Solar Cell Materials

Solar Cell Materials


Solar Cells can be made from a wide range of semiconductor materials .in the following sections,we will discuss:
*Silicon(Si) -including single-crystalline Si,multicrystalline Si,and amorphous si.
*Polycrystalline thin films-including copper indium diselenide (cis) ,cadmium telluride(cdTe),and thin-film silicon
*Single-crystalline thin-including high efficiency material such as gallium arsenide(GaAS)
First ,though,we provide an overview of aspects that relate to all materials.This discussion serves as a basis for the more detailed section on individual materials .The aspects we will cover are crystallinity,absortion,bandgap,and complesity of manufacturing.
Crystallinity
The crystallinity of a material indicates how perfectly ordered the atoms are in the crystal structures .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 smaler 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 trypically used to grow or deposit such material.

Thype of Silicon Abbreviation Crystal size range Deposition Method
Single-crystal Silicon sc-si >10cm Czochralski,Float zone
Multicrystalline silicon mc-si 1mm-10cm Cast,sheet,ribbon
Polycrystalline Silicon pc-si 1mm-10cm Chemical-vapor deposition
Microcrystalline silicon mc-si <1mm Plasma deposition

Absorbtion
The absorption coefficient of a material indicates how far light having a specific wavelength(or energy) can penetrate the material before being absorb .A samll 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 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 with an atom to a free state .This free state is where the electron can be involved in conduction .The lower ernergy 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 E9) is the energy difference between the conduction band and valence band .

Complexity of Manufacturing
The most importants parts of a solar cell are the semiconductor layers .Because this is where electrons are are freed and the electric current is created-it's the active layer"where the action is ," so to speak.Several different semiconductor materials are used to make the layers in different types of solar cells.and each material has its benefits and drawbacks.

The most and complexity of manufacturing may vary across these materials and device structures based on many factors,including deposition in a vacuum environment,amount and type of material utilized,number of steps involved,need to move cells into different deposition chambers or processing process,and others.

From: http://www1.eere.energy.gov/solar/solar_cell_materials.html