Lateral Electron Transport Research Articles

Silicon Heterojunction Solar Cells Featuring Localized Front Contacts

Throughout the development of silicon heterojunction (SHJ) solar cells, the transparent conductive oxide has been regarded as an essential component of their front electrode, facilitating lateral charge transport of photogenerated carriers toward the front metal grid fingers. In rear junction (RJ)‐SHJ solar cells, the (n)c‐Si bulk is known to support the lateral electron transport at maximum power point injection level, provided that the contact resistance of the front contact stack is sufficiently low. This enables experimental RJ‐SHJ solar cell architectures featuring a localized front carrier‐selective passivating contact exclusively covering the area contacted by the metal grid. Herein, a top‐down approach to the synthesis of this type of architecture is studied and its optical and electrical performance applied to different (n)‐type contacts are investigated. Additionally, the potential of the localized contact architecture through Cu‐plated RJ‐SHJ solar cells is demonstrated. These solar cell demonstrators feature high short‐circuit current density of 40.5 mA cm−2, without significantly compromising their open‐circuit voltage or fill factor, enabling efficiencies well above 23%, a 2%abs improvement compared to their state before localization of the front contact.

Josephson Spin-Valve Realization in the Magnetic Nodal-Line Topological Semimetal Fe3GeTe2

Three-dimensional van der Waals ferromagnet Fe3GeTe2 (FGT) is regarded as a candidate for the magnetic topological nodal line semimetal. We investigate lateral electron transport between two 3 μm spaced superconducting In leads beneath a thick three-dimensional FGT exfoliated flake. At a low temperature of 30 mK, we observe Josephson supercurrent that exhibits unusual critical current Ic suppression by the magnetic field B. The overall Ic(B) pattern is asymmetric in respect of the sign of the magnetic field B. We demonstrate, that the asymmetry is defined by the magnetic field sweep direction, so the Ic(B) pattern is strictly reversed (as magnetic field reversal) for the opposite sweeps. We also observe an interplay between maximum and minimum in Ic(B) in normal magnetic fields, while there are fast aperiodic Ic(B) fluctuations for the in-plane ones. These effects cannot be expected for homogeneous superconductor-ferromagnet-superconductor junctions, while they are known for Josephson spin valves. The mostly possible scenario for Josephson spin valve realization in FGT is the misalignment of spin polarizations of the Fermi arc surface states and ferromagnetic FGT bulk, but we also discuss possible influence of spin-dependent transport between magnetic domains.

Manipulation of laser-accelerated proton beam spatial distribution by laser machined microstructure targets

Experimental study of laser proton acceleration was carried out using laser machined line targets and cross targets with tens of micrometers scale. We have found that both the shape and material of the microstructure targets have significant influences on the distribution of the proton beam. For the aluminum line target, the proton beam spot expands in a direction perpendicular to the boundary of the target; while for the plastic line target, it expands parallel to the boundary of the target. Detailed PIC simulations of the aluminum target have been carried out, which show that due to the lateral transport of hot electrons and the sheath fields accumulated at the edge of the microstructure target, the divergence angle of the accelerated proton beam is changed accordingly, thereby modulating the spatial distribution: the elliptical beam spot can be obtained from the line target, and the quasi-square beam spot can be obtained from the cross target. Simulations of the plastic target indicate that the difference in the electron transport properties for the two types of targets may be the reason for the completely different beam spot shapes. This work shows that the microstructure targets can be a potential method to manipulate the spatial distribution and uniformity of the proton beam.

Transparent electrode employing deep-subwavelength monolithic high-contrast grating integrated with metal.

This paper demonstrates designs of transparent electrodes for polarized light based on semiconductor deep-subwavelength monolithic high-contrast gratings integrated with metal (metalMHCG). We provide theoretical background explaining the phenomena of high transmittance in the gratings and investigate their optimal parameters, which enable above 95% transmittance for sheet resistance of 2 ΩSq-1 and over 90% transmittance for extremely small sheet resistance of 0.04 ΩSq-1 in a broad spectral range below the semiconductor band-gap. The analysis is based on our fully vectorial optical model, which has been verified previously via comparison with the experimental characteristics of similar structures. The transparent electrodes can be realized in any high refractive index material used in optoelectronics and designed for light in spectral ranges starting from ultra-violet with no upper limit for the wavelength of the electromagnetic waves. They not only enable lateral transport of electrons but can also be used as an electric contact for injecting current into a semiconductor.

