Münster 1999 – scientific programme

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CP: Chemische Physik

CP 15: Elektro-optische und elektrische Eigenschaften organischer Materialien

CP 15.3: Talk

Tuesday, March 23, 1999, 14:30–14:45, Gg

Influence of space charges on the device characteristics of organic light-emitting devices — •H. Riel¹, W. Br"utting², T. Beierlein¹, E. Haskal¹, P. M"uller¹, and W. Rie"s¹ — ¹IBM Research Division, Zurich Research Laboratory, CH-8803 R"uschlikon, Switzerland — ²Experimentalphysik II, Universit"at Bayreuth, D-95440 Bayreuth, Germany

The role of space charges on the device characteristics of multilayer organic light-emitting devices (OLEDs) is investigated. We studied devices consisting of copper phthalocyanine (CuPc) as buffer and hole injection layer, N,N’-di(naphthalene-1-yl)-N,N’-diphenyl-benzidine (NPB) as hole transport layer and tris(8-hydroxyquinolinato)aluminum (Alq₃) as electron transport and emitting layer sandwiched between high and low work function metal electrodes. Current-voltage measurements show that the device characteristics at low forward bias is determined by the build-up of hole space charges within the organic layer, which in turn increases the electric field for electron injection at the cathode. This is manifested by low-frequency capacitance-voltage measurements revealing a voltage independent capacitance in reverse bias direction and a significant increase between 0 and 2 V in forward direction. Transient experiments show, that the delay and rise time of electroluminescence after application of a rectangular voltage pulse in these bilayer OLEDs strongly depends on the injection barriers of both contacts, indicating that the response is controlled by the build-up of hole space charges and delayed electron injection rather than by charge carrier transport through the organic layers.