Electron transport layers (ETLs) used as components of photocathodes for light-driven CO2 reduction (CO2R) in aqueous media should have good electronic transport, be stable under CO2R conditions, and, ideally, be catalytically inert for the competing hydrogen evolution reaction (HER). Here, using planar p-Si (100) as the absorbing material, we show that TaOx satisfies all three of the above criteria. TaOx films were synthesized by both pulsed laser deposition (PLD) and radio-frequency (RF) sputtering. In both cases, careful control of the oxygen partial pressure during growth was required to produce ETLs with acceptable electron conductivity. p-Si/TaOx photocathodes were interfaced with ca. 10 nm of a CO2R catalyst: Cu or Au. Under front illumination with simulated AM 1.5G in CO2-saturated bicarbonate buffer, we observed, for both metals, faradaic efficiencies for CO2R products of ∼50% and ∼30% for PLD TaOx and RF sputtered TaOx, respectively, at photocurrent densities up to 8 mA cm−2. p-Si/TiO2/Cu photocathodes were also evaluated but produced mostly H2 (>97%) due to reduction of the TiO2 to Ti metal under CO2R conditions. In contrast, a dual ETL photocathode (p-Si/TiO2/TaOx/Cu) was selective for CO2R, which suggests a strategy for separately optimizing selective charge collection and the stability of the ETL/water interface. The maximum photovoltage obtained with p-Si/TaOx/Cu devices was 300 mV which was increased to 430-460 mV by employing ion implantation to make pn+-Si/TaOx/Cu structures. Photocathodes with RF sputtered TaOx ETLs are stable for CO2R for at least 300 min. Techno-economic analysis shows that the reported system, if scaled, could allow for an economically viable production of feedstocks for chemical synthesis under the adoption of specific CO2 credit schemes, thus becoming a significant component of carbon-neutral manufacturing.

To date, the in situ fabrication of the large-scale van der Waals multi-heterojunction transition metal dichalcogenides (multi-TMDs) is significantly challenging using conventional deposition methods. In this study, vertically stacked centimeter-scale multi-TMD (MoS₂/WS₂/WSe₂ and MoS₂/WSe₂) thin films are successfully fabricated via sequential pulsed laser deposition (PLD), which is an in situ growth process. The fabricated MoS₂/WS₂/WSe₂ thin film on p-type silicon (p-Si) substrate is designed to form multistaggered gaps (type-II band structure) with p-Si, and this film exhibits excellent spatial and thickness uniformity, which is verified by Raman spectroscopy. Among various application fields, MoS₂/WS₂/WSe₂ is applied to the thin-film catalyst of a p-Si photocathode, to effectively transfer the photogenerated electrons from p-Si to the electrolyte in the photo-electrochemical (PEC) hydrogen evolution. From a comparison between the PEC performances of the homostructure TMDs (homo-TMDs)/p-Si and multi-TMDs/p-Si, it is demonstrated that the multistaggered gap of multi-TMDs/p-Si improves the PEC performance significantly more than the homo-TMDs/p-Si and bare p-Si by effective charge transfer. The new in situ growth process for the fabrication of multi-TMD thin films offers a novel and innovative method for the application of multi-TMD thin films to various fields.

NaTaO3 is a stable and wide bandgap n-type semiconductor material with many different applications. Here, a flux-mediated synthesis method is presented for NaTaO3 resulting in highly distinctive, substrate covering shapes via precursor chemistry variation at comparatively low temperatures. It is found that the microstructure of the resulting NaTaO3 films can be varied from nanocubes to smooth thin films. These shapes and surface chemistries can be correlated by employing density functional theory calculations and surface sensitive X-ray photoemission spectroscopy. This study provides guidance on how to synthesize the material and tailor its shape and surface termination for different applications. Finally, as a proof of concept of one possible application, NaTaO3 is applied to perovskite solar cells as the electron transport layer, resulting in conversion efficiencies of >19%. This study provides a new strategy for designing ternary oxide thin films for renewable energy applications.

Despite achievements in the remarkable photoelectrochemical (PEC) performance of photoelectrodes based on organometal halide perovskites (OHPs), the scaling up of small-scale OHP-based PEC systems to large-scale systems remains a great challenge for their practical application in solar water splitting. Significant resistive losses and intrinsic defects are major obstacles to the scaling up of OHP-based PEC systems, leading to the PEC performance degradation of large-scale OHP photoelectrodes. Herein, a scalable design of the OHP-based PEC systems by modularization of the optimized OHP photoelectrodes exhibiting a high solar-to-hydrogen conversion efficiency of 10.4% is suggested. As a proof-of-concept, the OHP-based PEC module achieves an optimal PEC performance by avoiding major obstacles in the scaling up of the OHP photoelectrodes. The constructed OHP module is composed of a total of 16 OHP photoelectrodes, and a photocurrent of 11.52 mA is achieved under natural sunlight without external bias. The successful operation of unassisted solar water splitting using the OHP module without external bias can provide insights into the design of scalable OHP-based PEC systems for future practical application and commercialization.

Fabrication of epitaxial FeSe_xTe_1-x thin films using pulsed laser deposition (PLD) enables improving their superconducting transition temperature (T _c) by more than ~40% than their bulk T _c. Intriguingly, T _c enhancement in FeSe_xTe_1-x thin films has been observed on various substrates and with different Se content, x. To date, various mechanisms for T _c enhancement have been reported, but they remain controversial in universally explaining the T _c improvement in the FeSe_xTe_1-x films. In this report, we demonstrate that the controversies over the mechanism of T _c enhancement are due to the abnormal changes in the chalcogen ratio (Se:Te) during the film growth and that the previously reported T _c enhancement in FeSe_0.5Te_0.5 thin films is caused by a remarkable increase of Se content. Although our FeSe_xTe_1-x thin films were fabricated via PLD using a Fe_0.94Se_0.45Te_0.55 target, the precisely measured composition indicates a Se-rich FeSe_xTe_1-x (0.6 < x < 0.8) as ascertained through accurate compositional analysis by both wavelength dispersive spectroscopy (WDS) and Rutherford backscattering spectrometry (RBS). We suggest that the origin of the abnormal composition change is the difference in the thermodynamic properties of ternary FeSe_xTe_1-x, based on first principle calculations.