Novel approaches of light management in thin-film silicon solar cells
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0910-A25-01
Novel approaches of light management in thin-film silicon solar cells J. Krc1, M. Zeman2, A. Campa1, F. Smole1, and M. Topic1 1 University of Ljubljana, Faculty of Electrical Engineering, Trzaska 25, Ljubljana, 1000, Slovenia 2 Delft University of Technology - DIMES, Delft, P.O. Box 5053 2600 GB, Netherlands
ABSTRACT In order to improve light trapping in thin-film silicon solar cells two novel approaches are investigated in this article: angle-selective management of light scattering inside the solar cell and wavelength-selective manipulation of high reflectance or transmittance of light. Diffraction gratings are analyzed as a representative of the first approach. Haze and angular distribution function of scattered (diffracted) light in reflection are measured for aluminum-based rectangular periodic gratings with different period and height of the rectangles. High haze values in specific wavelength region and scattering angles of the investigated gratings measured in air and water agree very well with the theoretical predictions. Considering the actual optical situation in microcrystalline silicon solar cells, optimal period and height of the rectangular gratings applied as a back reflector are calculated for obtaining the total reflection at the front interfaces. In the frame of the second approach, photonic-crystal-like structures are introduced. By means of optical simulations photonic-crystal-like structures are investigated for two possible applications: an intermediate reflector in a micromorph silicon solar cell with wavelength-selective reflectivity and a dielectric back reflector with a high reflectance in the long-wavelength region. The photonic crystal structure consisting of sequences of n-doped amorphous silicon and ZnO layers is designed for the efficient intermediate reflector. For the back reflector with a high reflectance the structures with intrinsic amorphous silicon, SiO2, MgF2 and TiO2 are proposed. INTRODUCTION For obtaining high conversion efficiency of thin-film solar cells the proper light management inside the solar cell structures is of great importance. In today’s thin-film solar cells light management is accomplished by implementing light-trapping techniques. The term light trapping is used to describe methods, which help to capture most of the light in a desired part of a solar cell without allowing it to escape. At present the light-trapping techniques are based on the introduction of surface-textured substrates and the use of special layers called back reflectors. The employment of randomly surface-textured substrates and the implementation of back reflectors lower the optical losses of a solar cell due to a decrease of the total reflection of the cell and due to an improvement of the reflectivity at the back contact of the cell, respectively. In addition, surface-textured substrates introduce roughness also to the internal interfaces of the solar cell. Incident light is scattered at rough interfaces, resulting in a larger average light path through the layers and an enh
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