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Photovoltaic cells, more commonly known as solar cells, are found in applications such as calculator and satellites. First used almost exclusively in space, photovoltaic cells are now used in more common applications. In simple terms, photovoltaic cells and devices convert light energy into electrical energy. Photovoltaic cells are available in many different shapes and sizes. When individual photovoltaic cells are joined, they form photovoltaic modules.
Materials Used for the Construction of Photovoltaic Cells
Special materials are used for the construction of photovoltaic cells. These materials are called semiconductors. The most commonly used semiconductor material for the construction of photovoltaic cells is silicon. Several forms of silicon are used for the construction; they are single-crystalline, multi-crystalline and amorphous. Other materials used for the construction of photovoltaic cells are polycrystalline thin films such as copper indium diselenide, cadmium telluride, and gallium arsenide.
Silicon - The Most Popular Material for Solar Cells
A number of the earliest photovoltaic (PV) devices have been manufactured using silicon as the solar cell material and it is still the most popular material for solar cells today.
The molecular structure of single-crystal silicon is uniform. This uniformity is ideal for the transfer of electrons efficiently through the material. However, in order to make an effective photovoltaic cell, silicon needs to be "doped" with other elements.
Multi-crystalline silicon is normally considered less efficient than single-crystal silicon. On the other hand, multi-crystalline silicon devices are less expensive to produce. The casting process is the most common means of producing multi-crystalline silicon on a commercial scale.
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 - Reducing Material Required in Solar Cells
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 energy conversion efficiency.
Copper Indium Diselenide
Copper indium diselenide, 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 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.
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 are highly resistant to damage from radiation. This makes it suitable for concentrator systems and space applications.
Several new solar cell materials have been developed recently. However, most of these are still in the research stages. Apart from inorganic materials, several polymer-based materials and light-absorbing dyes have been used.
Perovskite structured materials used in solar cells are generally hybrid organic-inorganic lead or tin-halide materials, such as methylammonium lead halide. These materials can be solution-processed, hence enable inexpensive and simple fabrication. The efficiency of perovskite-based solar cells has been steadily increasing and is reported to be more than 20% in the lab currently. One of the key advantages of these materials is their ability to absorb sunlight across the entire visible spectrum.
Nanoparticles, a few nm in size, called quantum dots are another type of emerging materials used in solar cells. They are low bandgap semiconductor materials such as CdS, CdSe, and PbS. Their bandgaps can be tuned over a wide range by changing the size of the particles. Many common materials used for fabricating quantum dots such as Cd and Pb are considered toxic, hence other alternative materials such as copper indium selenide are being developed.
Semiconducting polymers such as polyphenylene vinylene (PPV) and small organic small molecules such as phthalocyanines, polyacenes, and squarenes are also used in solar cells. These highly conjugated organic molecules have a broad absorption in the visible and near infrared region. These materials are deposited as thin films either by vacuum deposition methods or solution processing, and solar cells using these materials are usually thin and flexible. However, the efficiency of these cells is still low, just a little more than 10%, hence they have not been commercialized yet.
Another type of solar cell material is a small molecule dye, such as a ruthenium metalorganic dye, that can absorb a broad range of the visible region of sunlight. An inorganic mesoporous nanoparticle layer, usually titanium dioxide, increases the area for light absorption. Solar cells using these materials can be made using solution processing, making them inexpensive to fabricate.
Sources and Further Reading
This article was updated on 11th March 2019.