Broadband Antireflection Research Articles

Absorption Enhancement in Ultrathin Structures Based on Crystalline-Si/Ag Parabola Nanocones Periodic Arrays with Broadband Antireflection Property

In this study, we examine the optical properties and unique features of a novel design of a parabola nanocone consisting of a homogenous shell-like cover layer of crystalline silicon (c-Si) and an Ag core which provides an enhanced absorption efficiency and significant photocurrent conversion during exposure to an incident light. Determining the geometrical sizes of the c-Si/Ag parabola nanocone, we designed an antireflection nanostructure based on certain arrays of investigated cone arrays on a GaAs substrate. We proved that the examined nanostructure shows a low percentage of reflectance of 6.24 % and a significant short current density of ~37.2 mA/m 2 as well as broadband antireflection facility. This understanding paves the way for novel methods toward the use of a simple and two layer nanoparticle in designing efficient and high performance antireflection layers of photovoltaics and solar cells that are able to function over a wide range of spectrum.

Cicada-Wing-Inspired Self-Cleaning Antireflection Coatings on Polymer Substrates.

The cicada has transparent wings with remarkable self-cleaning properties and high transmittance over the whole visible spectral range, which is derived from periodic conical structures covering the wing surface. Here we report a scalable self-assembly technique for fabricating multifunctional optical coatings that mimic cicada-wing structures. Spin-coated two-dimensional non-close-packed colloidal crystals are utilized as etching masks to pattern subwavelength-structured cone arrays directly on polymer substrates. The resulting gratings exhibit broadband antireflection performance and superhydrophobic properties after surface modification. The dependence of the cone shape and size on the antireflective and self-cleaning properties has also been investigated in this study.

Fabrication of Moth-Eye Nanostructure Arrays Using Roll-to-Roll UV-Nanoimprint Lithography With an Anodic Aluminum Oxide Mold

Broadband antireflection is essential for improving the photocurrent generation of photovoltaic modules or the enhancement of visibility in optical devices. The moth-eye nanostructure is one of the most promising structures with potential for commercialization in the near future and roll-to-roll ultraviolet nanoimprint lithography (R2R UV-NIL) technique provides a solution for the commercialization due to its high speed, large area, high resolution, and high throughput. In this study, the anodic aluminum oxide (AAO) mold is used to produce the moth-eye nanostructure arrays upon a flexible polyethylene terephthalate substrate combining with R2R UV-NIL process, which provides a solution for continuous production of moth-eye nanostructure arrays cheaply. In addition, the influence of mold parameters and process parameters on the forming quality were investigated, respectively. The influence of process parameters was investigated by using the one-variable-at-a-time method, including feeding speed, imprinting pressure, and mold temperature. The qualitative and quantitative height evaluation for the moth-eye nanostructure arrays were carried out based on the atomic force microscope images. As a result, the feasibility is verified to produce moth-eye nanostructure arrays continuously with AAO mold and R2R UV-NIL technique. A rapid feeding speed of 20 m/min was possible to produce good moth-eye nanostructure arrays with mold 1, which were 300 nm in diameter, 300 nm in height. However, different forming results were observed as the mold parameters decreased including unsatisfactory demolding phenomenon and incomplete filling phenomenon. The incomplete filling phenomenon was improved to a certain extent by changing the process parameters and higher forming height was accomplished with lower feeding speed, higher imprinting pressure, and higher mold temperature. This research is beneficial to provide a direction for the industrial production of the moth-eye nanostructure arrays.

Subwavelength structures for high power laser antireflection application on fused silica by one-step reactive ion etching

In this paper we report a simple method to fabricate a novel subwavelength structure surface on fused silica substrate using one-step reactive ion etching with two-dimensional polystyrene colloidal crystals as masks. The etching process and the morphologies of the obtained structure are controlled. We show that the period of the obtained fused silica pillar-like arrays were determined by the initial polystyrene nanoparticle size. The height of pillar arrays can be adjusted by controlling the etching duration, which is proved to be of importance in tailoring the antireflection properties of subwavelength structures surface. The novel subwavelength structures surface exhibit excellent broadband antireflection properties, but the size of the pillar affects the antireflective properties in short wavelength region. We anticipate this method would offer a convenient and scalable way for inexpensive and high-efficiency high power laser field designs.

