Answer 1:
This is fundamentally a materials
problem. The most efficient form of a solar cell
tends to be single crystalline, which
minimizes defects and impurities that could cause
the carriers in the solar cell to recombine. Even
then the maximum theoretical limit (see
Shockley Queisser limit ) is less than 35%
efficiency.
Silicon itself is intrinsically inefficient
due to its electronic structure (it has an
indirect band gap). However, because it's
1)cheap,
2)abundant, and
3)so well understood as a material, it's a
difficult material to out-compete economically.
Nevertheless, a lot of effort has been made to
work around this theoretical limit (which relies
on several assumptions).
You can read about it
here and a pretty cool example
here .
You can also see a nice compilation of efforts
to improve solar cell efficiency around the world
with silicon and other materials
here .
But there are other factors that come into play,
such as toxicity, cost, ease of growth and
processing.
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