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Single junction solar cells The most common structure is a semiconductor material into which a large-area diode or p-n junction has been formed The fabrication processes tend to be traditional semiconductor approaches diffusion ion implantation and so on Electrical current is taken from the device through a grid contact structure on the front that allows the sunlight to enter the solar cell a contact on the back that completes the circuit and an anti-reflection coating that minimises the amount of sunlight reflecting from the device Figure 2 is a schematic depiction of a rudimentary solar cell that shows the important features The fabrication of the p-n junction is key to successful operation of the photovoltaic device as well as other important semiconductor devices We will assume that the semiconductor material is single-crystal silicon Although photovoltaic technologists today use many other varieties of semiconductors crystalline-silicon concepts represent a reasonable compromise for this discussion because they are well known and understood by physics students Silicon is representative of the diamond crystal structure Each atom is covalently bonded to each of its four nearest neighbors that is each silicon atom shares its four valence electronic with the four neighboring atoms forming four covalent bonds Silicon has atomic number 14 and the configuration of its 14 electrons is 1s22s22p63s23p2 The core electrons 1s2 2s2 and 2p6 are very tightly bound to the nucleus and at real-world temperatures do not contribute to the electrical conductivity At absolute zero as N silicon atoms are brought together to form the solid two distinct energy bands are formed-the lower valence band and the upper conduction band The valence band has 4N availability energy states and 4N valence electrons and is therefore filled Conversely the conduction band is completely empty at absolute zero Thus the semiconductor is a perfect insulator at absolute zero As the temperature of the solid is raised above absolute zero energy is transferred to the
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