Perovskite tandem provider integrates with existing solar manufacturing lines

Perovskites are expected to play a critical role in the advancement of solar energy worldwide. The market is maturing rapidly as research and development breakthroughs continue to occur.

As silicon-based solar cells are advancing incrementally, reaching towards their theoretical limits, perovskite tandems have emerged to push efficiencies higher.

Perovskites are a material that can be used as the active material in solar cells on their own, and can be deposited in extremely thin layers, opening the door for applications in many different locations. Today, perovskites are mainly being explored as tandems, deposited as a layer above conventional silicon solar cells.

The more efficient a solar panel is, the more total system costs are driven down, and the more power you can get out of a constrained area like a residential rooftop.

Longi currently holds the record for silicon solar cell efficiency, reaching just over 27%. It recently achieved 34.6% efficiency with a silicon-perovskite tandem. Researchers from Fraunhofer ISE model that tandems could reach a practical efficiency of up to 39.5%, far beyond what silicon can do on its own.

Perovskites tandems have applications for solar products and projects of all sizes, from mobile devices to utility-scale solar projects, but one of the most exciting immediate applications may be for rooftop solar, said Ernest “Charlie” Hasselbrink, the chief technology officer at Caelux.

“If you can add 30% [efficiency] in a residential panel, in a system costing $3 per watt, you add $0.90 per watt of value that you can pick up for pennies,” Hasselbrink told pv magazine USA.

pv magazine USA sat down for an interview with Hasselbrink, the CTO at Caelux, a specialist in four-terminal perovskite tandems to discuss differences in two leading development pathways.

2T or 4T

There are two main architectures currently being explored by the industry today: a two-terminal (2T) and four-terminal (4T) approach. The 4T tandem structure has autonomous electrical connectivity to its two cells, while the 2T architecture is series-connected.

*Image: Nanomaterials, Muhammad Asmontas*

While Longi’s record tandem was set with 2T architecture, which offers slightly higher boosts for efficiency, Hasselbrink sees better applications for 4T tandems for several reasons. As perovskite chemistries continue to be explored to boost reliability and long-term performance in the field, 4T may offer more flexibility in the options available to researchers.

First, 4T perovskites can be placed on a completely separate substrate from the silicon solar cell, while 2T cannot. This may prove important as it gives perovskite developers less constraints for what substrate they use, possibly leading to a more durable design. Durability of performance remains the central problem to solve for perovskite use in the field.

“Our approach is, let’s coat this on the glass, and then there is almost no change for any of our customers, traditional silicon module manufacturers,” said Hasselbrink. “They can build their panel as they would do normally.”

What’s more, 4T offers better flexibility for bandgap design. The bandgap is the minimum energy required to excite an electron in a semiconductor to a higher energy state. With 2T tandems, currents have to be matched, which means once you’ve picked a cell’s bandgap, there is limited flexibility for the bandgap of the perovskite cell. With 4T, the solution space is wider, enabling more flexibility to choose chemistries that are more stable.

Hasselbrink said that by being placed on their own substrate, 4T perovskites benefit from chemical, mechanical and electrical isolation. Often, 2T tandems are deposited onto the textured surface of the silicon cell, which results in slightly better light absorption, but it may also cause limitations for perovskite developers as they explore different chemical and mechanical configurations.

“Why constrain the solution space for a small benefit?” said Hasselbrink.

Another benefit for process and design flexibility is that 4T must be voltage-matched rather than current-matched, said Hasselbrink. More than about 10-15% current mismatch is a major challenge for durability, resulting in very hot cells if not properly bypassed. In contrast, 4T tandems are voltage-matched.

“The voltage of solar cells only weakly depends on the irradiance – even when the irradiance falls by half, the voltage falls only by ~5%,” said Hasselbrink. “This isn’t enough to create any major mismatch, performance losses, or reliability problems.”

Scalability

Hasselbrink said many of the headline-capturing leading efficiencies achieved by perovskites are made using non-economically viable processes and often ignore manufacturing defect issues that could arise at scale.

He said Caelux develops its lab processes with scalability in mind, using equipment that is either a smaller version or even manufactured by the same vendor as their production equipment. This approach streamlines the transition from research to manufacturing, he said.

Focus on durability

Hasselbrink warned that the research and development community may be too focused on efficiency gains, and more focus should be placed on durability and reliability, which remains the most important innovation factor for perovskites.

“The National Renewable Energy Laboratory efficiency chart gives academic teams a chance to get their name on the worldwide stage by achieving best efficiencies,” said Hasselbrink. “There could be tremendous value in creating similar charts for durability: best performances in curated outdoor tests, as well as in and in specific, standardized accelerated tests (which give much faster feedback).”

Labs placing an emphasis on reliability in reporting may steer the research community in a more important direction, he said.

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