Study of the Light Coupling Efficiency of OLEDs Using a Nanostructured Glass Substrate

We study theoretically the enhancement of the light extraction from an OLED (Organic Light-Emitting Diode) with nanoair-bubbles embedded inside a glass substrate. Due to such a nanostructure inside the substrate, the critical angle which limits the light extraction outside the substrate from the OLED is increased. The theoretical results show that the nanoair bubbles near by the substrate surface can improve the efficiency of the light extraction by 7%. Such a substrate may also be suitable for photovoltaic cells or display screens. 1. Introduction Over the last decade, organic materials have attracted much attention due to their huge potential of applications such as OLED (Organic Light-Emitting Diode) and organic photovoltaic cells. As promising materials, the enhancement of their efficiency has widely been studied [1–4]. Although the internal quantum efficiency of OLEDs can be achieved near 100% [5–8], the light emitted by the OLED is largely wasted because of the total internal reflection inside the substrate. The most common substrate is glass substrate coated with an ITO (Indium Tin Oxide) layer. The ITO layer is used as a transparent anode electrode, but its refractive index is higher than that of organic materials. Therefore, much of light is trapped in the layer due to the internal reflection. Moreover, the light must again cross the glass-air boundary. This produces a total reflection of the light emitted from the organic materials beyond the critical angle. Therefore only small amount of light is coupled out of the ITO layer and glass substrate, and the rest is trapped inside the ITO layer and substrate as wave-guided modes. It has been reported that the external coupling efficiency of light from conventional OLEDs is only 20% [6–8]. A number of studies have been done to improve the coupling efficiency from OLEDs or photovoltaic cells such as addition of a diffusive layer on the substrate [9], nanomesh electrodes [10], nanoparticles [11], and microlens array [12, 13]. Corrugated structure and a quasiperiodic buckling structure were also proposed with an excellent outcoupling efficiency [14, 15]. A low-index grid embedded in the organic layer and a monolayer of -microspheres are also suggested for this purpose [7, 16]. Recently, patterning nanostructures on the substrate surface has been developed in order to increase the light extraction efficiency in [17]. This method may also be applied to OLEDs on glass substrate. However, as they are exposed to air directly, such structures are vulnerable to external shocks. So the structure can be