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The cells exploit the narrow slice of sunlight that survives beneath the sea.
Ten meters (32 ft.) beneath the South China Sea, sunlight has already lost much of what makes it useful on land. Red and infrared wavelengths fade rapidly in water. What remains is a dimmer, bluer world.
Yet off China’s Weizhou Island, researchers lowered a small underwater robot carrying solar modules into that light and left it there for two hours. By the time they brought it back up, the modules had delivered 324 milliwatt-hours of energy into lithium-ion batteries—enough for the researchers to later power an LED panel.
This pushes underwater photovoltaics well beyond the very shallow depths explored in most previous experiments. More importantly, the researchers designed a perovskite solar cell specifically around the peculiar colors of sunlight that survive underwater. Essentially, they made specially designed underwater solar cells.
“This offers a route to continuously power the marine devices, addressing the long-standing bottleneck of short battery endurance for underwater devices,” Wen-Hua Zhang of Yunnan University, the study’s senior author, told ZME Science.
Ordinary solar cells benefit from a broad spectrum of sunlight. But water acts like a secondary optical filter, after it was already filtered by the atmosphere. It absorbs red and infrared light particularly strongly; by depths of around 5 to 10 meters, the available spectrum becomes dominated by blue-green wavelengths between roughly 400 and 600 nanometers.
That means a solar cell designed for the surface wastes much of its potential underwater.
The researchers instead used a wide-band-gap perovskite, with a band gap of about 1.96 electron volts, tuned to respond strongly to the surviving blue-green light. They also added a polymer called PHMG, which helped the perovskite form a more stable crystal structure and reduced defects that can drain electrical performance.
A 2020 analysis in the journal Joule calculated that underwater photovoltaics could remain useful at depths approaching 50 meters in exceptionally clear water, provided researchers use wider-band-gap materials better matched to the altered spectrum. The analysis predicted an optimal band gap of roughly 2.1 electron volts at intermediate depths, which is mighty close to the one used in the practical application documented by the Chinese researchers.
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They’re not alone. In 2024, researchers reported organic solar cells designed for underwater use that exceeded 25.6% efficiency at one meter. Other researchers have recently begun testing perovskites underwater as well.
What’s different this time is that the solar cells have been tested in open water, in contrast to the previous tests in shallow water.
In lab tests designed to mimic sunlight 10 meters underwater, the small solar cells converted 34.71% of the light reaching them into electricity. Under standard sunlight conditions, they converted 17.08%. Larger modules reached 29.4% underwater.
But that does not mean they produced more electricity underwater. Much less sunlight reaches a panel at that depth, so the total power output was still lower. The cells were simply better at using the blue-green light that remained.
Ten meters may only be a first stop.
“On basis of our present work, speculatively, the underwater photovoltaics may offer viable practical operation at depths of 20~30 m,” Zhang said. “The experiments are under way in our group.”
Besides depth, Zhang pointed to marine biofouling, floating debris, and seawater corrosion as additional obstacles that impede the efficiency of underwater solar. Organisms growing over the cells could block light, while saltwater could eventually attack their protective layers.
“Therefore, the practical operational depth is governed by multiple factors, rather than being determined solely by the light intensity,” he noted.
If researchers can solve these issues, Zhang sees solar cells becoming one component of a growing technological landscape beneath the waves.
“Underwater photovoltaics can be widely deployed as one piece of the broader push toward sub-sea infrastructure development,” he said, pointing in particular to autonomous underwater vehicles and other “smart ocean” technologies.
His group now plans to push deeper, test the modules for longer periods in real water, scale them up, and integrate them with autonomous underwater machines. The researchers also want to develop standardized testing protocols that would allow scientists to compare underwater solar technologies on equal terms.
The study was published in the journal Joule.
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