Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis, but the poor photovoltages often reported so far will severely limit their performance. Here we report a novel observation of photovoltage loss associated with a charge extraction barrier imposed by the protection layer, and, by eliminating it, achieve photovoltages as high as 630 mV, the maximum reported so far for water-splitting silicon photoanodes. The loss mechanism is systematically probed in metal-insulator-semiconductor Schottky junction cells compared to buried junction p(+)n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A leaky-capacitor model related to the dielectric properties of the protective oxide explains this loss, achieving excellent agreement with the data. From these findings, we formulate design principles for simultaneous optimization of built-in field, interface quality, and hole extraction to maximize the photovoltage of oxide-protected water-splitting anodes.

We report on the effects on water oxidation performance of varying (1) the nanoscale TiO 2 thickness and (2) the catalyst material in catalyst/TiO 2 /SiO 2 /Si anodes. Uniform films of atomic layer deposited TiO 2 are prepared in the thickness range $1-12 nm on degenerately-doped p + -Si, yielding water oxidation overpotentials at 1 mA cm À2 of 300 mV to 600 mV in aqueous solution (pH 0 to 14). Electron/hole transport through Schottky tunnel junction structures of varying TiO 2 thickness was studied using the reversible redox couple ferri/ferrocyanide. The dependence of the water oxidation overpotential on ALD-TiO 2 thickness, with all other anode design features unchanged, exhibits a linear trend corresponding to $21 mV of added overpotential at 1 mA cm À2 per nanometer of TiO 2 for TiO 2 thicknesses greater than $2 nm. For thinner TiO 2 layers, an approximately thickness-independent overpotential is observed.The linear behavior for anodes with thicker TiO 2 layers is consistent with the predicted effect of bulk TiO 2 -limited electronic conduction on the voltage required to sustain the current density across the TiO 2 /SiO 2 insulator stack. Eight different oxygen evolution catalysts of thickness 1-3 nm are studied. For the anodes investigated, 3 nm of Ir or Ru gave the best water oxidation performance, but both thinner layers and other catalysts can be quite effective, suggesting the potential for reduced materials cost. Lastly, a flat band voltage analysis of solid state thin film capacitors was done for five different gate metals on n-Si to probe junction energetics directly relevant to a photoanode. The results are consistent with a Schottky junction in which the Fermi level at the semiconductor surface is unpinned. Broader contextEnergy storage is likely to be a requirement for grid-scale replacement of fossil fuel sources by renewable energy. Its value lies not only in the ability to compensate for intermittency, but also in the economics of peak leveling and its national security implications. The possibility of synthesizing fuels in a renewable and clean fashion has long fascinated academic and industrial researchers alike, but the high cost and poor efficiencies of such processes have prevented practical implementation of solar fuel or electro-fuel synthesis. In an effort to address these problems, researchers have been pursuing the development of photoelectrochemical cells: all-in-one units that convert solar energy into energy stored in chemical bonds. However, materials that are stable under harsh reducing and, especially, oxidizing conditions are rarely optimized for solar absorption and transport of electronic carriers. The ability to combine the properties of wide bandgap and stable materials, like TiO 2 , and efficient small bandgap absorbers, like silicon, constitutes a major advance in making viable photoelectrochemical cells. This report looks further at a novel device structure for fuel synthesis in which high quality absorbers are coupled to high quality water oxidation catalysts via an ALD...

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State-of-the-art silicon water splitting photoelectrochemical cells employ oxide protection layers that exhibit electrical conductance in between that of dielectric insulators and electronic conductors, optimizing both built-in field and conductivity.

A fundamental challenge in developing photoelectrochemical cells for the renewable production of solar chemicals and fuels is the simultaneous requirement of efficient light absorption and robust stability under corrosive conditions. Schemes for corrosion protection of semiconductor photoelectrodes such as silicon using deposited layers were proposed and attempted for several decades, but increased operational lifetimes were either insufficient or the resulting penalties for device efficiency were prohibitive. In recent years, advances in atomic layer deposition (ALD) of thin coatings have made novel materials engineering possible, leading to substantial and simultaneous improvements in stability and efficiency of photoelectrochemical cells. The self-limiting, layer-by-layer growth of ALD makes thin films with low pinhole densities possible and may also provide a path to defect control that can generalize this protection technology to a large set of materials necessary to fully realize photoelectrochemical cell technology for artificial photosynthesis.

Atomic layer deposited (ALD) TiO2 protection layers may allow for the development of both highly efficient and stable photoanodes for solar fuel synthesis; however, the very different conductivities and photovoltages reported for TiO2-protected silicon anodes prepared using similar ALD conditions indicate that mechanisms that set these key properties are, as yet, poorly understood. In this report, we study hydrogen-containing annealing treatments and find that postcatalyst-deposition anneals at intermediate temperatures reproducibly yield decreased oxide/silicon interface trap densities and high photovoltage. A previously reported insulator thickness-dependent photovoltage loss in metal-insulator-semiconductor Schottky junction photoanodes is suppressed. This occurs simultaneously with TiO2 crystallization and an increase in its dielectric constant. At small insulator thickness, a record for a Schottky junction photoanode of 623 mV photovoltage is achieved, yielding a photocurrent turn-on at 0.92 V vs NHE or -0.303 V with respect to the thermodynamic potential for water oxidation.

In order to fulfill the information storage needs of modern societies, the performance of electronic nonvolatile memories (NVMs) should be continuously improved. In the past few years, resistive random access memories (RRAM) have raised as one of the most promising technologies for future information storage due to their excellent performance and easy fabrication. In this work, a novel strategy is presented to further extend the performance of RRAMs. By using only cheap and industry friendly materials (Ti, TiO2, SiOX, and n++Si), memory cells are developed that show both filamentary and distributed resistive switching simultaneously (i.e., in the same I–V curve). The devices exhibit unprecedented hysteretic I–V characteristics, high current on/off ratios up to ≈5 orders of magnitude, ultra low currents in high resistive state and low resistive state (100 pA and 125 nA at –0.1 V, respectively), sharp switching transitions, good cycle‐to‐cycle endurance (>1000 cycles), and low device‐to‐device variability. We are not aware of any other resistive switching memory exhibiting such characteristics, which may open the door for the development of advanced NVMs combining the advantages of filamentary and distributed resistive switching mechanisms.

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