Zeeman Relaxation of Cold Atomic Iron and Nickel in Collisions with 3He

We have measured the ratio of the diffusion cross-section to the angular momentum reorientation cross-section in the colliding Fe-3He and Ni-3He systems. Nickel (Ni) and iron (Fe) atoms are introduced via laser ablation into a cryogenically cooled experimental cell containing cold (< 1 K) 3He buffer gas. Elastic collisions rapidly cool the translational temperature of the ablated atoms to the helium temperature. The cross-section ratio is extracted by measuring the decays of the atomic Zeeman sublevels. For our experimental conditions, thermal energy is comparable to the Zeeman splitting. As a result, thermal excitations between Zeeman sublevels significantly impact the observed decay. To determine the cross-section ratio accurately, we introduce a model of Zeeman state dynamics that includes thermal excitations. We find the cross-section ratio for Ni-3He = 5 x 10^3 and Fe-3He <= 3 x 10^3 at 0.75 K in a 0.8 T magnetic field. These measurements are interpreted in the context of submerged shell suppression of spin relaxation as studied previously in transition metals and rare earth atoms.Comment: 10 pages, 5 figures; submitted to Phys. Rev.

Reactivation of sub-bandgap absorption in chalcogen-hyperdoped silicon

Silicon doped with nonequilibrium concentrations of chalcogens using a femtosecond laser exhibits near-unity absorption of sub-bandgap photons to wavelengths of at least 2500 nm. Previous studies have shown that sub-bandgap absorptance decreases with thermal annealing up to 1175 K and that the absorption deactivation correlates with chalcogen diffusivity. In this work, we show that sub-bandgap absorptance can be reactivated by annealing at temperatures between 1350 and 1550 K followed by fast cooling (>50 K/s). Our results suggest that the defects responsible for sub-bandgap absorptance are in equilibrium at high temperatures in hyperdoped Si:chalcogen systems.National Science Foundation (U.S.) (Contract CBET 0754227)National Science Foundation (U.S.) (Contract CHE-DMR-DMS 0934480

Proposal for a Safety Qualification Program for Vehicle-Integrated PV Modules

We propose a safety qualification program for vehicle-integrated photovoltaic (VIPV) modules, which could serve as a simplification, thereby accelerating the homologation process of new vehicle designs. The basis is the current photovoltaic (PV) module safety qualification, as defined in IEC 61730:2016, which is compared to automotive norms and regulations because additional safety requirements have to be considered for PV modules used in this application. Therefore, testing based on regulations that concern electrical and electronic equipment in vehicles (ISO 16750), rupture safety of glass and laminated glass in vehicles (ECE R43), and pedestrian safety (ECE R127) are assessed and compared in terms of severity. Additionally, optional testing concerning the long-term stability of VIPV modules is recommended, as a guideline for vehicle manufacturers. If assessed to be necessary, the qualification program of IEC 61730 is complemented by the respective tests to finally present a conclusive safety qualification program for VIPV modules in new vehicle designs

Magnetic Trapping of Silver and Copper, and Anomalous Spin Relaxation in the Ag-He System

We have trapped large numbers of copper (Cu) and silver (Ag) atoms using buffer-gas cooling. Up to 3×1012  Cu atoms and 4×1013  Ag atoms are trapped. Lifetimes are as long as 5 s, limited by collisions with the buffer gas. Ratios of elastic to inelastic collision rates with He are ≳106, suggesting Cu and Ag are favorable for use in ultracold applications. The temperature dependence of the Ag−He3 collision rate varies as T5.8±0.4. We find that this temperature dependence is inconsistent with the behavior predicted for relaxation arising from the spin-rotation interaction, and conclude that the Ag−He3 system displays anomalous collisional behavior in the multiple-partial wave regime. Gold (Au) was ablated into He3 buffer gas, however, atomic Au lifetimes were observed to be too short to permit trapping.Physic

Limiting Light Escape Angle in Silicon Photovoltaics: Ideal and Realistic Cells

Restricting the light escape angle within a solar cell significantly enhances light trapping, resulting in potentially higher efficiency in thinner cells. Using an improved detailed balance model for silicon and neglecting diffuse light, we calculate an efficiency gain of 3%_(abs) for an ideal Si cell of 3-µm thickness and the escape angle restricted to 2.767° under AM1.5 direct illumination. Applying the model to current high-efficiency cell technologies, we find that a heterojunction-type device with better surface and contact passivation is better suited to escape angle restriction than a homojunction type device. In these more realistic cell models, we also find that there is little benefit gained by restricting the escape angle to less than 10°. The benefits of combining moderate escape angle restriction with low to moderate concentration offers further efficiency gains. Finally, we consider two potential structures for escape angle restriction: a narrowband graded index optical multilayer and a broadband ray optical structure. The broadband structure, which provides greater angle restriction, allows for higher efficiencies and much thinner cells than the narrowband structure

