Energy question of the week: How does a solar cell work?
At present, commercially available solar cells made from polycrystalline silicon operate with an efficiency of 20 percent. Special solar cells composed of other semiconductors such as gallium arsenide have already passed the 40 percent efficiency barrier. In contrast, cells based on organic materials or pigments convert only 10 percent of the sunlight into electrical current at best. All of these use the photovoltaic effect, but what actually happens in the process?
The appeal of solar cells is the direct conversion of radiant energy from sunlight into electrical current. In the case of coal-fired and nuclear power stations, steam is produced, which then drives turbines attached to the generator. The basic principle behind photovoltaics is the ‘photoelectric effect’, discovered in the 19th century and then explained by Einstein, who described the underlying physical processes in 1905. Particles of light (photons) collide with the electrons of a solid body and eject them from their orbit around the atom nucleus.
Sunlight liberates electrons from their orbits
This is also exactly what happens with the ‘internal photoelectric effect’ in solar cells. The sunlight’s photons possess enough energy to be able to eject individual electrons from their fixed orbits around the nuclei in semiconductors, whether silicon, gallium arsenide or certain plastics. The previously bonded electron becomes a negatively charged free electron and leaves an ‘electron hole’ behind. In this manner, free electrons and electron holes always appear in pairs. These electron–hole pairs form free charge carriers, which move in opposing directions under the influence of an intrinsic electrical field that is induced by doping; that is, the introduction of foreign atoms into the semiconductor. When the electrons reach the electrical terminals of the solar cell, they cause a surplus negative charge. This surplus may be measured as an electrical voltage. Because of this voltage, the charge carriers can generate an electrical current in an external circuit and supply energy.
Emirate of Abu Dhabi: Construction of a 10-Megawatt photovoltaic power station to supply the planned ‘zero-emission city’ of Masdar City. Both images: Jan Oliver Löfken
However, not every electron in a semiconductor can be knocked out of its orbit by sunlight, because the energy of the photon must exceed the bonding energy of the electron in order to knock it out its orbit. Most solar cells utilise only a small portion of the usable solar spectrum, which extends from ultraviolet to infrared. One goal of photovoltaics researchers is to modify the sensitivity of their cells, so as to be able to generate electron-hole pairs using the largest possible portion of the sunlight. Thus, they stack different solar cells on top of each other, each utilising a different part of the spectrum.
How electron-hole pairs can disappear again
Another problem in the development of higher efficiency cells is the disappearance of electron–hole pairs before the charge carriers reach the electrical terminals of the cell. This occurs when electrons travelling through the semiconductor collide with a free hole and simply fall back into it. The creation of the thinnest possible light-sensitive semiconductor layer is a remedy for this electron recombination problem, but the rate of generation of electron-hole pairs also falls with reduced thickness. Therefore, photovoltaics researchers have to strike a delicate balance in order to achieve the highest possible efficiency.
Despite these difficulties, findings of laboratories from all around the world seem optimistic. Expensive cells using special materials continue to set new efficiency standards, and the yield from commercially-available silicon cells is increasing slowly while the production cost of the modules is dropping over the long-term. Today, photovoltaic generated electricity cannot compete with the cost for wind or coal-fired electricity generation, but the price differentials are shrinking steadily.
The DLR Energy question of the week in 'The future of energy' Year of Science
The Federal Ministry of Education and Research (BMBF) has given the Year of Science 2010 the motto 'The future of energy'. For this reason the science journalist Jan Oliver Löfken will this year answer a question on the subject of energy in his blog each week. Do you have a question about how our energy supply might look in the future? Or do you want to know, for example, how a wave power plant works and how it can efficiently generate electricity? Then send us your question by email. Science journalist Jan Oliver Löfken will investigate the answers and publish them each week in this blog.