Influence of Trapped and Interfacial Charges in Organic Multilayer Light-emitting Devices

The influence of trapped and interfacial charges on the device characteristics of organic multilayer light-emitting devices is investigated. We have studied devices consisting of 20-nm copper phthalocyanine as buffer and hole-injection layer, 50-nm N,N'-di(naphthalene-1-yl)-N,N'-diphenyl- benzidine (NPB) as hole-transport layer, and 65-nm tris(8-hydroxyquinolinato)aluminum (Alq3) as electron transport and emitting layer sandwiched between a high-work-function metal and a semi-transparent Ca electrode. Current-voltage measurements show that the device characteristics in negative bias direction and at low positive bias are influenced by charges trapped within the organic layers. This is manifested by a strong dependence of the current on the direction and speed of the voltage sweep in this range. Low-frequency capacitance-voltage and static charge measurements reveal a voltage-independent capacitance in negative bias direction and a significant increase between 0 and 2 V, indicating the presence of negative interfacial charges at the NPB/Alq3 interface. Transient experiments show that the delay time of electroluminescence under forward bias conditions is controlled by the buildup of internal space charges rather than by charge-carrier transport through the organic layers.
Keyword: Bruetting