependent complex impedance was measured under discrete bias voltages from −6 to +20 V, and the voltage-dependent impedance was measured at 102 Hz, 103 Hz, 104 Hz, and 105 Hz. A Cole–Cole plot shows that there is one relaxation, and a parallel capacitor–resistor network in series with a contact resistance could be considered as an equivalent electrical circuit to this device. As the temperature decreases, a radius in the Cole–Cole plot increases, which indicates an increase of resistance of the device.