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04/12/2025
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Since the late 1990s, high-efficiency III-V multi-junction solar cells have become the standard for satellite power systems due to their excellent efficiency and radiation tolerance. However, their production is resource- and energy-intensive, which conflicts with Green Space sustainability goals.
While material-efficient fabrication of terrestrial silicon solar cells is already commercially mature, such processes are generally incompatible with the harsh conditions of the space environment. To address this challenge, Fraunhofer ISE, supported by ESA’s Discovery & Preparation element, is developing a mask-and-plate microfabrication approach as a reliable alternative manufacturing method for III-V based solar cells for space applications.
For more than 30 years, III-V solar cells have become the standard power source for satellites thanks to their high efficiency and strong radiation tolerance compared with silicon-based solar cells.
These devices are fabricated by growing very thin semiconductor layers on germanium substrates using a process called epitaxy. After the layers are grown, the cells are manufactured using specific processes. This precise manufacturing approach is compatible with the harsh conditions in space, but it is resource and energy intensive, which clashes with Green Space sustainability goals.
This resource intensity stems from three main factors: the reliance on germanium (Ge) as the substrate; the energy-intensive epitaxial growth process; and the subsequent microfabrication, which involves photolithography and metal evaporation steps – both costly, time-consuming, and energy-demanding steps.
Promising work is underway to enable substrate re-use and efforts are targeting more efficient epitaxial processes. However, microfabrication is not really tackled yet for space solar cells. While material-efficient technologies are already available for conventional terrestrial silicon solar cells production, the requirements for space solar cells usually prevent their implementation, as some materials used in these technologies are incompatible with the reliability needs of the space environment.
A team from Fraunhofer ISE is developing an innovative mask-and-plate approach to microfabricate III-V space solar cells without the use of photolithography or metal evaporation, a solution supported by the European Space Agency through its Discovery & Preparation element. The idea was submitted via ESA’s Open Discovery Ideas Channel (OSIP).
AlternateSpace, the solution developed by the team, is a compelling alternative that addresses sustainability concerns by replacing photolithography with inkjet printing technology, a technique that is well-established in the graphics and TV screen manufacturing industries.
The use of hotmelt inks in this process offers several advantages: the technique does not rely on toxic or photoactive materials, it can be applied directly in a precisely controlled pattern and it eliminates wet-chemical development steps, significantly simplifying the process chain and reducing chemical waste.
For metal contact deposition, this innovative approach also replaces metal evaporation with electroplating. This allows metal to be deposited only on areas where the semiconductor material is not covered by ink with no subsequent lift-off steps required.
This alternative solution required extensive optimisation, including testing various inks and adjusting parameters, such as resolution and temperature, to achieve reliable small contact openings. The mask’s chemical compatibility was verified by testing the hotmelt ink across different electrolytes, temperatures and pH values.
The subsequent metallisation involved the evaluation of different metal stacks for electroplating and explored nickel-phosphorus plating as a non-ferromagnetic alternative to standard nickel. A final sample featuring silver front side contacts on nickel-phosphorus emerged as a space-compatible option.
After a fully defined process route that incorporates all the newly developed steps, a fully functional photolithography-free solar cell based on space-compatible electroplated metal contacts is expected in December.
“This work marks a key step toward cost-effective, sustainable and efficient III-V solar cell technology. It paves the way for a scalable and economically viable manufacturing route for next-generation III-V space photovoltaics. The results of the activity highlight the key role of ESA’s Discovery & Preparation programme in generating novel ideas that can drive the development of future space technologies”, commented Erminio Greco, Solar Generators Engineer at the European Space Agency.
“By replacing photolithography and metal evaporation with scalable inkjet printing and electroplating, Fraunhofer ISE demonstrates a simplified process with significantly reduced chemical waste. This approach aligns with the goals of green space sustainability and cost reduction. After the successful demonstration of this approach, we aim towards a collaboration with industry to further develop, stabilise and finally scale the process towards industrial realisation”, commented Oliver Höhn, Head of the III-V Semiconductor Technology Group at Fraunhofer ISE.
