Crystalline Silicon Solar Cells

Front page headlines in the New York Times and the Wall Street Journal in 1954 heralded to the world the demonstration of the first reasonably efficient solar cells, an event made possible by the rapid development of crystalline silicon technology for miniaturised electronics. Since that time, the majority of solar cells fabricated to date have been based on silicon in monocrystalline or large-grained polycrystalline form. There are two main reasons for this. One is that silicon is an elemental semiconductor with good stability and a well-balanced set of electronic, physical and chemical properties, the same set of strengths that have made silicon the preferred material for microelectronics. The second reason why silicon cells have been so dominant is that the success of silicon in microelectronics has created an enormous industry where the economies of scale directly benefit the presently smaller photovoltaics industry. Most silicon cells have been fabricated using thin wafers cut from large cylindrical monocrystalline ingots prepared by the exacting Czochralski (CZ) crystal growth process and doped to about one part per million with boron during ingot growth. A smaller but significant number use what are referred to as 'multicrystalline' wafers sliced from ingots prepared by a simpler casting (or, more generally, directional solid-ification) technique, which produces large-grained polycrystalline ingots. To produce a cell, these boron-doped starting wafers generally have phosphorus diffused at high temperatures a fraction of a micron into the surface to form the p-n junction required. Metal contacts to both the n-and the p-type side of the junction are formed by screen printing a metal paste that is then densified by firing at high temperature. Each cell is typically 10–15 cm either in diameter or along either side if square or rectangular.