Solar cell - surse de energie

The most common configuration of this device, the first generation photovoltaic, consists of a large-area, single layer p-n junction diode, which is capable of generating usable electrical energy from light sources with the wavelengths of solar light. These cells are typically made using silicon. However, successive generations of photovoltaic cells are currently being developed that may improve the photoconversion efficiency for future photovoltaics.

The second generation of photovoltaic materials is based on multiple layers of p-n junction diodes. Each layer is designed to absorb a successively longer wavelength of light (lower energy), thus absorbing more of the solar spectrum and increasing the amount of electrical energy produced.

The third generation of photovoltaics is very different from the other two, and is broadly defined as a semiconductor device which does not rely on a traditional p-n junction to separate photogenerated charge carriers. These new devices include dye sensitized cells, organic polymer cells, and quantum dot solar cells.

History
The term "photovoltaic" comes from the Greek phos meaning "light", and the name of the Italian physicist Volta, after whom the volt (and consequently voltage) are named. It means literally of light and electricity.

The photovoltaic effect was first recognised in 1839 by French physicist Alexandre-Edmond Becquerel. However it was not until 1883 that the first solar cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions.

The device was only around 1% efficient. Russell Ohl patented the modern solar cell in 1946 (US2402662, "Light sensitive device"). Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells.

The modern age of solar power technology arrived in 1954 when Bell Laboratories experimentation with semiconductors accidentally found that silicon doped with certain impurities was very sensitive to light. This resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6 percent.

Applications and implementations
Solar cells are often electrically connected and encapsulated as a module, termed a photovoltaic array or solar panel. Solar panels often have a sheet of glass on the front (sun up) side with a resin barrier behind, allowing light to pass while protecting the semiconductor wafers from the elements (rain, hail, etc). Solar cells are also usually connected in series in modules, creating an additive voltage.

Theory
Simple explanation
1.Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon.
2.Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. The complementary positive charges that are also created (like bubbles) are called holes and flow in the direction opposite of the electrons in a silicon solar panel.

3.An array of solar panels converts solar energy into a usable amount of direct current (DC) electricity.
Optionally:
1.The DC current enters an inverter.
2.The inverter turns DC electricity into 120 or 230-volt AC (alternating current) electricity needed for home appliances.
3.The AC power enters the utility panel in the house.
4.The electricity is then distributed to appliances or lights in the house.


Photogeneration of charge carriers
When a photon hits a piece of silicon, one of three things can happen:
1.the photon can pass straight through the silicon - this (generally) happens for lower energy photons,
2.the photon can reflect off the surface,
3.the photon can be absorbed by the silicon which either:

•Generates heat, OR
•Generates electron-hole pairs, if the photon energy is higher than the silicon band gap value.Note that if a photon has an integer multiple of band gap energy, it can create more than one electron-hole pair. However, this effect is usually not significant in solar cells. The "integer multiple" part is a result of quantum mechanics and the quantization of energy.
When a photon is absorbed, its energy is given to an electron in the crystal lattice.

Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighboring atoms, and hence unable to move far. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. The covalent bond that the electron was previously a part of now has one less electron - this is known as a hole.

The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs.