Modeling of Advanced Light Trapping Approaches in Thin-Film Silicon Solar Cells
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Modeling of Advanced Light Trapping Approaches in Thin-Film Silicon Solar Cells. Miro Zeman, Olindo Isabella, Klaus Jäger, Pavel Babal, Serge Solntsev, Rudi Santbergen Delft University of Technology, PVMD/DIMES, P.O. Box 5053, 2628 CD Delft, Netherlands ABSTRACT Due to the increasing complexity of thin-film silicon solar cells, the role of computer modeling for analyzing and designing these devices becomes increasingly important. The ASA program was used to study two of these advanced devices. The simulations of an amorphous silicon solar cell with silver nanoparticles embedded in a zinc oxide back reflector demonstrated the negative effect of the parasitic absorption in the particles. When using optical properties of perfectly spherical particles a modest enhancement in the external quantum efficiency was found. The simulations of a tandem micromorph solar cell, in which a zinc oxide based photonic crystal-like multilayer was incorporated as an intermediate reflector (IR), demonstrated that the IR resulted in an enhanced photocurrent in the top cell and could be used to optimize the current matching of the top and bottom cell. INTRODUCTION Thin-film silicon solar cell technology is a promising photovoltaic (PV) technology for delivering low-cost solar electricity. However, the efficiency of thin-film silicon solar cells has to achieve a level of more than 20% in order to stay competitive with bulk crystalline silicon solar cells and other thin-film solar cell technologies. Light management [1] is one of the key areas for improving the performance of thin-film silicon solar cells and decreasing the production costs by using less material for an absorber layer and shortening its deposition time. Performance improvement of a tandem micromorph silicon solar cell has introduced new challenges for light management. In the micromorph solar cell hydrogenated amorphous silicon (a-Si:H) is incorporated as the absorber layer in the top cell and hydrogenated micro-crystalline silicon (ȝc-Si:H) in the bottom cell. An intermediate reflector (IR) is applied between the top and bottom cells and is optimized to reflect a particular part of the solar spectrum back into the top aSi:H absorber. In this way the IR makes it possible to use thinner absorber layers which results in a higher stability of the double-junction cell under light exposure and thus a higher stabilized efficiency. Due to the use of ȝc-Si:H absorber in the bottom cell a high level of scattering caused by rough interfaces is required for a broad wavelength range up to 1100 nm. Additional light scattering that increases the absorption in the absorber layers is expected from the implementation of metal nanoparticles in the solar cell structure due to plasmonic effects. On the other hand, in order to eliminate the parasitic plasmonic absorption in the rough back metal electrode [2], layers of transparent conductive oxides (TCO), distributed Bragg reflectors based on dielectric materials and/or white paint are developed and tested in solar cell structures. Modeling of t
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