Showing papers by "Nathan S. Lewis published in 2016"
TL;DR: Lewis reviews the status of solar thermal and solar fuels approaches for harnessing solar energy, as well as technology gaps for achieving cost-effective scalable deployment combined with storage technologies to provide reliable, dispatchable energy.
Abstract: Major developments, as well as remaining challenges and the associated research opportunities, are evaluated for three technologically distinct approaches to solar energy utilization: solar electricity, solar thermal, and solar fuels technologies. Much progress has been made, but research opportunities are still present for all approaches. Both evolutionary and revolutionary technology development, involving foundational research, applied research, learning by doing, demonstration projects, and deployment at scale will be needed to continue this technology-innovation ecosystem. Most of the approaches still offer the potential to provide much higher efficiencies, much lower costs, improved scalability, and new functionality, relative to the embodiments of solar energy-conversion systems that have been developed to date.
1,416 citations
TL;DR: In this paper, a comparison between photoelectrochemical (PEC) and photovoltaic-electrolytic (PV-E) solar-hydrogen production of 3.65 kilotons H2 per year was performed to assess the economics of each technology, and to provide a basis for comparison within the broader energy landscape.
Abstract: A technoeconomic analysis of photoelectrochemical (PEC) and photovoltaic-electrolytic (PV-E) solar-hydrogen production of 10000 kg H2 day−1 (3.65 kilotons per year) was performed to assess the economics of each technology, and to provide a basis for comparison between these technologies as well as within the broader energy landscape. Two PEC systems, differentiated primarily by the extent of solar concentration (unconcentrated and 10× concentrated) and two PV-E systems, differentiated by the degree of grid connectivity (unconnected and grid supplemented), were analyzed. In each case, a base-case system that used established designs and materials was compared to prospective systems that might be envisioned and developed in the future with the goal of achieving substantially lower overall system costs. With identical overall plant efficiencies of 9.8%, the unconcentrated PEC and non-grid connected PV-E system base-case capital expenses for the rated capacity of 3.65 kilotons H2 per year were $205 MM ($293 per m2 of solar collection area (mS−2), $14.7 WH2,P−1) and $260 MM ($371 mS−2, $18.8 WH2,P−1), respectively. The untaxed, plant-gate levelized costs for the hydrogen product (LCH) were $11.4 kg−1 and $12.1 kg−1 for the base-case PEC and PV-E systems, respectively. The 10× concentrated PEC base-case system capital cost was $160 MM ($428 mS−2, $11.5 WH2,P−1) and for an efficiency of 20% the LCH was $9.2 kg−1. Likewise, the grid supplemented base-case PV-E system capital cost was $66 MM ($441 mS−2, $11.5 WH2,P−1), and with solar-to-hydrogen and grid electrolysis system efficiencies of 9.8% and 61%, respectively, the LCH was $6.1 kg−1. As a benchmark, a proton-exchange membrane (PEM) based grid-connected electrolysis system was analyzed. Assuming a system efficiency of 61% and a grid electricity cost of $0.07 kWh−1, the LCH was $5.5 kg−1. A sensitivity analysis indicated that, relative to the base-case, increases in the system efficiency could effect the greatest cost reductions for all systems, due to the areal dependencies of many of the components. The balance-of-systems (BoS) costs were the largest factor in differentiating the PEC and PV-E systems. No single or combination of technical advancements based on currently demonstrated technology can provide sufficient cost reductions to allow solar hydrogen to directly compete on a levelized cost basis with hydrogen produced from fossil energy. Specifically, a cost of CO2 greater than ∼$800 (ton CO2)−1 was estimated to be necessary for base-case PEC hydrogen to reach price parity with hydrogen derived from steam reforming of methane priced at $12 GJ−1 ($1.39 (kg H2)−1). A comparison with low CO2 and CO2-neutral energy sources indicated that base-case PEC hydrogen is not currently cost-competitive with electrolysis using electricity supplied by nuclear power or from fossil-fuels in conjunction with carbon capture and storage. Solar electricity production and storage using either batteries or PEC hydrogen technologies are currently an order of magnitude greater in cost than electricity prices with no clear advantage to either battery or hydrogen storage as of yet. Significant advances in PEC technology performance and system cost reductions are necessary to enable cost-effective PEC-derived solar hydrogen for use in scalable grid-storage applications as well as for use as a chemical feedstock precursor to CO2-neutral high energy-density transportation fuels. Hence such applications are an opportunity for foundational research to contribute to the development of disruptive approaches to solar fuels generation systems that can offer higher performance at much lower cost than is provided by current embodiments of solar fuels generators. Efforts to directly reduce CO2 photoelectrochemically or electrochemically could potentially produce products with higher value than hydrogen, but many, as yet unmet, challenges include catalytic efficiency and selectivity, and CO2 mass transport rates and feedstock cost. Major breakthroughs are required to obtain viable economic costs for solar hydrogen production, but the barriers to achieve cost-competitiveness with existing large-scale thermochemical processes for CO2 reduction are even greater.
