Researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have proposed enhancing cell and module efficiency to record levels, scale manufacturing, address reliability, and durability issues, and figure out the design of hybrid tandem solar modules to better commercialize them.

Tandem cells, which combine two or more junctions, are recognized for their potential to achieve much higher efficiencies—over 40% under ideal conditions—compared to single-junction cells. Despite this, the commercial scalability of these cells continues to be hindered by the need for improved material properties, better integration techniques, and solving the durability and reliability issues.

In a recently published study in the journal Joule, the researchers have outlined a pathway by reviewing the current state of photovoltaic (PV) technologies and proposed a roadmap for overcoming the challenges. Through this, the team was able to effectively map the fundamental aspects of the cells and also identify the technical hurdles that needed to be navigated to improve the commercial viability of the tandem cells.

As per the researchers, most solar modules currently in use depend on a single junction, which can only absorb a fraction of the solar spectrum, limiting their efficiency. Tandem solar cells, when stacked together, hold the potential to reach higher efficiencies.

Tandem cells also come with their own challenges. ‘For single-junction PV materials, modules are made by interconnecting individual cells laterally in series. For tandems, there are multiple options to interconnect the cells, which provides another layer of complexity to the design of tandem modules,” said co-author Emily Warren, a staff scientist in the High-Efficiency Crystalline Photovoltaics group at NREL.

The roadmap they charted focuses on combining two or more different PV technologies — hybrid tandems — to provide maximum efficiency. These are further divided into top cells and bottom cells.

The researchers have also found that only three single-junction PV technologies were successfully scaled up to at least a gigawatt production.

Inroads have been made by solar cells from copper indium gallium diselenide (CIGS) and cadmium telluride (CdTe). CIGS is suitable for a bottom cell and CdTe as a top.

However, the commercialization trajectories between silicon and thin-film cells made of CIGS and CdTe inform the challenges that must be overcome to establish a gigawatt-scale tandem technology. Silicon PV benefitted from a steady influx of investments by the semiconductor community, resulting in shared knowledge and standardized processes.

For CIGS and CdTe, companies have guarded their processes and deposition techniques as they attempt to make headway against silicon PV.

For the top cell, the roadmap also vouches for the potential of other materials such as Gallium arsenide (GaAs) and gallium indium phosphide (GaInP)- both of which are known for their highest efficiency in single-junction devices. However, they are expensive, and the researchers at NREL were keen to search for cheaper manufacturing methods.

“Metal halide perovskites provide high enough efficiencies as a top cell and are also cost-effective enough to incorporate that the tandem would have much higher efficiencies than the single-junction cells of either technology,” said Kirstin Alberi, the lead author of Joule.

For the bottom cell, silicon is an obvious choice that is currently dominating the solar industry. The researchers claim that a tandem cell consisting of these top and bottom cells currently holds the record for efficiency. Further, this can be an important driver for lowering the total system cost by reducing the area of the system and the associated balance of system costs.

With these benefits, the study has placed tandem cells on the roadmap of PV manufacturers and has predicted that the cells will reach 2% of the market share by 2030.

“Existing consortia have proven to be immensely helpful in the development and commercialization of single-junction PV technologies because they can help in information sharing, advocating for cross-cutting research that will help the field as a whole, and focusing larger sets of stakeholders to work together to solve problems that impact the entire field,” said Alberi.

The authors have sought to enable researchers and manufacturers to collaborate and address important aspects of tandem design, reliability, and scaling to facilitate progress in mass commercialization.

Last month, researchers from the National University of Singapore announced a record energy conversion efficiency of 27.1% in a triple-junction tandem solar cell that they developed.

Earlier this year, scientists from the University of Michigan also found that adding bulky additives can improve the stability of perovskite solar cells and make them last longer.