Showing papers by "Nathan S. Lewis published in 2012"

A quantitative assessment of the competition between water and anion oxidation at WO3 photoanodes in acidic aqueous electrolytes

Abstract: The faradaic efficiency for O2(g) evolution at thin-film WO3 photoanodes has been evaluated in a series of acidic aqueous electrolytes. In 1.0 M H2SO4, persulfate was the predominant photoelectrochemical oxidation product, and no O2 was detected unless catalytic quantities of Ag+(aq) were added to the electrolyte. In contact with 1.0 M HClO4, dissolved O2 was observed with nearly unity faradaic efficiency, but addition of a hole scavenger, 4-cyanopyridine N-oxide, completely suppressed O2 formation. In 1.0 M HCl, Cl2(g) was the primary oxidation product. These results indicate that at WO3 photoanodes, water oxidation is dominated by oxidation of the acid anions in 1.0 M HCl, H2SO4, and HClO4, respectively.

Modeling, simulation, and design criteria for photoelectrochemical water-splitting systems

Abstract: A validated multi-physics numerical model that accounts for charge and species conservation, fluid flow, and electrochemical processes has been used to analyze the performance of solar-driven photoelectrochemical water-splitting systems. The modeling has provided an in-depth analysis of conceptual designs, proof-of-concepts, feasibility investigations, and quantification of performance. The modeling has led to the formulation of design guidelines at the system and component levels, and has identified quantifiable gaps that warrant further research effort at the component level. The two characteristic generic types of photoelectrochemical systems that were analyzed utilized: (i) side-by-side photoelectrodes and (ii) back-to-back photoelectrodes. In these designs, small electrode dimensions (mm to cm range) and large electrolyte heights were required to produce small overall resistive losses in the system. Additionally, thick, non-permeable separators were required to achieve acceptably low rates of product crossover.

Hydrogen-evolution characteristics of Ni–Mo-coated, radial junction, n+p-silicon microwire array photocathodes

Abstract: The photocathodic H2-evolution performance of Ni–Mo-coated radial n+p junction Si microwire (Si MW) arrays has been evaluated on the basis of thermodynamic energy-conversion efficiency as well as solar cell figures of merit. The Ni–Mo-coated n+p-Si MW electrodes yielded open-circuit photovoltages (Voc) of 0.46 V, short-circuit photocurrent densities (Jsc) of 9.1 mA cm−2, and thermodynamically based energy-conversion efficiencies (η) of 1.9% under simulated 1 Sun illumination. Under nominally the same conditions, the efficiency of the Ni–Mo-coated system was comparable to that of Pt-coated n+p-Si MW array photocathodes (Voc = 0.44 V, Jsc = 13.2 mA cm−2, η = 2.7%). This demonstrates that, at 1 Sun light intensity on high surface area microwire arrays, earth-abundant electrocatalysts can provide performance comparable to noble-metal catalysts for photoelectrochemical hydrogen evolution. The formation of an emitter layer on the microwires yielded significant improvements in the open-circuit voltage of the microwire-array-based photocathodes relative to Si MW arrays that did not have a buried n+p junction. Analysis of the spectral response and light-intensity dependence of these devices allowed for optimization of the catalyst loading and photocurrent density. The microwire arrays were also removed from the substrate to create flexible, hydrogen-evolving membranes that have potential for use in a solar water-splitting device.

Thermally Stable N2-Intercalated WO3 Photoanodes for Water Oxidation

Abstract: We describe stable intercalation compounds of the composition xN(2)·WO(3) (x = 0.034-0.039), formed by trapping N(2) in WO(3). The incorporation of N(2) significantly reduced the absorption threshold of WO(3); notably, 0.039N(2)·WO(3) anodes exhibited photocurrent under illumination at wavelengths ≤640 nm with a faradaic efficiency for O(2) evolution in 1.0 M HClO(4)(aq) of nearly unity. Spectroscopic and computational results indicated that deformation of the WO(3) host lattice, as well as weak electronic interactions between trapped N(2) and the WO(3) matrix, contributed to the observed red shift in optical absorption. Noble-gas-intercalated WO(3) materials similar to xN(2)·WO(3) are predicted to function as photoanodes that are responsive to visible light.

