Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 253

wafers. For the above-discussed thin-film technologies it is the other way round.
Material costs are low. Production processes are complex and expensive. There-
fore thin-film technologies can compete on an economic basis only if large pro-
ductions sites are operated.
Thin film solar cells made of crystalline silicon. There are also attempts to utilise
the economic and process-related advantages of thin film solar cell technology,
such as low material consumption, integrated module manufacturing by structur-
ing of individual layers during manufacturing, for crystalline silicon. Due to the
indirect energy gap of crystalline silicon layer thicknesses of at least 20 µm are
required to sufficiently absorb the incident solar radiation. "Light-trapping", how-
ever, allows further reduction of the layer thickness. If a light beam reflecting
diffuser, or an inclined reflecting structure is superimposed on the reverse side of
the solar cell, or if a pyramid-type substrate is coated with a thin silicon film, also
for crystalline silicon, layers of only a few µm thickness are sufficient to nearly
entirely absorb the incident radiation. For silicon layers of only 2 µm thickness
and optimised "light-trapping", the efficiency potential has been calculated at
approximately 15 %. In practice, various methods of film deposition and posterior
treatment are being investigated with the aim to manufacture such silicon cells
under commercial circumstances.
A variation of the deposition parameters of the plasma-supported enhanced
chemical deposition from the gas phase allows depositing microcrystalline silicon.
Although the silicon crystals of this kind of material have a size of only a few
10 nm (and are therefore also referred to as nanocrystalline silicon) based on p-i-
n-structures electrical efficiencies above 10 % are achieved in the laboratory
scale. Since the deposition conditions and the deposition temperature of nanocrys-
talline silicon, ranging between 200 and 300 °C, are very similar to those of
amorphous silicon both materials may be combined as tandem cells that yield
efficiencies of over 10 % on a laboratory scale /6-23/. For silicon thin film solar
cells of higher efficiencies, silicon needs to be deposed at temperatures above
700 °C, so that cheap glass-substrates are inappropriate /6-24/. The higher growth
temperatures allow achieving grain sizes of up to 100 µm and thus a better photo-
voltaic quality of the polycrystalline silicon layers. The so-called transfer tech-
nologies are a promising alternative to manufacture even mono-crystalline thin
film solar cells (see also /5-25/, /6-26/). Typically, a mono-crystalline silicon layer
of 20 to 50 µm thickness is manufactured on a pre-treated mono-crystalline silicon
substrate which is subsequently detached and transferred to any kind of foreign
substrate. The silicon substrate may afterwards be used again for this purpose.
With 16.6 %, on a laboratory scale, solar cells made of mono-crystalline trans-
ferred silicon permit to achieve the highest efficiencies of thin film silicon on
foreign substrates.
Thin film solar cells with integrated serial circuit. The individual cells of a solar
module must be series-connected if the module voltage is to be superior to that of