Transient behaviour of charge carriers in multilayer organic light-emitting diodes:: experiment and theory

We have investigated multilayer organic light-emitting diodes (OLEDs) with a tris(8-hydroxy-quinolinolato)aluminium (Alq(3)) emitting layer under both cw and pulsed operation. We detected optically the positive charge carriers in the hole transport layer (HTL) consisting of 1,3,5-tris-(N, N-bis-(4,5-methoxy-phenyl)-aminophenyl)-benzene (TAPB) as a function of the applied bias. It is seen that the positive contact shows ohmic behaviour. Furthermore, transient electroluminescence (EL) measurements demonstrate a dependence of rise times on applied bias and the device history, which we attribute to the formation of a positive surface charge at the HTL/electron transport layer (ETL) interface. An analytical model is developed which successfully describes these observed transients and predicts the corresponding carrier concentrations quantitatively. (C) 2000 Elsevier Science S.A. All rights reserved.