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Low-Cost Solar Cells
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10457 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
2 / 1993 |
2,051 Words |
| Author
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Michael Gratzel
Michael Gratzel is professor of chemistry at the Institute of
Physical Chemistry of the Swiss Federal Institute of
Technology at Lausanne, Switzerland. |
The dream of tapping directly into the sun's plentiful energy supply continues to inspire pursuit. While fossil fuels provide access to the sun's energy that was stored in them millions of years ago and nuclear power extracts the energy that was locked into atoms from the beginnings of the universe, solar technology makes it possible to use the sun's energy directly and immediately.
For years, the technology to convert sunlight directly into electricity was available only for the space program. The crystalline silicon cells used in this process, though efficient, are far too expensive for common consumer applications. A second generation of solar cells, made of very thin films of light capturing materials, did reduce the cost some, but not enough to make them attractive for applications other than small consumer products.[See "Second-Generation Solar Cells," THE WORLD & I, May 1988].
Can solar collectors ever provide the substantial power necessary for our homes and offices? A new solar contender in this quest is a photovoltaic cell developed in Switzerland. This cell is modeled after the world's commonest solar energy processors: plants, which use a two-step process to convert sunlight into energy. While conventional solar cells try to do the job all in one step, the new cells separate the two main parts of energy conversion much as plants do. These cells are considerably cheaper and hold the promise of clean, plentiful solar energy.
Solar cell basics
A conventional photovoltaic (PV) cell consists of two layers of a semi conducting material, usually silicon. One layer is chemically treated to have an excess of electrons, and the other carries an excess of positively charged holes. When the two layers are brought in contact, electrons flow from the n to the p side, producing an electric field at the interface. The resulting solid-state device is called a junction.
When photons of light energy from the sun strike the semiconductor material they excite electrons, which, as they rise to higher energy levels, leave "holes" behind. These electron hole pairs recombine rapidly unless the electrons are carried away quickly to create useful electric current. When the electron hole pair is created near the pn junction, the built in electric field forces the positively charged holes to the p side and the negatively charged electrons to the n side. This movement of free charges causes a current between the positive p region and the negative n
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