Silicon-free tandem solar cells are a research topic for commercial, academic, and institutional laboratories in the United States. Researchers at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) have published a roadmap for tandem technology in the journal Joule including 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 laboratory-scale fully thin-film cells with conversion efficiencies greater than 27%. “Tandems made entirely of thin film are a logical place to go eventually,” says Andries Wantenaar, solar energy analyst at market intelligence company Rethink Research. “Universities are making remarkable efficiency gains. The first wave is a single junction, then a perovskite-silicon tandem and finally a completely thin film.”
Advantage of thin film
First Solar is the largest mainstream thin-film PV manufacturer. It produces utility-scale CdTe solar panels and is investing in increasing annual production capacity to 25 GW by 2026. The company is investing in R&D focused on higher efficiency cells and announced a 23.1% efficient CdTe panel in 2024 cell and a 23.6% efficient CdTe cell. %-efficient CIGS cell, setting records for both technologies. CEO Mark Widmar consistently emphasizes the importance of thin film in commercializing high-efficiency tandem devices, a message he reiterated when inaugurating 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 feet (120,000 m2).2) includes pilot production support for full-size prototypes of thin-film and tandem PV modules.
In May 2024, First Solar received $6 million to develop a perovskite top cell and CIGS bottom cell tandem device. The goal is a 27% efficient design that can be scaled up to “mini modules” with practical production processes. The funding was announced in May 2024 by the DOE’s Solar Energy Technologies Office initiative to advance U.S. thin-film solar manufacturing.
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 at gigawatt scale.”
ZSW is known for its thin film production processes for rigid and flexible substrates. “Our decades of experience with thin-film solar energy fit perfectly with tandems, which always contain at least one thin-film cell,” says 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%. Swedish university team leader Marika Edoff commented on the potential tandem solar panels. “For CIGS technology, which is known for its high reliability, a world record also means that it can offer a viable alternative for new applications in, for example, tandem solar cells,” she says.
In December 2023, researchers from First Solar published an industrial perspective on all-thin-film tandem solar cells in the Journal of Physics: Energy. The researchers concluded that there is a good chance that high-efficiency tandem solar cells will be produced in large numbers “in 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 vision is also positive about the future of silicon-free tandems.
“Thin-film-only tandems could be a future prospect, but there is still work to be done on the bottom cell, especially if it is going to be a perovskite bottom cell,” says John Iannelli, the founder of Caelux.
Iannelli said that, in contrast to the “tremendous improvements in stability and efficiency achieved with wide bandgap perovskite top cells,” research on bottom cell perovskites is “somewhat lacking.” Looking ahead, Iannelli sees a four-terminal (4T) approach 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 that allows manufacturers of crystalline silicon modules to increase the efficiency of standard-sized panels. For example, a conventional silicon panel with an efficiency of 21.5% becomes a tandem device with an efficiency of 27%, 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.”
Tandem PV CEO Scott Wharton expressed a similar view. “We definitely think a perovskite-perovskite tandem could be on our roadmap,” he noted. “But in the near term, we are focusing 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 are focusing on perovskite-silicon tandems, but in the future we could also see a full perovskite tandem product,” Jean said.
Swift Solar reported that progress has been made in the deposition speed of thin films, which is now ten times faster than two years ago. Jean also said the company “has by no means exhausted potential profits.”
Perovskite startup Verde Technologies has been on a silicon-free path since its inception. Founded in 2021, Verde is developing a 22% efficient single-junction perovskite technology, which it makes using a roll-to-roll process. A full thin-film tandem project is underway in Verde’s laboratory.
“We have a partnership with a commercial entity that is targeting tandems with 30% efficiency by 2027-2028,” said Chad Miller, Chief Technology Officer of Verde Technologies. “This is the kind of step change we expect 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 research teams from American universities investigating silicon-free tandem solar panels. For example, at the Wright Center for Photovoltaics Innovation and Commercialization (PVIC), at the University of Toledo, researchers demonstrated a 4T tandem made with the internal cadmium-selenium telluride bottom cell and a metal halide-perovskite top cell and back contact design. which yielded an efficiency of 25%. The same group also demonstrated an all-perovskite tandem solar cell with an efficiency of 27.8%.
“Tandem research is an area of growing importance and interest within the scientific community,” explains PVIC leader Zhaoning Song. “Many researchers and companies are looking for solutions to unlock the potential of tandem solar cells. Thin-film tandems hold 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 laboratory-sized all-thin film devices with improved stability and energy conversion efficiency of up to 28.1%. “In addition to the all-perovskite double junctions, triple-junction solar cells have great potential for even higher efficiency,” says Assistant Professor Bin Chen.
“To get there, we need to develop materials with a wider bandgap. The good news is that many of the techniques we used for double intersections can also work for triple intersections. So we expect to see a lot of progress in the efficiency of triple intersections in the coming years.”
Looking ahead
The commercialization of all-thin-film tandem and multijunction technology could enable much greater form factor flexibility, among other benefits. A wider range of form factors and module specifications could in turn unlock new applications for solar energy. The ability to generate more energy per unit area, thanks to highly efficient thin-film devices, would also make PV practical for many more applications, which could accelerate solar energy adoption. “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 co-author of the roadmap.
In the meantime, improving the stability and longevity of modules are important areas of research. If perovskites are to be used, there is a need for new models for predicting energy yield and new techniques for measuring module performance, said Timothy Silverman, who has led NREL’s field testing for perovskite modules at the DOE-funded PV since 2022 -accelerator. commercialization of technologies.
Similar qualification issues 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 subcells, 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 co-author of the roadmap. “As a national laboratory, the research on this technology needs to be done now so that it is possible in five or six years,” she said.
This content is copyrighted and may not be reused. If you would like to collaborate with us and reuse some of our content, please contact: editors@pv-magazine.com.
