Photovoltaics - Understanding Renewable Energy Systems - page 130

Figure 4.12 Structure of an Amorphous Silicon Solar Module
this is glass or a metal foil. A spray process deposits a thin layer of transparent
tin oxide on the glass substrate. This layer serves as the transparent front
contact. A laser cuts this layer into strips in order to create an integrated series
connection and vapour deposition at high temperatures adds silicon and
dopants to the substrate. A 10-nm p-layer and afterwards a 10-nm buffer layer
are deposited. An intrinsic 1000-nm layer of amorphous silicon and finally a
20-nm n-layer follow. Screen-printing processes add the back contacts, which
are mostly made of aluminium. The samples are then either laminated or coated
with a polymer to protect the solar module from climatic influences. Figure
4.12 shows the principle structure of an amorphous silicon solar module.
Silicon loses its crystalline structure during vapour depositing in amorphous
solar cells. The main advantage of amorphous silicon cells is that they are
thinner than crystalline cells by a factor of 100. This is only possible because
amorphous silicon is a direct semiconductor with a much higher absorption
coefficient than crystalline silicon. Due to its amorphous structure, the band
gap is higher than crystalline silicon, at 1.7 eV. The production process saves a
lot of material and even more efficient production is possible. Laboratory
efficiencies of 13 per cent have been reached. However, amorphous silicon solar
cells are mainly used in small applications such as pocket calculators or
wristwatches largely because the efficiency achieved in production is only
around 6-8 per cent, which is much lower than that of crystalline cells, which
reach up to 20 per cent. Furthermore, amorphous silicon solar cells exhibit a
degradation process. This reduces the efficiency in the first months of operation
by 10-20 per cent until the performance finally stabilizes.
Other thin film materials already have started to be commercialized.
Various materials and technologies are currently under development. Besides
the use of new materials such as cadmium telluride (CdTe) or copper indium
diselenide (CuInSe 2 ), microcrystalline thin film cells are under development.
Other promising developments are dye-sensitized cells on a base of titanium
dioxide (TiO 2 ). Which technology will become commercially viable depends
on further technological developments.