Comparison of the Photoelectrochemical Behavior of H‑Terminated and Methyl-Terminated Si(111) Surfaces in Contact with a Series of One-Electron, Outer-Sphere Redox Couples in CH_3CN

Abstract: The photoelectrochemical behavior of methyl-terminated p-type and n-type Si(111) surfaces was determined in contact with a series of one-electron, outer-sphere, redox couples that span >1 V in the Nernstian redox potential, E(A/A^−), of the solution. The dependence of the current vs potential data, as well as of the open-circuit photovoltage, V_(OC), on E(A/A^−) was compared to the behavior of H-terminated p-type and n-type Si(111) surfaces in contact with these same electrolytes. For a particular E(A/A^−) value, CH_3-terminated p-Si(111) electrodes showed lower V_(OC) values than Hterminated p-Si(111) electrodes, whereas CH_3-terminated n-Si(111) electrodes showed higher V_(OC) values than H-terminated n-Si(111) electrodes. Under 100 mW cm^(−2) of ELH-simulated Air Mass 1.5 illumination, n-type H−Si(111) and CH_3−Si(111) electrodes both demonstrated nonrectifying behavior with no photovoltage at very negative values of E(A/A^−) and produced limiting V_(OC) values of >0.5 V at very positive values of E(A/A^−). Illuminated p-type H−Si(111) and CH_3−Si(111) electrodes produced no photovoltage at positive values of E(A/A^−) and produced limiting V_(OC) values in excess of 0.5 V at very negative values of E(A/A^−). In contact with CH_3CN-octamethylferrocene^(+/0), differential capacitance vs potential experiments yielded a −0.40 V shift in flat-band potential for CH_3-terminated n-Si(111) surfaces relative to H-terminated n-Si(111) surfaces. Similarly, in contact with CH_3CN-1,1′-dicarbomethoxycobaltocene^(+/0), the differential capacitance vs potential data indicated a −0.25 V shift in the flat-band potential for CH_3-terminated p-Si(111) electrodes relative to H-terminated p-Si(111) electrodes. The observed trends in V_(OC) vs E(A/A^−), and the trends in the differential capacitance vs potential data are consistent with a negative shift in the interfacial dipole as a result of methylation of the Si(111) surface. The negative dipole shift is consistent with a body of theoretical and experimental comparisons of the behavior of CH_3−Si(111) surfaces vs H−Si(111) surfaces, including density functional theory of the sign and magnitude of the surface dipole, photoemission spectroscopy in ultrahigh vacuum, the electrical behavior of Hg/Si contacts, and the pH dependence of the current−potential behavior of Si electrodes in contact with aqueous electrolytes.

Evaluation and optimization of mass transport of redox species in silicon microwire-array photoelectrodes

Abstract: Physical integration of a Ag electrical contact internally into a metal/substrate/microstructured Si wire array/oxide/Ag/electrolyte photoelectrochemical solar cell has produced structures that display relatively low ohmic resistance losses, as well as highly efficient mass transport of redox species in the absence of forced convection. Even with front-side illumination, such wire-array based photoelectrochemical solar cells do not require a transparent conducting oxide top contact. In contact with a test electrolyte that contained 50 mM/5.0 mM of the cobaltocenium+/0 redox species in CH3CN–1.0 M LiClO4, when the counterelectrode was placed in the solution and separated from the photoelectrode, mass transport restrictions of redox species in the internal volume of the Si wire array photoelectrode produced low fill factors and limited the obtainable current densities to 17.6 mA cm-2 even under high illumination. In contrast, when the physically integrated internal Ag film served as the counter electrode, the redox couple species were regenerated inside the internal volume of the photoelectrode, especially in regions where depletion of the redox species due to mass transport limitations would have otherwise occurred. This behavior allowed the integrated assembly to operate as a two-terminal, stand-alone, photoelectrochemical solar cell. The current density vs. voltage behavior of the integrated photoelectrochemical solar cell produced short-circuit current densities in excess of 80 mA cm-2 at high light intensities, and resulted in relatively low losses due to concentration overpotentials at 1 Sun illumination. The integrated wire array-based device architecture also provides design guidance for tandem photoelectrochemical cells for solar-driven water splitting.

