The word photovoltaic
is a combination of the word "photo" which is Greek for Light, and the
name of the physicist Allesandro Volta. The photovoltaic (PV) cell also
known as solar cell
is a basic electronic device that generates electricity by the
photoelectric effect when exposed to sunlight. Sometimes the term PV
cell is used in a broader sense for a device that converts light from any source (not just sun) to electricity.
HOW PHOTOVOLTAIC CELLS WORK
Although there are various types of PV cells, they all operate on the same
photovoltaic effect's basic principle. PV cells contain light absorbing
materials (mainly semiconductors), which are specially treated to
produce p-n junctions. A p-n junction
contains two regions of a material with differing types of conductivity separated by a semi-conductive junction. When light falls on the top of the cell, it
can penetrate through the junction and transfer a portion of its energy
to some electrons, which can cross the junction into the n-type region.
At the same time, the complementary positive charges called holes are
created in the p-type region where the electrons departed. This creates
a voltage of 0.5 to 0.7 volts across the junction under open-circuit
condition (see solar
). If an external circuit is connected to the cell's terminals,
the cell will produce a direct current through this circuit. As the result, electric power is extracted. When the current flows, electrons recombine with holes in the p-type region. As the current increases, the voltage across PV cells drops. The maximum current of a PV cell depends on its surface area and intensity of light radiation. In general, the larger the area, the more power can be produced.
HOW ARE PV CELLS MADE?
The photovoltaic cells contain one or more p-n junctions. The "p" material is
normally connected to a metal base that becomes a positive terminal. The topside of the
wafer with "n" material usually has a grid of electrical contacts,
which do not cover the entire face of the cell since they are not
transparent to light. The contacts connected to n-regions are
paralleled and routed to a negative terminal. Sunpower Corp. has
recently developed a technology in which electrical contacts are on the
back of the cell, which leaves the entire front surface available for
sunlight yielding higher efficiency. To reduce the percentage of the
photons that are reflected off the semiconductor material and increase
the number of photons that will become absorbed, an anti-reflective
coating is usually put on the semiconductor.
To minimize the amount of photons that just pass through the
semiconductor, some semiconductors are made with many layers, each
having a different band gaps for broader match of the light spectrum.
Solar cells are made from a wide range of semiconductor materials.
Currently the two main solar technologies are silicon (Si) wafers and
thin films. Si wafers
are cut from a bulk material. They can be single-crystalline, multi-crystalline, and amorphous.
use thin layers of photovoltaic materials deposited onto a substrate. This PV material can be polycrystalline, such as copper indium diselenide (CIS), cadmium telluride (CdTe), and thin-film Si; single-crystalline, such as gallium arsenide (GaAs), as well as organic. Crystalline silicon continues to make up more than 90% of PV panels manufactured worldwide. However, in US more then 65% of the PV systems are currently produced with thin-films.
The production PV cells for power applications have sizes up to 30x30 cm (11x11"). A typical 10x10 cm (100 cm²) silicon cell has approximately 2-4 amps short-circuit current at "standard sun" (1000 W/m²) sunlight. Microelectronics use chip-size cells as small as 2x2 millimeter. Strings of miniature cells are often placed in the conventional packages used for integrated circuits, such as SO8 package for surface mount installations on printed circuits boards.
Today's commercially available PV cells have efficiency ranging anywhere from 6 to 21%. For more information on types, pricing and efficiency of various PV technologies see this overview