Robust Tunnel TaOx Passivating Interlayer Enables Long‐term Solar Water Oxidation

Silicon‐based photoelectrochemical (PEC) cells offer significant advantages for solar‐to‐clean fuel conversion but face challenges such as surface recombination and poor stability, necessitating a comprehensive understanding of device failure mechanisms and effective mitigation strategies. Using nickel decorated n‐type silicon (n‐Si/Ni) as a model photoelectrode, we demonstrate that surface restructuring during operation initially passivates surface recombination but eventually blocks photogenerated hole transfer, leading to device failure. To mitigate this challenge, an ultrathin (<1 nm) tantalum oxide (TaOx) interlayer is introduced via atomic layer deposition (ALD). This TaOx layer serves as both a passivating contact and a corrosion‐resistant coating. The n‐Si/TaOx/Ni electrode achieves an increased photovoltage from 150 mV to 500 mV and significantly extends the device lifespan, maintaining over 90% performance for 500 h in 1 M KOH at 1.52 V versus RHE. These results highlight the implementation of amorphous TaOx in enhancing both the efficiency and durability of silicon‐based PEC systems, providing a simultaneously passivating and protection strategy for solar‐to‐fuel conversion technologies under harsh conditions.