Periodic Structures for Improved Light Management in Thin-film Silicon Solar Cells
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Periodic Structures for Improved Light Management in Thin-film Silicon Solar Cells Janez Krc1, Andrej Campa1, Stefan Luxembourg2, Miro Zeman2, and Marko Topic1 1 University of Ljubljana, Faculty of Electrical Engineering, Ljubljana, 1000, Slovenia 2 Delft University of Technology - DIMES, Delft, 2600 GB, Netherlands ABSTRACT Advanced light management strategies in thin-film solar cells can increase the photocurrent of the cells and, thus, their conversion efficiencies. In the present work two types of periodic structures, one-dimensional diffraction gratings and photonic crystals, are analyzed in order to evaluate their potential for improved light trapping in thin-film silicon solar cells. The anti-reflective effects and enhanced scattering at the gratings with triangular and rectangular features are studied by means of two-dimensional optical simulations. Simulations of a microcrystalline silicon solar cell, in which grating features are applied at all interfaces, are presented. Critical issues in the design of gratings are pointed out in order to achieve and surpass the performance of the cells with optimized randomly textured interfaces. Reflectance of a photonic crystal stack designed using 12 layers of amorphous silicon nitride and amorphous silicon are presented and compared with simulations. High reflectance (up to 99 %) of the stack is measured for a broad wavelength spectrum. The potential of a simple photonic crystal structure for increasing the absorption in an amorphous silicon solar cell, when applied as a back reflector, is demonstrated by means of optical simulations.
INTRODUCTION In order to obtain high conversion efficiency of thin-film solar cells proper light management inside the solar cell structures is of great importance. High optical absorption in active layers leads to increased photocurrent, enhancing the efficiency of the solar cells. 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 the light in a desired part (active layers) of a solar cell. At present the light-trapping techniques are based on the introduction of surface-textured substrates and the use of special layers called back reflectors. Incident light is scattered at rough interfaces, resulting in a larger average light path through the layers and an enhancement of the light absorption inside the cell. The back reflector is essential to reflect the long-wavelength light, which was not absorbed in the first pass, back to the absorber layer, giving it another opportunity to be absorbed. These light trapping techniques have been first introduced in thin-film amorphous silicon (a-Si:H) solar cells in the 1980s [1] and resulted in an increase of short-circuit current density of 3 to 5 mA/cm2 in comparison to solar cells deposited on flat substrates and without back reflector. The surface texture of both, front glass/TCO (transparent conductive oxide) superstrates and back metal-reflector
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