Antireflective Structures Research Articles

Design Optimization of a Porous Box-Type Breakwater Subjected to Regular Waves

Breakwaters are used to suppress the energy of waves for providing shelter to coastal and offshore facilities. Very often, the conventional rubble mound breakwaters result in high construction cost and several environmental problems, such as water contamination and wave amplification in front of the structures due to severe wave reflection. One way to alleviate the above-mentioned problem is to appropriately increase the porosity of the breakwaters. This paper aims at developing the optimum design of a porous box-type breakwater comprising multiple scrapped pipelines via physical modelling approach. Herein, the best geometrical design of the breakwater under the governing factors of porosity, width and internal tube length is proposed. A series of experiments have been conducted under the influence of regular wave environment through the analysis of wave transmission, reflection and energy loss. Several geometrical design criteria were derived to maximize the hydraulic performance of the breakwater, when adopted at sites. The proposed breakwater is a reasonably good wave attenuator and anti-reflection structure as well as an effective energy dissipator.

Investigation of the properties of a 3-level broadband antireflective structure on silicon by THz time-domain spectroscopy

Investigation of the properties of a 3-level broadband antireflective structure on silicon by THz time-domain spectroscopy

Anti-reflective coating for visible light using a silver nanodisc metasurface with a refractive index of less than 1.0

Suppression of visible-light reflection from material surfaces is an important technology for many applications such as flat-panel displays, camera lenses, and solar panels. In this study, we developed an anti-reflective coating design based on a silver nanodisc metasurface. The effective refractive index of a 10-nm-thick monolayer of silver nanodiscs was less than 1.0, which enabled strong suppression of reflection from the underlying substrate. The nanodisc structure was easy to fabricate using a conventional roll-to-roll wet-coating method. The anti-reflective structure was fabricated over a large area.

A broadband antireflective nanostructure with Ag nanoparticles on SiO2 nanocolumns

A broadband antireflective nanostructure with Ag nanoparticles on SiO2 nanocolumns has been presented. Ag nanoparticles are located at the top of SiO2 nanocolumns which are deposited on Si substrates. SiO2 nanocolumns are fabricated by oblique angle deposition through electron beam evaporation, and Ag nanoparticles are fabricated by thermal evaporation. Experimental results show that the average reflection can reach 3.84% in the range of 400–700 nm and 5.75% in the range of 400–1100 nm, much lower than that of Ag islands on SiO2 thin films. The simulation shows that the broadband low reflection can be attributed to the localized surface plasmon resonance of Ag nanoparticles whose resonance wavelengths depends on the size of Ag nanoparticles. The different diameters of SiO2 nanocolumns determine the size distribution of Ag nanoparticles which resonate in a wide wavelength range and lead to a broadband low reflection. This provides a possible way to fabricate a broadband antireflection structure.

Low reflection Fresnel lenses via double imprint combined with vacuum-UV surface hardening

To improve the optical performance of Fresnel lenses, a technique for preparing them with antireflective structures of the moth-eye type is developed. Masters featuring such hierarchical structures are prepared in SU-8, a negative tone photoresist, by two consecutive thermal imprint steps. The moth-eye structures imprinted first are vacuum ultraviolet-treated at 172 nm to provide a surface-near the cross-linked layer that remains stable during the second imprint of the 100 μm sized Fresnel structures. A successful combination of both structure types is possible at an imprint temperature as low as 45 °C. This can be understood on the basis of the typical exposure and the crosslinking behavior of a chemically amplified negative tone resist like SU-8. The masters prepared in this way will be subjected to extrusion coating, the process of choice for future large area preparation of such structures in a single step.

