[Show abstract][Hide abstract] ABSTRACT: Si is an excellent absorber material for use in photoelectrochemical (PEC) hydrogen production. Only a few studies have been done using Si in alkaline electrolyte for hydrogen evolution due to its poor chemical stability in high pH electrolyte, indicating that a chemically stable protection layer is essential. Here we investigate thin TiO 2 films deposited by high power impulse magnetron sputtering (HiPIMS) as a protection layer for a p-type silicon photocathode for photoelectrochemical H 2 evolution in a high pH electrolyte. The X-ray reflectometry analysis reveals that the HiPIMS process provides improved film density for TiO 2 films (4.15 g/cm 3), and consequently results in a significantly less corroded Si surface. The Si photocathode protected by the HiPIMS grown TiO 2 film along with Pt as co-catalyst produced a photocurrent onset potential of $ 0.5 V vs. RHE in 1 M KOH and showed a 4% decay over 24 h in KOH. In contrast, the sample with the TiO 2 deposited using conventional DC sputtering technique of similar thickness shows 20% loss in photocurrent for the same time interval. Considering the fact that the experiments were carried out not in the cleanroom, much less corrosion loss can be obtained if done in dust-free condition. Hence, these results suggest the HiPIMS technique as an improved approach for the protection of photoelectrodes, which are unstable in alkaline solution.
Full-text · Article · Jan 2016 · Solar Energy Materials and Solar Cells
[Show abstract][Hide abstract] ABSTRACT: We report fabrication of nanostructured, laser-doped selective emitter (LDSE) silicon solar cells with power conversion efficiency of 18.1% and a fill factor (FF) of 80.1%. The nanostructured solar cells were realized through a single step, mask-less, scalable reactive ion etch (RIE) texturing of the surface. The selective emitter was formed by means of laser doping using a continuous wave (CW) laser and subsequent contact formation using light-induced plating of Ni and Cu. The combination of RIE-texturing and a LDSE cell design has to our knowledge not been demonstrated previously. The resulting efficiency indicates a promising potential, especially considering that the cell reported in this work is the first proof-of-concept and that the fabricated cell is not fully optimized in terms of plating, emitter sheet resistance and surface passivation. Due to the scalable nature and simplicity of RIE-texturing as well as the LDSE process, we consider this specific combination a promising candidate for a cost-efficient process for future Si solar cells.
Full-text · Article · Jan 2016 · Solar Energy Materials and Solar Cells
[Show abstract][Hide abstract] ABSTRACT: With electrocatalysts it is important to be able to distinguish between the effects of mass transport and reaction kinetics on the performance of the catalyst. When the hydrogen evolution reaction (HER) is considered, an additional and often neglected detail of mass transport in liquid is the evolution and transport of gaseous H2, since HER leads to the continuous formation of H2 bubbles near the electrode. We present a numerical model that includes the transport of both gaseous and dissolved H2, as well as mass exchange between them, and combine it with a kinetic model of HER at platinum (Pt) nanoparticle electrodes. We study the effect of the diffusion layer thickness and H2 dissolution rate constant on the importance of gaseous transport, and the effect of equilibrium hydrogen coverage and Pt loading on the kinetic and mass transport overpotentials. Gaseous transport becomes significant when the gas volume fraction is sufficiently high to facilitate H2 transfer to bubbles within a distance shorter than the diffusion layer thickness. At current densities below about 40 mA/cm2 the model reduces to an analytical approximation that has characteristics similar to the diffusion of H2. At higher current densities the increase in the gas volume fraction makes the H2 surface concentration nonlinear with respect to the current density. Compared to the typical diffusion layer model, our model is an extension that allows more detailed studies of reaction kinetics and mass transport in the electrolyte and the effects of gas bubbles on them.
No preview · Article · Jan 2016 · International Journal of Hydrogen Energy
[Show abstract][Hide abstract] ABSTRACT: Si is an excellent photoabsorber for use in dual band gap photoelectrochemical water splitting. We investigate photoanodes with n+pp+-Si configuration under back-side illumination which is suited to work in a tandem device stack. A co-sputtered NiCoOx film coupled to the Si was used as a protective catalyst for the water oxidation reaction in 1M KOH. The sample showed high photocurrent (21 mA cm-2) under the red-light (38.6 mW cm-2). Long-term stability test showed gradual decrease of activity in the beginning, and then the activity was increased, yielding a cathodic shift of the onset voltage (> 50 mV) likely due to divergent response of Ni and Co to Fe present in KOH. Once the activity of the sample is stabilized, no further degradation was observed for the following 6 days, indicating that the demonstrated back-illuminated photoanode configuration can be considered as a promising architecture to be applied as a bottom cell of the tandem water splitting device under alkaline conditions.