Magnetic relaxation in dysprosium-dysprosium collisions

The collisional magnetic reorientation rate constant gR is measured for magnetically trapped atomic dysprosium (Dy), an atom with large magnetic dipole moments. Using buffer gas cooling with cold helium, large numbers (>1011) of Dy are loaded into a magnetic trap and the buffer gas is subsequently removed. The decay of the trapped sample is governed by collisional reorientation of the atomic magnetic moments. We find gR=1.9±0.5×10-11 cm3 s-1 at 390 mK. We also measure the magnetic reorientation rate constant of holmium (Ho), another highly magnetic atom, and find gR=5±2×10-12 cm3 s-1 at 690 mK. The Zeeman relaxation rates of these atoms are greater than expected for the magnetic dipole-dipole interaction, suggesting that another mechanism, such as an anisotropic electrostatic interaction, is responsible. Comparison with estimated elastic collision rates suggests that Dy is a poor candidate for evaporative cooling in a magnetic trap.National Science Foundation (U.S.) (Grant No. PHY-0757157

Retrograde Melting and Internal Liquid Gettering in Silicon

Retrograde melting (melting upon cooling) is observed in silicon doped with 3d transition metals, via synchrotron-based temperature-dependent X-ray microprobe measurements. Liquid metal-silicon droplets formed via retrograde melting act as efficient sinks for metal impurities dissolved within the silicon matrix. Cooling results in decomposition of the homogeneous liquid phase into solid multiple-metal alloy precipitates. These phenomena represent a novel pathway for engineering impurities in semiconductor-based systems

Influence of defect type on hydrogen passivation efficacy in multicrystalline silicon solar cells

We examine the effectiveness of hydrogen passivation as a function of defect type and microstructure at grain boundaries (GBs) in multicrystalline silicon. We analyze a solar cell with alternating mm-wide bare and SiNx-coated stripes using laser-beam-induced current, electron backscatter diffraction, X-ray fluorescence microscopy, and defect etching to correlate pre- and post-hydrogenation recombination activity with GB character, density of iron-silicide nanoprecipitates, and dislocations. A strong correlation was found between GB recombination activity and the nature/density of etch pits along the boundaries, while iron silicide precipitates above detection limits were found to play a less significant role

Detection of ZnS phases in CZTS thin-films by EXAFS

Copper zinc tin sulfide (CZTS) is a promising Earthabundant thin-film solar cell material; it has an appropriate band gap of ~1.45 eV and a high absorption coefficient. The most efficient CZTS cells tend to be slightly Zn-rich and Cu-poor. However, growing Zn-rich CZTS films can sometimes result in phase decomposition of CZTS into ZnS and Cu2SnS3, which is generally deleterious to solar cell performance. Cubic ZnS is difficult to detect by XRD, due to a similar diffraction pattern. We hypothesize that synchrotron-based extended X-ray absorption fine structure (EXAFS), which is sensitive to local chemical environment, may be able to determine the quantity of ZnS phase in CZTS films by detecting differences in the second-nearest neighbor shell of the Zn atoms. Films of varying stoichiometries, from Zn-rich to Cu-rich (Zn-poor) were examined using the EXAFS technique. Differences in the spectra as a function of Cu/Zn ratio are detected. Linear combination analysis suggests increasing ZnS signal as the CZTS films become more Zn-rich. We demonstrate that the sensitive technique of EXAFS could be used to quantify the amount of ZnS present and provide a guide to crystal growth of highly phase pure films

Impact of defect type on hydrogen passivation effectiveness in multicrystalline silicon solar cells

In this work we examine the effectiveness of hydrogen passivation at grain boundaries as a function of defect type and microstructure in multicrystalline silicon. We analyze a specially prepared solar cell with alternating mm-wide bare and SiNx-coated stripes using laser beam-induced current (LBIC), electron backscatter diffraction (EBSD), synchrotron-based X-ray fluorescence microscopy (μ-XRF), and defect etching to correlate pre- and post-hydrogenation recombination activity with grain boundary character, density of iron-silicide nanoprecipitates, and dislocations. This study reveals that the microstructure of boundaries that passivate well and those that do not differ mostly in the character of the dislocations along the grain boundary, while iron silicide precipitates along the grain boundaries (above detection limits) were found to play a less significant role