Hydrogenated Nanocrystalline Silicon based Solar Cell with 13.6% Stable Efficiency
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Hydrogenated Nanocrystalline Silicon based Solar Cell with 13.6% Stable Efficiency Guozhen Yue, Baojie Yan, Laura Sivec, Tining Su, Yan Zhou, Jeff Yang, and Subhendu Guha United Solar Ovonic LLC, 1100 West Maple Road, Troy, Michigan, 48084, USA
ABSTRACT Multi-junction solar cells incorporating hydrogenated nanocrystalline silicon (nc-Si:H) exhibit a high current capability and low light-induced degradation. In this paper, we report our recent progress in developing nc-Si:H solar cells using a modified very-high-frequency glow discharge technique. We achieved a short-circuit current density >30 mA/cm2 and 10.6% conversion efficiency from single-junction solar cells. Using the improved nc-Si:H cells in an aSi:H/nc-Si:H/nc-Si:H triple-junction structure, we attained initial and stabilized efficiencies of 13.9% and 13.6%, respectively. Issues related to improving material properties and device structures are addressed. Besides using the conventional techniques, such as hydrogen dilution profiling, optimized Ag/ZnO back reflector, and buffer layers, we found that compensation from Boron and Oxygen micro-doping is also critical in obtaining the above achievements. INTRODUCTION Hydrogenated nanocrystalline silicon (nc-Si:H) solar cell is a critical component in the fabrication of high efficiency multi-junction thin-film silicon solar cells [1,2]. The intrinsic nanocrystalline material of such a component cell has a low bandgap and no light-induced degradation under red light illumination, making it the perfect choice for bottom cell in hydrogenated amorphous silicon (a-Si:H) and nc-Si:H based multi-junction solar cells [3]. Since the early study of nc-Si:H solar cells by Neuchâtel [4,5], a-Si:H and nc-Si:H based multi-junction solar cell efficiency has now exceeded the record efficiency of a-Si:H and amorphous silicon germanium alloy (a-SiGe:H) based multi-junction solar cells [6,7]. Compared to a-SiGe:H, nc-Si:H has several advantages, such as high photocurrent capability, good stability upon light soaking, and a fabrication process that does not require the use of GeH4. However, nc-Si:H solar cells also have challenges that need to be addressed before the cells can be incorporated into volume manufacturing. Critical challenges are high rate deposition, large-area uniformity, and high performance as a component cell in multi-junction structures. Efficiency improvement remains one of the most important tasks for the thin-film silicon photovoltaic community. One limiting factor for nc-Si:H cell efficiency is the presence of impurities in the nc-Si:H intrinsic layer [8-13]. It has been found that nc-Si:H is more sensitive to O and N impurities than a-Si:H. This is because O and N form weak donors in nc-Si:H materials, which move the Fermi level toward the conduction band edge [13]. nc-Si:H usually shows weak n-type conductivity unless the impurity levels are reduced significantly or compensated by ptype doping. In order to make high efficiency cells, it is generally recommended [2] to reduce O level to below 5 1018 at./cm3.
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