604 citations
TL;DR: In this article, the authors highlight the progress that has been made in this field and provide insights into the synthesis, characterization, and electrochemical behavior of transition metal phosphides as HER electrocatalysts.
Abstract: Hydrogen gas obtained by the electrolysis of water has long been proposed as a clean and sustainable alternative to fossil fuels. Noble metals such as Pt are capable of splitting water at low overpotentials, but the implementation of inexpensive solar-driven water-splitting systems and electrolyzers could benefit from the development of robust, efficient, and abundant alternatives to noble metal catalysts. Transition metal phosphides (MxPy) have recently been identified as a promising family of Earth abundant electrocatalysts for the hydrogen-evolution reaction (HER) and are capable of operating with low overpotentials at operationally relevant current densities while exhibiting stability under strongly acidic conditions. In this review, we highlight the progress that has been made in this field and provide insights into the synthesis, characterization, and electrochemical behavior of transition metal phosphides as HER electrocatalysts. We also discuss strategies for the incorporation of metal phosphides ...
477 citations
Massachusetts Institute of Technology1, University of California, Berkeley2, Utrecht University3, Imperial College London4, California Institute of Technology5, Environmental Change Institute6, University of Tennessee7, Navigant Consulting, Inc.8, University of Illinois at Chicago9, Princeton University10
TL;DR: In this article, ten leading experts in energy research share their vision of what challenges their respective fields need to address in the coming decades, from the search for better materials for fuels, to the design of energy policy and markets for the developing world.
Abstract: Great strides have been made over the past century in our ability to harness energy sources, leading to profound transformations — both good and bad — in society. Looking at the energy system of today, it is clear that meeting the energy needs of the world now and in the years to come requires the concerted efforts of many different actors across a range of technologies and approaches. In this Feature, ten leading experts in energy research share their vision of what challenges their respective fields need to address in the coming decades. The issues being faced are diverse and multifaceted, from the search for better materials for fuels, to the design of energy policy and markets for the developing world. However, a common theme emerges: changes to adapt and improve our energy system are greatly needed. By improving our mutual understanding of the issues faced by each area of energy research, these changes can happen more smoothly, efficiently and rapidly. Meeting the world's energy needs requires the collective efforts of many different actors across a range of technologies and approaches. In this Feature, ten leading experts in energy research share their vision of the challenges their respective fields must address in the coming decades.
225 citations
TL;DR: In this article, the electrocatalytic reduction of CO2 to the highly reduced C2 products, ethylene and ethane, as well as to the fully reduced C1 product, methane, on three different phases of nickel-gallium (NiGa, Ni3Ga, and Ni5Ga3) films prepared by drop-casting was reported.
