The development of photovoltaic technologies for underwater and marine applications presents unique challenges and opportunities. Unlike terrestrial solar cells, which operate under direct sunlight, underwater photovoltaics must contend with the spectral filtering effects of water, corrosion resistance, and the need for stable power delivery in submerged environments. Organic photovoltaics (OPVs) offer several advantages in this context, including tunable absorption spectra, mechanical flexibility, and potential for low-cost manufacturing.
Water absorbs light selectively, with longer wavelengths such as red and infrared being attenuated rapidly, while shorter wavelengths like blue and green penetrate deeper. The spectral composition of underwater light varies with depth and water clarity. In clear ocean water, the peak transmission occurs around 480 nm (blue-green), while in turbid or coastal waters, the available spectrum shifts toward green wavelengths. OPVs can be engineered to align with these spectral conditions by selecting donor and acceptor materials with appropriate bandgaps. For instance, low-bandgap polymers such as PTB7-Th and non-fullerene acceptors like IEICO-4F have demonstrated strong absorption in the 600-800 nm range, but for underwater use, materials absorbing between 400-550 nm may be more effective.
A critical consideration for underwater OPVs is encapsulation to prevent water ingress, which can degrade organic materials. Conventional encapsulation methods using glass or metal barriers may not be suitable for flexible or lightweight applications. Instead, multilayer barrier films incorporating polymers and inorganic coatings (e.g., Al₂O₃ or SiO₂) have been explored to enhance water resistance while maintaining flexibility. Accelerated aging tests in saline water have shown that properly encapsulated OPVs can retain over 80% of their initial efficiency after 1000 hours of immersion.
Power delivery in aquatic environments depends on the available irradiance, which diminishes exponentially with depth. At shallow depths (less than 10 meters), irradiance can range from 10-50% of surface sunlight, depending on water clarity. Below 20 meters, the irradiance drops to less than 1%, necessitating high-efficiency devices or energy-harvesting systems that operate under low-light conditions. OPVs have demonstrated promising performance under diffuse and low-light scenarios, with some devices achieving power conversion efficiencies (PCE) of 5-8% under simulated underwater spectra.
One application of underwater OPVs is in autonomous sensor networks for marine monitoring. These systems require continuous power for sensors measuring parameters such as temperature, salinity, and dissolved oxygen. Traditional battery-powered systems have limited lifespans and require frequent maintenance, whereas OPVs can provide sustained energy harvesting. In experimental deployments, OPV-integrated sensors have operated autonomously for months by storing excess energy in supercapacitors or small batteries.
Another emerging use is in underwater robotics, where lightweight and flexible OPVs can be integrated onto the surfaces of autonomous underwater vehicles (AUVs). Unlike rigid silicon or III-V solar cells, OPVs can conform to curved surfaces without adding significant weight. This integration allows AUVs to extend their mission durations by periodically surfacing to recharge or harvesting energy at shallow depths.
Challenges remain in optimizing OPVs for deep-water applications, where light availability is minimal. Possible solutions include combining OPVs with other energy-harvesting methods such as triboelectric nanogenerators (TENGs) that exploit mechanical energy from water currents. Hybrid systems could provide more consistent power output in varying conditions.
Material stability is another area requiring further development. While encapsulation mitigates water damage, prolonged exposure to biofouling—such as algae and microbial growth—can reduce light absorption. Antifouling coatings inspired by marine organisms, such as silicone-based polymers with low surface energy, are being investigated to maintain optical clarity over extended periods.
In summary, organic photovoltaics present a viable solution for underwater and marine energy harvesting due to their spectral tunability, flexibility, and potential for corrosion resistance. Advances in material design, encapsulation, and system integration will be crucial for expanding their use in aquatic environments. Future research should focus on improving efficiency under low-light conditions, enhancing long-term durability, and exploring hybrid energy-harvesting approaches to meet the demands of marine applications.