NRL scientists develop solar cell that works under water

Scientists at the US Naval Research Laboratory (NRL) are developing high-bandgap solar cells capable of producing enough power to operate electronic sensor systems at depths of 9m.

According to NRL, underwater autonomous systems and sensor platforms are severely limited by the lack of long-endurance power sources.

To date, these systems must rely on onshore power, batteries or solar power supplied by an above-water platform.

Attempts to use photovoltaics have had limited success, primarily due to the lack of penetrating sunlight and the use of solar cells optimised more towards the unimpeded terrestrial solar spectrum.

In a statement, Phillip Jenkins, head of the NRL Imagers and Detectors Section, said: ‘The use of autonomous systems to provide situational awareness and long-term environment monitoring under water is increasing.

‘Although water absorbs sunlight, the technical challenge is to develop a solar cell that can efficiently convert these underwater photons to electricity.’

Even though the absolute intensity of solar radiation is lower under water, the spectral content is narrow and lends itself to high conversion efficiency if the solar cell is well matched to the wavelength range.

Previous attempts to operate solar cells under water have focused on crystalline silicon solar cells and, more recently, amorphous silicon cells.

High-quality gallium indium phosphide (GaInP) cells are well suited to underwater operation. GaInP cells have high quantum efficiency in wavelengths between 400nm and 700nm (visible light) and intrinsically low dark current, which is critical for high efficiency in low-light conditions.

The filtered spectrum of the sun under water is biased toward the blue/green portion of the spectrum and thus higher-bandgap cells such as GaInP perform much better than conventional silicon cells, said Jenkins.

Preliminary results at a maximum depth of 9.1m reveal output to be 7W per square metre of solar cells, sufficient to demonstrate there is useful solar power to be harvested at depths commonly found in near-shore littoral zones.