6X acuros algorithm validation in the presence of inhomogeneities for VMAT treatment planning.

To validate the Acuros®XB (AXB) dose calculation algorithm for a 6 MV beam from the Varian TrueBeam treatment units. Currently Anisotropic Analytic Algorithm (AAA) is clinically used on authors' department but AXB could replace it for VMAT treatments in regions where inhomogeneities and free air are present. Two steps are followed in the validation process of a new dose calculation algorithm. The first is to check the accuracy of algorithm for a homogenous phantom and regular fields. Multiple fields of increasing complexity have been acquired using a Mapcheck diode array. The accuracy of the algorithm was evaluated using the gamma analysis method. The second is to validate the algorithm in the presence of heterogeneous media. Planar absolute dose was measured with GafChromic®EBT2 film and was compared with the dose calculated by AXB. Gamma analysis was performed between Mapcheck measurements and AXB dose calculations, at a range of clinical source-surface distance. For SSDs ranging from 80 to 100 cm, the results show a minimum pass rate of 95% between AXB and Mapcheck acquisition. For open 6 MV photon beam interacting with a phantom with an air gap, the agreement after the air gap between AXB and GafChromic®EBT2 is less than 1% in the 3 × 3cm2 field and less than 2% in the 10 × 10 cm2 field. AXB has advanced modelling of lateral electron transport that enables a more accurate dose calculation in heterogeneous regions and, compared with AAA, improves accuracy between different density interfaces. This will be of particular benefit for head/neck treatments.

Lateral Josephson effect on the surface of the magnetic Weyl semimetal Co3Sn2S2

We experimentally study lateral electron transport between two 5~$\mu$m spaced superconducting indium leads on a top of magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$. For the disordered magnetic state of Co$_3$Sn$_2$S$_2$ crystal, we only observe the Andreev reflection in the proximity of each of the leads, which is indicative of highly transparent In-Co$_3$Sn$_2$S$_2$ interfaces. If the sample is homogeneously magnetized, it demonstrates well-developed anomalous Hall effect state. In this regime we find the Josephson current that takes place even for 5~$\mu$m long junctions and shows the unusual magnetic field and temperature dependencies. As a possible reason for the results obtained, we discuss the contribution to the proximity-induced spin-triplet Josephson current from the topologically protected Fermi-arc states on the surface of Co$_3$Sn$_2$S$_2$.

Surface-Induced Formation and Redox-Dependent Staining of Outer Membrane Extensions in Shewanella oneidensis MR-1

The metal-reducing bacterium Shewanella oneidensis MR-1 produces extensions of its outer membrane (OM) and periplasm that contain cytochromes responsible for extracellular electron transfer (EET) to external redox-active surfaces, including minerals and electrodes. While the role of multi-heme cytochromes in transporting electrons across the cell wall is well established, their distribution along S. oneidensis OM extensions is also thought to allow lateral electron transport along these filaments. These proposed bacterial nanowires, which can be several times the cell length, would thereby extend EET to more distant electron acceptors. However, it is still unclear why these extensions form, and to what extent they contribute to respiration in living cells. Here, we investigate physical contributors to their formation using in vivo fluorescence microscopy. While previous studies focused on the display of S. oneidensis outer membrane extensions (OMEs) as a response to oxygen limitation, we find that cell-to-surface contact is sufficient to trigger the production of OMEs, including some that reach >100 µm in length, irrespective of medium composition, agitation, or aeration. To visualize the extent of heme redox centers along OMEs, and help distinguish these structures from other extracellular filaments, we also performed histochemical redox-dependent staining with transmission electron microscopy on wild type and cytochrome-deficient strains. We demonstrate that redox-active components are limited to OMEs and not present on other extracellular appendages, such as pili and flagella. We also observed that the loss of 8 functional periplasmic and outer membrane cytochromes significantly decreased both the frequency and intensity of redox-dependent staining found widespread on OMEs. These results will improve our understanding of the environmental conditions that influence the formation of S. oneidensis OMEs, as well as the distribution and functionality of EET components along extracellular appendages.

