Silicon, used to make some of the earliest photovoltaic (PV) devices—is still the most popular material for solar cells.Silicon must be refined to a purity of 99.9999%. In single-crystal silicon, the molecular structure—which is the arrangement of atoms in the material—is uniform, because the entire structure is grown from the same crystal. This uniformity is ideal for transferring electrons efficiently through the material.
To make an effective PV cell, however, silicon has to be “doped” with other elements to make it N-type and P-type. The crystalline of a material indicates how perfectly ordered the atoms are in the crystal structure. Silicon, as well as other solar cell semiconductor materials, can come in various forms: single-crystalline, multi crystalline, polycrystalline, or amorphous. In a single-crystal material, the atoms making up the framework of the crystal are repeated in a very regular, orderly manner from layer to layer.
In contrast, in a material composed of numerous smaller crystals, the orderly arrangement is disrupted moving from one crystal to another. One classification scheme for silicon uses approximate crystal size and also includes the methods typically used to grow or deposit such material. The absorption coefficient of a material indicates how far light having a specific wavelength (or energy) can penetrate the material before being absorbed. A small absorption coefficient means that light is not readily absorbed by the material. Again, the absorption coefficient of a solar cell depends on two factors: the material making up the cell, and the wavelength or energy of the light being absorbed.
Solar cell material has an abrupt edge in its absorption coefficient. The reason is that light whose energy is below the material’s bandgap cannot free an electron. And so, it isn’t absorbed. The bandgap of a semiconductor material is an amount of energy. Specifically, it’s the minimum energy needed to move an electron from its bound state within an atom to a free state. This free state is where the electron can be involved in conduction. The lower energy level of a semiconductor is called the “valence band.
And the higher energy level where an electron is free to roam is called the “conduction band.” The bandgap (often symbolized by EG) is the energy difference between the conduction band and valence band. N-Type Silicon – N-type silicon is created by doping (contaminating) the Si with compounds that contain one more valence electrons* than Si does, such as with either Phosphorus or Arsenic. Since only four electrons are required to bond with the four adjacent silicon atoms, the fifth valence electron is available for conduction.
silicon is created by doping with compounds containing one less valence electrons* than Si does, such as with Boron. When silicon (four valence electrons) is doped with atoms that have one less valence electrons (three valence electrons), only three electrons are available for bonding with four adjacent silicon atoms, therefore an incomplete bond (hole) exists which can attract an electron from a nearby atom. Filling one hole creates another hole in a different SI atom. This movement of holes is available for conduction. The photon’s energy transfers to the valence electron of an atom in the N-type SI layer. That energy allows the valence electron to escape its orbit leaving behind a hole.
silicon layer, the free electrons are called majority carriers whereas the holes are called minority carriers. As the term “carrier” implies, both are able to move throughout the silicon layer of the solar cell, in the P-type silicon layer, electrons are termed minority carriers and holes are termed majority carriers, The region in the solar cell where the N-type and P-type Si layers meet is called the P-N junction. As you may have already guessed, the P-type silicon layer contains more positive charges, called holes, and the N-type silicon layer contains more negative charges, or electrons.
When P-type and N-type materials are placed in contact with each other, current will flow readily in one direction (forward biased) but not in the other (reverse biased). An interesting interaction occurs at the P-N junction of a darkened solar cell. Extra valence electrons in the N-type layer move into the P-type layer filling the holes in the P-type layer forming what is called a depletion zone.
The depletion zone does not contain any mobile positive or negative charges. Moreover, this zone keeps other charges from the P-type and N-type layers from moving across it. So, to recap, a region depleted of carriers is left around the junction, and a small electrical imbalance exists inside the solar cell. This electrical imbalance amounts to about 0.6 to 0.7 volts. So due to the P-N junction, a built in electric field is always present across the solar cell. When photons hit the solar cell, freed electrons (-) attempt to unite with holes on the P-type layer.
The P-N junction
A one-way road, only allows the electrons to move in one direction. If we provide an external conductive path, electrons will flow through this path to their original (P-type) side to unite with holes. The electron flow provides the current ( I ), and the cell’s electric field causes a voltage ( V ). With both current and voltage, we have power ( P ), which is just the product of the two. Therefore, when an external load (such as an electric bulb) is connected between the front and back contacts, electricity flows in the cell, working for us along the way.
This electric field works as a diode, which allow electrons to flow from the P side to the N side, but not to other way around.
The basic concepts of solar cells and the requirements for photovoltaic solar energy conversion are reviewed. All present solar cells are found to follow the same principles. they consist of an absorber embedded between layers with selective transport properties, semi permeable membranes for electrons on one side and for holes on the other side. Their structure is shown to be a consequence of the absorption and transport properties of the materials.
Good transport properties of the absorber allow planar geometries as in Si solar cells, whereas bad transport properties require an interpenetration of the semi-permeable membranes as in the Graetzel cell or the organic solar cell.