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.