Enhanced light extraction efficiency of OLEDs using various light extraction structures

Abstract

Organic light-emitting diode (OLED) is promised lighting sources as lightings and display. In order for OLEDs to be utilized in the industry, the light extraction efficiency of OLEDs needs to be improved further. The light extraction structure for the display requires high light extraction efficiency improvement, low spectral change depending on the viewing angle, low pixel blur and high contrast ratio. In addition, fabrication process that does not damage the thin top electrode layer is required for top-emitting OLED (TEOLED). The light extraction structure for the lighting requires high light extraction efficiency improvement, low spectral change depending on the viewing angle, uniform emission pattern and high color quality. Bottom-emission OLED (BEOLED) are selected for lighting so, various light extraction method can be applied. High power efficiency and color quality are required to compete with other light sources, including LEDs. To enhance extraction efficiency, various light extraction methods have been reported such as, microlens array, photonic crystal, scattering film, corrugated device etc. Among various methods, fabrication of organic micro cone array by vacuum deposition has potential as light extraction structure for display because of damage-less fabrication method, high enhancement ratio and spectral stability. But, size of the organic structure needs to be reduced in order to meet the actual display manufacturing time. In addition, when the size of structure goes to nano, precise optical simulation is needed for calculating optimized structure. As light extraction structure for lighting vacuum nano hole array (VaNHA) has potential with electrical stability due to flat surface, high extraction efficiency in comibation with half-spherical lens. Green OLED with hexagonal VaNHA and half spherical lens was demonstrated to extract air, substrate and waveguide mode but, for uniform emission pattern, hexagonal pattern cant be applied in lighting. This thesis concerns two research topics: (1) extraction efficiency of TEOLED with organic nano lens array (NLA) for display, and (2) extraction efficiency and color quality of OLED with VaNHA for lightings. Our analysis shows that each method could reach the proper light extraction method for display and lightings. The paper consists of two parts as display and lighting. In Chapter 1, efficiency of OLED, the optical loss path, the light extraction methods, and finite difference time domain (FDTD) method are briefly reviewed. In Chapter 2, we first explain the process of calculating the optical property distribution of a top-emitting OLED with light extraction structure by FDTD method. We will discuss the computational complexity of the finite difference method, and compare it with the calculated value of the classical electromagnetic dipole model for the verification of the above calculation results. Next, the calculation results of the light extraction efficiency of the TEOLED with the hexagonal NLA are described. In Chapter 3, we discuss the TEOLEDs with organic nano lens array (NLA). Oragnic NLA can be fabricatied without damage to OLED, by organic vapor phase deposition (OVPD) method. Acheivable extraction efficiency of NLA was calculated by FDTD modelling. With simulation result, optimal distribution for extraction is suggested and with high refractive index material and height of NLA, higher extraction efficiency can be achieved than efficiency with organic material (n=1.8). TEOLED with NLA showed enhanced efficiency of 50% with low pixel blur and low spectral change with viewing angle. Blue emitting TEOLED with NLA also showed enhancement ratio of 1.5 with low spectral change with viewing angle. In Chapter 4, we discuss the white OLED with randomly distributed VaNHA with unprecedently high efficiency of EQE 78% and power efficiency of 164 lm/W. Random pattern consists of 3 kinds of pillars with different radius of 120, 150 190 nm. With combination of half-spherical lens, almost air, substrate and waveguide mode of light can be extracted. By mode analysis calculation, extractable light can be increased with thick distance from metal electrode and emitting dipole. By random distribution, white OLED with VaNHA showed uniform emission pattern and spectral stability with viewing angle. FDTD calculations were performed to investigate the origin of high light extraction efficiency enhancement of Si3N4 and air extraction structures. The larger the difference between the refractive index of the dielectric layer and the nano hole array, the larger the light extraction efficiency. In Chapter 5, we discuss the theoretically achievable color quality of white tandem devices with various light extraction structure. First, color quality according to the proportion of the emitting dyes was calculated which is ideal value whole photolumeniscent (PL) spectrum is extracted. In consideration of micro cavity, achievable color quality of device without light extraction layer, device with half spherical lens, device with high refractive index and lens, device with high refractive index substrate and lens and elongated distance with dipole and metal electrode. Simulation result is compared with experiment result and matched well. By based this model, highly efficient tandem OLED with high color temperature ( > 5,000 K) can be fabricated. The WOLEDs showed maximum EQEs of 45.6 and 79.4% and maximum LE of 48.9 and 94.0 lm/W for WOLED without and with lens, respectively.