Electroluminescence image analysis of a photovoltaic module under accelerated lifecycle testing.

Electroluminescence (EL) imaging of Si-based photovoltaic (PV) modules is used widely to spatially detect and characterize electrical defects, including handling and degradation-induced cracking of the component Si cells that are associated with reductions in module performance. In the present study, a commercial polycrystalline silicon PV module was subjected to accelerated lifecycle test environmental conditions and examined as a function of environmental exposure time using EL imaging. The approach followed pixel intensity distributions over each individual PV cell and confirmed a positive correlation between module conversion efficiency and results of the image analysis. Overall, an average of a 2.5% reduction in normalized EL intensity was correlated to a 0.35% reduction in actual power conversion efficiency (or a 2.3% decrease in relative efficiency). The imaging analysis technique offers a rapid, unsupervised means to assess EL data in lieu of conventional visual interpretation.

Structure of ZnCl2 Melt. Part II: Fragile-to-Strong Transition in a Tetrahedral Liquid.

The fraction of edge- and corner-sharing tetrahedra in liquid ZnCl2 is quantified as a function of temperature using Raman spectroscopy and ab initio molecular dynamic simulations. Two distinct regimes are found in the temperature dependence of the change in these structural units. This behavior is consistent with the existence of a fragile-to-strong transition in liquid ZnCl2 as suggested by calorimetric and viscosity measurements. The structural origin of this transition is rationalized in terms of a constraint counting formalism. It is suggested that the ratio of edge- to corner-sharing tetrahedra controls the configurational entropy and in turn the viscosity of the melt. The temperature dependence of this ratio above the melting point is also found to be qualitatively consistent with neutron diffraction data. The observation of a similar fragile-to-strong transition in the isostructural GeSe2 melt indicates that it may be a common feature of tetrahedral liquids.

Structure of ZnCl2 Melt. Part I: Raman Spectroscopy Analysis Driven by Ab Initio Methods.

The structure of molten ZnCl2 is investigated using a combination of computer simulation and experimental methods. Ab initio molecular dynamics (AIMD) is used to model the structure of ZnCl2 at 600 K. The structure factors and pair distribution functions derived from AIMD show a good match with those previously measured by neutron diffraction (ND). In addition, Raman spectroscopy is used to investigate the structure of liquid ZnCl2 and identify the relative fractions of constituent structural units. To ascertain the assignment of each Raman mode, a series of ZnCl2 crystalline prototypes are modeled and the corresponding Raman modes are derived by first-principles calculations. Curve fitting of experimental Raman spectra using these mode assignments shows excellent agreement with both AIMD and ND. These results confirm the presence of significant fractions of edge-sharing tetrahedra in liquid ZnCl2. The presence of these structural motifs has significant impact on the fragility of this tetrahedral glass-forming liquid. The assignment of Raman bands present in molten ZnCl2 is revised and discussed in view of these results.

Interfacial effects on the optical behavior of Ge:ITO and Ge:ZnO nanocomposite films.

Nanophase semiconductors are of interest for their unique, size-tunable solar spectral absorption characteristics as well as their potential to contribute to the improved energy conversion efficiency of photovoltaics (PV). Embedding these nanoparticles within electrically active transparent conductive oxides (TCO) can also provide an opportunity for enhanced, long-range carrier transport. However, differences in the atomic and electronic structure, dielectric behavior, and chemistry between the matrix and semiconductor phases highlight the influence of interfacial effects on the optical absorption properties of the composite. In this work, nanocomposites of Ge:indium tin oxide (Ge:ITO) and Ge:ZnO were fabricated with sequential RF-magnetron sputtering and annealed at temperatures from 310 to 550 °C to investigate the impact of matrix identity on this interface and its contribution to nanostructure-mediated optical absorption. Transmission electron microscopy showed a decrease in Ge nanocrystal size relative to the initial semiconductor domain size in both matrices that was correlated with an increase in absorption onset energy after annealing. The effect was particularly pronounced in Ge:ITO composites in which Raman spectroscopy indicated the presence of germanium oxide at the semiconductor-ITO interface. These results support the primary contribution of carrier confinement in the Ge nanophase to the shifts in absorption onset energies observed.

Intrinsic electronic transitions of the absorption spectrum of (OPy)2Ti(TAP)2: implications toward photostructural modifications.