Multifunctional, angle dependent antireflection, and hydrophilic properties of SiO2 inspired by nano-scale structures of cicada wings

Inspired by the multifunctional properties of cicada wings, we have precisely replicated biomorphic SiO2 with antireflective structures (ARSs) using a simple, inexpensive, and highly effective sol-gel ultrasonic method. The biomorphic replica of SiO2 was directly achieved from a cicada template at high calcination. The biomorphic SiO2 not only inherited the ARS effectively but also exhibited the excellent angle dependent antireflective properties over a wide range of incident angles (10°–60°). The change in reflectance spectra (visible wavelength) of biomorphic SiO2 was observed from 0.3% to 3.3% with the increasing incident angles. The smooth surface of the SiO2 crystal without nanostructures showed a high reflection of 9.2% compared to the biomorphic SiO2 with ARS. These excellent antireflective properties of biomorphic SiO2 can be attributed to the nanoscale structures which introduce a gradient in the refractive index between air and the material surface via ARS. In the meantime, biomorphic SiO2 demonstrates high hydrophilic properties due to the existence of nanostructures on its surface. These multifunctional properties of biomorphic SiO2, angle dependent antireflective properties, and hydrophilicity with high thermal stability may have potential applications in solar cells and antifogging optical materials.

Analysis of random antireflective structures fabricated by silver dewetting to enhance transmission

The random antireflective structures are modeled by the analysis of the random morphology distribution. According to the effective medium theory, the transmission of the antireflective structure is calculated by dividing the structure into multilayer, and the dependence on parameters of the subwavelength is analyzed in detail. In the single-variable condition, etching depth, half breadth of distribution, and median of distribution get a positive correlation with the transmittance where the etching depth plays a most important part in enhancing the transmittance, whereas the angle of structures gets a negative correlation. The experimental results coincide well with the calculation and analysis. The analysis offers a theory guidance to fabricate random subwavelength antireflected structures using metal dewetting.

Enhanced photocatalytic hydrogen production on three-dimensional gold butterfly wing scales/CdS nanoparticles

Abstract Photocatalytic water splitting via utilizing various semiconductors is recognized as a promising way for hydrogen production. Plasmonic metals with sub-micrometer textures can improve the photocatalytic performance of semiconductors via a localized surface plasmon resonance (LSPR) process. Moreover, arrays of multilayer metallic structures can help generate strong LSPR. However, artificial synthesis has difficulties in constructing novel multilayer metallic arrays down to nanoscales. Here, we use three dimensional (3D) scales from Morpho didius forewings (M) to prepare 3D Au-wings with intact hierarchical bio-structures. For comparison, we use Troides helena forewings (T) which are known for their antireflection quasi-honeycomb structures resulting in strong light absorbing ability. Results show that multilayer rib structures of Au-M can significantly amplify the LSPR of 3D Au and thus can efficiently help the photocatalytic process (9-fold increase). This amplification effect is obviously more superior to the straightforward enhancement of the absorption of incident light (Au-T, 5-fold increase). Thus, our study provides the possibility to prepare highly efficient plasmonic photocatalysts (possessing 3D multilayer rib structures) via an easy method. This work will also be revealing for plasmonic applications in other fields.

Nano/micro dual-textured antireflective subwavelength structures in anisotropically etched GaAs.

Light trapping by surface texturing is widely used to improve the performance of optoelectronic devices. In this Letter, we demonstrate nano/micro dual-scale textured GaAs by integrating triangular GaAs by orientation-dependent wet etching and subwavelength nanoholes by metal-assisted chemical etching (MacEtch). This is the first report on nano/micro dual-scale textured GaAs. The reflectance was adjusted by controlling the aspect ratio of the nanoholes by varying the MacEtch duration. The combination of the microstructure and subwavelength structures significantly reduced the solar-weighted reflectance of a bare GaAs substrate by 72%.

Anti-reflective microstructure array and its performance evaluation in thin film flexible solar cells

The anti-reflective (AR) structure greatly reduces the light reflection. When it is applied on solar cells, it enables more light to be absorbed by the cells, increasing the energy of the incident light and improving the light-to-electricity conversion efficiency. In this study, the optical properties of AR microstructures are investigated followed by the performance evaluation of solar cells. The AR microstructure is arrayed in a uniform and periodic fashion. When it is applied on PMMA, only 1.0% of the light is reflected away while 2.6% of the light is reflected on glass. The angular dependence performance is also improved with AR structure with 9.4% more light absorption, which can increase the effective energy generation duration for the solar cell. The AR structure is applied to amorphous silicon thin film solar cells by nano-imprinting technology. The solar cell with AR structure gained 8.63% more power compared to the conventional solar cells.

