Ultrathin HfO₂ passivated silicon photocathodes for efficient alkaline water splitting

HfO₂ has many favorable characteristics for use in energy conversion devices including high thermodynamic stability, good chemical stability in corrosive electrolytes, high refractive index, and wide bandgap. Here, we report surface passivation of a c-Si photocathode by ultrathin HfO₂ prepared using atomic layer deposition as an effective approach for enhancing its photoelectrochemical (PEC) performance. The effect of the thickness of HfO₂, deposition temperature, and annealing in forming gas on the passivation performance are systematically investigated. We demonstrate that the Si photocathode with a p⁺/n/n⁺ structure decorated with a Ni₃N/Ni cocatalyst and an HfO₂ interlayer follows a metal–insulator–semiconductor mechanism with thicker HfO₂ films proving detrimental to the PEC performance. The Si photocathode passivated with a 1 nm HfO₂ layer exhibits an enhancement in the onset potential by 100 mV, an applied-bias photon-to-current efficiency of 9%, and improved operational stability. This work provides insights into the application of HfO₂ as a passivating layer for Si photoelectrodes for solar hydrogen production.