A wide-bandgap perovskite solar cell engineered at Yunnan University has converted 34.71% of the available light into electricity at a simulated depth of 10 metres — a record for underwater photovoltaics, according to results published in Joule. In an open-sea trial off Weizhou Island in the South China Sea, scaled modules built on the same chemistry charged lithium-ion batteries underwater, delivering 324 mWh of stored energy.

The team, led by Wen-Hua Zhang, attacked a problem that has stalled autonomous marine instrumentation for years: water is a brutal optical filter. Wavelengths at 630 nm and above absorb within a few metres of descent, leaving only a faint 400–600 nm blue-orange band as illumination. Conventional silicon, cadmium telluride and standard 1.55 eV perovskite cells absorb photons that never arrive, making them poor fits for deep-sea sensors and environmental monitors.

The Chinese researchers matched their absorber to the light that does penetrate. They built a mixed-anion lead halide perovskite with a bandgap of roughly 1.96 eV, tuned to the narrow blue-green window of deep water. Wide-bandgap perovskites normally fail on land because intense light and heat drive halide-ion migration, phase separation and defect accumulation. Underwater conditions help — temperatures below 25°C and dim light suppress thermal degradation — but internal defects and ion migration remain.

To close that gap, the group added polyhexamethylene guanidine hydrochloride (PHMG), a crystallization additive, to the perovskite precursor. The polymer carries a water-repelling backbone plus guanidinium cations that anchor into gaps in the crystal lattice and bind lead and halide ions through hydrogen bonding. The measured effect is concrete: the activation energy barrier for ion migration tripled from 0.07 eV to 0.21 eV, effectively halting phase segregation.

Ultraviolet photoelectron spectroscopy showed a second consequence of the additive. PHMG shifted the film's surface conductivity from p-type to n-type, creating band bending that accelerates electron extraction and cuts non-radiative recombination losses at the interface.

From simulator to seawater

The numbers split cleanly between measured results and projections. Under standard terrestrial sunlight, the modified cell achieved a certified power conversion efficiency of 16.79% — modest by terrestrial standards. Under a custom underwater solar simulator using multilayer interference optical filters replicating 10 m depth, the 0.0895 cm² cell reached 34.71%.

Scaling followed. The team fabricated modules with an active area of nearly 29 cm², encapsulated in polyisobutylene, cover glass and epoxy resin to survive hydrostatic pressure and salt-water corrosion. Mounted on a miniature robot in the South China Sea, the modules generated stable power across varying depths and at 10 m produced 324 mWh to charge lithium-ion batteries, which subsequently powered an LED panel.

Zhang stresses the gap between bench and ocean. "In contrast to the lab-based static underwater solar simulator, waves, turbidity and suspended organic matter in real oceans can change the light intensity and spectral composition. This leads to substantial fluctuations in output power and reduces the power-generation stability of the cells," he told Physics World. "Therefore, in-field experiments of the large-area solar modules, as we carried out in the South China Sea, are indispensable to assess their real-world power generation."

Durability figures warrant scrutiny. Accelerated thermal aging tests combined with Arrhenius degradation kinetics — a modelling projection, not a field measurement — predict a T80 lifetime of roughly 48,094 hours, or 5.49 years of continuous operation at 25°C. Mass spectrometry detected negligible lead leakage into surrounding water, an important datum for any environmental deployment permit.

Biofouling remains unresolved. "Biofouling is a critical challenge for the long-term operation of underwater photovoltaics. However, this issue involves multidisciplinary efforts and requires collaborative research from more scientists," Zhang said.

For R&D managers weighing marine power portfolios, the work demonstrates a viable materials route — bandgap tuning plus polymer-stabilized crystallization — to self-sustaining submerged energy. Zhang's team points toward integrated photovoltaic-storage systems, deep-sea research cameras and submerged Internet-of-Things nodes as the next deployment targets, with field validation at scale the outstanding hurdle.