The clearest way to make solar power cheaper than fossil fuels is to find ways to inexpensively increase the efficiency of solar cells—to convert more of the sun’s energy into electricity. That’s the promise of a class of materials called perovskites. These abundant minerals are superior to silicon at absorbing light and have electronic properties that could make them more efficient than silicon at generating electricity.
Researchers made the first solar cell out of perovskites nine years ago. It converted just over 2 percent of the energy it captured from light into electricity. Since then, perovskite solar cell performance has improved at an astonishing rate and is already approaching that of silicon. Research teams all over the world are scrambling to make devices out of the material, and one startup is promising a commercial product by 2017.
But is there room in the market for a brand new solar material? The price of silicon solar panels has been falling dramatically for several years. It could soon be cheap enough that the potential advantages of perovskites will no longer be as compelling to investors and consumers. Nonetheless, the electronic characteristics of perovskites are simply too exciting for researchers to ignore. Recent research suggests that perovskites might not actually have to compete with silicon—instead, they could work in tandem with silicon in high-efficiency cells that feature both materials.
Why all the hype?
In short, perovskite solar cells are exciting because they could be both highly efficient and cheap to make.
The term “perovskite” refers to a distinct chemical structure that gives rise to unique electronic properties. For example, when one material containing this structure, called methylammonium lead halide, absorbs light, charges (electrons and their positively charged counterparts) are excited within it for a much longer time than in common thin-film solar materials. That means more charge can be turned into electricity, says Tonio Buonassisi, a professor of mechanical engineering at MIT.
The efficiency of the silicon panels on the market now ranges from about 17 to 20 percent. In the lab, perovskite cells have already surpassed 20 percent, and many researchers are confident that 25 percent could be reached soon. Last month, researchers reported in Science that they’d used high-powered microscopes to pinpoint previously unknown defects in the material. University of Washington chemistry professor David Ginger, who led the research, says the results reveal opportunities to modify the chemistry on the surface of the cell to enhance performance.
Unlike silicon, perovskites don’t have to be in a highly pure form to work. Whereas the equipment needed to make silicon cells is expensive, perovskites can be put into a solution that then can be deposited as a thin film on top of a variety of surfaces relatively easily.
Teaming up with silicon
The ease and low cost of fabrication is why perovskites’ most compelling commercial opportunity could be in a so-called tandem cell, in which the new material would be combined with silicon to boost the efficiency. Because each material can absorb different wavelengths of light, a tandem cell could lead to devices capable of efficiencies higher than the theoretical limit of silicon or any other solar material on its own. Researchers have previously used materials other than perovskites to build tandem cells with efficiencies of more than 47 percent, well above silicon’s maximum theoretical efficiency of around 30 percent. But making them has been expensive because it requires growing high-purity crystals of multiple semiconductor materials, one on top of the other, which is technically difficult.
In the past two years several academic research groups have begun pursuing silicon/perovskite tandem cells. In March, a group led by MIT’s Buonassisi and Michael McGehee, a professor of materials science and engineering at Stanford, demonstrated a promising design, and the researchers say they have identified key opportunities for improvement that could make the cell reach efficiencies greater than 25 percent. Oxford Photovoltaics, a company spun out of the University of Oxford lab of Henry Snaith, who is responsible for much of the pioneering development of perovskite solar cells, recently demonstrated a tandem cell unofficially rated at 20 percent efficiency. The company is aiming to have a commercial product ready by 2017, says CTO Chris Case.
Big potential roadblocks
It’s not all good news, though. Perovskites have several substantial drawbacks that could eventually spoil their commercial promise. Perhaps the greatest concern is that they tend to degrade in the presence of moisture, calling into question whether the material is suitable for solar panels, which need to last for decades. Overcoming this challenge will require that perovskites be encapsulated in some protective material. The optimal way to do that is still an open question, and will add to the cost. Oxford Photovoltaics encapsulates its cells with glass, says Case.
Another downside is that the most popular perovskite material contains lead, which is toxic. Case acknowledges that this is “unfortunate,” but points out that it’s a relatively small amount compared with certain other widely used products, such as lead-acid automotive batteries. Many research groups, including those at Oxford Photovoltaics, are exploring alternatives that contain substitutes, such as tin, but have not come close to replicating the outstanding performance of the material that contains lead.
Perovskite solar cells have promise, but they are still very early in their development and face significant challenges to commercialization. If they do make it to the market, it’s likely the first products will be tandem cells made of both a perovskite material and silicon.
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