A thin-film solar cell is made by depositing one or more thin layers of PV material on a supporting material such as glass, plastic, or metal. There are two main types of thin-film PV semiconductors on the market today: cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS). Both materials can be deposited directly onto either the front or back of the module surface.
Perovskite solar cells are a type of thin-film cell and are named after their characteristic crystal structure. Perovskite cells are built with layers of materials that are printed, coated, or vacuum-deposited onto an underlying support layer, known as the substrate. They are typically easy to assemble and can reach efficiencies similar to crystalline silicon. In the lab, perovskite solar cell efficiencies have improved faster than any other PV material, from 3% in 2009 to over 25% in 2020. To be commercially viable, perovskite PV cells have to become stable enough to survive 20 years outdoors, so researchers are working on making them more durable and developing large-scale, low-cost manufacturing techniques.
While all solar cells with more than one bandgap are multijunction solar cells, a solar cell with exactly two bandgaps is called a tandem solar cell. Multijunction solar cells that combine semiconductors from columns III and V in the periodic table are called multijunction III-V solar cells.
Concentration PV, also known as CPV, focuses sunlight onto a solar cell by using a mirror or lens. By focusing sunlight onto a small area, less PV material is required. PV materials become more efficient as the light becomes more concentrated, so the highest overall efficiencies are obtained with CPV cells and modules. However, more expensive materials, manufacturing techniques, and ability to track the movement of the sun are required, so demonstrating the necessary cost advantage over today's high-volume silicon modules has become challenging.
Harnessing the massive amounts of energy that reaches the Earth from the Sun is a challenging but necessary step to ensure that the future energy needs of the planet can be met. Photovoltaic cells composed of various semiconductor materials are springing up all over the world to convert light energy directly into electricity with zero emissions.
When light reaches a solar panel or photovoltaic (PV) cell, it can either be reflected, absorbed or pass right through it. At the heart of a solar cell is a semiconductor layer, which is unequivocally the most important part of the cell. This material combines the properties of metals and insulators to yield a substance uniquely skilled at converting sunlight to electricity. When the semiconductor absorbs light, photons transfer their energy to electrons which flow through the material as an electrical current towards metal contacts above and below the semiconductor layer, from where it can travel to the power grid.
So, what does the future hold? Many researchers are working with a material called perovskite. Perovskites have a particular crystalline structure and can consist different materials but lead is an especially common choice. This material is currently placed over conventional silicon solar cells to boost power output by converting certain parts of the solar spectrum into electricity more efficiently than silicon. However, perovskites have a limited lifespan and alternative to the toxic lead are necessary.
A PV cell is made of semiconductor material. When photons strike a PV cell, they may reflect off the cell, pass through the cell, or be absorbed by the semiconductor material. Only the absorbed photons provide energy to generate electricity. When the semiconductor material absorbs enough sunlight (solar energy), electrons are dislodged from the material's atoms. Special treatment of the material surface during manufacturing makes the front surface of the cell more receptive to the dislodged, or free, electrons so that the electrons naturally migrate to the surface of the cell.
The photoelectric effect was first noted by a French physicist, Edmund Bequerel, in 1839, who found that certain materials would produce small amounts of electric current when exposed to light. In 1905, Albert Einstein described the nature of light and the photoelectric effect on which photovoltaic technology is based, for which he later won a Nobel prize in physics. The first photovoltaic module was built by Bell Laboratories in 1954. It was billed as a solar battery and was mostly just a curiosity as it was too expensive to gain widespread use. In the 1960s, the space industry began to make the first serious use of the technology to provide power aboard spacecraft. Through the space programs, the technology advanced, its reliability was established, and the cost began to decline. During the energy crisis in the 1970s, photovoltaic technology gained recognition as a source of power for non-space applications.
The diagram above illustrates the operation of a basic photovoltaic cell, also called a solar cell. Solar cells are made of the same kinds of semiconductor materials, such as silicon, used in the microelectronics industry. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. When light energy strikes the solar cell, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of an electric current -- that is, electricity. This electricity can then be used to power a load, such as a light or a tool.
A number of solar cells electrically connected to each other and mounted in a support structure or frame is called a photovoltaic module. Modules are designed to supply electricity at a certain voltage, such as a common 12 volts system. The current produced is directly dependent on how much light strikes the module.
While in use, solar panels safely generate electricity without creating any air emissions. However, like any source of energy, there are associated wastes that need to be properly recycled or disposed of when solar panels reach their end of life. As the solar photovoltaic (PV) market grows, so will the volume of end-of-life panels. By 2030, the United States is expected to have as much as one million total tons of solar panel waste. For comparison, the total generation of U.S. municipal solid waste (MSW) in 2018 was 292.4 million tons. By 2050, the United States is expected to have the second largest number of end-of-life panels in the world, with as many as an estimated 10 million total tons of panels. For more information on these and other solar panel waste projections, visit the International Renewable Energy Agency (IRENA) report on end-of-life solar panel management.
Thin-film solar cells contain thin layers of semiconductor material, such as cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS), layered on a supporting material such as glass, plastic, or metal. CdTe is the second-most common PV material after silicon, and cells can be made using low-cost manufacturing processes, but their efficiencies aren't as high as silicon solar PV.
Hazardous waste testing on solar panels in the marketplace has indicated that different varieties of solar panels have different metals present in the semiconductor and solder. Some of these metals, like lead and cadmium, are harmful to human health and the environment at high levels. If these metals are present in high enough quantities in the solar panels, solar panel waste could be a hazardous waste under RCRA. Some solar panels are considered hazardous waste, and some are not, even within the same model and manufacturer. Homeowners with solar panels on their houses should contact their state/local recycling agencies for more information on disposal/recycling.
The discarded solar panel, which is now considered solid waste, may then also be regulated under RCRA Subtitle C as hazardous waste if it is determined to be hazardous. The most common reason that solar panels would be determined to be hazardous waste would be by meeting the characteristic of toxicity. Heavy metals like lead and cadmium may be leachable at such concentrations that waste panels would fail the toxicity characteristic leaching procedure (TCLP), a test required under RCRA to determine if materials are hazardous waste. If the generator of the solar panels knows from previous experience that the material would fail the TCLP test, they can determine that the waste is hazardous without the need for testing.
While heavy metals are present in most solar panels, there are a variety of manufacturers and models, with different materials used as semiconductors. Because of the variation in design and components, testing has shown that some solar panels may pass the TCLP while others fail.
Hazardous waste solar panels that are recycled may be able to use regulatory exclusions available under RCRA, including the transfer-based exclusion (Title 40 of the Code of Federal Regulations section 261.4(a)(24)) in states that have adopted the 2015 or 2018 Definition of Solid Waste Rule. The transfer-based exclusion is a regulatory exclusion for hazardous secondary material that is recycled, as long as certain criteria laid out in the regulations are followed. This conditional exclusion is designed to encourage recycling of materials by third parties while still providing a regulatory framework that prevents mismanagement.
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