Power density of GaInP and crystalline silicon cells, underwater, as a function of depth. (U.S. Naval Research Laboratory)
Scientists at the U.S. Naval Research Laboratory, Electronics Science and Technology Division, have developed high-band-gap solar cells capable of producing sufficient power to operate electronic sensor systems at water depths of 9 meters.

Underwater autonomous systems and sensor platforms are limited by the lack of long endurance power sources. Such systems must rely on on-shore 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 optimized more towards the unimpeded terrestrial solar spectrum.

Even though the absolute intensity of solar radiation is lower underwater, the spectral content is narrow and lends itself to high conversion efficiency if the solar cell is well matched to the wavelength range. High-quality gallium indium phosphide (GaInP) cells are well-suited for underwater operation. GaInP cells have high quantum efficiency in wavelengths between 400 and 700 nanometers (visible light) and intrinsically low dark current, which is critical for high efficiency in lowlight conditions.

Preliminary results at a maximum depth of 9.1 meters reveal output to be 7 watts per square meter of solar cells - sufficient to demonstrate there is useful solar power to be harvested at depths commonly found in nearshore zones.

(U.S. Naval Research Laboratory)