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Photovoltaic Cells and How They Work

Photovoltaics is the conversion of sunlight directly into electricity (rather than converting the sun’s heat into electricity).  The word itself signifies this: the prefix “photo” meaning “produced by light” combined with the suffix “voltaic”, which means “producing electricity by chemical action”.  When sunlight hits the PV cell, it interacts with semiconductor materials in the cell, freeing electrons which are captured as an electric current flowing through the cell.  Many of the cells can be combined to produce enough electricity to power a home, a sign, a calculator or even a satellite orbiting the earth.

There are several different types of solar cells depending on what they are made of.  The most common solar cells used in residential solar power are crystalline silicon cells.  Typical silicon solar cells produce an efficiency around 15% meaning  this is the percent of the sun's energy is converted to electrical energy.  These are what is called single bandgap cells, meaning the semiconductor material converts electricity most efficiently at a wavelength around 1200nm (1.1eV). Colors of light that don't match the bandgap of the cell can still be converted to electricity, but at a much decreased efficiency. There are solar cells that are called triple junction or triple bandgap cells which have three different materials, each with a different bandgap, that convert light into electricity.  These can operate at much high efficiencies (the record is 41%); however, these are also more than 300 times more expensive then silicon cell; and therefore are mostly used in satellite arrays or concentrating solar power systems.

The primary reason why solar cells are not 100% efficient is because the semiconductors do not respond to the entire spectrum of sunlight. Photons with energy less than silicon's bandgap pass through the cell and are not absorbed, which wastes about 18% of incoming energy. The energy content of photons above the bandgap will be wasted surplus re-emitted as heat or light. This accounts for an additional loss of about 49%. Thus about 67% of the energy from the original sunlight is lost, or only 33% is usable for electricity in an ideal solar cell and more like 15% in a cell once it is manufactured.  If you are intereted in a much more detailed understanding of the physics of how a solar cell works, check out this really great presentation on how solar cells work.

While silicon solar cells have been around for quite some time, there has been a more recent industry shift towards "thin film" solar cells based on CdTe (Cadmium Telluride) or CIGS (copper, indium, gallium, and selenium) materials.  In fact, the largest solar panel manufacturer in the world, First Solar, uses CdTe as the photovoltaic material.  You won't, however, see thin film solar panels used much in the residential solar market because their efficiencies are still lower (around 11.7% - First Solar) compared to silicon technologies (15.6% - SunTech).  While First Solar plans to have the efficiency of its CdTe cells up to 14% by 2014, they still lag behind silicon.

 


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