Effective Diffusion Length Research Articles

"Secondary Growth" in Hydrothermal Synthesis of Aligned ZnO Nanostructures and Its Application in Dye-Sensitized Solar Cells.

One-dimensional (1D) aligned ZnO nanostructures were prepared on ZnO film seeded substrates using a low-temperature hydrothermal method, and zinc nitrate and hexamethylenetetramine (HMT) precursors. It was observed that increasing the concentration ratio of Zn2+/HMT from 1 to 100 led to a "secondary growth," and a change in the morphologies of the ZnO nanostructures from arrays of thick nanorods to arrays of thin nanorod-step-thick nanorods. The morphological evolution of ZnO nanostructures with increased growth time at high Zn2+/HMT concentration ratios showed the same transformation. Dye-sensitized solar cells (DSSCs) were fabricated using ZnO nanostructures as the photoanodes, and the electron transport properties were determined by electrochemical impedance spectroscopy (EIS). Although the DSSCs showed low power conversion efficiencies due to the short lengths, the arrays of the thin nanorods demonstrated excellent electron transport with an electron diffusion coefficient (Dn) of 1.57 x 10(-3) cm2/s, and an effective diffusion length (L) of 140 µm.

CO and C3H8 oxidation activity of Pd/ZnO nanowires/cordierite catalyst

Using nanowires grown on monolith cordierite as catalyst wash-coat is a new concept. ZnO nanowires array has different pore-region diffusion of reactants in the catalyst media because it can have greater effective porosity and average diffusion length scale than traditional catalyst carrier. Also, thickness of ZnO nanowires array is less than conventional wash coat. ZnO nanowires were grown on monolith cordierite channels, and Pd was impregnated on the nanowires. The catalyst structure was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and atomic absorption spectrometry (AAS). The activity performance of Pd/ZnO nanowires catalyst for C3H8 and CO was examined under lean, stoichiometric and rich conditions. T50 was achieved under stoichiometric condition of gas mixture for C3H8 and CO at 400 °C and 235 °C, respectively.

Effective spin Hall properties of a mixture of materials with and without spin-orbit coupling: Tailoring the effective spin diffusion length

We study theoretically the effective spin Hall properties of a composite consisting of two materials with and without spin-orbit (SO) coupling. In particular, we assume that SO material represents a system of grains in a matrix with no SO. We calculate the effective spin Hall angle and the effective spin diffusion length of the mixture. Our main qualitative finding is that, when the bare spin diffusion length is much smaller than the radius of the grain, the effective spin diffusion length is strongly enhanced, well beyond the "geometrical" factor. The physical origin of this additional enhancement is that, with small diffusion length, the spin current mostly flows around the grain without suffering much loss. We also demonstrate that the voltage, created by a spin current, is sensitive to a very weak magnetic field directed along the spin current, and even reverses sign in a certain domain of fields. The origin of this sensitivity is that the spin precession, caused by magnetic field, takes place outside the grains where SO is absent.

Evaluation of spin diffusion length of AuW alloys using spin absorption experiments in the limit of large spin-orbit interactions

The knowledge of the spin diffusion length, ${\ensuremath{\lambda}}_{A}$, is a prerequisite for the estimation of the spin Hall angle of given materials. We investigate the spin-current absorption of materials with a short ${\ensuremath{\lambda}}_{A}$ using AuW stripes inserted in Cu-based lateral spin valves. Width variations of the AuW stripe lead to drastic changes of the spin absorption which cannot be explained by a conventional analysis. We show that the spin-current polarization and the spin-accumulation attenuation in Cu in the vicinity of the spin absorber must be precisely taken into account for an accurate estimation of ${\ensuremath{\lambda}}_{A}$. We propose an analytical extension of the standard diffusion model of spin transport and spin absorption based on the existence of an effective spin diffusion length for Cu being in direct contact with AuW. The calculations are supported by numerical investigations which allow us to extract proper values of ${\ensuremath{\lambda}}_{A}$.

Inkjet printed highly porous TiO2 films for improved electrical properties of photoanode

The aim of presented work is to show the improvements obtained in the properties of TiO2 films for dye sensitized solar cells fabricated by inkjet printing using an innovative methodology. We describe the development and properties of TiO2-based inks used in a lab-scale printer, testing various commercial TiO2 pastes. The porosity of the deposited inkjet printed TiO2 films is much higher than using the conventional “doctor blade” deposition technique, as the ink solvent evaporates during the droplet fly from the nozzle to the substrate due to its picoliter volume and the applied heating of a printing stage (70°C). Thanks to higher surface area, the dye sensitized solar cells incorporating inkjet printed TiO2 film gave higher efficiencies (ηmax≈3.06%) than the more compact films obtained by the “doctor blade” method (ηmax≈2.56%). Furthermore, electrochemical analysis indicates that for whole tested thickness range, the inkjet printed layers have higher effective electron diffusion length indicating their better transport properties.