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In 2017, the early leaders in energy storage made an audacious bet: 35 gigawatts of the new grid technology would be installed in the United States by 2025.
That goal sounded improbable even to some who believed that storage was on a growth trajectory. A smattering of independent developers and utilities had managed to install just 500 megawatts of batteries nationwide, equivalent to one good-size gas-fired power plant. Building 35 gigawatts would entail a 70-fold growth in just eight years.
The number didn’t come out of thin air, though. The Energy Storage Association worked with Navigant Research to model scenarios based on a range of assumptions, recalled Praveen Kathpal, then chair of the ESA board of directors. The association decided to run with the most aggressive of the defensible scenarios in its November 2017 report.
In 2021, ESA agreed to merge with the American Clean Power Association and ceased to exist. But, somehow, its boast proved not self-aggrandizing but prophetic.
The U.S. crossed the threshold of 35 gigawatts of battery installations this July and then passed 40 gigawatts in the third quarter, according to data from the American Clean Power Association. The group of vendors, developers, and installers who just eight years ago stood at the margins of the power industry is now second only to solar developers in gigawatts built per year. Storage capacity outnumbers gas power in the queues for future grid additions by a factor of 6.5, according to data compiled by Lawrence Berkeley National Laboratory.
“Storage has become the dominant form of new power addition,” Kathpal said. “I think it’s fair to say that batteries are how America does capacity.”
Back in 2017, I was covering the young storage industry for an outlet called Greentech Media, a beat that was complicated by how little was happening. There was much to write about the “enormous potential” of energy storage to make the grid more reliable and affordable, but it required caveats like “if states change their grid regulations to allow this new technology to compete fairly on its merits, yada yada yada.”
Those batteries that did get built in 2017 look tiny by today’s standards. The locally owned utility cooperative in Kauai built a trailblazing 13-megawatt/52-megawatt-hour battery, the first such utility-scale system designed to sit alongside a solar power plant. And 2017 saw the tail end of the Aliso Canyon procurement, a foundational trial for the storage industry in which developers built a series of batteries in Southern California in just a handful of months to shore up the grid after a record-busting gas leak — adding up to about 100 megawatts.
“You saw green shoots of a lot of where the industry has gone,” said Kathpal.
California passed a law creating a storage mandate in 2010, then found a pressing need for the technology to neutralize the threat of summertime power shortages. Kauai’s small island grid quickly hit a saturation point with daytime solar, so the utility wanted a battery to shift that clean power into the nighttime. These installations weren’t research projects; they were solving real grid problems. But they were few and far in between.
Kathpal recalled one moment that encapsulated the storage industry’s early lean era. At the time, he was developing storage projects for the independent power producer AES. One night around midnight, he parked a rented Camry off a dirt road and pointed a flashlight through a sheet of rain. It was his last stop on a trip to evaluate potential lease sites for grid storage ahead of a utility procurement — looking at available space, proximity to the grid, and stormwater characteristics. But once the utility saw the bids, it decided not to install any batteries after all.
“The storage market is built not only from Navigant reports but also from moments like that,” he said. “We had to lose a lot of projects before we started winning.”
Now that same utility is putting out a call for storage near its substations — exactly the kind of setting Kathpal had toured in the rain all those years ago.
Indeed, many of the projects connected to the grid this year started with developers anticipating future grid needs and putting money on the line for storage back around the time ESA was formulating its big goal, said Aaron Zubaty, CEO of early storage developer Eolian.
“Eolian began developing projects around major metro areas in the western U.S. starting in 2016 and putting the queue positions in that then became operational in 2025,” Zubaty said. The 200-megawatt Seaside battery site at a substation in Portland, Oregon, is one example.
Though the storage industry pioneers somehow nailed the 35-gigawatt goal, market growth defied their expectations in several important ways.
ESA had expected more of a steady ramp to the 35 gigawatts, said Kelly Speakes-Backman, who served as its chief executive officer from 2017 to 2021. But the storage market ran into plenty of false starts, such as when states passed mandates to install batteries but never enforced them, and when federal regulators ordered wholesale markets to incorporate storage but regional implementation dragged on for years.