// ARS TECHNICA — MODA & SOCIETÀ
A new solar cell could generate electricity underwater
The trick isn’t using perovskites—the trick is making them last.
In one episode of the Hanna-Barbera cartoon Birdman, the eponymous hero struggles to fight the evil Dr. Shark aboard a submarine without solar energy to recharge his powers. So Birdman would surely appreciate the new perovskite solar cells developed by a team led by Simin Ma at Yunnan University, since they are designed to work underwater.
Solar cells made from perovskites rather than silicon are always a tale of trade-offs. They can be made cheaply, they can take interesting forms (like thin, flexible, transparent films), and they can convert substantially more of the incoming solar energy into electricity. The difficulty is that they tend to degrade quite quickly.
Moisture is particularly destructive to perovskites, making them a seemingly odd choice for an underwater solar panel. But these materials have another critical superpower: They can be tuned to work with different wavelengths of light. Water quickly blocks the wavelengths of light that silicon solar panels absorb, but a carefully designed perovskite cell could still make electricity in the deep blue sea. And actually, the lower light levels and cooler temperatures should help it live longer.
Tuning perovskites just requires tweaking some of their chemistry during production, so getting something that absorbs the wavelengths present a few meters deep was not a challenge. The real task was making the cells durable. The team found a particularly effective additive (polyhexamethylene guanidine hydrochloride) that helped in several ways.
It built a water-repelling layer around the material, for one. But part of the compound also gets involved with the perovskite crystal lattice, helping larger crystals form and preventing ions from moving around in the lattice structure. The additive limits some of the common ways that perovskites break down, while also improving the solar cell’s electricity production.
When the researchers tested the cells under light filtered to match an ocean depth of about 10 meters, they were remarkably efficient, converting about 35 percent of that light energy to electricity. (Silicon solar panels are generally closer to 20 percent efficiency.)
After building a proper little solar panel by sandwiching the perovskite in some protective layers, the team ran real-world durability tests. First, they submerged it in seawater (using their filtered light) for about 40 days, at which point it was still at 99.6 percent of its original efficiency. Based on that, they estimate it should last 5.5 years in seawater before it drops to 80 percent—what is typically considered its useful lifetime. While terrible compared to silicon, it’s well beyond what most perovskites have achieved.
They also tested a panel in the South China Sea, attaching it to a small vehicle that could maintain a specific depth and position. The panel charged some coin cell batteries for a couple of hours each at 2, 6, and 10 meter depths. There were no surprises in performance beyond noticing that power fluctuated pretty strongly at 2 meters thanks to sunlight interacting with the surface waves. At the deeper depths, the increased scattering made the light hitting the solar panel much more consistent, though dimmer. At 10 meters, it produced a little less than a quarter as much energy as it did at 2 meters.
The researchers say their design could enable “autonomous marine power systems and submerged Internet of Things infrastructure”—think uncrewed underwater vehicles and sensors, for example. Of course, if you go much deeper, there won’t be enough light to work with. Birdman is still on his own down there.