Dim ocean light could keep sensors running longer and help researchers collect continuous data from hard-to-reach areas.
Seawater takes the red out of sunlight in the first few feet, then the orange. By 33 feet (10 meters) down, only a dim blue-green light is left, and a rooftop solar panel is designed to capture wavelengths that seawater has already filtered out.
A recent study found that a cell tuned to the light that does survive turned 34.71% of it into electricity. That’s a bigger share than the same cell converts in full sunlight, and it isn’t due to more power.
Far less light arrives down there, and the only question is how much a cell can use.
Panels built the same way then charged batteries for two hours at that depth in the South China Sea, the only part of the work done in real seawater.
Wen-Hua Zhang of Yunnan University in Kunming led the study, with co-authors at the Chinese Academy of Sciences and at EPFL in Switzerland.
In an interview with Earth.com, Zhang said the solar cells would suit reef monitoring and fish farms, along with underwater vehicles, detectors, lighting and communication gear.
Past about 730 nanometers, just beyond the deepest red the eye can see, water absorbs sunlight completely. The light that’s left between 16 and 33 feet (5 and 10 meters) down is mostly blue and green.
That leaves an ordinary solar panel with the wrong range. Silicon collects light far into the infrared, and so does cadmium telluride.
So does the usual version of the material this team works with: a low-cost printable crystal called a perovskite, painted on as an ink and dried into a film.
Earlier attempts at submerged solar all worked at about 6.5 feet (2 meters) or less, Zhang said.
Zhang’s team set this crystal’s range to stop near orange, so almost everything it absorbs is light that actually arrives at depth. Crystals tuned that way have a known weakness: the iodide and bromide inside them drift when light hits, the film separates into patches, and the voltage falls.
So the researchers stirred a polymer into the ink before the film formed. Its repeating units bond to lead atoms and to the iodide around them.
Its bulky parts fill gaps where a building block is missing from the crystal, and its long, greasy backbone makes the dried film repel water. Grains came out larger and the surface smoother.
Moving an ion through the film took three times the energy it took before. In full sunlight the treated cell reached 17.08%, and an independent certification came back at 16.79%. Across 50 cells, the spread of results narrowed.
To test a cell at depth without getting it wet, the team built a lamp behind filters. Each one was cut to match the light at 6.5, 16, or 33 feet (2, 5, or 10 meters).
A national testing center in Fuzhou checked them against the light calculated for each depth.
At the deepest setting the cell received less than a quarter of the light it gets in full sun. It put out a little under half as much power.
Both are true at once, and the ratio between them is the efficiency: 34.71%, against 17.08% in full sunlight.
Efficiency records on land come from squeezing more out of the whole spectrum. This one came from ignoring the part that never arrives.
Cells with wider collecting ranges managed only about 27% efficiency under the same filtered light, and a 2.8-inch (7 cm) square module reached 29.40% at the 33-foot (10-meter) setting.
For the sea trial, the modules were sealed under an aluminum oxide barrier, a rubbery sealant and a cover glass, then set in epoxy. They came through six months underwater without cracking. Over 200 days in a tank, the lead in the surrounding water stayed under 1.15 parts per billion.
A small robot carried two sets of four modules down off Weizhou Island in the South China Sea, holding each depth for two hours.
Each set had about 18 square inches (115 square centimeters) of working surface. A timer kept the circuit open for the first forty minutes, so nothing charged on the way down.
At 6.5 feet (2 meters), the panels put 1,416 milliwatt-hours (mWh) into a stack of coin-cell lithium-ion batteries. Twenty feet down, the same panels put 752 mWh into the batteries. Each step deeper cut the total roughly in half.
The 33-foot (10-meter) run, with the least light of any of them, managed 324 mWh. That’s enough to keep a one-watt LED burning for about 20 minutes. The charged batteries lit a panel of red letters spelling out the university’s name.
Zhang did not expect results at that depth.
“Owing to the limited solar photon flux in underwater environment, we had anticipated substantially lower charging output, for instance, 50~100 mWh at 10 m water depth,” Zhang told Earth.com.
The team had expected the cells would cease to charge at a depth of between 16 and 20 feet (5 and 6 meters). “The in-field data (> 300 mWh) at 10 meters depth underwater do really surprise us!”
On a roof, heat and ultraviolet light are what wear this kind of cell out. Underwater the light is weaker, the ultraviolet is gone and the water stays cool.
A sealed cell ran 1,000 hours in simulated seawater under light matched to 33 feet (10 meters), and still had 99.58% of its starting efficiency. Cells stored dry in a nitrogen box for 300 days kept 96%, where untreated ones were down to 85% after 200 days.
Neither test is a year in the ocean, but both point the same way.
Heating cells to between 95 and 225°F (35 and 105°C) let the team age them fast and work backward. At 77°F (25°C), the average water temperature near the test site, they put the moment a cell falls to 80% of its output at 48,094 hours, or about 5.5 years.
Asked what else stood out from the study, Zhang picked that number.
“The underwater environment itself delivers intrinsic stabilizing influence on perovskite solar cells,” Zhang said. The missing ultraviolet and the cool water both help; on land the two stresses arrive together and break these cells down.
Nobody has watched one of these panels work through a year at sea. The 5.5-year figure comes from heated cells and an equation, and the longest continuous immersion ran about six weeks.
In the sea, the record is two hours at a time. Zhang’s team lists weather, water clarity, surface waves and turbulence as things that change what a submerged panel returns. None of that was tracked across a season.
Light near the sea floor can change fast, which is what an underwater sensor would be there to record. The lead figure came from a tank as well, not from the sea.
Asked by Earth.com how deep the cells can go, Zhang said each depth needs its own tuning, and this one is at its best at 33 feet (10 meters).
“The experiment is under way to assess the power generation capability of the submerged solar cells for work at underwater 20-30 meter marine environments, at which depths we think it is possible for our underwater solar cells to work well,” Zhang added. That range is 66 to 98 feet (20 to 30 meters).
Until those results arrive, its performance farther below the surface remains unproven.
The full study was published in the journal Joule.
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