Using nanoimprint techniques and micro-lens structures to improve the efficiency of white-light solid-state lightening devices
Date Issued
2010
Date
2010
Author(s)
Lee, Yang-Chun
Abstract
Improving the efficiency of the light-emitting diodes (LED) is one of the important issues for the incoming energy shortage. In this thesis, we studied the factors that influence the efficiency of phosphor-converted white-light solid-state lightening device. There were two things we need to consider : first, the extraction efficiency of the pumping blue light-emitting diode and second, the conversion efficiency of phosphor.
In the first part of this thesis, the extraction efficiency improvement of the blue LED with surface micro-lens arrays was analyzed by Finite-Difference Time Domain (FDTD) method. The extraction behaviors of point light sources at different relative position were also studied by computing the power flux that leaves the GaN and air interface.We found the optimal structure is the non-closed packed micro-lens array because the extraction will be decreased dramatically by changing the light emission direction to the oblique direction when the point source is at the position below the tangent between two closed-packed lens. We also found that the extraction efficiency of micro-lens array with optimal period and radius of curvature can be enhanced more than 4 times than planar blue light-emitting diodes.
In the second part of this thesis, the nanoimprint techniques and optical thin films were applied in the phosphor encapsulation to improve the conversion efficiency of the phosphor-converted LED. The extraction efficiency of various periodic nano-scale structures were first analyzed by Rigurous Coupled-Wave Analysis (RCWA) method. We found the best structure is inverse cone with optimal period and depth, which can increase the transmittion due to elimination of reflection and diffractive transmittion when the angle of incidence is more than critical angle. We successfully fabricated two kind of the inverse cone structure on the front surface of the phosphor encapsulation by nanoimprint techniques. The optical thin films with high transmittance at wavelength 450nm and high reflectance at wavelength 570nm were also deposited at the backside surface of the phosphor encapsulation to reduce the absorption loss when the light incident to blue-LED or the sidewall of device. Considering the conversion between blue light and yellow light, the best structure was the combination of the inverse cone with highest yellow light extraction efficiency and optical thin films. More details about the colour temperature and colour-rendering index of the phosphor-converted white-light LED will be studied in the future.
Subjects
Gallium Nitride blue light-emitting diodes
surface micro-lens structures
phosphor-converted white light-emitting diodes
nanoimprint techniques
light extraction efficiency
Type
thesis
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