Metal Blacks As Scattering Centers To Increase The Efficiency Of Thin Film Solar Cells

Metal nano particles are investigated as scattering centers on front surface of thin-film solar cells to improve efficiency. The principle is that scattering, which is enhanced near the plasmon resonance frequency of the particle and depends on particle size, increases the effective optical path length of incident light, leading to more light absorption in active layer of thin film solar cell. The particular types of particles investigated here are known as metal-black , well known as an IR absorber for bolometric infrared detectors. Gold-black was deposited on commercial thin-film solar cells using a thermal evaporator in a nitrogen ambient at pressures of ~1 Torr. We suggest that the broad range of length scales for gold black particles, as quantified by scanning electron microscopy, gives rise to efficient scattering over a broad range of wavelengths across the solar spectrum. The solar cell efficiency was determined both as a function of wavelength and for a solar spectrum produced by a Xe lamp and appropriate filters. Up to 20% increase in short-circuit photocurrent, and a 7% increase in efficiency at the maximum power point, were observed

Characterization of gold black and its application in un-cooled infrared detectors

Gold black porous coatings were thermally evaporated in the chamber backfilled with inert gas pressure and their optical properties were studied in near-far-IR wavelengths. The porosities of coatings were found to be extremely high around ~ 99%. Different approaches of effective medium theories such as Maxwell-Garnett, Bruggeman, Landau-Lifshitz-Looyenga and Bergman Formalism were utilized to calculate refractive index (n) and extinction coefficient (k). The aging induced changes on electrical and optical properties were studied in regular laboratory conditions using transmission electron microscopy, Fourier transform infrared spectroscopy, and fore-probe electrical measurements. A significant decrease in electrical resistance in as deposited coating was found to be consistent with changes in the granular structure with aging at room temperature. Electrical relaxation model was applied to calculate structural relaxation time in the coatings prepared with different porosities. Interestingly, with aging, absorptance of the coatings improved, which is explained using conductivity form of Bergman Formulism. Underlying aim of this work was to utilize gold blacks to improve sensitivity in un-cooled IR sensors consist of pixel arrays. To achieve this, fragile gold blacks were patterned on sub-mm length scale areas using both stenciling and conventional photolithography. Infrared spectral imaging with sub-micron spatial resolution revealed the spatial distribution of absorption across the gold black patterns produced with both the methods. Initial experiments on VOx-Au bolometers showed that, gold black improved the responsivity by 42%. This work successfully establishes promising role of gold black coatings in commercial un-cooled infrared detectors

Far-Infrared Absorber Based On Standing-Wave Resonances In Metal-Dielectric-Metal Cavity

Thin-film resonant absorbers for the far-IR spectral range were fabricated, characterized, and modeled. The 3-μm-thick structure comprises a periodic surface array of metal squares, a dielectric spacer and a metallic ground plane. Up to 95% absorption for the fundamental band at ∼53.5μm wavelength (5.6 THz) is achieved experimentally. Absorption bands are independent of the structure period and only weakly dependent on polarization and incident angle. The results are well explained in terms of standing-wave resonances within individual metal-dielectric-metal cavities. The structure has application as a wavelength selective coating for far-IR bolometers

Patterning Of Oxide-Hardened Gold Black By Photolithography And Metal Lift-Off

A method to pattern infrared-absorbing gold black by conventional photolithography and lift-off is described. A photo-resist pattern is developed on a substrate by standard photolithography. Gold black is deposited over the whole by thermal evaporation in an inert gas at ∼1 Torr. SiO2 is then deposited as a protection layer by electron beam evaporation. Lift-off proceeds by dissolving the photoresist in acetone. The resulting sub-millimeter size gold black patterns that remain on the substrate retain high infrared absorption out to ∼5 μm wavelength and exhibit good mechanical stability. This technique allows selective application of gold black coatings to the pixels of thermal infrared imaging array detectors. © 2013 Elsevier B.V. All rights reserved

Patterning And Hardening Of Gold Black Infrared Absorber By Shadow Mask Deposition With Ethyl Cyanoacrylate