Photoelectrochemical behavior of planar and microwire-array Si | GaP electrodes

Abstract: Gallium phosphide exhibits a short diffusion length relative to its optical absorption length, and is thus a candidate for use in wire array geometries that allow light absorption to be decoupled from minority carrier collection. Herein is reported the photoanodic performance of heteroepitaxially grown gallium phosphide on planar and microwire-array Si substrates. The n-GaP|n-Si heterojunction results in a favorable conduction band alignment for electron collection in the silicon. A conformal electrochemical contact to the outer GaP layer is produced using the ferrocenium/ferrocene (Fc^+/Fc) redox couple in acetonitrile. Photovoltages of ∼750 mV under 1 sun illumination are observed and are attributed to the barrier formed at the (Fc^+/Fc)|n-GaP junction. The short-circuit current densities of the composite microwire-arrays are similar to those observed using single-crystal n-GaP photoelectrodes. Spectral response measurements along with a finite-difference-time-domain optical model indicate that the minority carrier diffusion length in the GaP is ∼80 nm. Solid-state current–voltage measurements show that shunting occurs through thin GaP layers that are present near the base of the microwire-arrays. The results provide guidance for further studies of 3D multi-junction photoelectrochemical cells.

Band alignment of epitaxial ZnS/Zn3P2 heterojunctions

Abstract: The energy-band alignment of epitaxial zb-ZnS(001)/α-Zn_(3)P_(2)(001) heterojunctions has been determined by measurement of shifts in the phosphorus 2p and sulfur 2p core-level binding energies for various thicknesses (0.6–2.2 nm) of ZnS grown by molecular beam epitaxy on Zn_(3)P_(2). In addition, the position of the valence-band maximum for bulk ZnS and Zn3P2 films was estimated using density functional theory calculations of the valence-band density-of-states. The heterojunction was observed to be type I, with a valence-band offset, ΔE_V, of −1.19 ± 0.07 eV, which is significantly different from the type II alignment based on electron affinities that is predicted by Anderson theory. n^(+)-ZnS/p-Zn_(3)P_(2) heterojunctions demonstrated open-circuit voltages of >750 mV, indicating passivation of the Zn_(3)P_(2) surface due to the introduction of the ZnS overlayer. Carrier transport across the heterojunction devices was inhibited by the large conduction-band offset, which resulted in short-circuit current densities of <0.1 mA cm^(−2) under 1 Sun simulated illumination. Hence, constraints on the current density will likely limit the direct application of the ZnS/Zn_(3)P_(2) heterojunction to photovoltaics, whereas metal-insulator-semiconductor structures that utilize an intrinsic ZnS insulating layer appear promising.

Design and Construction of Absorption Cells for Precision Radial Velocities in the K Band Using Methane Isotopologues

Abstract: We present a method to optimize absorption cells for precise wavelength calibration in the near-infrared We apply it to design and optimize methane isotopologue cells for precision radial velocity measurements in the K band We also describe the construction and installation of two such cells for the CSHELL spectrograph at NASA’s IRTF We have obtained their high-resolution laboratory spectra, which we can then use in precision radial velocity measurements and which can also have other applications In terms of obtainable RV precision, methane should outperform other proposed cells, such as the ammonia cell (^(14)NH_3) recently demonstrated on CRIRES/VLT The laboratory spectra of the ammonia and methane cells show strong absorption features in the H band that could also be exploited for precision Doppler measurements We present spectra and preliminary radial velocity measurements obtained during our first-light run These initial results show that a precision down to 20-30 m s^(-1)can be obtained using a wavelength interval of only 5 nm in the K band and S/N ∼ 150 This supports the prediction that a precision down to a few meters per second can be achieved on late-M dwarfs using the new generation of NIR spectrographs, thus enabling the detection of terrestrial planets in their habitable zones Doppler measurements in the NIR can also be used to mitigate the radial velocity jitter due to stellar activity, enabling more efficient surveys on young active stars

Photoanodic behavior of vapor-liquid-solid–grown, lightly doped, crystalline Si microwire arrays

