![]() ![]() "They are all needed to monolithically integrate the three power-generating junctions and to ensure that the maximum amount of light enters the solar cell and is not reflected at the surface. "In practice each triple-junction solar cell has about 30 individual film layers," says Mr Gerlach. The resulting leap in efficiency from single to triple junction cells is on the order of ten percentage points, although designing and building them is hard. Forming a single overall structure, these layers are stacked in series based on their sensitivity: the topmost junction reacts to highest-energy blue light while letting other light through the middle layer absorbs medium-energy green light then the bottom layer makes its contribution from the remaining lowest-energy red light. These allow the placing of multiple junctions on top of one another, each tuned to different segments of the overall light spectrum. The top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by lower-band-gap cells.Silicon was once the semiconductor of choice but it is increasingly supplanted by gallium arsenide (GaAs) –based semiconductors which reach higher power levels. Multijunction devices can achieve a higher total conversion efficiency because they can convert more of the energy spectrum of light to electricity.Īs shown below, a multijunction device is a stack of individual single-junction cells in descending order of band gap (Eg). These are referred to as "multijunction" cells (also called "cascade" or "tandem" cells). 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. 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. 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. 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. The current produced is directly dependent on how much light strikes the module. Modules are designed to supply electricity at a certain voltage, such as a common 12 volts system. This electricity can then be used to power a load, such as a light or a tool.Ī number of solar cells electrically connected to each other and mounted in a support structure or frame is called a photovoltaic module. 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. When light energy strikes the solar cell, electrons are knocked loose from the atoms in the semiconductor material. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. Solar cells are made of the same kinds of semiconductor materials, such as silicon, used in the microelectronics industry. The diagram above illustrates the operation of a basic photovoltaic cell, also called a solar cell. During the energy crisis in the 1970s, photovoltaic technology gained recognition as a source of power for non-space applications. Through the space programs, the technology advanced, its reliability was established, and the cost began to decline. In the 1960s, the space industry began to make the first serious use of the technology to provide power aboard spacecraft. It was billed as a solar battery and was mostly just a curiosity as it was too expensive to gain widespread use. The first photovoltaic module was built by Bell Laboratories in 1954. 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 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. When these free electrons are captured, an electric current results that can be used as electricity. Some materials exhibit a property known as the photoelectric effect that causes them to absorb photons of light and release electrons. Photovoltaics is the direct conversion of light into electricity at the atomic level. Back to the story "The Edge of Sunshine" What is Photovoltaics? ![]()
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