A simple fabrication procedure leads to unusually structured, highly crystalline films of perovskite semiconductors widely used for making experimental solar cells, according to a study (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b03144). The advance may boost the efficiency with which solar cells based on methylammonium lead trihalide perovskite compounds convert sunlight to electricity, spurring the low-cost devices toward commercialization. Light absorption in a photovoltaic device generates positive and negative charges, which must migrate to opposite electrodes to produce electric current. Because the charges often get trapped and neutralized at interfaces between crystalline regions—so-called grain boundaries—researchers strive to minimize these imperfections. But controlling crystallinity is difficult with solution-phase synthesis methods. Kai Xiao of Oak Ridge National Laboratory and coworkers report that depositing solutions of the trihalide precursors via spin coating, exposing the films to humid air, and then heating them in nitrogen yields perovskite crystals with vertically oriented grain boundaries, instead of randomly oriented crystallites. The team notes that those factors, together with segregation of a positive-charge-conducting material near the grain boundaries, led to a conversion efficiency in unoptimized test cells of more than 16%, which approaches the perovskite solar-cell record of about 20%.