The photovoltaic effect occurs in semiconductors such as silicon or germanium. Semiconductors, as their name implies, are materials through which charged particles move more slowly than through conductors (e.g. metals), but faster than through insulators (e.g., plastics). Infusing small amounts of specific impurities into the crystals of semiconductors enhances their ability to move charges. Certain impurities, such as phosphorus, arsenic, or antimony, facilitate the movement of electrons and other negative charges (n-type), whereas other impurities, such as boron or aluminum, facilitate the movement of positive charges (p-type). Layering n-type and p-type semiconductors upon one another forms a depletion region.
When sunlight strikes a depletion region, electrons in the region become excited and may migrate into the n-type layer while an equal number of positive charges migrate into the p-type layer. This establishes an electrical potential that can do work such as illuminate a light bulb. Electrodes on the surfaces of the n-type and p-type layers collect the negative and positive charges for current to flow through an external circuit.
The efficiency with which photovoltaic cells convert solar electromagnetic energy into electricity has doubled over the past 30 years and now can exceed 40% under ideal conditions. Lifespans of photovoltaic cells have lengthened to over 20 years.  Moreover, photovoltaic cell manufacturing has become more efficient, and so after about 2 years in a sunny environment, a cell generates enough electricity to recoup the energy expended in its manufacture. Installation costs for photovoltaic cells have declined, but they are still higher than for most other forms of electricity. Operating costs for photovoltaic cells, however, are as low as any energy source, and the reliability of photovoltaic cells is unmatched under conditions as extreme as outer space. The photovoltaic cells can even be incorporated into buildings or airplanes.
Time required for photovoltaic solar cells of different types to recoup the energy expended in their manufacture, given present and future efficiencies of the cells (%). “Other” includes the energy required to manufacture the inverter, support, and cable. “Laminate” includes the energy required to produce cell modules. Future modules probably will not require aluminum frames. [After Alsema et al. 2006.]
 Alsema, E. A., M. J. de Wild-Scholten, and V. M. Fthenakis. 2006. Environmental Impacts of PV Electricity Generation – a Critical Comparison of Energy Supply Options. Paper read at 21st European Photovoltaic Solar Energy Conference Sept. 4 – 8, 2006, at Dresden,Germany.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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