Scientists at the U.S. Naval Research Laboratory, Electronics Science and
Technology Division, dive into underwater photovoltaic research to develop high
bandgap solar cells capable of producing sufficient power to operate electronic
sensor systems at depths of 9 m.
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 optimized more towards
the unimpeded terrestrial solar spectrum.
“The use of autonomous systems to provide situational awareness and
long-term environment monitoring underwater is increasing,” said Phillip
Jenkins, head, NRL Imagers and Detectors Section. “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 underwater,
the spectral content is narrow and thus lends itself to high conversion
efficiency if the solar cell is well matched to the wavelength range. Previous
attempts to operate solar cells underwater have focused on crystalline silicon
solar cells and more recently, amorphous silicon cells.
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 nm (visible light) and intrinsically low dark current,
which is critical for high efficiency in lowlight conditions.
The filtered spectrum of the sun underwater 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, states Jenkins.
Preliminary results at a maximum depth of 9.1 m reveal output to be 7 W per
square meter of solar cells, sufficient to demonstrate there is useful solar
power to be harvested at depths commonly found in nearshore littoral zones.
Source: U.S. Naval Research Laboratory
