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Essay Name       : 1161.txt 
Uploader         : John Brown
Email Address    : 
Language         : English
Subject          : Chemistry
Title            : Our Solar System
Grade            : 98%
School System    : Los Angeles
Country          : U.S.A
Author Comments  : 12th grade essay on chemical bonds in our solar system.
Teacher Comments : Well Researched!
Date             : 11-17-96
Site found at    : Browsing the web.
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     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 and semiconductors there is an "energy gap", in which no electron orbits can exist,
between the inner valence band and outer conduction band [Book 1]. Valence electrons help to
bind together the atoms in a solid by orbiting 2 adjacent nucleii, while conduction electrons,
being less closely bound to the nucleii, are free to move in response to an applied voltage or
electric field. The fewer conduction electrons there are, the higher the electrical resistivity of
the material.
     In semiconductors, the materials from which solar sells are made, the energy gap Eg is
fairly small. Because of this, electrons in the valence band can easily be made to jump to the
conduction band by the injection of energy, either in the form of heat or light [Book 4]. This
explains why the high resistivity of semiconductors decreases as the temperature is raised or the
material illuminated. The excitation of valence electrons to the conduction band is best
accomplished when the semiconductor is in the crystalline state, i.e. when the atoms are
arranged in a precise geometrical formation or "lattice".
     At room temperature and low illumination, pure or so-called "intrinsic" semiconductors
have a high resistivity. But the resistivity can be greatly reduced by "doping", i.e. introducing
a very small amount of impurity, of the order of one in a million atoms. There are 2 kinds of
dopant. Those which have more valence electrons that the semiconductor itself are called
"donors" and those which have fewer are termed "acceptors" [Book 2].
     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].
     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
layer of negatively-charged impurity atoms. The opposite action also takes place, holes from
acceptor atoms in the p-region crossing into the n-region, combining with electrons and
producing positively-charged impurity atoms [Book 4]. The net result of these movements is the
disappearance of conduction electrons and holes from the vicinity of the junction and the
establishment there of a reverse electric field, which is positive on the n-side and negative on
the p-side. This reverse field plays a vital part in the functioning of the device. The area in
which it is set up is called the "depletion area" or "barrier layer"[Book 4].
     When light falls on the front surface, photons with energy in excess of the energy gap
(1.1 eV in crystalline silicon) interact with valence electrons and lift them to the conduction
band. This movement leaves behind holes, so each photon is said to generate an "electron-hole
pair" [Book 2]. In the crystalline silicon, electron-hole generation takes place throughout the
thickness of the cell, in concentrations depending on the irradiance and the spectral composition
of the light. Photon energy is inversely proportional to wavelength. The highly energetic photons
in the ultra-violet and blue part of the spectrum are absorbed very near the surface, while the
less energetic longer wave photons in the red and infrared are absorbed deeper in the crystal and
further from the junction [Book 4]. Most are absorbed within a thickness of 100 æm.
     The electrons and holes diffuse through the crystal in an effort to produce an even
distribution. Some recombine after a lifetime of the order of one millisecond, neutralizing their
charges and giving up energy in the form of heat. Others reach the junction before their lifetime
has expired. There they are separated by the reverse field, the electrons being accelerated
towards the negative contact and the holes towards the positive [Book 5]. If the cell is connected
to a load, electrons will be pushed from the negative contact through the load to the positive
contact, where they will recombine with holes. This constitutes an electric current. In crystalline
silicon cells, the current generated by radiation of a particular spectral composition is directly
proportional to the irradiance [Book 2]. Some types of solar cell, however, do not exhibit this
linear relationship.
     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. 
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