Evening out the heat The conversion of precursors into the active layer of perovskite solar cells normally occurs by heating the underlying substrate. Conversion tends to occur near the top of the film, where solvent is lost, and unwanted preheating of reactants occurs near the substrate before the reaction. Li et al . show that the use of a surrounding heat transfer oil (anisole) leads to more rapid and even heating, removes solvent, and avoids air and water contamination effects. The larger grains and more uniform films led a much greater retention of efficiency in moving from small-area to large-area devices. —PDS