Photovoltaic cell

A photovoltaic cell is a Electronics component which, exposed to the Lumière (Photon S), generates a electric tension (Volt) (this effect is called the photovoltaic effect). The running obtained is a D.C. current and the tension obtained is about 0,5 V.

The photovoltaic cells consist of Semi-conducteur S containing Silicium (If), of Sulfure of cadmium (CdS) or Tellurure of cadmium (CdTe). They are appeared as two fine plates in close contact. Another name is “photo-galvanic”.

This semiconductor is sandwiched between two electrode S metal and the whole is protected by a pane.

Principle of operation

It can be illustrated for the following example, which presents the case of a cell to silicon:

  • the roadbase of the cell is made up of silicon doped by an element of higher valence in the periodic Classification, i.e. which has more electron S on its Couche of valence that silicon. Silicon has 4 electrons on its layer of valence: one can thus use elements of the column 15, for example the Phosphore (P). This layer will thus have on average a quantity of electrons higher than a layer of pure silicon. It is about a semiconductor of type NR.

  • the sub-base of the cell is made up of silicon doped by an element of valence lower than silicon. It can be a question of Bore (B) or another element of the column 13. This layer will thus have on average a quantity of electrons lower than a layer of pure silicon. It is about a semiconductor of type P.

When one puts these two semiconductors in contact (so that there can be conduction), one creates a junction PN, which must allow the passage of the electrons between the two plates. However, in the case of a photovoltaic cell, the gap of the semiconductor of the type NR is calculated so that the current cannot only be established: it is necessary that there is a contribution of energy, in the form of a Photon of light, so that an electron of the layer NR is torn off and comes to be placed in the layer P, thus creating a modification of the total burden-sharing in the building.

Two electrodes are placed, one on the level of the roadbase and the other than the level of the sub-base: a potential difference electric and an electric current are created.

Technique of manufacture

Silicon is currently the material more used to manufacture the photovoltaic cells available to an industrial level. Various treatments of sand make it possible to purify the silicon which then is heated and reduced in a furnace. The product obtained is a silicon known as metallurgical, pure to 98% only. This silicon is then purified chemically and ends in the electronic silicon of quality which arises in liquid form. Thereafter, this pure silicon will be enriched in elements doping (P, As, Sb or B) at the time of the stage of doping, in order to be able to transform it into semiconductor of the type P or NR.

The production of the photovoltaic cells requires energy, and it is estimated that a photovoltaic cell must function approximately 2 to 3 years following its technology to produce the energy which was necessary to its manufacture (energy return of the module).

The techniques of manufacture and the characteristics of the principal types of cell are described in the 3 following paragraphs. There currently exist other types of cells being studied, but their use is currently practically negligible.

Amorphous silicon cell

Silicon at the time of its transformation, produces a gas, which is projected on a glass leaf. The cell is gray very dark. It is the cell of the computers and the watches known as " solaires".

  • advantages:

    • functions with a weak illumination (even in covered weather or inside a building),
    • less expensive than the others.
    • less sensitive to the high temperatures than the crystalline cells mono or poly
  • disadvantages:
    • weak yield full sun, of 60Wc/m ²,
    • performances which decrease appreciably with time.

Single-crystal silicon cell

During cooling, molten silicon is solidified by forming one crystal of great dimension. One cuts out then the crystal in fine sections which will give the cells. These cells are in general of a uniform blue.

  • favors:
    • very good output, of 150 Wc/m ² or more (2007).
  • disadvantages:
    • high cost,
    • weak output under a weak illumination.

Polycrystalline silicon cell

During the cooling of silicon, it is formed several crystals. This kind of cell is also blue, but not uniform, one distinguishes from the reasons created by the various crystals.