Antireflective Nanostructures Research Articles

Hot-rolled embossing of microlens arrays with antireflective nanostructures on optical glass

sGlass is an excellent material for optical devices requiring optical and mechanical properties better than those of a polymer. Because traditional micro- or nano-fabrication on glass substrates is typically slow and costly, the fabrication of micron- or nanometer-scale structures on glass has been a challenge. For a novel continuous fabrication of glass microlens arrays with antireflective nanostructures (AR-MLA), we have designed and implemented a hot-rolled embossing facility with synergetic heating. The mold of a hybrid micro/nanostructure was fabricated in two steps. First, a micron-scale lens cavity was embossed on a nearly pure aluminum sheet with an electroplated nickel mold; an anodic aluminum-oxide process was used to create a nanostructure on the microlens array cavity. The mold was then mounted on a roller and used in a hot embossing machine to fabricate AR-MLA on optical glass. The optical properties of the fabricated microlens with nanostructures have been verified. The image of focused spots of fabricated AR-MLA shows an effective converging ability, and the reflectance at 550 nm was decreased from 7.63 % to 0.53 % with the fabricated nanostructures. This proposed technique has proven its potential for hot-rolled embossing of glass components with micro/nanostructures.

The application of magnetic fluid bag micro-roll imprinting technology in optical anti-reflective and solar concentrator soft film replication

This study developed the magnetic fluid bag micro-roll imprinting system equipment, and compared the roll imprinting behavior of the general rigid roller and soft roller imprinting systems. Using the precision equipment system, this study developed optical anti-reflective micro Nano-structure roll imprinting replication and conducted an inspection of the optical features. The research results suggested that the proposed magnetic fluid bag micro-roll imprinting equipment and processing technology can provide even distribution of the magnetic fluid inside the bag. The magnetic force can effectively increase the imprinting contact area of the roller and substrate during the roll imprinting process, while effectively enhancing roll imprinting velocity. In addition, the experiment confirmed that the precision system equipment and processing methodology of this innovative design can successfully print optical anti-reflective film and solar concentrator (Fresnel lens micro-structure). Moreover, it has very good performance in the optical anti-reflective experiment, sunlight intensity analysis, and inspection.

Black metal thin films by deposition on dielectric antireflective moth-eye nanostructures.

Although metals are commonly shiny and highly reflective, we here show that thin metal films appear black when deposited on a dielectric with antireflective moth-eye nanostructures. The nanostructures were tapered and close-packed, with heights in the range 300-600 nm, and a lateral, spatial frequency in the range 5–7 μm−1. A reflectance in the visible spectrum as low as 6%, and an absorbance of 90% was observed for an Al film of 100 nm thickness. Corresponding experiments on a planar film yielded 80% reflectance and 20% absorbance. The observed absorbance enhancement is attributed to a gradient effect causing the metal film to be antireflective, analogous to the mechanism in dielectrics and semiconductors. We find that the investigated nanostructures have too large spatial frequency to facilitate efficient coupling to the otherwise non-radiating surface plasmons. Applications for decoration and displays are discussed.

Gradient index antireflection coatings on glass containing plasma-etched organic layers

The plasma etching process was originally developed to produce antireflective (AR) nanostructures on polymer substrates. A common vacuum coating system equipped with a plasma source was used. The technique is now applied to deposit and etch organic films on glass. The resulting organic nanostructured layers exhibit a low effective refractive index that can be tuned down to approximately 1.1. Broadband AR coatings were developed by combining inorganic materials and organic nanostructured layers in such a way that the effective index decreases in a stepwise manner or gradually from the index of the substrate to that of the ambient medium. They exhibit AR properties from 400 nm to 1200 nm at normal and oblique light incidence.

Design and fabrication of sunlight-redirecting and infrared-insulating microstructure

Air-conditioning and artificial lighting consume over 50% of the energy in a building. Decreasing thermal loading and introducing sunlight into a building can effectively save electricity and reduce greenhouse effect. This study reports a design for a passive prismatic sunlight-redirecting microstructure coupled with infrared (IR) insulation. The infrared insulation is an anti-reflection nanostructure with a silver thin film. The base design uses a quadrangular prism with a 45° vertex angle and a height and pitch of 50μm to direct the high intensity sunlight deep into the room; unfortunately, this design produces harmful glare, and the area lighted by the redirected sunlight drifts as the varies sunlight during the day. The prism is further modified using an inclined-curved complex, which has 18.3μm pitch and 20.91μm height. The curved plane can diffuse the sunlight into a wide-angle exiting sunlight beam, which provides uniform sunlight with a stable daylighting area. The experimental results show that the daylighting redirection efficiency reaches 70% at solar elevation angles of 40–70° and that the daylighting area can reach up to 7m deep into the room. The average transmittance is lower than 15.46%, and the reflectance is higher than 75.64% at infrared wavelengths.

