Combination of Scattering Layers and Color Conversion Layers for White Organic Light-emitting Diodes

• Main Authors: LIU, TZU-HAO, 劉子豪
• Other Authors: YANG, SU-HUA
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/uw67vk

碩士 === 國立高雄應用科技大學 === 電子工程系 === 106 === This study focused on the light extraction and color conversion layer preparations for a white organic light-emitting diode (OLED). Based on this motivation, a blue OLED combined with a color conversion layer was fabricated to achieve white light emission. The blue OLED had a simple structure, which showed a high luminance of 10,800 cd/m2 with peak wavelength at 465 nm. Furthermore, light extraction was implemented with a scattering layer along with ground substrate. This approach effectively increase light extraction efficiency and improve luminance efficiency of the device. In the first section, the negative transparent photoresist and polyacrylic acid were used as the host materials of the color conversion layer. Different concentrations of phosphor were doped in the host and, the color conversion property of the device was analysized. When the optimal concentration of phosphor was obtained, a maximum luminescence was reached; the CIE coordinates of the white OLED was (0.29, 0.27) and its color rendering index was 80.9. However, when a higher phosphor concentration was increased, the efficiency of the OLED was decreased. It is observed that color conversion ability of the negative transparent photoresist is better than that of the polyacrylic acid. Accordingly, the negative transparent photoresist was selected as the host material for the following experiments. In the second section, a ground glass substrate was used as the scattering layer for light extraction. In this condition, the light confined in substrate was scattered by the roughness surface and then extracted out of the device. The effect of substrate grinding time on the electroluminescence (EL) properties of the device was studied. From measurement, it was found that the luminance was increased to 11,700 cd/m2 attributing to the light extraction when a grinding time of 120 s was used. The performanance of the device was improved accordingly. Nevertheless, a longer grinding time caused a deep-rough surface and resulted in less light extraction. In the third section, the negative transparent photoresist and polyacrylic acid were used as the host materials of the scattering layer. Different concentrations of silicon dioxide (SiO2) and titanium dioxide (TiO2) nanoparticles were doped in the host, and the light scattering property of the scattering layer was analysized. When the concentration of nanoparticles reached the optimal value, a scattering layer with high scattering ability and OLED with high luminance were achieved. If the nanoparticles concentration was increased further, the efficiency of OLED was decreased. For the light extraction configuration used in this study, SiO2 is a more suitable substance than the TiO2 based on the consideration of refractive index. Hence, SiO2 was selected as the nanoparticles for the following experiments. Meanwhile, the penetration rate of negative transparent photoresist is better than that of the polyacrylic acid, thus, the negative transparent photoresist was selected as the host material and used in the following experiments. In this section of light extraction experiments, the luminance of the OLED was increased to 14,850 cd/m2, and the corresponding efficiencies were enhanced noticeably. In the last section, the optimal values of parameters obtained from the above sections were combinational used to fabricate the OLEDs, and the EL characteristics of the devices were analyzed and discussed. It was found that the luminance was decreased to 13,450 cd/m2 and other efficiencies were slightly declined though, the devices maintained working with high efficiency. A white OLED (WOLED) with CIE coordinates of (0.27, 0.25) and color rendering index of 77.8 was obtained. A high efficiency and high color rendering index of WOLED was achieved in this study.