Radiation defect dynamics in Si at room temperature studied by pulsed ion beams

The evolution of radiation defects after the thermalization of collision cascades often plays the dominant role in the formation of stable radiation disorder in crystalline solids of interest to electronics and nuclear materials applications. Here, we explore a pulsed-ion-beam method to study defect interaction dynamics in Si crystals bombarded at room temperature with 500 keV Ne, Ar, Kr, and Xe ions. The effective time constant of defect interaction is measured directly by studying the dependence of lattice disorder, monitored by ion channeling, on the passive part of the beam duty cycle. The effective defect diffusion length is revealed by the dependence of damage on the active part of the beam duty cycle. Results show that the defect relaxation behavior obeys a second order kinetic process for all the cases studied, with a time constant in the range of ∼4–13 ms and a diffusion length of ∼15–50 nm. Both radiation dynamics parameters (the time constant and diffusion length) are essentially independent of the maximum instantaneous dose rate, total ion dose, and dopant concentration within the ranges studied. However, both the time constant and diffusion length increase with increasing ion mass. This demonstrates that the density of collision cascades influences not only defect production and annealing efficiencies but also the defect interaction dynamics.

Improving the Short-Wavelength Spectral Response of Silicon Solar Cells by Spray Deposition of YVO4 :Eu3+ Downshifting Phosphor Nanoparticles

Europium-doped yttrium vanadate downshifting phosphor nanoparticles (NPs) have been coated on top of monocrystalline silicon solar cells, having efficiency more than 15%, by a spray deposition technique. The effects of phosphor NPs on solar cells with antireflection coating (ARC) have been studied. The optimized quantity of phosphor NPs provides a photocurrent enhancement of ~2.1% for monosilicon solar cells. External quantum efficiency data of high-efficiency ARC layer-coated silicon solar cells conclusively show that the enhancement in short-wavelength spectral response is mainly due to downshifting effects of the phosphor NPs. A small increment in a long-wavelength spectral response is also observed due to the scattering effects of phosphor NPs, which also results in a small enhancement of effective diffusion length of minority carriers in the base region.

Morphology control of ZnO nanostructures for high efficient dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) were fabricated based on different ZnO nanostructures with controllable morphologies synthesized via capping agent-assisted hydrothermal method. The capping agent hindered the growth along (001) direction, so the morphology evolved from rods to nanosheets with increasing the amounts of capping agents. The kinetics of the photoelectrochemical properties in DSSCs were also evaluated via electrochemical impedance spectroscopy with focusing on exploring the electron lifetime, electron diffusion coefficient and effective diffusion length of the cells in a quantitative manner based on an equivalent circuit model. The physical properties, such as specific surface area, photoluminescence and ultraviolet–vis diffuse reflectance spectra (DRS), were also extensively characterized in detail. DSSCs fabricated with ZnO nanosheets/rod hierarchical structures as photoanodes showed significantly improved photovoltaic performance with open-circuit voltage increasing from 0.45V to 0.73V, as well as the overall conversion efficiency 5.69 times better than that of ZnO rod. It was attributed to higher surface area (21.96m2·g−1) and more efficient light absorption, as well as more efficient electron transfer process.

Pt–Pd catalytic nanoflowers: Synthesis, characterization, and the activity toward electrochemical oxygen reduction

Abstract

Lifetime analysis of laser crystallized silicon films on glass

Only recently, the quality of liquid phase crystallized silicon directly on glass substrates made a huge leap towards the quality of multi-crystalline wafers with open circuit voltages well above 600 mV. In this paper, we investigate the material quality in order to identify the factors limiting further performance improvements. We employ photoluminescence imaging on a state of the art test structure with lifetime calibration by transient photoluminescence. The resulting lifetime map is converted into an effective diffusion length map and the origin of regions with short lifetimes is investigated with electron backscattering and transmission electron microscopy. High local dislocation densities in areas with dissociated coincidence site lattice boundaries were found to be responsible for the localised quenching of the photoluminescence signal.

