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 organ ic light-emitting devices (OLEDs) is investigated. We studied OLEDs consist ing 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 electro des. Detailed current-voltage measurements show that the device characteristics at low bias depend strongly on sweep direction as well as on sweep speed, i ndicating that space charges accumulate within the organic layers. On the o ne hand these space charges increase the electric field for electron inject ion at the cathode, on the other hand they screen the applied electrical fi eld and thus determine the steepness of the current-voltage characteristics . Reducing these space charges by fabricating optimized structures where th e limiting interfaces between the different organic layers are graded resul ts in a significantly enhanced current flow and higher brightness at a give n voltage. (C) 2000 Elsevier Science S.A. All rights reserved.