Hole injection or blocking? The role of CuPc in Alq3-based organic light-emitting devices

It is known in tris-(8-hydroxyqunoline) aluminum-based organic light-emitting device, that copper-phthalocyanines (CuPc) inserted between the anode and the hole transport layer (HTL), has two opposing roles: improving and reducing the current injection. Despite of its controversy, no work has been done to clear the matter. Here, we addressed this controversy with experiments and simulations. Our results showed that, CuPc inserted between indium-tin-oxide (ITO) and HTL, played dual roles of hole-blocking and hole-injection. CuPc layer improves holes injection, when the amount of oxygen at the CuPc/HTL interface is small. However, CuPc layer switches to impede holes injection, when the oxygen diffusion from ITO to CuPc/HTL interface is severe.