Abstract: We report the electrocatalytic reduction of CO2 to the highly reduced C2 products, ethylene and ethane, as well as to the fully reduced C1 product, methane, on three different phases of nickel–gallium (NiGa, Ni3Ga, and Ni5Ga3) films prepared by drop-casting. In aqueous bicarbonate electrolytes at neutral pH, the onset potential for methane, ethylene, and ethane production on all three phases was found to be −0.48 V versus the reversible hydrogen electrode (RHE), among the lowest onset potentials reported to date for the production of C2 products from CO2. Similar product distributions and onset potentials were observed for all three nickel–gallium stoichiometries tested. The onset potential for the reduction of CO2 to C2 products at low current densities catalyzed by nickel–gallium was >250 mV more positive than that of polycrystalline copper, and approximately equal to that of single crystals of copper, which have some of the lowest overpotentials to date for the reduction of CO2 to C2 products and metha...
218 citations
TL;DR: In this paper, the authors present the fundamentals of water splitting and describe practices which distinguish commercial water-electrolysis systems from simple laboratory-scale demonstrations, as well as a comparison between the two types of water-splitting systems.
Abstract: Efforts to develop renewable sources of carbon-neutral fuels have brought a renewed focus to research and development of sunlight-driven water-splitting systems. Electrolysis of water to produce H2 and O2 gases is the foundation of such systems, is conceptually and practically simple, and has been practiced both in the laboratory and industrially for many decades. In this Focus article, we present the fundamentals of water splitting and describe practices which distinguish commercial water-electrolysis systems from simple laboratory-scale demonstrations.
179 citations
158 citations
TL;DR: This work discusses the development of an approach that is based on an architecture that combines arrays of semiconducting microwires with flexible polymeric membranes, and highlights the key steps that have been taken towards delivering a fully functional solar fuels generator.
Abstract: An artificial photosynthetic system that directly produces fuels from sunlight could provide an approach to scalable energy storage and a technology for the carbon-neutral production of high-energy-density transportation fuels. A variety of designs are currently being explored to create a viable artificial photosynthetic system, and the most technologically advanced systems are based on semiconducting photoelectrodes. Here, I discuss the development of an approach that is based on an architecture, first conceived around a decade ago, that combines arrays of semiconducting microwires with flexible polymeric membranes. I highlight the key steps that have been taken towards delivering a fully functional solar fuels generator, which have exploited advances in nanotechnology at all hierarchical levels of device construction, and include the discovery of earth-abundant electrocatalysts for fuel formation and materials for the stabilization of light absorbers. Finally, I consider the remaining scientific and engineering challenges facing the fulfilment of an artificial photosynthetic system that is simultaneously safe, robust, efficient and scalable.
151 citations
TL;DR: In this article, a planar n-Si(100) photoanodes with a thin ∼50 nm film of cobalt oxide fabricated using atomic-layer deposition (ALD) was used for photovoltaic-biased solar fuels generators.
Abstract: Heterojunction photoanodes, consisting of n-type crystalline Si(100) substrates coated with a thin ∼50 nm film of cobalt oxide fabricated using atomic-layer deposition (ALD), exhibited photocurrent-onset potentials of −205 ± 20 mV relative to the formal potential for the oxygen-evolution reaction (OER), ideal regenerative solar-to-O_2(g) conversion efficiencies of 1.42 ± 0.20%, and operated continuously for over 100 days (∼2500 h) in 1.0 M KOH(aq) under simulated solar illumination. The ALD CoO_x thin film: (i) formed a heterojunction with the n-Si(100) that provided a photovoltage of 575 mV under 1 Sun of simulated solar illumination; (ii) stabilized Si photoanodes that are otherwise unstable when operated in aqueous alkaline electrolytes; and, (iii) catalyzed the oxidation of water, thereby reducing the kinetic overpotential required for the reaction and increasing the overall efficiency relative to electrodes that do not have an inherently electrocatalytic coating. The process provides a simple, effective method for enabling the use of planar n-Si(100) substrates as efficient and durable photoanodes in fully integrated, photovoltaic-biased solar fuels generators.