[Show abstract][Hide abstract] ABSTRACT: Line-focusing compound silicon x-ray lenses with structure heights exceeding 300 μm were fabricated using deep reactive ion etching. To ensure profile uniformity over the full height, a new strategy was developed in which the perimeter of the structures was defined by trenches of constant width. The remaining sacrificial material inside the lens cavities was removed by etching through the silicon wafer. Since the wafers become fragile after through-etching, they were then adhesively bonded to a carrier wafer. Individual chips were separated using laser micro machining and the 3D shape of fabricated lenses was thoroughly characterized by a variety of means. Optical testing using synchrotron radiation with a photon energy of 56 keV yielded a 300 μm wide beam with a waist of 980 nm (full width at half maximum) at a focal length of 1.3 m. Optical aberrations are discussed in the context of the shape analysis, where a slight bowing of the lens sidewalls and an insufficiently uniform apex region are identified as resolution-limiting factors. Despite these, the proposed fabrication route proved a viable approach for producing x-ray lenses with large structure heights and provides the means to improve the resolution and capabilities of modern x-ray techniques such as x-ray microscopy and 3D x-ray diffraction.
Full-text · Article · Dec 2015 · Journal of Micromechanics and Microengineering
[Show abstract][Hide abstract] ABSTRACT: A novel and economical approach for fabricating compound refractive lenses for the purpose of focusing hard X-rays is described. A silicon master was manufactured by UV-lithography and deep reactive ion etching (DRIE). Sacrificial structures were utilized, which enabled accurate control of the etching profile and were removed after DRIE. By electroplating, an inverse nickel sample was obtained, which was used as a mold insert in a commercial polymer injection molding machine. A prototype lens made of polyethylene with a focal length of 350 mm was tested using synchrotron radiation at photon energies of 17 keV. A 55 µm long line focus with a minimal waist of 770 nm (FWHM) and a total lens transmittance of 32% were measured. Due to its suitability for cheap mass production, this highly efficient optics may find widespread use in hard X-ray instruments.
Full-text · Article · Nov 2015 · Optical Materials Express
[Show abstract][Hide abstract] ABSTRACT: This article describes the realization of complex high-aspect ratio silicon
structures with feature dimensions from 100 μm to 100 nm by deep reactive ion etching using the Bosch process. As the exact shape of the sidewall profiles can be crucial for the proper functioning of a device, the authors investigated how sacrificial structures in the form of guarding walls and pillars may be utilized to facilitate accurate control of the etch profile. Unlike other sacrificial structuring approaches, no silicon-on-insulator substrates or multiple lithography steps are required. In addition, the safe removal of the sacrificial structures was accomplished by thermal oxidation and subsequent selective wet etching. The effects of the dimensions and relative placement of sacrificial walls and pillars on the etching result were determined through systematic experiments. The authors applied this process for exact sidewall control in the manufacture of x-ray
lenses that are very sensitive to sidewall shape nonuniformities. Compound kinoform lenses for focusing hard x-rays with structure heights of 200 μm were manufactured, and the lenses were tested in terms of their focusing ability and refracting qualities using synchrotron radiation at a photon energy of 17 keV. A 180 μm long line focus with a waist of 430 nm at a focal length of 215 mm was obtained.
Full-text · Article · Nov 2015 · Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics
[Show abstract][Hide abstract] ABSTRACT: We show experimentally as well as theoretically that patterned magnetic tunnel junctions can be characterized using the current-in-plane tunneling (CIPT) method, and the key parameters, the resistance-area product (RA) and the tunnel magnetoresistance (TMR), can be determined. The CIPT method relies on four-point probe measurements performed with a range of different probe pitches and was originally developed for infinite samples. Using the method of images, we derive a modified CIPT model, which compensates for the insulating boundaries of a finite rectangular sample geometry. We measure on square tunnel junction pads with varying sizes and analyze the measured data using both the original and the modified CIPT model. Thus, we determine in which sample size range the modified CIPT model is needed to ensure validity of the extracted sample parameters, RA and TMR. In addition, measurements as a function of position on a square tunnel junction pad are used to investigate the sensitivity of the measurement results to probe misalignment.
