January 02, 2024
By Tina Casey

Perovskite Solar Cells Are Going Lead-Free, Eventually

June 21, 2021- NREL researcher Mengjin Yang holds a rigid glass perovskite module. (Photo by Dennis Schroeder / NREL)

Perovskite solar cells first bubbled up through the corridors of research laboratories back in 2006 with the promise of low cost materials and high solar conversion efficiency. They haven’t materialized on the mainstream market quite yet, but one key barrier to widespread adoption is finally on the verge of falling.

Why Perovskite Solar Cells Need Lead…

Perovskites are lab-grown optical materials based on the structure of the naturally occurring mineral perovskite. The material is fragile on its own, so lead is commonly used as a stabilizer. The American Institute of Physics lists CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ among the most common formulas used to fabricate perovskite solar cells.

The presence of lead in perovskite solar cells is not a surprise. Though lead is a toxic substance, it has been the go-to stabilizer for the the solder used in all sorts of electronic devices, which typically consists of 27% lead and 63% tin.

Electronic devices do not leach lead into the environment like other known hazards, such as leaded paint, gasoline, cosmetics, certain types of ammunition, and the soldering used in water systems. Nevertheless, misuse or improper disposal of e-waste can pose hazards. The EU banned tin-lead solder for consumer devices in 1999 and the US followed suit in 2006, though it is still permitted for other uses.

…For Now

Assuming that some markets for perovskite solar cells will be similarly permitted, several companies already appear close to commercializing solar cells that pair perovskite with silicon or other photovoltaic materials, aiming for a balance between durability, solar conversion efficiency, and cost.

In 2021 the US Department of Energy also launched a competition called the Perovskite Start-Up Prize to stimulate activity among American innovators.

In addition to startups, legacy electronic firms like Panasonic are also jumping into the pool. Last August Panasonic provided an update of its work on building-integrated perovskite solar cells. Called “Energy-Generating Glasses,” the new technology is being pilot-tested through 2024 in Panasonic’s “Future Co-Creation FINECOURT III” model house.

The perovskite market already shows signs of rapid expansion. “The global perovskite solar cell market size is estimated to surpass around USD 2,479.2 million by 2032, increasing from USD 135.6 million in 2023,” the firm Precedence Research reported in December.

“However, challenges related to long-term stability and environmental concerns need to be addressed for widespread commercial adoption,” they advised.

Still, the search for lead-free perovskite solar cells has continued apace, motivated by the potential for driving down the cost of solar power in broader markets.

One key material of focus is tin, which shares the same Group 14 space on the Periodic Table of Elements with lead.

Back in 2014 CleanTechnica took note of research at Northwestern University and Oxford University, indicating that tin could be a workable substitute for lead. At the time, though, the solar conversion efficiency of lead-containing perovskite solar cells was about 17%, while Northwestern’s lead-free version came in at just 5%. Oxford fared slightly better at 6%.

It’s been slow going since then, but new insights into the growth of tin perovskite crystals has helped to move things along, and signs of progress have appeared in the past few months. Last August, for example, the journal Applied Materials & Interfaces published a study of a tin perovskite formula that achieved 9.7% power conversion efficiency while retaining stability.

In November, Applied Materials & Interfaces published another study under the title, “Interfacial Molecular Lock Enables Highly Efficient Tin Perovskite Solar Cells,” in which the authors report a power conversion efficiency of 14.08% with minimal degradation.

On January 2 of this year, the journal Angewandte Chemie published another study under the tantalizing title, “Electronically Manipulated Molecular Strategy Enabling Highly Efficient Tin Perovskite Photovoltaics.” The authors reported a “a champion efficiency of 14.67%” for their lead-free formula.

Another development is the emergence of tandem solar cells, in which a conventional silicon solar cell is treated with a layer of perovskites. Lead-free perovskites are difficult to adhere to the silicon layer without a catastrophic loss of solar efficiency, but the upcoming April 2024 issue of Next Materials describes a tandem device that solves the problem.

Many Roads To The Perovskite Solar Cell Of The Future

Meanwhile, research on leaded perovskite solar cells continues apace. That includes Northwestern University, where a research team reported yet another breakthrough last November in the form of a two-molecule approach.

You can get all the technical details in the journal Science under the title, “Bimolecularly passivated interface enables efficient and stable inverted perovskite solar cells.” The Northwestern comms team also provides this handy plain-language explainer:

“By incorporating first, a molecule to address something called surface recombination, in which electrons are lost when they are trapped by defects — missing atoms on the surface, and a second molecule to disrupt recombination at the interface between layers, the team achieved a National Renewable Energy Lab (NREL) certified efficiency of 25.1% where earlier approaches reached efficiencies of just 24.09%.”

In even plainer language, the problem is that perovskites are really good at trapping solar energy, but they don’t want to let all of it go. The perovskite layer of a solar cell tends to re-absorb some electrons that could otherwise be shuttled out through the transport layer.

Previous attempts to solve the problem relied on the molecule PDAI2 (propane diammonium diiodide) to prevent re-absorbtion. The new research tapped sulfur to work with PDAI2 for maximum efficiency.

The research team anticipates that their findings can be applied to tandem perovskite-silicon solar cells. Meanwhile, next steps include exploring additional molecular combinations to boost perovskite solar cell efficiency.

Beyond Tin

To the extent that these findings could also be applied to lead-free perovskite solar cells, tin is not the only option. Last December, the journal Results in Engineering published a state-of-the-science review of lead-free alternatives, primarily consisting of metals including germanium, titanium, silver, bismuth, and copper as well as tin.  The journal Nature published a similar study in November, focusing on germanium as well as tin and several other formulations.

Something must be cooking in the germanium field because the journal Synthetic Materials also chipped in its two cents around the same time, with a review focused specifically on the ability of a germanium-tin combo to resolve stability issues with lead-free perovskite solar cells.

“Numerous techniques and materials are being explored to increase the stability of SnGe PSCs [tin-germanium perovskite solar cells] against moisture, temperature, oxygen, and ultraviolet radiation,” they concluded, though apparently the material is not quite ready for prime time.

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Photo (cropped): Perovskite solar cells were the  (courtesy of US Department of Energy.