Joshua D. Winans

Plasmonic effects in ultrathin amorphous silicon solar cells: performance improvements with Ag nanoparticles on the front, the back, and both

Thin-film hydrogenated amorphous silicon (a-Si:H) solar cells that are free-standing over a 2x2 mm area have been fabricated with thicknesses of 150 nm, 100 nm, and 60 nm. Silver nanoparticles (NPs) created on the front and/or back surfaces of the solar cells led to improvement in performance measures such as current density, overall efficiency, and external quantum efficiency. The effect of changing silver nanoparticle size and incident light angle was tested. Finite-Difference Time-Domain simulations are presented as a way to understand the experimental results as well as guide future research efforts.

On the scaling behavior of dipole and quadrupole modes in coupled plasmonic nanoparticle pairs

We investigate shifts of localized surface plasmon resonance (LSPR) caused by coupling between Ag nanospheres in close proximity to one another by using three-dimensional (3D) finite-difference time-domain (FDTD) simulations and exact Mie solutions. Our findings agree well with previous reports of universal scaling in coupled nanostructures where the relative fractional shift in dipole plasmon resonance wavelength decays over an inter-particle gap with the same universal trend independent of particle size, shape and material composition. To expand upon this, we investigate universal scaling of the dipole mode in coupled particle pairs greater than 100 nm in diameter where higher-order modes of resonance (i.e. both dipole and quadrupole modes of resonance) are present. It is shown that fractional shifts of the quadrupole mode in coupled sphere-pairs do not follow a universal scaling trend independent of particle size. Rather, the fractional shifts are dependent on a predetermined set of particle sizes defined by the particle-pair spacing at which the onset of shifts follow bands that are dependent on the center-to-center particle distance.

Ultrathin Membrane Fouling Mechanism Transitions in Dead-End Filtration of Protein

Ultrathin membranes will likely see great utility in future membrane-based separations, but key aspects of the performance of these membranes, especially when they are used to filter protein, remain poorly understood. In this work we perform protein filtrations using new nanoporous silicon nitride (NPN) membranes. Several concentrations of protein are filtered using dead end filtration in a benchtop centrifuge, and we track fouling based on the amount of filtrate passed over time. A modification of the classic fouling model that includes the effects of using a centrifuge and allow for the visualization of a transition between pore constriction and cake filtration demonstrate that for a range of protein concentrations, cake filtration supersedes pore constriction after ∼30 seconds at 690 g.Copyright

Silver Nanoparticles to Enhance the Efficiency of Thin-Film Solar Cells

Silver nanoparticles are applied to the front surface of thin-film a-Si to reduce reflection and transmission losses. We use FDTD simulations to investigate the effect of varying particle size and shape on the absorption of light.

A new technique for localized formation of SOI active regions

The oxidation of electrochemically etched porous silicon (PSi) has demonstrated success in the formation of device quality localized SOI for CMOS applications [1,2]. A primary advantage with a localized SOI formation is the ability to integrate novel device structures and optoelectronics (i.e. optical switches, waveguides) with bulk silicon CMOS. The formation of PSi can be done selectively by controlling the Fermi level in areas to be etched or not etched, which is typically done by adjusting the level of doping [1]. An alternative method is to introduce a reversible donor species such as protons [2] or fluorine (this work) for the selective formation of islands of crystalline silicon surrounded by porous silicon. Implanted fluorine in silicon has demonstrated a donor effect upon annealing at low temperature (600°C), which is reversible as the fluorine outdiffuses during higher temperature annealing (1000°C). Crystalline silicon islands that were fabricated with this technique have a thickness of about 300nm and are completely surrounded by oxidized porous silicon. Further study will investigate the device quality of the localized SOI structures for microelectronic and optoelectronic applications.