Optimization of large-area OLED current distribution grids with self-aligned passivation

Abstract

The luminance homogeneity of large-area organic light emitting diodes (OLEDs) is limited by the sheet resistance of the transparent electrode. A large lateral voltage drop inside the electrode and thus brightness inhomogeneity result from the high sheet resistance of transparent conductors. To improve sheet resistance, a low-resistance metal grid is often included. To prevent shorting, the grid needs to be passivated. However, since passivation further decreases the device active area, accurate alignment of the passivation layer is crucial. We report simulations of a Joule-heating-based self-alignment method for the passivation layer. The Joule heating model was divided into two sub-models: a current model to study current distribution on grid scale during Joule heating, and thermoelectric model to study heat transfer in the system. Grid design rules – minimum line pitch and the geometry of the grid – necessary for a successful Joule heating process were studied. The line group design was found to be the best option for a current distribution grid. The minimum line pitch limited by heat transfer was 0.8mm on an indium tin oxide (ITO) coated polyethylene terephthalate (PET) substrate. With the line group design, maximum luminance output was achieved with a pitch of 4mm, when the sheet resistance of the metal lines was 0.01Ω/□. This fulfils the demands placed by the Joule heating process.