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OLED의 다층 박막 플렉서블 투명 전극의 구조적 설계

OLED의 다층 박막 플렉서블 투명 전극의 구조적 설계
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Flexible optoelectronic devices have attracted great attention for realization of next generation of displays, solid state lighting and energy generating devices. In order to have competitive advantage over other devices, high efficiency, low power consumption and long life time is very important. In typical optoelectronic devices are composed by transparent electrodes, active semiconductor layer, and reflective electrodes. In order to realization of devices with high efficiency, it is important to develop the semiconductors having high internal quantum efficiency and transparent electrodes having high transmittance and excellent charge injection or extraction properties. In recent years, various semiconductors for active layers of optoelectronic devices have been developed having high internal quantum efficiency. Each semiconductor had characteristic emission or absorption wavelengths which are decided by its electronic band structure. Therefore, in order to maximize its efficiency, it is essential to pump or inject appropriate wavelength of light and inject or extract charge carriers efficiently according band structure of semiconductors. Transparent electrodes with sub-wavelength nano structures (grating or hole array) has been investigated for electrodes with wavelength tuning properties via control of periodicity and geometry of nano features. However, several problems limits the adoption to transparent electrodes including high cost for e-beam lithography, complex fabrication, rough surface and low transmittance (~70 %). In this work, we investigated simple fabrication method for flexible transparent electrodes with wavelength tuning and efficient charge injection properties using metal based multilayer electrodes and provided a universal design rule. The metal based multilayer structure consisted of tree components, metal conducting layer, charge injection dielectric layer, and wavelength tuning dielectric layer, was investigated and role of three components are systematically investigated. Firstly, we could observe that the chemical composition of transition metal oxides could be controlled by deposition rate during thermal evaporation, and mixed phase could induce the gap-states in band gap of metal oxides. The Gap states could act as a hole transport path, and hole injection properties of inner dielectric layer could be improved. Secondly, we also demonstrated that the peak wavelength of transmitted light of electrodes could be tuned by adjusting phase optical thickness via control the refractive index of outer dielectric layer. As the refractive indices of out dielectric layers increased from 1.8 to 2.5, the peak wavelength shifted from 470 nm to 610 nm. Finally, in order to enhanced light extraction in OLEDs with dielectric/metal/dielectric electrodes by utilizing a graded index layer with a nano-facet structure. Using a nano-facet structured MgO (n=1.73) layer and a ZrO2 (n=1.84) layer as a graded index layer, the luminance of OLED with dielectric/metal/dielectric electrodes was enhanced by 24% compared to that of device with ITO.
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