This study concentrates on solar light absorption power in a silicon solar cell using a double diffraction triangular
nano-grating. The first grating is located on top of the solar cell and the second grating is located on bottom of the
solar cell above a reflective metallic substrate of Ar (Si3 N4 ) (Argon gas mixed with Silicon Nitride). We simulate the
solar cell behavior over varying grating parameters as it absorbs sunlight and compare the average power output
absorbed at the center of the solar cell. Each case simulates a period (At ) that varies from 100nm to 800nm in 100nm
interval for the top lattice, while maintaining the bottom lattice at a constant period (Ab ). We repeat this procedure
for the bottom lattice, changing the lattice period from 100nm to 800nm in 100nm interval in order to find the
optimized case. We also consider solar spectrum irradiation under wavelengths ranging from 300nm to 1100nm in
50nm intervals. The total power absorption improvement is about 170% compared to the non-grating case, occurring
in the weighted solar cell simulation with top grating period greater than 300nm and bottom grating period of
500nm.
In this paper, we use a Finite-Difference Time-Domain GaN LED model to study constant wave (CW)
average power of extracted light. The structure simulated comprises of a 200nm-thick p-GaN substrate,
50nm-thick MQW, 400nm-thick n-GaN substrate, and a 200nm n-GaN two-dimensional Photonic
Crystal(2PhC) grating. We focus on optimizing three design parameters: grating period (A), grating height
(d), and fill factor (FF). In the primary set of simulations, we fix the fill factor at 50% and simulate ten
different grating periods (100 to 1000nm in steps of 100nm) and four different grating heights (50 to
200nm in steps of 50nm), and calculate the average power output of the device. The results from these
simulations show that for both conical and cylindrical gratings, the maxmium light extraction improvement
occurs when A =100nm. In the second set of simulations, we maintain a constant grating period A = 100nm
and sweep the fill factor from 25 to 75%. The results of these simulations show that the fill factor affects
clyindrical and conical gratings differently. As a whole, we see that the nano-structure grating mostly
depends on period, but also depends on height and fill factor. The grating structure improves light
extraction in some cases, but not all.
We study solar-cell designs using nano-grating on both top (transmission) and bottom (reflection) of the
solar cell. First, we perform simulations based on rigorous coupled wave analysis (RCWA) to evaluate
the diffraction top gratins. In RCWA method, we calculate up to 20 harmonics, and sweep the launch
angle of incident light from 0 to 90 degree. The incident light varies from100nm to 1200nm wavelength.
Triangular grating can achieve higher light absorption compared to the rectangular grating. The best top
grating is around 200nm grating period, 100nm grating height, and 50% filling factor, which responses to
37% improvement for triangular grating and 23% for rectangular grating compared to non-grating case.
Then, we use Finite-Difference Time-Domain (FDTD) to simulate transmission/reflection double grating
cases. We simulated triangular-triangular (top-bottom) grating cases and triangular-rectangular (top-bottom)
grating case. We realize solar cell efficiency improvement about 42.4%. For the triangular-triangular
(top-bottom) grating case, the 20% efficiency improvement is achieved. Finally, we present
weighted-light simulation for the double grating for the first time and show the best grating can achieve
104% light improvement, which is quite different from traditional non-weighted calculation.
We study the top transmission grating's improvement on GaN LED light extraction efficiency. We use the finite
difference time domain (FDTD) method, a computational electromagnetic solution to Maxwell's equations, to measure
light extraction efficiency improvements of the various grating structures. Also, since FDTD can freely define
materials for any layer or shape, we choose three particular materials to represent our transmission grating: 1) non-lossy
p-GaN, 2) lossy indium tin oxide (ITO), and 3) non-lossy ITO (α=0). We define a regular spacing between unit
cells in a crystal lattice arrangement by employing the following three parameters: grating cell period (Α), grating cell
height (d), and grating cell width (w). The conical grating model and the cylindrical grating model are studied. We
also presented in the paper directly comparison with reflection grating results. Both studies show that the top grating
has better performance, improving light extraction efficiency by 165%, compared to that of the bottom reflection
grating (112%), and top-bottom grating (42%). We also find that when grating cells closely pack together, a
transmission grating maximizes light extraction efficiency. This points our research towards a more closely packed
structure, such as a 3-fold symmetric photonic crystal structure with triangular symmetry and also smaller feature sizes
in the nano-scale, such as the wavelength of light at 460 nm, half-wavelengths, quarter wavelengths, etc.
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