Influence of SiNx:H film properties according to gas mixture ratios for crystalline silicon solar cells

The Hydrogenated silicon nitride (SiN_x:H) using plasma enhanced chemical vapor deposition is widely used in photovoltaic industry as an antireflection coating and passivation layer. In the high temperature firing process, the SiN_x:H film should not change the properties for its use as high quality surface layer in crystalline silicon solar cells. For optimizing surface layer in crystalline silicon solar cells, by varying gas mixture ratios (SiH₄ + NH₃ + N₂, SiH₄ + NH₃, SiH₄ + N₂), the hydrogenated silicon nitride films were analyzed for its antireflection and surface passivation (electrical and chemical) properties. The film deposited with the gas mixture of SiH₄ + NH₃ + N₂ showed the best properties in before and after firing process conditions.The single crystalline silicon solar cells fabricated according to optimized gas mixture condition (SiH₄ + NH₃ + N₂) on large area substrate of size 156 mm × 156 mm (Pseudo square) was found to have the conversion efficiency as high as 17.2%. The reason for the high efficiency using SiH₄ + NH₃ + N₂ is because of the good optical transmittance and passivation properties. Optimized hydrogenated silicon nitride surface layer and high efficiency crystalline silicon solar cells fabrication sequence has also been explained in this study.     

Universal classes of magnetic-field- and electric-current-induced magnetic domain-wall dynamics in one and two dimensional regimes

Magnetic domain-wall (DW), interface between different magnetic domains, has received great attention due to its opportunities toward memory and logic devices as well as its abundant physical properties as a driven interface. Since recent advances of fabrication techniques allow us to scale down the devices, we are facing lower dimensional properties that should be elucidated undoubtedly. Here, we review recent progresses on DW dynamics in ferromagnetic nanowires and our recent experimental observation on the dimensionality transition of the DW dynamics driven by magnetic field and/or current. Our results show that the DW dynamics shows a transition from two to one dimensional behavior as the wire width decreases. In addition, we also demonstrate that the magnetic-field- and electric-current-driven DW dynamics in metallic ferromagnetic nanowires belong to the same universal class.     

Study of alkyl chain length dependent characteristics of imidazolium based ionic liquids [CnMIM]+[TFSA]− by Brillouin and dielectric loss spectroscopy

This study examined the acoustic phonon mode of ionic liquids consisting of 1-alkyl-3-methyl-imidazolium family (C_nMIM) cations with n values ranging from 2 to 10 and bis(trifluoromethylsulfonyl)amide (TFSA) anion in the temperature range from 300 K to 100 K. [C_nMIM]⁺[TFSA]^? showed depolarized (VH) components of Brillouin peaks at temperatures below the glass transition temperature when n is larger than 4. On the other hand, in the case of ionic liquids with different anions, such as [C₄MIM]⁺[BF₄]^?, [C₄MIM]⁺[PF₆]^? and [C₈MIM]⁺[BF₄]^?, the VH component of Brillouin peaks was not observed in the temperature range investigated. The dielectric loss spectra showed that the temperature dependence of alkyl chain domain relaxation of all ionic liquids followed the Arrhenius law and showed an increase in activation energy at the temperature where the VH component of Brillouin peak appeared. These results suggest that the observed depolarized component of Brillouin peak might originate from uniquely induced polarization in the 2nd domain composed of head groups of cations and anions.     

Sputter-deposited ZnO thin films consisting of nano-networks for binder-free dye-sensitized solar cells

ZnO nano-network structures with high porosity were prepared for use in the photoelectrodes of binder-free dye-sensitized solar cells (DSSCs) by DC sputtering and subsequent thermal oxidation. Zn thin films prepared at 100 °C showed nano-network structures with high porosity, while those prepared at 25 °C did not. This was partially attributed to the high mobility of sputter-deposited particles that arrived at the surface of the substrate and partially to a supersaturation mechanism. The prepared nano-network Zn was successfully transformed to ZnO without a morphological change via subsequent annealing in air. The power conversion efficiency of DSSCs based on the ZnO nano-network structures exhibited 10 times higher efficiency than those based on ZnO film prepared at 25 °C because of its large surface area for adsorption of dye molecules. The thickness of the ZnO nano-network structures increased linearly at 10 μm h^?1 as a function of sputter time. As the film thickness increased, the power conversion efficiency of DSSCs increased from 1.09% to 1.82%.     

