FREE ESSAY ON SOLAR POWER

SOLAR POWER

Solar Energy
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 ?m. 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.