No preview · Article · Oct 2015 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: We report angle resolved characterization of nanostructured and conventionally textured silicon solar cells. The nanostructured solar cells are realized through a single step, mask-less, scalable reactive ion etching (RIE) texturing of the surface. Photovoltaic properties including short circuit current, open circuit voltage, fill factor (FF) and power conversion efficiency are each measured as function of the relative incident angle between the solar cell and the light source. The relative incident angle is varied from 0° to 90° in steps of 10° in orthogonal axes, such that each solar cell is characterized at 100 different angle combinations. The angle resolved photovoltaic properties are summarized in terms of the average, angle-dependent electrical power output normalized to the power output at normal incidence and differently textured cells on different silicon substrates are compared in terms of angle resolved performance. The results show a 3% point improvement in average electrical power output normalized with respect to normal incidence power output of RIE textured, multicrystalline Si cells compared to conventional multicrystalline Si cells and above 1% point improvement of RIE textured monocrystalline Si cells compared to conventional monocrystalline Si cells. &
Full-text · Article · Sep 2015 · Solar Energy Materials and Solar Cells
[Show abstract][Hide abstract] ABSTRACT: The formation of self-assembled contacts between vapor-liquid-solid grown silicon nanowires and flat silicon surfaces was imaged in situ using electron microscopy. By measuring the structural evolution of the contact formation process, we demonstrate how different contact geometries are created by adjusting the balance between silicon deposition and Au migration. We show that electromigration provides an efficient way of controlling the contact. The results point to novel device geometries achieved by direct nanowire growth on devices.
[Show abstract][Hide abstract] ABSTRACT: Thin-film sheet resistance measurements at high spatial resolution and on small pads are important and can be realized with micrometer-scale four-point probes. As a result of the small scale the measurements are affected by electrode position errors. We have characterized the electrode position errors in measurements on Ru thin film using an Au-coated 12-point probe. We show that the standard deviation of the static electrode position error is on the order of 5 nm, which significantly affects the results of single configuration measurements. Position-error-corrected dual-configuration measurements, however, are shown to eliminate the effect of position errors to a level limited either by electrical measurement noise or dynamic position errors. We show that the probe contact points remain almost static on the surface during the measurements (measured on an atomic scale) with a standard deviation of the dynamic position errors of 3 Å. We demonstrate how to experimentally distinguish between different sources of measurement errors, e.g. electrical measurement noise, probe geometry error as well as static and dynamic electrode position errors.
No preview · Article · Sep 2015 · Measurement Science and Technology
[Show abstract][Hide abstract] ABSTRACT: This paper presents a novel apparatus for extracting volatile species from liquids using a “sniffer-chip.” By ultrafast transfer of the volatile species through a perforated and hydrophobic membrane into an inert carrier gas stream, the sniffer-chip is able to transport the species directly to a mass spectrometer through a narrow capillary without the use of differential pumping. This method inherits features from differential electrochemical mass spectrometry (DEMS) and membrane inlet mass spectrometry (MIMS), but brings the best of both worlds, i.e., the fast time-response of a DEMS system and the high sensitivity of a MIMS system. In this paper, the concept of the sniffer-chip is thoroughly explained and it is shown how it can be used to quantify hydrogen and oxygen evolution on a polycrystalline platinum thin film in situ at absolute faradaic currents down to ∼30 nA. To benchmark the capabilities of this method, a CO-stripping experiment is performed on a polycrystalline platinum thin film, illustrating how the sniffer-chip system is capable of making a quantitative in situ
measurement of <1 % of a monolayer of surface adsorbed CO being electrochemically stripped off an electrode at a potential scan-rate of 50 mV s−1.
No preview · Article · Jul 2015 · The Review of scientific instruments
[Show abstract][Hide abstract] ABSTRACT: Full-field x-ray microscopy using x-ray objectives has become a mainstay of
the biological and materials sciences. However, the inefficiency of existing
objectives at x-ray energies above 15 keV has limited the technique to weakly
absorbing or two-dimensional (2D) samples. Here, we show that significant gains
in numerical aperture and spatial resolution may be possible at hard x-ray
energies by using silicon-based optics comprising 'interdigitated' refractive
silicon lenslets that alternate their focus between the horizontal and vertical
directions. By capitalizing on the nano-manufacturing processes available to
silicon, we show that it is possible to overcome the inherent inefficiencies of
silicon-based optics and interdigitated geometries. As a proof-of-concept of
Si-based interdigitated objectives, we demonstrate a prototype interdigitated
lens with a resolution of ~255 nm at 17 keV.
Full-text · Article · Jun 2015 · Optics Communications
[Show abstract][Hide abstract] ABSTRACT: We have used replica molding and large-range atomic force microscopy to characterize the three-dimensional shape of high aspect ratio microstructures. Casting inverted replicas of microstructures using polydimethylsiloxane (PDMS) circumvents the inability of AFM probes to measure deep and narrow cavities. We investigated cylindrical deep reactive ion etched cavities in silicon wafers and determined the radius of curvature (ROC) of the sidewalls as a function of depth. Statistical analysis verified the reliability and reproducibility of the replication procedure. The mean ROC was determined as (6.32 ± 0.06) μm, i.e., with 1% accuracy, while the ROC linearly increases by (0.52 ± 0.03) μm from the top to the bottom of the sidewalls. Nanometer sized surface defects are also well replicated. In addition, the method allows combining multiple features from differently processed wafers into a single sample, accelerating characterization in process optimization tasks. To access the sidewall shape samples needed to be cleaved. The method was applied to study X-ray refractive optics, whose performance is crucially affected by their three dimensional shapes.
