Moth-eye Antireflection Research Articles

Improve anti-glare ability of helicopters based on the anti-reflective structure of bionic moth eyes

This thesis focuses on the issue of glare during helicopter night navigation, which is caused by the high reflectivity of cockpit instruments, displays, and windshield surfaces. This can result in pilot eye fatigue and pose a serious threat to flight safety. To address this problem, a bionic moth eye antireflection structure is designed and prepared on the windshield glass surface using a combination of self-assembly and reactive ion beam technology. The aim is to reduce the surface reflectivity of the windshield glass and prevent glare. The anti-reflective structure comprises of conical structures with a size cycle of 15 nm and a structure height of 2000 nm. The surface of both sides is microstructured, resulting in a reduction of surface reflectivity from 8% to 0.5% at the wavelength of 300 nm to 800 nm. The passing rate is increased from 92% to 99.5%, and the applicable angle is greater than 50°). The anti-glare ability is improved by about 16 times. This thesis proposes a solution to the problem of glare during helicopter night navigation by designing and implementing a bionic moth eye antireflection structure on the windshield glass surface using self-assembly and reactive ion beam technology. The results show a significant reduction in surface reflectivity and improved anti-glare ability, which can contribute to enhancing flight safety.

Numerical Study on Overcoming the Light-Harvesting Limitation of Lead-Free Cs2AgBiBr6 Double Perovskite Solar Cell Using Moth-Eye Broadband Antireflection Layer

Lead-free Cs2AgBiBr6 double perovskite has emerged as a promising new-generation photovoltaic, due to its non-toxicity, long carrier lifetime, and low exciton binding energies. However, the low power conversion efficiency, due to the high indirect bandgap (≈2 eV), is a challenge that must be overcome and acts as an obstacle to commercialization. Herein, to overcome the limitations through the light trapping strategy, we analyzed the performance evaluation via FDTD simulation when applying the moth-eye broadband antireflection (AR) layer on top of a Cs2AgBiBr6 double perovskite cell. A parabola cone structure was used as a moth-eye AR layer, and an Al2O3 (n: 1.77), MgF2 (n: 1.38), SiO2 (n: 1.46), and ZnO (n: 1.9) were selected as investigation targets. The simulation was performed assuming that the IQE was 100% and when the heights of Al2O3, MgF2, SiO2, and ZnO were 500, 350, 250, and 450 nm, which are the optimal conditions, respectively, the maximum short-circuit current density improved 41, 46, 11.7, and 15%, respectively, compared to the reference cell. This study is meaningful and innovative in analyzing how the refractive index of a moth-eye antireflection layer affects the light trapping within the cell under broadband illumination until the NIR region.

Optical enhancement in perovskite thin films using moth-eye anti-reflection film

Perovskite solar cells represent an emerging photovoltaic technology. With the latest record efficiencies exceeding 25% coupled with low fabrication cost and tunable transparency, perovskite solar cells demonstrate promising applications as solar windows. For these applications, the perovskite solar cells need to be semi-transparent to allow incident light to pass through the windows while generating electricity. However, semi-transparency compromises light absorption in the solar cells. In this work, optical enhancement in perovskite thin films using moth-eye anti-reflection (AR) film is investigated. Perovskite thin films with different thicknesses are used and the thickness is controlled by varying the spin speeds during the deposition of the perovskite precursor. The optical effects in the perovskite films without and with moth-eye AR film are studied. From the findings, the reflection loss is reduced by maximum of ∼4%, which contributes to a higher light absorption in the perovskite layer, due to the AR effect of the moth-eye structure. These results imply that the moth-eye film can be a promising and facile alternative for developing a highly efficient and semi-transparent perovskite solar cell with a thin absorber layer.

