Influence of space charges on the current-voltage characteristic of organic light-emitting devices

The role of space charges on the device characteristics of multilayer organic light-emitting devices (OLEDs) is investigated. We studied OLEDs consisting of copper phthalocyanine (CuPc) as buffer and hole injection layer, N,N'-di(naphthalene-1-yl)-N,N'-diphenylbenzidine (NPB) as hole transport layer, and tris(8-hydroxyquinolinato)aluminum (Alq(3)) as electron transport and emitting layer sandwiched between high and low work function metal electrodes. Detailed current-voltage measurements show that the device characteristics at low bias depend strongly on sweep direction as well as on sweep speed, indicating that space charges accumulate within the organic layers. On the one hand these space charges increase the electric field for electron injection at the cathode, on the other hand they screen the applied electrical field and thus determine the steepness of the current-voltage characteristics. Reducing these space charges by fabricating optimized structures where the limiting interfaces between the different organic layers are graded results in a significantly enhanced current flow and higher brightness at a given voltage. (C) 2000 Elsevier Science S.A. All rights reserved.