Environmental Engineering Reference
In-Depth Information
8
Third-Generation Concepts, Survey of Efficiency
What can be done to make solar cells more ef cient? We have already described the
main classes of solar cells that are now available. These can be characterized in terms
of the number of distinct energy gaps, according to the physical form, and according
to the use for single sun illumination versus concentrated illumination.
New conceptual approaches, which may become available in practice, may lead to
higher ef ciency and lower cost. These were suggested in Figure 5.6 in the category
advanced thin lms. The agreed criterion for third generation designation is that
the ef ciency exceeds that for the single-junction cell, about 30%. At present, the only
cells to achieve third-generation ef ciency are, in fact, single-crystal tandem cells
and spatially dispersed multijunction cells, at most
41%. The tandem cell is a
proven device, as we saw in Chapter 7, but at high demonstrated ef ciency it has been
available neither in thin- lm form nor at low cost.
The rst new concept that may alter this situation is the intermediate band cell.
The second is some form of carrier multiplication, as a means of harvesting in the
external circuit the photon energy in excess of the bandgap.
8.1
Intermediate Band Cells
Ef ciency comes fromutilizing a larger portion of the solar spectrum. The idea of the
intermediate band cell is suggested by the improvement experimentally realized in
the two-bandgap tandem cell. The generalization is that some homogeneous semi-
conductor might be found to have three characteristic energies, E C , E V , and E IB ,so
that the differences among these energies could represent two or three separate gaps,
possibly acting to intercept a larger fraction of the solar spectrum.
A pioneering calculation along these lines is summarized in Figure 8.1, based on a
hypothetical semiconductor as illustrated in Figure 8.2. (We will see in Section 8.3 that
dilute magnetic semiconductors may be workable examples of such a band structure.)
The concept of the intermediate band semiconductor is shown in Figure 8.2.
The Fermi level lies in the intermediate band , which is metallic, and is partially filled.
In semiconductor physics, this situation arises in a semiconductor with a large
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