Antireflection coatings for solar panel power output enhancement

ABSTRACTThe impact of nanostructured broadband antireflection (AR) coatings on solar panel performance has been projected for a broad range of panel tilt angles at various locations. AR coated films have been integrated on test panels and the short-circuit current has been measured for the entire range of panel tilts. The integration of the AR coatings resulted in an increase in short-circuit current of the panels by eliminating front sheet reflection loss for a broad spectrum of light and wide angle of light incidence. The short-circuit current enhancement is 5% for normal light incidence and approximately 20% for off-angle light incidence. The National Renewable Energy Laboratory (NREL) System Advisor Model (SAM) predicts that this AR coating can yield at least 6.5% improvement in solar panel annual power output. The greatest enhancement, approximately 14%, is predicted for vertical panels. The AR coating’s contributions to vertical mount panels and building-integrated solar panels are significant. This nanostructured broadband AR coating thus has the potential to lower the cost per watt of photovoltaic solar energy.

Nanostructured Antireflection Coatings for Optical Detection and Sensing Applications

ABSTRACTOptical components such as lenses, glass windows, and prisms are subject to Fresnel reflection due to the mismatch between the refractive indices of the air and glass. An optical interface layer, i.e., antireflection (AR) layer, is needed to eliminate this unwanted reflection at the air/glass interface. Nanostructured broadband and wide-angle AR structures have been developed using a scalable self-assembly process. Ultra-high performance of the nanostructured AR coatings has been demonstrated on various substrates such as quartz, sapphire, polymer, and other materials typically employed in optical lenses. AR coatings on polycarbonate lead to optical transmittance enhancement from approximately 90% to almost 100% for the entire visible, and part of the near-infrared (NIR), band. The AR coatings have also been demonstrated on curved surfaces. AR coatings on n-BK7 lenses enable ultra-high light transmittance for the entire visible, and most of the NIR, spectrum. Nanostructured oxide layers with step-graded index profiles, deposited onto the optical elements of an optical system, can significantly increase sensitivity, and hence improve the overall performance of the system.

Advanced Flexible CIGS Solar Cells Enhanced by Broadband Nanostructured Antireflection Coatings

ABSTRACTFlexible copper indium gallium diselenide (CIGS) solar cells on lightweight substrates can deliver high specific powers. Flexible lightweight CIGS solar cells are also primary candidates for building-integrated panels. In all applications, CIGS cells can greatly benefit from the application of broadband and wide-angle AR coating technology. The AR coatings can significantly improve the transmittance of light over the entire CIGS absorption band spectrum. Increased short-circuit current has been observed after integrating AR coated films onto baseline solar panels. NREL’s System Advisor Model (SAM) has predicted up to 14% higher annual power output on AR integrated vertical or building-integrated panels. The combination of lightweight flexible substrates and advanced device designs employing nanostructured optical coatings together have the potential to achieve flexible CIGS modules with enhanced efficiencies and specific power.

Design of antireflective nanostructures and optical coatings for next-generation multijunction photovoltaic devices.

The successful development of multijunction photovoltaic devices with four or more subcells has placed additional importance on the design of high-quality broadband antireflection coatings. Antireflective nanostructures have shown promise for reducing reflection loss compared to the best thin-film interference coatings. However, material constraints make nanostructures difficult to integrate without introducing additional absorption or electrical losses. In this work, we compare the performance of various nanostructure configurations with that of an optimized multilayer antireflection coating. Transmission into a four-junction solar cell is computed for each antireflective design, and the corresponding cell efficiency is calculated. We find that the best performance is achieved with a hybrid configuration that combines nanostructures with a multilayer thin-film optical coating. This approach increases transmitted power into the top subcell by 1.3% over an optimal thin-film coating, corresponding to an increase of approximately 0.8% in the modeled cell efficiency.

