Photovoltaic cells

Electricity is produced from solar energy when photons or particles of light are absorbed by semiconductors. This is the basis of the photovoltaic (PV) cell. Most solar cells in current use are built from solid-state semiconducting material. Semiconductors are at the centre of the electronic revolution of the last century and it is worth a moment to consider how they function.

Silicon is a typical semiconductor material in that its electrical properties can be influenced in a number of ways. The electronic structure of a solid, that is, the disposition of its electrons, falls into bands separated by 'band gaps'. A flow of electrons represents an electric current. The ability of electrons to move is determined by the extent to which a particular band is filled. Electrons in filled bands are relatively static. So solids with fully filled bands cannot conduct electricity - there is no space to allow electrons to move. Such materials are insulators. Materials with partially filled bands like most metals are conductors.

In all solids the saturated electronic band that has the highest energy density is called the 'valence band'. The next band above the valence band is the 'conduction band'. In insulators this band is empty; in metals, partially filled. The electronic band structure of silicon is similar to that of insulators. The valence band is completely filled and the conduction band empty. What distinguished a semiconductor from a pure insulator is the size of the band gap. In silicon it is small enough for a few electrons in the valence band to pick up enough thermal energy to hop into the conduction band where they have the space to move. This leaves a vacancy or hole in the valence band which has a real existence with an electrical charge opposite to that of an electron. It is, in effect, a virtual particle. So, in silicon, an electrical current is carried by a few energetic particles in the conduction band moving in one direction and by positively charged 'holes' in the valence bands moving in the opposite direction. This movement of particles is activated by the application of heat. The charged particles are thermally excited.

The conductivity of semiconductors can be improved by the addition of certain foreign atoms that provide extra charge carriers. These atoms are called 'dopants'. In the case of silicon an atom of arsenic replacing an atom of silicon results in the material acquiring an extra electron. The valence band being full means that this extra electron sits within the band gap which in turn means that it takes less energy to enable it to gravitate to the conduction band. In other words it more readily becomes a thermally excited charge carrier. This kind of doping introduces negative charge carriers, hence its description 'n-type' doping.

Conversely, using a dopant that has one fewer valence electrons than silicon creates a hole in the valence band. This hole behaves as a positive charge carrier. This is described as p-type doping. This manipulation of the electronic properties of silicon by doping has provided the basis of silicon microelectronics. At the heart of this technology is the so-called p-n junction or interface.

In the case of photovoltaic cells a layer of semiconductor material lies back to back with another semiconductor. One is p-doped and the other n-doped. This sets up an electrical field at the interface. When light falls on the cell, the energy from the photons frees some electrons in the semiconductors which are propelled to the extremities of the two-layer structure. This creates a difference in potential which generates an electrical current. Metal electrodes are attached to the two faces of the cell to complete an electrical circuit (see Fig. 6.1).

At present in most cells the p-doped and n-doped cells are formed within a monolithic piece of crystalline silicon. To reduce efficiency loss through reflection, most crystalline cells are chemically etched to roughen the surface. The absorption bandwidth of these cells is from 350 nm, the ultraviolet part of the spectrum, to the near-infrared 1.1 ^m. The fundamental conversion efficiency limit of crystalline silicon is said to be 29%.

A characteristic of such cells is that heat is generated when electrons are propelled to the boundary of the n-doped semiconductor, heat which needs to be dissipated, otherwise the efficiency of the cell is reduced.

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