Impedance and Electroluminescent Performance of OLEDs Driven by DC and AC Voltage

Abstract

Organic light-emitting diodes (OLEDs) are generally operated under the DC voltage. However, since the mobility of hole is different from that of electron in organics, long time DC operation of the device accumulates charge carriers in bulk or at organic-organic interface. The charge accumulation forms a space charge. Since the formed space charge generates an electric field opposite to that of the applied voltage, it prevents a current in the device. It accelerates a degradation of the device. In order to resolve these problems, AC-driven OLEDs are getting attention. An application of small AC voltage to the device generates additional charges, and injects them to the device. The injected charges modify the space-charge region in the device, and it is expected to reduce the accumulation of charges. In this study, in order to release the space charge and to improve the performance of the device, we have investigated impedance and electroluminescent performance of the OLEDs driven by DC and AC voltages, and focused on the characteristic time, impedance, and space charge. Device was fabricated into a ITO(180 nm)/TPD(40 nm)/Alq3(60 nm)/Liq(2 nm)/Al (100 nm) structure. And in order to observe the electroluminescent performance of the device, summing amplifier that adds DC and AC voltage was fabricated. The applied voltage and frequency range are 0 ~ 9 V and 100 Hz ~ 100 kHz, respectively. The applied amplitude of AC voltage was 200 mV and 500 mV. And time-dependent applied voltage and current were simultaneously measured by using the oscilloscope. Compared to that of DC-driven OLEDs, small AC voltage is less affected to electroluminescent performance. However, a phase difference between the applied voltage and current occurs by the capacitive effect caused by AC signal. Impedance analyzer also provides (DC+AC) voltage, and it can be confirmed charge injection and transport of the OLEDs. To understand charge transport and space-charge distribution, we monitored impedance, phase difference, and capacitance of the device using the impedance analyzer (Agilent 4294A) at several voltages. The applied DC voltage and frequency are 0 ~ 8 V and 40 ~ 100 kHz, respectively. And the amplitude of AC voltage was 500 mV. In low-voltage region, the space charge was formed by the accumulated holes. As the frequency increases, the phase difference occurs by the capacitive effect. And the capacitance of the device gradually decreases. It means a decrease of accumulation of charges, and the decrease indicates that the space charge is released. This kind of operation of the device not only reduces the degradation of the device, but also is expected to improve the performance and lifetime of the device.