General Strategy toward Dual-Scale-Controlled Metallic Micro-Nano Hybrid Structures with Ultralow Reflectance.

Functional metal surfaces with minimum optical reflection over a broadband spectrum have essential importance for optical and optoelectronic devices. However, the intrinsically large optical impedance mismatch between metals and the free space causes a huge obstacle in achieving such a purpose. We propose and experimentally demonstrate a general pulse injection controlled ultrafast laser direct writing strategy for fabricating highly effective antireflection structures on metal surfaces. The presented strategy can implement separate and flexible modifications on both microscale frame structures and nanoscale particles, a benefit from which is that optimized geometrical light trapping and enhanced effective medium effect reducing the surface reflection can be simultaneously achieved within one hybrid structure. Thus, comprehensively improved antireflection performances can be realized. Hybrid structures with substantial nanoparticles hierarchically attached on regularly arrayed microcones are generally constructed on different metal surfaces, achieving highly efficient light absorption over ultraviolet to near-infrared broadband spectrum regions. Reflectance minimums of 1.4%, 0.29%, and 2.5% are reached on Cu, Ti, and W surfaces, respectively. The presented strategy is simple in process, adaptable for different kinds of metals, reproduceable in dual-scale structural features, and feasible for large-area production. All these advantages make the strategy as well as the prepared antireflection structures excellent candidates for practical applications.

Rapid fabrication of antireflective pyramid structure on polystyrene film used as protective layer of solar cell

A promising strategy is proposed for facilely and successively replicating randomly arranged pyramids on an etched silicon wafer to polystyrene (PS) surface. First, a nickel mold insert with negative pyramids is fabricated via combining electroless plating and subsequent electroplating with an etched silicon wafer as a template. Then, by using microinjection compression molding with the nickel mold insert as a template, the pyramids on the insert are accurately transcribed to the PS surface. The pyramids on the PS replica surface have the hemlines of about 1–7µm and heights of about 1–4µm. The PS replica surface exhibits a reflectance of about 5% in the wavelength range of 400–900nm and an enhanced transmittance. That is, it exhibits an enhanced light trapping effect, which originates from a combination of staggered arrangement and dense distribution of the pyramids with arc-shaped top and different sizes. Especially, a thin film solar cell covered with the PS replica exhibits increased power conversion efficiency enhancement of up to 7.9% under normal incidence comparing to that covered with the PS counterpart. The solar cell covered with the PS replica also shows an excellent photovoltaic performance over a wide range of incident angles (0–70°). The proposed fast and efficient replication strategy can be an excellent candidate for developing antireflective protective layers without complicated procedures and expensive materials.

Analysis of long-term monitoring data of PV module with SiOx-based anti-reflective patterned protective glass

The PV modules are inevitably installed outdoors and receive direct sunlight. Therefore, protective glass or film is required to protect the surface of the PV module from the wind, rain, snow and dust, and other environments. However, the incident light is reflected from the surfaces of these protective layers, resulting in a considerable efficiency loss. In this study, we developed a moth-eye structure with HSQ using a direct-transfer printing process to fabricate anti-reflective coatings on the protective glass. In addition, we also conducted long-term field tests to monitor and measure the impact of the fabricated anti-reflective structures and durability against environmental factors. The transmittance of the glass substrate increased to 95.0% as a result of the fabricated SiOx-based moth-eye patterns. In addition, the PV module with the moth-eye-patterned protective glass generated 7.88% more electricity compared to that without the moth-eye-patterned glass during 1-year field test.

