Radial Junction Architecture: A New Approach to Stable and Highly Efficient Silicon Thin Film Solar Cells
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Radial Junction Architecture: A New Approach to Stable and Highly Efficient Silicon Thin Film Solar Cells S. Misra1, M. Foldyna1, I. Florea1, L. Yu1, 2 and P. Roca i Cabarrocas1 1 LPICM-CNRS, Ecole Polytechnique, 91128 Palaiseau, France. 2 School of Electronics Science and Engineering, Nanjing University, 210093, Nanjing, People’s Republic of China. ABSTRACT Incorporation of properly designed nanostructures in solar cells improves light trapping and consequently their power conversion efficiencies. Due to its unique structure, a silicon nanowire (SiNW) matrix provides excellent light trapping and thus offers a promising approach for cost-effective, stable and efficient silicon thin film photovoltaics. Moreover, by decoupling the light absorption and carrier collection directions, radial junction solar cells built around the SiNWs allow the use of very thin active layers. As a matter of fact, radial PIN junctions with 9.2% power conversion efficiency have already been demonstrated on glass substrates with only 100 nm thick intrinsic hydrogenated amorphous silicon layers. The most straightforward way to further improve the short circuit current density is to use an active layer with a lower band gap. In this work, the performances of devices with two different low band gap materials, e.g., hydrogenated microcrystalline silicon (μc-Si:H) and hydrogenated amorphous silicon germanium alloy (a-SiGe:H) are presented. To the best of our knowledge, this is the first demonstration of a-SiGe:H radial junction solar cell. INTRODUCTION Incorporation of nanostructures is one of the recent trends to improve the light absorption and consequently the efficiency of the solar cells. In this regard, SiNWs provide an attractive research platform for a new generation of cost-effective and efficient solar cells [1]. This is particularly advantageous for silicon thin film solar cells. Hydrogenated a-Si PIN radial junction solar cells built over a dense matrix of SiNWs benefit from a very strong light trapping and efficient absorption [1, 2]. By decoupling the light absorption and carrier collection directions they allow the use of very thin intrinsic layer, which results in efficient collection thanks to strong built-in field. Another advantage of using very thin a-Si:H absorber layers is the minimization of the light-induced degradation [3, 4]. Thus, arrays of radial junction solar cells bring advantages of high efficiency with reduced material consumption. The goal of using this architecture is to fabricate a tandem a-Si:H/μc-Si:H radial junction solar cell with a potential power conversion efficiency of 15%, while retaining a low production cost. There are many different ways to fabricate SiNWs. Highly ordered Si nanowires or microwires can be synthesized from crystalline Si wafers by using substrate selective etching using a mask [5, 6]. Although reasonable solar cells with ~ 10% power conversion efficiency have been demonstrated for Si nanopillars fabricated by using these methods [7, 8], crystalline Si wafer itself limits the scope o
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