Improved performance of flip-chip white light emitting diodes with conformal phosphor layer prepared by model conformal coating technique

Autor: Hsin-Yi Chiang, 江欣怡
Rok vydání: 2015
Druh dokumentu: 學位論文 ; thesis
Popis: 103
In this study, the white light emitting diodes (LEDs) were fabricated by combining the flip-chip device structure and conformal phosphor layer. To improve the angular correlated color temperature (CCT) uniformity of white LEDs, the model conformal coating (MCC) technique has been proposed to prepare the phosphor layer on the flip-chip LED. Firstly, by designing the model shapes, four phosphor layers with various geometries can be formed. These four geometries of the MCC-prepared phosphor layers consisted of conventional cubic, convex-surface cubic, conventional cylinder, and convex-surface cylinder shapes. Through the design of the geometry of the MCC-prepared phosphor layer to the conventional cubic shape, the fabricated white LED would have the lowest CCT deviation than those of the other devices. This is attributed to the relatively low difference of optical paths between the lights radiated from normal and lateral directions of the LED chip to the conventional cubic phosphor layer. Therefore, this white LED possesses higher CCT uniformity. Subsequently, the conventional pulsed spray coating (PSC) and the MCC methods were both used for the preparation of phosphor layers on the LED chips. These two devices possessed similar CCT deviation. Obviously, these two white LEDs both has good CCT uniformity at a lower CCT (5100 K). On the other hand, when the CCT was near 5800 K, the CCT deviations of these two devices were measured to be 460 and 800 K. From the cross-sectional image of white LED by scanning electron microscopy, it can be found that the MCC-prepared phosphor layer has higher thickness uniformity than that of PSC-prepared phosphor layer. This is the reason why the white LED contained with the MCC-prepared phosphor layer possesses better CCT uniformity. In other words, the smoother surface of the MCC-prepared phosphor layer can cause more uniform phosphor conversion efficiency at various light extraction angles, leading to better CCT uniformity of white LED. Especially at higher CCT value (5800 K), i.e. the lower phosphor concentration in the phosphor layer, the surface roughness would apparently affect the CCT deviation. In addition to the CCT characteristics, the light emission and thermal propertiesof the white LEDs were also investigated in this research. In this part, the phosphor layers were formed by the MCC, PSC, and dispensing methods. According to the measured results, the white LED with the dispensing-prepared phosphor layer has higher light output power than those with the MCC- and the PSC-prepared phosphor layers. Because the phosphor powders of MCC- and PSC-prepared phosphor layers were both conformally coated on LED chips, the back-scattered lights generated from the device were easily absorbed by the chip, leading to the increment in the junction temperature and the reduction in the light emission efficiency. On the other hand, as the dispensing-prepared phosphor layer was formed on the LED chip, the phosphor powders were uniformly distributed in the glue. In this case, the back-scattered lights sourced from the device were less absorbed by the chip, i.e., the heat created from the phosphor powders would induce less effect on the LED chip, resulting in higher light emission efficiency in this device. Even though the white LED with the dispensing-prepared phosphor layer has better light emission characteristic than those of the other two samples, its inferior CCT uniformity would lower the possibility of optoelectronic applications. Moreover, the effect of encapsulated material for packaging process on the device characteristics of white LEDs was also analyzed. It is well known that the silicone is a common material for LED package. Nevertheless, there is still room for improvement in the heat dissipation of the device packaged with silicone. Thus, in this work, a novel inorganic encapsulated material has been presented for the LED package. Apparently, the inorganic glue indeed has better heat dissipation, inducing higher light emission in the device. Finally, the 2 × 2 white LED array, which fabricated by adhering the LED chips to the phosphor plate, was also presented in this research. In the white LED array, the distance between LED chips was varied from 1 to 2.5 mm, and the effect of this distance on the light emission characteristic and the CCT uniformity of the white LED array was discussed. It can be found that the optimum distance between LED chips is 1.5 mm. When the distance between LED chips is less than 1.5 mm, this short distance could cause the interactive influence of heats generated from various chips, leading to the increase in the device temperature and the decrease in the light emission efficiency of the white LED array. With increasing the distance between LED chips above 1.5 mm, the white LED array is similar to the combination of four light sources rather than single light source. In this white LED array, the light emission efficiency and the CCT uniformity both become worse.
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