Photovoltaic cells currently in production for energy generation employ any of a variety of technologies. Two ways of classifying those technologies are by physical form and by elemental composition.
- Physical form
- Elemental composition
The physical form of a solar cell -- whether its constituents are crystalline or amorphous, the thickness of the materials used to form the junctions, and the microstructure of those junctions -- reflect the processes used to manufacture the cell.
A bulk process of manufacturing solar cells involves making a block of crystalline material and slicing it into wafers that are then subjected to various surface treatments.
Most PV panels use solar cells manufactured from bulk silicon. A crystalline ingot of highly purified silicon is grown -- a process requiring intense heat to melt the silicon -- and then sawed into thin wafers. Each wafer is then treated to make it a P/N junction -- a form of diode -- by adding a p-type dopant to one side, and an n-type dopant to the opposite side.
Currently silicon is the only element used for for making bulk-type solar cells that end up in commercial PV panels.
A thin-film process of manufacturing solar cells involves the deposition of layers of semiconductor materials on a substrate.
Nano-crystal composites embed PV nano-structures within a thin-film matrix, typcally to increase energy conversion efficiency by either or both of two methods:
- Producing a spread of band-gap energies within nano-particles of different sizes
- Reducing internal reflection by trapping light between the nanostructures
Many different types of semiconductors can be used to fabricate the solar cells that form PV panels and other PV solar energy devices. By far the most common types of cells are made of silicon in the form of a thin layer whose sides are doped to form a P-N junctions. Numerous other types of solar cells are made by combining and sandwiching layers of several different elements to form P-N junctions. Whereas Silicon belongs to group 4 of the periodic table, the elements that comprise the other types of solar cells straddle group 4.
Silicon solar cells manufactured from bulk silicon currently make up the most efficient solar panels, with conversion efficiencies of up to about 20 percent. Because silicon has a bandgap of 1.1 eV, corresponding to a wavelength of 1127 nm, the conversion efficiency of a silicon cell has an upper theoretical limit of about 31 percet. 1
|Energy absorbtion spectra of triple-junction III-V cell|
Group III-V solar cells combine elements from columns 3 and 5 of the periodic table -- typically Gallium, Arsenic, Indium, Germanium, and Phosphorus. Cells of this type provde the highest conversion efficiencies of any type of solar cell.
Group II-VI Semiconductors
Group II-VI semiconductors, like CdTe, use much less expensive elements than Group III-V semiconductors, like GaAs. Will they allow the manufacture of high-efficiency multi-junction cells, thereby avoding the resource limitations of Group III-V cells?
Cadmium telluride (CdTe) is a crystalline compound formed from cadmium and tellurium with a zinc blende (cubic) crystal structure (space group F43m). In the bulk crystalline form it is a direct bandgap semiconductor. CdTe is also a strong solar cell material. It is usually sandwiched with cadmium sulfide to form a pn junction photovoltaic solar cell.
Group I-III-VI solar cells combine elements from columns 1, 3, and 6 of the Periodic Table.
The most common type of Group I-III-VI PV semiconductors are Copper indium gallium selenide (CIGS). The material is a solid solution of copper indium selenide (CIS) and copper gallium selenide (CGS), having the chemical formula CuInxGa(1-x)Se2, where x can vary from 1 (pure CIS) to 0 (pure CGS). It is a tetrahedrally-bonded semiconductor, with the chalcopyrite crystal structure, and a bandgap varying continuously with x from about 1.0eV (for copper indium selenide) to about 1.7eV (for copper gallium selenide).
Its main use is for photovoltaic cells in the form of polycrystalline thin films. Unlike P-N junction silicon cells, the CIGS cells have a complex heterojunction structure. The best efficiency achieved as of December 2005 was 19.5% reported by Contreras et al. 2
2. Diode Characteristics of State-of-the art ZnO/CdS/Cu(In1-xGax)Se2 Solar Cells, www3.interscience.wiley.com,