Solar cells

 From time immemorial, human beings have revered the sun as one of the life giving principle. Life as we know it, would simply not be possible without the light and heat received from this insignificant, fairly ordinary star in the Milky Way galaxy. Yet, all the energy that we consume on earth is coming from the sun; whether it is in the form of biomass or oil or hydro, the ultimate source is the radiation from the sun.

To put it in perspective, the amount of energy received by earth from the sun is mind boggling. We receive about 10^18 kWh of solar energy annually while humanity's total energy consumption is around 10^14 kWh, i.e. we consume only about .01% of the energy we receive from the sun. If we can harness even a minute fraction of this colossal amount of energy into more useful forms of energy, it could be significant. What is more, solar energy would be less polluting, safe and environmentally friendly compared with the traditional forms of energy like fossil fuels, nuclear or even hydro.

One of the areas in which a lot of research effort has gone in is solar cells or devices which convert the solar radiation into electricity. This solar cell may function as a light detector, as in the exposure meter of a camera or may form a part of a complicated system for electric power generation. The basic principle behind the working of a solar cell is the effect. This effect, discovered by A.C.Becquerel in 1839 is the conversion of light energy (which we know is in the forms of bundles or quanta called photons) into an electric voltage by a material. The material is in most cases a semiconductor though originally the effect was observed in an electrolyte solution.

Semiconductors are the materials which are responsible for the electronics revolution. Every single electronic device, from a simple transistor radio to a supercomputer, depends on these materials for its operation. When we talk about the conduction of electricity in materials, ordinarily we talk of two kinds of materials; conductors like metals and insulators like porcelain. Semiconductors are substances which have a conductivity ( at room temperature) in between conductors and insulators. Basically the crystal structure of these materials is such that only a limited movement of electrons (the elementary particles responsible for the conduction of electric current) is allowed. Elements which are semiconductors include silicon and germanium. These are called intrinsic semiconductors. It turns out that one can add certain impurities (called dopants) to the semiconductor and radically change the conductivity. If an impurity is added which contributes negatively charged electrons ( for instance adding arsenic to silicon) to the original substance, then we get an N- type extrinsic semiconductor while if the impurity contributes positively charged holes ( boron in silicon) we get a P-type extrinsic semiconductor. It is important to remember that even in a N(P) type semiconductor there are , apart from the majority electrons (holes), also holes(electrons) as minority carriers.

 With these two types of semiconductors, we can build various devices like diodes and transistors which are at the heart of all electronic equipment. The simplest device which is used for a solar cell is a pn junction. This, as the name suggests is simply an electrical junction formed from a p and an n type semiconductor. The two sides of the junction have different concentrations of carriers and this leads to their diffusion across the junction. Near the junction, the holes from the P type diffuse into the N region leaving behind negatively charged ions in the P region. Similarly, electrons from the N region diffuse into the P region leaving behind positive ions. Thus a region is created at the junction which has a negative charge on one side (P-type) and a positive charge on the other side (N-type). This region is called a depletion layer. The charges in the depletion layer oppose the motion of the majority carriers across the junction. For instance, holes from the P type wanting to diffuse into the N side are repelled by the positively charged ions.

The conversion of solar energy into electricity in a solar cell involves three processes. Firstly, the radiation has to be absorbed by the semiconductor material. The absorbed photons generate free electron-hole pairs in the semiconductor. If these pairs can be separated into different regions of the cell, then we will have a voltage across the cell which can be used to generate an electric current.
The amount of light absorbed by the solar cell is dependent upon the intensity and frequency of the light, the characteristics of the material of the cell, the reflection properties of the cell and the thickness of the layer of the semiconductor material. An important quantity here is called the bandgap and is simply the minimum amount of energy which the material can absorb. Silicon has one of the lowest bandgaps in the semiconductors, thus explaining its widespread use in solar cells. To improve the efficiency of absorption of light, various techniques like use of Fresnel lenses for concentration of solar radiation are being used.

Once the light is absorbed in the material, free electron-hole pairs are formed. In a pn junction, these pairs get separated because of the existence of the depletion layer, with the holes going towards the negatively charged ions in the p region and the electrons moving in the opposite direction. This separation of charges is what causes a voltage (somewhat like a battery) to develop and can be used to derive a current. In some applications like calculators or watches, the current derived from such a cell is used directly, while in most applications where conventional electrical power is required, the output is converted to an alternating current through a power conditioning system. Since the individual solar cells are limited in area (because of the difficulty in producing pure semiconductor material) the basic handicap in their use has been the small amount of power generated. Typically, a silicon cell can deliver about 0.6 W at 0.5V in full sunlight. The answer to this problem has been to use a large number of cells in huge arrays. Another area of research has been in improving the efficiency of the material used. Silicon has a typical efficiency (the percentage of incident power converted to electricity) of 15% while newer materials like single crystal gallium arsenide gives a higher theoretical efficiency. Other advances are in the use of thin films of semiconductor materials which are better absorbers of light than silicon. An increase in efficiency can also be brought about by using a multi layered device. This is basically a stack of different materials with each layer being sensitive to a different part of the spectrum ( i.e having a different bandgap), thereby making optimal use of the incident energy. Typical materials used are gallium aluminum arsenide, gallium arsenide and silicon. With these multi layered devices, efficiencies of upto 40% are possible.

Even with the tremendous improvement in the technology of solar cells in the last two decades, it is still not feasible for large scale commercial use. Significant material and efficiency enhancements leading to a much lower cost, is required before their widespread use becomes a possibility. Nevertheless, there are several areas where solar cell arrays have been used to supply power. Remote areas where the cost of setting up transmission lines is prohibitive, navigational aids, remote radio repeaters are some examples. Of course, to all this must be added the use of solar arrays in satellites where there use is almost universal. Maybe as our nonrenewable sources of energy are depleted and the environmental costs of conventional energy sources are taken into the cost-benefit analysis, this clean, renewable and inexhaustible source will become more and more attractive for supplying electric power.