Environmental Engineering Reference
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additional process steps are required, mainly to improve light trapping and the
electric properties of surfaces and boundary areas of the photovoltaic cell. For
instance, for high-efficiency solar cells made of mono-crystalline silicon multi-
stage phosphorus diffusion is applied for the p-n-junction and for structuring of
the rear side contact (either point or line-shaped contact) /6-7/, /6-8/. Since the
high-performance processes mentioned above imply a multitude of additional
complex and expensive process steps (e.g. photolithography), they are currently
not cost-efficient despite the enhanced cell efficiencies. Only if the specific costs
with regard to the overall photovoltaic system decrease significantly, high-
efficiency cells are expected to be manufactured on a broader commercial scale.
Another concept to increase cell efficiency is the metal insulator semiconductor
(MIS) solar cell. The name of this solar cell type stems from the impact of a layer
of fixed positive charges, located at the surface of a p-doped layer. This layer is
referred to as inversion layer since the part of the p-layer close to the surface vir-
tually acts as an n-layer, due to the electrical field created by the fixed surface
charges; the p-layer close to the surface is thus quasi inverted. The advantage of
these cells is that they only require six manufacturing steps at a relatively low
temperature level. In large-scale production the electrical cell efficiency amounts
to roughly 16 %.
For a similar concept, aimed at a significant simplification of the manufactur-
ing process /6-19/, the so-called Hetero-junction with Intrinsic Thin-layer (HIT)
structure, the rectifying front contact to a mono-crystalline silicon wafer (n-
doped) is created by deposition of a double layer consisting of un-doped (intrin-
sic) and p-doped amorphous silicon. Thus, a rectifying p-n-hetero-contact be-
tween n-conducting crystalline and p-conducting amorphous silicon is created.
The overall thickness of both layers only amounts to a few 10 nm, so that amor-
phous silicon does not contribute to the photocurrent. The actual photovoltaic
absorber material still is the mono-crystalline silicon wafer. The sophisticated and
energy-consuming manufacturing process of the p-n-junction by diffusion is re-
placed by the comparatively simple and energy-saving deposition of the amor-
phous silicon double layer. This new technology allows obtaining electrical cell
efficiencies of above 20 % (in the laboratory).
Above all, current research is aimed at reducing the costs of solar cell produc-
tion while maintaining maximum electrical cell efficiencies. For this purpose the
applied materials as well as the overall manufacturing process chain are examined
over and over again for cost reduction potentials. Besides the new concepts men-
tioned above, current investigations focus on the use of thinner silicon wafers
(thicknesses as small as 70 µm). Such thin wafers have already obtained good
efficiencies in the laboratory. However, disadvantages are the expensive manufac-
turing of thin wafers, on the one hand, and their poor stability within the industrial
manufacturing process on the other.
Thin-layer amorphous silicon (a-Si:H) solar cells.
In the mid 1970's hydrogen-
passivated amorphous silicon (a-Si:H) was first applied as a base material for
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