These small solar photovoltaic (PV) cells replace the silicon typically used with methylammonium tin halide, a move that could one day greatly reduce the cost of solar energy. © Nakita Noel/ University of Oxford
Research into a new solar photovoltaic cell structure shows promise in reducing the costs of the systems and the electricity they produce.
May 13, 2014—Researchers at several key institutions, including Northwestern University and the University of Oxford, are exploring a technology with the potential to dramatically reduce the cost of solar photovoltaic (PV) cells by replacing the silicon currently used with methylammonium tin halide. The cost reduction that this replacement could produce could make solar energy more cost-competitive with fossil fuels.
Although market factors and recent advances have boosted the efficiency of PV cells—and, to a degree, reduced their costs—electricity generated from solar sources is still significantly more expensive than that generated by fossil fuels.
The silicon that traditional PV cells employ to convert sunlight to energy represents a significant manufacturing cost of PV cells and panels. Seeking to address this, researchers have been developing PV cells from mineral structures known as perovskites.
Researchers first focused on lead perovskites because the material can be easily processed on a bench, shares important patterns in electron configuration with silicon, and is inherently more stable than alternative metals. Early work in the field showed promise. The efficiency achieved by lead perovskite PV cells—just 3.8 percent in 2009—has since reached a verified 17.9 percent, with reports of efficiencies approaching nearly 20 percent. Typical residential or commercial panels currently operate with less efficiency than that today.
There is a nagging concern about lead perovskite PV cells, however: their toxicity. “The lead is easier to work with and that’s where most of the activity came,” says Mercouri Kanatzidis, Ph.D., a professor of chemistry at Northwestern University who has been researching perovskites since 2009. “But we know, sooner or later the lead is going to cause issues. Because these compounds …can dissolve, the lead could be lost into the environment. So that may cause some hurdles in commercialization and investment.”
Looking to other options within the group 14 elements—the carbon group—researchers arrived at tin, a nontoxic alternative to lead that shares patterns in electron configuration that are beneficial to PV operations. Two new research papers explore tin in Perovskite solar cells. Work at Northwestern University is detailed in the paper “Lead-Free Solid-State Organic-Inorganic Halide Perovskite Solar Cells,” coauthored by Kanatzidis and published in the journal Nature Photonics this month. Work by a team at the University of Oxford is detailed in “Lead-Free Organic-Inorganic Tin Halide Perovskites for Photovoltaic Applications,” published by the journal Energy & Environmental Science this month.
Both teams have successfully demonstrated that a tin-based solar cell is viable, though their approach differs chemically. The efficiency ratings are 5.73 percent for the Northwestern cell and 6.4 percent for the Oxford cell. The key challenges that remain for researchers are how to improve efficiency, how to create systems that have a life span exceeding 20 years in an outdoor environment, and how to effectively develop the technology for mass production
“One of the main problems that we have with tin is [that] it is inherently unstable in its 2+ oxidation state,” says Nakita Noel, a Ph.D. candidate at the University of Oxford. Noel was the lead author of the Oxford research paper and works in the Photovoltaic and Optoelectronic Device Group at Oxford led by Henry Snaith, Ph.D., a lecturer at the institution.
“In order to get around that, we’ve tried to seal it,” Noel says, noting that early efforts haven’t been effective and work is under way for a better sealing solution. “But I don’t necessarily think that’s really solving the problem. The main problem that we have to look at is finding a way to stabilize the material itself.”
Although researchers in the field are encouraged by the possibilities of a tin perovskite cell, they are exploring other metals and haven’t eliminated lead, Snaith noted in written comments to Civil Engineering online.
“We are very excited about having an alternative to lead, but we should not rule lead out of the equation simply because we now have [a] tin cell working,” Snaith said. “There are many advantages of lead, specifically stability, and I would predict that the first commercial products are based on lead perovskites, with tin potentially coming in for the second generation.”
Kanatzidis and Noel both say they expect rapid progress toward boosting the efficiency of tin perovskites, similar to what was seen with lead perovskite PV cells. To verify the system’s longevity, the tin perovskite PV cells will then be put through a pressure cooker—literally. “The industry has adopted protocols that they will accept as accelerated aging tests,” Kanatzidis explains. “For example, they include testing the cells inside pressure cookers, with very high humidity and very high temperatures, to see how long they last within a period of days or weeks. Then they can extrapolate to years.”
The potential for this technology to lower the cost of solar power is high, Kanatzidis says. “All the chemicals are readily available. The processes are relatively straight forward. That’s why people are excited about it. They can see a path to very low-cost applications.”
“It will be interesting to see what happens in the next year,” he says, predicting that if the stability issues don’t prove intractable, large panels could be produced in a matter of years.
“We are hoping that one of the implications would be that we become less dependent on fossil fuels,” Noel says. “We have such a huge problem right now with climate change. Our population keeps growing. Our energy demands keep growing. And we are looking to something that’s a finite resource. The sun can prove to be an abundant source of clean energy.”