Formation of Surface Silver Nano-network Structures through Hot Electron Regulated Diffusion-limited Aggregation

A surface Ag nano-network pattern is formed by first depositing Ag nanoparticles (NPs) on a conductive template, which has a certain defect structure, and then illuminating the Ag NPs with ultraviolet (UV) light in a moist environment. Such an Ag nano-network pattern consists of multiple connected Brownian trees (BTs), which are produced through the diffusion-limited aggregation (DLA) process. In the DLA process, diffuse Ag+ ions, which are generated by UV light illumination and dissolved by a thin adsorbed water layer on the surfaces of the Ag NPs and used GaN template, settle to form a BT through the combination with excited hot electrons migrating into the template from the Ag NPs. The lateral transport of hot electrons in the template is regulated by the distributions of threading dislocation and point defect cluster in the template, which eventually become the centers of BTs. The structure of a surface Ag nano-network can potentially serve as a transparent conductor.

Abstract ID: 377 The photon dose calculation algorithms

Purpose An overview of the photon dose calculation algorithm is given according to the classification in type “a” (no lateral electron transport included), type “b” (with lateral electron transport estimation), type “c” (true Monte Carlo simulations or Boltzmann transport equation solvers). Methods Performances of the different algorithms for common clinical treatment sites were presented: prostate, head and neck, breast and lung, planned with conformal technique. To better focus on the heterogeneities, different phantom studies were proposed. Finally, the overview focused on the most critical case of small treatment fields on low density tissue. Results The dose calculation models belonging to type “a” (pencil beam based) presented quite high inaccuracies in the small fields and low density regions, while type “b” were more congruent in the clinical situations with the type “c”, where Monte Carlo is considered the benchmark for dose estimation. This congruence is lowered for the critical clinical case of e.g. a stereotactic lung treatment. Additional point of discussion, especially for type “c” algorithms, concerned the report of the estimated dose as dose to medium, or dose to water in different human tissues. Conclusions The clinical use of a dose calculation system should be correlated with the degree of accuracy of the algorithm in each specific clinical situation.

Validation of the 6X Acuros algorithm version 13.7 in the presence of inhomogeneities for VMAT treatment planning

Purpose the aim of this study was to validate the Acuros®XB (AXB) dose calculation algorithm for a Varian TrueBeam Linac against absolute dose measurements in heterogeneous phantoms. Subsequently, the results were benchmarked against the clinically used Anisotropic Analytic Algorithm (AAA). Methods and materials the algorithm was validated in a homogenous water phantom using MapCheck diode array measurements for a range of field sizes and SSDs. To examine AXB in heterogeneous media, two slab phantoms, representing head/neck and abdominal regions, have been created and absolute dose measurements were performed with GafChromic®EBT2 films. The phantoms simulate water, muscle, bone and free air. The setup was modelled in Eclipse and the dose calculated with AXB and AAA. Results The homogenous phantom results met the gamma criteria of 3%, 2 mm. The planar dose differences between AXB and films after an air gap, for a 6 MV beam, are less than 1% in 3 × 3 cm2 field and 2% in 10 × 10 cm2 field. In the case of a 6MV beam, 5 × 5 cm2 field, interacting with a phantom of different densities and an air gap, AXB underestimates the dose after the free air gap, while AAA overestimates the dose. Measurements at 5 mm from the air/water interface show a difference in dose of 3% for AXB and 7% for AAA. Conclusion AXB was validated in a homogeneous phantom. AXB has advanced modelling of lateral electron transport that enables a more accurate dose calculation in heterogeneous regions. As a result, AXB can replace AAA for dose calculations for head/neck and abdominal VMAT plans.

Geometric Effects in Current-Voltage Characteristics of a Cross-Shaped MDM Ni/NiO/Fe Structure

The current–voltage (I–U) characteristics of a cross-shaped metal-dielectric-metal (MDM) Ni/NiO/Fe structures with NiO layers of different thicknesses are investigated. The dependence of the sign of the differential resistance on the current flowing through the structure and on the thickness of the NiO in a four-contact measuring system of potentials is revealed. In a region 8–10 nm thick, the voltage on the structure changes sign thrice and has the form of an N-shaped curve. This dependence is explained by the geometric effect, which appears due to the competition between the vertical and lateral electron transport, when the ratio of the resistances of the dielectric interlayer and metallic leading electrodes changes under the influence of the current. The numerical calculation confirms the experimentally observed I–U dependences.

Impedance spectroscopy of single bacterial nanofilament reveals water-mediated charge transfer.