Monolithically integrated micro- and nanostructured glass surface with antiglare, antireflection, and superhydrophobic properties.

Hierarchical micro- and nanostructured surfaces have previously been made using a variety of materials and methods, including particle deposition, polymer molding, and the like. These surfaces have attracted a wide variety of interest for applications including reduced specular reflection and superhydrophobic surfaces. To the best of our knowledge, this paper reports the first monolithic, hierarchically structured glass surface that combines micro- and nanoscale surface features to simultaneously generate antiglare (AG), antireflection (AR), and superhydrophobic properties. The AG microstructure mechanically protects the AR nanostructure during wiping and smudging, while the uniform composition of the substrate and the micro- and nanostructured surface enables ion exchange through the surface, so that both the substrate and structured surface can be simultaneously chemically strengthened.

Resist-free antireflective nanostructured film fabricated by thermal-NIL.

Resist-free antireflective (AR) nanostructured films are directly fabricated on polycarbonate (PC) film using thermal-nanoimprint lithography (T-NIL) and the moth-eye shape of AR nanostructure is elaborately optimized with different oxygen reactive ion etching conditions. Anodic aluminum oxide (AAO) templates are directly used as master molds of T-NIL for preparation of AR nanostructures on PC film without an additional T-NIL resist. AR nanostructures are well arranged with a period of about 200 nm and diameter of about 150 nm, which corresponds to those of the AAO template mold. The moth-eye AR nanostructures exhibit the average reflectance of 2% in wavelength range from 400 to 800 nm. From the results, highly enhanced AR properties with simple direct imprinting on PC film demonstrate the potential for panel application in the field of flat display, touch screen, and solar cells.

Ultrabroadband and Wide-Angle Hybrid Antireflection Coatings With Nanostructures

Ultrabroadband and wide-angle antireflection coatings (ARCs) are essential to realizing efficiency gains for state-of-the-art multijunction photovoltaic devices. In this study, we examine a novel design that integrates a nanostructured antireflection layer with a multilayer ARC. Using optical models, we find that this hybrid approach can reduce reflected AM1.5D power by 10-50 W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> over a wide angular range compared to conventional thin-film ARCs. A detailed balance model correlates this to an improvement in absolute cell efficiency of 1-2%. Three different ARC designs are fabricated on indium gallium phosphide, and reflectance is measured to show the benefit of this hybrid approach.

Breakthroughs in Photonics 2013: Organic Nanostructures for Antireflection

The processing of organic substances by vacuum deposition is opening new possibilities for the properties of optical surfaces. Organic small molecules can be evaporated and deposited like optical thin films. Plasma etching, which has been successfully applied to generate nanostructures on polymer substrates, is now used for obtaining antireflective nanostructures on top of interference stacks on glass. The latest results in achieving excellent antireflective properties for a wide range of light incidence angles were accomplished through multiple etching of polymer substrates and organic layers.

Injection moulding antireflective nanostructures

We present a method for injection moulding antireflective nanostructures on large areas, for high volume production. Nanostructured black silicon masters were fabricated by mask-less reactive ion etching, and electroplated with nickel. The nickel shim was antistiction coated and used in an injection moulding process, to fabricate the antireflective surfaces. The cycle-time was 35s. The injection moulded structures had a height of 125nm, and the visible spectrum reflectance of injection moulded black polypropylene surfaces was reduced from 4.5±0.5% to 2.5±0.5%. The gradient of the refractive index of the nanostructured surfaces was estimated from atomic force micrographs and the theoretical reflectance was calculated using the transfer matrix method and effective medium theory. The measured reflectance shows good agreement with the theory of graded index antireflective nanostructures.

A double nanostructure for wide-angle antireflection on optical polymers

Direct plasma etching is a powerful method for producing antireflective nanostructures on optical polymers, such as cycloolefin polymers. The approved process requires the deposition of a very thin initial layer followed by etching. The structure depth achievable in this way is limited to approximately 100 nm. Due to this limitation, the reflectance performance of materials produced by plasma etching is sufficient in the visible spectral range for normal light incidence on planar substrates only. By depositing and etching an additional organic layer on top of the structure, its antireflective performance can be significantly broadened. This type of double structure is adequate for light incidence angles of up to 60° on planar and curved substrates.

