Chapter 5. Simulation
5.2 Structure in front side
Because the few enhancement of absorption in a-Si thin film solar cell with PhC structures in rear side. We try to another way to enhance the efficiency of a-Si thin film solar cell by PhC structures. Because of the light scattering only in long wavelength region by structure in rear side, thus we change the position of our structures to front side, just like figure 5.10. We predict the absorption can be enhanced not only in IR region but also in short wavelength region by this change. Next part we observe the effect of ITO PhC structure in front side, and solve its problem, finally optimize the structure to enhance a-Si thin film solar cell.
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Figure 5.10 (a) The simulation model with ITO/TiO2 PhC structure in front side. (b) Hole structure. (c) Rod structure.
5.2.1 Spectrum analysis
In this section, we change the location of ITO/a-Si PhC structure to front of active layer.
Before incident light enter the active layer, PhC structure can provide the light scattering effect to enhance optical path length. The result is shown in figure 5.11.
Light
ITO 80nm
ITO/TiO
2PhC 250nm Al 500nm
Glass a-Si 430nm
(b) (c)
(a)
TiO
2(n=2.49)
ITO
(n=2.01)
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Figure 5.11 The absorption of ITO/a-Si PhC structure in front side of a-Si thin film solar cell, parameters: period 600nm, thickness 250nm, ITO ratio 50%.
From figure 5.11, we can know when ITO/a-Si PhC structure in front side and the absorption by a-Si couldn’t be ignored. Therefore, chosen a-Si between ITO PhC structure in front side is not good. Thus, we find TiO2 (n=2.49) replace a-Si to design ITO/TiO2 PhC structure in front side (shown in figure 5.9). The effect is shown in figure 5.12.
0.3 0.4 0.5 0.6 0.7 0.8 solar cell, parameters: period 600nm, thickness 250nm, ITO ratio 50%.
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From figure 5.12, we can confirm the effect by ITO/TiO2 PhC structure in a-Si thin film solar cells. Both holes and rods structure show significant improvement comparing to reference structure. For incident light with wavelength less than 600 nm, ITO/TiO2 PhC structure plays the role of anti-reflective layer, increasing incident light that goes into active layer. Incident light of wavelength shorter than 350nm is unavoidably absorbed by TiO2, yet the anti-reflective effect increases more photon flux into a-Si active layer. In the long wavelength region where wavelength longer than 600 nm, light scattering by the PhC structures increase the optical path length of incident light, resulting in improvement of absorption.
5.2.2 Optimized Structure
After we confirm the absorption enhancement from ITO/TiO2 PhC structures, then we want to know the best parameters, period, thickness, ITO ratio, in hole/rod structures. In optimization, we observe the results in different thickness. In this way, we design the structure in front side, we consider not only light scattering effect, but also compare anti-reflective effect. Thus we calculated the integrated current density by equation (5-1) and average reflection by the following equation (5-2), and compared the results to find the best thickness of our structures at front.
(5-2)
45 thickness of PhC structure with period 600nm, ITO ratio 50%.
Result in figure 5.13, we find that both of the highest current densities and the lowest average reflections located at a thickness of PhC structure of 200nm. It can be explained by effective medium theory. The two different materials with two different refractive indices in periodic array which are close to optical wavelength provide an equivalent refractive index in PhC structure layer. Not only the light scattering effect is achieved, but also an anti-reflective
(a)
(b)
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layer is obtained. Figure 5.13(b) show that thickness in 200nm is optimum value of ITO/TiO2
PhC structures for anti-reflective layer. After we found that the thickness in 200nm is the optimum value, the integrated current density is analyzed with different period and ITO ratio of PhC structures in front side.
Figure 5.14 Integrated current density in different period and ITO ratio of (a) hole PhC structure and (b) rod PhC structure in front side. The thickness of PhC structures are 200nm both.
Hole
Rod
(a)
(b)
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Figure 5.15 The electric field by finite-difference time-domain (FDTD) method in ITO/TiO2 PhC structures (a) hole, (b) rod.
When we find the results in different period and ITO ratio are shown in figure 5.14, hole’s result has higher integrated current densities than rod’s result generally. In figure 5.15, we can explain why of this trend. In simulation result by finite-difference time-domain (FDTD) method in two different PhC structures, the ITO hole structure can trap the energy in higher refractive index material, TiO2. But in the ITO rod structure, TiO2 is spread in PhC layer that not easy to concentrate the energy to transfer to active layer. That the reason we can assertion hole structure is better than rod structure in our design for a-Si thin film solar cells.
To find the result from figure 5.14, the optimum parameters of ITO/TiO2 PhC structures is shown in table 5.2.
Table 5.2 The optimum result of ITO/TiO2 PhC structure in front side.
Period (nm) Thickness
On the other hand, we simulate the electric field result in a-Si back ground with optimum ITO/TiO2 structures we found as substrates in figure 5.16. Resonance in a-Si active layer by
(a) (b)
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material has higher refractive index, TiO2, can be observed. In hole structure, we can see more strong electric field enhanced by TiO2 than rod structure. The reason can be explained in figure 5.15, Hole structure concentrate the energy in ITO hole that filled with TiO2. So that the optimum parameters of ITO ratio is ITO higher than TiO2 in hole structure, but the ratio is ITO less than TiO2 in rod structure. Not only in figure 5.15, has figure 5.16 also showed me the enhancement is hole structure is better than rod structure.
Figure 5.16 The electric field by finite-difference time-domain (FDTD) method in a-Si back ground with optimum ITO/TiO2 PhC structures (a) hole, (b) rod.
Finally, we found the parameters of ITO/TiO2 PhC structures in front side with the highest absorption enhancement about 19% and 17%. In other hand, we simulate the effect of optimum structures in other optical properties in figure 5.14.
(a) (b)
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Figure 5.16 The optical properties versus different incident angle of light, the reflection of (a) reference, (c) hole structure, (e) rod structure, and the transmission of (b) reference, (d) hole structure, (f) rod structure.
The simulated reflection and transmission result are shown in figure 5.16. ITO/TiO2 PhC structure provide higher and wider anti-reflective and light scattering effect than reference in large incident angle. They exhibited characteristic of omnidirectional absorption.
(a) (b)
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