Preparation of structured surfaces for full-spectrum photon management in photovoltaic-thermoelectric systems

Micro-structured surfaces for broadband and omnidirectional photon management in photovoltaic-thermoelectric systems are fabricated on the silicon wafer by inductively coupled plasma etching with self-assembly polystyrene spheres mask, KOH solution etching and magnetron sputtering method. The Si antireflection structure coated with SiO2 film is fabricated on the top surface of Si substrate to enhance the absorption of the photons with wavelengths of 0.3–1.1µm is increase the electron-hole pairs. The TiO2/SiO2 films as the back-antireflection coating are deposited at the back of Si wafer to improve the transmission of solar energy within the wavelength range of 1.1–2.5µm. The absorbed photons within the wavelength range of 0.3–1.1µm can directly be converted into electricity power by Si solar cells while the photons with wavelengths of 1.1–2.5µm transmit through the cells and are utilized by thermoelectric modules. The effects of etching parameters on the spectral features are investigated to get the optimal etching parameters. With the optimal etching parameters, the surface absorption for the wavelengths of 0.3–1.1µm is significantly increased to above 0.95 due to the effective graded refractive index and the transmission is obviously enhanced for the wavelengths of 1.1–2.5µm. The incident angle-sensitivity and polarization-sensitivity of the fabricated structured surfaces are analyzed. The results indicate that these surfaces can be selected for photovoltaic-thermoelectric systems due to their high omnidirectional absorption and transmission of the photons as well as the favorable polarization-insensitivity within the expected wavelength ranges.

Antireflection subwavelength structures based on silicon nanowires arrays fabricated by metal-assisted chemical etching

In this paper, we have obtained a series of large-area and different diameters nanosphere lithography (NSL) to obtain the required silicon nanowires (SiNWs) arrays. The single-crystalline SiNWs have been presented by combining nanosphere lithography (NSL) and metal-assisted chemical etching (MACE). The period of SiNW arrays can be controlled by adjusting the original diameter of polystyrene nanosphere (PSs) and the etching time during the NSL process. The special SiNWs structure obtained can be demonstrated to be significant for improving the antireflection properties of silicon substrate. The results show that SiNW arrays with various parameters, such as diameter, distance and height can be obtained by controlling the key etching parameter during the MACE process, which are important to obtain the structures of different parameters to adapt an appropriate value to decrease the light scattering. For a wide wavelength range of 300–1200 nm, the reflectance is below 10% or less, which is due to an ultra-high surface area. Especially, the reflectance of antireflection structure (ARS) surface reduces below 1% over a wavelength range of 300–400 nm. Furthermore, the silicon nanowire (SiNW) arrays with highly efficient antireflection obtained by MACE exhibit different surface roughness from the bottom to the top part of SiNWs by high resolution images, which is benefit for further improving the ARS of SiNWs.

Organic Halides and Nanocone Plastic Structures Enhance the Energy Conversion Efficiency and Self-Cleaning Ability of Colloidal Quantum Dot Photovoltaic Devices

This paper presents solid-state ligand exchange of spin-coated colloidal lead sulfide quantum dot (PbS QD) films by methylammonium iodide (MAI) and integration of them in depleted heterojunction solar (DHS) devices having an antireflecting (AR) nanocone plastic structure. Time-resolved photoluminescence measurements determine a shorter lifetime of the charge carries on a semiconductor (TiO2) electron transfer layer for the MAI-passivated QD films as compared with those with long-chain aliphatic or short thiol ligands. Consequently, the DHS device yields improved power conversion efficiency (>125%) relative to oleic-acid-passivated PbS QD films. Using anodized aluminum oxide templates, an inverted nanocone polydimethylsiloxane structure was also prepared and utilized as an AR layer in the DHS device. The solar cells exhibit an energy conversion efficiency of 7.5% with enhanced water-repellant ability.

Broadband Pillar-Type Antireflective Subwavelength Structures for Silicon and Alumina

A broadband antireflective subwavelength structure was developed suitable for large diameter silicon or alumina lenses used in the optical systems of superconducting cameras. In this study, a broadband antireflective subwavelength structure that can be easily fabricated with a dicing blade was designed. The model comprised both a pyramidal section (550 $\mu$ m in depth) and a straight section (150 $\mu$ m in depth) to simplify fabrication with a metal bonded V-shaped blade and a normal straight dicing-blade. The simulated transmittance between 97 and 334 GHz was > $\text{90}\%$ , which is comparable to that with a 700- $\mu$ m-depth tapered structure. The structure was fabricated on a flat surface, and transmittance at cryogenic temperatures was measured using a Fourier transform spectrometer. The measured average transmittance between 186 and 346 GHz was approximately 95 $\%$ , and the experimental result is in good agreement with the simulation results.

