Introduction
Photovoltaic s (PV) comprises the technology to convert sunlight directly into electricity.
The term “photo” means light and “voltaic,” electricity. A photovoltaic (PV) cell, also known as “solar cell,” is a semiconductor device that generates electricity when light falls on it .
Although photovoltaic effect was observed in 1839 by the French scientist Edmund Becquerel, it was not fully comprehensible until the development of quantum theory of light and solid state physics in early to
middle 1900s.
middle 1900s.
While most PV cells in use today are silicon-based, cells made of other semiconductor materials are expected to surpass silicon PV cells in performance and cost and become viable competitors in the PV marketplace. This paper surveys the major types of PV cell materials including silicon- and non-silicon-based
materials, providing an overview of the advantages and limitations of each type of materials.
materials, providing an overview of the advantages and limitations of each type of materials.
Photovoltaics and Photovoltaic Cells
When sunlight strikes a PV cell, the photons of the absorbed sunlight dislodge the electrons from
the atoms of the cell. The free electrons then move through the cell, creating and filling in holes in
the cell. It is this movement of electrons and holes that generates electricity.
the atoms of the cell. The free electrons then move through the cell, creating and filling in holes in
the cell. It is this movement of electrons and holes that generates electricity.
The physical process in which a PV cell converts sunlight into electricity is known as the photovoltaic effect.
One single PV cell produces up to 2 watts of power, too small even for powering pocket
calculators or wristwatches. To increase power output, many PV cells are connected together to
form modules, which are further assembled into larger units called arrays.
One single PV cell produces up to 2 watts of power, too small even for powering pocket
calculators or wristwatches. To increase power output, many PV cells are connected together to
form modules, which are further assembled into larger units called arrays.
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Types of PV cell materials
PV cells are made of semiconductor materials. The major types of materials are crystalline and thin
films, which vary from each other in terms of light absorption efficiency, energy conversion
efficiency, manufacturing technology and cost of production. The rest of the paper discusses the
characteristics, advantages and limitations of these two major types of cell materials.
films, which vary from each other in terms of light absorption efficiency, energy conversion
efficiency, manufacturing technology and cost of production. The rest of the paper discusses the
characteristics, advantages and limitations of these two major types of cell materials.
1 . Crystalline Materials
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| Single-Crystal silicon Solar Cell |
1 . 1 Single-crystal silicon
Single-crystal silicon cells are the most common in the PV industry. The main technique for producing single-crystal silicon is the Czochralski (CZ) method.
High-purity polycrystalline is melted in a quartz crucible. The ingots are then sawed into thin wafers about 200-400 micrometers thick (1 micrometer = 1/1,000,000 meter). The conversion efficiency for single-silicon commercial modules ranges between 15-20%. Not only are they energy efficient, single-silicon modules are highly reliable for outdoor power applications.
1 . 2 Polycrystalline silicon Consisting of small grains of single-crystal silicon, polycrystalline PV cells are less energy efficient than single-crystalline silicon PV cells. The grain boundaries in polycrystalline silicon hinder the flow of electrons and reduce the power output of the cell. The energy conversion efficiency for a commercial module made of polycrystalline silicon ranges between 10 to 14%.
High-purity polycrystalline is melted in a quartz crucible. The ingots are then sawed into thin wafers about 200-400 micrometers thick (1 micrometer = 1/1,000,000 meter). The conversion efficiency for single-silicon commercial modules ranges between 15-20%. Not only are they energy efficient, single-silicon modules are highly reliable for outdoor power applications.
1 . 2 Polycrystalline silicon Consisting of small grains of single-crystal silicon, polycrystalline PV cells are less energy efficient than single-crystalline silicon PV cells. The grain boundaries in polycrystalline silicon hinder the flow of electrons and reduce the power output of the cell. The energy conversion efficiency for a commercial module made of polycrystalline silicon ranges between 10 to 14%.
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| Poly crystalline Solar Cell |
1.3 Gallium Arsenide (GaAs)
A compound semiconductor made of two elements: gallium (Ga) and arsenic (As), GaAs has a crystal structure similar to that of silicon. An advantage of GaAs is that it has high level of light absorptivity. To absorb the same amount of sunlight, GaAs requires only a layer of few micrometers thick while crystalline silicon requires a wafer of about 200-300 micrometers thick.3 Also, GaAs has a much higher energy conversion efficiency than crystal silicon, reaching about 25 to 30%.