Theoretical calculations based on time-dependent density functional theory are used to characterize the electronic absorption spectrum of a heteroleptic Ti-alkoxide molecule, (OPy)(2)Ti(TAP)(2) [OPy = pyridine carbinoxide, TAP = 2,4,6 tris(dimethylamino)phenoxide] under investigation as a photosensitive precursor for use in optically initiated solution synthesis of the metal oxide. Computational results support the assignment of UV absorption features observed in solid-state precursor films to key intrinsic ground-state transitions that involve ligand-to-metal charge transfer and pi-pi* transitions within the cyclic ligand moieties present. The nature of electron density redistribution associated with these transitions provides early insight into the excitation wavelength dependence of photostructural modification previously observed in this precursor system.

Direct fabrication of physical relief structures via patterned photodeposition of a titanium alkoxide solution.

Ultraviolet (lambda=248 nm) excitation of a photosensitive Ti alkoxide solution was found to generate a metal-oxide-based insoluble film on substrates in contact with the solution during illumination. Patterned deposition of 100 microm wide lines of material was demonstrated using a slit-shaped aluminum shadow mask during exposure. Stylus profilometry confirmed that the average thickness of the photodeposited film monotonically varied with accumulated UV fluence, exhibiting thicknesses of 10 to 310 nm for fluences of 12 and 192 J/cm(2), respectively. Moreover, the surface profile of the film surface at fluences greater than 12 J/cm(2) was found to reproduce the near-field Fresnel diffraction pattern anticipated from the slit mask used.

Biologically inspired sensing: infrared spectroscopic analysis of cell responses to an inhalation health hazard.

This work describes the development of a biologically based sensing technique to quantify chemical agents that pose inhalation health hazards. The approach utilizes cultured epithelial cells (A549 human type II pneumocytes) of the lung, exposed to potential toxins and monitored through the noninvasive means of infrared spectroscopy to quantify changes to cell physiology and function. Cell response to Streptolysin O, a cholesterol-binding cytolysin, is investigated here. Infrared spectra display changes in cell physiology indicative of membrane damage, altered proteins, and some nucleic acid damage. Methods to improve cell adhesion through modification of support surface properties are detailed. This spectroscopic approach not only provides a robust means to detect potential toxins but also provides information on modes of damage and mechanisms of cellular response.

Vacuum-ultraviolet spectroscopy of poly(methylphenylsilylene).

The first vacuum-ultraviolet spectrum of a polysilylene (chain-type polysilane) with aromatic substituents is presented. Assignments of the absorption bands of the model compound poly(methylphenylsilylene) are based on previous experimental data and theoretical electronic band structure calculations for poly(alkylsilylenes) and on ultraviolet spectra of phenyl-containing monomers and polymers. Although aryl orbitals mix with the sigma-conjugated orbitals located along the catenated silicon backbone, some transitions are largely localized on the phenyl groups. These assignments elucidate the nature of the bonding in polysilylenes and should be useful in understanding photodegradation mechanisms and in the design of related new optical materials.

Optically defined multifunctional patterning of photosensitive thin-film silica mesophases.

Photosensitive films incorporating molecular photoacid generators compartmentalized within a silica-surfactant mesophase were prepared by an evaporation-induced self-assembly process. Ultraviolet exposure promoted localized acid-catalyzed siloxane condensation, which can be used for selective etching of unexposed regions; for "gray-scale" patterning of refractive index, pore size, surface area, and wetting behavior; and for optically defining a mesophase transformation (from hexagonal to tetragonal) within the film. The ability to optically define and continuously control both structure and function on the macro- and mesoscales is of interest for sensor arrays, nanoreactors, photonic and fluidic devices, and low-dielectric-constant films.

Photosensitivity of solgel-derived germanosilicate planar waveguides.

The effects of controlled atmosphere heat treatments on the photosensitivity and UV absorption of GeO(2)-SiO(2) solgel planar waveguides are presented. Photoinduced refractive-index changes in the samples are deduced from photosensitive grating writing experiments and are found to be of the order of those seen in some fiber experiments. A correlation is seen between the sample sensitivity and the strength of the absorption at 242 nm, providing insight into the possible mechanisms governing photosensitive processes in these materials.