Environmental Engineering Reference
In-Depth Information
Figure 6.9 Empirical relation [73b] between
open-circuit voltage and grain size for
polycrystalline (closed circles) and nanocrystalline
(open circles) silicon solar cells. Note that the
grain size is as large as 10mm for the
Mitsubishi polycrystalline cells. Manufacturers
and originating laboratories are indicated in the
figure.
bonds, which allows the nanocrystalline
films have
the advantage that lower processing temperatures may suf
ce, making fabrication
easier.
Recombination reduces output voltage attainable from a solar cell. Recombination
is characterized by a velocity that may vary from 10
7
films to be viable. Nanocrystalline
to 1000 cm/s, as shown in
Figure 6.9 [73b].
Thin-
lm solar cells may also be made of organic compounds and polymers, as
suggested in Figure 6.10b. Such cells are more easily made
flexible and are
potentially much cheaper. They have historically had low ef
ciency and, further-
more, usually have short lifetimes against degradation by the suns rays. As we will
see, several types of organic solar cells, including tandem cells, are well advanced
in technology and have ef
ciencies approaching 10%. A basic problem with
organics is that the electron and hole mobilities are low, so the materials are of
low electrical conductivity. Sometimes, the polymers are loaded with conductive
molecules such as C
60
to improve the electrical conductivity. Such additions add to
the cost.
Major opportunity for supplying competitive electric power from sunlight seems
centered on thin-film devices, which are appropriate to reach a lower cost. (This
conventional assumption may be challenged by the combination of large concen-
trating mirrors with extremely high-ef
ciency multiple junction tandem cells, as will
be discussed later.) Thin
lms usemuch lessmaterial, and polycrystalline silicon cells
are well known. (Polymer and dye-sensitized cells are inexpensive but seem to be
limited in ef
ciency, as are amorphous silicon cells, at about 10%.) We here focus on
perceived opportunities to reach a largemarket.We describe next a class of device that
is capable of mass production.
