Encapsulation of Organic Light Emitting Diodes

Abstract

Organic Light Emitting Diodes (OLEDs) are extremely attractive candidates for flexible display and lighting panels due to their high contrast ratio, light weight and flexible nature. However, the materials in an OLED get oxidized by extremely small quantities of atmospheric moisture and oxygen. To obtain a flexible OLED device, a flexible thin-film barrier encapsulation with low permeability for water is necessary.

Water permeates through a thin-film barrier by 4 modes: microcracks, contaminant particles, along interfaces, and through the bulk of the material. We have developed a flexible barrier film made by Plasma Enhanced Chemical Vapor Deposition (PECVD) that is devoid of any microcracks. In this work we have systematically reduced the permeation from the other three modes to come up with a barrier film design for an operating lifetime of over 10 years.

To provide quantitative feedback during barrier material development, techniques for measuring low diffusion coefficient and solubility of water in a barrier material have been developed. The mechanism of water diffusion in the barrier has been identified. From the measurements, we have created a model for predicting the operating lifetime from accelerated tests when the lifetime is limited by bulk diffusion.

To prevent the particle induced water permeation, we have encapsulated artificial particles and have studied their cross section. A three layer thin-film that can coat a particle at thicknesses smaller than the particle diameter is identified. It is demonstrated to protect a bottom emission OLED device that was contaminated with standard sized glass beads.

The photoresist and the organic layers below the barrier film causes sideways permeation that can reduce the lifetime set by permeation through the bulk of the barrier. To prevent the sideways permeation, an impermeable inorganic grid made of the same barrier material is designed. The reduction in sideways permeation due to the impermeable inorganic grid is demonstrated in an encapsulated OLED.

In this work, we have dealt with three permeation mechanisms and shown solution to each of them. These steps give us reliable flexible encapsulation that has a lifetime of greater than 10 years.