Uphill Water Transport on a Wettability-Patterned Surface: Experimental and Theoretical Results.

In nature, there exist many functional water-controlling surfaces, such as the water-repellent surface of lotus leaves, the superhydrophobic water-adhesive surface of rose petals, the water-harvesting surface of a beetle's back, and the water-transporting surface of the legs of Ligia exotica. These natural surfaces suggest that surface chemistry and hierarchical structures are essential for controlling the water behavior. We have reported the preparation of superhydrophobic and antireflection silicon nanospike-array structures using self-organized honeycomb-patterned films as three-dimensional dry-etching masks. Moreover, the surface wettability of the silicon nanospike-array structures can be easily transformed from superhydrophobic to superhydrophilic by changes in the surface chemistry. In this report, we show the preparation of water-controlling surfaces, such as water-harvesting and water-transporting surfaces, by the wettability patterning of silicon nanostructured surfaces. We prepared honeycomb-patterned films for dry-etching masks made from polystyrene and an amphiphilic polymer by casting a chloroform solution. After the fixation of the top layer of the honeycomb-patterned films on a single-crystal silicon substrate, reactive ion etching was performed. The as-prepared silicon nanospike-array structure showed superhydrophobicity, and the water contact angles were over 170°. After UV-O3 treatment with photomasks, only the UV-irradiated surfaces showed superhydrophilicity, suggesting that we can obtain superhydrophobic- and superhydrophilic-patterned surfaces for which the patterns are the same as those of the photomasks. On the basis of these wettability-patterned surfaces, we demonstrated water harvesting by superhydrophilic dot-patterned surfaces and water transportation against gravity by superhydrophilic triangular-patterned surfaces. In particular, we investigated uphill water transport through the motion of droplets on tilting slopes based on the equation of motion. These results suggested that we can obtain superior microfluidic devices suitable for various applications through the use of optional wettability patterns.

Experimental study of the pyramidal texturization on the surface of photovoltaic silicon with cemented carbide micro-milling tool

Some anti-reflection structures can be fabricated on the surface of high-efficiency silicon solar cells. The anti-reflection efficiency can be increased and hence the photoelectric conversion efficiency can be increased. The effects of varied anti-reflection structure are altered on the optical reflectance. The forward stochastic pyramid structure can be fabricated mechanically. Before processing the pyramid structure, a unidirectional V-groove array must be fabricated firstly. The research content of this paper was based on our previous researches. The fabrication of V-groove with different depth on the surface of silicon wafers was explored. Micro-milling tool constituted by cemented carbide was used. The quality of completed V-grooves was measured and analyzed. Then the effect of the structure for the reflectance was evaluated. The calculated reflectance of accomplished V-grooves was compared with that of the theoretical model. A satisfactory value was obtained by contrast with existing technologies. Furthermore, the tools were observed after the processing, to evaluate the tool wear. Results indicated that processing the pyramid structure using the micro-milling tool is feasible.

A simple and low-cost chemical etching method for controllable fabrication of large-scale kinked silicon nanowires

In this paper, large-area and uniform kinked silicon nanowires (SiNWs) in Si(111) have been fabricated by an alternating metal-catalyzed chemical etching (AMCCE) method. The etching direction alternating between [111] and [100] is highly controllable in this system, and the morphology of kinked SiNWs can be customized easily through controlling the alternating frequency. Good anti-reflection properties were proved by UV–vis spectrophotometer, the average reflectance at 200–1100nm is less than 10%, and moreover, the reflectance can be as low as 6% in the visible region (380–780nm), which shows a promising potential application as an anti-reflection structure in the photovoltaic field.