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GAO-22-105378 1 (2021-11-04)

handle is hein.gao/gaomjb0001 and id is 1 raw text is: Science, Technology Assessment, and Analytics SCIENCE & TECH SPOTLIGHT: ALTERNATIVE MATERIALS FOR SOLAR CELLS What is it? Most solar cells (the components that generate electricity from sunlight) are currently produced with crystalline silicon in a process that is complex, expensive, and energy-intensive. Alternative materials-such as cadmium telluride, amorphous silicon, perovskites, and organic (carbon- containing) compounds-applied in thin layers of film may perform better and be easier and cheaper to manufacture. How does it work? As sunlight shines on a solar cell, some of the energy is absorbed to generate electricity either for immediate use or for storage in batteries (see fig. 1). The more readily a given solar cell absorbs light and transforms it into electricity, the higher its efficiency. The electric current generated by sunlight flows through wires that connect the front and back contacts of the solar cell. Sunlight Back contact Source: GAO analysis of NASA information. GAO-22-105378 Some alternative materials absorb light 10 to 100 times more strongly than crystalline silicon, allowing them to produce electricity using less material. In turn, solar cells made with these materials are typically thinner and weigh less. In addition, these thin film solar cells can be manufactured quickly, reducing cost. How mature is it? The maturity of these alternative materials varies widely, with some currently used to manufacture solar cells and others in the early stages of research and development. For example, cadmium telluride cells and copper indium gallium diselinide cells together account for roughly 10 percent of current solar cells and they are already cost- competitive with crystalline silicon cells. Novel solar cells under development use a variety of materials. Among them is amorphous silicon, which is non-crystalline and can be deposited as a thin film. Perovskites are an emerging class of materials with rapidly increasing efficiencies. Organic materials offer yet another option for thin films. They consist of carbon-containing compounds, either long chains or molecules, tailored to absorb specific wavelengths of light. Researchers are also investigating the use of quantum dots-microscopic particles of compounds such as cadmium telluride, cadmium selenide, indium phosphide, or zinc selenide, that are able to produce electricity from light. Although these diverse materials differ in their chemical composition, they all fall under the category known as thin films because of the extremely thin layer-comparable in thickness to a red blood cell-in which they are applied (see fig. 2). In addition to being easy to produce and relatively inexpensive, these materials can be deposited on a variety of substrates, including flexible plastics in some cases. Emerging Crystalline thin film Human hair silicon in a (0.6 pm) (100 Nm) solar cell y Commercial (100 pm) thin film Red blood Pencil tip (3 pm) cell (7 pm) (1,000 pm) I l I l l I ;1 0 10 100 1 micrometer (pm) millimeter Source: GAO analysis of data from Massachusetts Institute of Technology, The Future of Solar Energy, 2015, and National Cancer Institute information. I GAO-22-105378 Fgre 2. Crr and p tentl amtrnaiv mateil Lsl ei r ple in In addition to absorbing light, solar cells must convert it to electricity. While promising, commercial thin film solar cells currently average a conversion efficiency in the range of 12 to 15 percent, compared to 15 to 21 percent for crystalline silicon, according to a Massachusetts Institute of Technology (MIT) study. In addition, they require appropriate sealing materials to protect them from ambient oxygen and moisture. As a result, many alternative solar cell materials are currently under development or limited to specialized applications. * Reduced land use. Large solar farms can reduce usable farmland and affect previously undisturbed habitats. These new materials could reduce the amount of land needed for solar arrays by increasing efficiency and offering the flexibility to add more solar GAO-22-105378 Alternative Materials for Solar Cells

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