Solar Photovoltaic (PV)

Definition:

Photovoltaic refers to a technology which uses a device (usually a solar panel) to produce free electrons when exposed to light, resulting in the production of an electric current.

Additional Photovoltaic Information:

• Photovoltaic is commonly referred to as "PV";
• Solar power is the common term for this technology;
• PV cells or solar power panels are used to harness the energy from the sun;
• A PV device is a semi-conductor cell which, when exposed to sunlight, converts the suns rays into an electrical current.

General Information

• Photovoltaic technology has been around for many years, the definition you see above relates to the use of solar power or energy in a modern day environment.
• The reason why many of the solar power plants across the world are efficient, lies in the advances of PV or "photovoltaic" technologies over the years.
• PV energy is set for bigger and better things in the future, and we should all attempt to benefit from this reliable technology if our surrounding environment is suitable.
• Some areas of the world are not able to benefit from photovoltaic energy due to the climate, weather patterns, or high levels of pollution, but many areas have seen outstanding efficiency from the use of solar cells.
• Solar technology or PV energy does not release any greenhouse gases into our atmosphere when in use, as the process absorbs light energy as opposed to the burning of a fuel in order to generate the output of electricity.
• The sun is the only fuel which we need to make efficient use of the available photovoltaic technologies, and as it is already burning, why don't we take advantage of this.
• It makes clear sense to take advantage of an energy source which is clean, free and available to a large portion of the globe.
• Not only will this allow us the opportunity to have a cleaner environment, it allows us to become less dependent on the ever decreasing levels of fossil fuels.

How do Photovoltaics Work?

Photovoltaics is the direct conversion of light into electricity at the atomic level. Some materials exhibit a property known as the photoelectric effect that causes them to absorb photons of light and release electrons. When these free electrons are captured, an electric current results that can be used as electricity.

The photoelectric effect was first noted by a French physicist, Edmund Bequerel, in 1839, who found that certain materials would produce small amounts of electric current when exposed to light. In 1905, Albert Einstein described the nature of light and the photoelectric effect on which photovoltaic technology is based, for which he later won a Nobel prize in physics. The first photovoltaic module was built by Bell Laboratories in 1954. It was billed as a solar battery and was mostly just a curiosity as it was too expensive to gain widespread use. In the 1960s, the space industry began to make the first serious use of the technology to provide power aboard spacecraft. Through the space programs, the technology advanced, its reliability was established, and the cost began to decline. During the energy crisis in the 1970s, photovoltaic technology gained recognition as a source of power for non-space applications.

The diagram above illustrates the operation of a basic photovoltaic cell, also called a solar cell. Solar cells are made of the same kinds of semiconductor materials, such as silicon, used in the microelectronics industry. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. When light energy strikes the solar cell, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of an electric current -- that is, electricity. This electricity can then be used to power a load, such as a light or a tool.

A number of solar cells electrically connected to each other and mounted in a support structure or frame is called a photovoltaic module. Modules are designed to supply electricity at a certain voltage, such as a common 12 volts system. The current produced is directly dependent on how much light strikes the module.

Multiple modules can be wired together to form an array. In general, the larger the area of a module or array, the more electricity that will be produced. Photovoltaic modules and arrays produce direct-current (dc) electricity. They can be connected in both series and parallel electrical arrangements to produce any required voltage and current combination.

Today's most common PV devices use a single junction, or interface, to create an electric field within a semiconductor such as a PV cell. In a single-junction PV cell, only photons whose energy is equal to or greater than the band gap of the cell material can free an electron for an electric circuit. In other words, the photovoltaic response of single-junction cells is limited to the portion of the sun's spectrum whose energy is above the band gap of the absorbing material, and lower-energy photons are not used.

One way to get around this limitation is to use two (or more) different cells, with more than one band gap and more than one junction, to generate a voltage. These are referred to as "multijunction" cells (also called "cascade" or "tandem" cells). Multijunction devices can achieve a higher total conversion efficiency because they can convert more of the energy spectrum of light to electricity.

As shown below, a multijunction device is a stack of individual single-junction cells in descending order of band gap (Eg). The top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by lower-band-gap cells.
 

Much of today's research in multijunction cells focuses on gallium arsenide as one (or all) of the component cells. Such cells have reached efficiencies of around 35% under

concentrated sunlight. Other materials studied for multijunction devices have been amorphous silicon and copper indium diselenide.

As an example, the multijunction device above uses a top cell of gallium indium phosphide, "a tunnel junction," to aid the flow of electrons between the cells, and a bottom cell of gallium arsenide.

Sources:

Clean Energy Ideas - http://www.clean-energy-ideas.com/articles/photovoltaic_definition.html

Gil Karnier - Science @ NASA - http://science.nasa.gov/headlines/y2002/solarcells.htm

Images courtesy of their respective owners.