Lighter satellites thanks to new technology

A high-speed data line via cable or mobile communication systems is not available to ev - eryone in the world. Yet where this is the case, satellites can provide a broadband Internet connection. Elon Musk's Starlink is the best-known example of this approach. Most mod - ern satellites are powered by solar cells, whereby the greater their efficiency and the more durable they are, the longer a satellite can remain active. The University of Stuttgart's Institute for Photovoltaics (IPV) and Institute of Space Systems (IRS) are conducting research on solar cells that have the potential to make satellite operations more cost-effective.

As engineer Dr. Claudiu Mortan, a researcher at the IPV, explains: "The solar cells used in current satellites are based on gallium arsenide, not silicon like the photovoltaic modules installed on rooftops. This semiconductor material has been shown to be much more robust in the extreme conditions of outer space." However, another class of compounds, known as the perovskites, could prove to be even more suitable in the future and are already being used for terrestrial solar cells as an alternative or complement to silicon cells.

"Per - ovskites,” as Mortan explains, are metal halide compounds.” Dieter Weber of the Univer - sity of Stuttgart's Institute of Inorganic Chemistry published the first paper on perovskites back in 1978." This was followed by the first scientific publication on a perovskite solar cell in 2009. "Some 13,000 scientific publications have now been published on the subject," says Mortan, highlighting the enormous increase in interest. IPV head Prof. Michael Saliba is one of the world's most cited researchers in the field. "By contrast," Mortan says, "only around 40 papers have been published on perovskite solar cells for use in space.” The IPV and IRS launched the PÆROSPACE project to dedicate themselves to this subfield.

The IPV received 50,000 euros of start-up funding for this purpose in 2022 through the University of Stuttgart's "Terra incognita" research funding program, which is intended to help develop previously undefined research fields using interdisciplinary approaches and to facilitate pioneering research. Current solar cells made of semiconductor materials are produced on wafers in a complex and therefore expensive manufacturing process. The use of perovskites, on the other hand, is much simpler as they can be painted onto a substrate as a solution, after which the solvent evaporates leaving behind the desired layer.

The active layer of perovskite cells is much thinner than that of cells made of semiconductor materials, which, as Mor - tan explains: "has a positive effect on the weight-specific electrical output." "Whereas conventional solar cells made of gallium arsenide can only produce three watts per gram, perovskite solar cells can achieve 30 watts per gram.” The lower a satellite's weight, the less expensive it is to launch. "Another benefit is that perovskite solar cells can easily be applied to films or foils," Mortan says and these can be rolled up. The upshot is that a satellite equipped with per - ovskite cells would not only be lighter, but would also have smaller dimensions at launch. Gallium arsenide solar modules are too thick to roll up and can only be folded and hence require much more space.

The objective of the PÆROSPACE project is to find suitable perovskite solar cells for use in space. "However," as Mortan further explains, "solar cells in are exposed to extreme conditions in space that cause them to age faster than they do on Earth." On the one hand, temperatures can fluctuate greatly, from minus 50 degrees Celsius in the Earth's shadow to 150 degrees Celsius in direct sunlight and any satellite orbiting at a similar altitude to the International Space Station experiences this temperature change every one and a half hours, which can cause cracking in the cell. On the other hand, the vacuum in space could cause unwanted outgassing of atoms from the cell, which would impair its function. And, last but not least, solar cells can be hit by energetic particles, e.g., from the sun, resulting in a deterioration of their optoelectronic properties. For all of these reasons, perovskite solar cells must first be qualified for use in space.

Mortan's team is working on a housing whose surface area is about the size of two smartphones placed side by side, which will contain four perovskite solar cells produced at the IPV, each a few square centimeters in size, which are to be tested in a stratospheric balloon at an altitude of 35 kilometers, where whilst the prevailing conditions are not the same as in outer space, they are much more extreme than on Earth. The balloon will be launched by KSat, the University of Stuttgart's Student-based Small Satellites Group. During the several-hour-long flight, Mortan's team will record changes in the character - istic properties of the four cells. Overlapping with this balloon-based experiment, preparations for a satellite test of perovskite cells are being made in collaboration with the IRS. The funding application has already been submitted. A small satellite will be launched into a 2,500-kilometer orbit in 2025 to test the IPV's perovskite solar cells under space conditions for at least a year. "As yet,” Mortan explains, “no such long-term analytical data is available.”

Author: Michael Vogel