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 (Alq3) 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.