2 . Thin Film Materials
Thin Film Solar Cell
In a thin-film PV cell, a thin semiconductor layer of PV materials is deposited on low-cost supporting layer such as glass, metal or plastic foil. Since thin-film materials have higher light absorptivity than crystalline materials, the deposited layer of PV materials is extremely
thin, from a few micrometers to even less than a micrometer (a single amorphous cell can be as thin as 0.3 micrometers). Thinner layers of material yield significant cost saving. Also, the deposition techniques in which PV materials are sprayed directly onto glass or metal substrate are cheaper. So the manufacturing process is faster, using up less energy and mass production is made easier than the ingot-growth approach of crystalline silicon.
thin, from a few micrometers to even less than a micrometer (a single amorphous cell can be as thin as 0.3 micrometers). Thinner layers of material yield significant cost saving. Also, the deposition techniques in which PV materials are sprayed directly onto glass or metal substrate are cheaper. So the manufacturing process is faster, using up less energy and mass production is made easier than the ingot-growth approach of crystalline silicon.
However, thin film PV cells suffer from poor cell conversion efficiency due to non-singlecrystal
structure, requiring larger array areas and increasing area-related costs such as
mountings. Constituting about 4% of total PV module shipments of US4, the PV industry sees great
potentials of thin-film technology to achieve low-cost PV electricity.
structure, requiring larger array areas and increasing area-related costs such as
mountings. Constituting about 4% of total PV module shipments of US4, the PV industry sees great
potentials of thin-film technology to achieve low-cost PV electricity.
Materials used for thin film PV modules are as follows:
2 . 1 Amorphous Silicon (a-Si)
Used mostly in consumer electronic products which require lower power output and cost of production, amorphous silicon has been the dominant thin-film PV material since it was first discovered in 1974. Amorphous silicon is a non-crystalline form of silicon i.e. its silicon atoms are disordered in structure. A significant advantage of a-Si is its high light absorptivity, about 40 times higher than that of single-crystal silicon. One is the low cell energy conversion efficiency, ranging between 5-9%, and the other is the outdoor reliability problem in which the efficiency degrades within a few months of exposure to sunlight, losing about 10 to 15%.
2 . 2 Cadmium Telluride (CdTe)
As a polycrystalline semiconductor compound made of cadmium and tellurium, CdTe has a high light absorptivity level -- only about a micrometer thick can absorb 90% of the solar spectrum. Another advantage is that it is relatively easy and cheap As a polycrystalline semiconductor compound made of cadmium and tellurium, CdTe has a high light absorptivity level -- only about a micrometer thick can absorb
90% of the solar spectrum. Another advantage is that it is relatively easy and cheap
90% of the solar spectrum. Another advantage is that it is relatively easy and cheap
2 . 3 Copper Indium Diselenide (CuInSe2, or CIS)
A polycrystalline semiconductor compound of copper, indium and selnium, CIS has been one of the major research areas in the thin film industry. The reason for it to receive so much attention is that CIS has the highest “research” energy conversion efficiency of 17.7% in 1996 is not only the best among all the existing
thin film materials, but also came close to the 18% research efficiency of the polycrystalline silicon PV cells. (A prototype CIS power module has a conversion efficiency of 10%.) Being able to deliver such high energy conversion efficiency without suffering from the outdoor degradation problem, CIS has demonstrated that
thin film PV cells are a viable and competitive choice for the solar industry in the future .
thin film materials, but also came close to the 18% research efficiency of the polycrystalline silicon PV cells. (A prototype CIS power module has a conversion efficiency of 10%.) Being able to deliver such high energy conversion efficiency without suffering from the outdoor degradation problem, CIS has demonstrated that
thin film PV cells are a viable and competitive choice for the solar industry in the future .
Conclusion
Crystalline silicon has been the workhorse of the PV cells for the past two decades. However, recent progress in the thin-films technology has led many industry experts to believe that thin-films PV cells will eventually dominate the marketplace one day and realize the goals of PV -- a low price and reliable source of energy supply.
Crystalline silicon has been the workhorse of the PV cells for the past two decades. However, recent progress in the thin-films technology has led many industry experts to believe that thin-films PV cells will eventually dominate the marketplace one day and realize the goals of PV -- a low price and reliable source of energy supply.
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