Assessment of Luminescent Downshifting Layers for the Improvement of Light‐Harvesting Efficiency in Dye‐Sensitized Solar Cells

Luminescence downshifting (LDS) of light can be a practical photon management technique to compensate the narrow absorption band of high‐extinction‐coefficient dyes in dye‐sensitized solar cells (DSSCs). Herein, an optical analysis on the loss mechanisms in a reflective LDS (R‐LDS)/DSSC configuration is reported. For squaraine dye (550–700 nm absorption band) and CaAlSiN₃:Eu²⁺ LDS material (550–700 nm emission band), the major loss channels are found to be non‐unity luminescence quantum efficiency (QE) and electrolyte absorption. By using an ideal LDS layer (QE=100 %), a less absorbing electrolyte (Co‐based), and antireflection coatings, approximately 20 % better light harvesting is obtained. If the absorption/emission band of dye/LDS is shifted to 800 nm, a maximal short‐circuit current density (J_sc) of 22.1 mA cm^?2 can be achieved. By putting the LDS layer in front of the DSSC (transmissive mode), more significant loss channels are observed, and hence a lower overall efficiency than the R‐LDS configuration.     

First principle study of hydrogen storage on the graphene-like aluminum nitride nanosheet

Employing density functional calculations including an empirical dispersion term, we investigated the hydrogenation of an aluminum nitride nanosheet (h-AlN) with atomic and molecular hydrogen. It was found that atomic H prefers to be adsorbed on an N atom rather than Al, releasing energy of 21.1 kcal/mol. The HOMO/LUMO energy gap of the sheet is dramatically reduced from 107.9 to 44.5 kcal/mol, upon the adsorption of one hydrogen atom. The adsorption of atomic H on the h-AlN presents properties which are promising for nanoelectronic applications. The molecular H₂ was found to be adsorbed collinearly on an N atom and dissociated to two H atoms on Al–N bond. Calculated barrier and adsorption energies for this dissociation process are about +18.9 and ?1.9 kcal/mol. We predict that each nitrogen atom in an AlN sheet can adsorb two hydrogen molecules on opposite sides of the sheet, and thus the gravimetric density for hydrogen storage on AlN sheet is evaluated to be about 8.9 wt%.     

Variational formulations for nonlinear wave propagation and unsteady transonic flow

A variational principle developed recently for constrained vector fields is applied to nonlinear waves and to unsteady, transonic flow. The mathematical conditions on the admissible functional form of the speed of propagation is compatible with physical considerations for the first systems and the treatment of the mixed derivative is indicative for a proper way to discretize the second for numerical calculations. The variational formulation provides a framework for stability analysis and finite element approximations for the nonlinear systems considered.

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Zusammenfassung Ein Variationsprinzip, das in letzter Zeit für Vektorfelder mit einschränkenden Bedingungen entwickelt worden ist, wird auf nichtlineare Wellen und auf nichtstationäre transonische Strömung angewendet. Die mathematische Bedingung für die zulässige funktionale Form der Fortpflanzungsgeschwindigkeit ist verträglich mit den physikalischen Bedingungen für das erste System, und die Behandlung der gemischten Ableitung deutet an, wie das zweite System für numerische Behandlung diskretisiert werden soll. Die variationale Formulierung gibt einen Rahmen für eine Stabilitätsuntersuchung und für eine Näherung durch finite Elemente für das nichtlineare System.

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Work partially supported by Grant AFOSR-73-2561.     

Multiuser quantum key distribution over telecom fiber networks

We report five-user quantum key distribution (QKD) over switched fiber networks in both star and tree configurations, using the BB84-protocol [1] with phase encoding. Both setups implement polarization insensitive phase modulators, necessary for birefringent single mode fiber (SMF) networks. In both configurations we have achieved transmission distances between 25 km and 50 km with quantum bit error rates between 1.24% and 5.56% for the mean photon number μ=0.1. The measurements have showed feasibility of multiuser QKD over switched fiber networks, using standard fiber telecom components.     

Using co-rotational method for cracked frame analysis

A cracked element is formulated using the two-filed Hellinger–Reissner functional. Due to utilization of the linear elastic fracture mechanics, only geometrical nonlinearities can be considered for the cracked element. A clear step-by-step algorithm for the element state determination is also presented. The element flexibility matrix is derived in a basic coordinate system. Co-rotational approach is used to transform the element stiffness matrix and the resisting force vector from the basic system to the global one. The suggested element is applicable for static and dynamic analysis, as well as, the stress intensity factor calculation, and also inverse crack detection. Various numerical problems verify accuracy of the proposed element for linear and nonlinear structural analysis.     

