Crystalline silicon (c-Si) solar cells have been dominating the photovoltaic (PV) market for decades, and c-Si based photoelectrochemical (PEC) cells are regarded as one of the most promising routes for water splitting and renewable production of hydrogen. In this work, we demonstrate a nanoscale tantalum oxide (TaO x , ∼6 nm) as an electron-selective heterocontact, simultaneously providing high-quality passivation to the silicon surface and effective transport of electrons to either an external circuit or a water-splitting catalyst. The PV application of TaO x is demonstrated by a proof-of-concept device having a conversion efficiency of 19.1%. In addition, the PEC application is demonstrated by a photon-to-current efficiency (with additional applied bias) of 7.7%. These results represent a 2% and 3.8% absolute enhancement over control devices without a TaO x interlayer, respectively. The methods presented in this Letter are not limited to c-Si based devices and can be viewed as a more general approach to the interface engineering of optoelectronic and photoelectrochemical applications.

Realizing solar‐to‐hydrogen (STH) efficiencies close to 20% using low‐cost semiconductors remains a major step toward accomplishing the practical viability of photoelectrochemical (PEC) hydrogen generation technologies. Dual‐absorber tandem cells combining inexpensive semiconductors are a promising strategy to achieve high STH efficiencies at a reasonable cost. Here, a perovskite photovoltaic biased silicon (Si) photoelectrode is demonstrated for highly efficient stand‐alone solar water splitting. A p+nn+ ‐Si/Ti/Pt photocathode is shown to present a remarkable photon‐to‐current efficiency of 14.1% under biased condition and stability over three days under continuous illumination. Upon pairing with a semitransparent mixed perovskite solar cell of an appropriate bandgap with state‐of‐the‐art performance, an unprecedented 17.6% STH efficiency is achieved for self‐driven solar water splitting. Modeling and analysis of the dual‐absorber PEC system reveal that further work into replacing the noble‐metal catalyst materials with earth‐abundant elements and improvement of perovskite fill factor will pave the way for the realization of a low‐cost high‐efficiency PEC system.

In this work, we report on the photoelectrochemical (PEC) investigation of n-GaN nanopillar (NP) photoanodes fabricated using metal organic chemical vapour deposition and the top-down approach. Substantial improvement in photocurrents is observed for GaN NP photoanodes compared to their planar counterparts. The role of carrier concentration and NP dimensions on the PEC performance of NP photoanodes is further elucidated. Photocurrent density is almost doubled for doped NP photoanodes whereas no improvement is noticed for undoped NP photoanodes. While the diameter of GaN NP is found to influence the onset potential, carrier concentration is found to affect both the onset and overpotential of the electrodes. Optical and electrochemical impedance spectroscopy characterisations are utilised to further explain the PEC results of NP photoanodes. Finally, improvement in the photostability of NP photoanodes with the addition of NiO as a co-catalyst is investigated.

Photoelectrochemical (PEC) reduction of CO 2 with H 2 O is a promising approach to convert solar energy and greenhouse gas into valueadded chemicals or fuels. However, the exact role of structures and interfaces of photoelectrodes in governing the photoelectrocatalytic processes in terms of both activity and selectivity remains elusive. Herein, by systematically investigating the InP photocathodes with Au−TiO 2 interfaces, we discover that nanostructuring of InP can not only enhance the photoresponse owing to increased light absorption and prolonged minority carrier lifetime, but also improve selectivity toward CO production by providing more abundant interfacial contact points between Au and TiO 2 than planar photocathodes. In addition, theoretical studies on the Au−TiO 2 interface demonstrate that the charge transfer between Au and TiO 2 , which is locally confined to the interface, strengthens the binding of the CO* intermediate on positively charged Au interfacial sites, thus improving CO 2 photoelectroreduction to form CO. An optimal Au−TiO 2 /InP nanopillar-array photocathode exhibits an onset potential of +0.3 V vs reversible hydrogen electrode (RHE) and a Faradaic efficiency of 84.2% for CO production at −0.11 V vs RHE under simulated AM 1.5G illumination at 1 sun. The present findings of the synergistic effects of the structure and interface on the photoresponse and selectivity of a photoelectrode provide insights into the development of III−V semiconductor-based PEC systems for solar fuel generation.

The scalable synthesis of highly transparent and robust sub‐monolayers of Co3O4 nano‐islands, which efficiently catalyze water oxidation, is reported. Rapid aerosol deposition of Co3O4 nanoparticles and thermally induced self‐organization lead to an ultra‐fine nano‐island morphology with more than 94% light transmission at a wavelength of 500 nm. These transparent sub‐monolayers demonstrate a remarkable mass‐weighted water oxidation activity of 2070–2350 A gCo3O4−1 and per‐metal turnover frequency of 0.38–0.62 s−1 at an overpotential of 400 mV in 1 m NaOH aqueous solution. This mixed valent cobalt oxide structure exhibits excellent long‐term electrochemical and mechanical stability preserving the initial catalytic activity over more than 12 h of constant current electrolysis and 1000 consecutive voltammetric cycles. The potential of the Co3O4 nano‐islands for photoelectrochemical water splitting has been demonstrated by incorporation of co‐catalysts in GaN nanowire photoanodes. The Co3O4‐GaN photoanodes reveal significantly reduced onset overpotentials, improved photoresponse and photostability compared to the bare GaN ones. These findings provide a highly performing catalyst structure and a scalable synthesis method for the engineering of efficient photoanodes for integrated solar water‐splitting cells.