124 citations
TL;DR: This Review focuses on the modeling- and simulation-guided development and implementation of solar-driven water-splitting prototypes from a holistic viewpoint that explores the various interplays between the components.
Abstract: An integrated cell for the solar-driven splitting of water consists of multiple functional components and couples various photoelectrochemical (PEC) processes at different length and time scales. The overall solar-to-hydrogen (STH) conversion efficiency of such a system depends on the performance and materials properties of the individual components as well as on the component integration, overall device architecture, and system operating conditions. This Review focuses on the modeling- and simulation-guided development and implementation of solar-driven water-splitting prototypes from a holistic viewpoint that explores the various interplays between the components. The underlying physics and interactions at the cell level is are reviewed and discussed, followed by an overview of the use of the cell model to provide target properties of materials and guide the design of a range of traditional and unique device architectures.
113 citations
TL;DR: In this article, an efficient, stable, and intrinsically safe solar water-splitting device is demonstrated using a III-V tandem junction photoanode, an acid-stable, earth-abundant hydrogen evolution catalyst, and a bipolar membrane.
Abstract: An efficient, stable, and intrinsically safe solar water-splitting device is demonstrated using a III–V tandem junction photoanode, an acid-stable, earth-abundant hydrogen evolution catalyst, and a bipolar membrane. The integrated photoelectrochemical cell operates under a steady-state pH gradient and achieves ≈10% solar-to-hydrogen conversion efficiency, >100 h of stability in a large (>1 cm^2) photoactive area in relation to most previous reports.
TL;DR: The interaction of a high-contrast short-pulse laser with a flat target via periodic Si microwires yields a substantial enhancement in both the total and cutoff energies of the produced electron beam.
Abstract: We report on the first successful proof-of-principle experiment to manipulate laser-matter interactions on microscales using highly ordered Si microwire arrays. The interaction of a high-contrast short-pulse laser with a flat target via periodic Si microwires yields a substantial enhancement in both the total and cutoff energies of the produced electron beam. The self-generated electric and magnetic fields behave as an electromagnetic lens that confines and guides electrons between the microwires as they acquire relativistic energies via direct laser acceleration.
TL;DR: In this article, the stabilization of various semiconductor photoanodes is reviewed in the context of the electronic characteristics and a mechanistic analysis of the TiO_2 films, along with a discussion of the optical, catalytic, and electronic nature of NiO_x films for stabilization of semiconductor photos for water oxidation.
TL;DR: In this article, photoelectrochemical measurements of n-Si/TiO2/Ni interfaces in contact with a series of one-electron, electrochemically reversible redox systems indicated that the structure acted as a buried junction whose photovoltage was independent of the formal potential of the contacting electrolyte.
Abstract: Solid-state electrical, photoelectrochemical, and photoelectron spectroscopic techniques have been used to characterize the behavior and electronic structure of interfaces between n-Si, n+-Si, or p+-Si surfaces and amorphous coatings of TiO2 formed using atomic-layer deposition. Photoelectrochemical measurements of n-Si/TiO2/Ni interfaces in contact with a series of one-electron, electrochemically reversible redox systems indicated that the n-Si/TiO2/Ni structure acted as a buried junction whose photovoltage was independent of the formal potential of the contacting electrolyte. Solid-state current–voltage analysis indicated that the built-in voltage of the n-Si/TiO2 heterojunction was ∼0.7 V, with an effective Richardson constant ∼1/100th of the value of typical Si/metal Schottky barriers. X-ray photoelectron spectroscopic data allowed formulation of energy band-diagrams for the n-Si/TiO2, n+-Si/TiO2, and p+-Si/TiO2 interfaces. The XPS data were consistent with the rectifying behavior observed for amorpho...
TL;DR: In this article, the authors discuss the issues that intertwine science, technology, legal, and policy issues critical to the implementation of any global carbon emissions control regime: (i) What to regulate and at what levels; (ii) Regulating short-term versus long-term emissions; (iii) Validation of compliance in a regulated global emissions regime; and, (iv) Legal aspects of geoengineering.
