To build better solar cells, scientists need to design materials from the bottom up, placing chains of molecules just where they are needed. Scientists devised a new way to grow neatly arranged, densely packed bundles of molecular chains, specifically semiconducting polymer chains. The chains folded on themselves to make bundles that stretched from the growth plate to the film’s surface. The bundles were packed side-by-side with neighboring bundles. This bottom-up approach, starting with the building blocks and creating the larger structure, was used to make large-area films with tiny patterns. The films have controlled architecture, uniform assembly and excellent stability.