Design and applications of organic light-emitting diodes based on top metal transparent electrodes

Abstract

Organic light-emitting diodes (OLEDs) are considered a promising candidate for next-generation displays that can lead to futuristic applications that are not readily available through conventional technologies ？ augmented reality, mutual interactive displays, and invisible displays seamlessly integrated with various objects, to name a few. Transparent OLEDs (TrOLEDs) and top-emitting OLEDs can be easily achieved by replacing their top, thick metal electrodes with a transparent one. A seemingly straightforward transparent electrode for such a purpose may be transparent conductive oxides (TCOs), such as indium tin oxide (ITO) or indium zinc oxide (IZO)

however, the sputtering deposition process used for TCOs can easily damage the underlying organic layers, making it difficult, though not impossible, to adopt them as a top electrode. For this reason, thin metallic films (e.g. Ag or Au) have particularly been popular because they can be deposited by thermal evaporation with little damage to the underlying organic layers. In the first part, strategies to achieve efficient TrOLEDs are presented. The emission zone position is carefully adjusted by monitoring the optical phase change upon reflection from the top electrode, which is significant when the thickness of the capping layer changes. With the proposed design strategy, external quantum efficiency and transmittance values as high as 15% and 80% are demonstrated simultaneously. The effect of surface plasmon polaritons (SPP) loss from thin metal electrodes is also taken into account to correctly describe the full scaling behavior of the efficiency of TrOLEDs over key optical design parameters. In the second part, a systematic methodology for suppressing the white angular dependence (WAD) in top-emitting OLEDs is presented. In particular, optical simulation is modified to include the effect of an encapsulation layer, which involves both optically thin and thick multilayers. When this encapsulation structure is also taken into account, the refractive index of the passivation capping layer is shown to plays a major role in WAD and efficiency control

with a high-index capping layer inserted in between the top-metal electrode and the encapsulation structure, we demonstrate top-emitting OLEDs in which both efficiency and color stability characteristics are simultaneously improved. The optically equivalent structure is then fabricated to prove the main concept of the proposed method.