With the development of lighting technology, high-power white LEDs will become the future mainstream technology. At present, there are still many problems in optical stability, reliability, heat dissipation. This paper focuses on the optical properties of vertical high-power white LEDs. The measurement results of the luminous flux output, correlated color temperature and color coordinates of the white and blue LEDs after 1100 hours of aging life testing under the drive current of 350 mA show us that the color coordinates of the white LED changed from (0.3362,0.3700) to (0.3359,0.3698) and shifts to the blue color coordinates direction. The luminous flux output of the white LED is declined from 95.002lm to 93.046lm. The color temperature declines from 5403k to 5383k. After the rapid temperature changes, the aging speed of white LED is faster than the blue LED’s. Analyzing the results, it can be concluded that phosphor aging rate greatly affects the optical stability of the device, how to improve the stability of phosphor is the key to improve optical stability for the vertical high-power white LED.
The III-V nitride material such as InGaN has many favorable physical properties including a wide direct band-gap (0.7- 3.4eV), high absorption coefficients (105 cm-1), and high radiation resistance. As such, InGaN has been chosen as an excellent material for full-solar-spectrum photovoltaic applications utilizing its wide and tunable band-gap. The refractive index of GaN is about 2.5 in the full-solar-spectrum. According to the Fresnel formula, there is a high reflection of ~18.4% as the sun light entering GaN. Anti-reflection films could be used on InGaN/GaN solar cell to decrease the reflection loss. The photonic crystal structure is a kind of anti-reflection based on the effective medium theory without any limitations, for example the mismatched thermal expansion coefficient. In this paper, we reported our research work on the design and fabrication of photonic crystal structure on the surface of GaN. FDTD Solutions is used to simulate the reflectivity on the surface of GaN with hexagonal close-packed pillar which has different period-a, diameter-d and height-h. When the parameters a is 500nm, d is 300nm, the reflectivity reached the lowest point of 4.18%. The self-assembly method was used to fabricate the photonic crystal structure on the GaN surface and the fabrication process was also researched. The photonic crystal structures on the surface of p-GaN were obtained and their characteristics of the antireflective film will be discussed in detail.
In this paper, we used FDTD Solutions software to simulate the reflectance of two-dimensional photonic crystal by changing duty ratio from 40% to 70%. The different models, which are referred by Stavenga, Southwell and Grann et al, were used for obtaining the equivalent refractive index. And then the simulations of double layer dielectric structure designed in FDTD Solutions was used for getting the reflectance curves by importing the data of the equivalent refractive index. After comparing these curves we could obtain that the accuracy of different models are related to the ratio of the period（P） and wavelength（λ). Thus the equivalent refractive theory has its most applicable models of micro structure in different period scales. The imprecise quantitative explaining of micro structures’ antireflection by using equivalent refractive theory models was solved in this paper, which has important significance in many areas.
The III-N material system (including alloys of InN, AlN, and GaN) has several characteristics which give it key advantages over the existing solar cell materials, for example, the high absorption coefficient and high carrier mobility, more important, the wide range of band gap energies which spans nearly the entire solar spectrum. In this paper, we fabricated a multiple quantum well (MQW) InGaN/GaN solar cell. The photovoltaic characteristics of the device was demonstrated that, the short circuit current density (JSC) is about 0.43mA/cm2 , the open circuit voltage is about 2.2 eV, and the fill factor is about 81%. But the peak external quantum efficiency (EQE) is not very high, only 30%. And the conversion efficiency is about 0.83%, so more work for device design should be done in the future.