Spatially Quantitative Profiling of Defect Density for Hydrogen-Induced Passivation Mechanism of Metal–Insulator–Semiconductor Photoanodes

Silicon-based metal-insulator-semiconductor (MIS) photoelectrodes provide a built-in field for photoelectrochemical water splitting but suffer from inefficient charge transport due to carrier recombination at defect sites within the space-charge region. The insulating layer fabricated by atomic layer deposition (ALD) is critical for defect passivation and requires carefully engineered deposition and annealing processes for optimal performance. This necessitates the quantitative analysis of defect density distribution. However, such analysis remains impeded by intrinsic limitations of conventional single-capacitance techniques and the influence of tunneling leakage currents. This paper describes an integrated multicapacitance methodology that combines drive-level capacitance profiling (DLCP) and capacitance-voltage (C-V) techniques to quantify both interface (<i>N</i>_it) and bulk defect charge density (<i>N</i>_DLCP) within the space-charge region, using the space-charge capacitance refined by MIS-adapted multicomponent equivalent circuit modeling. This approach reveals that Al₂O₃ reduces the <i>N</i>_it at n-Si/ITO interface from approximately 2.9 × 10¹⁶ to 1.4 × 10¹⁶ cm^-3, while simultaneously profiling the variation of <i>N</i>_DLCP and depletion width. Furthermore, systematic DLCP/C-V analysis enables the design of a tailored passivation protocol by tuning ALD deposition (<i>T</i>_dep) and annealing temperatures to leverage the hydrogen-induced mechanism. Optimal passivation is achieved at a moderate <i>T</i>_dep (∼160 °C) followed by 350 °C annealing, which balances sufficient hydrogen content and suitable structural properties of the Al₂O₃ film without introducing additional bulk defects during annealing. Eventually, the optimized n-Si/SiO_<i>x</i>/Al₂O₃/ITO/Ni photoanode exhibits an onset potential of 0.88 V vs RHE and an applied bias photon-to-current efficiency (ABPE) of 2.91%, with a long-term stability of 120 h.