Photovoltaic cells or solar cells generate a voltage when radiant energy falls on the boundary between dissimilar substances.  The photovoltaic process converts the energy of the sun directly into electricity using solar cells. This conversion is called the photovoltaic effect. The light hitting the solar cell produces both voltage and current to generate electrical power. 

How photovoltaic cells work. Image Credit: inventors.about.com

Solar cells must be built with special materials, usually silicon crystal, in which the absorption of light releases electrons. A solar panel, also called a photovoltaic panel, is a group of photovoltaic cells that are enclosed to keep the cells safe and so that the voltage obtained from each cell can be combined. They are usually made using semiconductor materials in the form of a p-n junction (a typical diode).

Semiconductors are differentiated by the band gap energy and the band gap type. Band gap energy is the energy needed to allow an electron in an atom's shell to break away from the atom and flow freely in the material.  The higher the band gap energy the higher the energy of light required to release an electron so it conducts current. If the band gap is too high then few photons have enough energy, which results in low current and low power. If the band gap energy is too low then all the photons produce the same low voltage which also leads to low power.

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There are two types of band gap: direct and indirect. Direct band gap has higher efficiency in creating free electrons since only a photon is required to release an electron. This type is also thinner than indirect band gap material. Indirect band gap requires a small amount of kinetic energy that cannot come from a photon, but comes from the momentum of other particles. It is less efficient and therefore requires thicker material.

Photovoltaic Cell Materials

Solar cell materials are generally group IV elements on the periodic table (titanium, zirconium, hafnium, rutherfordium).

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• Silicon is the second most abundant element in the Earth's crust and it is therefore much cheaper than other semiconductor materials. Since it links well with the energy of visible sunlight it is the dominant material used for solar cells.
  • Advantages: Abundant and inexpensive to process, non-toxic, and used in electronics so it is well studied and understood.
  • Disadvantages: weak absorber of light and a band gap lower than ideals for solar spectrum. Silicon is indirect, but it is much cheaper than other elements.
  • Examples include: Amorphous silicon, monocrystalline silicon, polycrystalline silicon, ribbon silicon.
• Germanium is widely used in electronics and was the dominant material in the early market. Now it is being combined with silicon.
  • Advantages: Germanium has low impurities that can reduce cell output and can be used in amorphous and crystalline forms.
  • Disadvantage: Poor semiconductor property of indirect band gap; band gap is too small for high efficiency cells.
• Gallium arsenide cells are multifunction cells that are more efficient than silicon but have a high cost and toxicity.
  • Advantages: Electrons have longer lifetime and generates current easily. It is more efficient due to direct band gap.
  • Disadvantages: Gallium arsenide has no natural insulating layer to prevent impurities from shorting the cell. It is also expensive.
• Selenium is expensive and difficult to obtain and therefore too expensive for commercial use. It is well-suited to the solar spectrum since most photons have sufficient energy and few have too much.

Silicon Photovoltaic Cells

There are three basic types of photovoltaic cells: mono-crystalline cells, polycrystalline cells, and amorphous cells. Crystalline silicon is the most common material for commercial applications. It has a well-known standard process because silicon is abundant. However, it requires expensive highly pure silicon and competes for silicon with electronics industry.

• A mono-crystalline cell is the most efficient, but also the most costly photovoltaic cell.
• A polycrystalline cell is more common and less expensive, but also less efficient.
• An amorphous cell is relatively inexpensive, but produces much less power. As such, the solar panels made from these photovoltaic cells must be larger to produce the same amount of energy. A UV-sensitive photo cell converts UV radiation to electrical energy.

Photovoltaic Cell Specifications

A photovoltaic system contains individual solar panels that convert the solar energy into usable direct current (DC) electricity that can then be distributed through an inverter to the electric grid or the utility panels at industrial sites or even in houses. Photovoltaic cells are generally connected to form solar panels. Solar panels can also be combined to produce currents used in a variety of applications. A photovoltaic system is composed of a cell, panel, and array.

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Specifications include:

• Power - The output power of the solar cell.
• Efficiency - The efficiency of the solar cell.
• Open circuit voltage - The open circuit voltage is the maximum voltage of the cell when the device is under infinite load, or in an open-circuit situation.
• Short circuit current- The short circuit current is the maximum current when the cell is under zero load.
• Peak voltage - The maximum voltage produced by the panel or cell.
• Peak current - The maximum current produced by the panel or cell.

 In order to design a photovoltaic system there are several steps that must be considered. The total load current and operational time should be determined and then system losses can be added. Load refers to all the devices in the system that consume electrical power. Additional design rules include determining the solar irradiation in daily equivalent sun hours, the total solar array current requirements, optimum module arrangement for the solar array, and battery size for recommended reserve time.

Applications

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Photovoltaic power is reliable, creates no pollution, and can be quickly installed. A photovoltaic cell manufacturer or a solar cell manufacturer can produce this type of cell for many applications, ranging from calculators to satellites to telephones and vehicles. The expected lifetime for photovoltaic cells can be up to 40 years. The time to recoup the cost of a solar panel averages five years, but can range between one to thirty years depending on the type of solar panel and its larger photovoltaic system.

Photovoltaic Standards

Resources

Northeast Advanced Technological Education Center - Abe Michelen