Solar cells today are mostly made of silicon, one of the most commonelements on Earth.
The crystalline silicon solar cell was one of the firsttypes to be developed and it is still the most common type in use today. They donot pollute the atmosphere and they leave behind no harmful waste products.Photovoltaic cells work effectively even in cloudy weather and unlike solarheaters, are more efficient at low temperatures.
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They do their job silently andthere are no moving parts to wear out. It is no wonder that one marvels on howsuch a device would function.To understand how a solar cell works, it is necessary to go back to somebasic atomic concepts. In the simplest model of the atom, electrons orbit acentral nucleus, composed of protons and neutrons. each electron carries onenegative 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 thewhole, it is electrically neutral. The electrons have discrete kinetic energylevels, which increase with the orbital radius. When atoms bond together to forma 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 valenceband and outer conduction band Book 1.
Valence electrons help to bind togetherthe 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 anapplied voltage or electric field. The fewer conduction electrons there are, thehigher the electrical resistivity of the material.In semiconductors, the materials from which solar sells are made, theenergy gap Eg is fairly small. Because of this, electrons in the valence bandcan 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 highresistivity of semiconductors decreases as the temperature is raised or thematerial illuminated. The excitation of valence electrons to the conduction bandis 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 greatlyreduced by “doping”, i.e. introducing a very small amount of impurity, of theorder of one in a million atoms. There are 2 kinds of dopant. Those which havemore valence electrons that the semiconductor itself are called “donors” andthose which have fewer are termed “acceptors” Book 2.
In a silicon crystal, each atom has 4 valence electrons, which are sharedwith a neighbouring atom to form a stable tetrahedral structure. Phosphorus,which has 5 valence electrons, is a donor and causes extra electrons to appearin the conduction band. Silicon so doped is called “n-type” Book 5. On theother hand, boron, with a valence of 3, is an acceptor, leaving so-called”holes” in the lattice, which act like positive charges and render thesilicon”p-type”Book 5. The drawings in Figure 1.2 are 2-dimensional representationsof n- and p-type silicon crystals, in which the atomic nucleii in the latticeare indicated by circles and the bonding valence electrons are shown as linesbetween the atoms.
Holes, like electrons, will remove under the influence of anapplied voltage but, as the mechanism of their movement is valence electronsubstitution from atom to atom, they are less mobile than the free conductionelectrons Book 2.In a n-on-p crystalline silicon solar cell, a shadow junction is formed bydiffusing phosphorus into a boron-based base. At the junction, conductionelectrons from donor atoms in the n-region diffuse into the p-region and combinewith holes in acceptor atoms, producing a layer of negatively-charged impurityatoms. The opposite action also takes place, holes from acceptor atoms in the p-region crossing into the n-region, combining with electrons and producingpositively-charged impurity atoms Book 4. The net result of these movements isthe disappearance of conduction electrons and holes from the vicinity of thejunction and the establishment there of a reverse electric field, which ispositive on the n-side and negative on the p-side. This reverse field plays avital part in the functioning of the device. The area in which it is set up iscalled the “depletion area” or “barrier layer”Book 4.
When light falls on the front surface, photons with energy in excess of theenergy gap (1.1 eV in crystalline silicon) interact with valence electrons andlift them to the conduction band. This movement leaves behind holes, so eachphoton is said to generate an “electron-hole pair” Book 2. In the crystallinesilicon, electron-hole generation takes place throughout the thickness of thecell, in concentrations depending on the irradiance and the spectral compositionof the light.
Photon energy is inversely proportional to wavelength. The highlyenergetic photons in the ultra-violet and blue part of the spectrum are absorbedvery near the surface, while the less energetic longer wave photons in the redand infrared are absorbed deeper in the crystal and further from the junctionBook 4. Most are absorbed within a thickness of 100 m.The electrons and holes diffuse through the crystal in an effort to producean even distribution. Some recombine after a lifetime of the order of onemillisecond, neutralizing their charges and giving up energy in the form of heat.
Others reach the junction before their lifetime has expired. There they areseparated by the reverse field, the electrons being accelerated towards thenegative contact and the holes towards the positive Book 5. If the cell isconnected to a load, electrons will be pushed from the negative contact throughthe load to the positive contact, where they will recombine with holes. Thisconstitutes an electric current.
In crystalline silicon cells, the currentgenerated by radiation of a particular spectral composition is directlyproportional to the irradiance Book 2. Some types of solar cell, however, donot 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 powerplants are quite modular and can respond to sudden changes in solar input whichoccur 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 withconversion efficiencies of 20% to 30%. With time, both of these problems will besolved through mass production and new technological advances in semiconductors.Bibliography1) Green, Martin Solar Cells, Operating Principles, Technology and SystemApplications. New Jersey, Prentice-Hall, 1989.
pg 104-1062) Hovel, Howard Solar Cells, Semiconductors and Semimetals. New York, AcademicPress, 1990. pg 334-3393) Newham, Michael ,”Photovoltaics, The Sunrise Industry”, Solar Energy, October1, 1989, pp 253-2564) Pulfrey, Donald Photovoltaic Power Generation. Oxford, Van Norstrand Co.
,1988. pg 56-615) Treble, Fredrick Generating Electricity from the Sun. New York, PergamonPress, 1991. pg 192-195 Category: Science