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. A novel way to directly measure buried dipoles at metal–organic interfaces in organic electronic devices has been demonstrated. It is critical to match the energy levels at metal–organic interfaces in organic electronic devices such as organic solar cells and light-emitting diodes. To determine the most suitable material for the interfacial layer to achieve this, the work function of the cathode needs to be measured before and after depositing the interfacial layer, but this is difficult in conventional devices. Jian Wang and co-workers at South China University of Technology have solved this problem through forming an ultrathin metal layer by titling a shadow mask at a small angle during the evaporation process. Unlike thick metal layers, this ultrathin layer does not screen dipoles, allowing dipoles to be directly measured at interfaces. By thermally evaporated through a tilted shadow mask, metal vapor scatters and lands on the substrate to form an ultrathin layer. Absence of screening effect from bulk metal enables direct measurement of the buried dipole formed at the organic/metal interface. It is found that polar species of the organic, Fermi-level pinning and interface morphology can all influence the dipole orientation. The proposed method provides a quick and easy approach to assess interfacial materials and design electrode interfaces in organic electronic devices.