Doping in semiconductor photoelectrodes controls defect formation and carrier transport that critically determine the device performance. Here we report an unconventional carrier transport relation that is tuned by extrinsic molybdenum (Mo) doping in BiVO4 photoanodes. Using the single-crystalline thin film approach, we identify that Mo doping significantly condenses the optimization regime between carrier transport and photon collection. For Mo-doped BiVO4 films, an unprecedentedly thin layer (50 nm), less than one-third of the pristine BiVO4 thickness, delivers larger photocurrents by overcoming the charge transport limitation, representing a regime not covered in conventional models. We provide direct evidence that Mo doping improves electron transport by boosting not only the donor density but also the electron mobility in the form of a small polaron, with the latter applying substantial impact on the photoelectrochemical performance. Density functional theory calculations reveal that fully ionized Mo dopants establish a strong electrostatic interaction with a small polaron, which helps reduce its hopping barrier by minimizing the local lattice expansion. Our results deliver mechanistic insights on the interplay between extrinsic doping and carrier transport, and provide guidance in developing advanced semiconductor photoelectrodes.

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Carrier transport in semiconductor photoelectrodes strongly correlates with intrinsic material characteristics including carrier mobility and diffusion length, and extrinsic structural imperfections including mobile charged defects at domain boundaries, which collectively determines the photoelectrochemistry (PEC) performance. Here we elucidate the interplay between intrinsic carrier transport, domain-boundaryinduced conductivity, and PEC water oxidation in the model photoanode of bismuth vanadate (BiVO 4 ). In particular, epitaxial single-domain BiVO 4 and c-axis-oriented multidomain BiVO 4 thin films are fabricated using pulsed laser deposition to decouple the intrinsic and extrinsic carrier transport. In addition to the low intrinsic conductivity that is due to the small-polaron transport within BiVO 4 domains, we identify anomalously high electrical conductivity arising from vertical domain boundaries for multidomain BiVO 4 films. Local domain-boundary conduction compensates the inherently poor electron transport by shortening the transport distance for electrons diffused into the domainboundary region, therefore suppressing the photocurrent difference between front and back illumination. This work provides insights into engineering carrier transport through coordinating structural domain boundaries and intrinsic material features in designing modulated water-splitting photoelectrodes.

Here a novel ultrathin lutetium oxide (Lu 2 O 3 ) interlayer is integrated with crystalline bismuth vanadate (BiVO 4 ) thin film photoanodes to facilitate carrier transport through atomic-scale interface control. The epitaxial Lu 2 O 3 interlayer fabricated by pulsed laser deposition features very few structural defects at the back contact of the heterojunction, and forms a unique band alignment that favors photohole blocking. An optimized interlayer thickness of 1.4 nm significantly enhances charge separation efficiency and photocurrent. Combined with photoelectrochemical characterization, solid-state electronic, and localized conductive atomic force microscopy measurements, it is revealed that the Lu 2 O 3 interlayer modulates the electronic conduction pathways along structural grain boundaries and determines the overall device performance. This study sheds light on the nature of interface-engineered carrier transport for efficient photoelectrode heterostructure design.