Thin-film tandem solar in the U.S. Tandem solar-cell technology featuring silicon has been widely researched but materials such as perovskites, paired with established thin-film solar or with other perovskite cells, are pointing to a new development path. September 19, 2024 Valerie Thompson Modules & Upstream Manufacturing Technology and R&D United States Image: NREL Share Silicon-free tandem solar cells are a topic of research for commercial, academic, and institutional labs in the United States. Researchers at the US Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) have published a tandem technology roadmap in the journal Joule which included emerging all-organic, all-perovskite tandem combinations and cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) devices. Multiple teams in the United States have announced lab-scale, all-thin-film cells with conversion efficiencies above 27%. “Tandems that are all-thin-film are a logical place to eventually go,” said Andries Wantenaar, solar analyst at market intelligence company Rethink Research. “Universities are achieving remarkable efficiencies. The first wave is single junction, then perovskite-silicon tandem and then, eventually, all-thin-film.” Thin-film advantage First Solar is the largest mainstream thin-film PV manufacturer. It produces CdTe panels for utility-scale solar and is investing in raising its annual production capacity to 25 GW in 2026. The company is investing in R&D focused on higher-efficiency cells and in 2024 announced a 23.1%-efficient CdTe cell and a 23.6%-efficient CIGS cell, setting records for both technologies. CEO Mark Widmar consistently stresses the importance of thin film in commercializing high-efficiency tandem devices, a message he repeated at the commissioning of First Solar’s new research center in Ohio in July 2024. The Jim Nolan Center is part of an approximately $500 million R&D investment by First Solar. The 1.3 million square foot (120,000 m2) facility includes pilot manufacturing support for full-sized prototypes of thin-film and tandem PV modules. In May 2024, First Solar was awarded $6 million to develop a perovskite top cell and CIGS bottom cell tandem device. The aim is a 27%-efficient design to be scaled to “mini modules” with practical manufacturing processes. Funding was announced in May 2024 by the DOE’s Solar Energy Technologies Office initiative to advance US thin-film solar production. A month earlier, First Solar announced a partnership with the Zentrum für Sonnenenergie und Wasserstoff Forschung (ZSW), in Germany, to focus on performance and potential to “develop and optimize all-thin-film tandem technologies on a gigawatt scale.” ZSW is known for its thin-film manufacturing processes for rigid and flexible substrates. “Our decades of experience with thin-film photovoltaics fits perfectly for tandems, which always contain at least one thin-film cell,” said ZSW researcher Stefan Paetel. In February 2024, the First Solar European Technology Center and Sweden’s Uppsala University achieved the CIGS cell efficiency record of 23.6%. The Swedish university’s team leader, Marika Edoff, commented on the potential tandem solar holds. “For the CIGS technology, which is known for high reliability, a world record also means that it may offer a viable alternative for new applications in, for example, tandem solar cells,” she said. In December 2023, First Solar researchers published an industrial perspective on all-thin-film tandem solar cells in the Journal of Physics: Energy. The researchers concluded there is a good probability that high-efficiency tandem solar cells will be manufactured at high volume “within the foreseeable future” despite less-than-ideal device and material quality. New ventures Several US-based startups are working on perovskite-silicon tandem devices, including CubicPV, Caelux, Swift Solar, and Tandem PV. The shared outlook is also positive about the future of silicon-free tandems. “All-thin-film tandems could be a long-term prospect but there is still work to be done on the bottom cell, especially if it is going to be a perovskite bottom cell,” said John Iannelli, the founder of Caelux. Iannelli said that unlike the “vast improvements in stability and efficiency made with wide-bandgap perovskite top cells,” research on perovskites for the bottom cell is “somewhat lacking.” Looking ahead, Iannelli sees a four-terminal (4T) approach as likely for all-perovskite tandems. That would be the same method used in the CaeluxOne product line, an active glass solution with perovskite on the inside of the cover glass to enable crystalline silicon module manufacturers to increase efficiency of standard-sized panels. For example, a conventional silicon panel with 21.5% efficiency becomes a 27% efficient tandem device, according to Iannelli. At CubicPV, a company spokesperson said perovskite-perovskite tandems have potential, “but the development time