2, ZHAW, Winterthur, , Switzerland
Recent portable electronic devices such as flagship smartphones and wearable VR/AR have adopted organic light-emitting diodes (OLEDs) as a display panel due to their lightness, high color purity, contrast, high-speed, and applicability to versatile designs. Those emerging devices often need even greater efficiency than conventional devices because the capacity of their battery is often severely limited. In this regard, unlocking the full optical potential of OLEDs is strongly desired for long operational hours and, not to mention, for performance improvements.
It is well known that most of the generated photons in an OLED are trapped in the substrate and organic layer as a waveguide mode or dissipated in the metal surface as a surface plasmon polariton (SPP) mode. The portion of energy loss induced by SPP can be as high as 30% in conventional OLEDs, and its extraction or recovery has been considered as the most challenging among the various loss mechanisms. For this reason, many researchers have focused on extracting waveguide or SPP mode using nano- or micro- structures; however, applying internal structure in an OLED easily causes electrical shorts and makes it subject to rather expensive fabrication cost. Increasing the distance between the metal and emitter is another way to reduce SPP loss, however, this approach increases the portion of waveguided modes and weakens Purcell effect in cavity–resonance structures, tending to render its efficiency to drop. To place low-index materials between the emitter and metal electrode is an alternative way to reduce SPP. By introducing the low-index medium adjacent to metal, the portion of dissipated power from SPP loss is suppressed and shifted into a smaller in-plane wavevector region, contributing to the improvement in outcoupling efficiency. From this perspective, developing a low-refractive-index material with proper electrical properties (alignment of energy levels and carrier mobility) for the buffer layer is becoming an important agenda.
In this study, we propose a very low-refractive index layer based on a conducting polymer with perfluorinated ionomer, which can effectively mitigate SPP mode generated along the dielectric/ metal interfaces. In addition, by adopting an anisotropic material for transport layer having a lower extra-ordinary refractive (ne) index, the SPP loss associated with semitransparent metal layers is also suppressed effectively. Based on this combinatory method, we demonstrate a highly efficient top-emitting OLED by suppressing SPP loss from metal electrodes. The theoretical predictions are verified in an experiment that compares OLEDs with and without the proposed low-index buffer layer and anisotropic transport layer.