A thin film solar cell is a second generation of solar cells that is made by depositing one or more thin layers, or thin film (TF) of photovoltaic material on a substrate, such as glass, plastic or metal.
The thickness of the film varies from a few nanometers (nm) to tens of micrometers (µm). The film is much thinner than the rival technology of the thin film, the first generation conventional crystalline silicon solar cell (c-Si), which uses wafers up to 200 µm thick. This allows thin film cells to be flexible and of less weight. It is used in the construction of integrated photovoltaic systems and as a semi-transparent photovoltaic glazing material that can be laminated in windows. Other commercial applications use rigid thin film solar panels (sandwiched between two glass panels) in some of the world's largest photovoltaic plants.
Thin film technology has always been cheaper but less efficient than conventional c-Si technology. However, it has improved significantly over the years. The efficiency of the laboratory cell for CdTe and CIGS now exceeds 21 percent, surpassing multicrystalline silicon, the dominant material currently used in most photovoltaic solar systems. Accelerated life tests of thin film modules under laboratory conditions measured a somewhat faster degradation compared to conventional PV, while a useful life of 20 years or more is generally expected. Despite these improvements,
Other thin film technologies that are still in an initial stage of ongoing research or with limited commercial availability are often classified as emerging or third generation photovoltaic cells and include organic and dye-sensitized, as well as quantum dots, copper sulfide tin zinc, nanocrystals, micromorphs and perovskite solar cells.
Types of thin film photovoltaic cells
Many of the photovoltaic materials are manufactured with different deposition methods on a variety of substrates. Thin film solar cells are generally classified according to the photovoltaic material used. According to these criteria are the following types of thin-film photovoltaic cells.
- Amorphous silicon (a-Se), and other thin-film silicones (TF-Se)
- Cadmium Tellurium (CdTe)
- Indian gallium and semenium copper (CIS or CIGS)
- Color sensitive solar cells (DSC) and other organic solar cells.
The use of cadmium telluride in the production of thin films is the most advanced thin film technology. Approximately half of the world production of photovoltaic panels and more than half of the thin film market are in the hands of this technology. The efficiency of the cell phone in vitro has increased dramatically in recent years and is in line with the thin film CIGS and close to the efficiency of multicrystalline silicon. Cadmium telluride also has the lowest energy recovery time of all mass production technologies, and in desirable situations it can be as short as eight months.
While environmental concerns about cadmium toxicity can be completely remedied by recycling cadmium at the end of its period, there are still doubts about the technology and public opinion is skeptical. The use of scarce materials can also be a problem for the economic viability of cadmium thin film technology.
Indian gallium and semenium copper
The possible compounds of the elements of group XI, XIII, XVI in the periodic photovoltaic table are: copper, silver, gold, aluminum, gallium, indium, silicon, selenium, tellurium. A photovoltaic cell of selenium, gallium or CIGS uses an adsorbent of selenium, gallium, indium and copper, the other types of free gallium are abbreviated CIS.
This technology is one of the three main streams of thin film technology, the other two being amorphous silicon cadmium telluride, which has a laboratory efficiency of 5% and a market share of 5%.
Amorphous silicon is a multiple form of non-crystalline silicon and has been the most advanced thin film technology to date. While CIS and CdTe photovoltaic cells have worked successfully in vitro, the industry is still focusing on thin-film silicon-based cells.
Silicon-based products are less problematic than CIS and CdTe products, for example, the toxicity and moisture problems of CdTe cells and the low production of CIS products do not arise due to the complexity of the materials associated with the products of silicon. In addition, there is no objection to the use of standard silicon as a result of political resistance to the use of non-green materials in the production of solar energy. The silicon modules are divided into three categories:
- Amorphous silicon photovoltaic cells
- Multicrystalline tandem photovoltaic cells
- Thin film of multicrystalline silicon on glass
Efficiencies of the thin film photovoltaic cell
Incremental improvements in efficiency began with the invention of the first modern silicon solar cell in 1954. In 2010, these constant improvements had resulted in modules capable of converting 12 to 18 percent of solar radiation into electricity. Efficiency improvements have continued to accelerate in the years since 2010, as shown in the attached table.