For decades respiratory chain and photosystems were the main firing field of the studies devoted to mechanisms of electron transfer in proteins. The concept of conjugated lateral electron and transverse proton transport during cellular respiration and photosynthesis, which was formulated in the beginning of 1960-s, has been confirmed by thousands of experiments. However, charge transfer in recently discovered bacterial nanofilaments produced by various electrogenic bacteria is regarded currently outside of electron and proton conjugation concept. Here we report the new study of charge transfer within nanofilaments produced by Shewanella oneidensis MR-1 conducted in atmosphere of different relative humidity (RH). We utilize impedance spectroscopy and DC (direct current) transport measurements to find out the peculiarities of conductivity and Raman spectroscopy to analyze the nanofilaments’ composition. Data analysis demonstrates that apparent conductivity of nanofilaments has crucial sensitivity to humidity and contains several components including one with unusual behavior which we assign to electron transport. We demonstrate that in the case of Shewanella oneidensis MR-1 charge transfer within these objects is strongly mediated by water. Basing on current data analysis of conductivity we conclude that the studied filaments of Shewanella oneidensis MR-1 are capable of hybrid (conjugated) electron and ion conductivity.

Optimal design of current collectors for microfluidic fuel cell with flow-through porous electrodes: Model and experiment

Design optimization of current collectors has been performed to reduce the significant ohmic resistance observed in microfluidic fuel cell (MFC) with flow-through porous electrodes. A three-dimensional computational model is developed to investigate the electron transport characteristics in the porous electrodes, where lateral electron transport is found to encounter high resistance. Influences of different current collector design parameters on the transport resistances are examined and analyzed. The modeling results indicate that current collector position is the most influential factor due to the non-uniform flow rate distribution. Optimal current collector position is located at the high flow rate region instead of the conventional exposed end of the porous electrode. Experimental studies are performed to support the modeling analysis. The experimental results demonstrate that the optimized current collector position can boost the maximum power density by 61%. This study highlights the significance of the current collector design in achieving high performance MFC with flow-through porous electrodes. Based on the results, some general rules have been set for the current collector designs in this energy system, which can provide useful guidance for the future development of MFC.

Photoelectrochemical and Impedance Spectroscopic Analysis of Amorphous Si for Light-Guided Electrodeposition and Hydrogen Evolution Reaction.

For more efficient photoelectrochemical water splitting, there is a dilemma that a photoelectrode needs both light absorption and electrocatalytic faradaic reaction. One of the promising strategies is to deposit a pattern of electrocatalysts onto a semiconductor surface, leaving sufficient bare surface for light absorption while minimizing concentration overpotential as well as resistive loss at the ultramicroelectrodes for faradaic reaction. This scheme can be successfully realized by "maskless" direct photoelectrochemical patterning of electrocatalyst onto an SiOx/amorphous Si (a-Si) surface by the light-guided electrodeposition technique. Electrochemical impedance spectroscopy at various pHs tells us much about how it works. The surface states at the SiOx/a-Si interface can mediate the photogenerated electrons for hydrogen evolution, whereas electroactive species in the solution undergo outer-sphere electron transfer, taking electrons tunneling across the SiOx layer from the conduction band. In addition to previously reported long-distance lateral electron transport behavior at a patterned catalyst/SiOx/a-Si interface, the charging process of the surface states plays a crucial role in proton reduction, leading to deeper understanding of the operation mechanisms for photoelectrochemical water splitting.

Design Principles of Current Collectors in Microfluidic Fuel Cell with Flow-through Porous Electrodes

Abstract Computational and experimental studies have been performed to investigate the effects of current collector design on the performance of microfluidic fuel cell (MFC) with flow-through porous electrodes. Characteristics of electron transport in MFC with flow-through porous electrodes are investigated based on a three-dimensional computational model. The lateral electron transport in the porous electrode is found to encounter high resistance. To improve the cell performance, influences of different current collector design parameters on the transport resistances are examined. Physical origins for the influences of different design parameters are also discussed. The results demonstrate that current collector position is the most influential factor due to the non-uniform flow rate distribution. In the experimental study, cell performances revealed maximum power density when current collectors were located at the high flow rate region. An increase of 61% was observed when the current collectors moved from the conventional exposed ends of the electrodes to the high flow rate regions in the electrode active area. Based on the results, some general rules are set for the current collector designs of MFCs.

