While many technical advances have made solar cells more efficient and affordable, a disadvantage remains in the fact that solar cells produce no power when it’s raining. Now, however, researchers from the Ocean University of China (Qingdao) and Yunnan Normal University (Kunming, China) have developed an all-weather solar cell that is triggered by both sunlight and raindrops by combining an electron-enriched graphene electrode with a dye-sensitized solar cell.
The new solar cell can be excited by incident light on sunny days and raindrops on rainy days, yielding an optimal solar-to-electric conversion efficiency of 6.53% under AM 1.5 irradiation and current over microamps as well as a voltage of hundreds of microvolts by simulated raindrops. Their work is published as a “Very Important Paper” in the journal Angewandte Chemie.
Graphene is a two-dimensional form of carbon in which the atoms are bonded into a honeycomb arrangement. It can readily be prepared by the oxidation, exfoliation, and subsequent reduction of graphite. Graphene is characterized by its unusual electronic properties: it conducts electricity and is rich in electrons that can move freely across the entire layer (delocalized).
In aqueous solution, graphene can bind positively charged ions with its electrons (Lewis acid-base interaction). This property is used in graphene-based processes to remove lead ions and organic dyes from solutions.
This phenomenon inspired researchers working with Qunwei Tang to use graphene electrodes to obtain power from the impact of raindrops.
Raindrops are not pure water; they contain salts that dissociate into positive and negative ions. The positively charged ions, including sodium, calcium, and ammonium ions, can bind to the graphene surface.
At the point of contact between the raindrop and the graphene, the water becomes enriched in positive ions and the graphene becomes enriched in delocalized electrons. This results in a double-layer made of electrons and positively charged ions, a feature known as a pseudocapacitor. The difference in potential associated with this phenomenon is sufficient to produce a voltage and current.
On rainy days, the new solar cells can be reversed with rGO film upward, creating current and voltage outputs under he persistent dropping of raindrops (NaCl aqueous solution with concentration of 0.6m, 1m, or 2m is used as simulated rain). … Moreover, each droplet can yield voltage and output power of 152.6 mV and 7.17 pW for 9.46 mm, 54.7 mV and 20.7 pW for 7.58 mm, and 25.1 mV and 84.7 pW for 4.52 mm.
… When dropping onto a rGO surface, simulated raindrops will quickly spread to the periphery. In this fashion, the cations are adsorbed at the front end, advancing the pseudocapacitor forward and drawing electrons in the rGO. The migration of drawing electrons creates current. Subsequently, the cations are desorbed at the rear end of the droplets during the shrinking process due to a hydrophobic rGO surface … discharging the pseudocapacitor and releasing the p-electrons to rGO. This discharging process results in a reduced current.—Tang et al.
The research team suggested that the work opens a viable concept of developing all-weather solar cells that can persistently generate electricity.
Tang, Q., Wang, X., Yang, P. and He, B. (2016) “A Solar Cell That Is Triggered by Sun and Rain.” Angew. Chem. Int. Ed. doi: 10.1002/anie.201602114