New Light Management Concepts for Thin-Film Silicon Solar Cells
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New Light Management Concepts for Thin-Film Silicon Solar Cells Helmut Stiebig1,2, Christian Haase2, Silvia Jorke2, Philipp Obermeyer2, Etienne Moulin2, and Melanie Schulte2 1 Malibu GmbH, Bielefeld, D-33609, Germany 2 IEF-5, Forschungszentrum Juelich, Juelich, D-52425, Germany ABSTRACT An efficient utilization of the sun spectrum is a key issue in the field of thin-film silicon solar cell technology. Therefore, different strategies for enhanced light absorption were presented in the last years. In order to achieve a better understanding of light scattering at nanotextured interfaces the optical properties of a large variety of samples were studied. The angle resolved scattering behavior was analyzed by means of a developed ray tracing model. As an alternative to randomly textured substrates, the influence of periodically textured substrates on the light propagation in solar cells was experimentally and numerically studied with respect to improved light in-coupling and light trapping. Based on a deeper understanding a new tandem cell structure with a diffractive element between the top and bottom cell was developed. Finally, the influence of metallic nanoparticles on the cell performance was studied. INTRODUCTION Thin-film solar cells based on amorphous (a-Si:H) and microcrystalline silicon (µc-Si:H) require efficient light absorption concepts, in order to achieve a better utilization of the sun spectrum. In particular for µc-Si:H cells with an absorber layer thickness in the µm-range, the penetration depth of the light is significantly longer than the absorber layer thickness. Therefore, light trapping schemes based on randomly textured transparent conductive oxides (TCO) [1-4], transparent grating couplers [5-7], SiO2 spheres [8], textured glass substrates [9] new concepts based on photonic crystals [10] or metallic nanoparticles [11] were studied. The interaction principle of light waves incident on a surface with a sub-wavelength nanostructure or on nanoparticles is a key question in the development of high efficient solar cells. Thus, different strategies for enhanced light absorption in thin-film silicon solar cells will be discussed. The standard concept is the application of randomly textured TCO in combination with a highly reflective back contact. An improved light in-coupling at the front side and scattering within the solar cells take place. The TCO morphology mainly determines the scattering behavior of the substrate. The texturing can be realized either directly by APCVD (LPCVD) growth of SnO2 (ZnO) or by a post-etching step of sputtered ZnO. For the light path enhancement which leads to a higher absorption in the silicon layer (i) the amount of scattered light - the haze - and (ii) the angle distribution of scattered light are of importance. These two key parameters are correlated with the TCO morphology. Although the optical properties have been investigated in the last years, a general understanding of the influence of the feature size of randomly textured TCOs on the light scattering
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