Differential capacitance of hybrid organic/inorganic CdSe/ZnS quantum dots light-emitting device

We have studied the differential capacitance of self-assembly hybrid organic/inorganic quantum dot (QD) light-emitting devices (QD-OLEDs) at room temperature in the frequency range of 1x10(3) to 1x10(6) Hz. Self-assembly CdSe/ZnS QDs monolayer was used to fabricate trilayer hybrid QD-OLEDs on indium tin oxide coated glass substrates. We observed negative differential capacitance (NDC) in both OLED and QD-OLEDs for the first time in the intermediate frequency range of 5x10(3) to 5x10(5) Hz. The onset and frequency range of NDC are strongly depended on the applied bias voltage. This behavior is described by a mathematical model. The simulated results showed that the NDC was due to the time-dependent transient current from the QDs and organic/metal interfacial states in the device. The probable mechanisms of NDC in organic devices are discussed.