Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 249

photovoltaic cells. This material is directly derived from decomposed silane
(SiH 4 ) at temperatures between 80 and 200 °C by plasma-supported enhanced
chemical deposition from the gas-phase. Due to the fact that amorphous silicon
forms a direct semiconductor, very thin active layers in the range of 1 µm are
required. Therefore very little material is needed. Additionally this process is
characterised by very low deposition temperatures and thus small energy con-
sumption. As a consequence the costs of solar cell manufacturing are reduced
tremendously compared with crystalline silicon solar cells.
The assembly of a a-Si:H-solar cell is completely different compared to a crys-
talline silicon photovoltaic cell. Instead of a p-n-junction, p-i-n-structures are
used; i.e. the major portion of the photovoltaically active layer of a thickness of
several 100 nm consists of un-doped (intrinsic) hydrogen-passivated amorphous
silicon (a-Si:H) with n-doped layers of a few 10 nm on top and underneath. The
electric field of such a structure thus covers the entire absorber region and ensures
the separation of electron-hole-pairs created by the absorbed solar irradiation at all
places.
Fig. 6.10 shows the layer sequence of typical a-Si:H solar cells. According to
this, different substrate technologies are distinguished. The layers are deposited in
the direction from the shaded side to the light-exposed side (Fig. 6.10, left). Start-
ing on top of a conductive (non-transparent) substrate, such as e.g. a stainless-
steel foil, a layer sequence consisting of n-doped, un-doped and p-doped hydro-
gen-passivated amorphous silicon (a-Si:H) is deposited from the gas phase. Even-
tually, a transparent conductive oxide (TCO) acts as a contact on the light-
exposed side. Superstrate technologies start with deposition on the light-exposed
side; i.e. first the conductive oxide acting as a transparent contact and subse-
quently the layer sequence consisting of hydrogen-passivated silicon (a-Si:H) and
lastly the metallic rear side contact need to be deposited (Fig. 6.10, centre).
Besides solar cells provided with individual p-i-n-junctions, also tandem cells
and even triple cells are in use. For these applications two or three p-i-n-layers are
piled on top of each other, whereas the junction between highly n-doped and
highly p-doped material bypasses both layers (so-called tunnel contact). The volt-
ages of the two or three piled p-i-n-layers add up. For an optimum utilisation of
the solar spectrum frequently the energy gap of one of these p-i-n-structures is
enhanced or reduced by alloying the hydrogen-passivated amorphous silicon (a-
Si:H) with amorphous carbon or amorphous germanium. The cell that is closest to
the light-exposed side should show the largest energy gap. As this cell only ab-
sorbs the short-wave radiation of the solar spectrum it better exploits the photon
energy. Such a cell thus supplies higher voltages with regard to its energy gap.
The cell located on the side shielded from the sun light has the smallest energy
gap and can thus still use part of the low-energy photons that have not been ab-
sorbed by the first cell. Fig. 6.10, right, shows the example of a tandem cell where
two p-i-n-structures of hydrogen-passivated amorphous silicon (a-Si:H) and a a-
SiGe:H alloy have been combined.