Page 59 - Alternative Energy Systems in Building Design

36 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

out of the P-type and into the N-type material, they lose energy while moving or
circling through an external circuit. Eventually, when they enter back into the P-type
material, they recombine with the valence-band hole (at lower-energy potential) they
left behind, therefore permitting sunlight energy to be converted into electric current.
Some of the disadvantages of the conventional solar power technology discussed
earlier include a large bandgap difference in energy between the valence and conduc-
tion bands. The bandgap creates a situation in which only photons with that amount
of energy can overcome the bandgap, the potential difference that contributed to pro-
ducing a current.
Another shortcoming of conventional semiconductor-based solar power technology
is that higher-energy photons, at the blue and violet ends of the solar spectrum, have
more than enough energy to cross the bandgap. Even though a small fraction of this
energy is transferred into the electrons, a much larger portion of it becomes wasted as
heat, reducing cell efficiency owing to ohmic drop.
Another issue is that in order to have a reasonable chance of capturing a photon in
the P-type layer, it has to be fairly thick, which, in turn, promotes the recombination
of electrons and holes within the gap material before reaching the PN junction. These
limitations result in an upper limit on the efficiency of silicon solar cells, which at
present is about 12–15 percent for production-type solar cells and somewhat closer to
40 percent under ideal laboratory test conditions.
Besides these physical impediments, the most important disadvantage of semicon-
ductor solar cells is their production cost because production requires a thick layer of
silicon in order to have reasonable photon capture rates. The silicon necessary to
accomplish this is becoming rare and thus is a very expensive commodity.
Some of the measures undertaken to reduce the use of semiconductor-based solar cells
have resulted in the development of thin-film approaches, which, as discussed earlier,
involve the use of P- and N-type semiconductor paste in a type of lithographic printing
process. At present, owing to the loss of electrons and molecular decomposition, film-
based solar cell technologies have had limited applications.
Another design approach that shows great promise for improvement in efficiency is
the multijunction approach. This process involves stacking several layers of junctions
of solar cells that capture a much wider spectrum of solar energy. However, at present,
this type of cell is very expensive to produce and will be marketed mainly for large
commercial applications.

BASIC PRINCIPLES OF DSCs
DSCs essentially separate the two functions provided by silicon in a conventional
semiconductor type of solar cell. Under normal conditions, the silicon in a semicon-
ductor cell both acts as the source of photoelectrons and also forms the potential barrier
that allows for the separation of charges that creates current.
In contrast, in the DSC, the semiconductor is used for charge separation only; the
photoelectrons are provided from a separate photosensitive dye. Furthermore, charge
separation is not provided only by the semiconductor but also works in concert with a
third element of the cell, an electrolyte, that is kept in contact with both the semicon-
ductor and the dye.