Converting sunlight into electricity

Jan 9, 2008 15:50 GMT  ·  By

The Sun is relatively generous when it comes to providing the Earth with power, as massive amounts of energy are daily radiated towards our planet. It has been calculated that, on a sunny day, the Sun is able to produce about 1,000 watts of pure energy per square meter on the surface of the Earth. If only we were able to efficiently capture at least a fraction of this energy, we could finally say 'goodbye' to fossil fuels.

Sunlight can be converted into electrical energy with the help of so-called photovoltaic cells. The photovoltaic effect was discovered in 1839 by Alexandre-Edmond Becquerel who laid the bases for the construction of the first true photovoltaic cell by Charles Fritts, in 1884. There was only one problem though - the device had an efficiency of only 1 percent. Although technology has evolved a great deal since then, and the efficiency greatly increased to about 40 percent, they are still considered extremely unproductive by today's standards.

Furthermore, at the time the effect was discovered, nobody was able to explain the basic principle that made it work. By quantifying the light into small packets of energy we currently know as photons, Albert Einstein succeeded in proving that energy in the form of light particles is absorbed in semiconductor materials and released in the form of electric energy. Einstein eventually received the Nobel Prize for physics, but it was for explaining the Photoelectric Effect, and not for the Theory of Relativity as you might think.

Most solar panel designs involve using a silicon base structure, which is separated into multiple layers, each with a different role. Other solar panel designs may use either thin-films, light absorbing dyes, cadmium telluride thin films or organic and polymer films. Depending on the fabrication process of the silicon film, different silicon allotropes may result, such as amorphous silicon, protocrystalline silicon and nanocrystalline silicon.

The basic modern photovoltaic cell design consists of several layers, each with different tasks. To give you an idea on how it works, let's consider, for example, a photon of light striking a solar cell. The photon may suffer one of the following outcomes: the photon passes right through the whole mass of silicon material without releasing its energy, which usually happens with the low energy photon particles, or it may be reflected on the surface.

As you can see, in neither of the upper cases, energy is produced. The second category of possible outcomes involves energy producing interactions, thus a photon may be absorbed in the silicon mass, in which case it will be converted into heat, or it may strike a silicon atom to release an electron, but only if the photon has enough energy to exceed that of the silicon band gap.

Thus, the solar cell is designed to optimize its power gathering capabilities. The top part represents an anti-reflection layer, consisting of silicon dioxide material, followed by an 'n' doped silicon layer and a 'p' doped silicon layer which, with the one in the middle, forms a so-called p-n junction that lies on an aluminum substrate.

The photon is usually absorbed in the 'p' layer of the silicon thin film and releases its energy, which in turn triggers the ejection of an electron that is collected at the cathode of the solar cell in the form of electric energy. Though highly efficient in space, as the light intensity outside Earth's atmosphere is ten times greater than on the surface, the invention that promised to provide us a cheap non-polluting source of energy seems to be quickly fading into darkness.

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Field of solar panels generating electric energy
Simple sketch of the design of a silicon thin-film frequently used in silicon solar cells
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