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2 When light is absorbed in the semiconductor, a negatively charged electron and positively charged hole are created. The heart of the solar cell is the electrical junction which separates these electrons and holes from one another after they are created by the light. An electrical junction may be formed by the contact of: a metal to a semiconductor (this junction is called a Schottky barrier); a liquid to a semicon- ductor to form a photoelectrochemical cell; or two semicon- ductor regions (called a pn junction). The fundamental principles of the electrical junction can be illustrated with the silicon pn junction. Pure silicon to which a trace amount of a fifth-column element such as phos- phorus has been added is an n-type semiconductor, where electric current is carried by free electrons. Each phospho- rus atom contributes one free electron, leaving behind the phosphorus atom bound to the crystal structure with a unit positive charge. Similarly, pure silicon to which a trace amount of a column-three element such as boron has been added is a p-type semiconductor, where the electric current is carried by free holes. Each boron atom contributes one hole, leaving behind the boron atom with a unit negative charge. The interface between the p- and n-type silicon is called the pn junction. The fixed charges at the interface due to the bound boron and phosphorus atoms create a per- manent dipole charge layer with a high electric field. When photons of light energy from the Sun produce electron-hole pairs near the junction, the built-in electric field forces the holes to the p side and the electrons to the n side (Fig. 2). This displacement of free charges results in a voltage differ- ence between the two regions of the crystal, the p region being plus and the n region minus. When a load is connect- ed at the terminals, electron current flows in the direction of the arrow, and electrical power is available at the load. Applications Although the photovoltaic effect was discovered by French scientist Antoine César Becquerel in 1839 CE, practical solar cells made of silicon crystals were not developed until 1955. Beginning with Vanguard 1, launched in 1958, silicon solar cell arrays have become the almost exclusive power source for satellites. Solar cell arrays were initially used primarily to power small remote electrical loads that would otherwise be impractical or uneconomical to power by conventional means such as storage batteries or motor-generator sets. As the costs of solar cells fell dramatically and their efficiencies improved, solar cells were scaled up as large arrays that power facilities Solar Cell (continued) Fig. 2: Cross-sectional view of a silicon pn junction solar cell, illustrating the creation of electron pairs by photons of light energy from the Sun. 0.0001" = 2.5 µm; 0.04" = 1 mm; 1.0" = 25 mm. Fig. 3: Rooftop solar panel installation. (Credit: iStock.) or feed into a utility grid. The first of these so-called solar parks or farms at megawatt-scale were built in the 1980s in the Unit- ed States. For residential solar energy systems, PV modules and arrays can be mounted on a structural surface, such as a rooftop and faced toward the Sun (Fig. 3). When powering loads that require alternating current (ac) voltage, a power inverter is used to convert the direct current (dc) voltage from the solar cell ar- ray into usable ac power. + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com This article was originally published by McGraw Hill's AccessScience. Click here to view and find more articles like this.