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 Solar Cell Types

Semiconductors:
The most commonly used semiconductor materials for the Special materials are used for the construction of photovoltaic cells. These materials are called construction of PV cells is Silicon. Several forms of Silicon are used for the construction viz:

Silicon :

Monocrystalline - Silicon solar cells can differ in their crystal structure. In monocrystalline solar cells, the silicon is in the form of a single crystal with a uniform crystal lattice structure. This homogeneous form of the crystal permits them to generate more energy from sunlight than crystals with a non-uniform crystal structure. Monocrystalline silicon is, however, relatively expensive to manufacture, and more energy has to be expended in the fabrication of these solar cells. This in turn affects the ‘energy return time’.

Multicrystalline - The polycrystalline variant is different. Here, the silicon consists of numerous small individual crystals. The solar cells are cheaper to manufacture and the energy return time is considerably shorter and their efficiency is somewhat less than the monocrystalline

Amorphous Silicon - Amorphous silicon can absorb 40 times more solar radiation than single-crystal silicon. This is one of the main reasons why amorphous silicon can reduce the cost of photovoltaics. Amorphous silicon can be coated on low-cost substrates such as plastics and glass. This makes amorphous silicon ideal for building-integrated photovoltaic products.

Polycrystalline thin films :
Numerous thin-film technologies are currently being developed to decrease the amount of light absorbing material required to produce solar cells. This could lead to a reduction in the processing costs; however it could also lead to a reduction in the energy conversion efficiency.

Copper Indium Diselenide - Copper indium diselenide or CIS for short, has an extremely high absorptivity. This means that 99% of the light illuminated on CIS will be consumed in the first micrometer of the material. The addition of a small amount of gallium will improve the efficiency of the photovoltaic device. This is commonly referred to as copper indium gallium diselenide or CIGS photovoltaic cell.

Cadmium Telluride - Cadmium telluride or CdTe is another well-known polycrystalline thin-film material. Similar to copper indium diselenide, CdTe also has a very high absorptivity and can be produced using low-cost techniques. The properties of CdTe can be altered by the addition of alloying elements such as mercury and zinc.

Single crystalline thin films including high efficiency materials like Gallium Arsenide (GaAs)

Gallium Arsenide -
Gallium arsenide or GaAs is a compound of two elements: gallium and arsenic. Gallium is rarer than gold and is a byproduct of the smelting of other metals, particularly aluminum and zinc. Arsenic, on the other hand, is not rare, however it is poisonous. Gallium arsenide also has a very high absorptivity and it only requires a cell of a few microns thick to absorb sunlight. GaAs cells are unaffected by heat and is highly resistant to damage from radiation. This makes it suitable for concentrator systems and space applications. GaAs is especially suitable for use in multijunction and high-efficiency solar cells for several reasons: The GaAs band gap is 1.43 eV, nearly ideal for single-junction solar cells. GaAs has an absorptivity so high it requires a cell only a few microns thick to absorb sunlight. (Crystalline silicon requires a layer 100 microns or more in thickness.) Unlike silicon cells, GaAs cells are relatively insensitive to heat. (Cell temperatures can often be quite high, especially in concentrator applications.) Alloys made from GaAs using aluminum, phosphorus, antimony, or indium have characteristics complementary to those of gallium arsenide, allowing great flexibility in cell design. GaAs is very resistant to radiation damage. This, along with its high efficiency, makes GaAs very desirable for space applications.

One of the greatest advantages of gallium arsenide and its alloys as PV cell materials is the wide range of design options possible. A cell with a GaAs base can have several layers of slightly different compositions that allow a cell designer to precisely control the generation and collection of electrons and holes. (To accomplish the same thing, silicon cells have been limited to variations in the level of doping.) This degree of control allows cell designers to push efficiencies closer and closer to theoretical levels. For example, one of the most common GaAs cell structures uses a very thin window layer of aluminum gallium arsenide. This thin layer allows electrons and holes to be created close to the electric field at the junction.

Basic Factors on which the performance of the different types of Solar cell materials
depends upon are:

Crystallinity - The crystallinity 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, multicrystalline, 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.

Absorption - 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.

Bandgap - 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.

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