To date, the dominant solar cell technology has been crystalline silicon cells. Following the success of crystalline technology, many solar scientists and engineers started developing alternative, lower cost PV technologies, which led to the development of thin-films. As the name suggests, thin-film cells are based on using thinner semiconductor layers to absorb and convert sunlight to electricity.
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How thin you ask?
Compared to silicon-wafer cells, which have light-absorbing layers that are generally 350 microns thick, thin-film solar cells have light-absorbing layers that are just one micron thick (1 micron = one-millionth of a meter).
How are they manufactured?
Thin-film solar cells are created by depositing several layers of a light-absorbing material (a semiconductor) onto a substrate such as coated glass, metal, or plastic. These semiconductor layers don’t have to be thick because they can absorb solar energy very efficiently. As a result, thin-film solar cells require less materials to manufacture, are flexible, and are therefore suitable for many applications that crystalline cells are not. Thin-film can also be manufactured in a large-area process, which can be an automated, continuous production process, and therefore has the potential to significantly reduce manufacturing costs.
So why haven’t thin-film cells taken over the solar market?
Thin-film solar cells are currently not as efficient as crystalline cells and are still more expensive to manufacture for most solar applications.
Are there different types of thin-film cells?
There are three main types of thin-film cells, Amorphous Silicon (a-Si), Cadmium Telluride (CdTe) and Copper Indium, Gallium, and Selenium (CIGS) cells.
Amorphous silicon cells are a thinner version of the traditional silicon-wafer cell. One of the biggest problems with a-Si solar cells is their efficiency. These cells are subject to significant degradation in power output when exposed to the sun. By reducing the thickness of the cells, these degradation issues can be overcome, however, thinner layers also absorb sunlight less efficiently. As a result, a-Si cells are perfect for smaller-scale applications, such as calculators, but less than ideal for larger-scale applications, such as solar-powered buildings.
Moving on down the efficiency scale we come to CdTe cells. The basic structure and function of these cells is as follows:
The front and back of the module are made of laminated glass sheets. This glass is heat-strengthened to withstand transport and thermally-induced stresses. This ensures the modules durability over its 25+ year life.
The semiconductor is a CdTe compound semiconductor that is applied in a very thin layer and forms the active photovoltaic cells, which convert sunlight into electricity.
The laminate material or EVA is an adhesive used to bond the cover glass to the substrate. This seals the cell from the environment.
While CdTe thin-film cells are still not as efficient as crystalline, these cells are being used in large-scale commercial solar developments. As of 2008, First Solar, a leading CdTe manufacturer, reported that over 300 MW of First Solar PV modules had been installed worldwide.
Compared to CdTe cells, CIGS has been able to reach higher efficiencies and requires less toxic cadmium to produce.These cells operate similarly to conventional crystalline silicon solar cells. When light hits the cell it is absorbed in the CIGS and thus creates free electrons and holes. These electrons diffuse in the CIGS grains until they reach the electric field within the junction region. At this point they are driven into the Cadmium Sulfide / Zinc Oxide (ZnO), which leads to a build up of voltage between the ZnO electrode and the Molybdenum (Mo) base.
CIGS exhibits a few characteristics that make it a valuable solar PV material. The first is its absorption coefficient, which is rated among the highest for all semiconductor materials. This means that ninety-nine percent of the light that hits CIGS is absorbed in the first micrometer, which allows these cells to remain thin yet efficient. In addition, CIGS has a high current density and as a result, has the potential to produce high current outputs.
Over 35 companies are currently developing CIGS technology, with the most notable being Solyndra who received a $535 million federal loan from the Department of Energy to build a state of the art CIGS manufacturing plant. Most recently, Solar Frontier (a Showa Shell company), announced it is building a 900 Megawatt factory in Japan. This $1 billion investment in the plant will provide Solar Frontier with a manufacturing capacity that could elevate the company to a CIGS market leader.
The health concerns with thin-films focus on the use of cadmium. Cadmium is a highly toxic substance that, like mercury, can accumulate in food chains. Many companies like First Solar have recognized the issue and have created recycling programs to deal with the solar cells at the end of their useful life. Many claim that this is a significant issue for the technology especially as it often included as part of the green energy revolution. Due to these health concerns, the National Renewable Energy Laboratory and several other agencies and companies are currently investigating cadmium-free thin-film solar cells.
With investment dollars pouring into thin-film one could conclude that it is only a matter a time before thin-films dominate the solar market. The applications for these flexible solar cells are endless. Entire buildings could be blanketed with cells and they could help facilitate a new generation of solar-powered cars and trucks.