Metal oxides are generally very stable in aqueous solutions and cheap, but their photochemical activity is usually limited by poor charge carrier separation. Here we show that this problem can be solved by introducing a gradient dopant concentration in the metal oxide film, thereby creating a distributed n þ -n homojunction. This concept is demonstrated with a lowcost, spray-deposited and non-porous tungsten-doped bismuth vanadate photoanode in which carrier-separation efficiencies of up to 80% are achieved. By combining this state-ofthe-art photoanode with an earth-abundant cobalt phosphate water-oxidation catalyst and a double-or single-junction amorphous Si solar cell in a tandem configuration, stable shortcircuit water-splitting photocurrents of B4 and 3 mA cm À 2 , respectively, are achieved under 1 sun illumination. The 4 mA cm À 2 photocurrent corresponds to a solar-to-hydrogen efficiency of 4.9%, which is the highest efficiency yet reported for a stand-alone water-splitting device based on a metal oxide photoanode.

BiVO4 is considered to be a promising photoanode material for solar water splitting applications. Its performance is limited by two main factors: slow water oxidation kinetics and poor charge separation. We confirm recent reports that cobalt phosphate (Co-Pi) is an efficient water oxidation catalyst for BiVO4 and report an AM1.5 photocurrent of 1.7 mA/cm2 at 1.23 V vs RHE for 100 nm spray-deposited, compact, and undoped BiVO4 films with an optimized Co-Pi film thickness of 30 nm. The charge separation of these films depends strongly on light intensity, ranging from 90% at low light intensities to less than 20% at intensities corresponding to 1 sun. These observations indicate that the charge separation efficiency in BiVO4 is limited by poor electron transport and not by the presence of bulk defect states, interface traps, or the presence of a Schottky junction at the back-contact.

We unravel for the first time the origin of the poor carrier transport properties of BiVO 4 , a promising metal oxide photoanode for solar water splitting. Time-resolved microwave conductivity (TRMC) measurements reveal an (extrapolated) carrier mobility of ∼4 × 10 −2 cm 2 V −1 s −1 for undoped BiVO 4 under ∼1 sun illumination conditions, which is unusually low for a photoanode material. The poor carrier mobility is compensated by an unexpectedly long carrier lifetime of 40 ns. This translates to a relatively long diffusion length of 70 nm, consistent with the high quantum efficiencies reported for BiVO 4 photoanodes. Tungsten (W) doping is found to strongly decrease the carrier mobility by introducing intermediate-depth donor defects as carrier traps. At the same time, the increased carrier density improves the overall photoresponse, which confirms that bulk electronic conductivity is one of the main performance bottlenecks for BiVO 4 .

The main role of a cobalt phosphate catalyst on BiVO4 is to improve the photocurrent by passivating the surface and suppressing recombination.

Bismuth vanadate (BiVO4) thin film photoanodes for light‐induced water oxidation are deposited by a low‐cost and scalable spray pyrolysis method. The resulting films are of high quality, as indicated by an internal quantum efficiency close to 100 % between 360 and 450 nm. However, its performance under AM1.5 illumination is limited by slow water oxidation kinetics. This can be addressed by using cobalt phosphate (Co‐Pi) as a water oxidation co‐catalyst. Electrodeposition of 30 nm Co‐Pi catalyst on the surface of BiVO4 increases the water oxidation efficiency from ≈30 % to more than 90 % at potentials higher than 1.2 V vs. a reversible hydrogen electrode (RHE). Once the surface catalysis limitation is removed, the performance of the photoanode is limited by low charge separation efficiency; more than 60 % of the electron‐hole pairs recombine before reaching the respective interfaces. Slow electron transport is shown to be the main cause of this low efficiency. We show that this can be remedied by introducing W as a donor type dopant in BiVO4, resulting in an AM1.5 photocurrent of ≈2.3 mA cm−2 at 1.23 V vs. RHE for 1 % W‐doped Co‐Pi‐catalyzed BiVO4.

CuBi2O4 is a multinary p-type semiconductor that has recently been identified as a promising photocathode material for photoelectrochemical (PEC) water splitting. It has an optimal bandgap energy (∼1.8 eV) and an exceptionally positive photocurrent onset potential (>1 V vs RHE), making it an ideal candidate for the top absorber in a dual absorber PEC device. However, photocathodes made from CuBi2O4 have not yet demonstrated high photoconversion efficiencies, and the factors that limit the efficiency have not yet been fully identified. In this work we characterize CuBi2O4 photocathodes synthesized by a straightforward drop-casting procedure and for the first time report many of the quintessential material properties that are relevant to PEC water splitting. Our results provide important insights into the limitations of CuBi2O4 in regards to optical absorption, charge carrier transport, reaction kinetics, and stability. This information will be valuable in future work to optimize CuBi2O4 as a PEC material. In addition, we report new benchmark photocurrent density and IPCE values for CuBi2O4 photocathodes.

Bismuth vanadate (BiVO4) is a promising semiconductor material for photoelectrochemical water splitting showing good visible light absorption and a high photochemical stability. To improve the performance of BiVO4, it is of key importance to understand its photophysics upon light absorption. Here we study the carrier dynamics of BiVO4 prepared by the spray pyrolysis method using broadband transient absorption spectroscopy (TAS), in thin films as well as in a photoelectrochemical (PEC) cell under water-splitting conditions. The use of a dual-laser setup consisting of electronically synchronized Ti:sapphire amplifiers enable us to measure the femtosecond to microsecond time scales in a single experiment. On the basis of this data, we propose a model of carrier dynamics that includes relaxation and trapping rates for electrons and holes. Hole trapping occurs in multiple phases, with the majority of the photogenerated holes being trapped with a time constant of 5 ps and a small fraction of this hole trapping taking place within the instrument response of 120 fs. The induced absorption band that represents the trapped holes is modulated by an oscillation of 63 cm–1, which is assigned to the coupling of holes to a phonon mode. We find electrons to undergo a relaxation with a time constant of 40 ps, followed by deeper trapping on the 2.5 ns time scale. On time scales longer than 10 ns, trap-limited recombination that follows a power law is found, spanning time scales up to microseconds. Finally, we observe no spectral or kinetic differences by applying a bias voltage to the PEC cell, indicating that the effect of a voltage and the charge transfer processes between BiVO4 and the electrolyte occurs on longer time scales. Our results therefore provide new insights into the carrier dynamics of BiVO4 and further expand the application window of TAS as an analytical tool for photoanode materials.

Several application fields can benefit from solar-hydrogen technologies via specific short-term and long-term pathways.