Abstract: Arrays of n-Si microwires have to date exhibited low efficiencies when measured as photoanodes in contact with a 1-1′-dimethylferrocene (Me2Fc+/0)–CH3OH solution. Using high-purity Au or Cu catalysts, arrays of crystalline Si microwires were grown by a vapor-liquid-solid process without dopants, which produced wires with electronically active dopant concentrations of 1 × 1013 cm−3. When measured as photoanodes in contact with a Me2Fc+/0–CH3OH solution, the lightly doped Si microwire arrays exhibited greatly increased fill factors and efficiencies as compared to n-Si microwires grown previously with a lower purity Au catalyst. In particular, the Cu-catalyzed Si microwire array photoanodes exhibited open-circuit voltages of ∼0.44 V, carrier-collection efficiencies exceeding ∼0.75, and an energy-conversion efficiency of 1.4% under simulated air mass 1.5 G illumination. Lightly doped Cu-catalyzed Si microwire array photoanodes have thus demonstrated performance that is comparable to that of optimally doped p-type Si microwire array photocathodes in photoelectrochemical cells.

Wafer-Scale Growth of Silicon Microwire Arrays for Photovoltaics and Solar Fuel Generation

Abstract: Silicon microwire arrays have recently demonstrated their potential for low-cost, high-efficiency photovoltaics and photoelectrochemical fuel generation. A remaining challenge to making this technology commercially viable is scaling up of microwire-array growth. We discuss here a technique for vapor–liquid–solid growth of microwire arrays on the scale of six-inch wafers using a cold-wall radio-frequency heated chemical vapor deposition furnace, enabling fairly uniform growth over large areas with rapid cycle time and improved run-to-run reproducibility. We have also developed a technique to embed these large-area wire arrays in polymer and to peel them intact from the growth substrate, which could enable lightweight, flexible solar cells with efficiencies as high as multicrystalline Si solar cells. We characterize these large-area microwire arrays using scanning electron microscopy and confocal microscopy to assess their structure and fidelity, and we test their energy-conversion properties using a methyl viologen (MV $^{2+/+}$ ) liquid junction contact in a photoelectrochemical cell. Initial photoelectrochemical conversion efficiencies suggest that the material quality of these microwire arrays is similar to smaller (∼1 cm $^2$ ) wire arrays that we have grown in the past, indicating that this technique is a viable way to scale up microwire-array devices.

Passivation of Zn3P2 substrates by aqueous chemical etching and air oxidation

Abstract: Surface recombination velocities measured by time-resolved photoluminescence and compositions of Zn_(3)P_2 surfaces measured by x-ray photoelectron spectroscopy (XPS) have been correlated for a series of wet chemical etches of Zn_(3)P_2 substrates. Zn_(3)P_2 substrates that were etched with Br_2 in methanol exhibited surface recombination velocity values of 2.8 × 10^4 cm s^(−1), whereas substrates that were further treated by aqueous HF–H_(2)O_2 exhibited surface recombination velocity values of 1.0 × 10^4 cm s^(−1). Zn_(3)P_2 substrates that were etched with Br_2 in methanol and exposed to air for 1 week exhibited surface recombination velocity values of 1.8 × 10^3 cm s^(−1), as well as improved ideality in metal/insulator/semiconductor devices.

Comparison between the electrical junction properties of H-terminated and methyl-terminated individual Si microwire/polymer assemblies for photoelectrochemical fuel production

Abstract: The photoelectrical properties and stability of individual p-silicon (Si) microwire/polyethylenedioxythiophene/polystyrene sulfonate:Nafion/n-Si microwire structures, designed for use as arrays for solar fuel production, were investigated for both H-terminated and CH3-terminated Si microwires. Using a tungsten probe method, the resistances of individual wires, as well as between individual wires and the conducting polymer, were measured vs. time. For the H-terminated samples, the n-Si/polymer contacts were initially rectifying, whereas p-Si microwire/polymer contacts were initially ohmic, but the resistance of both the n-Si and p-Si microwire/polymer contacts increased over time. In contrast, relatively stable, ohmic behavior was observed at the junctions between CH3-terminated p-Si microwires and conducting polymers. CH3-terminated n-Si microwire/polymer junctions demonstrated strongly rectifying behavior, attributable to the work function mismatch between the Si and polymer. Hence, a balance must be found between the improved stability of the junction electrical properties achieved by passivation, and the detrimental impact on the effective resistance associated with the additional rectification at CH3-terminated n-Si microwire/polymer junctions. Nevertheless, the current system under study would produce a resistance drop of ∼20 mV during operation under 100 mW cm−2 of Air Mass 1.5 illumination with high quantum yields for photocurrent production in a water-splitting device.

