Short-diode like diffusion capacitance of organic light emission devices

A prototypical organic light-emitting diode (OLED), anode/organic layers/cathode, is investigated via bias-dependent capacitance measurement of two-layer (HTL/ETL) and four-layer (HIL/HTL/EML/ETL) structure at low frequency. As applied voltage was larger than the built-in voltage, the capacitance is augmented by diffusion capacitance with increasing the forward bias voltage. In contrast, the capacitance dropped quickly. It is possibly due to the voltage drop in the bulk and conductivity modulation occurred by high carrier concentration for much higher forward biases, eventually showing negative for two-layer OLEDs. We infer that the phenomena were resulted from the extremely thin OLED structure, just like short p–n semiconductor diodes. This diffusion capacitance exhibiting like short-diode behavior also can be measured in C–V curves of single-layer structure OLED. D 2005 Elsevier B.V. All rights reserved.