"Raindrops are not pure water. They contain salts that split up into positive and negative ions. To manipulate that bit of chemistry, the Ocean University researchers turned to graphene, the one-atom-thick sheets of carbon. Graphene's electrons can attract the positively charged ions, such as sodium, calcium and ammonium. The result: separated layers of positive and negative ions that act much like a capacitor to store energy."
"...the scientists added graphene to a dye-sensitized solar cell, a kind of inexpensive thin-film solar cell, then put them on a flexible, transparent backing of indium tin oxide and plastic. The resulting flexible solar cell demonstrated a solar-to-electric conversion efficiency of up to 6.53 percent, and generated hundreds of microvolts from slightly salty water that was used to simulate rainwater"
21 March online edition of the journal Angewandte Chemie International Edition
A Solar Cell That Is Triggered by Sun and Rain