Embedded Nanostructures for High Efficiency Organic Light Emitting Devices

Abstract

Organic light emitting diodes (OLEDs) have been successfully applied to various mobile electronics because the display, which utilizes a spontaneous emission of light from organic molecules by nature, is a very energy efficient way to produce vibrant images. However, only around 20-30% of the light generated by a typical bottom emission OLED is extracted into the air because the light intensity is decreased by a factor of 1/2n2 as the light passes through each layers, where n is the refractive index (RI). The light extraction efficiency, which is one of the most important factors, can be enhanced by applying photonic implements to each layer. In this thesis, a nano-hole array (NHA) embedded OLEDs were proposed for light extraction. The NHA structure is proposed as a strategy for escaping the loss mode wasted by the substrate, waveguided and surface plasmon polaritons. Various methods have been proposed to convert the loss modes into the radiation mode, e.g., polymer/Si3N4 nano hole array embedded OLEDs (chapter 3), vacuum/Si3N4 nano hole array embedded OLEDs (chapter 4 and chapter 7), corrugated OLEDs (chapter 5) and their combinations (chapter 6). The enhanced performance of the nanostructure embedded OLEDs is evaluated by using in terms of photoluminescence (PL) and electroluminescence (EL), and the finite difference time domain (FDTD) simulation was carried out to analyze the optical performance of the NHA structure for extraction of the emission. We explored the effect of the NHA structure on the extraction improvement converted from waveguide mode by measuring EL intensities of the devices with hemisphere lens. The reduction of power dissipation to waveguide and surface plasmon modes by applying the NHA structure leads to strongly enhance the out-coupling efficiency of OLEDs. Especially the robust reverse transfer process was newly developed for confining the nano hole array in the vacuum state. The periodic nano hole array is inserted in the vacuum state to maximize the refractive index contrast of the PhC slab for a given background high-refractive index material. In addition, the transfer process employed in VNHA fabrication yielded extremely low surface roughness, and thus outstanding electrical characteristics. We obtained an extremely high efficiency OLEDs with over 50% of external quantum efficiency (EQE) and low roll-off by inserting vacuum nano hole array (VNHA) into phosphorescent OLEDs (PhOLEDs), and it is the highest EQE for bottom-emitting OLEDs.