Solar Cell Design and Operation

Solar cells, or photovoltaic (PV) cells convert electromagnetic energy into electrical energy. Designs of solar-cells have greatly changed since 1954 when Pearson, Chapin, and Fuller, researchers at Bell Labs, first reported their discovery of a silicon solar cell converting 6% of incident sunlight into electricity.

All solar cells work on the same basic principle: when light strikes the cell it is absorbed by semiconductor layers. A metallic grid then collects electrons from the semiconductor creating an electric current. There are two main parts to the cell, an n-type side and a p-type side. The "n" in n-type stands for negative and means that it has an extra unbonded electron due to the nature of the doping material, usually phosphorous which has 5 valence electrons in its outer orbital. The "p" in p-type stands for positive and conversely means that the doping material, usually boron, has bonds filled by only 3 valence electrons, which leave "holes." 

Image courtesy of ACRE - Australian CRC for Renewal Energies Limited

When these two materials are aligned opposite to each other, a natural electric field is created as the unbound electrons in the phosphorus rush to fill in the holes left by the boron atoms. As this is happening a positive charge builds up on the n-plate, and a negative charge builds up on the p-plate. This electric field causes the plates to have a voltage difference.

Images courtesy of How Stuff Works

Once electrons build up on the p-type plate it gets very hard for more electrons to cross the junction. If a photon of light having sufficient energy, called bandgap energy, strikes an electron on the p-type side, the electron is knocked free, crosses the gap and collects on the n-plate. An electron which has crossed the gap is called a photoelectron.

Images courtesy of How Stuff Works

If we connect the cell to an external circuit, then the photoelectrons will move through that circuit in order to return to the holes in the p-plate. This process creates a current and does electrical work. The power of a PV cell is determined by the product of its voltage times the current produced.

PV cells are used in flat plate collectors or concentrators that can either remain stationary or track the sun's position. The fundamental types of photovoltaic cells currently in use are single crystalline silicon, amorphous silicon, and polycrystalline thin films.

Single crystalline silicon cells use silicon wafers much like those used in electronics, and are therefore rather expensive to manufacture. These cells are doped with phosphor to make n-type plates and with boron to make p-type plates. Although expensive, the upside to these cells is that they are the best semiconductors available.

Amorphous silicon cells are much cheaper to manufacture than single crystalline silicon cells, but they also have disadvantages. They do not conduct nearly as well as single crystalline silicon cells, and have many bonding irregularities which create "holes" on the n-type plate rendering them less efficient.

Thin film cells are made by applying the proper materials in layers to a surface, usually glass or plastic. The sequence of materials is superstrate (opposite of substrate), transparent conductor, n-type layer, p-type layer, and then a metal conductor to finish the sandwich. These cells are quite cheap to manufacture but have the disadvantage of not being as efficient as silicon cells. However, there is a dramatic reduction in the amount of semiconducting material required and the embedded energy content of the modules in comparison with crystalline technologies.

Titania cells by Sustainable Technologies Australia, Ltd. are thin film cells that utilize a revolutionary technology. Instead of using silicon for the semiconductor they use titanium dioxide impregnated with organic dyes. This increases the efficiency of the cells since different dyes can be tuned to a wide variety of band gaps. Titania cells are less dependent on the angle of incidence of the sun's rays,  less sensitive to partial shading, and can operate at a wide range of temperatures. Titania solar cell manufacture uses non-vacuum processing equipment and produces no toxic gas emissions.

Another semiconductor at the cutting edge of solar cell research are those made with gallium arsenide. This very efficient semi-conductor can be tuned whatever specific voltage the designer needs. The huge drawbacks are that gallium is more rare than gold, and arsenic is poisonous so manufacturing these cells is difficult.

Other materials used include gallium arsenide, copper indium diselenide and cadmium telluride. Since different materials have different band gaps, they seem to be "tuned" to different wavelengths, or photons of different energies. 

The amount of power drawn from PV cells depends on their configuration into modules. Most cells produce 0.5 volts, and can be connected in series into modules of 12 volts or higher. One way efficiency has been improved is to use two or more layers of different materials with different band gaps. The higher band gap material is on the surface, absorbing high energy photons while allowing lower energy photons to be absorbed by the lower band gap material beneath. This technique can result in much higher efficiencies. The peak wattage depends on the plate's area, the incident radiation's intensity, frequency, angle of incidence, average average daily insolation (incoming solar radiation) or ESH (Equivalent Sun Hours), as well as the ambient temperature and such things as the air mass above the array, wind, cloudiness, dust and pollution. (email)