Efficiency Limits of Underwater Solar Cells

Operation of underwater vehicles and autonomous systems is currently limited by the lack of long-lasting power sources. These systems could potentially be powered using underwater solar cells, but the material requirements to achieve their full potential are not well understood. Using detailed-balance calculations, we show that underwater solar cells can exhibit efficiencies from ∼55% in shallow waters to more than 65% in deep waters, while maintaining a power density >5 mW cm⁻². We show that the optimum band gap of the solar cell shifts by ∼0.6 eV between shallow and deep waters and plateaus at ∼2.1 eV at intermediate depths, independent of geographical location. This wide range in optimum band-gap energies opens the potential for a library of wide-band-gap semiconductors to be used for high-efficiency underwater solar cells. Our results provide a roadmap for proper choice of underwater solar cell materials, given the conditions at points of use. Most attempts to use solar cells to power underwater systems have had limited success due to the use of silicon, which has a relatively narrow band gap and absorbs ultraviolet (UV), visible, and infrared (IR) light. Because of absorption by water, most of the IR light from the sun is absorbed at relatively shallow depths, and wider band-gap semiconductors, which primarily absorb visible light, should therefore be used. To understand how efficient underwater solar cells can be and what band gaps are optimum in deep waters, we combined oceanographic data with detailed balance calculations to show that solar cells can harvest useful power at water depths down to 50 m with very high efficiencies. Our findings show that underwater solar cells can efficiently generate useful power in very deep waters but should employ much wider band-gap semiconductors than what are currently used today. Most attempts to use solar cells to power underwater systems have had limited success due to the use of materials with relatively narrow band gaps such as silicon. We performed detailed balance calculations combined with oceanographic data to show that, if wide-band-gap semiconductors are employed, underwater solar cells can operate at efficiencies up to ∼65% while still generating useful power. Finally, we propose both organic and inorganic materials that could be used to maximize underwater power generation and efficiency.