Solar Cell Types
Several types of solar cells have been developed with the aims of reducing costs and improving efficiencies.
- C rystalline Silicon Solar Cells
Benefiti ng from the manufacturing experience of the semiconductor industry, crystalline silicon is the leading solar cell material, though still relatively expensive. Monocrystalline cells are cut from single crystals of high purity electronics grade silicon. These cells are about 25 percent efficient at best. Using the easier to manufacture polycrystalline silicon cut from from a block of crystals or less pure, so called "solar grade" silicon, efficiencies may be only about 15% or 16% due to the effect of grain boundaries or impurities but they cost a fraction of single crystal electronics grade cells.
- Amorphou s Silicon Solar Cells
Amorphou s Silicon has been employed for many years in the manufacture the solar cells used for powering electronic calculators and watches and promises the possibility of low cost, higher power cells. Amorphous material appears like a solid but has no regular crystal lattice structure. Glass is an example of such materials. The presence of controlled quantities of certain "impurity" elements such as hydrogen and the random crystal lattice formation actually enhance the otherwise very low conversion efficiency. Typical cell efficiencies range from 5% to 10%.
Manufacturing yield is still a problem and the cells suffer from degradation when exposed to the sun.
- Thin Film Silicon Solar Cells
Thin film cells are made by depositing the active photovoltaic material, such as amorphous silicon or other semiconductor onto a glass or other substrate together with the necessary current collecting contacts. The cell construction is much less costly than using semiconductor wafers and the manufacturing process is also simpler as well as being suitable for making cells with a much larger area and hence current carrying capability. Efficiencies of 11% to 14% have been achieved with this construction.
PV systems on flexible polymer substrates have also been made using Copper Indium Gallium Selenide (CIGS) active material with efficiencies of 10%.
Intensiv e research is being carried out by various teams on the use of organic semiconductors in the construction of PV cells. Devices can be fabricated by means of a printing process from single or double layer organic polymer films sandwiched between a pair of electrodes. Manufacturing does not involve the high energy consumption associated with crystalline semi conductors and brings the possibility of high volume, low cost products printed onto flexible films. Currently, conversion efficiencies are rather low at around 12% but this is expected to improve.
- Multi Layer (Tandem) Solar Cells
Better conversion efficiencies are possible by using multiple layers of differing semiconductor materials, optimised for different wavelengths, in a single device. This can raise the theoretical efficiency limit, currently about 30% for a single junction device, to about 45% for a three junction cell.
Efficiencies of over 33% have already been achieved in practical devices.
Mate rials such as Gallium Arsenide, Copper Indium Diselenide, Cadmium Telluride and Indium Nitride have been employed to provide particular characteristics to optimise solar cells for specific applications.
Gallium Arsenide is used for military and aerospace applications in a variety of cells in combination with other elements because of it's suitability for capturing high energy photons (ultra violet radiation), high potential conversion efficiency and its ability to withstand high temperatures. It is however more difficult to manufacture and cells using Gallium Arsenide can be 100 times more expensive than commercial silicon based cells.
Copper Indium Diselenide and Cadmium Telluride are used in polycrystalline form in low cost thin film cells because of their ease of manufacture and reasonable yields. Efficiencies are however low ranging from 8% to 14%
Indium Nitride is suitable for capturing low energy photons (infra red radiation) making it suitable for full spectrum devices when used in tandem solar cells in combination with other materials such as Gallium Arsenide which capture the high energy photons.
- Electrochemica l Solar Cells - Dye Sensitised Solar Cells (DSSC or Grätzel Cells)
Relatively new, these cells are low cost devices which use dye sensitised Titanium dioxide in combination with a liquid electrolyte to generate the current. Up to now they are only available in small sizes with efficiencies between 7% and 10%.