Effects of combined dimension reduction and tabulation on the simulations of a turbulent premixed flame using a large-eddy simulation/probability density function method

A turbulent lean-premixed propane–air flame stabilised by a triangular cylinder as a flame-holder is simulated to assess the accuracy and computational efficiency of combined dimension reduction and tabulation of chemistry. The computational condition matches the Volvo rig experiments. For the reactive simulation, the Lagrangian Large-Eddy Simulation/Probability Density Function (LES/PDF) formulation is used. A novel two-way coupling approach between LES and PDF is applied to obtain resolved density to reduce its statistical fluctuations. Composition mixing is evaluated by the modified Interaction-by-Exchange with the Mean (IEM) model. A baseline case uses In Situ Adaptive Tabulation (ISAT) to calculate chemical reactions efficiently. Its results demonstrate good agreement with the experimental measurements in turbulence statistics, temperature, and minor species mass fractions. For dimension reduction, 11 and 16 represented species are chosen and a variant of Rate Controlled Constrained Equilibrium (RCCE) is applied in conjunction with ISAT to each case. All the quantities in the comparison are indistinguishable from the baseline results using ISAT only. The combined use of RCCE/ISAT reduces the computational time for chemical reaction by more than 50%. However, for the current turbulent premixed flame, chemical reaction takes only a minor portion of the overall computational cost, in contrast to non-premixed flame simulations using LES/PDF, presumably due to the restricted manifold of purely premixed flame in the composition space. Instead, composition mixing is the major contributor to cost reduction since the mean-drift term, which is computationally expensive, is computed for the reduced representation. Overall, a reduction of more than 15% in the computational cost is obtained.     

Marked length rigidity of rank one symmetric spaces and their product

In this paper we show that if two Zariski dense representations, from a group G into Iso(X) where X is rank one symmetric space, have the proportional marked length spectrum, then they are conjugate. As a generalization we show that a Zariski dense representation into the isometry group of the product of rank one symmetric spaces is determined by the marked cross ratio.     

Normal analytic polyhedra in ℂ2 with a noncompact automorphism group

Let P be a bounded analytic polyhedron in ℂ² whose boundary is smooth except for normal crossing singularities. We show that P is a holomorphic quotient of the bidisc, if its automorphism group is noncompact.     

An optimal service rate in a Poisson arrival queue with two-stage service policy*

We consider a two-stage service policy for a Poisson arrival queueing system. The idle server starts to work with ordinary service rate when a customer arrives. If the number of customers in the system reaches N, the service rate gets faster and continues until the system becomes empty. Otherwise, the server finishes the busy period with ordinary service rate. After assigning various operating costs to the system, we show that there exists a unique fast service rate minimizing the long-run average cost per unit time.This work was supported by Korea Research Foundation Grant(KRF-2002-070-C00021).     

A Comparative Study on Dynamic and Static Sparsity Patterns in Parallel Sparse Approximate Inverse Preconditioning

Sparse approximate inverse (SAI) techniques have recently emerged as a new class of parallel preconditioning techniques for solving large sparse linear systems on high performance computers. The choice of the sparsity pattern of the SAI matrix is probably the most important step in constructing an SAI preconditioner. Both dynamic and static sparsity pattern selection approaches have been proposed by researchers. Through a few numerical experiments, we conduct a comparable study on the properties and performance of the SAI preconditioners using the different sparsity patterns for solving some sparse linear systems. This revised version was published online in July 2006 with corrections to the Cover Date.