What is Photovoltaics?
Photovoltaics is a high-technology approach to converting sunlight
directly into electrical energy. The electricity is direct current
and can be used that way, converted to alternating current or stored
for later use.
Conceptually, in its simplest form a photovoltaic device is a
solar-powered battery whose only consumable is the light that fuels
it. There are no moving parts; operation is environmentally benign;
and if the device is correctly encapsulated against the environment,
there is nothing to wear out.[2] Because sunlight is universally
available, photovoltaic devices have many additional benefits that
make them usable and acceptable to all inhabitants of our planet.
Photovoltaic systems are modular, and so their electrical power
output can be engineered for virtually any application, from
low-powered consumer uses-wristwatches, calculators and small
battery chargers-to energy-significant requirements such as
generating power at electric utility central stations (see figure
1). Moreover, incremental power additions are easily accommodated in
photovoltaic systems, unlike more conventional approaches such as
fossil or nuclear fuel, which require multimegawatt plants to be
economically feasible.
To understand the many facets of photovoltaic power, one must
understand the fundamentals of how the devices work. Although
photovoltaic cells come in a variety of forms, the most common
structure is a semiconductor material into which a large-area diode,
or p-n junction, has been formed. The fabrication processes tend to
be traditional semiconductor approaches-diffusion, ion implantation
and so on. Electrical current is taken from the device through a
grid contact structure on the front that allows the sunlight to
enter the solar cell, a contact on the back that completes the
circuit, and an antireflection coating that minimizes the amount of
sunlight reflecting from the device. Figure 2 is a schematic
depiction of a rudimentary solar cell that shows the important
features.
The fabrication of the p-n junction is key to successful operation
of the photovoltaic device (as well as other important semiconductor
devices). We will assume that the semiconductor material is
single-crystal silicon. Although photovoltaic technologists today
use many other varieties of semiconductors, crystalline-silicon
concepts represent a reasonable compromise for this discussion
because they are well known and understood by physics students.
Silicon is representative of the diamond crystal structure. Each
atom is covalently bonded to each of its four nearest neighbors;
that is, each silicon atom shares its four valence electronic with
the four neighboring atoms, forming four covalent bonds. Silicon has
atomic number 14, and the configuration of its 14 electrons is
1s22s22p63s23p2. The core electrons, 1s2, 2s2 and 2p6, are very
tightly bound to the nucleus and, at real-world temperatures, do not
contribute to the electrical conductivity. At absolute zero, as N
silicon atoms are brought together to form the solid, two distinct
energy bands are formed-the lower, "valence" band and the upper,
"conduction" band. The valence band has 4N availability energy
states and 4N valence electrons and is therefore filled. Conversely,
the conduction band is completely empty at absolute zero. Thus the
semiconductor is a perfect insulator at absolute zero.