Recent rapid progress in efficiencies for solar water splitting by photoelectrochemical devices has enhanced its prospects to enable storable renewable energy. Efficient solar fuel generators all use tandem photoelectrode structures, and advanced integrated devices incorporate corrosion protection layers as well as heterogeneous catalysts. Realization of near thermodynamic limiting performance requires tailoring the energy band structure of the photoelectrode and also the optical and electronic properties of the surface layers exposed to the electrolyte. Here, we report a monolithic device architecture that exhibits reduced surface reflectivity in conjunction with metallic Rh nanoparticle catalyst layers that minimize parasitic light absorption. Additionally, the anatase TiO 2 protection layer on the photocathode creates a favorable internal band alignment for hydrogen evolution. An initial solar-to-hydrogen efficiency of 19.3 % is obtained in acidic electrolyte and an efficiency of 18.5 % is achieved at neutral pH condition (under simulated sunlight). Main TextAdvances in the field of artificial photosynthesis 1 have led to the development of functional prototypes for photoelectrochemical water splitting 2 , featuring improved photoelectrode stability through the use of corrosion protection layers 3 and the realization of systems for unassisted water splitting 4-6 in integrated monolithic devices. The requirement for the device operating voltage under illumination to exceed the thermodynamic potential difference for water dissociation of 1.23 V imposes constraints on the energy bandgaps for the photoelectrode absorber layers and their combined operating potential in a series-connected tandem configuration. Several strategies have been followed. Early prototypes used single absorber

Photosynthesis is nature's route to convert intermittent solar irradiation into storable energy, while its use for an industrial energy supply is impaired by low efficiency. Artificial photosynthesis provides a promising alternative for efficient robust carbon-neutral renewable energy generation. The approach of direct hydrogen generation by photoelectrochemical water splitting utilizes customized tandem absorber structures to mimic the Z-scheme of natural photosynthesis. Here a combined chemical surface transformation of a tandem structure and catalyst deposition at ambient temperature yields photocurrents approaching the theoretical limit of the absorber and results in a solar-to-hydrogen efficiency of 14%. The potentiostatically assisted photoelectrode efficiency is 17%. Present benchmarks for integrated systems are clearly exceeded. Details of the in situ interface transformation, the electronic improvement and chemical passivation are presented. The surface functionalization procedure is widely applicable and can be precisely controlled, allowing further developments of high-efficiency robust hydrogen generators.

Theoretical limiting efficiencies have a critical role in determining technological viability and expectations for device prototypes, as evidenced by the photovoltaics community's focus on detailed balance. However, due to their multicomponent nature, photoelectrochemical devices do not have an equivalent analogue to detailed balance, and reported theoretical efficiency limits vary depending on the assumptions made. Here we introduce a unified framework for photoelectrochemical device performance through which all previous limiting efficiencies can be understood and contextualized. Ideal and experimentally realistic limiting efficiencies are presented, and then generalized using five representative parameters—semiconductor absorption fraction, external radiative efficiency, series resistance, shunt resistance and catalytic exchange current density—to account for imperfect light absorption, charge transport and catalysis. Finally, we discuss the origin of deviations between the limits discussed herein and reported water-splitting efficiencies. This analysis provides insight into the primary factors that determine device performance and a powerful handle to improve device efficiency.

These authors contributed equally to the work. MFL, SH and MHR contributed to the design, execution, and analysis of the experiment; EJC was critical in the design, building and testing of the end station that allows atmospheric pressure XPS data collection on a solution under potentiostatic control. ‡ Corresponding authors:

Efficient solar energy conversion with CuInS2 thin films is reported. The copper-rich p-type absorber is prepared by thermal coevaporation. A copper to indium ratio between 1.0 and 1.8 can be tolerated with small (≤10%) solar-to-electrical conversion losses. Copper excess phases (CuS) are removed chemically. The cell structure glass/Mo/p-CuInS2/n-CdS/n+-ZnO/Al delivers 10.2% at simulated AM 1.5 conditions. The device properties are discussed based on its energy band diagram.

Anodic oxidation of Ru and RuO2 electrodes in 1N H2SO4 has been investigated using x‐ray photoelectron spectroscopy. During O2 evolution on Ru a highly defective hydrated oxide film is formed as a result of corrosion. At a temperature of 310°C in vacuum this film decomposes to metallic ruthenium. The surface of RuO2 anodes prepared by thermal decomposition of RuCl3 contains some RuO3 , which is stable during anodic polarization. A reaction path for O2 evolution and corrosion on Ru and RuO2 anodes is proposed.

The performance of a series of n-WSe2 photoanodes differing in surface morphology has been investigated. A solar to electrical conversion efficiency of 3.7% is reached on a smooth photoanode in the n-WSe2|2 M KI-0.05 M I21C cell. A correlation exists between the short circuit current, the open circuit voltage, and the fill factor on the one side and surface perfection, determined by scanning electron microscopy, on the other. We propose that an electric field component which parallels the layers is introduced by the steps. Because of the highly anisotropic conductivity of layered compounds the holes, on their way to the interface, are deflected and move to recombination sites at the edges of steps.

A hybrid photovoltaic/photoelectrochemical (PV/PEC) water‐splitting device with a benchmark solar‐to‐hydrogen conversion efficiency of 5.2 % under simulated air mass (AM) 1.5 illumination is reported. This cell consists of a gradient‐doped tungsten–bismuth vanadate (W:BiVO4) photoanode and a thin‐film silicon solar cell. The improvement with respect to an earlier cell that also used gradient‐doped W:BiVO4 has been achieved by simultaneously introducing a textured substrate to enhance light trapping in the BiVO4 photoanode and further optimization of the W gradient doping profile in the photoanode. Various PV cells have been studied in combination with this BiVO4 photoanode, such as an amorphous silicon (a‐Si:H) single junction, an a‐Si:H/a‐Si:H double junction, and an a‐Si:H/nanocrystalline silicon (nc‐Si:H) micromorph junction. The highest conversion efficiency, which is also the record efficiency for metal oxide based water‐splitting devices, is reached for a tandem system consisting of the optimized W:BiVO4 photoanode and the micromorph (a‐Si:H/nc‐Si:H) cell. This record efficiency is attributed to the increased performance of the BiVO4 photoanode, which is the limiting factor in this hybrid PEC/PV device, as well as better spectral matching between BiVO4 and the nc‐Si:H cell.