Solar Cells and Photovoltaics for Space Applications

Spacecraft depend on a reliable supply of electrical power to operate almost every system onboard, from communication equipment, sensors and cameras to computers, navigation systems and scientific instruments. Unlike systems on Earth, spacecraft cannot simply connect to an external power source or refuel whenever energy is needed. This is why solar photovoltaic systems are the primary power source for many satellites and spacecraft. Solar cells convert sunlight directly into electrical energy through the photovoltaic effect, while batteries store excess energy for periods when sunlight is unavailable. The basic energy pathway is solar radiation → photovoltaic cell → electrical power → spacecraft systems. However, generating electricity in space is far more demanding than on Earth. Solar cells must operate in vacuum, withstand energetic radiation, tolerate extreme temperature variations and continue producing useful power throughout a mission, all while adding as little mass as possible to the spacecraft.

The energy that powers these solar cells ultimately comes from nuclear fusion inside the Sun. In the Sun’s core, hydrogen nuclei combine to form helium, converting a small amount of mass into an enormous amount of energy. This energy eventually reaches the Sun’s surface and is emitted into space as electromagnetic radiation, including visible, ultraviolet and infrared radiation. Solar cells do not directly use the Sun’s heat; instead, they use the energy carried by photons. The energy of an individual photon is given by E = hf = hc/λ, meaning shorter-wavelength photons carry more energy than longer-wavelength photons. The amount of sunlight available to a spacecraft also depends on its distance from the Sun. Near Earth’s orbit, solar irradiance is approximately 1361 W/m², although it varies slightly. Solar irradiance follows an inverse-square relationship with distance, so spacecraft farther from the Sun receive considerably less power per unit area. This becomes particularly important for missions travelling to destinations such as Mars, Jupiter and beyond, where the available sunlight is much weaker.

At the heart of a solar cell is a semiconductor, commonly made from materials such as silicon, gallium arsenide (GaAs), or compound semiconductor materials used in advanced multi-junction cells. Semiconductors contain a valence band and a conduction band separated by a forbidden energy gap called the bandgap. When a photon with energy equal to or greater than the bandgap is absorbed, it can excite an electron from the valence band into the conduction band, creating an electron-hole pair. Photons with energy below the bandgap generally cannot produce this transition, while photons with energy significantly above the bandgap lose their excess energy as heat through thermalization. A solar cell also contains a p-n junction, formed by joining p-type and n-type semiconductor regions. When these regions meet, electrons and holes diffuse across the junction and recombine, leaving behind charged ions and creating a depletion region with an internal electric field. When sunlight generates electron-hole pairs, this electric field, together with carrier diffusion and other transport mechanisms, helps separate the charges and reduce their immediate recombination. Electrons and holes then move toward their respective electrical contacts, allowing current to flow through an external circuit. The resulting electrical power is given by P = VI.

A single solar cell can produce only a limited amount of electrical power, so spacecraft connect many cells together to form panels and larger solar arrays. Cells connected in series increase voltage, while parallel connections increase the available current. A typical spacecraft solar-power system includes solar cells, interconnects, protective coverglass, structural supports, deployment mechanisms, wiring and power-conditioning electronics. During periods of sunlight, the solar array powers the spacecraft and charges its batteries. When a spacecraft passes into the shadow of a planet or moon, known as an eclipse, the batteries supply power to the onboard systems. Advanced spacecraft increasingly use multi-junction solar cells, which contain multiple semiconductor layers with different bandgaps. Each layer is designed to absorb a different portion of the solar spectrum, reducing some of the losses associated with a single-junction cell and enabling much higher efficiency. Materials such as GaAs, indium gallium phosphide and germanium are commonly associated with these advanced structures. Although they are more complex and expensive than conventional silicon cells, their high efficiency, power density and radiation performance make them particularly valuable for space missions.

Designing solar-power systems for space is ultimately a balance between efficiency, reliability, mass and lifetime. Spacecraft are continuously exposed to energetic particles such as protons, electrons and heavy ions, which can create defects in semiconductor materials and gradually reduce electrical performance. This radiation degradation can lower current, voltage, maximum power and overall efficiency, so spacecraft solar cells may use radiation-resistant materials, protective coverglass and carefully designed end-of-life power margins. Temperature is another major challenge because solar cells experience heating from direct sunlight and cooling during eclipse, with temperature changes affecting voltage and efficiency. Engineers must also account for micrometeoroid impacts, shadowing from spacecraft structures, deployment failures, electrical losses and the increasing power requirements of modern communication, sensing, computing and propulsion systems. Ultimately, a spacecraft solar array is much more than a collection of photovoltaic cells: it is an engineered power system designed to continuously transform the Sun’s nuclear energy into usable electrical power under some of the harshest conditions imaginable. As spacecraft become more capable and missions travel farther from Earth, advances in high-efficiency, radiation-resistant and lightweight solar technologies will remain essential to exploring and operating in space.

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