Broadband antireflection coatings for optical lenses

Broadband antireflection coatings for optical lenses

Random Au–Ag Composite Nanoparticles as Broadband Antireflection Coating for Si Surface

Random Au–Ag Composite Nanoparticles as Broadband Antireflection Coating for Si Surface

Enhancement of antireflection characteristic of resonant sinusoidal grating

Broadband antireflective characteristic is frequently achieved by sinusoidal grating in the sub-wavelength domain. Also, in the resonance domain the enhancement of antireflection for broadband spectrum and wide-angle spectrum can be obtained. For external reflection grating, the total transmittance can reach to 99.9% when the aspect ratio is larger than 1.5 for both incident polarization states. For internal reflection grating, the antireflective property in relatively narrow broadband spectrum is revealed for TE polarization. But for TM polarization, broadband antireflection at appropriate grating period and groove depth is shown. It is found that the aspect ratio of grating is a significant parameter for improving antireflection performance.

Novel-type nanostructured SiO2 antireflection coatings and their application in Cu(In,Ga)Se2 solar cells

Broadband antireflection coatings (ARCs) are a considerable way to sufficiently harvest more solar light into the solar cells due to dramatically reducing the reflection losses at the surface of substrate. Novel structures consisting of continuous SiO2 film and monodispersed SiO2 nanospeheres (NSs) array are proposed to fabricate ARCs according to mimicking the moth-eye-like nanostructure with a periodic sub-wavelength protuberance. Three distinct architectures of coatings based on SiO2 film and SiO2 NSs were successfully obtained by sol–gel dip-coating and electrostatic self-assembly technique, and the structural, morphological and optical properties of the coatings were investigated. Antireflection functionality of the coatings is achieved over the solar spectrum, and the solar transmittance of the substrate in visible region is increased by 7.02% on average with maximum of 8.21% by introducing coating of SiO2 NSs/SiO2 film. The power conversion efficiency of Cu(In,Ga)Se2 solar cells is clearly increased from 11.66% to 12.59% by using with as-prepared ARCs. The novel-type nanostructured ARCs are promising materials in both optical and photovoltaic applications.

Ultrathin broadband antireflection structure composed of multilayer hollow nanopillar arrays of c-Si and Ag shells embedded in a GaAs substrate

In this study, we investigated the spectral response and properties of compositional multilayer hollow nanopillars in the certain arrays that are embedded in a GaAs substrate to design a broadband antireflection subwavelength structure. Considering the arrays of multilayer hollow nanopillars (crystalline-Si/Ag/air) with the appropriate geometrical sizes, three kinds of optical modes at the absorption profile in the wide range of the spectrum are reported. Fabry–Perot (FP), hybrid, and plasmon resonance modes are the three appeared optical modes at the calculated absorption cross-sectional profile using numerical method. Quantifying the reflectance of the subwavelength structure, this parameter is measured as approximately 6.69% in the range of λ ∼ 400–2900nm. We proved that proposed nanostructure is able to provide high ratio of optical energy absorption due to the existence of GaAs, c-Si and Ag substances that play fundamental role in this enhancement as the absorber materials.

Broadband-antireflective hybrid nanopillar array for photovoltaic application

Subwavelength structures such as nanopillars, nanoholes, and nanodomes have recently attracted considerable attention as antireflective structures for solar cells. Recent studies on the optical property of nanopillar array revealed that the reflection minimum is related to the diameter, the pitch, and the height of nanopillars. Here, we investigate the “hybrid” nanopillar array, which is composed of different diameters of nanopillars. Finite differential time domain simulations revealed that the photogeneration in a hybrid nanopillar array is spatially heterogeneous: carriers are generated mainly in the narrower pillars for short-wavelength incident light and in the thicker pillars for long-wavelength light, respectively. Hybrid silicon nanopillar arrays fabricated by using electron beam lithography and dry etching show excellent broadband antireflection property. Hybrid nanopillar array is thus highly promising for next-generation antireflection for photovoltaic applications.

Formation of broadband antireflective and superhydrophilic subwavelength structures on fused silica using one-step self-masking reactive ion etching.

Fused silica subwavelength structures (SWSs) with an average period of ~100 nm were fabricated using an efficient approach based on one-step self-masking reactive ion etching. The subwavelength structures exhibited excellent broadband antireflection properties from the ultraviolet to near-infrared wavelength range. These properties are attributable to the graded refractive index for the transition from air to the fused silica substrate that is produced by the ideal nanocone subwavelength structures. The transmittance in the 400–700 nm range increased from approximately 93% for the polished fused silica to greater than 99% for the subwavelength structure layer on fused silica. Achieving broadband antireflection in the visible and near-infrared wavelength range by appropriate matching of the SWS heights on the front and back sides of the fused silica is a novel strategy. The measured antireflection properties are consistent with the results of theoretical analysis using a finite-difference time-domain (FDTD) method. This method is also applicable to diffraction grating fabrication. Moreover, the surface of the subwavelength structures exhibits significant superhydrophilic properties.

