The word Photovoltaic is a combination of the Greek word for light and the name of the physicist Allesandro Volta. It identifies the direct conversion of sunlight into energy by means of solar cell. The conversion process is based on the photoelectric effect discovered by Alexander Bequerel in 1839. The photoelectric effect describes the release of positive and negative charge carriers in a solid state when light strikes its surface.
How a Solar Cell Works
Solar cells are composed of various semiconducting materials. Semiconductors are materials which become electrically conductive when supplied with light or heat, but which operate as insulators at low temperatures.
Over 95% of all the solar cells produced worldwide are composed of the semiconductor material Silicon (Si). As the second most abundant element in the Earth’s crust, silicon has the advantage of being available in sufficient quantities, and additionally, processing the material does not burden the environment. To produce a solar cell, the semiconductor is contaminated or “doped”. “Doping” is the international introduction of chemical elements, with which one can obtain a surplus of either positive charge carriers (p-conducting semiconductor layer) or negative charge carriers (n-conducting semiconductor) from the semiconductor material. If two differently contaminated semiconductor layers are combined, then a so-called p-n-junction results on the boundary of the layers.
At this junction, an interior electric field is built up which leads to the separation of the charge carriers that are released by light.
Through metal contacts, an electric charge can be tapped. If the outer circuit is closed, meaning a consumer is connected, than direct current flows. Silicon cells are approximately 10 cm by 15 cm. A transparent anti reflection film protects the cell and decreases reflective loss on the cell surface.
Characteristics of a Solar Cell
The usable voltage from solar cells depends on the semiconductor material. In silicon it amounts to approximately 0.5 V. Terminal voltage is only weakly dependent on light radiation, while the current intensity increases with higher luminosity. A 100 cm² silicon cell, for example, reaches a maximum current intensity of approximately 2 A when radiated by 1000 W/m².
The output of a solar cell is temperature dependent. Higher cell temperatures lead to lower output, and hence to lower efficiency.
Types of Solar Cells
One can distinguish three cell types according to the type of crystal: monocrystalline, polycrystalline and amorphous. To produce a monocrystalline silicon cell, absolutely pure semiconducting material is necessary. Monocrystalline rods are extracted from melted silicon and then sawed into thin plates. This production process guarantees a relatively high level of efficiency. The production of polycrystalline cells is more cost-efficient. In this process, liquid silicon is poured into blocks that are subsequently sawed into plates. During the solidification of the material, crystal structures of varying sizes are formed, at whose borders defects emerge. As a result of this crystal defect, the solar cell is less efficient.
If a silicon film is deposited on glass or another substrate material, this is a so-called amorphous cell, or thin layer cell. The layer thickness amounts to less than 1 micro meter (thickness of a human hair: 50-100 micro meters), so the production costs are lower due to the low material costs. However, the efficiency of amorphous cells is much lower than that of the other two cell types. Because of this, they are primarily used in low power equipment or facade elements.
From Cell to Module
In order to make the appropriate voltages and outputs available for different applications, single solar cells are interconnected to form larger units. Cells connected in series have a higher voltage, while those connected in parallel produce more electric current. The interconnected solar cells are usually embedded in transparent Ethyl-Vinyl-Acetate, fitted with an aluminium or stainless steel frame and covered with transparent glass on the front side.
The typical power ratings of such solar modules are between 10 Wpeak and 100 Wpeak. The characteristic data refer to the standard test conditions of 1000 W/m² solar radiation at a cell temperature of 25 ° Celsius. The manufacturer’s standard warranty of ten or more years is quite long and shows the high quality standards and life expectancy of today’s products.
Natural Limits of Efficiency
In addition to optimizing the production processes, work is also being done to increase the level of efficiency in order to lower the costs of solar cells. However, different loss mechanisms are setting limits on these plans. Basically, the different semiconductor materials or combinations are suited only for specific portion of the radiant energy cannot be used, because the light quanta (photons) do not have enough energy to activate the charge carriers. On the other hand, a certain amount of surplus photon energy is transformed into heat rather than into electrical energy. In addition to that, there are optical losses, such as the shadowing of the cell surface or reflection of incoming rays on the connecting cable. The disrupting influence of material contamination, surface effects and crystal defects, however, are also significant. Single loss mechanisms cannot be further improved because of inherent physical limits imposed by the materials themselves. This leads to a theoretical maximum level of efficiency, i.e. approximately 28 % for crystal silicon.