Spectrally selective antireflection of nanoimprint lithography-formed 3D spherical structures on film coated with a silver layer

We fabricate moth-eye antireflection (AR) coatings using high-resolution and low-cost UV nanoimprint lithography with polyethylene terphthalate (PET) molds. Several various thicknesses of silver films placed on the moth-eye structure were analyzed for reflectance and transmission. On PET, the conical nanostructured surface arrays had a spatial period length of approximately 250 nm, a diameter of approximately 200 nm, and a height of approximately 160 nm. After them, a silver (Ag) layer of 18 nm is deposited satisfactorily on the PET substrate surface. The never-ending moth-eye formations of the imprinted mold were fabricated by Ni mold electroplating, interference lithography, and replication. We found that an Ag layer of suitable thickness on AR film in the spectrum range that can be seen has high transmittance (Highest value is 72%) while in the infrared spectrum it has high reflectance (At least 60%). For an optical film with a silver coating has been placed on an anti-reflection subwavelength-structured (ASS) surface, such properties, including heat insulation, have obvious applications in windows for homes and vehicles.

Omnidirectional and broadband photon harvesting in self-organized Ge columnar nanovoids

Highly porous Germanium surfaces with uniformly distributed columnar nanovoid structures are fabricated over a large area (wafer scale) by large fluence Sn+ irradiation through a thin silicon nitride layer. The latter represents a one-step highly reproducible approach with no material loss to strongly increase photon harvesting into a semiconductor active layer by exploiting the moth-eye antireflection effect. The ion implantation through the nitride cap layer allows fabricating porous nanostructures with high aspect ratio, which can be tailored by varying ion fluence. By comparing the reflectivity of nanoporous Ge films with a flat reference we demonstrate a strong and omnidirectional reduction in the optical reflectivity by a factor of 96% in the selected spectral regions around 960 nm and by a factor of 67.1% averaged over the broad spectral range from 350 to 1800 nm. Such highly anti-reflective nanostructured Ge films prepared over large-areas with a self-organized maskless approach have the potential to impact real world applications aiming at energy harvesting.

Omnidirectional Current Enhancement From Laminated Moth-Eye Textured Polymer Packaging for Large-Area, Flexible III-V Solar Modules

Epitaxial lift-off (ELO) processes have allowed for cheaper development of mechanically flexible, ultra-thin, and high-efficiency III-V solar cells. ELO solar cells are natural candidates for applications where solar cells must conform to curved surfaces and provide high efficiency and high specific power generation (W/kg). Such examples include power generation for unmanned aerial vehicles, electric vehicles, and portable electrical power. However, when considering these mobile solar applications, large variations in angle of incidence (AOI) that inevitably occur can greatly decrease overall system efficiency due to significant Fresnel reflections. In this article, we demonstrate the integration of moth-eye antireflection nanostructures on the polymer packaging layer of ELO solar cell arrays using a low-cost, colloidal self-assembly process. The moth-eye structures mitigate Fresnel reflections and increase photocurrent generation over all measured angles of incidence relative to ELO solar cell arrays with traditional untextured polymer packaging. The nanostructures survive a commercial lamination procedure, an important criterion that must be met to ensure the feasibility of integration into commercial processing. Outdoor solar characterization measurements are performed and, under direct optical illumination, moth-eye textured solar cell arrays show a maximum I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> enhancement of ~58% at 79° AOI relative to traditional untextured polymer packaged solar cell arrays, and when exposed to both direct and diffuse optical illumination a maximum I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> enhancement of ~23% at 79° AOI is observed.

Terahertz broadband anti-reflection moth-eye structures fabricated by femtosecond laser processing

Anti-reflection (AR) coatings aiming at the reduction of Fresnel reflection losses has come into demand in the terahertz (THz) region. Implementation of such a coating in practice is a difficult task, partially because the broad spectrum of the THz signal is difficult to control. Here, we propose and demonstrate a moth-eye AR structure capable of suppressing reflection losses in the range of 0.3 to 2.5 THz for high-resistivity silicon, resulting in a maximum transmission of 91%. The structure comprises of pyramid-like structures with a height of about 100 μm created on the material surface by femtosecond laser processing. We demonstrate experimentally and theoretically that such micromachining considerably increases transmittance of the silicon in the spectral range of 0.3–2.5 THz. We also demonstrate experimentally that such a structure allows one to improve performance of the THz source based on the LiNbO3 crystal.