Fabrication of Sub-25 nm Diameter GaSb Nanopillar Arrays by Nanoscale Self-Mask Effect.

GaSb individual nanowires and nanowire arrays are considered as intriguing candidates for electronic and photonic applications. In this paper, we report a new mask-free method to fabricate large area GaSb nanopillar arrays through reactive ion etching of GaSb substrates facilitated by O2 plasma. We have shown that nanoscale oxide self-masks could form thereby facilitating the formation of GaSb nanopillars. We have achieved GaSb nanowires with diameters less than 25 nm and an aspect ratio of 24. Additionally, GaSb nanopillar arrays with desired heights, diameters, and density can be obtained by choosing the plasma chemistry and/or controlling etching parameters, such as bias power and pressure. The nanopillar arrays prepared also exhibit tunable broadband antireflection properties.

Design principles for morphologies of antireflection patterns for solar absorbing applications.

Two-dimensional surface texturing is a widespread technology for imparting broadband antireflection, yet its design rules are not completely understood. The dependence of the reflectance spectrum of a periodically patterned glass film on various structural parameters (e.g., pitch, height, shape, and fill factor) has been investigated by means of full-vectorial numerical simulations. An average weighted reflectivity accounting for the AM1.5G solar spectrum (λ=300-1000 nm) was sinusoidally modulated by a rod pattern's height, and was minimized for pitches of 400-600 nm. When a rationally optimized cone pattern was used, the average weighted reflectivity was less than 0.5%, for incident angles of up to 40° off normal. The broadband antireflection of a cone pattern was reproduced well by a graded refractive index film model corresponding to its geometry, with the addition of a diffraction effect resulting from its periodicity. The broadband antireflection ability of optimized cone patterns is not limited to the glass material, but rather is generically applicable to other semiconductor materials, including Si and GaAs. The design rules developed herein represent a key step in the development of light-absorbing devices, such as solar cells.

Artificial dispersion of all-dielectric gradient nanostructures: Frequency-selective interfaces and tunneling-assisted broadband antireflection coatings

The non-locality of optical properties of gradient dielectric nanofilms, stipulated by smooth spatial distributions of refractive index, is shown to create the peculiar plasma-like dispersion of non-polar dielectric films, determined by the shapes and sizes of these distributions. Gradient all-dielectric nanostructures, characterized by the artificial heterogeneity-induced nonlocal dispersion and providing the broadband antireflection tunneling regime of energy transport in the visible and infrared ranges, are designed and tested. The wave energy flow in these structures is supported due to interference of evanescent and antievanescent modes formed by the non-Fresnel reflections of these modes on the discontinuities of gradient of refractive index on the boundaries of adjacent nanofilms. The transmittance spectra of these structures in the visible and infrared ranges, characterized by strong dispersion nearby the red edge of visible range, almost constant high transmittance in the near infrared range and weak dependence of tunneling energy flow upon the multilayer structure thickness, are calculated; the experimental verifications of these effects are presented. The perspectives to use the tunneling of light in gradient media for reconsideration of Hartman paradox are shown. Potential of periodical gradient all-dielectric nanostructures for optimized design of optical dispersive elements and broadband antireflection coatings for the visible and IR spectral range, respectively, is discussed.

High refractive index composite for broadband antireflection in terahertz frequency range

In this study, titania–polymer composites with a very high refractive-index tenability and high transparency in the terahertz region were prepared. By controlling the blending ratio of the titania particle, a broad refractive-index tuning range from 1.5 to 3.1 was realized. Then, the composites were used to fabricate antireflective (AR) layers of high-resistivity silicon (HR-Si). By utilizing the thermoplasticity of the titania–polymer composite, a graded-index structure was fabricated via a hot-embossing method. Because of the good refractive-index matching between the composite and the HR-Si substrate, a broadband (0.2–1.6 THz, 7% reflection) AR layer was fabricated.

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.

