The influence of the internal interface energy barrier and the device dimensions on the transient electroluminescence lifetime of bi-layer OLEDs

We present a study on the dependence of the transient electroluminescence lifetime in polyfluorene bi-layer light-emitting diodes on the internal energy barrier at the organic/organic interface and on the device dimensions. Using a homemade specially designed driver, facilitating a switching time of ∼5 ns and thus minimizing effects on the charge carriers' distribution, allowed us to study the physical processes with a well-defined charge distribution imposed by the steady-state operation. We observe an unexpected asymmetric behavior between the dependence of the lifetime on the hole transporting layer (HTL) thickness or the electron transporting layer (ETL) thickness. The lifetime decreases when the ETL thickness is reduced but increases when the HTL thickness is reduced. We associate this observation to the highly asymmetric barrier at the HTL:ETL interface and the lower mobility of the electrons in the ETL. We verify it using a steady-state simulation based on the “Marburg model”. We further show that the additional interface, (i.e. the injection barrier or injection efficiency) has a one-to-one correspondence with the measured lifetime and can reduce it by approximately 75 percent, but does not affect the asymmetry. This behavior is also validated and explained via the Marburg model simulation.