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TFSi-Thin Film Silicon

The thin-film silicon solar cell technology is based on a versatile set of materials and alloys, in both amorphous and microcrystalline form, grown from precursor gases by means of a capacitively coupled plasma. It is a mature and reliable photovoltaic technology with the advantages of large-area, low-cost of manufacturing, abundance of raw materials, and aesthetics of products. However, the conversion efficiency needs to be improved to be competitive with respect to other technologies.

A special attention to light management within the device and the exploitation of multi-junctionarchitectures are the main aspects under exploration to increase the efficiency. Since the technology relies on thin-films of a weakly absorbing material, the application of light-management concepts is crucial in order to maximize light absorption in the active regions with a minimum of parasitic optical losses in the supportive layers.

Various strategies are being investigated, going from improved lightscattering textures to advanced schemes based on nanopillars or plasmonics, accompanied by material research toward reduced parasitic losses [1]. In addition, for a more efficient use of light across the solar spectrum, multi-junction architectures have to be considered, by stacking thin component cells dedicated to the absorption of specific portions of the spectrum. Within the thin-film Si technology

the highest efficiencies are indeed obtained with multi-junction devices, already starting with the very promising micromorph (amorphous silicon/microcrystalline silicon—a-Si:H/_c-SiH) tandem combination, for which a confirmed record efficiency of 12.3% has been demonstrated on large area modules (1.4 m2) [2].

The highest conversion efficiencies have so far been obtained with triple-junction cells: 16.3% initial, shown by United Solar with the a-Si:H/a-SiGe:H/_c-Si:H combination [3], and 13.6% stable, demonstrated by the AIST group with the a-Si:H/_c-Si:H/_c-Si:H configuration [4]. In these cases, a-Si:H is used as top cell absorber. However, theoretical analysis suggests that the highest efficiency for a triple junction can be actually obtained when the bandgap of the top cell absorber is around 2.0 eV,owing to the increase in open circuit voltage (VOC) [5]. The development of wide bandgap Si-based absorber materials is then a crucial factor for exploiting the efficiency potential of multi-junction solar cells. In this context, hydrogenated amorphous silicon oxide (a-SiOx:H) appears to be a promising candidate, since its energy gap can be significantly widened by adjusting the oxygen content [6–8]. Silicon oxide alloys can be useful also as more transparent inactive layers, thus helping to reduce the parasitic light absorption. In this case mixed-phase nanocrystalline doped silicon oxide (nc-SiOx:H) is demonstrating interesting capabilities, and various versions are now being successfully used as superior doped layers and/or reflecting layers in multi-junction thin-film Si solar cells [3,9–14]

As common aspects for existing thin-film technologies, TFSi deals at production level with reliable, cost-effective production equipment, low-cost packaging solutions both for rigid and flexible modules, and more reliable modules through better quality assurance procedures (advanced module testing, and improved assessment of module performance). Alternatives for shortage of chemical elements (metals) and the more expensive sealant/encapsulant  and recycling of materials and modules that have reached the end of their lives is starting to become very important.

Published by ENEA


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