A one-step wet-chemistry procedure cross-links and stabilizes crystals of methylammonium lead triiodide, a perovskite material widely studied for application in photovoltaics (Nat. Chem. 2015, DOI: 10.1038/nchem.2324). The advance may improve the durability of solar cells based on trihalide perovskite compounds and spur these low-cost devices, which convert sunlight to electricity, toward commercialization. The rapid rise in perovskite solar cells’ power conversion efficiency (PCE) has thrust these experimental devices into the photovoltaics limelight. But their tendency to degrade upon exposure to heat and humidity leaves them with an uncertain future. A team led by Michael Grätzel of ETH Lausanne may have a way to stop the degradation. The group reports that spin coating a solution of the perovskite precursors and butylphosphonic acid 4-ammonium chloride forms well-ordered perovskite crystals cross-linked via hydrogen bonds. In tests conducted at 55% relative humidity, the PCE of solar cells treated with the additive slowly fell from 14% to 10% over 40 days. The PCE of untreated cells fell from 7% to 1% in just one week. Other tests showed that unlike untreated cells, treated ones remained fairly stable when exposed to 85 °C temperatures for 350 hours.