Journal
JOURNAL OF APPLIED PHYSICS
Volume 89, Issue 3, Pages 1704-1712Publisher
AMER INST PHYSICS
DOI: 10.1063/1.1332088
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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 (Alq(3)) as electron transport and emitting layer sandwiched between a high-work-function metal and a semitransparent 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/Alq(3) 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. (C) 2001 American Institute of Physics.
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