Environmental Engineering Reference
In-Depth Information
texturing of the surfaces to increase the distance a photon could travel inside the
solar cell structure. Amorphous silicon can easily be deposited in thin layers and
has a much higher optical absorption. This means that an amorphous silicon cell
need not be as thick as a crystalline solar cell, a double advantage in reducing the
cost of such a cell. The amorphous form of silicon, however, has inferior electrical
properties, compared to crystalline silicon. Primarily, the carriers are less mobile
because thematerial is not periodic in the location of the atoms. Related to the p
-
i
-
n
natureofthejunctionsinthesecells,onecanrefertoFigure6.2a.Thediffusion
length described in Chapter 3 is short in the amorphous material, and a gradient of
doping acting as a local electrical
field is introduced to separate the photogenerated
holes and electrons.
Several forms of amorphous silicon can be rather easily deposited that have
characteristically different bandgaps. This facilitates making a tandem cell, based on
the same principles as were described in connection with the GaAs triple-junction
solar cells. Microcrystalline silicon, amorphous silicon, amorphous silicon passiv-
ated with hydrogen a-Si:H, and Si:Ge all can be incorporated and have differing
energy gap values. Amorphous silicon is slightly unstable, tending to the more stable
crystalline state, and requires steps including hydrogen passivation, to
ll dangling Si
bonds in the incompletely
filled bond set around the occasional atom, and also
postfabrication annealing to allow a part of the reconstruction to occur before the
device is put to use.
It should be said that these various forms of thin-
lm silicon solar cells are widely
produced and employed in devices of all sorts. Where the ef
ciency is not a key
parameter, useful products
find many applications, from hand calculators to wrist-
watches to semitransparent glass panels that may be used in sky-lights or in a sun roof
for a car. Also, especially connected with the United Solar Ovonics, LLC, company of
Troy, Michigan, are inexpensive roo
ng tiles with built-in solar cells. The cells shown
in Figure 7.12 are among the most ef
cient, approaching 11% ef
ciency.
The structures described here are wide-area devices produced on
flexible stainless
steel backing of 5 mil (0.127mm) thickness and 35.6 cm width. This is a production
process in which rolls of stainless steel foil are sequentially processed. Noting from
Figure 7.12, the re
ective coating Ag/ZnO is
first applied, to re
ect back into the cell
light that has not been absorbed. The second full roll process was deposition of the
three-layer silicon structure, as the foil advances down a line of processing stations
including a radio frequency (rf) glowdischarge to deposit Si at a thickness rate of 3A/s
and a linear foil speed of 2.3
0
/min. This was followed by a full roll deposition of the
transparent conductive oxide (TCO) to cover the structure. The roll was then cut into
individual cells of dimension 35.6 cm
23.6 cm, towhichwere appliedwires and bus
bars to collect current. The efficiencies measured on these large-area junctions were
near 10.4% under one sun illumination, corresponding to open-circuit voltage 2.2 V
and short-circuit current 5.7 A. (The best ef
ciency for cells of the type shown in
Figure 7.13 is reported as 13% [94].) The large-area cells [93] are joined to form
laminatedmodules for which the stable output power under one sun is expected to be
151W. The larger open-circuit voltage, 2.2 V, coming from the series connection of
junctions, enables a single cell to electrolyze water, which requires about 1.93 V, as