Abstract: The delegates to COP21 in Paris, in conjunction with nationally formulated commitments and pledges, resolved that countries should take actions to “hold the increase in global temperature to well below 2 °C above pre-industrial levels” and to achieve “a balance between anthropogenic emissions by sources and removal by sinks of greenhouse gases in the second half of this century”. This resolution for action suggests a step towards a global carbon emissions-control regime which, due to regional variabilities and remaining uncertainties as to the exact effects of atmospheric CO2 concentrations, must be considered within the purview of risk management. In this Opinion, four topics are discussed that intertwine science, technology, legal, and policy issues critical to the implementation of any global carbon emissions-control regime: (i) What to regulate and at what levels; (ii) Regulating short-term versus long-term emissions; (iii) Validation of compliance in a regulated global emissions regime; and, (iv) Legal aspects of geoengineering.
TL;DR: In this article, the spatial variation in photoelectrochemical performance for the reduction of an aqueous one-electron redox couple, Ru(NH_3)_6^(3+/2+), and for the evolution of H_2(g) from 0.5 M H 2SO_4(aq) at the surface of bare or Pt-decorated p-type WSe_2 photocathodes has been investigated in situ using scanning photocurrent microscopy (SPCM).
Abstract: The spatial variation in the photoelectrochemical performance for the reduction of an aqueous one-electron redox couple, Ru(NH_3)_6^(3+/2+), and for the evolution of H_2(g) from 0.5 M H_2SO_4(aq) at the surface of bare or Pt-decorated p-type WSe_2 photocathodes has been investigated in situ using scanning photocurrent microscopy (SPCM). The measurements revealed significant differences in the charge-collection performance (quantified by the values of external quantum yields, Φ_(ext)) on various macroscopic terraces. Local spectral response measurements indicated a variation in the local electronic structure among the terraces, which was consistent with a non-uniform spatial distribution of sub-band-gap states within the crystals. The photoconversion efficiencies of Pt-decorated p-WSe_2 photocathodes were greater for the evolution of H_2(g) from 0.5 M H_2SO_4 than for the reduction of Ru(NH_3)_6^(3+/2+), and terraces that exhibited relatively low values of Φ_(ext) for the reduction of Ru(NH_3)_6^(3+/2+) could in some cases yield values of Φ_(ext) for the evolution of H_2(g) comparable to the values of Φ_(ext) yielded by the highest-performing terraces. Although the spatial resolution of the techniques used in this work frequently did not result in observation of the effect of edge sites on photocurrent efficiency, some edge effects were observed in the measurements; however the observed edge effects differed among edges, and did not appear to determine the performance of the electrodes.
TL;DR: In this article, mixed methyl/TFPA-terminated (MMTFPA) n- and p-type Si(111) surfaces were synthesized and characterized by electrochemical methods.
Abstract: Functionalization of semiconductor surfaces with organic moieties can change the charge distribution, surface dipole, and electric field at the interface. The modified electric field will shift the semiconductor band-edge positions relative to those of a contacting phase. Achieving chemical control over the energetics at semiconductor surfaces promises to provide a means of tuning the band-edge energetics to form optimized junctions with a desired material. Si(111) surfaces functionalized with 3,4,5-trifluorophenylacetylenyl (TFPA) groups were characterized by transmission infrared spectroscopy, X-ray photoelectron spectroscopy, and surface recombination velocity measurements. Mixed methyl/TFPA-terminated (MMTFPA) n- and p-type Si(111) surfaces were synthesized and characterized by electrochemical methods. Current density versus voltage and Mott–Schottky measurements of Si(111)–MMTFPA electrodes in contact with Hg indicated that the barrier height, Φb, was a function of the fractional monolayer coverage o...
