In summary, we design photonic crystal (PhC) structures in a-Si thin film solar cells to increase optical path length and improve the absorption of transmitted light. In this study, we design PhC structures for a-Si thin film solar cells. Due to the periodic change in refractive index with period ranging dimension close to optical wavelength, light scattering can be achieved. Then we use the three-dimensional Rigorous Coupled-Wave Analysis (RCWA) method to simulate the effect brought by PhC structures in a-Si thin film solar cells.
At first, we design ITO/a-Si PhC structures and achieve optimization. The results for hole structure are 300nm in period, 150nm in thickness, 55% in ITO ratio, and that for rod structure are 300nm in period, 150nm in thickness, 65% in ITO ratio. We found ITO/a-Si PhC structure can change the absorption in long wavelength and enhance efficiency in different parameters. Although the enhancements are only 3.26% and 2.89% comparing to the reference, yet we discover that we can reduce the thickness of active layer yet still achieve the same device performance of the reference cell to achieve material saving about 35%.
Next, we change the position of PhC structure from rear side of the cells to front side and overcome the problem that enhancement only occurs in long wavelength. But when we use ITO/a-Si PhC design in front side, we couldn’t ignore the absorption by a-Si in PhC structure, thus we use another material, TiO2, to replace a-Si then complete our design and optimize.
As a result, we found the optimum parameters of ITO/TiO2 PhC structures for hole are 400nm in period, 200nm in thickness, 45% in ITO ratio, and that for rod are 450nm in period, 200nm in thickness, 25% in ITO ratio. When we change the thickness of TiO2 in PhC structure, because of ITO/TiO2 structure with an equivalent refractive index, and 50nm ITO and 200nm equivalent refractive index layer has the best anti-reflective effect in our design.
On the other hand, light scattering effect enhanced the absorption by change the direction of incident light, and the optimum structure prove more smooth transmittance curve in long
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wavelength region. So that, front structures not only provide light scattering effect, but also play a role of anti-reflective layer. It increases the optical path length and traps incident light in active layer, reduce energy loss by omnidirectional anti-reflective effect and by high transmittance of PhC structure. We believe that when apply the ITO PhC structure in an actual a-Si thin film solar cell, the efficiency of it can be enhanced.
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