Full-text · Article · Jun 2015 · Microelectronic Engineering
[Show abstract][Hide abstract] ABSTRACT: Stabilizing efficient photoabsorbers for solar water splitting has recently shown significant progress with the development of various protection layers. Suitable protection layers for tandem devices should be conductive, transparent, and stable in strongly acidic or alkaline solutions. This paper shows that under certain conditions n-type semiconductors, such as TiO2, can be used as protection layers for Si-based photoanodes. It also provides evidence that even in a photoanode assembly TiO2 is conducting only electrons (not holes as in p-type protection layers), and therefore TiO2 can be described as a simple ohmic contact. This renders n-type semiconductors, such as TiO2, to be versatile and simple protection layers, which can be used for photoanodes and as previously shown for photocathodes. The ohmic behavior of n-type TiO2 in a Si/TiO2–photoanode assembly is demonstrated under dark and illuminated conditions by performing the oxygen evolution reaction (OER) and using the Fe(II)/Fe(III) redox couple. These measurements reveal that the performance of the Si/TiO2–photoanode assembly is strongly dependent on the TiO2/electrolyte interaction. Finally, the conditions and requirements that make TiO2 generally applicable for photoanode assemblies, and thus for protecting tandem devices, are outlined and quantitatively shown by band diagram calculations. The results presented here provide the understanding required for the design of highly efficient and stable photoelectrochemical water splitting devices.
Full-text · Article · Jun 2015 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: The electrocatalytic performance for hydrogen evolution has been evaluated for radial-junction n+p-Si microwire (MW) arrays with Pt or cobalt phosphide, CoP, nanoparticulate catalysts in contact with 0.50 M H2SO4(aq). The CoP-coated (2.0 mg cm-2) n+p-Si MW photocathodes were stable for over 12 h of continuous operation and produced an open-circuit photovoltage (Voc) of 0.48 V, a light-limited photocurrent density (Jph) of 17 mA cm-2, a fill factor (ff) of 0.24, and an ideal regenerative cell efficiency (ηIRC) of 1.9% under simulated 1 Sun illumination. Pt-coated (0.5 mg cm-2) n+p-Si MW-array photocathodes produced Voc = 0.44 V, Jph = 14 mA cm-2, ff = 0.46, and η = 2.9% under identical conditions. Thus, the MW geometry allows the fabrication of photocathodes entirely comprised of earth-abundant materials that exhibit performance comparable to that of devices that contain Pt.Keywords: silicon; cobalt phosphide; hydrogen evolution; platinum; microwires; solar fuel
No preview · Article · May 2015 · Journal of Physical Chemistry Letters
[Show abstract][Hide abstract] ABSTRACT: Cu2ZnSnS4 films prepared by pulsed laser deposition at different temperatures are characterized by spectroscopic ellipsometry. The focus is on confirming results from direct measurement techniques, by finding appropriate models of the surface overlayer for data fitting, and extracting the dielectric function of the films. It is found that the surface overlayer changes with film thickness and deposition temperature. Adopting different ellipsometry measurements and modeling strategies for each film, dielectric functions are extracted and compared. As the deposition temperature is increased, the dielectric functions exhibit additional critical points related to optical transitions in the material other than absorption across the fundamental band gap. In the case of a thin film < 200 nm thick, surface features observed by scanning electron microscopy and atomic force microscopy are accurately reproduced by ellipsometry data fitting.
No preview · Article · May 2015 · Thin Solid Films
[Show abstract][Hide abstract] ABSTRACT: We describe a process for the fabrication of a Ni stamp that is applied to the microstructuring of polymers by hot embossing. The target devices are microcontainers that have a potential application in oral drug delivery. Each container is a 3D, cylindrical, high aspect ratio microstructure obtained by defining a reservoir and a separating trench with different depths of 85 and 125 µm, respectively, in a single embossing step. The fabrication of the required two leveled stamp is done using a modified DEEMO (dry etching, electroplating and molding) process. Dry etching using the Bosch process and electroplating are optimized to obtain a stamp with smooth stamp surfaces and a positive sidewall profile. Using this stamp, hot embossing is performed successfully with excellent yield and high replication fidelity.
Full-text · Article · May 2015 · Journal of Micromechanics and Microengineering