Ga1-xInxP is a technologically important III-V ternary semiconductor widely utilized in commercial and record-efficiency solar cells. We report the growth of Ga1-xInxP by water-vapor-mediated close-spaced vapor transport. Because growth of III-V semiconductors in this system is controlled by diffusion of metal oxide species, we find that congruent transport from the mixed powder source requires complete annealing to form a single alloy phase. Growth from a fully alloyed source at water vapor concentrations of ∼7000 ppm in H2 at 850 °C affords smooth films with electron mobility of 1070 cm2 V-1 s-1 and peak internal quantum efficiency of ∼90% for carrier collection in a nonaqueous photoelectrochemical test cell.

Cascade photoelectrocatalysis (PEC) is a possible method to improve the selectivity of solar-driven CO2 reduction (CO2R). This concept can be realized by coupling different CO2R catalysts to different subcells in a multijunction photovoltaic (PV) stack. Efficient implementation will require careful tuning of the photocurrents and design of the photovoltages provided by the subcells to the CO2R catalysts in such a way as to facilitate the target reaction. Here, we outline the design principles of the tandem PEC approach using two-step conversion of CO2 to ethylene in aqueous electrolyte, via a CO intermediate, as a model system. To perform this reaction, the first coupled PV-catalyst component should provide 4 electrons to reduce 2 molecules of CO2 to CO; the second component should provide 8 electrons to reduce 2 CO molecules to C2H4. Based on known CO2R catalysts, the overpotential required to produce CO can be less than that required to reduce it to ethylene, creating the opportunity for improved efficiency. Cascade PEC can be realized in a three-terminal tandem (3TT) configuration using III-V-semiconductor based subcells coupled to Au (produces CO intermediate) and Cu (converts CO to ethylene). The current to each catalyst can be controlled by the area of the subcell exposed to the electrolyte, and the photovoltage is determined by the materials selected and device configuration. Operating conditions are found by simulating the coupled system using the open-source circuit simulator SPICE (Simulation Program with Integrated Circuits Emphasis). We identify conditions under which a 3TT configuration can have a higher solar to chemical conversion efficiency compared to a two-terminal two-junction tandem (2T 2J) with the same absorbers and a Cu catalyst only. We also show that 3TT PEC devices can be less sensitive to variations in catalyst activity compared to 2T devices. Finally, we discuss the applications of cascade PEC to CO2 reduction, using different intermediates, and to other chemical networks.

Photoelectrochemical fuel generation is a promising route to sustainable liquid fuels produced from water and captured carbon dioxide with sunlight as the energy input. Development of these technologies requires photoelectrode materials that are both photocatalytically active and operationally stable in harsh oxidative and/or reductive electrochemical environments. Such photocatalysts can be discovered based on co-design principles, wherein design for stability is based on the propensity for the photocatalyst to self-passivate under operating conditions and design for photoactivity is based on the ability to integrate the photocatalyst with established semiconductor substrates. Here, we report on the synthesis and characterization of zinc titanium nitride (ZnTiN₂) that follows these design rules by having a wurtzite-derived crystal structure and showing self-passivating surface oxides created by electrochemical polarization. The sputtered ZnTiN₂ thin films have optical absorption onsets below 2 eV and n-type electrical conduction of 3 S/cm. The band gap of this material is reduced from the 3.36 eV theoretical value by cation-site disorder, and the impact of cation antisites on the band structure of ZnTiN₂ is explored using density functional theory. Under electrochemical polarization, the ZnTiN₂ surfaces have TiO₂- or ZnO-like character, consistent with Materials Project Pourbaix calculations predicting the formation of stable solid phases under near-neutral pH. These results show that ZnTiN₂ is a promising candidate for photoelectrochemical liquid fuel generation and demonstrate a new materials design approach to other photoelectrodes with self-passivating native operational surface chemistry.