The Effects of Content-Based Academic Paired Conversations on the Speaking Skills of Fifth-Grade English Learners

The purpose of this study was to determine the effects of implementation of content-based paired academic conversations among fifth-grade English learners (ELs) at an advanced proficiency level. The major influences for the instructional methods used in this study came from Zwiers and Crawford (2011) and Kinsella and Hancock (2015). The research method was a mixed methods study conducted in an ESL pull-out class during spring 2015. The data collected consisted of a written post-test for academic vocabulary, recordings made of paired academic conversations which were assessed using a rubric, and focused observations of paired academic conversations based on a rubric. The major findings were that ELs can learn general academic vocabulary from a focused curriculum, but that they do not necessarily use those words in other contexts. Also, they benefit from explicit instruction and practice in paired academic conversation skills, even if the instruction is of a short duration

Predicted efficiency of Si wire array solar cells

Solar cells based on arrays of CVD-grown Si nano- or micro-wires have attracted interest as potentially low-cost alternatives to conventional wafer-based Si photovoltaics [1-6], and single-wire solar cells have been reported with efficiencies of up to 3.4% [7]. We recently presented device physics simulations which predicted efficiencies exceeding 17%, based on experimentally observed diffusion lengths within our wires [8]. However, this model did not take into account the optical properties of a wire array device - in particular the inherently low packing fraction of wires within CVD-grown wire arrays, which might limit their ability to fully absorb incident sunlight. For this reason, we have combined a device physics model of Si wire solar cells with FDTD simulations of light absorption within wire arrays to investigate the potential photovoltaic efficiency of this cell geometry. We have found that even a sparsely packed array (14%) is expected to absorb moderate (66%) amounts of above-bandgap solar energy, yielding a simulated photovoltaic efficiency of 14.5%. Because the wire array comprises such a small volume of Si, the observed absorption represents an effective optical concentration, which enables greater operating voltages than previously predicted for Si wire array solar cells

Growth of vertically aligned Si wire arrays over large areas (>1 cm^2) with Au and Cu catalysts

Arrays of vertically oriented Si wires with diameters of 1.5 µm and lengths of up to 75 µm were grown over areas >1 cm^2 by photolithographically patterning an oxide buffer layer, followed by vapor-liquid-solid growth with either Au or Cu as the growth catalyst. The pattern fidelity depended critically on the presence of the oxide layer, which prevented migration of the catalyst on the surface during annealing and in the early stages of wire growth. These arrays can be used as the absorber material in novel photovoltaic architectures and potentially in photonic crystals in which large areas are needed

10 µm minority-carrier diffusion lengths in Si wires synthesized by Cu-catalyzed vapor-liquid-solid growth

The effective electron minority-carrier diffusion length, L_(n,eff), for 2.0 µm diameter Si wires that were synthesized by Cu-catalyzed vapor-liquid-solid growth was measured by scanning photocurrent microscopy. In dark, ambient conditions, L_(n,eff) was limited by surface recombination to a value of ≤ 0.7 µm. However, a value of L_(n,eff) = 10.5±1 µm was measured under broad-area illumination in low-level injection. The relatively long minority-carrier diffusion length observed under illumination is consistent with an increased surface passivation resulting from filling of the surface states of the Si wires by photogenerated carriers. These relatively large L_(n,eff) values have important implications for the design of high-efficiency, radial-junction photovoltaic cells from arrays of Si wires synthesized by metal-catalyzed growth processes

Dynamically Stable Radiation Pressure Propulsion of Flexible Lightsails for Interstellar Exploration

Lightsail spacecraft, propelled to relativistic velocities via photon pressure using high power density laser radiation, offer a potentially new route to space exploration within and beyond the solar system, extending to interstellar distances. Such missions will require meter-scale lightsails of submicron thickness, posing substantial challenges for materials science and engineering. We analyze the structural and photonic design of flexible lightsails, developing a mesh-based multiphysics simulator based on linear elastic theory, treating the lightsail as a flexible membrane rather than a rigid body. We find that flexible lightsail membranes can be spin stabilized to prevent shape collapse during acceleration, and that certain lightsail shapes and designs offer beam-riding stability despite the deformations caused by photon pressure and thermal expansion. Excitingly, nanophotonic lightsails based on planar silicon nitride membranes patterned with suitably designed optical metagratings exhibit both mechanically and dynamically stable propulsion along the pump laser axis. These advances suggest that laser-driven acceleration of membrane-like lightsails to the relativistic speeds needed to access interstellar distances is conceptually feasible, and that fabrication of such lightsails may be within the reach of modern microfabrication technology.Comment: 14 pages, 6 figures; plus 18-page SI with figures and linked video

Effects of bulk and grain boundary recombination on the efficiency of columnar-grained crystalline silicon film solar cells

Columnar-grained polycrystalline silicon films deposited at low temperatures are promising materials for use in thin-film photovoltaics. We study the effects of recombination at grain boundaries, bulk intragranular recombination, grain size, and doping in such structures with two-dimensional device physics simulations, explicitly modeling the full statistics and electrostatics of traps at the grain boundary. We characterize the transition from grain-boundary-limited to bulk-lifetime-limited performance as a function of intergranular defect density and find that higher bulk lifetimes amplify grain boundary recombination effects in the intermediate regime of this transition. However, longer bulk lifetimes ultimately yield higher efficiencies. Additionally, heavier base doping is found to make performance less sensitive to grain boundary defect density

Improved red-response in thin film a-Si:H solar cells with soft-imprinted plasmonic back reflectors

The impact of controlled nanopatterning on the Ag back contact of an n-i-p a-Si:H solar cell was investigated experimentally and through electromagnetic simulation. Compared to a similar reference cell with a flat back contact, we demonstrate an efficiency increase from 4.5% to 6.2%, with a 26% increase in short circuit current density. Spectral response measurements show the majority of the improvement between 600 and 800 nm, with no reduction in photocurrent at wavelengths shorter than 600 nm. Optimization of the pattern aspect ratio using electromagnetic simulation predicts absorption enhancements over 50% at 660 nm

Fundamental Limit of Nanophotonic Light-trapping in Solar Cells

Establishing the fundamental limit of nanophotonic light-trapping schemes is of paramount importance and is becoming increasingly urgent for current solar cell research. The standard theory of light trapping demonstrated that absorption enhancement in a medium cannot exceed a factor of 4n^2/ sin^2(\theta), where n is the refractive index of the active layer, and \theta is the angle of the emission cone in the medium surrounding the cell. This theory, however, is not applicable in the nanophotonic regime. Here we develop a statistical temporal coupled-mode theory of light trapping based on a rigorous electromagnetic approach. Our theory reveals that the standard limit can be substantially surpassed when optical modes in the active layer are confined to deep-subwavelength scale, opening new avenues for highly efficient next-generation solar cells