Robust Tunnel TaO _x Passivating Interlayer Enables Long‐Term Solar Water Oxidation

Abstract 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 (TaO x ) interlayer is introduced via atomic layer deposition (ALD). This TaO x layer serves as both a passivating contact and a corrosion‐resistant coating. The n‐Si/TaO x /Ni electrode achieves an increased photovoltage from 150 to 500 mV during the activation process, and then 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 TaO x in enhancing both the efficiency and durability of silicon‐based PEC systems, providing a simultaneously passivating and protective strategy for solar‐to‐fuel conversion technologies under harsh conditions.