TL;DR: In this article, the authors reported ordered, high aspect ratio, tapered Si microwire arrays that exhibit an extremely low angular (0° to 50°) and spectrally averaged reflectivity of < 1% of the incident 400-1100 nm illumination.
Abstract: We report ordered, high aspect ratio, tapered Si microwire arrays that exhibit an extremely low angular (0° to 50°) and spectrally averaged reflectivity of <1% of the incident 400–1100 nm illumination. After isolating the microwires from the substrate with a polymer infill and peel off process, the arrays were found to absorb 89.1% of angular averaged incident illumination (0° to 50°) in the equivalent volume of a 20 μm thick Si planar slab, reaching 99.5% of the classical light trapping limit between 400 and 1100 nm. We explain the broadband absorption by enhancement in coupling to waveguide modes due to the tapered microstructure of the arrays. Time-resolved microwave photoconductivity decay measurements yielded charge-carrier lifetimes of 0.75 μs (more than an order of magnitude higher than vapor–liquid–solid-grown Si microwires) in the tapered microwires, resulting in an implied Voc of 0.655 V. The high absorption and high aspect ratio in these ordered microwire arrays make them an attractive platform...
TL;DR: In this paper, a multiphysics model that accounts for the performance of electrocatalysts and triple-junction light absorbers, as well as for the transport properties of the electrolyte and dissolved CO2, was used to evaluate the spatial and light-intensity dependence of product distributions in an integrated photoelectrochemical CO2 reduction (CO2R) cell.
Abstract: A multiphysics model that accounts for the performance of electrocatalysts and triple-junction light absorbers, as well as for the transport properties of the electrolyte and dissolved CO2, was used to evaluate the spatial and light-intensity dependence of product distributions in an integrated photoelectrochemical CO2 reduction (CO2R) cell. Different sets of band gap combinations of triple-junction light absorbers were required to accommodate the optimal total operating current density relative to the optimal partial current density for CO2R. The simulated product distribution was highly nonuniform along the width of the electrode and depended on the electrode dimensions as well as the illumination intensity. To achieve the same product selectivity as in a potentiostatic, “half-cell” configuration, the electrocatalyst must retain its selectivity over a range of cathode potentials, and this range is dependent on the transport losses and current–voltage relationship of the light absorbers, the geometric pa...
TL;DR: Tandem-junction microwave array photoelectrodes have been fabricated by coating np^+-Si radial homojunction microwire arrays sequentially with fluorine-doped tin oxide (FTO) and titanium dioxide (TiO_2).
Abstract: Tandem-junction microwire array photoelectrodes have been fabricated by coating np^+-Si radial homojunction microwire arrays sequentially with fluorine-doped tin oxide (FTO) and titanium dioxide (TiO_2). These photoelectrodes effected unassisted water splitting under simulated 1 Sun conditions with an open-circuit potential (E_(oc)) of −1.5 V vs the formal potential for oxygen evolution, E^(0′)(OH^−/O_2), a current density at E = E^(0′)(OH^−/O_2) of 0.78 mA cm^(−2), a fill factor ( ff ) = 0.51, and a photovoltaic-biased photoelectrochemical ideal regenerative cell efficiency of 0.6%.
TL;DR: In this article, the authors used synchrotron radiation photoelectron spectroscopy (SRPES) in the tender X-ray energy range to obtain information on the energy-band relations of semiconductor and metal-covered semiconductor surfaces while in direct contact with aqueous electrolytes under potentiostatic control.
TL;DR: The electrical and spectroscopic properties of the TiO_2/Ni protection layer system, which enables stabilization of otherwise corroding photoanodes, have been investigated in contact with electrolyte solutions by scanning-probe microscopy, electrochemistry and in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS).
