Laminated transparent top electrodes for organic light-emitting diodes

Abstract

Transparent electrodes are essential for the implementation of optoelectronic devices. In recent years, there have been increasing demands for an electrode having flexibility as well as transparency for realizing next-generation flexible optoelectronic devices. In top-emitting organic light-emitting diodes (OLEDs) and transparent OLEDs, semi-transparent thin metal layers are widely used as a top electrode because they can be easily formed by thermal evaporation and can control the microcavity effect for a wide color gamut. However, the high reflectance that originates from the nature of the metal itself often produces reflective images, which lowers visibility and immersion. In addition, the relatively large angular color distortion is caused by the strong microcavity effect, and strong surface plasmon polariton mode light loss is inevitable at the interface of the thin metal layer. Nonmetal-based materials such as the graphene and conducting polymers have the potential to overcome the limitations of metals. However, there are many difficulties in forming such nonmetal materials as the top electrode in OLEDs without inducing damages to soft organic layers because of the necessary harsh process conditions. In this study, we introduce a vacuum lamination technique which can overcome the limitations of conventional thin metal layers and stably form top electrodes. This technique makes it possible to form transparent top electrodes that could not be formed due to present process limitations and meets requirements such as large-scale compatibility, sufficient flexibility, high visibility, and improved efficiency for future displays. Through the proposed technique, we successfully demonstrated multi-layered graphene (MLG)-laminated OLEDs, and the device performance was improved by lowering the hole injection barrier through the chlorine doping onto the MLG. Laminated large-area OLEDs were also demonstrated using the conducting polymer film with the embedded auxiliary mesh to show the large-area compatibility of the proposed technique. In addition, we enhanced the light extraction with little optical blurring through the lamination of the structured top electrode film based on the conducting polymer. We also verified the experimental results and suggested a way to further improve the light outcoupling efficiency using the optical analysis. Moreover, we have studied adhesion mechanisms and key control parameters through a variety of analyses including a mechanical simulation for the ultimate realization of the reproducible and controllable vacuum lamination technique.