Perovskite solar cell. Perovskite is a material that has the same crystal structure as calcium titanium oxide. Scientists at the NREL (National Renewable Energy Laboratory) have developed an interdigitated (interlocked) back contact solar cell that can have solutions of metals and other materials printed or sprayed on to it. Back contact cells are more efficient as there are no front-mounted contacts to cast a shadow. Perovskite solar cells are more efficient than silicon solar cells. Photographed at the National Renewable Energy Laboratory, Golden, Colorado, USA.