Modulated surface-textured substrates with high haze for thin-film silicon solar cells
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Modulated surface-textured substrates with high haze for thin-film silicon solar cells O. Isabella1, P. Liu1, B. Bolman1, J. Krþ2, M. Zeman1 1 Delft University of Technology, EEC Unit / DIMES, 2600 GB Delft, The Netherlands 2 University of Ljubljana, Faculty of Electrical Engineering, SI-1000 Ljubljana, Slovenia ABSTRACT Modulated surface-textured substrates for thin-film silicon solar cells exhibiting high haze in a broad range of wavelengths were fabricated. Glass substrates coated with different thicknesses of a sacrificial layer were wet-etched allowing the manipulation of the surface morphology with surface roughness ranging from 200 nm up to 1000 nm. Subsequently, zinc-oxide layers were sputtered and then wet-etched constituting the final modulated textures. The morphological analysis of the substrates demonstrated the surface modulation, and the optical analysis revealed broad angle intensity distributions and high hazes. A small anti-reflective effect with respect to untreated glass was found for etched glass samples. The performance of solar cells on high-haze substrates was evaluated. The solar cells outperformed the reference cell fabricated on a randomly-textured zincoxide-coated flat glass. The trend in the efficiency resembled the increased surface roughness and the anti-reflective effect was confirmed also in solar cell devices. INTRODUCTION Thin-film silicon solar cells is one of the major photovoltaic technologies. The short energy payback time and a small amount of needed materials offer a market profitable performance-price ratio. To sustain the growth of this technology an increase in the efficiency is mandatory. One way to approach this challenge is to enhance the photocurrent. Photocurrent can be increased by applying different light management techniques. One of them is light scattering at internal interfaces, which prolongs the light path and consequently increases the absorption in the absorber layers. Nowadays thin-film silicon solar cells are formed by two junctions stacked on top of each other with rough internal interfaces for light scattering. Amorphous silicon (a-Si:H) and microcrystalline silicon (ȝc-Si:H) are used as absorber layers in the top and bottom component cells, respectively, forming a tandem device. To evaluate scattering properties of a surface texture the wavelength-dependent haze parameter is commonly used [1]. When the surface morphologies exhibit features smaller than the wavelength of incident light, the amount of scattered light will exponentially decrease with the haze parameter approaching zero (scalar scattering approach) [2], leading to poor absorption in the long wavelength region. The bottom cell that is optically active up to 1100 nm, especially suffers from this problem, as the photons passing through the ȝc-Si:H absorber are not efficiently scattered. To boost the absorption in the bottom cell a different family of interface morphology is needed that ensures high scattering also at long wavelengths (Ȝ > 700nm). Advanced surface morphologies have been recentl
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