UNDERSTANDING SOLAR CELLS
THE BASICS
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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 power basics). 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.
Thin films 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.