Patterning of gold-black infrared absorbing films by stencil lithography and hardening by polymer infusion is reported. Gold black nano-structured films are deposited through a thin metal shadow mask in a thermal evaporator in ~400 mTorr pressure of inert gas, followed by ethyl cyanoacrylate fuming through the same mask to produce rugged IR absorptive patterns of ~100 micron scale dimensions. Infrared absorptivity is determined by transmission and reflectivity measurements using a Fourier spectrometer and infrared microscope. Results indicate that the optimized hardening process reduces the usual degradation of the absorptivity with age. This work has potential application to infrared array bolometers. © 2013 SPIE

Enhanced Performance Of VoX-Based Bolometer Using Patterned Gold Black Absorber

Patterned highly absorbing gold black film has been selectively deposited on the active surfaces of a vanadium-oxide-based infrared bolometer array. Patterning by metal lift-off relies on protection of the fragile gold black with an evaporated oxide, which preserves gold black\u27s near unity absorption. This patterned gold black also survives the dry-etch removal of the sacrificial polyimide used to fabricate the air-bridge bolometers. Infrared responsivity is substantially improved by the gold black coating without significantly increasing noise. The increase in the time constant caused by the additional mass of gold black is a modest 14%

Aging Of Nano-Morphology, Resistivity, And Far-Infrared Absorption In Gold-Black

Gold black is a highly porous, extremely fragile, infrared-absorbing film used primarily as a coating for bolometers. Long term stability of its absorbance is a significant practical concern. This paper reports on the aging of morphological, electrical, and optical properties of gold black samples prepared with different initial porosities. An observed two-fold decrease in electrical resistance after 90 days at room-temperature is correlated with an increase in nano-crystalline grain size. Much larger resistance drops were observed after isothermal annealing at temperatures up to 100°C. Aging and annealing tended to improve the far-infrared absorption. Samples with the highest initial porosity have the fastest structural relaxation

Chalcogenide Glass Thin-Film Optics For Infrared Applications

Chalcogenide glasses are increasingly used in infrared-transparent optical systems for space applications due to their relatively low density (compared to Ge or ZnSe), tunable spectral and thermo-mechanical properties, and molding capability. Remaining challenges include their application to thin-film optics and coatings. The high refractive indices of chalcogenide glasses (n \u3e2.7) suggest the possibility for high reflecting coatings based on few periods of alternating layers with high index contrast. As2Se3 thin film deposited by thermal evaporation is investigated using ellipsometry which show optical properties consistent with bulk material. Also we demonstrate a novel method for fabrication of antireflection coating using porous chalcogenide. Possibility of negligible extinction coefficient and low refractive index of this porous coating promises broadband suppression of undesired Fresnel reflections at the interface from infrared optics. © 2014 SPIE

Metal-Black Scattering Centers To Enhance Light Harvesting By Thin-Film Solar Cells

Small metal particles are investigated as scattering centers to increase the effective optical thickness of thin-film solar cells. The particular type of particles used is known as metal-black , well known as an IR absorber for bolometric infrared detectors. Gold-black was deposited on commercial thin-film solar cells using a thermal evaporator in nitrogen ambient at pressures of ∼1 Torr. A broad range of length scales, as revealed by scanning electron microscope images gives rise to effective scattering over a range of wavelengths across the solar spectrum. The solar cell efficiency was determined both as a function of wavelength and for a solar spectrum produced by a Xe lamp and appropriate filters. Up to 20% increase in short-circuit photo-current, and a 5% increase in efficiency at the maximum power point, were observed. © 2011 SPIE

Chalcogenide glass thin-film optics for infrared applications

Chalcogenide glasses are increasingly used in infrared-transparent optical systems for space applications due to their relatively low density (compared to Ge or ZnSe), tunable spectral and thermo-mechanical properties, and molding capability. Remaining challenges include their application to thin-film optics and coatings. The high refractive indices of chalcogenide glasses (n \u3e2.7) suggest the possibility for high reflecting coatings based on few periods of alternating layers with high index contrast. As2Se3 thin film deposited by thermal evaporation is investigated using ellipsometry which show optical properties consistent with bulk material. Also we demonstrate a novel method for fabrication of antireflection coating using porous chalcogenide. Possibility of negligible extinction coefficient and low refractive index of this porous coating promises broadband suppression of undesired Fresnel reflections at the interface from infrared optics. © 2014 SPIE