Solar Cells – Small but Powerful

A solar system consists of a number of interconnected solar modules, and a module comprises many solar cells. The cell is therefore at the heart of the photovoltaic system. Photovoltaic processes take place within the system that convert the energy radiated by the sunlight into electrical energy. Solar cells mostly consist of an inorganic semiconductor material, generally silicon. Cells may consist of other semiconductor materials, for example gallium arsenide, cadmium telluride, copper-indium diselenide or even organic hydrocarbon compounds. Silicon solar cells are, however, the most common type in use.

Monocrystalline or polycrystalline

Silicon solar cells can differ in their crystal structure. In monocrystalline solar cells, the silicon is in the form of a single crystal with a uniform crystal lattice structure. This homogeneous form of the crystal permits them to generate more energy from sunlight than crystals with a non-uniform crystal structure. Monocrystalline silicon is, however, relatively expensive to manufacture, and more energy has to be expended in the fabrication of these solar cells. This in turn affects the ‘energy return time’.

The polycrystalline variant is different. Here, the silicon consists of numerous small individual crystals. The solar cells are cheaper to manufacture and the energy return time is considerably shorter. Because of this, polycrystalline cells offer a good price/performance ratio, although their efficiency is somewhat less than the monocrystalline variety.

Crystalline or thin film

The semiconductor material used is another factor that differentiates types of solar cells. Both monocrystalline and polycrystalline cells belong to the category of the silicon-based solar cell. An alternative is the so-called thin film process, in which a very thin coating of amorphous silicon (a-Si) is applied to a glass plate as plasma. This saves both material and energy. A further advantage of the thin film module is that it generally converts even weak and diffuse light into electricity better than its crystalline counterpart does. Moreover, the current output remains constant on hot summer days, whereas for crystalline modules the output decreases as the temperature increases. Thin film modules can also be tailored to meet individual requirements in size, design and power output. This is why they are often employed in larger architectural projects, the solar cells being frequently integrated directly into the shell of the building.

Because thin film modules are less efficient than crystalline modules at the current state of the art, thin film modules need a larger surface area to give the same energy yield. This involves, at least partially, higher system costs, for example, for module attachment. For this reason, a successor process based on micromorphous technology is already in development that is expected to increase the energy yield per unit of surface area.

According to estimates made by the State Bank of Baden-Wuerttemberg (Germany) and other market observers, the market for thin film modules will continue to grow in coming years, but in the long run, the modules will not displace the crystalline technology. It is more likely that both production processes will coexist and complement each other in specific application areas in order to meet the individual desires and requirements of users¹⁾.

Source: 1) LBBW Sector Report, 8/2007.