Broadband antireflective silicon nanostructures produced by spin-coated Ag nanoparticles

We report the fabrication of broadband antireflective silicon (Si) nanostructures fabricated using spin-coated silver (Ag) nanoparticles as an etch mask followed by inductively coupled plasma (ICP) etching process. This fabrication technique is a simple, fast, cost-effective, and high-throughput method, making it highly suitable for mass production. Prior to the fabrication of Si nanostructures, theoretical investigations were carried out using a rigorous coupled-wave analysis method in order to determine the effects of variations in the geometrical features of Si nanostructures to obtain antireflection over a broad wavelength range. The Ag ink ratio and ICP etching conditions, which can affect the distribution, distance between the adjacent nanostructures, and height of the resulting Si nanostructures, were carefully adjusted to determine the optimal experimental conditions for obtaining desirable Si nanostructures for practical applications. The Si nanostructures fabricated using the optimal experimental conditions showed a very low average reflectance of 8.3%, which is much lower than that of bulk Si (36.8%), as well as a very low reflectance for a wide range of incident angles and different polarizations over a broad wavelength range of 300 to 1,100 nm. These results indicate that the fabrication technique is highly beneficial to produce antireflective structures for Si-based device applications requiring low light reflection.

Morphology-Tuned Fabrication of Highly Transparent and Flexible Nanostructures at Room Temperature

Shape controlled fabrication of highly transparent and flexible nanostructures (nanocone, nanoneedle, nanowalls) onto nafion substrate were performed at room temperature by simple ion irradiation method. By varying the ion incidence angle and irradiated ion, the surface morphology and alignment were gradually changed from nanocone to nanowall pattern. Interestingly, ion irradiation onto the nafion surface led to the systematic pattern without surface modification by external addition of any material onto the substrate prior to ion irradiation. The antireflective performance of the nanostructures made the surface more transparent compared to the bare substrate. The growth mechanism of the nanostructures arrays with different shapes is also discussed briefly. This straightforward and fast method is thought to be very enticing for surface engineering and fabrication of antireflective nanostructures of controlled dimension onto different kinds of plastic substrates. These kinds of transparent nanostructures might have a remarkable role for promising commercial impact in wide variety of areas such as dust-free devices, see-through devices, nanofluidics, and drug delivery.

P.115: Micropyramid Array with Antireflective Nanostructure Surfaces for Light Extraction Efficiency Enhancement of Organic Light Emitting Devices

AbstractA novel and efficient light extraction technique of OLEDs by attaching hierarchical micro‐pyramid array with antireflective nanostructures was demonstrated. The hierarchical micropyramid arrays were nanoimprinted on polycarbonate films by using silicon concave mold incorporated with micropyramid and nanoporous surface, which were made by alkaline texturing and nanoparticle‐catalyzed etching processes respectively. The additional antireflective nanostructures can effectively reduce the loss for the light normally incident at the inclined surface. After applying the subwavelength antireflective surface at the conventional micropyramids, the luminous efficiency enhancement was improved from 37.7% to 84.2%.

Characteristics of terahertz pulses from antireflective GaAs surfaces with nanopillars

We investigated the characteristics of terahertz pulses generated from antireflective GaAs surfaces with nanopillars under femtosecond laser excitation. Although the antireflective nanostructures contribute to the enhancement of free photocarrier excitation in GaAs, they could reduce the transient photocurrent density and advance the start time of the photocurrent decay. Thus, the relative amplitudes of the high-frequency spectral components of terahertz pulses increased, whereas the energies of the pulses decreased. However, we showed that thinly distributed nanopillar structures could generate a short terahertz pulse without a reduction in the pulse energy.

Large scale antireflective glass texturing using grid contacts in anodization methods

Antireflection coatings on glass are highly desirable, since glasses are widely used as a packaging material in the solar industry due to their transparency and durability. In this report, we propose a novel anodizing method using grid line contacts for antireflective nanostructures on a large area glass substrate. The nanoholes on the glass substrate exhibit broadband and omnidirectional antireflective properties, leading to an 11.34% improvement in the short-circuit current and enhanced power conversion efficiency from 7.9% to 8.57%. Moreover, the enhanced optical properties persist for 3 months in an outdoor environment. The proposed anodizing method can be considered as an alternative technique for the fabrication of large area nanostructures.