Page 35 - Solar Power in Building Design The Engineer's Complete Design Resource

SOLAR CELL PHYSICS 5

use of power; second, there are no conversion losses resulting from the use of dc power
which allows maximum harvest of solar irradiance energy potential.

Solar Cell Physics

Most solar cells are constructed from semiconductor material, such as silicon (the
fourteenth element in the Mendeleyev table of elements). Silicon is a semiconductor
that has the combined properties of a conductor and an insulator.
Metals such as gold, copper, and iron are conductors; they have loosely bound
electrons in the outer shell or orbit of their atomic configuration. These electrons can
be detached when subjected to an electric voltage or current. On the contrary, atoms
of insulators, such as glass, have very strongly bonded electrons in the atomic config-
uration and do not allow the flow of electrons even under the severest application of
voltage or current. Semiconductor materials, on the other hand, bind electrons midway
between that of metals and insulators.
Semiconductor elements used in electronics are constructed by fusing two adja-
cently doped silicon wafer elements. Doping implies impregnation of silicon by posi-
tive and negative agents, such as phosphor and boron. Phosphor creates a free electron
that produces so-called N-type material. Boron creates a “hole,” or a shortage of an
electron, which produces so-called P-type material. Impregnation is accomplished by
depositing the previously referenced dopants on the surface of silicon using a certain
heating or chemical process. The N-type material has a propensity to lose electrons
and gain holes, so it acquires a positive charge. The P-type material has a propensity
to lose holes and gain electrons, so it acquires a negative charge.
When N-type and P-type doped silicon wafers are fused together, they form a PN
junction. The negative charge on P-type material prevents electrons from crossing the
junction, and the positive charge on the N-type material prevents holes from crossing
the junction. A space created by the P and N, or PN, wafers creates a potential barrier
across the junction.
This PN junction, which forms the basic block of most electronic components, such
as diodes and transistors, has the following specific operational uses when applied in
electronics:

In diodes, a PN device allows for the flow of electrons and, therefore, current in one
direction. For example, a battery, with direct current, connected across a diode
allows the flow of current from positive to negative leads. When an alternating sinu-
soidal current is connected across the device, only the positive portion of the wave-
form is allowed to pass through. The negative portion of the waveform is blocked.
In transistors, a wire secured in a sandwich of a PNP-junction device (formed by
three doped junctions), when properly polarized or biased, controls the amount of
direct current from the positive to the negative lead, thus forming the basis for cur-
rent control, switching, and amplification, as shown in Figure 1.2.