AlGaInAs MOVPE selective area growth for photonic integrated circuits

Abstract We developed a generic photonic integration platform based on selective area growth (SAG) by metal organic vapor-phase epitaxy (MOVPE) of AlGaInAs/InP multiple quantum well (MQW) material. For efficient and predictive band gap engineering of photonic integrated circuits, different SAG zones of active and passive function heterostructures are precisely modeled and characterized. With the vapor-phase diffusion model, using numerical simulations of finite volumes, we extracted the three effective diffusion lengths of Al, Ga, and In species. In our growth conditions, these lengths were 32, 65, and 14 μm, respectively. The Kardar-Parisi-Zhang (KPZ) equation, a classic approach to describe the growing interface profile, is used. AlGaInAs MQW properties are then simulated in terms of thickness, composition, band gap, and biaxial strain variations. Highly resolved μ-photoluminescence and optical interferometer microscopy measurements confirm the validity of the band gap and thickness variations for both bulk and MQW layers. A new diffractometer, with a submillimeter X-ray spot, was used to study the structural properties of the MQW in the center of the SAG area. As an application, we present the realization and operation of full-monolithic high-speed advanced modulation format transmitters based on novel prefixed optical phase switching by fast electro-absorption modulators.

Structural, morphological, electrical and electron transport studies in ZnO–rGO (wt% = 0.01, 0.05 and 0.1) based dye-sensitized solar cell

Zinc oxide–reduced graphene oxide (ZnO–rGO) thin films were fabricated on a fluorine-doped tin oxide glass substrate by a chemical bath deposition method. The thin films were immersed in the Eosin Y dye for 24 and 48 h to be fabricated as a dye-sensitized solar cell. ZnO hexagonal wurtzite structures were analyzed by X-ray diffraction analysis. Field effect scanning electron microscope showed the images of ZnO nanoparticles and nanobranches. The atomic force microscopy analysis estimated the average roughness of ZnO–rGO films doped with 0.01, 0.05 and 0.1 wt% which varied from 178 to 267 nm. The electrical or photovoltaic performance of ZnO–rGO were measured and compared accordingly by considering their power conversion efficiency, η, photocurrent density, J sc , open-circuit voltage, V oc and fill factor, FF. The cell’s efficiency of ZnO with 0.01 wt% rGO, 0.05 wt% rGO and 0.1 wt% rGO reached η = 2.36, 0.78 and 0.38 %, respectively. EIS analysis estimated the effective electron lifetime, τ eff , effective electron chemical diffusion coefficient, D eff , effective electron diffusion length of the photoanode, L n , charge transport resistance, R ct and transport resistance, R t .

Efficient Exciton Harvesting through Long-Range Energy Transfer.

Efficient exciton collection at charge-generation sites is one of the key requirements for the improvement in power conversion efficiency (PCE) of organic solar cells, because only excitons arriving at a donor/acceptor interface can be dissociated into free charge carriers. We evaluated the effective diffusion length in poly(3-hexylthiophene) (P3HT) by using donor/acceptor bilayers with two different exciton-quenching acceptors. One is an insoluble fullerene polymer (p-PCBVB), which is an efficient electron-accepting material with negligible absorption in the visible region. The other is a low-bandgap polymer, poly[(4,4-bis(2-ethylhexyl)-dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl], (PSBTBT). This polymer has a large absorption band in the near-IR region, which overlaps well with the emission band of P3HT. The effective diffusion length of P3HT excitons is evaluated to be 15 nm for P3HT/p-PCBVB bilayers and improved to 30 nm for P3HT/PSBTBT bilayers. This improvement is ascribed to long-range energy transfer from P3HT to PSBTBT. This finding suggests that the effective diffusion length of P3HT excitons can be increased through long-range energy transfer by incorporating PSBTBT into P3HT/PCBM blends.

Electrochemical Kinetics Investigation of Li4Ti5O12/Reduced Graphene Oxide Nanocomposite Using Voltammetric Charge Analysis

The electrochemical kinetic properties of a Li4Ti5O12 (LTO)/reduced graphene oxide (RGO) nanocomposite were systematically investigated using cyclic voltammetry (CV). Since there are several factors that influence the electrochemical kinetics of LTO depending on the potential scan rate, the kinetic properties of LTO in the nanocomposite were evaluated by analyzing the scan rate dependence of the voltammetric charge, as measured by CV. When we formed a nanocomposite of LTO and electrically conductive RGO, the number of surface electrochemical reaction sites increased and that of the bulk electrochemical reaction sites decreased. This led to an improvement in the kinetics related to Li+ ion diffusion in LTO. The range of scan rates for which the voltammetric charge was decreased by the kinetic limitations ascribed to diffusion was reduced because of the decrease in the effective diffusion length and the increase in the diffusion area, compared to commercial LTO particles. Thus, unlike previous studies, we report that the improved rate capability of the composite with RGO is ascribed to enhanced diffusion properties in the bulk electrochemical reaction sites rather than an increase in the electrical conductivity, which is a surface property.

