Recent advances have been made since 2017 by Alta Devices, where their flexible solar cells exceed efficiencies of 30%, aerial densities of 170 g/ m, and are 30 μ m thick. Experiments with UAVs and solar cells have been around for over 20 years, and there is constant progress. For civilian use, they are used in flying cell phone towers and communications. UAVs can then serve for mapping, surveillance, border patrol, or search and rescue. They thus replace launching satellites into orbits, which are usually covered by considerable expenses. Structure and Composition of GaAs Solar CellsĮxperimental high-altitude long-endurance UAVs are aircraft that are covered mainly with flexible solar cells because of stay in the air for up to months. Since 1977, when the cost per watt was around 76 dollars, it is now approximately 36 cents. In contrast, the prices of silicon cells are very affordable today. state that compared to silicon, the prices of GaAs cells are up to ten times higher. The high price is influenced not only by the cost of the wafer but also by subsequent production-expensive equipment. Prices may vary depending on the complexity of the technology-the number of junctions. However, from a practical point of view, this type of solar cell is expensive for common use. They are also used in the aviation and military due to their flexibility and weight, which can be used especially for unmanned aerial vehicles (UAVs) and last but not least for concentrators, thanks to which solar cells can operate at very high temperatures. The main reason is their wide spectral coverage, which is much larger in space than on Earth. The most common field using GaAs-based solar cells is the aerospace industry. Germanium is often used as a substrate, which is suitable for its high mechanical strength and atomic lattice spacing very similar to GaAs. Single crystals of GaAs are very brittle. Due to the temperature gradient acting as mechanical stress, more crystalline defects are created: a standard diameter of 6″ wafers is used compared to 12″ for silicon. The production of wafers is generally more difficult and expensive. Therefore, GaAs solar cells have also become the standard for use in demanding temperature conditions. With higher temperatures, the thermal generation of carriers becomes more dominant over the intentionally doped level of carriers. The advantage of a wide bandgap is also the fact that the material remains more semiconductive at higher temperatures, such as in silicon, which has a bandgap of 1.12 eV. It is often extended by so-called alloying, i.e., precise melting of two elements together, in this case, with aluminum, to give Al xGa 1 − xAs. The direct bandgap of GaAs of 1.42 eV is also suitable for diode and photovoltaic (PV) cell applications. However, hole mobility, in contrast to much higher electron mobility, is similar to silicon-the response times are the same for devices that require cooperation between the motion of holes and electrons. In contrast to silicon, it has become very popular in high electron mobility transistor (HEMT) structures since it does not require any momentum change in the transition between the maximum of the valence band and the minimum of the conductivity band, and does not require a collaborative particle interaction. Gallium arsenide is a material widely used mainly in semiconductor technologies due to its attractive properties, where it has found many uses.
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