Cells made of newer materials tend to be less efficient than bulk silicon, but their production is less expensive. Its quantum efficiency is also lower due to the reduced number of charge carriers collected per incident photon.
The performance and potential of thin film materials are high, reaching cell efficiencies of 12-20%; prototypes of module efficiencies from 7 to 13%; and production modules in the 9% range. The thin film cell prototype with the best efficiency produces 20.4% (First Solar), comparable to Panasonic's best conventional solar cell prototype efficiency of 25.6%.
The solar frontier has achieved a new record efficiency of thin-film solar cells of 22.3%, the world's largest cis solar energy provider. In a joint investigation with the New Energy and Industrial Technology Development Organization (NEDO) of Japan, Solar Frontier achieved a conversion efficiency of 22.3% in a 0.5 cm 2 cell using its CIS technology. This is an increase of 0.6 percentage points over the previous record of the thin film of the industry of 21.7%.
Emerging photovoltaic energy
An experimental silicon-based solar cell developed at Sandia National Laboratories
The National Renewable Energy Laboratory (NREL) classifies a series of thin film technologies as emerging photovoltaics; most of them have not yet been commercially applied and are still in the research or development phase. Many use organic materials, often organometallic compounds, as well as inorganic substances. Although their efficiencies had been low and the stability of the absorbent material was often too short for commercial applications, much research is invested in these technologies, as they promise to achieve the goal of producing low cost and high efficiency. Solar cells.
Emerging photovoltaic energy, often called third generation photovoltaic cells, includes:
- Copper tin zinc sulfide solar cell (CZTS) and CZTSe and CZTSSe derivatives
- Dye-sensitized solar cell, also known as "Grätzel cell"
- Organic solar cell
- Perovskite solar cell
- Quantum dot solar cell
Especially the achievements in the research of perovskite cells have received great attention from the public, as their research efficiencies recently soared above 20 percent. They also offer a wide spectrum of low cost applications. In addition, another emerging technology, the photovoltaic concentrator (CPV), uses high efficiency multiple junction solar cells in combination with optical lenses and a tracking system.
Absorption of solar radiation by the thin film solar cell
Multiple techniques have been used to increase the amount of light entering the cell and reduce the amount that escapes without absorption. The most obvious technique is to minimize the upper contact coverage of the cell surface, reducing the area that prevents light from reaching the cell.
The weakly absorbed long wavelength light can be obliquely coupled to the silicon and passes through the film several times to improve absorption.
Multiple methods have been developed to increase absorption by reducing the amount of incident photons that are reflected away from the cell surface. An additional anti-reflective coating can cause destructive interference within the cell by modulating the refractive index of the surface coating. Destructive interference eliminates the reflective wave, causing all incident light to enter the cell.
Surface texturing is another option to increase absorption, but it increases costs. By applying a texture to the surface of the active material, the reflected light can be refracted to strike the surface again, thereby reducing the reflectance. For example, the texture of black silicon by reactive ionic etching (RIE) is an effective and economical approach to increase the absorption of thin-film silicon solar cells. A textured rear reflector can prevent light from escaping from the back of the cell.
In addition to the surface texture, the plasmonic light capture scheme attracted a lot of attention to help improve the photocurrent in thin-film solar cells. This method uses the collective oscillation of excited free electrons in nanoparticles of noble metals, which are influenced by the shape of the particles, the size and the dielectric properties of the surrounding medium.
In addition to minimizing the reflective loss, the solar cell material itself can be optimized to have a greater chance of absorbing a photon that reaches it. Thermal processing techniques can significantly improve the crystal quality of silicon cells and, therefore, increase efficiency. The thin film cell layer to create a multiple junction solar cell can also be made. The band interval of each layer can be designed to better absorb a different range of wavelengths, so that together they can absorb a greater spectrum of light.
Further progress in geometric considerations can exploit the nanomaterial's dimensionality. Large parallel nanowire matrices allow long absorption lengths along the length of the cable while maintaining short diffusion lengths of minor carriers along the radial direction. Adding nanoparticles between the nanowires allows conduction. The natural geometry of these matrices forms a textured surface that catches more light.