| Work Detail |
Tandem solar cell technology with silicon has been widely researched, but materials such as perovskites, combined with established thin-film solar cells or other perovskite cells, point to a new avenue of development.
Silicon-free tandem solar cells are a hot topic of research for commercial, academic, and institutional laboratories in the United States. Researchers at the U.S. Department of Energys (DOE) National Renewable Energy Laboratory (NREL) have published a tandem technology roadmap in the journal Joule that includes emerging all-organic perovskite tandem combinations and cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) devices.
Several US teams have announced lab-scale thin-film cells with conversion efficiencies above 27%. “Thin-film tandems are the logical way to go,” says Andries Wantenaar, a solar analyst at market intelligence firm Rethink Research. “Universities are achieving remarkable efficiencies. The first wave is single-junction, then perovskite-silicon tandem, and finally thin-film.”
The advantage of thin layer
First Solar is the largest thin-film PV manufacturer. It produces CdTe panels for utility-scale solar power and is investing to increase its annual production capacity to 25 GW by 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 launch of First Solar’s ??new Ohio research center in July 2024.
The Jim Nolan Center is part of an estimated $500 million investment in R&D by First Solar. The 1.3 million square foot (120,000 m2) facility includes pilot manufacturing 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/CIGS bottom-cell tandem device. The goal is a 27% efficient design that can 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 boost thin-film solar production in the United States.
A month earlier, First Solar announced a partnership with the Zentrum fur Sonnenenergie und Wasserstoff Forschung (ZSW) in Germany to focus on the performance and potential to “develop and optimize gigawatt-scale thin-film tandem technologies.”
A month earlier, First Solar announced a partnership with the Zentrum fur Sonnenenergie und Wasserstoff Forschung (ZSW) in Germany to focus on the performance and potential to “develop and optimize gigawatt-scale thin-film tandem technologies.”
ZSW is known for its thin film manufacturing processes for rigid and flexible substrates. “Our decades-long experience in thin-film photovoltaics is an ideal fit for tandems, which always contain at least one thin-film cell,” says Stefan Paetel, researcher at ZSW.
In February 2024, the European Technology Centre First Solar and Sweden’s Uppsala University achieved a record efficiency for a CIGS cell with 23.6%. The team leader at the Swedish university, Marika Edoff, commented on the potential of tandem solar. “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 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 that high-efficiency tandem solar cells are very likely to be manufactured in large quantities “in the foreseeable future,” even though device and material quality is not ideal.
New companies
Several US-based startups are working on perovskite-silicon tandem devices, including CubicPV, Caelux, Swift Solar and Tandem PV. Shared outlooks are also positive about the future of silicon-free tandems.
“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, founder of Caelux.
According to Iannelli, unlike the “huge improvements in stability and efficiency achieved with wide-bandgap perovskite top cells,” research on perovskites for the bottom cell is “somewhat sparse.” Looking ahead, Iannelli sees a four-terminal (4T) approach for all-perovskite tandems as likely. This 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 the efficiency of standard-size panels. For example, a conventional silicon panel with 21.5% efficiency becomes a tandem device with 27% efficiency, 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 a bottom layer in a module.”
Scott Wharton, CEO of Tandem PV, expressed a similar view. “We think a perovskite-perovskite tandem could be on our roadmap,” he said. “But in the near term we are focused on perovskite-silicon as the most practical approach.”
Swift Solar is currently developing highly efficient perovskite tandem PV systems, according to CEO Joel Jean. “Right now we are focused on perovskite-silicon tandems, but in the future we could see an all-perovskite tandem product,” Jean said.
Swift Solar reported advances in thin film deposition speed, which is now 10 times faster than two years ago. Jean also said the company has “by no means exhausted the potential gains.”
Verde Technologies, a perovskite startup, has been following a silicon-free path from the start. Founded in 2021, Verde is developing a 22% efficient single-junction perovskite technology, which it manufactures using a roll-to-roll process. A thin-film tandem project is underway in Verde’s lab.
“We have a partnership with a commercial entity that aims to achieve 30% efficient tandems by 2027-28,” says Chad Miller, chief technology officer at Verde Technologies. “That’s 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 universities
Several US university teams are 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 their own selenium cadmium telluride bottom cell and a perovskite metal halide top cell and back-contact design, which achieved 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 in the scientific community,” explains Zhaoning Song, director of PVIC. “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 they also need further research and development to realize their full potential.”
At Northwestern University, within the Sargent Group in the Department of Chemistry, recent collaborations have resulted in laboratory-sized, all-thin-film devices with improved stability and power conversion efficiencies of up to 28.1%. “In addition to perovskite double junctions, triple-junction solar cells have great potential to achieve even higher efficiencies,” says Associate Professor Bin Chen.
“To achieve this, we need to develop wider bandgap materials. The good news is that many of the techniques we have used for double junctions also work for triple junctions, so we expect to see many advances in triple junction efficiency in the coming years.”
Looking to the future
Commercialization of all-thin-film tandem and multijunction technology could enable much greater form factor flexibility, among other advantages. In turn, a greater variety of module form factors and specifications could open up new applications for solar. The ability to generate more power per unit area, thanks to high-efficiency thin-film devices, would also make photovoltaics practical for many more applications, potentially accelerating solar adoption. “What’s needed for the next five terawatts of photovoltaics adoption is not the same as what was needed for the first five,” said Joseph Berry, a researcher at NREL and co-author of the roadmap.
Meanwhile, improving module stability and lifetime are key research areas. If perovskites are to be used, new models for predicting power output and new techniques for measuring module performance are needed, according to Timothy Silverman, who is leading NREL’s field testing of perovskite modules starting in 2022 at the DOE-funded photovoltaic accelerator for technology commercialization.
Similar qualification issues have had to be overcome in the past with other thin-film technologies, Silverman said. “That’s why it’s important to start looking at it now,” he added.
To overcome the challenges of tandem technology related to the complexity of subcell integration, thermalization issues, and the need for efficient deposition technology, among others, consortia of commercial and research organizations are likely to drive progress in the coming years, according to Emily Warren, a researcher at NREL and co-author of the roadmap. “As a national laboratory, research on this technology needs to be done now for it to be a possibility in five or six years,” she said. |
