The photovoltaic effect
Photovoltaic technology allows to directly convert energy from solar radiation directly into electric energy, with an overall efficiency between 16% and 18% for a single monocrystalline photovoltaic cell .
This technology makes use of the photovoltaic effect which is based on the properties of certain semiconducting materials which can convert solar radiation energy into electric energy without the use of moving mechanical parts and without the use of fuel (Fig. 1).
These devices are made from semiconductor, materials, such as silicon (Si), gallium arsenide (GaAs) and copper sulphate (Cu2S). In a photovoltaic cell, the photons of the incidental solar radiation break the ties of the semiconductor's electrons, thus allowing the electrons to move freely in the semiconductor. The positions left free by the electrons act as positive charges and take the name of "holes". The photovoltaic cells generally consists of two thin regions, one above the other, each with specially added impurities called dopants. The result is that one region is of "type n", with an excess of electrons (negative), while the other is of "type p", with an excess of positive holes. This 2-region structure, called a p-n junction, produces an internal electric field. When the photons create free electrons and holes in proximity to the p-n junction, the internal electric field makes them move in opposite directions: the electrons move towards the side n and the holes move towards the side p. So a tension (electromotive force, e.m.f.) is generated between the p and n regions, with p positive and n negative. Using wires, the side p and n are connected to a "load", e.g. a light bulb, and an electric current runs through the load.
Silicon in crystalline form is the
material most commonly used to make photovoltaic cells, which typically measure
12cm x 12cm. The cells are assembled together to obtain
with a surface of approximately half a metre squared (Fig. 2).
Photovoltaic systems are made up of many panels connected in series and in parallel; this modularity permits the systems to be highly flexible. A photovoltaic system can be a stand alone system or a grid connected) system. In both cases it is necessary to convert the continuous electric current supplied by the cells into alternate current through the use of an inverter .