Rain helps solar cells operate efficiently by washing away dust and dirt that block the sun’s rays. Still, photovoltaic cells depend on light to produce electricity, and so generate a negligible amount of power when there are clouds overhead.
But researchers in China wondered whether it would be possible to create all-weather solar cells. “We would like to develop a solar cell that can be triggered by sun and rain,” says study lead author Qunwei Tang, a materials scientist at Ocean University of China in Qingdao.
So, how do you generate current from rain? 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 acapacitor to store energy.
With that in mind, 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. “Future solar cells may produce electricity in all weather,” Tang says.
The scientists detailed their findings on the possibility of a rainy day solar cell in the 21 March online edition of the journal Angewandte Chemie International Edition.
Future research will investigate how to handle the variety of ions found in rain, as well how to generate electricity from the low concentrations of these ions typically seen in rainwater, Tang says.