Organic light-emitting diodes (OLEDs) have become one of the most popular displays on the markets. They have advanges such as wide viewing angle, fast response time, and the potential to be transparent and flexible. However, their low out-coupling efficiency of ~20% due to structural limitation is their main draw-back that needs to be overcome.
Indium tin oxide (ITO), a transparent conducting oxide (TCO) that is generally used as a transparent electrode, has a low electrical resistivity and high optical transparency. However, the brittle nature of ITO makes it susceptible to cracking when bent. This is a critical issue for ITO in flexible OLED (FOLED) applications, and many studies have attempted to develop an alternative electrode to replace ITO.
In this study, we fabricated a metal thin film electrode with a nanometer size hole pattern on a poly-ethylene terephthalate (PET) substrate. The goal for this electrode was to not only acheive a high optical transparency and electrical resistivity, lower than that of ITO, but also to retain flexibility for FOLED applications. In addition, we aimed to enhance the low out-coupling efficiency of an OLED when the electrode is applied to the device.
By using the Langmuir-Blodgett (LB) method, a colloidal lithography mask with nanometer size polystyrene (PS) beads was made. Through the mask, a silver (Ag) thin film electrode with a quasi-periodic hexagonal hole pattern was fabricated. This nanometer size hole pattern is effective when used to extract waveguide modes and surface plasmon (SP) polaritons in an OLED device. Through this pattern, we enhanced the optical transparency of the metal thin film electrode and out-coupling efficiency when applied to the device.
Various parameters, such as metal film thickness, hole periodicity, aperture ratio, and index matching layer, were optimized to achieve low electrical resistivity and high optical transparency. During optimization, a bending test was carried out for every sample, and it was proven that the proposed metallic nano-hole transparent electrode can retain flexibility at 2.5% tensile strain, which usual ITO cannot bear.
Green fluorescence FOLEDs with ITO and the metallic nano-hole transparent electrode as an anode, respectively, were fabricated to study the effect of the electrode on a FOLED. The efficiency and EL intensity at the luminous wavelength with the metallic nano-hole transparent electrode increased compare to the OLED with ITO anode due to SP-mode out-coupling. It was also proved that the OLED with the metallic nano-hole transparent electrode could sustain the tensile strain that the OLED with ITO could not bear.
유기 발광 소자는 디스플레이 시장에서 각광받는 디스플레이 중 하나로써, 시야각이 넓고, 빠른 반응 속도와 투명, 플렉시블화의 잠재성을 장점으로 가지고 있다. 하지만 구조적 한계로 인한 20%에 못 미치는 저조한 광 추출 효율 문제는 OLED가 극복해야할 가장 큰 문제점 중 하나이다.
ITO는 투명 전도 산화막의 일종으로써 OLED의 투명 양극으로써 널리 사용되는 물질이다. ITO는 광학적 특성이 매우 우수하나, 세라믹한 특성에 의하여 구부림 시 표면에 크랙이 생기게 된다.이는 OLED로 미래의 디스플레이로써 주목받고 있는 투명 플렉시블 디스플레이를 개발하는데에 있어서 치명적인 문제이며, 이러한 ITO의 문제점들을 해결하기 위한 여러 대체 전극 연구들이 진행되고 있다.
본 논문에서는 PET기판에 나노 미터 단위의 구멍 구조를 가진 금속 박막을 전극으로써 개발하였다. 이를 통하여, 투과도와 낮은 면저항을 확보하는 동시에 플렉시블 유기 발광 소자에 적합한 유연성을 확보하고자 하였다. 또한 본 전극을 소자에 적용하여, 소자의 낮은 광 추출 효율 문제도 해결하고자 하였다.
