One photon of sufficient energy (or frequency) must hit an electron for it to be ejected multiple photons of lesser energy cannot combine to eject an electron. That is, the amount of energy required to excite an electron enough to exit the material is a discrete value. The quantization of light into photons also illuminates another important point: the quantization of atomic energy levels. This is actually the reason that they travel at the fastest speed to have momentum, massless particles must have infinite velocity, while massive particles can never travel as fast as light! Photons, unlike particles such as electrons, are massless. The energy of a photon is given by the following equation, where E is the energy of the photon, f is the light’s frequency and h is Planck’s constant: This also explains why the brightness of the light has no effect. This explains why the blue light (higher frequency) ejects electrons, while the red light (lower frequency) does not. These light particles, called photons, are discrete quanta of light energy, whose energy is associated with their frequency (color) instead of their amplitude (the number present). The top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by lower-band-gap cells.\( \newcommand\): : Shining blue light on the negatively-charged plate causes the ejection ofĮlectrons, but the red light doesn't. 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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