Image courtesy of Fraunhofer.

Solving The Silicon-Perovskite Tandem Solar Cell Puzzle

December 16, 202416 hours ago Tina Casey 0 Comments

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Tandem silicon-perovskite solar cells combine the familiar silicon PV technology with the not-so-familiar material perovskite, and they have been grabbing attention for their potential to deliver more solar energy over a smaller area, at less cost. So, what’s not to like about a tandem solar cell? End-of-life questions are one key area under exploration, and solutions are emerging.

Perovskites & The Tandem Solar Cell Revolution

A silicon-perovskite tandem solar cell combines the best of both photovoltaic worlds. Silicon has set the gold standard for commercial-scale solar conversion efficiency, but silicon is expensive relative to other photovoltaic materials. Perovskite has a lower conversion efficiency but it yields a less expensive solar cell, partly because it is more amenable to high volume, low cost manufacturing methods than silicon (see more perovskite background here).

Perovskite is a finicky material on its own, so combining it with silicon in a tandem solar cell can bring down the overall cost of a solar cell while achieving a high level of solar conversion efficiency. Some PV stakeholders are already pursuing the tandem solar cell path to commercialization, one example being the UK firm Oxford PV.

Chasing The Tandem Solar Cell Beyond The Schockley-Queisser Border, Or Not

The rapid progress of tandem solar cell R&D was underscored last year, when the Chinese firm LONGi announced a new tandem solar cell with a record-breaking solar conversion efficiency of 33.9%. The 33.9% figure is significant because it edges past the theoretical limit for solar conversion efficiency in a silicon solar cell. The limit been established at 33.7% based on calculations by solar researchers William Shockley and Hans-Joachim Queisser back in 1961.

In terms of commercialization, though, one over-arching question is the cost of manufacturing. The most efficient tandem solar cell is not necessarily the most competitive one to produce. The emerging circular economy also needs to be taken into consideration. That includes the sustainability of the material supply chain and the manufacturing process along with end-of-life treatments.

A multi-branched team at the leading German solar research institution Fraunhofer has been engaged in a soup-to-nuts analysis of the most competitive pathways for commercial-scale development of a silicon-perovskite tandem solar cell, under the somewhat awkward title of “MaNiTU.” Their conclusion, released last week, indicates that breaking the Shockley-Queisser barrier is a neat trick but not necessarily the most commercially viable one.

With the current state of industrial technology in mind, the researchers developed a new perovskite-silicon tandem solar cell along with a bespoke production process aimed at teasing solar conversion efficiency up while containing supply chain and manufacturing costs. They concluded that a solar conversion efficiency of 31.6% presents the most viable and sustainable pathway to mass adoption for high-efficiency tandem solar cells.

Sustainability & The Silicon-Perovskite Tandem Solar Cell Of The Future

The sustainability angle is a crucial one because the Fraunhofer team advocates for a tandem solar cell that deploys a perovskite formula based on lead, a material commonly used in the perovskite field as a stabilizer. The Fraunhofer team also examined non-leaded perovskite formulas, but concluded that none were commercially viable at this time.

“The research team was unable to produce tandem solar cells of sufficient efficiency from any of the lead-free materials that they theoretically and experimentally analyzed, however, as the intrinsic material qualities were simply not high enough,” Fraunhofer explained in a press release dated December 11.

Lead was once a key ingredient in many consumer products including gasoline, house paint, toys, and dishware. It has been banned or significantly reduced for these and other uses here in the US, particularly in use cases where human exposure and environmental leaching is a risk.

Those risks are not present in a tandem solar cell. For that matter, the US Environmental Protection Agency notes that lead is still used commonly used to produce sheet metals and other alloys. Lead also plays a ubiquitous role in electronics manufacturing, and battery manufacturers also continue to use lead, one example being the lead-acid batteries used in practically every gas-powered vehicle on the road.

Recycling & The Circular Economy

As a key part of the MaNiTU project, the Fraunhofer team developed a lifecycle analysis of their 31.6% efficient tandem solar cell, which determined that “advanced recycling processes” would support lifecycle energy efficiency. The press release did not include details, so I’m reaching out to the institution for more information. In the meantime, other researchers have also been developing perovskite recycling solutions.

“When it comes to perovskite solar modules that contain toxic and water soluble lead, going to landfill is not an option due to its threat to ecosystem and human health,” note the authors of a 2021 study that explored a recycling solution that combines a heat treatment to de-laminate perovskite films, along with a carboxylic acid cation-exchange resin adsorbent to capture the lead ions.

“Different from previous lead-trapping studies via strongly acidic cation-exchange resin, the weakly acidic cation-exchange resin showed better lead recycling efficiency due to easy release of Pb²⁺ ions from carboxylic acid functional group compared with sulfonic acid,” the researchers emphasized.

The Lead-Free Tandem Solar Cell Of The Future

Last spring, a research team in The Netherlands issued detailed lifecycle comparison between a tandem solar cell and a silicon-only solar cell that provides some further insights. The study included recovering silicon and silver from the tandem solar cells as well as lead. “Climate change impacts per kilowatt hour were projected to decrease by two-thirds over time,” the researchers concluded, while noting that the overall results would depend on the ability of tandem solar cells that to outlast their silicon-only counterparts.

In the meantime, the search for a lead-free alternative continues. Tin has emerged as one possible pathway to lead-free perovskites, since it occupies the same space on the periodic table. The idea first crossed the CleanTechnica radar back in 2014. Catching up in January of this year, we find that researchers are beginning to unlock the secrets of growing tin perovskite crystals.

Another lead-adjacent element, bismuth, has also been the subject of study. Last spring a research team in China synthesized a lead-free CsBiSCl₂ perovskite formula for use as a solar energy harvester. They concluded that the stability factor was promising enough to warrant further exploration, so keep an eye open for more on the bismuth side.

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Photo (cropped): Researchers have designed a new silicon-perovskite tandem solar cell to maximize solar conversion efficiency and lifecycle sustainability together (courtesy of Fraunhofer).