Fault-Tolerant Design of a Shift Register at the Nanoscale Based on Quantum-dot Cellular Automata

Quantum-dot Cellular Automata (QCA) as a novel technology in the nanometer scale has been considered as one of the substitutes to CMOS technology. The QCA helps to create faster computers with lower power consumption. On the other hand, a shift register as one of the most important logical circuit in the digital systems consists of a line of latches. Also, the QCA-based designs have more advantages compared to the conventional CMOS designs. However, some deposition defects are possible to occur in the QCA-based designs, which have necessitated the fault-tolerant structures. Therefore, this paper aims to design an optimized 2-bit universal shift register based on QCA technology through the optimized multiplexer and D flip-flop. This paper studies the functionality and the fault tolerance of the proposed universal shift register in the presence of the QCA deposition faults. The structure of the 2-bit universal register is extendable to 4-bit, 8-bit and higher. The proposed design has better performance regarding fault tolerant, complexity and area consumption compared to the current designs based on the achieved results via QCADesigner.     

Plasma coating of nanoparticles in the presence of an external electric field

Film deposition onto nanoparticles by low-pressure plasma in the presence of an external electric field is studied numerically. The plasma discharge fluid model along with surface deposition and heating models for nanoparticles, as well as a dynamics model considering the motion of nanoparticles, are employed for this study. The results of the simulation show that applying external field during the process increases the uniformity of the film deposited onto nanoparticles and leads to that nanoparticles grow in a spherical shape. Increase in film uniformity and particles sphericity is related to particle dynamics that is controlled by parameters of the external field like frequency and amplitude. The results of this work can be helpful to produce spherical core-shell nanoparticles in nanomaterial industry.     

The amide rotational barrier in isonicotinamide: Dynamic NMR and ab initio studies

We report use of dynamic nuclear magnetic resonance (NMR) to measure the amide rotational barrier in isonicotinamide. A significant challenge to obtaining good transition rates from dynamic NMR data is suppression of errors due to inherent line widths associated with transverse relaxation. We address this challenge with a fitting procedure that incorporates transverse relaxation over the temperature range of interest simply and reliably. The fitting model is nonlinear in only one of the fit parameters, namely, the activation enthalpy. This reduces parameter estimation to solution of a single transcendental equation, which avoids both a fine search over a multidimensional parameter space and extrapolation of a "limiting line width" solely from slow-exchange data. The activation enthalpy Delta H++ measured for isonicotinamide, +14.1 +/- 0.2 kcal/mol, falls between those of its regioisomers picolinamide and nicotinamide, which were reported in an earlier study. In that study, ab initio calculations of the rotational barriers helped to discern the relative importance of steric, electronic, and hydrogen-bonding effects in this biochemically significant combination of pyridine-ring and carboxamide moieties. A direct comparison between isonicotinamide and nicotinamide, where steric and hydrogen-bonding effects differ only slightly, permits a closer study of electronic considerations.     

2-Harmonic Curves Isometrially Immersed in a Surface

In this paper,the author proves the necessary and sufficient condition for the existenceof 2-harmonically and isometrically immersed curves in a 2-dimensinonal surface N■IE~3.     

Mixed‐Halide CH3NH3PbI3−xXx (X=Cl,Br, I) Perovskites: Vapor‐Assisted Solution Deposition and Application as Solar Cell Absorbers

There have been recent reports on the formation of single‐halide perovskites, CH₃NH₃PbX₃ (X=Cl, Br, I), by means of vapor‐assisted solution processing. Herein, the successful formation of mixed‐halide perovskites (CH₃NH₃PbI_3?xX_x) by means of a vapor‐assisted solution method at ambient atmosphere is reported. The perovskite films are synthesized by exposing PbI₂ film to CH₃NH₃X (X=I, Br, or Cl) vapor. The prepared perovskite films have uniform surfaces with good coverage, as confirmed by SEM images. The inclusion of chlorine and bromine into the structure leads to a lower temperature and shorter reaction time for optimum perovskite film formation. In the case of CH₃NH₃PbI_3?xCl_x, the optimum reaction temperature is reduced to 100 °C, and the resulting phases are CH₃NH₃PbI₃ (with trace Cl) and CH₃NH₃PbCl₃ with a ratio of about 2:1. In the case of CH₃NH₃PbI_3?xBr_x, single‐phase CH₃NH₃PbI₂Br is formed in a considerably shorter reaction time than that of CH₃NH₃PbI₃. The mesostructured perovskite solar cells based on CH₃NH₃PbI₃ films show the best optimal power conversion efficiency of 13.5 %, whereas for CH₃NH₃PbI_3?xCl_x and CH₃NH₃PbI_3?xBr_x the best recorded efficiencies are 11.6 and 10.5 %, respectively.