De‐Pinning Fermi Level and Accelerating Surface Kinetics with an ALD Finish Boost the Fill Factor of BiVO4 Photoanodes to 44%

Abstract With the rapid development of performance and long‐term stability, bismuth vanadate (BiVO 4 ) has emerged as the preferred photoanode in photoelectrochemical tandem devices. Although state‐of‐the‐art BiVO 4 photoanodes realize a saturated photocurrent density approaching the theoretical maximum, the fill factor (FF) is still inferior, pulling down the half‐cell applied bias photon‐to‐current efficiency (HC‐ABPE). Among the major fundamental limitations are the Fermi level pinning and sluggish surface kinetics at the low applied potentials. This work demonstrates that the plasma‐assisted atomic layer deposition technique is capable of addressing these issues by seamlessly installing an angstrom‐scale FeNi‐layer between BiVO 4 and electrolyte. Not only this ultrathin FeNi layer serves as an efficient OER cocatalyst, more importantly, it also effectively passivates the surface states of BiVO 4 , de‐pins the surface Fermi level, and enlarges the built‐in voltage, allowing the photoanode to make optimal use of the photogenerated holes for achieving high FF up to 44% and HC‐ABPE to 2.2%. This study offers a new approach for enhancing the FF of photoanodes and provides guidelines for designing efficient unassisted solar fuel devices.