ABSTRACT
As discussed throughout this book, the combination of a highly reflective cathode on one end together with a semitransparent anode on the other side forms a submicron-scale optical microcavity where certain wavelengths or modes of light are enhanced and others are trapped inside depending on the total device thickness. Such a microcavity effect occurs mainly along the longitudinal, forward direction (normal to organic light-emitting diode [OLED]- emitting surface), leading to a directional-or angular-dependent emission profile. This is further exacerbated by the fact that the refractive index of organic materials and standard indium tin oxide (ITO) anode (n ≈ 1.7-1.9) are higher than that of standard glass and plastic substrates (n ≈ 1.5), thereby inducing even more severe total internal reflections, as shown in Fig. 9.1. These trapped modes are considered organic waveguide modes which constitute one of three major optical loss channels. Another main optical loss channel is the substrate modes which arise from refractive index mismatch between the glass/plastic substrate (n ≈ 1.5) and the air (n ≈ 1), resulting in considerable total internal reflections within the relatively thick substrate. The third major optical loss is attributed to surface plasmon modes taking place on the highly reflective cathode. This is induced by the close proximity between the emissive
layer (EML) of the OLED and the reflective metal surface, separated only by the electron transport layer (ETL). These emissive modes are easily coupled to the surface plasmon modes, thereby forming evanescent waves that dissipate as heat. In the following, a number of the recently reported techniques to address these three types of optical out-coupling losses are presented.