Broadband antireflection sub-wavelength structure of InGaP/InGaAs/Ge triple junction solar cell with composition-graded SiNx

This work reports a fabrication strategy to improve the antireflective ability of a InGaP/GaAs/Ge triple-junction solar cell, by combining a nano-templating technique and a chemical-synthesis approach. SiH4 and N2 were used as ammonia-free reaction gases in a plasma-enhanced chemical vapor deposition (PECVD) to prepare Si3N4 as an original antireflective coating (ARC) layer with better chemical stability. Composition-graded SiNx was successfully integrated with sub-wavelength structure by modulating SiH4/N2 ratio during PECVD deposition, and followed by a controllable gold-nanoparticle masking technique on top of the solar cell. Finite-difference time-domain solution was employed to simulate and optimize the aspect-ratio of the ARC, under the condition of variable refractive index over a broad wavelength window, and followed by the masking technique to obtain the desired ARC dimension. This enabled a low light reflectance (<10%) over a broad spectral bandwidth (300–1800 nm) for the solar cell with excellent stability, because of the triple advantages of structural optimization, better chemical stability and graded refractive index of the ARC. The solar cell’s performance was tested and showed great competitiveness to those of forefront studies, suggesting the feasibility of the proposed technology.

FDTD Analysis for Devices with Glass Substrates and Its Application to Antireflection Coating on Organic Solar Cells

Organic photovoltaics (OPVs) have been receiving growing interest, because they can provide large area and low cost solar cells for next generation. A power conversion efficiency (PCE) of more than 10 % under AM 1.5 illumination has been reported for the OPVs, although higher level of PCE is required for large-scale commercialization. Since the maximum thickness of light absorbing layer (around 100 nm) is determined by the low carrier mobility of organic semiconductors, it is important to develop antireflection technologies that can enhance light trapping in the thin absorbing layer. A popular method for analyzing light propagation within solar cells is the finite-difference time-domain (FDTD) simulation. In the FDTD method, the electric and magnetic field are spatially discretized on the grid and the temporal evolution of the field intensities is obtained through numerical integration. An advantage of the FDTD method is that it can obtain optical response for a wide wavelength range in a single simulation by Fourier transforming the results obtained by a brief input pulse. However, the FDTD simulation suffers from a considerable increase in calculation time when there is a glass substrate in the light path, which is much thicker than the other material layers within solar cells. To overcome this problem, in this study, we explore a new technique for FDTD simulation, to significantly reduce computational cost in the presence of a thick substrate, and apply this technique to the optical design of moth eye antireflection coatings on OPVs. In the proposed method, the absorption spectrum is averaged to eliminate the interference effect of light passing through a thick glass substrate and reproduce incoherent light addition within the substrate. We compare the proposed averaging method with the other two types of averaging methods, and show that the proposed algorithm gives the most accurate solution. In addition, our method is quite robust and independent of detailed values of control parameters. The validity of the results is checked by the comparison with the results obtained by the characteristic matrix method. In addition, we apply the proposed method to optimize the geometric pattern of moth eye array and maximize light trapping in OPVs. The results show that the combination of the FDTD simulation with our averaging method is quite useful to design broadband antireflection coatings for solar cells having thick glass substrates.

Broadband and omnidirectional antireflection of SiN composite nanostructures–decorated Si surface for highly efficient Si solar cells

SiN composite nanostructures (CNs), composed of SiN nanorods and the underlying SiN film, are formed on Si substrate through SiN deposition, nanosphere lithography, and dry etching. The antireflection performance of Si samples decorated by the SiN CNs with different morphology is experimentally investigated. All the SiN CNs decorated Si samples exhibit antireflection over 300 to 1000 nm and a wide view. Their antireflection performance varies with the height of the nanorods (H) and the thickness of the underneath film (T). A reflectivity of less than 10% over 300 to 1000 nm and an incident angle of 8 deg and 65 deg are achieved in the optimal antireflection structures with H = 240 nm, T = 750 nm and H = 500 nm, T = 300 nm, respectively. Furthermore, antireflection behavior in the SiN CNs decorated Si sample with H = 500 nm, T = 300 nm is compared with that in Si samples decorated by 565- and 60-nm thick SiN film. A weighted reflectance of about 5% is achieved in an SiN CNs decorated Si sample in any incident angle, which is much lower than that in any SiN film coated Si sample. Moreover, such a performance is beyond the limitation of interface reflectivity of Si and SiN materials, and should benefit Si solar cells to simultaneously enhance the absorption and surface passivation. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)

Broadband antireflective coatings based on two-dimensional arrays of subwavelength nanopores

A broadband thin-film antireflective coating (ARC)—a regular array of nanopores in a dielectric substrate—is studied theoretically and experimentally. Tuning the geometrical parameters of the array allows for strong suppression of reflection. For a fused silica substrate, reflectivity is lower than 1% over a range with a relative bandwidth, 60%–70%, reaching 0.05% in the minimum. The underlying physics is the spatial dispersion in the porous medium which enables phase compensation for the partially reflected waves in a broad band. This allows for more broadband antireflection than any flat homogeneous single-layer ARC can provide. Importantly, the studied ARC is universal for any transparent dielectric substrate.