is very long … There are significant scientific challenges associated with perovskite as the bottom layer in a module.” Scott Wharton, the CEO of Tandem PV, expressed a similar view. “We definitely think that a perovskite-perovskite tandem could be on our roadmap,” he commented. “But in the near-term we are focused on perovskite-silicon as a more practical approach.” Swift Solar is currently developing high-efficiency perovskite tandem photovoltaics, according to CEO Joel Jean. “Right now, we’re focused on perovskite-silicon tandems but we could also see an all-perovskite tandem product in the future,” Jean said. Swift Solar reported making strides in thin-film deposition speed, which is now 10 times faster than it was two years ago. Jean also said the company has “by no means exhausted the potential gains.” Perovskite startup Verde Technologies has pursued a silicon-free path from the start. Founded in 2021, Verde is developing 22%-efficient single-junction perovskite technology, which it makes with a roll-to-roll process. An all-thin-film tandem project is underway in Verde’s lab. “We have a partnership with a commercial entity aiming for tandems with 30% efficiency by 2027-28,” said Chad Miller, the chief technology officer of Verde Technologies. “It is the kind of step change we are expecting for the PV industry.” Miller added that such an outcome would avoid the conventional silicon PV supply chain “with all its challenges.” Research and academia There are several US university research teams investigating silicon-free tandem solar devices. For example, at the Wright Center for Photovoltaics Innovation and Commercialization (PVIC), at the University of Toledo, researchers demonstrated a 4T tandem made with its in-house cadmium selenium telluride bottom cell and a metal halide perovskite top cell and back contact design, which delivered 25% efficiency. The same group also demonstrated an all-perovskite tandem solar cell with 27.8% efficiency. “Tandem research is an area of growing importance and interest within the scientific community,” explained PVIC leader Zhaoning Song. “Many researchers and companies are seeking solutions to unlock the potential of tandem solar cells. Thin-film tandems have great promise for future applications but also need more research and development to realize their full potential.” At Northwestern University, within the Department of Chemistry’s Sargent Group, recent collaborations resulted in lab-sized, all-thin-film devices with improved stability and power conversion efficiencies of up to 28.1%. “In addition to all-perovskite double junctions, triple-junction solar cells have great potential for even higher efficiencies,” said assistant professor Bin Chen. “To get there, we need to develop wider-bandgap materials. The good news is that many of the techniques we’ve used for double junctions can also work for triple junctions so we expect to see a lot of progress in triple-junction efficiencies in the next few years.” Looking ahead The commercialization of all-thin-film tandem and multijunction technology could enable much greater form-factor flexibility, among other benefits. A greater range of form factors and module specifications could in turn unlock new applications for solar. The ability to generate greater power per unit area, due to high-efficiency thin-film devices, would also make PV practical for many more applications, which could accelerate the adoption of solar. “What is needed for the next five terawatts of PV adoption is not the same as what was needed for the first five,” said Joseph Berry, NREL researcher and roadmap co-author. In the meantime, improving stability and module lifetimes are key research areas. If perovskites are to be used, there is a need for new models for predicting energy output and for new techniques to measure module performance, according to Timothy Silverman, who has led NREL’s perovskite module field testing since 2022 at the DOE-funded PV accelerator for commercializing technologies. Similar qualification problems had to be overcome in the past with other thin-film technologies, according to Silverman. “So it is important to start studying it now,” he added. To overcome the challenges for tandem technology related to the complexity of integrating sub cells, thermalization issues, and the need for efficient deposition technology and other challenges, it is likely that consortia of commercial and research organizations will drive progress in the coming years, according to Emily Warren, NREL researcher and roadmap co-author. “As a national lab, the research on this technology should be done now to make it a possibility in five or six years’ time,” she said. This content is protected by copyright and may not be reused. If you want to cooperate with us and would like to reuse some of our content, please contact: editors@pv-magazine.com. 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