Abstract: The electrical and spectroscopic properties of the TiO_2/Ni protection layer system, which enables stabilization of otherwise corroding photoanodes, have been investigated in contact with electrolyte solutions by scanning-probe microscopy, electrochemistry and in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Specifically, the energy-band relations of the p+-Si/ALD-TiO_2/Ni interface have been determined for a selected range of Ni thicknesses. AP-XPS measurements using tender X-rays were performed in a three-electrode electrochemical arrangement under potentiostatic control to obtain information from the semiconductor near-surface region, the electrochemical double layer (ECDL) and the electrolyte beyond the ECDL. The degree of conductivity depended on the chemical state of the Ni on the TiO2surface. At low loadings of Ni, the Ni was present primarily as an oxide layer and the samples were not conductive, although the TiO_2 XPS core levels nonetheless displayed behavior indicative of a metal-electrolyte junction. In contrast, as the Ni thickness increased, the Ni phase was primarily metallic and the electrochemical behavior became highly conductive, with the AP-XPS data indicative of a metal-electrolyte junction. Electrochemical and microtopographical methods have been employed to better define the nature of the TiO_2/Ni electrodes and to contextualize the AP-XPS results.
TL;DR: In this paper, the photoelectrochemical behavior of n-type GaAs(100) electrodes coated with a single layer of graphene was compared with the behavior of bare, freshly etched n-Type GaAs100 electrodes, both for electrodes in contact with an aqueous solution containing K3[Fe(CN)6]/K4[Fe (CN) 6] and for electrodes with nonaqueous solutions containing a series of one-electron redox couples selected such that the Nernstian solution potentials spanned a range greater than 1 V.
Abstract: The photoelectrochemical behavior of n-type GaAs(100) electrodes coated with a single layer of graphene was compared with the behavior of bare, freshly etched n-type GaAs(100) electrodes, both for electrodes in contact with an aqueous solution containing K3[Fe(CN)6]/K4[Fe(CN)6] and for electrodes in contact with nonaqueous solutions containing a series of one-electron redox couples selected such that the Nernstian solution potentials spanned a range greater than 1 V. Under simulated 1 Sun illumination, the graphene-coated electrodes produced a short-circuit photocurrent density of 20 mA cm–2 for up to 8 h of continuous operation in nonaqueous electrolytes (H2O concentration 0.1%, v/v), while, under the same conditions, the unprotected n-GaAs electrodes showed a rapid decay of the photocurrent density within ∼400 s. Although the graphene monolayers enhanced the stability of n-GaAs photoanodes in nonaqueous electrolytes, the graphene did not fully protect photoanodes operated in contact with Fe(CN)63–/4–(aq...
TL;DR: The behavior of n-Si(111) photoanodes covered by monolayer sheets of fluorinated graphene (F-Gr) was investigated under a range of chemical and electrochemical conditions and showed that oxide formation at the Si surface was significantly inhibited for Si electrodes coated with F-Gr relative to bare Si electrodes exposed to the same conditions.
Abstract: The behavior of n-Si(111) photoanodes covered by monolayer sheets of fluorinated graphene (F–Gr) was investigated under a range of chemical and electrochemical conditions. The electrochemical behavior of n-Si/F–Gr and np+-Si/F–Gr photoanodes was compared to hydride-terminated n-Si (n-Si−H) and np+-Si−H electrodes in contact with aqueous Fe(CN)63-/4- and Br2/HBr electrolytes as well as in contact with a series of outer-sphere, one-electron redox couples in nonaqueous electrolytes. Illuminated n-Si/F–Gr and np+-Si/F–Gr electrodes in contact with an aqueous K3(Fe(CN)6/K4(Fe(CN)6 solutions exhibited stable short-circuit photocurrent densities of ∼10 mA cm–2 for 100,000 s (>24 h), in comparison to bare Si electrodes, which yielded nearly a complete photocurrent decay over ∼100 s. X-ray photoelectron spectra collected before and after exposure to aqueous anodic conditions showed that oxide formation at the Si surface was significantly inhibited for Si electrodes coated with F–Gr relative to bare Si electrodes e...
