Steering Directional Charge Migration via Ordered Self-Assembled Monolayers for High-Efficiency BiVO4-Silicon Artificial Leaf

The BiVO ₄ -silicon artificial leaf represents a promising unbiased photoelectrochemical (PEC) water splitting device, yet its efficiency remains limited by sluggish charge transport in the BiVO ₄ photoanode. To overcome these limitations, we propose a self-assembled molecular-bridge strategy to establish ordered and fast pathways for efficient charge transport between BiVO ₄ and NiFeCoO _x oxygen evolution cocatalysts (OECs). To clarify the role of dipole strength, we compare two self-assembled monolayers (SAMs) with distinct dipole moments: [4-(9H-carbazol-9-yl)ethyl]phosphonic acid (4PACZ ~ 1.83 Debye) and (4-(7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (4PADCB ~ 2.54 Debye). The larger dipole of 4PADCB generates a stronger interfacial electric field (IEF), which drives directional hole extraction and suppresses recombination. Consequently, the optimized BiVO ₄ /4PADCB/NiFeCoO _x photoanode achieves a photocurrent density of 6.35 mA cm⁻ ² at 1.23 V versus a reversible hydrogen electrode (V _RHE ), ranking among the best BiVO ₄ -based systems modified with organic interfacial layers. Furthermore, by integrating the photoanode with a silicon solar cell, a standalone artificial leaf (1.5 cm ² area) was constructed, achieving a solar-to-hydrogen conversion efficiency of 8.75% and stable operation for over 15 hours. This work establishes a clear link between molecular dipole strength and interfacial charge separation, providing a generalizable design principle for molecular interface engineering in PEC water splitting.