A research team has developed perovskite solar cells that can operate underwater at depths of up to 10 metres, in a technology that could power marine robots, sensors, cameras and communication systems. The cells recorded an efficiency of 34.71% when simulating the light available at a depth of 10 metres, according to a study published in the journal Cell–Joule.
Designing cells to capture blue and green light
The researchers designed the cells to use the blue and green light that penetrates to relatively shallow depths, as water absorbs red light and infrared radiation more rapidly. The higher efficiency underwater does not mean the cells produce more electricity than they would at the Earth’s surface; rather, they harness a larger share of the limited light available at depth.
Simin Ma, a researcher at Yunnan University and the study’s co-first author, said the team chose a material with a wide bandgap of about 1.96 electron volts, matching the light spectrum at depths of between 5 and 10 metres. The researchers then adjusted the material to suit the light available at depth, rather than trying to operate a conventional solar cell in an aquatic environment.
The team used an additive known by the abbreviation PHMG to improve crystal formation and reduce defects that impede the movement of electrical charges, while also limiting ion migration associated with the decline in perovskite-cell performance over time. The material also helped increase the cells’ resistance to moisture.
Encapsulating and testing the modules near Weizhou Island
The small cells achieved an efficiency of 17.08% under standard sunlight simulating conditions at the Earth’s surface, while the certified efficiency reached 16.79%. To move from the laboratory to the marine environment, the researchers built larger modules and encapsulated them in layers to protect against water pressure and salinity, then tested them near Weizhou Island in the South China Sea.
Stability tests showed that the modified cells retained 96% of their initial efficiency after being stored for 300 days in a nitrogen atmosphere. Accelerated ageing tests estimated that the operating period before a 20% loss of efficiency would be about 48094 hours, or approximately 5.49 years, under simulated submerged illumination at a depth of 10 metres and a temperature of 25 degrees Celsius.
Most tests relied on laboratory simulations of the underwater light spectrum, while the sea trial lasted only two hours. Weather conditions, water clarity, waves, turbidity and currents could affect actual performance during long-term operation. The cells also contain lead; although very low leakage was recorded after encapsulation, environmental safety remains a prerequisite before they can be used widely in the seas.