Magnetic Field Alignment of Randomly Oriented, High Aspect Ratio Silicon Microwires into Vertically Oriented Arrays

Abstract: External magnetic fields have been used to vertically align ensembles of silicon microwires coated with ferromagnetic nickel films X-ray diffraction and image analysis techniques were used to quantify the degree of vertical orientation of the microwires The degree of vertical alignment and the minimum field strength required for alignment were evaluated as a function of the wire length, coating thickness, magnetic history, and substrate surface properties Nearly 100% of 100 μm long, 2 μm diameter, Si microwires that had been coated with 300 nm of Ni could be vertically aligned by a 300 G magnetic field For wires ranging from 40 to 60 μm in length, as the length of the wire increased, a higher degree of alignment was observed at lower field strengths, consistent with an increase in the available magnetic torque Microwires that had been exposed to a magnetic sweep up to 300 G remained magnetized and, therefore, aligned more readily during subsequent magnetic field alignment sweeps Alignment of the Ni-coated Si microwires occurred at lower field strengths on hydrophilic Si substrates than on hydrophobic Si substrates The magnetic field alignment approach provides a pathway for the directed assembly of solution-grown semiconductor wires into vertical arrays, with potential applications in solar cells as well as in other electronic devices that utilize nano- and microscale components as active elements

Proton exchange membrane electrolysis using water vapor as a feedstock

Abstract: A light-driven electrolytic cell that uses water vapor as the feedstock and that has no wires or connections whatsoever to an external electrical power source of any kind. In one embodiment, the electrolytic cell uses a proton exchange membrane (PEM) with an IrRuOx water oxidation catalyst and a Pt black water reduction catalyst to consume water vapor and generate molecular oxygen and a chemical fuel, molecular hydrogen. The operation of the electrolytic cell using water vapor supplied by a humidified carrier gas has been demonstrated under varying conditions of the gas flow rate, the relative humidity, and the presence or absence of oxygen. The performance of the system with water vapor was also compared to the performance when the device was immersed in liquid water.

Nickel-based electrocatalytic photoelectrodes

Abstract: A photoelectrode, methods of making and using, including systems for water-splitting are provided. The photoelectrode can be a semiconductive material having a photocatalyst such as nickel or nickel-molybdenum coated on the material.

In situ nanomechanical measurements of interfacial strength in membrane-embedded chemically functionalized si microwires for flexible solar cells

Abstract: Arrays of vertically aligned Si microwires embedded in polydimethylsiloxane (PDMS) have emerged as a promising candidate for use in solar energy conversion devices. Such structures are lightweight and concurrently demonstrate competitive efficiency and mechanical flexibility. To ensure reliable functioning under bending and flexing, strong interfacial adhesion between the nanowire and the matrix is needed. In situ uniaxial tensile tests of individual, chemically functionalized, Si microwires embedded in a compliant PDMS matrix reveal that chemical functionality on Si microwire surfaces is directly correlated with interfacial adhesion strength. Chemical functionalization can therefore serve as an effective methodology for accessing a wide range of interfacial adhesion between the rigid constituents and the soft polymer matrix; the adhesion can be quantified by measuring the mechanical strength of such systems.

A tandem solar cell using a silicon microwire array and amorphous silicon photovoltaic layer

Abstract: This invention relates to photovoltaic cells, devices, methods of making and using the same.

