Robust Tunnel TaO_x 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 (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.