[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. &
Solar Energy Materials and Solar Cells 09/2015; 140. DOI:10.1016/j.solmat.2015.04.001 · 5.34 Impact Factor
[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.
[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.
The Review of scientific instruments 07/2015; 86(7):075006. DOI:10.1063/1.4923453 · 1.61 Impact Factor
[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.
[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.
[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.
The Journal of Physical Chemistry C 06/2015; 119(27). DOI:10.1021/acs.jpcc.5b04407 · 4.77 Impact Factor
[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
[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.
Journal of Micromechanics and Microengineering 05/2015; 25(5). DOI:10.1088/0960-1317/25/5/055021 · 1.73 Impact Factor
[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.
Thin Solid Films 05/2015; DOI:10.1016/j.tsf.2014.11.075 · 1.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: While investigating uniformity of magnetic tunnel junction (MTJ) stacks we find experimentally and analytically that variation in the resistance area product (RA) is more important to monitor as compared to the tunnel magnetoresistance (TMR), which is less sensitive to MTJ variability. The standard Current In-Plane Tunneling (CIPT) method measures both RA and TMR, but the usefulness for uniformity mapping, e.g. for tool optimization, is limited by excessive measurement time. Thus, we develop and demonstrate a fast complementary static magnetic field method focused only on measurement of RA. We compare the static field method to the standard CIPT method and find perfect agreement between the extracted RA values and measurement repeatability while the static field method is several times faster. The static field CIPT method is demonstrated for 200 mm wafer mapping showing radial as well as asymmetrical variations related to the MTJ deposition conditions.
[Show abstract][Hide abstract] ABSTRACT: A photocatalytic model reactor system has been devised to assess the capacity and feasibility of a photocatalytic unit for the removal of trace amounts of organic contaminants in air. Realistic operating conditions are applied, and a mathematical model based on Langmuir-Hinselwood adsorbtion permits the capacity of the removal unit to be extrapolated to extreme operating conditions. A radial flow reactor system allows parameters such as gas velocity, contaminant concentration and relative humidity to be accurately controlled. Ethene photooxidation in fruit containers is studied as an example of application. A runaway ethene production from a full shipment of fruit in a 40’ container can be prevented from a starting ethene concentration of 0.5 ppm with a few m2 of the porous photocatalyst at a 254 nm irradiance of 37.5 mW cm−2.
Chemical Engineering Journal 02/2015; 262:648-657. DOI:10.1016/j.cej.2014.10.008 · 4.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Si is an excellent absorber material for use in 2-photon photoelectrochemical hydrogen production. So far nearly all studies of silicon photoelectrodes have employed frontal illumination despite the fact that in most water-splitting 2-photon device concepts the silicon is the “bottom” cell in the tandem stack and therefore illuminated from the back with respect to the electrolyte. In the present work, we investigate back-illuminated Si photoelectrodes experimentally, as well as by modelling, the dependence of induced photocurrent on various parameters, such as carrier diffusion length (Le) and surface recombination velocity (vs) to quantify their relative importance. A bifacial light absorbing structure (p+pn+ Si) is tested under back-illumination conditions which mimic the actual working environment in a tandem water splitting device. The thickness of the absorbing Si layer is varied from 30 to 350 μm to assess the impact of the diffusion length/thickness ratio (Le/L) on photocatalytic performance. It is shown how the induced photocurrent (JL) of a back-illuminated sample increases as wafer thickness decreases. Compared to the 350 μm thick sample, a thinned 50 μm thick sample shows a 2.7-fold increase in JL, and consequently also a higher open circuit voltage. An analytical model is developed to quantify how the relative Le/L-ratio affects the maximum JL under back-illumination, and the result agrees well with experimental results. JL increases with the Le/L-ratio only up to a certain point, beyond which the surface recombination velocity becomes the dominant loss mechanism. This implies that further efforts should to be focused on reduction of surface recombination. The present study is the first experimental demonstration of a Si wafer based photocathode under back-illumination. Moreover, the comparative experimental and theoretical treatment also highlights which photoabsorber properties merit the most attention in the further development towards full tandem water splitting devices.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that the quasi-one-dimensional (1D) current transport, experimentally observed in graphene as measured by a collinear four-point probe in two electrode configurations A and B, can be interpreted using the sensitivity functions of the two electrode configurations (configurations A and B represents different pairs of electrodes chosen for current sources and potential measurements). The measured sheet resistance in a four-point probe measurement is averaged over an area determined by the sensitivity function. For a two-dimensional conductor, the sensitivity functions for electrode configurations A and B are different. But when the current is forced to flow through a percolation network, e.g., graphene with high density of extended defects, the two sensitivity functions become identical. This is equivalent to a four-point measurement on a line resistor, hence quasi-1D transport. The sensitivity analysis presents a formal definition of quasi-1D current transport, which was recently observed experimentally in chemical-vapor-deposition graphene. Our numerical model for calculating sensitivity is verified by comparing the model to analytical calculations based on conformal mapping of a single extended defect.
Physical Review B 12/2014; 90(24-24). DOI:10.1103/PhysRevB.90.245432 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: While silicon is an anisotropic material it is often in literature treated as an isotropic material when it comes to plate calculations. This leads to considerable errors in the calculated deflection. To overcome this problem, we present an in-depth analysis of the bending behavior of thin crystalline plates. An analysis of the compliance tensor for the 32 different crystal classes shows, that for thin plates, only 5 different types of plates exist. An anisotropic plate equation valid for crystalline thin plates is derived and solved for circular, elliptic, rectangular and square plates using both exact analytical expressions and approximate expressions calculated by the Galerkin method. The results are applied to plates made on silicon (001), (011) and (111) substrates, respectively, and analytical equations for the deflection, strain energy and resonance frequency of such plates are presented. These expressions are in excellent agreement with anisotropic finite element calculations. The calculated deflection differs less than 0.1%, for both circular and rectangular plates, compared to finite element calculations. The results are presented as ready-to-use facilitating accurate analytical models involving crystalline plates, such as those often found in the field of micro electro mechanical systems. The effect of elastic boundary conditions is taken into account by using an effective radius of the plate.
