Environmental Engineering Reference
In-Depth Information
The degradation of properties of amorphous silicon on light exposure was
becoming a serious hindrance to improving the light-to-electricity conversion
efficiency of the solar cells and was considered a major obstacle to the successful
commercialization of this technology. By the mid 1980s, several Japanese groups
were exploring the use of hydrogen dilution to improve the stability of the mate-
rial against light exposure. Since the cause of this light-induced degradation was
assumed to be somehow related to the presence of hydrogen, how the use of
excess hydrogen in the starting gas mixture can reduce degradation was very
puzzling.
Several industries took a two-pronged attack. The first aim was to reduce the
degradation by optimizing the amount of hydrogen dilution and other deposition
parameters empirically; simultaneously, a dedicated research effort was directed to
have an understanding of this phenomenon.
Guha and his colleagues were addressing the problem both empirically
and analytically, with considerable success. In 1987, the US Department
of Energy, which was interested in the potential of solar energy, requested
proposals in the field of solar cells using new thin-film materials. Guha's
team responded, and their project was awarded a three-year, $6.26 million
cost-sharing grant through the National Renewable Energy Laboratory
in Golden, Colorado. It was the beginning of a relationship with
DOE/NREL that continues to this day.
Gradually, Guha's responsibilities expanded beyond research and devel-
opment to include administration, manufacturing, and sales as an executive
of United Solar Systems, a joint venture company started in 1990 by ECD
and Canon of Japan to manufacture solar cells.
At that time, conventional solar energy technology consisted of photo-
voltaic cells made with crystalline silicon. Crystalline silicon, like amor-
phous silicon, is a semiconductor.When sunlight falls on a semiconductor,
it is absorbed to create charges (positive or negative) that are separated and
then channeled to produce an electric current.The regular atomic structure
of the crystalline silicon actually makes absorption of light more difficult
than in the looser arrangement of amorphous silicon. The conventional
solar cells were therefore thick, required a rigid and heavy support, and
were expensive to produce.They were not cost effective, since it took 7 or
8 years of operation to recoup the energy spent in their production. ECD
and United Solar Systems were interested in developing lighter, less expen-
sive solar cells using amorphous silicon, which could be 100 times thinner
than crystalline silicon with equal capacity for light absorption. Guha notes
that hydrogen dilution was the key:
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