Fire / Feu

Cellules photovoltaïques La science du sauvetage de la terre

Les cellules énergétiques solaires (photovoltaïques) sont parmi les nombreuses technologies progressistes dans lesquelles les nations doivent investir et, sur lesquelles les ingénieurs doivent porter leurs attentions si nous voulons nous départir de notre dépendance morbide envers les combustibles fossiles.

Photovoltaic Cells
The Science of Saving the World

Jacob Moeller
Argosy Publications Inc.
February 2004

There was more than one reason for me to write an article on solar energy. Of course the most obvious has to do with the problems associated with our economy's dependence on non renewable energy sources.

Photovoltaic Cell

The UN's Intergovernmental Panel on Climate Change predicts that average global temperatures will rise as much as 5.8 degrees celcius by the end of the century, almost double their previous forecast. The ice caps are melting faster than expected, raising global water levels; natural disasters are predicted to continue to become more frequent and severe as the planet warms; coral reefs are struggling to survive; polar bears are under pressure; asthma caused by air pollution in our cities has dramatically increased; many people predict the coming of an environmentally catalyzed economic collapse. 2002 was the second warmest year on record, beaten only by 1998. It was the 24th warmest year in a row.

The solar energy cell (photovoltaic) is one, among many, of the progressive technologies that nations must invest in, and engineers must take interest in moving the industry forward if we are ever to kick our lethal fossil fuel habit. Even though the environmental reason is enough to necessitate an article on this subject, I do have other reasons-I'm doing a physics independent studies course on photovoltaic cells. So this week I get to rant about my physics independent studies topic while we learn about a technology that is generally accepted to be an integral part of what has to be done to prevent a global catastrophe.

The most common type of photovoltaic cell is made primarily from silicon, an element with an outer shell containing 4 electrons. Since 8 electrons fit snugly into the outer shell, when you stick a lot of silicon atoms together they will arrange themselves so that they share each other's outer electrons and have full outer orbitals. When an electron is in the outer orbital of an atom in a solid, it is said to be in a valence band. But, given enough energy, electrons can leave the restrictive valence band and move into the conduction band, where they are able to move around and carry charge. This moving charge is called electric current - the stuff that we're after. Conduction band electrons are either extra electrons that don't fit into an already full valence band or have been coaxed into moving from the valence band to the higher energy conduction band.

There are a number of ways to make it easier to coax electrons into the conduction band. One way is to dope the silicon. Doping a pure material is rather similar to doping a person. In both cases a foreign substance must be injected into the lump of the material at hand. In the case of silicon we can add phosphate, which has 5 outer electrons. Remember that the silicon atoms have arranged themselves in such a way so that each atom has a full valence orbital. Now where there is a phosphate atom, there will be an extra electron. This electron is then easily moved to the conduction band and becomes a little charge carrying slave. This is called an n-type semiconductor (the n refers to the extra negative charge). The reverse is also true: we can add a atom with 3 electrons in its outer orbital creating a positive charge carrying "hole." A silicon material doped in this way is called a p-type semiconductor.

When n-type and p-type semiconductors are put together, an "intrinsic electric field" is created in a thin layer at the junction between the two materials. This electric field acts to make the semiconductor a diode - a device in which the current can only move in one direction. At equilibrium, the currents associated with these conduction band electrons and holes will be balanced. However, if you orient the p-type material towards the sunlight, a photon (light quanta) may transfer its energy to an electron in the valence band and cause that electron to move to the conduction band. Since the internal electric field in the p-n junction acts to make the material a diode, all the electrons that move to the conduction band can travel in only one direction. When this happens, a useful electrical current is created and the imaginations of those who see a technologically advanced but environmentally sound future for the world, are set alight.