The efficiency with which silicon solar cells convert sunlight to electricity can be boosted by forming a tandem structure that couples a standard silicon cell with one based on a tailor-made perovskite compound, a study shows (Science 2016, DOI: 10.1126/science.aad5845). The aim in designing these tandem cells is to capture a broader portion of the solar spectrum by coupling two or more materials that absorb complementary wavelength regions. Researchers have tried altering the composition of perovskite-type metal halides to absorb light in the high-energy blue portion of the solar spectrum in order to pair the material with silicon, which absorbs lower-energy light. But the perovskites prepared thus far, most of which are related to methylammonium lead triiodide, decompose on prolonged exposure to light and heat. A team led by University of Oxford physicist Henry J. Snaith may have come up with a solution. The researchers report that [HC(NH2)2]0.83Cs0.17Pb(I0.6Br0.4)3 absorbs light strongly in the blue region and is highly stable. By combining that compound with a 19% efficient silicon cell, the researchers showed that they could produce a tandem cell with a conversion efficiency of more than 25%.