We suggest a new paradigm for solar cells that uses a nanostructured crystalline collector (silicon) in an amorphous absorber matrix (hydrogenated amorphous silicon). Previously amorphous absorbers have received no serious consideration because of their low carrier mobilities. Specifically, we demonstrate that carriers generated in the amorphous region are transported out of this region before losing their energy to heat. This result establishes the possibility of using a wide range of nanostructured amorphous matrices to dramatically increase the efficiencies of solar cells. The use of an amorphous absorber provides a highly desirable and flexible approach to producing low-cost, hot carrier solar cells. Since amorphous materials can be grown over a much wider composition space than crystalline materials, this surprising result greatly broadens the absorbing materials that can be used to dramatically increase the efficiencies of solar cells.
Polarimetric imaging using micropolarizers integrated on focal plane arrays has previously been limited to the linear
components of the Stokes vector because of the lack of an effective structure with selectivity to circular polarization. We
discuss a plasmonic micropolarizing filter that can be tuned for linear or circular polarization as well as wavelength
selectivity from blue to infrared (IR) through simple changes in its horizontal geometry. The filter consists of a patterned
metal film with an aperture in a central cavity that is surrounded by gratings that couple to incoming light. The aperture
and gratings are covered with a transparent dielectric layer to form a surface plasmon slab waveguide. A metal cap
covers the aperture and forms a metal-insulator-metal (MIM) waveguide. Structures with linear apertures and gratings
provide sensitivity to linear polarization, while structures with circular apertures and spiral gratings give circular
polarization selectivity. Plasmonic TM modes are transmitted down the MIM waveguide while the TE modes are cut off
due to the sub-wavelength dielectric thickness, providing the potential for extremely high extinction ratios. Experimental
results are presented for micropolarizers fabricated on glass or directly into the Ohmic contact metallization of silicon
photodiodes. Extinction ratios for linear polarization larger than 3000 have been measured.
Silicon wire arrays have been synthesized through a two-step metal-assisted electrode-less etching from an n-type silicon
wafer with (100) orientation. Field Emission Scanning Electron microscope (FESEM), Ultra violet-Visible-Near infrared
(UV-VIS-NIR) spectrophotometer and Resonance-coupled photoconductivity decay (RCPCD) have been used to
characterize the morphological, optical, and electrical properties of Si wires at varying etching times. The reflectivity of
the wire arrays decreased with increasing etching time because of light scattering from the micro-roughness of the
Silicon wire surfaces. The effective carrier lifetime decreased with increasing wire length due to the increased surface
area. We also created smoother wire surfaces by thermal oxidation followed by HF dipping. From FESEM cross
sectional images and reflectivity results, this treatment removes the micro-roughness, but the effective lifetime is lower
than the as-grown wire arrays. A photoluminescence peak observed only in the smoother wires suggests that the lower
effective lifetime is due to the diffusion of residual Ag atoms from the wire surfaces into the bulk during the thermal
oxidation process.
Nanomaterials have the potential to revolutionize photovoltaics with the promise of new physics, novel architectures
and low cost synthesis. Silicon quantum dots, relative to their II-VI counterparts, are understudied
due to the difficulty of solution synthesis and chemical passivation. However, silicon is still an attractive solar
cell material, providing an optimal band gap, low toxicity, and a very solid body of physical understanding of
bulk silicon to draw from. We have synthesized silicon quantum dots with plasma enhanced chemical vapor
deposition, and have developed a method for chemical passivation of these silicon quantum dots that can be
used on particles created in a variety of ways. This versatile method utilizes oxidation via wet chemical etch and
subsequent siloxane bond formation. The attachment of a silane to the SiOx shell leads to stability of the silicon
core for over a month in air, and individual particles can be seen with TEM; thus a stable, colloidal suspension
is formed. The future for this technique, including increasing quantum yield of the particles by changing the
nature of the oxide, will be discussed.
Enhanced transmission through structures consisting of linear gratings surrounding a single subwavelength aperture in an opaque gold film is modeled using a commercial finite element model (FEM). The stability of the FEM and boundary conditions are discussed, and different field visualizations are explored to gain insight into field behavior. The results from the FEM were compared with experimental results, yielding excellent agreement. This lends confidence that the FEM is giving an accurate representation of the field behavior around the structure. The FEM was then used to examine how transmission enhancement depends on geometric properties of the structure and to gain insight into the mechanisms of transmission enhancement.
We present a study of optical transmission in the visible and
near-infrared regimes through subwavelength apertures in gold
films. Samples consisting of single, ~100 nm wide, 50 micron long, linear apertures, centered between two finite grating structures, were prepared using electron-beam lithography with
subsequent broad-beam argon-ion milling. The period and number of
the corrugations that make up the grating structures was constant,
while the distance between the gratings on each side of the
aperture was varied. Spectrally resolved far-field transmission
measurements were obtained for normal incidence with a
spectrometer-coupled optical microscope configured for
transmission measurements. Transmission through these structures
was significantly enhanced relative to an isolated aperture at
resonant wavelengths for transverse magnetic polarized incident
light, in agreement with the literature. Wavelengths where the
transmission was suppressed relative to an isolated aperture were
also observed. The wavelengths of maximum transmission and of
suppression were found to depend on the spacing between the
grating arrays and the aperture. Measured spectra were consistent
with modeled results and can be interpreted in terms of the
interference between the incident light and surface plasmon
polaritons (SPP) as well as cavity resonances of the SPPs.
The Near-Field Scanning Optical Microscope (NSOM) is a tool that combines the spatial resolution of scanning probe microscopy with optical characterization techniques. Using this technique, we have generated high-resolution spatial intensity maps of the output from vertical-cavity surface-emitting lasers (VCSELs) in the near-field region of the facet as a function of operating current. The VCSELs studied were proton implanted, gain guided devices designed to operate at ~850nm. Optical signals that have been spatially imaged include total intensity, the spectrally resolved intensity of individual transverse modes, and the derivative of intensity with respect of operating current. Deviations from expected mode patterns in the devices have been qualitatively linked to unacceptable levels of noise in operating lasers. These deviations can be observed at operating currents below the actual onset of unacceptable noise. We have also found that derivative spectroscopy can be used to sensitively detect the cutoff points of transverse modes. Using the spatial intensity profile at the cutoff point of an allowed mode, a first approximation to the index of refraction profile can be made that is in good agreement with prior work. A series of index profile estimates from the cutoff points of a VCSEL can provide information on the evolution of the index profile and the thermal lens as the power is ramped up.
We have grown Si/Sii_Ger multiple quantum wells (r 8%) lattice-matched to silicon with well thicknesses between 3 and 20 nni using UHV-CVD. The sample parameters were obtained accurately with high-resolution X-ray diffraction ( rocking curves) and transmission electron microscopy. From an analysis of the band-edge related photoluminescence energies we find a blue-shift due to confinement for thin wells.
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