Solar cellsSolar 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 do not pollute the atmosphere and they leave behind no harmful wasteproducts.
Photovoltaic cells work effectively even in cloudy weather andunlike solar heaters, are more efficient at low temperatures. They do theirjob silently and there are no moving parts to wear out. It is no wonderthat one marvels on how such a device would function. To understand how a solar cell works, it is necessary to go back tosome basic atomic concepts.
In the simplest model of the atom, electronsorbit a central nucleus, composed of protons and neutrons. each electroncarries one negative charge and each proton one positive charge. Neutronscarry no charge. Every atom has the same number of electrons as there areprotons, so, on the whole, it is electrically neutral. The electrons havediscrete kinetic energy levels, which increase with the orbital radius. When atoms bond together to form a solid, the electron energy levels mergeinto bands.
In electrical conductors, these bands are continuous but ininsulators and semiconductors there is an “energy gap”, in which noelectron orbits can exist, between the inner valence band and outerconduction band Book 1. Valence electrons help to bind together the atomsin 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,the higher 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 valenceband can easily be made to jump to the conduction band by the injection ofenergy, either in the form of heat or light Book 4.
This explains why thehigh resistivity of semiconductors decreases as the temperature is raisedor the material illuminated. The excitation of valence electrons to theconduction band is best accomplished when the semiconductor is in thecrystalline state, i. e. when the atoms are arranged in a precisegeometrical formation or “lattice”.
At room temperature and low illumination, pure or so-called”intrinsic” semiconductors have a high resistivity. But the resistivity canbe greatly reduced by “doping”, i. e. introducing a very small amount ofimpurity, of the order of one in a million atoms. There are 2 kinds ofdopant.
Those which have more valence electrons that the semiconductoritself are called “donors” and those which have fewer are termed”acceptors” Book 2. In a silicon crystal, each atom has 4 valence electrons, which areshared with a neighbouring atom to form a stable tetrahedral structure. Phosphorus, which has 5 valence electrons, is a donor and causes extraelectrons 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 anacceptor, leaving so-called “holes” in the lattice, which act likepositive charges and render the silicon “p-type”Book 5.
The drawings inFigure 1. 2 are 2-dimensional representations of n-and p-type siliconcrystals, in which the atomic nucleii in the lattice are indicated bycircles and the bonding valence electrons are shown as lines between theatoms. Holes, like electrons, will remove under the influence of an appliedvoltage but, as the mechanism of their movement is valence electronsubstitution from atom to atom, they are less mobile than the freeconduction electrons Book 2. In a n-on-p crystalline silicon solar cell, a shadow junction isformed 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 ofnegatively-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 atomsBook 4.
The net result of these movements is the disappearance ofconduction electrons and holes from the vicinity of the junction and theestablishment there of a reverse electric field, which is positive on then-side and negative on the p-side. This reverse field plays a vital part inthe 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 excessof the energy gap (1. 1 eV in crystalline silicon) interact with valenceelectrons and lift them to the conduction band. This movement leaves behindholes, so each photon is said to generate an “electron-hole pair” Book 2.
In the crystalline silicon, electron-hole generation takes place throughoutthe thickness of the cell, in concentrations depending on the irradianceand the spectral composition of the light. Photon energy is inverselyproportional 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 areabsorbed deeper in the crystal and further from the junction Book 4. Mostare absorbed within a thickness of 100 m. The electrons and holes diffuse through the crystal in an effort toproduce an even distribution. Some recombine after a lifetime of the orderof one millisecond, neutralizing their charges and giving up energy in theform of heat.
Others reach the junction before their lifetime has expired. There they are separated by the reverse field, the electrons beingaccelerated towards the negative contact and the holes towards the positiveBook 5. If the cell is connected to a load, electrons will be pushed fromthe negative contact through the load to the positive contact, where theywill recombine with holes. This constitutes an electric current.
Incrystalline silicon cells, the current generated by radiation of aparticular spectral composition is directly proportional to the irradianceBook 2. Some types of solar cell, however, do not exhibit this linearrelationship. The silicon solar cell has many advantages such as high reliability,photovoltaic power plants can be put up easily and quickly, photovoltaicpower plants are quite modular and can respond to sudden changes in solarinput which occur when clouds pass by. However there are still some majorproblems with them. They still cost too much for mass use and arerelatively inefficient with conversion efficiencies of 20% to 30%.
Withtime, both of these problems will be solved through mass production and newtechnological 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,Academic Press, 1990.
pg 334-3393) Newham, Michael ,”Photovoltaics, The Sunrise Industry”, Solar Energy,October 1, 1989, pp 253-256 4) Pulfrey, Donald Photovoltaic PowerGeneration. Oxford, Van Norstrand Co. , 1988. pg 56-615) Treble, Fredrick Generating Electricity from the Sun. New York, PergamonPress, 1991. pg 192-195
