Influence of trapped and interfacial charges in organic multilayer light-emitting devices

The influence of trapped and interfacial charges on the device characterist ics of organic multilayer light-emitting devices is investigated. We have s tudied 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 (Alq(3)) as electron transport and emitting layer sandwiched between a hig h-work-function metal and a semitransparent Ca electrode. Current-voltage m easurements show that the device characteristics in negative bias direction and at low positive bias are influenced by charges trapped within the orga nic 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 capa citance-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 N PB/Alq(3) interface. Transient experiments show that the delay time of elec troluminescence under forward bias conditions is controlled by the buildup of internal space charges rather than by charge-carrier transport through t he organic layers. (C) 2001 American Institute of Physics.