Thin, free-standing Cu 2 O substrates via thermal oxidation for photovoltaic devices

Abstract: Cu 2 O is a promising, earth-abundant alternative to traditional photovoltaic materials (CIGS, CdTe, etc.) because of its low cost, high availability, and straightforward processing. We report a method to fabricate Cu 2 O substrates with thicknesses of less than 20 microns which may be handled and processed into devices. Development of thinner Cu 2 O substrates is essential as extrinsic doping has been impossible thus far, and intrinsic Cu 2 O is highly resistive. Hall measurements indicate that the substrates had Hall mobilities of 10–20 cm2V−1s−1 and carrier concentrations on the order of 1014 cm−3. Current-voltage characteristics of these Cu 2 O substrates were derived from liquid junction Schottky barrier device measurements which indicate open circuit voltages of Voc ∼ 600 mV.

Chemical Stability of Organic Monolayers Formed in Solution

Abstract: The H- Si surface is important to the electronics and photovoltaics industries because Si-H is the starting point for many Si-based devices. In tum, the electronic and chemical properties of the H-terminated Si surface affect the properties of subsequent Si surfaces and interfaces [1-3]. Dangling or weak bonds present at the surface will affect minority-carrier ("excited-state") processes, and will thus affect devices such as field-effect transistors and pbotovoltaics. Furthermore, the topography of, as well as the presence of adsorbed chemical contaminants on, a H-terminated Si surface greatly affects the electronic properties of Si/SiO_x interfaces formed from this initial H-terminated Si surface [4, 5]. Organic contaminants are difficult to remove from the Si surface, and different cleaning procedures have been shown to result in a variety of contaminant fingerprints. Hence, significant effort has been directed to understand the reactivity of the H-Si surface. This section will explore the reactivity of the H-terminated Si surface with O_2, H_2O, alcohols, metals, amines, and thiols. The synthesis, as well as the physical and electronic characterization, of the H-Si surfaces will be reviewed briefly, but the reader is encouraged to consult Chapter 3 for a more detailed account of the preparation and characterization of H-terminated Si surfaces.

Design and Construction of Absorption Cells for Precision Radial Velocities in the K Band using Methane Isotopologues

Abstract: We present a method to optimize absorption cells for precise wavelength calibration in the near-infrared. We apply it to design and optimize methane isotopologue cells for precision radial velocity measurements in the K band. We also describe the construction and installation of two such cells for the CSHELL spectrograph at NASA's IRTF. We have obtained their high-resolution laboratory spectra, which we can then use in precision radial velocity measurements and which can also have other applications. In terms of obtainable RV precision methane should out-perform other proposed cells, such as the ammonia cell ($^{14}$NH$_{3}$) recently demonstrated on CRIRES/VLT. The laboratory spectra of Ammonia and the Methane cells show strong absorption features in the H band that could also be exploited for precision Doppler measurements. We present spectra and preliminary radial velocity measurements obtained during our first-light run. These initial results show that a precision down to 20-30 m s$^{-1}$ can be obtained using a wavelength interval of only 5 nm in the K band and S/N$\sim$150. This supports the prediction that a precision down to a few m s$^{-1}$ can be achieved on late M dwarfs using the new generation of NIR spectrographs, thus enabling the detection of terrestrial planets in their habitable zones. Doppler measurements in the NIR can also be used to mitigate the radial velocity jitter due to stellar activity enabling more efficient surveys on young active stars.

Photoelectrochemical characterization of Si microwire array solar cells

Abstract: Many proposed next-generation photovoltaic devices have complicated nano- and micro-structured architectures that are designed to simultaneously optimize carrier collection and light absorption. Characterization of the electrical properties of these highly structured materials can be challenging due to the difficulty of creating electrical contacts, as well as the need to decouple the properties of the contact from that of the semiconductor. Regenerative photoelectrochemistry is a powerful technique to characterize the electrical properties of such systems, providing a conformal liquid contact that can be ohmic or rectifying, depending on the system used. We demonstrate the use of the methyl viologen regenerative electrochemical system to characterize different stages of the fabrication of radial junction Si microwire (SiMW) solar cells. Photoelectrochemical characterization, combined with other more traditional measurements allows evaluation of how the different processing steps affect the device performance, without having to construct a fully integrated device. We describe the operating principle of this technique, and demonstrate that it can be applied to semiconductor materials with complex architectures.

