Ten years ago, Explorer Raphaël Domjan set out to fly a fully solar‑powered aircraft into the stratosphere, a goal many believed was not technically possible. In August 2025, SolarStratos turned this vision into reality by climbing to 9,521 meters and setting a world record for manned solar‑electric flight.
Aviation is one of the most difficult sectors to make climate-friendly because aircraft must carry all their energy on board, with every extra kilogram reducing how far and how high they can fly. Because SolarStratos flies only on sunlight and batteries, it had to be as light and efficient as possible, with wings that generate electricity, stay light, and remain smooth for safe flight in thin air. The batteries also had to be light, safe, and reliable, even in extreme cold.
SolarStratos is first and foremost the brainchild of Explorer Raphaël Domjan, who imagined an aircraft reaching the upper atmosphere using only the sun. To turn that idea into a real aircraft, he needed technologies that did not yet exist. That is where CSEM came in as a long-term partner, providing the solar and battery expertise needed to transform a daring concept into a record‑breaking aircraft.
From the early design phases, CSEM was asked to develop solar modules that were much lighter than anything on the market and could be built directly into the aircraft wings and tail. Using its materials know-how, CSEM also created a protective layer, known as an encapsulant, that shields the solar cells inside the modules so they keep working despite strong sunlight, cold, and vibration. “Our engineers then produced and installed more than 20 square meters of flexible solar modules with minimal weight and a surface quality indistinguishable from the aircraft’s own structure,” explains Matthieu Despeisse, Group Leader Sustainable Energy, CSEM. Mounted on wing sections, the modules were tested under demanding conditions and have now powered more than 100 hours of flight, including the record flight.
CSEM also helped define the battery approach for SolarStratos. Through a project funded by the Swiss Federal Office of Energy (SFOE), CSEM compared current lithium-ion batteries with lighter future options such as lithium-sulfur and lithium-silicon and showed that these could cut weight by about one third. This work continues to guide key design choices and supports lighter battery packs for future phases of the mission. CSEM also invested in a state-of-the-art dry room where sensitive advanced batteries can be safely developed. As Andreas Hutter, Group Leader Sustainable Energy, CSEM explains: “Energy density is key, and every gram matters in aviation, especially when you want to reach the stratosphere. We make sure that batteries are not only light but also deliver the required performance while remaining safe.”
SolarStratos is a flagship for clean mobility. It shows the public, decision-makers, and industry that solar-electric aviation is no longer science-fiction but a credible way to cut emissions. The technologies developed with CSEM, from ultra-light solar modules and advanced protective materials to lighter battery concepts, are already being explored for vehicle-integrated solar roofs, high‑altitude observation platforms, and self-powered transport containers. Over more than a decade, Raphaël Domjan, CSEM, and their partners have taken calculated risks, invested in new materials, and remained committed. The 2025 record flight symbolizes this achievement and represents a legacy of solar and battery solutions that can help aviation reach cleaner and higher horizons.
Every time SolarStratos climbs higher, I feel we are sending a message to the world: solar energy belongs to aviation’s future. CSEM’s innovations and relentless hard work have made that message louder. Together, we are demonstrating that innovation and a pioneering spirit are key drivers of the energy transition to protect our climate.
Visit CSEM’s Batteries and Integrated Lightweight Photovoltaics pages to see how we combine battery intelligence, sensing, and lightweight solar integration to create more efficient energy solutions. Or contact us through our form to start the conversation.