how solar cells work

A solar cell, sometimes called a photovoltaic cell, is a device that converts light energy into electrical energy. A single solar cell creates a very small amount of energy (about .6 volts DC) so they are usually grouped together in an integrated electrical panel called a solar panel. Sunlight is a somewhat diffuse form of energy and only a portion of the light captured by a solar cell is converted into electricity.  The current generation of solar cells convert only 12 to 15 per cent of the sun's light into electricity.  However in recent years there have been significant advances in their design.  Some new cells on the market now are around 20% efficient and some laboratory prototypes are reaching as high as  30 per cent.  Given this it is likely that their efficiency will continue to improve over time.

Theory Behind Solar Cells

A solar cell is based upon the "photovoltaic effect" discovered in 1839 by Edmund Becquerel, a French physicist.  In his experiments he found that certain materials would produce small amounts of electric current when exposed to sunlight.  Sunlight is made up of packets of energy called photons.  When the photons strike the semi-conductor layer (usually silicon) of a solar cell a portion of the photons are absorbed by the material rather than bouncing off of it or going through the material.  When a photon is absorbed the energy of that photon is transferred to an electron in an atom of the cell causing the electron to escape from its normal position.  This creates, in essence, a hole in the atom.  This hole will attract another electron from a nearby atom now creating yet another whole, which in turn is again filled by an electron from another atom.  This hole filling process is repeated a few zillion times and voila, an electric current is formed. 

Structure of a Solar Cell

A typical solar cell is a multi-layered material. Let's review what the layers are:

• Cover Glass - this is a clear glass layer that provides outer protection from the elements.
• Transparent Adhesive - this holds the glass to the rest of the solar cell.
• Anti-reflective Coating - this substance is designed to prevent the light that strikes the cell from bouncing off so that the maximum energy is absorbed into the cell.
• Front Contact - transmits the electric current.
• N-Type Semiconductor Layer - This is a thin layer of silicon which has been doped with phosphorous.
• P-Type Semiconductor Layer - This is a thin layer of silicon which has been doped with boron.
• Back Contact - transmits the electric current.

Types of Solar Cells

Because of the extensive research being done on solar energy there are now many types of solar cells on the market.  All of them follow the principles described when it comes to generating an electric current.  However, many different approaches are now used to create the structures in order to reduce the costs of production. These approaches involve a tradeoff between lower manufacturing costs versus lower efficiency in converting sunlight to electricity.  The three most common approaches are summarized below:  

• Monocrystalline Silicon - This type of solar cell uses a single layer of silicon for the semi-conductor. In order to produce this type of silicon it must be extremely pure which means it is the most expensive type of solar cell to produce.
• Polycrystalline Silicon - To make polycrystalline silicon cells liquid silicon is poured into blocks that are subsequently sawed into plates. This type of approach produces some degree of degradation of the silicon crystals which makes them less efficient.  However, this type of approach is easier and cheaper to manufacture.
• Amorphous Thin Film Silicon - This type of solar cell uses layers of semiconductor that are only a few micrometers thick (about 1/100th the thickness of a human hair).  This lower the material cost but makes it even less efficient than the other types of silicon. However, because it is so thin this type of cell has the advantage that it can be placed on a wide variety of flexible materials in order to make things like solar shingles or roof tiles.

Another way of looking at solar cells is in terms of the types of materials they are made with.  While silicon is the most commonly used crystal a number of other materials can be used as well.  These include the following:

• gallium arsenide
• copper indium diselenide
• cadmium telluride

Different types of substances perform better under certain light conditions.  Some cells perform better outdoors (i.e., optimized for sunlight), while others perform better indoors (optimized for fluorescent light).