Study on high extraction efficiency polymer light-emitting diodes using titania inverse opal monolayer

Abstract

Light-emitting diodes (LEDs) such as Organic LEDs(OLEDs) and Polymer LEDs(PLEDs) have attracted wide attention due to their promising advantages as displays and interior lightings such as high efficiency, high contrast ratio, long life time and low power consumption. Recently, much effort has been devoted to increase the external quantum efficiency of the devices because in conventional device structure, below 20% of the generated light can only escape from the device as useful radiation and the other generated light is trapped within and waveguided out in the device owing to the multiply layered structure with difference refractive indices. In order to solve the waveguide problem, several strategies have been reported such as the use of modified substrates, microlenses, silica microspheres, silica aerogel and photonic crystals. Among them, photonic crystals having periodic dielectric structure have considered as one of the best candidate for the efficient light extraction. Although several structures having photonic crystal patterns have been proposed, almost reported structures have been fabricated by using complicated and expensive way and they do not have enough refractive index contrast to diffract the waveguided light effectively. In this thesis, novel structure of inverse opal monolayer is proposed as a photonic crystal for high extraction efficiency PLEDs. In order to apply the structure to the device, we modified the conventional inverse opal into the inverse opal monolayer which has cylindrical void space resembling slab structure and covered with thin flat film for fabricating a device on the structure. It can be fabricated by an easy way without any special equipment and easy to control the periodic size by varying the size of opal template nanoparticles. Furthermore, it creates strong diffraction due to the high refractive index contrast between the void air space and the titania wall. By introducing the structure into the PLED, the enhancement o...