The mechanism of lifetime extension due to CuPc injection layer in organic light-emitting diodes

The remarkable effect on lifetime improvement of copper phthalocianine (CuPc) coated indium tin oxide (ITO) anode of organic light emitting diodes (OLED's) is experimentally well approved. Also known are the electrode morphology, with and without CuPc coating, the energy levels of the used materials, important for charge injection and conduction, the carrier mobility etc. Based on this knowledge we suggest the model that explains the mechanism behind the lifetime improvement. We argue that the charge accumulation at the interface between the CuPc and the hole transport layer is responsible for screening out of the electric field variations leading to current density homogenization across the OLED surface. The variation of the injection field, introduced by electrode roughness, is estimated for typical indium tin oxide morphology used in OLED production. Without the CuPc hole injection layer a substantial current channeling occurs in OLED's, leading to accelerated device degradation