A Photovoltaic cell is the minimum unit of a solar panel, it is the heart of the panel, which produces electricity when the sun’s rays hit it.
A solar cell is a device capable of converting energy from solar radiation into electrical energy. The vast majority of solar cells that are currently commercially available are mono or polycrystalline silicon.
The first type is more widespread and although its elaboration process is more complicated, it usually presents better results in terms of its efficiency.
On the other hand, experimentation with materials such as Cadmium Tellurium or Indio-Copper Diselenide is leading the cells manufactured with these substances to situations close to commercial applications, with the advantages of being able to work with thin sheet technologies .
The word photovoltaic is composed of two terms: Photo = Light, Voltaic = Electricity. It is a device that converts sunlight directly into electricity.
What material are the photovoltaic cells made of?
Photovoltaic cells are made of special materials called semiconductors such as silicon, which is the most used material. When sunlight hits the cell a certain portion of it is absorbed into the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy hits the free electrons allowing them to flow freely.
All fv cells have one or more electric fields that act to force electrons released by the action of light to flow in a certain direction. This flow of electrons is a current and by putting the metal contacts on the top and bottom of the cell fv we can draw the current to use it externally. This current, together with the cell voltage, defines the power that the solar cell can deliver.
Silicon has some special chemical properties, especially in its crystalline form. A silicon atom has 14 electrons arranged in three different layers. The first two layers, the closest to the center are completely filled. The outer shell is only semi full, because it has only four electrons.
A silicon atom will always seek to fill its last layer with 8 electrons. To do this he will share four electrons of his neighboring atom. This process forms the crystalline structure and this structure turns out to be important for this type of fv cells.
This pure silicon does not serve as a conductor, so silicon with impurities is used. Normally silicon structures with phosphorus are used since this having 5 electrons, leaves a free one not attached to the structure. By applying energy, for example in the form of heat, this electron is released from its position more easily than in a pure silicon structure.
What is silicon doping?
This process of adding impurities to silicon is called Dopping. When silicon is doping with phosphorus, a silicon called N-type results, because free electrons prevail. N-type silicon is much better conductor than pure silicon.
When silicon is doping with boron, which has 3 electrons in the last layer, it is called P-type silicon. P-type silicon, instead of having free electrons, has free holes. The holes are absence of electrons, thus carrying a charge opposite to that of the electron, that is, a positive charge. These move the same way electrons do.
How do photovoltaic cells work ?
The PV cells without an electric field would not work. This electric field can be formed by contacting an N-type silicon and a P-type silicon. At the junction there is a barrier that makes it difficult for electrons on the N side to cross to the P side, we have an electric field that separates the 2 sides. This field acts as a diode allowing electrons to flow from the P to N. side with the application of external energy.
When the photon-shaped light collides with our cell, it releases electron-hole pairs. Each photon will release exactly one electron leaving a free hole. If this happens close enough to the electric field, this will cause an electron to be sent to the N side and a hole to the P side. This causes the breakdown of the electrical neutrality. If an external path is also provided, the electrons will flow to their original side (P-type side) to join the holes.
The electrons that flow constitute the current and the electric field of the cell constitutes the voltage. With the current and voltage we have the power of the cell.
How much solar energy does the PV cell absorb?
Most cells can absorb about 25% and more likely 15% or less. This is because visible light is only part of the electromagnetic spectrum. And electromagnetic radiation is not monochromatic. The light can be separated into different wavelengths.
The light that hits has photons with a great variety of energy, it turns out that some do not have enough energy to form the hollow electron pair.
While other photons have much more energy.
Only a certain amount of energy measured in electron volt is required to strike a free electron (1.1 eV is required for crystalline silicon).
This is called a band of energy interval of a material.
There are other losses, such as the internal resistance of the cell called the series resistance.
To minimize these losses, the cell is covered by a metal grid, an antireflective cover is placed on the grid and a glass cover on it is protected. This reduces losses by 5%.
There is also polycrystalline silicon, but it is not more efficient than crystalline silicon.
These materials have different band ranges and appear to be sitonized at different wavelengths or at photons with different energies.
It has been proven that the use of two or more layers of different materials with different ranges of energy bands turns out to be very efficient.
The material with greater band is placed on the surface and below those that require photons with less energy. These cells are called multijuntiras, and can have more than one electric field.
Densely crystalline materials
Simple crystalline silicon
Sliced from simple grown silicon crystal, these cells are 200 microns thick. The investigated cell has reached 24% efficiency, commercial modules exceed 15%.
Slice of silicon mold blocks, these cells are less expensive to manufacture and less efficient than simple silicon crystal cells. The investigated cells reach 18% efficiency and the commercial modules reach 14%.
A film of simple silicon crystals taken from molten silicon, like a soap bubble, between two dendritic crystals.
Arsenide Gallium (GaAs)
A semiconductor III-V material from which they make high-efficiency PV cells, are used in concentrator systems and space power systems. Research says they reach 25% efficiency under sunlight and 28% under concentrated sunlight. The multi-joint cells are based on GaAs and related to III-V alloys have exceeded 30% efficiency.
Thin film materials
Amorphous Silicon (a-Si)
The amorphous silicon that is a non-crystalline structure. First use in PV materials in 1974. In 1996, amorphous silicon constituted more than 15% of the worldwide PV production. Small experimental modules of Si-a exceed 10% efficiency, in commercial modules a range between 5-7% is reached. Used in consumer products, Si-a is the great promise for the construction of intergraded systems, replacing tinted glass with semi-transparent modules.
Cadmium Telluride (CdTe)
A thin film of polycrystalline material, deposited by electrodeposition. Small laboratories have approached 16% efficiency, and with a commercial size module (7200-cm2) they measured 8.34% efficiency, and 7% module production.
Indian Copper Diselenide (CuInSe2, or CIS)
A film of polycrystalline material, which reaches an efficiency of 17.7%, in 1996, with a prototype power module reaches 10.2%. The difficulty in taking this technology is the difficulty of preventing the formation of defects during deposition that prevent the formation of uniform layers.
The concentrator system uses lenses to focus the light inside the solar cells. The Lenses, with a concentration radius of 10x to 500x, typically linear or point spotlights. The cells are usually silicon. GaAs cells and other materials have high conversion efficiency at high temperatures, but they are very expensive. The efficiency of the modules exceeds 17%, and concentrators are designed for a conversion efficiency that exceeds 30%.
The Reflectors can be used to increase the power output, increasing the intensity of the light in the modules, or prolonging their time.
Concentrator System: The lenses cannot center the scattered light, limiting their use of areas, such as desert areas, with a substantial number of clear days in the year.
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