Broadband Absorptance High Efficiency Silicon Nanowire Fractal Arrays for Photovoltaic Applications
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Broadband Absorptance High Efficiency Silicon Nanowire Fractal Arrays for Photovoltaic Applications 1
Omar H. Alzoubi, 2Husam Abu-Safe, 3Khalid Alshurman, 1Hameed A. Naseem
1
Department of Electrical Engineering, University of Arkansas, Fayetteville, AR 72701, U.S.A. Natural Resources Engineering and Management, German-Jordanian University, Madaba, Jordan 3 Microelectronics and photonics program, University of Arkansas, Fayetteville, AR 72701, U.S.A. 2
ABSTRACT Nanowire arrays have been proposed to enhance light trapping, increase efficiencies, and reduced material cost in photovoltaic solar cells. In this work we present a new crystalline silicon nanowire array structure, inspired by fractal geometry. The array structure is assumed to be an infinite 2D array in the x and y directions, and composed of vertically aligned SiNW suspended in air. Hexagonal fractal-like geometry is adapted in arranging cylindrical SiNW in these arrays. Full-wave finite element method 3D simulation is used to compute reflectance, transmittance and absorptance of the array for a normal incidence plane wave. The proposed fractal-like distribution of SiNW arrays yield broad absorption spectrum and enhanced efficiency while using less material. The efficiency of the proposed fractal-like SiNW arrays achieve ~100% enhancement over that of the equivalent thickness flat c-Si film, and ~18% enhancement over an equivalent height hexagonal array. The proposed optimized structures achieved a filling ratio ~25%, which is ~33% less than the corresponding hexagonal array. INTRODUCTION Solar energy is the most abundant and environmentally clean source of energy on earth. Photovoltaic (PV) cells convert solar energy directly into electrical energy and are considered attractive candidate that would contribute significantly to future renewable energy sources. The efficiency of the commercial PV cells and the ($/Watt) cost make PV cells poor competitive to other less expensive energy sources1. Therefore, reducing the material cost of PV cells, and increasing their efficiency have been the focus of numerous research efforts 1,2. Thin film PV cells, with thickness in the order of few hundreds of nanometers up to few micrometers, have been proposed to achieve such goals3. Thin films suffer from high optical losses due to reflection and transmission, especially for silicon. Therefore, light trapping enhancement inside thin films and reduction of reflection losses are required to achieve reasonable efficiency. Different techniques for enhancing light trapping in thin films and boosting efficiency have been proposed4-6. Nanowires (NW) arrays are one of the most promising techniques that can achieve low reflection loss and high light trapping, and have potential to produce thin film PV solar cells with reasonable efficiency. Numerous researches have been conducted on NW arrays of different array structures7-9. In this work, we introduce new SiNW array structures inspired by fractal geometry, where the distribution of the SiNW in the arrays forms a fractal-like
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