TL;DR: Computer modeling of the morphologies of films grown photoelectrochemically using light from two simultaneous sources that had mutually different linear polarizations successfully reproduced the experimental morphologies and quantitatively agreed with the pattern orientations observed experimentally, indicating that the resultant morphology is a function of all illumination inputs despite differing polarizations.
Abstract: The template-free growth of well ordered, highly anisotropic lamellar structures has been demonstrated during the photoelectrodeposition of Se–Te films, wherein the orientation of the pattern can be directed by orienting the linear polarization of the incident light. This control mechanism was investigated further herein by examining the morphologies of films grown photoelectrochemically using light from two simultaneous sources that had mutually different linear polarizations. Photoelectrochemical growth with light from two nonorthogonally polarized same-wavelength sources generated lamellar morphologies in which the long axes of the lamellae were oriented parallel to the intensity-weighted average polarization orientation. Simulations of light scattering at the solution–film interface were consistent with this observation. Computer modeling of these growths using combined full-wave electromagnetic and Monte Carlo growth simulations successfully reproduced the experimental morphologies and quantitatively...
TL;DR: Au was photoelectrochemically deposited onto cylindrical or tapered p-Si microwires on Si substrates to profile the photoinduced charge-carrier generation in individual wires in a photoactive semiconductor wire array to determine the spatially resolved carrier generation profiles.
Abstract: Au was photoelectrochemically deposited onto cylindrical or tapered p-Si microwires on Si substrates to profile the photoinduced charge-carrier generation in individual wires in a photoactive semiconductor wire array. Similar experiments were repeated for otherwise identical Si microwires doped to be n-type. The metal plating profile was conformal for n-type wires, but for p-type wires was a function of distance from the substrate and was dependent on the illumination wavelength. Spatially resolved charge-carrier generation profiles were computed using full-wave electromagnetic simulations, and the localization of the deposition at the p-type wire surfaces observed experimentally correlated well with the regions of enhanced calculated carrier generation in the volumes of the microwires. This technique could potentially be extended to determine the spatially resolved carrier generation profiles in a variety of mesostructured, photoactive semiconductors.
TL;DR: The collective data indicated that the photoelectrodeposition process exhibits sensitivity toward the coherency, relative phase, and polarization orientations of all optical inputs and that this sensitivity is physically expressed in the morphology of the deposit.
Abstract: Highly anisotropic and ordered nanoscale lamellar morphologies can be spontaneously generated over macroscopic areas, without the use of a photomask or any templating agents, via the photoelectrodeposition of Se–Te alloy films. To form such structures, the light source can be a single, linearly polarized light source that need not necessarily be highly coherent. In this work, the variation in the morphologies produced by this deposition process was evaluated in response to differences in the coherence and relative phase between multiple simultaneous linearly polarized illumination inputs. Specifically, the morphologies of photoelectrodeposits were evaluated when two tandem same-wavelength sources with discrete linear polarizations, both either mutually incoherent or mutually coherent (with defined phase differences), were used. Additionally, morphologies were simulated via computer modeling of the interfacial light scattering and absorption during the photoelectrochemical growth process. The morphologies ...
05 Jun 2016
TL;DR: In this paper, the authors reported that ICPRIE etched tapered silicon microwires are ideal light trapping structures with extremely low (1.08% between 400 nm-1100 nm under normal incidence) reflectivity.
Abstract: We report cryogenic inductively coupled plasma reactive ion etching (ICPRIE) etched tapered silicon microwires are ideal light trapping structures with extremely low (1.08% between 400 nm–1100 nm under normal incidence) reflectivity. We show that these tapered microwire arrays absorb 90.12% of incident light under normal incidence in an effectively 20 μm thick silicon when embedded in a polymer and peeled off the substrate, making them an attractive alternative for achieving high efficiency in thin film crystalline silicon solar cells. We show that microwave photoconductivity decay measurements as a simple quick way to measure carrier lifetimes in etched microwires under various liquid surface passivation techniques to estimate surface recombination velocities. The etched structures demonstrate >1 μs lifetimes.