This is an illustration of a pervoskite crystal Penn and Drexel researchers fabricated to test the "bulk" photovoltaic effect for visible light. The crystal is a composite of potassium niobate and barium nickel niobate. The blue spheres are potassium atoms; green spheres are barium atoms. The octahedrons have oxygen atoms on their vertices, and their colors indicate which other metal atom is inside: orange contains niobium and gray contains nickel. Getting the right ratios of these atoms in the final crystal was the goal of the research, as the product needed to be able to absorb visible light but remain polar so that the energy from the light could move electrons and generate electricity.