Localized deposition of polymer film on nanocantilever chemical vapor sensors by surface-initiated atom transfer radical polymerization

Abstract: Cantilever chemical vapor sensors that can be tailored to respond preferentially in frequency by controlling the location of deposition of an adsorbing layer. Cantilever chemical vapor sensor having a base, one or more legs and a tip are fabricated using a gold layer to promote deposition of a sorbing layer of a polymeric material in a desired location, and using a chromium layer to inhibit deposition of the sorbing layer in other locations. Sorbing layers having different glass temperatures Tg and their effects are described. The methods of making such cantilever chemical vapor sensors are described.

Methodology for forming pnictide compositions suitable for use in microelectronic devices

Abstract: The present invention provides methods for making pnictide compositions, particularly photoactive and/or semiconductive pnictides. In many embodiments, these compositions are in the form of thin films grown on a wide range of suitable substrates to be incorporated into a wide range of microelectronic devices, including photovoltaic devices, photodetectors, light emitting diodes, betavoltaic devices, thermoelectric devices, transistors, other optoelectronic devices, and the like. As an overview, the present invention prepares these compositions from suitable source compounds in which a vapor flux is derived from a source compound in a first processing zone, the vapor flux is treated in a second processing zone distinct from the first processing zone, and then the treated vapor flux, optionally in combination with one or more other ingredients, is used to grow pnictide films on a suitable substrate.

Modeling the Behavior of a Solar-Hydrogen Generator

Abstract: Solar radiation is the most abundant energy source available but it is distributed and intermittent, thereby necessitating its storage via conversion to a fuel (e.g. hydrogen or carbohydrates). A viable low-temperature route for the production of solar fuels is photosynthesis. However, natural photosynthesis exhibits low (~1%) solar-to-fuel efficiencies, and thus it is the aim of artificial photosynthesis to increase this efficiency. Artificial photosynthetic devices use light-capturing semiconductors attached to electrodes covered by catalysts that generate oxygen and hydrogen or hydrocarbons by electrochemical reactions. Essential to the system is a minimum-ohmic-loss ionic pathway between the electrodes. Additionally, a selectively permeable separator placed between the electrodes helps to minimize fuel crossover losses, thereby increasing system performance, stability, yield, and safety. Although much effort has been devoted to the development of suitable robust and scalable materials for solar-driven electrolysis, relatively little attention has been paid to the electrochemical-system engineeringdesign aspects. These are crucial because the material combinations that provide optimal performance in such a system depend significantly on the architecture of the system itself. In this talk, we will investigate the system engineering aspects using a validated multi-physics numerical model – solving for charge and species conservation, fluid dynamics, light absorption, semiconductor transport, and electrochemical reaction – to analyze solar-driven photo electrochemical (PEC) devices and understand the system response to changes in design and materials. In this study the focus is primarily on transport phenomena including separator transport properties and ohmic (both electronic and ionic) losses. In addition, the impact of the kinetic parameters and light absorption will also be mentioned. Figure 1, for example, depicts ionic potential losses and hydrogen yield efficiencies as a function of separator porosity for various design dimensions, showing the importance of separator porosity for system performance. From the analysis, design criteria and guidelines can be established that account for the various performance tradeoffs such that practical artificial-photosynthetic solar-fuel generators can be realized. Acknowledgements This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.

Separator design criteria for a solar-hydrogen electrochemical generator

Abstract: Introduction Solar irradiation is the most abundant energy source available but it is distributed and intermittent, thereby necessitating its storage via conversion to a fuel (e.g., hydrogen). One possible route for direct solar-hydrogen production is through an integrated electrochemical device that uses light-capturing semiconductors in contact with electrodes to generate oxygen and hydrogen [1,2]. Key in such a device is balancing product crossover with Ohmic losses in the solution which necessitate higher photovoltages. Often these requirements are accomplished using a polymer-electrolyte separator [3], yet the exact material-property design targets are not definitively known. In this presentation, a validated multi-physics numerical model of an electrochemical solar-hydrogen generator is used to study its performance. Advantages and limitations concerning current efficiency, required photovoltage, and safety are investigated as a function of separator transport parameters which leads to general design guidelines. Systems including both porous and nonporous photoactive components are examined.