Study of the spatial distribution of minority carrier diffusion length in epiplanar detector structures

AbstractOne of the key parameters determining detection properties of silicon PIN detector structures (pThe paper presents a method for measuring the spatial distribution of effective carrier diffusion length in silicon detector structures, based on the measurement of photoelectric current of a non-polarised structure illuminated (spot diameter of 250 μm) with monochromatic radiation of two wavelengths λ

Internal quantum efficiency analysis of plasmonic textured silicon solar cells: surface plasmon resonance and off-resonance effects

Silver nanoparticles (Ag NPs) of various sizes and concentration were integrated on textured silicon solar cells for further confinement of incident light, generated photocurrent modifications were investigated using spectrally resolved short-circuit current measurements. Internal quantum efficiency (IQE) spectra were used for quantifying the effective minority carrier diffusion lengths (Leff) of plasmonic cells in the long wavelength region (850 < λ < 1020 nm). The Leff of an optimized plasmonic solar cell enhanced to 431 µm compared to 338 µm of the bare cell, which is due to interacting Ag NPs' scattered fields, leading to enhanced light absorption in the plasmonic cell. Despite the enhanced Leff values, the overall generated photocurrent reduced with Ag NPs which is due to the significant losses near the surface plasmon resonant region. Reduced IQE of plasmonic cells near and below the surface plasmon resonant region is due to size-dependent parasitic absorption and enhanced back scattering of Ag NPs, and a modified surface recombination process due to Ag NPs' strong near-fields.

Enhanced accessibility and utilization efficiency of acid sites in hierarchical MFI zeolite catalyst for effective diffusivity improvement

The determination of utilization degree and effective diffusivity is very important to the design of a new hierarchical zeolite catalyst. By employing Thiele modulus analysis in the cumene cracking reaction, it is found the effective diffusion length in hierarchical MFI zeolite was obviously shortened. The enhanced adsorption capacity and accelerated diffusion rate of cumene molecules in hierarchical MFI zeolite remarkably promoted accessibility to the internal surface and obviously improved the utilization of acid sites.

Relationship between Molecular Structure and Electron Transfer in a Polymeric Nitroxyl-Radical Energy Storage Material

In recent years, stable organic radical functional groups have been incorporated into a variety of polymeric materials for use as electrodes within energy storage devices, for example, batteries and capacitors. With the complex nature of the charge-transfer processes in a polymer matrix, the morphologies of the polymer films can have a significant impact on the redox behavior of the organic-based radical. To elucidate possible effects of packing on electron-transport mechanisms, theoretical modeling of the well-characterized cathode material poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) was conducted. Polymer morphologies were modeled using classical molecular dynamics simulations, and subsequently, the electronic-coupling matrix element between each radical site was calculated. Building on a previously derived treatment of diffusion in inhomogeneous materials, an expression for an effective electron diffusion length and an effective electron diffusion rate was derived in terms of an electro...

Oxygen incorporation at the three-phase boundary of LSCF–SDC composite

Oxygen reduction at LSCF–SDC (La0.6Sr0.4Co0.2Fe0.8O3 − δ–Sm0.2Ce0.8O1.9) composite cathode might take place at the LSCF-gas two-phase interface (2 PB) and the LSCF–SDC-gas three-phase boundary (3 PB). A method is developed to determine the oxygen incorporation rate and the amount of incorporated oxygen at 3 PB using the electrical conductivity relaxation technique. The obtained results are powerful supports to the viewpoint that oxygen incorporation at 3 PB is much more facile than that at 2 PB. The amount of oxygen incorporated absolutely at 3 PB plays a dominating role in the total process, which actually contributes an average of more than 70% when only 4.5 vol. % SDC is added to LSCF. The incorporation rate at 3 PB is found to be affected not only by the 3 PB length but also the diffusion of oxygen species on the LSCF surface. An effective surface diffusion length of about 1.5 μm is suggested by the relation between the rate, 3 PB length and LSCF surface area.

Role of bulk-magnon transport in the temporal evolution of the longitudinal spin-Seebeck effect

We present temporal evolution of the spin Seebeck effect in a YIG|Pt bilayer system. Our findings reveal that this effect is a sub-microseconds fast phenomenon governed by the temperature gradient and the thermal magnons diffusion in the magnetic materials. A comparison of experimental results with the thermal-driven magnon-diffusion model shows that the temporal behavior of this effect depends on the time development of the temperature gradient in the vicinity of the YIG|Pt interface. The effective thermal-magnon diffusion length for YIG|Pt systems is estimated to be around 700nm.