Dipole formation at organic/metal interfaces with pre-deposited and post-deposited metal

In organic electronic devices, the interfacial dipole at organic/metal interfaces is critical in determining the carrier injection or extraction that limits the performance of the device. A novel technique to enable the direct measurement of underburied dipoles is developed and demonstrated. By tilting the shadow mask by a small angle, metal atoms diffuse into the opening slit to form an ultrathin metal layer during the evaporation process. As the ultrathin metal layer cannot screen out the dipole-induced surface work function change, the dipole strength and direction at the organic/metal interface can be revealed. It was found that the polarity of the organic material, the Fermi-level pinning and the interface morphology all play important roles in dipole formation. By comparing the energy level shifts at the organic/pre-deposited metal and organic/post-deposited metal interfaces, the dipole formed by molecular interactions could be distinguished from the dipole formed by Fermi-level pinning.