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scale. Antisolvents are typically added during spin coating to rapidly saturate the perovskite crystals out of the solution, which gives uniform films. But the window for this processing step is only a few seconds, making it difficult for large-scale manufacturing. Kai Zhu and Maikel van Hest, at the National Renewable Energy Laboratory, formulated an ink of CH3NH3I and PbI2 in a N-methyl-2-pyrrolidinone: N,Ndimethylformamide solvent and added an excess of methylammonium chloride.

The ink can be deposited on a substrate using blade coating, an easily scalable processing method used to make large-area films on rigid or flexible surfaces in which a blade spreads liquid on a moving substrate. Instead of seconds, the precursor ink film can be processed for up to eight minutes. Further, it only needs a minute of heat treatment. All of this would be attractive for manufacturing. The researchers used the process to make 1.2 cm2 cells that had an efficiency of 17.33%.

n ink developed for perovskite thin films reported recently in Nature Energy (doi:10.1038/nenergy.2017.38) should allow the manufacture of perovskite films on large areas at high volumes, important for producing solar photovoltaic modules commercially. Most researchers make perovskite films using spin coating, which involves depositing a precursor solution on a fast-spinning substrate, evaporating the solvent, and then heating the film. This is hard to accomplish quickly on a large

PS1

Energy Focus

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research team led by Claire Villevieille of the Paul Scherrer Institute used operando x-ray diffraction (XRD) to observe soluble polysulfide intermediates in a lithium-sulfur battery. This study, which expands the batterycharacterization capabilities of operando XRD, was published recently in Nature Energy (doi:10.1038/nenergy.2017.69). Lithium-sulfur batteries are a potential replacement for the ubiquitous lithiumion batteries that are found in cell phones, computers, and electric cars. That is, if the formation of parasitic polysulfide intermediates can be characterized and controlled in these batteries. “To date, we haven’t found a proper solution to the problem in lithium-sulfur batteries:

Time (h)

Operando XRD captures soluble polysulfide intermediates in lithium-sulfur batteries

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Potential (V) (versus Li*/Li)

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2θ (°) (λ = Cu Kα)

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PS2

reactive superoxide species, which can quickly degrade CH3NH3PbI3. For the study, they combined experimental and computational methods for a microscopic assessment of the mechanism. They exposed a 500-nmthick CH3NH3PbI3 film to dry air for 20 minutes while recording its weight. The film saturated with oxygen within 10 minutes. Time-of-flight mass spectrometry showed the gas was uniformly distributed in the film. Films made of 100 nm crystals had much more superoxide yield than

Li2S

nderstanding the mechanism of perovskite degradation is key to designing more stable perovskite solar cells. In a recent Nature Communications (doi:10.1038/nco