Highest solar-to-hydrogen conversion efficiency in Cu2ZnSnS4 photocathodes and its directly unbiased solar seawater splitting

A study in Nano-Micro Letters led by Shuo Chen, Liming Ding and Guangxing Liang from Shenzhen University introduces precursor-seed-layer engineering (PSLE) that shatters the performance ceiling of earth-abundant Cu₂ZnSnS₄ (CZTS) photocathodes, delivering a record half-cell solar-to-hydrogen (HC-STH) efficiency of 9.91 % and enabling the first unbiased CZTS-BiVO₄ tandem cell to achieve 2.20% STH in natural seawater.

Why This Research Matters
• Overcoming Defect-Limited Efficiency: Conventional CZTS devices stall below 8 % STH because bulk Cu_Zn antisites and interface traps devour carriers. PSLE-controlled nucleation creates dense, vertically aligned grains, slashes defect density to 9.88 × 10¹⁵ cm^-3, lengthens minority-carrier lifetime to 4.40 ns and drives photocurrent to an unprecedented 29.44 mA cm^-2 at 0 V vs RHE—within 3 % of the theoretical 30.49 mA cm^-2 limit.
• Seawater-Splitting Ready: Moving beyond pristine acidic electrolytes, the champion photocathode sustains 16.54 mA cm^-2 and 2.56 % HC-STH in filtered natural seawater—highest among all CZTS reports—while retaining 85 % activity after 5 h and >95 % Faradaic efficiency for hydrogen, proving corrosion resistance against chloride ions and dissolved oxygen.

Innovative Design and Mechanisms
• Grain Engineering via Seed Tuning: A precisely timed 150s hot-plate anneal converts clustered precursors into well-separated granular seeds that guide sulfurization toward large, crack-free crystals with spike-like (+0.06 eV) CZTS/CdS band alignment, boosting built-in voltage to 0.66 V and suppressing recombination.
• Scalable Device Architecture: A simple spin-coat + sulfurization process (<620 °C) fabricates uniform Mo/CZTS/CdS/TiO₂/Pt stacks on 4 × 4 cm² substrates; coupling with BiVO₄ yields a 16 cm² tandem module that evolves >190 µmol cm^-2 h^-1 H₂ and 95 µmol cm^-2 h^-1 O₂ under AM 1.5 G without external bias, verified by real-time gas chromatography.

Applications and Future Outlook
Leveraging only earth-abundant Cu, Zn, Sn and S, PSLE-enabled CZTS photocathodes cut material costs by >70 % versus In/Ga-based chalcogenides, require no rare co-catalysts, and are compatible with roll-to-roll coating. The strategy opens a direct, gigawatt-scale pathway to low-cost green hydrogen produced directly from seawater, positioning CZTS as the keystone for sustainable solar fuels and circular hydrogen economies.