Scientists have found a way to spice up those dull solar cells. Silicon dioxide used to make glass optical fibres, and titanium dioxide, widely used to absorb harmful ultraviolet rays in sunscreens, are layered alternately to create a structure called photonic crystal that can interact with light. A layer of perovskite, the light-harvesting material, is placed on top of the photonic crystal. The colours appear as a result of a combination of light reflected by the photonic crystal and absorbed by the perovskite; they can be changed by varying the thickness of individual silicon or titanium layers in the photonic crystal. However, the coloured cells are less efficient than their black counterparts, converting only 9% of the sunlight they receive into electricity, versus 25% for traditional cells. Still, the team hopes that the colourful cells will boost solar adoption on buildings and other structures.