How do Photovoltaic Cells Work

While you might be familiar with solar powered devices like solar calculators or even homes that use solar panels on the roof, have you ever wondered how do photovoltaic cells work?

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Photovoltaic cells, also known as PV cells, are the scientific term used for solar panels. They are basically large, flat devices that collect solar radiation and convert that energy to electrical energy. While the potential for photovoltaic technology to power the entire world for free remains a very real possibility, the technology still isn't advanced enough for that.

The easiest way to understand how do photovoltaic cells work is through reviewing how these devices were invented, and how the technology has advanced throughout the history of solar power. A French physicist by the name of Edmund Bequerel discovered the photoelectric effect in 1839 when he discovered that when specific materials (in this case, two metal electrodes) were exposed to sunlight (or any light), their atoms would release electrons. That release of electrons is what produces electricity. After his discovery, physicists soon discovered the photovoltaic effects of selenium, and in 1904 Einstein published a paper describing the photoelectric effect that later won him the Nobel prize in 1923.

The following timeline describes the progression of photovoltaic technology based on what physicists call the "photoelectric effect" of various elements.

• In 1954, Bell Laboratories produced the first photovoltaic device. It was the worlds first "solar battery" using silicon solar cells, but the materials used and the manufacturing procedures required to build it made the device far too expensive to be practical. The "efficiency," or the amount of solar energy the solar cell could convert to equivalent electricity, was only about 6 percent by the end of the year.
• By 1958, Hoffman Electronics produced photovoltaic cells that were 9 percent efficient, and then the following year those cells were used to power Vanguard I, the first satellite to use PV technology for power.
• For the next ten years the PV technology, running at about 14 percent efficiency, were used in the space industry to power almost all satellite systems.
• Throughout the 1970s and 1980s, both government and private industry produced a number of solar arrays using photovoltaic technology, such as NASA's 3.5 kilowatt (kW) system on the Papago Indian Reservation in Arizona in 1979 and the 90.4 kW system used to power Beverly High School in Beverly, Massachusetts in 1981. By 1985, solar cells reached 20 percent efficiency.

Photovoltaic cells, also known as PV or solar cells, are created from semiconductor material such as silicon. The process of solar conversion works as follows:

1. When light strikes the cell, a specific portion or "band" of the light wave is absorbed by the material.
2. That solar energy causes the semiconductor material to release electrons.
3. The semiconductor material is positioned within an electrical field by using negatively and positively charged silicon (n-type and p-type), so that all electrons set "free" from the material are forced to flow - generating electrical current.

The simplest, early photovoltaic cells were built using the configuration described above, called "single-junction" cells. The problem with these early cells was that they could only convert a certain "band" or wavelength of solar radiation. That wavelength is called the "band gap" of the type of semiconductor material used to make the cell. More recently, engineers improved on the efficiency of photovoltaic cells by using several different cells with multiple band gaps, known as "multijunction" cells. By taking this approach, cells are stacked so that as light passes through the cells, the lower solar cells convert the band of light that the higher cells can't - increasing the efficiency of the entire photovoltaic cell tremendously.

The potential uses for photovoltaic cells throughout the world in order to convert the world to a more "green" existence are limited only by the size and efficiency of the technology. In 1997, the General Motors "Sunracer" vehicle won the Pentax World Solar Challenge with a speed of 71 km/h, about 41 miles per hour. Modern PV technology is called "thin-film silicon photovoltaics," and most experts within alternative energy industries consider it as the fastest growing energy technology. Thin-film PV cells include:

• Amorphous Silicon (a-Si)
• Cadmium Telluride (CdTe)
• Copper Indium Diselenide/Copper Indium Gallium Diselenide (CIS/CIGS)
• Organic materials

While thin-film PV technology is preferred because they take up far less space, their efficiency remains at 20 percent, while non thin-film photovoltaic technology, such as crystalline silicon, was last achieved at 42.8 percent by the researchers at the University of Delaware in 2007. The Defense Advanced Research Projects Agency (DARPA) has set a milestone for solar technology to reach 50 percent efficiency.

Even through the use of multijunction cells and the most advanced semiconductor materials, solar cells have only advanced from 20 percent efficiency in the 1980s to just under 50 percent efficiency in 2008. Creating commercial products with small, practical and efficient solar panels remains a significant industry challenge. However, as semiconductor technologies advance, and as scientists come up with additional unique methods of boosting solar cell efficiency, a world powered mostly by the sun's energy is quickly becoming a very real possibility.

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