TechFocusThin-film solar cell
Harnessing solar energy more efficiently
There is an immense shortage of electricity in our country and the world over as conventional power plants -- power turbines, wind turbines, hydroelectric plants and solar panels -- cannot meet the demand for it. Power turbines -- the most used method -- use coal, oil, and gas for producing electricity. But these fossil fuels are becoming less available as days roll by, entailing a spike in their production cost. Installing a wind-powered turbine or hydroelectric plant, on the other hand, is also very high. All these methods are not environment-friendly either. Many scientists have been working on developing alternative energy called "green energy". Green power is the solution to creating a cleaner, sustainable energy system. The sun, wind, plants and moving water are renewable energy sources and they hold the promise of meeting our energy needs and protecting the environment as well. Among these, solar energy is one of the greenest forms of all renewable energy. But producing electricity from the sunlight has its drawbacks. The main setback for first-generation solar cells is that they are not cost-effective as the panels use single-layer p-n junction diode -- which occupies a large area -- using a silicon wafer. The production cost of these cells is also very high. That is why conventional solar cells have not gained much worldwide popularity. The efficiency of these cells, which have 86 percent market share, is about 7-13%. Thin-film solar cell is the second-generation technology in producing power from the sunlight. Various thin-film technologies currently being developed reduce the amount (or mass) of light-absorbing material required in creating a solar cell. This may lead to reduced processing costs. They, however, also tend to reduce energy conversion efficiency, although many multi-layer thin films have more efficiency than that of bulk silicon wafers. The main reason why thin-film solar cells are held to be the solution to ever-growing electricity needs is their being cost-friendly. The second-generation of photovoltaic materials is based on the use of thin-film deposits of semiconductors. These devices were initially designed to be high-efficiency multiple-junction photovoltaic cells. Later, the advantage of using a thin-film of material was noted, reducing the mass of material required for cell design. This led to the prediction of greatly reduced costs for thin film solar cells. There are currently a number of technologies/semiconductor materials under development or in mass production. Examples include amorphous silicon, polycrystalline silicon, microcrystalline silicon, cadmium telluride, copper indium selenide /sulfide. Typically, the efficiency of thin-film solar cells is lower compared with silicon (wafer-based) solar cells, but their manufacturing costs are also lower so a lower cost per watt can be achieved. Another advantage of the reduced mass is that less support is needed when placing panels on rooftops and it allows fitting panels on light materials or flexible materials, even on textiles. Some of the thin-film materials and their efficacy are given below. 1) CdTe: Efficiencies of approximately 10 percent were achieved using both p-i-n and p-n structures. 2) CIGS: The best efficiency of a thin-film solar cell as of December 2005 was 19.5% with CIGS. Higher efficiency (around 30%) can be obtained by using optics to concentrate the incident light. 3) CIS: This film can achieve 13.5% efficiency. 4) Gallium arsenide (GaAs) multijunction: GaAs triple-junction cells reach efficiency above 28.3%. They are also some of the most expensive cells per unit area (up to US$40/cm²). 5) Light absorbing dyes: Typically a Ruthenium metal organic dye (Ru-centred) used as a monolayer of light-absorbing material. 6) Organic/polymer solar cells: Energy conversion efficiencies achieved to date using conductive polymers are low at 4-5% efficiency for the best cells to date. 7) Silicon: Silicon thin-films are mainly deposited by chemical vapour deposition (typically plasma enhanced (PE-CVD)) from silane gas and hydrogen gas. The best conversion efficiency of 15.12% (AM1.5G, 24.5/spl deg/C) has been achieved without the cell's surface texture. The cells, called thin-film solar cells, are 100 times thinner and potentially lighter than today's silicon cells. Because they require less semiconductor material than other solar cells, lots of thin-film solar cells can be made for less money. However, the new cells have a much more complex structure and are more difficult to make, limiting their production and commercialisation so far. "They have a more complicated structure and require more complicated processing," said Tim Anderson, chairman of chemical engineering department at the University of Florida (UF) and member of the research team. "Our role is to better understand and transfer the technology to industry." The advantage of thin-film solar cells is their ability to produce electrical power without harmful emissions, according to the scientists. They can also generate power for a house or small business on site, reducing electrical demand on power plants and electrical grids. The thin-film cells can be created with pennies worth of material on flexible surfaces such as plastic. "The material cost is very minimal," said Sheng Li, a UF professor of electrical and computer engineering and part of a four-member UF faculty team at work on the process. "This is a very promising technology for solar cells." Li expects CIS cells to be widely available in less than 10 years. Times Square, the tallest skyscraper built in the 1990s in New York City, incorporates more energy-efficient building techniques than any other commercial skyscraper and also includes building-integrated photovoltaic (BIPV) panels on the 37th through 43rd floors on the south-and west-facing facades to produce part of the building's power. The building's most advanced feature is the photovoltaic skin, a system that uses thin-film PV panels to replace traditional glass cladding material. This cell has a vast array of usages. It has the potential of being used almost anywhere. The high cost of crystalline silicon wafers (they make up 40-50% of the cost of a finished module) has led the industry and scientists to look at cheaper materials to make solar cells. And thin films are one of those materials. In this era of ever increasing consumption of electricity, thin-film technology is a breath of fresh air. This technology-based cell's efficiency can reach up to 25-30%. With the use of thin film, solar cell price can decrease to 40-50 percent. The future of meeting the electricity need is tied to the expansion of thin-film cell technologies. The author studies electrical & telecommunication engineering at Independent University, Bangladesh.
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