How solar energy transform to electricity

[Angela]

In 1839, French scientist Edmund Becquerel discovered that certain
materials would give off a spark of electricity when struck with
sunlight. This photoelectric effect was used in primitive solar cells
made of selenium in the late 1800s. In the 1950s, scientists at Bell
Labs revisited the technology and, using silicon, produced solar cells
that could convert four percent of the energy in sunlight directly to
electricity.

When light shines on the panel it creates an electric field across
layers of silicon in the cell, causing electricity to flow. The
greater the intensity of the light, the greater the flow of
electricity. Photovoltaic energy is the conversion of sunlight into
electricity usually made from silicon alloys.

Sunlight is composed of photons, or particles of solar energy. These
photons contain various amounts of energy corresponding to the
different wavelengths of the solar spectrum. When photons strike a
photovoltaic cell, they may be reflected, pass right through, or be
absorbed. Only the absorbed photons provide energy to generate
electricity. When enough sunlight is absorbed by the material (a
semiconductor), electrons are dislodged from the material's atoms.

The most important components of a PV cell are two layers of
semiconductor material generally composed of silicon crystals. On its
own, crystallized silicon is not a very good conductor of electricity,
but when impurities are intentionally added—a process called doping—
the stage is set for creating an electric current. The bottom layer of
the PV cell is usually doped with boron, which bonds with the silicon
to facilitate a positive charge (P). The top layer is doped with
phosphorus, which bonds with the silicon to facilitate a negative
charge (N).

The surface between the resulting "p-type" and "n-type" semiconductors
is called the P-N junction. Electron movement at this surface produces
an electric field that only allows electrons to flow from the p-type
layer to the n-type layer.

When the electrons leave their position, holes are formed. When many
electrons, each carrying a negative charge, travel toward the front
surface of the cell, the resulting imbalance of charge between the
cell's front and back surfaces creates a voltage potential like the
negative and positive terminals of a battery. When the two surfaces
are connected through an external load, electricity flows.

When sunlight enters the cell, its energy knocks electrons loose in
both layers. Because of the opposite charges of the layers, the
electrons want to flow from the n-type layer to the p-type layer, but
the electric field at the P-N junction prevents this from happening.
The presence of an external circuit, however, provides the necessary
path for electrons in the n-type layer to travel to the p-type layer.
Extremely thin wires running along the top of the n-type layer provide
this external circuit.

The photovoltaic cell is the basic building block of a photovoltaic
system. Individual cells can vary in size from about 1 centimeter
(1/2 inch) to about 10 centimeter (4 inches) across. However, one
cell only produces 1 or 2 watts, which isn't enough power for most
applications. To increase power output, cells are electrically
connected into a packaged weather-tight module.

The three basic types of solar cells made from silicon are single-
crystal, polycrystalline, and amorphous.

Single-crystal cells are made in long cylinders and sliced into round
or hexagonal wafers. While this process is energy-intensive and
wasteful of materials, it produces the highest-efficiency cells—as
high as 25 percent in some laboratory tests. Because these high-
efficiency cells are more expensive, they are sometimes used in
combination with concentrators such as mirrors or lenses.
Concentrating systems can boost efficiency to almost 30 percent.
Single-crystal accounts for 29 percent of the global market for PV.
Polycrystalline cells are made of molten silicon cast into ingots or
drawn into sheets, then sliced into squares. While production costs
are lower, the efficiency of the cells is lower too—around 15 percent.
Because the cells are square, they can be packed more closely
together. Polycrystalline cells make up 62 percent of the global PV
market.
Amorphous silicon (a-Si) is a radically different approach. Silicon is
essentially sprayed onto a glass or metal surface in thin films,
making the whole module in one step. This approach is by far the least
expensive, but it results in very low efficiencies—only about five
percent.

A number of exotic materials other than silicon are under development,
such as gallium arsenide (Ga-As), copper-indium-diselenide (CuInSe2),
and cadmium-telluride (CdTe). These materials offer higher
efficiencies and other interesting properties, including the ability
to manufacture amorphous cells that are sensitive to different parts
of the light spectrum. By stacking cells into multiple layers, they
can capture more of the available light. Although a-Si accounts for
only five percent of the global market, it appears to be the most
promising for future cost reductions and growth potential.