Current Research

• GaAs cells and their composition
• How are they built for space applications? What’s different about the way it is fabricated?

1. Introduction

• Gallium arsenide is a material widely used mainly in semiconductor technologies due to its attractive properties, where it has found many uses.
• In contrast to silicon, it has become prevalent 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. It does not require a collaborative particle interaction. However, in contrast to much higher electron mobility, hole mobility is similar to silicon—the response times are the same for devices that require cooperation between the motion of holes and electrons. The direct bandgap of GaAs of 1.42 eV is also suitable for diode and photovoltaic (PV) cell applications. It is often extended by so-called alloying, i.e., precise melting of two elements together, in this case, with aluminum, to give AlxGa1−xAs.
• 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. With higher temperatures, the thermal generation of carriers becomes more dominant over the intentionally doped level of carriers. Therefore, GaAs solar cells have also become the standard for use in demanding temperature conditions.
• The production of wafers is generally more difficult and expensive. 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. Single crystals of GaAs are very brittle. Germanium is often used as a substrate, which is suitable for its high mechanical strength and atomic lattice spacing very similar to GaAs.
• For GaAs-based solar cells, performance can also be tuned by layering, where one solar cell can contain up to eight thin layers, each absorbing light at a specific wavelength. Such photovoltaic cells are called multi-junction or cascade solar cells. They use tandem fabrication, so they can also be found under the name tandem cells. Each layer contains a different composition and material with a specific bandgap that absorbs light in a particular spectral region. Usually, the top layer has a large bandgap and absorbs most of the visible spectrum up to the bottom layer with a low bandgap, which absorbs light in the infrared region.
• By covering a wide spectral electromagnetic range, maximum efficiency can be achieved. Other layers are commonly used, such as GaAs, AlGaAs, InP, InGaP, and GaInAs. Due to the mentioned mechanical strength and oriented growth of the Ge crystal lattice, it is possible to make very thin layers, reducing the overall weight of the PV cell.
• Multi-junction solar cells or thin-layer solar cells are referred to as the second generation of solar cells, which have also already been successfully commercialized. It is, therefore, not an experimental technology but a very mature and mastered technology that is already used in many areas. Thanks to such a multi-layered construction, they achieve higher efficiency than conventional single-layer solar cells.
• In March 2016, Yamaguchi et al. developed the triple-junction PV cell with 37.9% efficiency under 1 Sun, and 44.4% efficiency together with a concentrator under 246–302 Suns. In April 2020, a study was published in Nature Energy, where the authors of the six-junction PV cell achieved an efficiency of 39.2% and a value of 47.1% at 143 Suns, using the concentrator which was also certified by NREL. They also claimed that further reduction in the limiting series resistance should result in efficiencies over 50%.
• The most common field using GaAs-based solar cells is the aerospace industry. The main reason is their wide spectral coverage, which is much larger in space than on Earth. 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.
• However, from a practical point of view, this type of solar cell is expensive for common use. Prices may vary depending on the complexity of the technology—the number of junctions. The high price is influenced not only by the cost of the wafer but also by subsequent production—expensive equipment. Prices of GaAs cells are up to ten times higher. In contrast, the prices of silicon cells are very affordable today. Since 1977, when the cost per watt was around 76 dollars, it is now approximately 36 cents.

2. Structure and Composition of GaAs Solar Cells

• As mentioned in the introduction, not only have single-junction solar cells been developed for a long time, but multi-junction structures are being created to achieve the highest possible performance. The composition of these structures depends on the specific use. Thus, it is clear that, for example, the light of a different spectral range than on Earth will fall on the surface of Mars due to its atmosphere. Therefore, the Earth’s atmosphere filters not only harmful radiation for humans but also radiation that the solar cell can use.
• For multilayer structures, emphasis is placed on high crystal perfection in order to avoid recombination of generated minority carriers at cracks and other defects. By default, production takes place by growing on a doped substrate. The specific substrate is chosen depending on the next layer that will grow on it to induce an ideal lattice within the epitaxy.

3. Applications of Solar Cells

• Experimental 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. They thus replace launching satellites into orbits, which are usually covered by considerable expenses. UAVs can then serve for mapping, surveillance, border patrol, or search and rescue. For civilian use, they are used in flying cell phone towers and communications. Experiments with UAVs and solar cells have been around for over 20 years, and there is constant progress. 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 um thick. Their solar cells are widely used for aerospace purposes. Microlink Devices Inc. also supplies solar cells to the UAV sector. For example, for Airbus Zephyr —a solar high-altitude platform station operating in the stratosphere with >29% AM0 efficiency. Last but not least is the Thales Stratobus airship capable of flying at an altitude of 20 km, which previously used a transparent envelope section that allows sunlight reflection in concentrator mirrors, which were directed to solar arrays inside the UAV. However, since 2018, this system has been abandoned and replaced by flexible multi-junction arrays installed on the top surface.

3.3. Probes, Satellites and Other Space Objects