The performance of solar cell newcomer materials called perovskites has soared in recent months. But they still have big problems when it comes to working in real-world settings. For starters, the best perovskites contain lead, which is highly toxic. Now, two independent research groups report making solar cells with lead-free perovskites. So far, the new devices convert only about 6% of the energy in sunlight to electricity, less than one-third as much as lead-containing versions do. But that’s about how efficient leaded devices were only 3 years ago. So if the lead-free versions improve as quickly as their leaded cousins did, it could help propel perovskites into the marketplace.
Perovskites are a broad class of crystalline minerals that have been known for well over a century. But their ability to convert solar energy to electricity came to light only in 2009. Since then, the efficiency of perovskite solar cells has climbed from 3.8% to 19.3%, a pace of improvement unmatched by any other solar technology. By comparison, crystalline silicon solar cells, the leading commercial technology, convert about 25% of solar energy to electricity.
But lead could be a perovskite killer. In large part, that’s because the compounds are saltlike minerals that readily dissolve in water or even humid air. And the prospect of dissolved lead dripping onto the rooftops of homeowners with solar panels isn’t a pleasant one to companies looking to commercialize the technology.
The new reports could offer the beginnings of a solution. Both groups of researchers take a similar approach, replacing lead in the complex crystalline structure with tin, a metal that sits just above lead in the periodic table and thus shares a similar electronic structure. In a paper posted online on 1 May in Energy & Environmental Science, Henry Snaith, a physicist at the University of Oxford in the United Kingdom, and colleagues report making tin perovskite solar cells that achieve a maximum efficiency of 6.4% . And in an article posted today in Nature Photonics, Robert Chang, a materials scientist at Northwestern University in Evanston, Illinois, and colleagues report similar solar cells with efficiencies up to 5.73%.
“Both are fantastic work,” says Yang Yang, a materials scientist at the University of California, Los Angeles, whose group last week reported a record-breaking efficiency of 19.3%  in lead-based perovskite solar cells. Yang cautions that to be commercially viable, perovskite solar cells will have to be cheap, safe, durable, and capable of being manufactured in large panels. So research in this area is just getting started, he says. Nevertheless, he adds, “Both demonstrations represent a significant step towards the realization of low-cost, high-efficiency, environmentally benign next-generation solid-state solar cells.”