About the non-consistency of PTV-based prescription in lung

PurposeThe goal of this study is to show that the PTV concept is inconsistent for prescribing lung treatments when using type B algorithms, which take into account lateral electron transport. It is well known that type A dose calculation algorithms are not capable of calculating dose in lung correctly. Dose calculations should be based on type B algorithms. However, the combination of a type B algorithm with the PTV concept leads to prescription inconsistencies. MethodsA spherical isocentric setup has been simulated, using multiple realistic values for lung density, tumor density and collimator size. Different prescription methods are investigated using Dose-Volume-Histograms (DVH), Dose-Mass-Histograms (DMH), generalized Equivalent Uniform Dose (gEUD) and surrounding isodose percentage. ResultsIsodose percentages on the PTV drop down to 50% for small tumors and low lung density. When applying the same PTV prescription to different patients with different lung characteristics, the effective mean dose to the GTV is very different, with factors up to 1.4. The most consistent prescription method seems to be the D50%DMH (PTV) DMH point, but is also limited to tumors with size over 1cm. ConclusionsEven when using the different prescription methods, the prescription to the PTV is not consistent for type B-algorithm based dose calculations if clinical studies should produce coherent data. This combination leads to patients’ GTV with low lung density possibly receiving very high dose compared to patients with higher lung density. The only solution seems to remove the classical PTV concept for type B dose calculations in lung.

Single crystal organic photovoltaic cells using lateral electron transport

The operation of single crystal organic photovoltaic cells using lateral electron transport was demonstrated. The spacing between the collection electrodes, which is determined by the lateral range of the electrons, was estimated to be 30 μm. The possibility of electron ranges of the millimeter order was indicated.

Backward terahertz radiation from intense laser-solid interactions.

We report a systematic study on backward terahertz (THz) radiation generation from laser-solid interactions by changing a variety of laser/plasma parameters. We demonstrate a high-energy (with an energy flux density reaching 80 μJ/sr), broadband (>10 THz) plasma-based radiation source. The radiation energy is mainly distributed either in the >10 THz or <3 THz regions. A radial surface current formed by the lateral transport of low-energy electrons (LEE) is believed to be responsible for the radiation in the high-THz region (>10 THz), while high-energy surface fast electrons (SFE) accelerated along the target surface mainly contribute to lower frequency (<3 THz) radiation. The unifying explanation could be applied to backward THz radiation generation from solid targets with presence of relative small preplasmas.

Textured AZO for Thin-Film Si Solar Cells: Towards Understanding the Effect of AZO Film Thickness on the Surface Texturing Properties

Sputter-deposited Al-doped ZnO (AZO) films have been widely used as the front transparent conductive electrode for superstrate thin-film silicon (Si) solar cells. The AZO layer must be conductive enough to enable the lateral transport of electrons and sufficiently transparent to incoming light. In addition, the front AZO layer requires a rough surface texture morphology for light trapping, which improves the solar cell's current and efficiency. In this work, three types of AZO films with different layer thicknesses are prepared by pulsed DC magnetron sputtering. The post-deposition texturing is performed by using diluted hydrochloric (HCl) acid. After the HCl texturing, we find that not only the electrical performance, but also the optical scattering and morphological properties of the surface-etched AZO films can greatly depend on the initial AZO layer thickness. The AZO layer needs to be thick enough (∼700nm) in order to obtain good surface texturing properties for thin-film Si solar cell applications.

Band Alignment in WSe2-Graphene Heterostructures.

Using different types of WSe2 and graphene-based heterostructures, we experimentally determine the offset between the graphene neutrality point and the WSe2 conduction and valence band edges, as well as the WSe2 dielectric constant along the c-axis. In a first heterostructure, consisting of WSe2-on-graphene, we use the WSe2 layer as the top dielectric in dual-gated graphene field-effect transistors to determine the WSe2 capacitance as a function of thickness, and the WSe2 dielectric constant along the c-axis. In a second heterostructure consisting of graphene-on-WSe2, the lateral electron transport shows ambipolar behavior characteristic of graphene combined with a conductivity saturation at sufficiently high positive (negative) gate bias, associated with carrier population of the conduction (valence) band in WSe2. By combining the experimental results from both heterostructures, we determine the band offset between the graphene charge neutrality point, and the WSe2 conduction and valence band edges.