Solar cells
Solar cells today are mostly made of silicon, one of the most common elements on Earth. The crystalline silicon solar cell was one of the first types to be developed and it is still the most common type in use today. They do not pollute the atmosphere and they leave behind no harmful waste products. Photovoltaic cells work effectively even in cloudy weather and unlike solar heaters, are more efficient at low temperatures. They do their job silently and there are no moving parts to wear out. It is no wonder that one marvels on how such a device would function.To understand how a solar cell works, it is necessary to go back to some basic atomic concepts. In the simplest model of the atom, electrons orbit a central nucleus, composed of protons and neutrons. each electron carries one negative charge and each proton one positive charge. Neutrons carry no charge. Every atom has the same number of electrons as there are protons, so, on the whole, it is electrically neutral. The electrons have discrete kinetic energy levels, which increase with the orbital radius. When atoms bond together to form a solid, the electron energy levels merge into bands. In electrical conductors, these bands are continuous but in insulators
In a silicon crystal, each atom has 4 valence electrons, which are shared with a neighbouring atom to form a stable tetrahedral structure. Phosphorus, which has 5 valence electrons, is a donor and causes extra electrons to appear in the conduction band. Silicon so doped is called "n-type" [Book 5]. On the other hand, boron, with a valence of 3, is an acceptor, leaving so-called "holes" in the lattice, which act like positive charges and render the silicon "p-type"[Book 5]. The drawings in Figure 1.2 are 2-dimensional representations of n- and p-type silicon crystals, in which the atomic nucleii in the lattice are indicated by circles and the bonding valence electrons are shown as lines between the atoms. Holes, like electrons, will remove under the influence of an applied voltage but, as the mechanism of their movement is valence electron substitution from atom to atom, they are less mobile than the free conduction electrons [Book 2]. The silicon solar cell has many advantages such as high reliability, photovoltaic power plants can be put up easily and quickly, photovoltaic power plants are quite modular and can respond to sudden changes in solar input which occur when clouds pass by. However there are still some major problems with them. They still cost too much for mass use and are relatively inefficient with conversion efficiencies of 20% to 30%. With time, both of these problems will be solved through mass production and new technological advances in semiconductors. In a n-on-p crystalline silicon solar cell, a shadow junction is formed by diffusing phosphorus into a boron-based base. At the junction, conduction electrons from donor atoms in the n-region diffuse into the p-region and combine with holes in acceptor atoms, producing a la
Some common words found in the essay are:
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Approximate Word count = 1183
Approximate Pages = 5 (250 words per page double spaced)
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