Environmental Engineering Reference
In-Depth Information
According to the Economist [91], arrays of multijunction concentrating solar cells
are available from Sunrgi, Inc., which are reportedly 37% ef
cient at 1600 sun
concentration and can produce electricity at 5 c/kWh. (By comparison, the price of
electricity inNewYork is 14 c/kWh.) It appears that the junctions are similar in design
to those shown in Figure 7.3. As stated, this is a very favorable cost
figure, which
awaits con
rmation.
7.1.3
Low-Cost Tandem Technology: Advanced Tandem Semiconducting Polymer Cells
The idea of a polymer solar cell was introduced in Figure 6.10, while a simple example
of such a cell was shown in Figure 6.19. Such cells are usually considered among the
least expensive to produce, and thereby are important in making solar energy more
available. An improvement in this latter type of cell, to an ef
ciency of 6%, is our next
topic [92].
The improved cell is of the tandem type, in which two cells are connected in series.
Each of the constituent cells resembles that shown in Figure 6.10, but differs in that a
network of internal heterojunctions pervades the charge-separating layers. The
overall structure is similarly built on ITO-coated conductive glass and connected
on the top with a thin metal electrode. The title of the article, Ef
cient Tandem
Polymer Solar Cells Fabricated by All-Solution Processing, makes a claim for
ef
ciency and for a low-cost process.
The materials used in these cells are characterized as semiconducting polymers
and fullerene derivatives. Typically, semiconducting polymers have a framework
of alternating single and double C
N) bonds. Delocalization of
the electrons in double bonds over the entire polymer molecule produces a
molecular bandgap in the range of 1.5
-
3 eV. Light absorption in such a material
produces an exciton or coupled electron
-
hole pair, which may diffuse but has to
be dissociated in order to get a photocurrent. It is found that ef
cient exciton
dissociation occurs if, instead of a single polymer, two separate polymers, called
donor and acceptor polymers, are in contact. (This may be a well-de
C (sometimes C
ned interface,
as in Figure 6.10 right panel, or a mixture of the polymers can create a network of
such interfaces.) The exciton is generated in the donor material and charge
separation occurs at the interface if the acceptor material has an empty energy
level that is lower than the LUMO (lowest unoccupied level) of the donor (this is
the level that is transiently occupied by a photoelectron). The exciton dissociation
gives an electron in the acceptor material that is thus separated from the hole that
remains in the donor. This charge separation gives an effective potential differ-
ence, induced by light, leading to a photovoltaic effect. In the work described [92],
the acceptor polymers are loaded with fullerene molecules to improve the
electrical conduction. The donor polymers, absorbing the light at different
wavelengthsbyvirtueofthechemicalstructure,actlikedyes.Theroleofadye
was discussed in Figures 6.16
-
6.18. Exciton propagation is important in these
materials, so that a dissociating interface can be reached, or else no external
current will result.
