The biomimetic glass substrates were fabricated by nanosphere lithography and RIE technique. We apply this technique on amorphous silicon thin film solar cells.
Compared with previous research, which place a metal grating back reflector to scatter the long wavelength light only and further to enhance the power conversion efficiency, we use the biomimetic structure glass substrate to deposit semiconductor thin film further to get the conformal structure and obtain both of the anti-reflective characteristic and light scattering characteristic. Because the conformal structure can provide a periodic grating back reflector to further scatter the light and a graded refractive index interface to reduce the Fresnel reflection, both of the light scattering and anti-reflective characteristic are good for solar energy harvesting.
From the reflectance measurement result, we observed good anti-reflective characteristic and light scattering characteristic of biomimetic glass substrates. From the power conversion efficiency measurement, both the anti-reflective characteristic and scattering light characteristic could be contributed to enhancing the power conversion efficiency. From the RCWA simulation results, it shows that higher height of the biomimetic structure which result in the higher photo current. However, from experiments result, open circuit voltage and the electrical property will decrease when the H/D ratio of biomimetic structures increase. This result means the biomimetic glass substrate with better optical properties will decrease the electrical property and the open circuit voltage is almost a linear decay. We believe that higher H/D ratio of biomimetic structure will result in uneven coverage of thin film growth. Because the thickness of transparent conductive layer is only about 100nm, and amorphous silicon thin film of p-layer and n-layer thickness are only 12nm/20nm, higher biomimetic structure H/D ratio may result in uneven coverage of a-Si thin film. We must consider both of the optical property and electrical property. In the result of solar cell electrical
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parameter (open voltage, short circuit current, fill factor, power conversion efficiency) verse the H/D ratio of biomimetic structure analysis, we conclude the best H/D ratio for biomimetic glass structure applies in a-Si thin film solar cell is around 0.2~0.3.
Finally, the biomimetic glass substrate has been demonstrated with a notable improvement on antireflection and light scattering property, and these two mechanisms are beneficial for light harvesting. The JSC of biomimetic glass substrate cell (D=1um H=200nm) shows 8% enhancement and 51.6% enhancement compared to Asahi-U glass substrate and flat glass substrates cell respectively. And the power conversion efficiency of biomimetic glass substrate cell also shows 3.1%
enhancement and 48.4% enhancement compared to Asahi-U glass substrate and flat glass substrates cell respectively.
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