Sensors and Actuators A Physical 12/2014; 220. DOI:10.1016/j.sna.2014.09.007 · 1.90 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The electrical performance of graphene synthesized by chemical vapour deposition and transferred to insulating surfaces may be compromised by extended defects, including for instance grain boundaries, cracks, wrinkles and tears. In this study we experimentally investigate and compare the nano- and micro-scale electrical continuity of single layer graphene grown on cm-size single crystal copper with that of previously studied graphene films, grown on commercially available copper foil. The electrical continuity of the graphene films is analysed using two non-invasive conductance characterization methods: ultra-broadband terahertz time-domain spectroscopy and micro four-point probe, which probe the electrical properties of the graphene film on different length scales; 100 nm and 10 µm, respectively. Ultra-broadband terahertz time-domain spectroscopy allows for measurement of the complex conductance response in the frequency range 1-15 terahertz, covering the entire intraband conductance spectrum, and reveals that the conductance response for the graphene grown on single crystalline copper intimately follows the Drude model for a barrier-free conductor. In contrast, the graphene grown on commercial foil copper shows a distinctly non-Drude conductance spectrum that is better described by the Drude-Smith model, which incorporates the effect of preferential carrier backscattering associated with extended, electronic barriers with a typical separation on the order of 100 nm. Micro four-point probe resistance values measured on graphene grown on single crystalline copper in two different voltage-current configurations show close agreement with the expected distributions for a continuous 2D conductor, in contrast with previous observations on graphene grown on commercial copper foil. The terahertz and micro four-point probe conductance values of the graphene grown on single crystalline copper shows a close to unity correlation, in contrast with those of the graphene grown on commercial copper foil, which we explain by the absence of extended defects on the microscale in CVD graphene grown on single crystalline copper. The presented results demonstrate that the graphene grown on single crystal copper is electrically continuous on the nano-, micro-, as well as intermediate scales.
[Show abstract][Hide abstract] ABSTRACT: We derive exact, analytic expressions for the sensitivity of sheet resistance and Hall sheet resistance measurements to local inhomogeneities for the cases of nonzero magnetic fields, strong perturbations, and perturbations over a finite area, extending our earlier results on weak perturbations. We express these sensitivities for conductance tensor components and for other charge transport quantities. Both resistive and Hall sensitivities, for a van der Pauw specimen in a finite magnetic field, are a superposition of the zero-field sensitivities to both sheet resistance and Hall sheet resistance. Strong perturbations produce a nonlinear correction term that depends on the strength of the inhomogeneity. Solution of the specific case of a finite-sized circular inhomogeneity coaxial with a circular specimen suggests a first-order correction for the general case. Our results are confirmed by computer simulations on both a linear four-point probe array on a large circular disc and a van der Pauw square geometry. Furthermore, the results also agree well with Nahlik et al. published experimental results for physical holes in a circular copper foil disc. (C) 2014 AIP Publishing LLC.
[Show abstract][Hide abstract] ABSTRACT: Modeling of energy harvesting devices is complicated by the coupling between electrical and mechanical domains. In this paper, we present a coupled electromechanical model for electret-based resonant energy harvesters where the two output pads are placed on the same device side (single-sided). An analytical analysis is complemented by 2-D finite element method simulations, where the fringing field effect on a plane capacitor is studied and accounted for by an effective area that is well fitted by a sinusoidal function of the displacement of the proof mass. From analytical calculations, we prove that the electrostatic transducer force is related to the voltage output and cannot be approximated by viscous damping or a Coulomb force as reported previously. The coupled model with two simultaneous differential equations is numerically solved for the voltage output and transduction force with given parameters. The model was verified both by practical measurements from our own fabricated device and results from a reference. An optimization study is carried out using this model to achieve the maximum output power by tuning the allowable movement (X M) of the proof mass. Finally, the effect of a standard power-conditioning circuit is investigated for both continuous and burst power supply applications. [2013-0207]
Journal of Microelectromechanical Systems 10/2014; 23(5). DOI:10.1109/JMEMS.2014.2306963 · 1.75 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sputter deposition of 50 nm thick NiO films on p+-n-Si and subsequent treatment in Fe-containing electrolyte yielded highly transparent photoanodes capable of water oxidation (OER) in alkaline media (1 M KOH) with high efficiency and stability. The Fe-treatment of NiO thin films enabled Si-based photoanode assemblies to obtain a current density of 10 mA/cm2 (requirement for >10% efficient devices) at 1.15 V vs. RHE (Reversible Hydrogen Electrode) under red-light (38.6 mW/cm2) irradiation. Thus the photoanodes were harvesting ~80 mV of free energy (voltage), which places them among the best performing Si-based photoanodes in alkaline media. The stability was proven by chronoamperometry at 1.3 V vs. RHE for 300 h. Furthermore, measurements with electrochemical quartz crystal microbalances coupled with ICP-MS showed minor corrosion under dark operation. Extrapolation of the corrosion rate showed stability for more than 2000 days of continuous operation. Therefore, protection by Fe-treated NiO films is a promising strategy to achieve highly efficient and stable photoanodes.