Investigation of Textured Back Reflectors for Microcrystalline Silicon Based Solar Cells
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Microcrystalline silicon (kc-Si:H) solar cells require an effective light trapping in the near
infrared (NIR) to enhance the long wavelength spectral response. For this purpose we investigated back reflectors based on texture-etched ZnO/Ag stacks prepared on glass substrates by magnetron sputtering. With decreasing sputter pressure the resulting surface texture of the glass/Ag/ZnO substrates after etching exhibits a larger feature size and root mean square roughness. The increase in feature size corresponds to an increase of diffuse reflectivity. Applied in microcrystalline solar cells prepared by VHF plasma enhanced chemical vapour deposition (PECVD), the reflectors showing the largest feature size (prepared at the lowest possible sputter pressure) yielded the highest long wavelength spectral
response. The pc-Si n-i-p cells prepared on the latter back reflector exhibited efficiencies of 2 6.9 % (short circuit current density jc= 18.8 mA/cm 2) and 7.5 % (j&c=25 mA/cm ) for an i-layer thickness of 1 pm and 3.5 pm, respectively. INTRODUCTION High efficiency amorphous and microcrystalline silicon based solar cells rely on an improvement of the light absorption due to an optical confinement of the long wavelength light (light trapping). In the ideal case, the solar radiation is scattered, repeatedly reflected within the multi-layered thin film solar cell and almost completely absorbed after multiple passes through the intrinsic layer which generates the photo-current. An effective light trapping yields high currents and consequently high efficiencies even if thin i-layers are used. In state-of-the-art n-i-p (substrate) technology, the front TCO film provides low absorption and reflectance losses, while the light scattering is provided by the natural surface texture of the intrinsic silicon film [1] or/and a textured back reflector [2,31 for which a combination of ZnO and Ag is often used. In the latter case, the texture of the ZnO/Ag back reflector strongly influences the device performance. The required texture for optimized scattering can be obtained by depositing Ag and ZnO at high temperatures [2,3]. Textured ZnO/Ag back reflectors can also be realized by post deposition chemical etching of sputtered ZnO films [4,5]. Chemical etching of sputtered ZnO:Al films e.g. in diluted hydrochloric acid (HCI) allows to adjust the surface morphology depending on the structural properties of the initial film and the etching time [6-8]. This paper deals with the preparation and characterization of texture-etched ZnO/Ag back reflectors for jac-Si based n-i-p solar cells. The following questions were addressed: Firstly, how does the surface morphology (feature size) of texture-etched ZnO/Ag back reflectors depend on the deposition conditions of the ZnO sputter process? Here, the deposition pressure turned out to be the key parameter. Secondly, how do the resulting different surface textures obtained after etching influence the diffuse reflectivity of the ZnO/Ag back reflectors? The final goal is to find correlations between the
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