Solar Power in Building Design
Author:
Peter Gevorkian, Ph.D., P.E.,
ASIN: B00121SIIM
Publisher: McGraw-Hill
Professional
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
configuration 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 adjacently doped silicon wafer elements. Doping implies
impregnation of silicon by positive 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 sinusoidal
current is connected across the device, only the positive portion of
the waveform is allowed to pass through. The negative portion of the
waveform is blocked.
(Continues...)