Moth-eye antireflection nanostructure on glass for CubeSats

A CubeSat is a type of miniaturized and modular satellite designed for space research or technology demonstration. By filling the unused capacity of major launch vehicles, CubeSats significantly lower the cost of entry to the space. To reduce the energy loss at the solar panel coverglass on CubeSats, the authors investigated the potential of the biomimetic moth-eye antireflection (MEAR) technique which features a gradient refractive index. A novel fabrication process combining nanosphere lithography, reactive-ion etching, and dry oxidation for patterning glass with moth-eye structures was reported that allows for the production of well-defined nanopillars with tunable sidewall profiles. The thermal annealing treatment of colloidal particles was introduced for making MEAR structures for which partial deformation of nanospheres was achieved. The broadband and omnidirectional antireflective performance of fabricated glasses was experimentally demonstrated by using a spectrophotometer and ellipsometer. A significant 10% decrease of reflectance throughout the measured spectral range was observed at a high incident angle of 75°.

Transparent Long-Pass Filter with Short-Wavelength Scattering Based on Morpho Butterfly Nanostructures

We combine the principles of moth-eye antireflection, Bragg scattering, and thin-film interference to design and fabricate a short-wavelength scattering/long-pass filter with sharp cutoff, high transmission of infrared light, and strong reflection of visible light into high angles. Based on the lamellae-edge features on Morpho didius butterfly wings, nanostructures are self-assembled via sequential one-chamber chemical vapor deposition, metal nanoparticle formation, and wet-chemical etching. Finite-element modeling demonstrates strong (>45%) reflection into the first diffracted order for short wavelengths, while retaining >80% transmission for longer wavelengths. Fabricated nanostructures couple more than 50% of reflected light into angles of >10° while enabling broadband long-pass transmission. Such structures have potential applications in light trapping for tandem solar cells, stealth, and signals processing.

Efficiency Enhancement of the Solar Cells through Subwavelength Scaled Biomimiced Moth Eye Patterning using Lumerical Solutions

Recent studies have shown that biomimicry of moth eye structure reduces reflections over a wide range of wavelengths and angles of incidence thereby enabling good absorption at the broadband frequencies. Moth eyes consist of certain bump like nanostructures that ensure that refractive index gradually increases (from 1 at the top to around 3.5 at the bottom) as light travels from air to the optical nerve of the insect. Silicon and Gallium Arsenide (GaAs) have been used as Moth Eye antireflection coatings (ARCs). In the case of Silicon, when the incident irradiation or temperature is high, the temperature of the solar cell increases substantially leading to rapid decrease in efficiency, whereas using GaAs retains efficiency over a wide temperature spectrum.

モスアイ型反射防止フィルムのR to R 製造技術の開発

モスアイ型反射防止フィルムのR to R 製造技術の開発

Enhanced Absorption in Laser-Crystallized Silicon Thin Films on Textured Glass

This paper reports enhanced absorption in laser-crystallized Si thin films on abrade-textured glass. The Si film on textured glass has large high-quality grains of several hundred micrometers in width and up to several centimeters in length. A combination of glass texturing, rear Si texturing, white paint rear reflector, and front moth-eye antireflection foil on a 10-μm-thick Si film (on $\text{SiO}_{x}/\text{SiN}_{x}/\text{SiO}_{x}$ intermediate layer) shows a large broadband absorption enhancement with potential short-circuit current densities up to 29.5 mA/cm2 (26.1% enhancement as compared with a planar reference).

FDTD Analysis for Light Passing Through Glass Substrate and Its Application to Organic Photovoltaics with Moth Eye Antireflection Coating

To improve the performance of organic photovoltaics (OPVs), it is useful to trap light by using the antireflection nanotexture such as moth eye structure. The finite-difference time-domain (FDTD) method is frequently used to analyze the optical properties of nanotexture. However, in the case that FDTD is applied to OPVs, the existence of a glass substrate generates a strong oscillation in optical response, which does not exist in the actual device. To remove such oscillatory components and accurately simulate optical response, we study an FDTD-based computational algorithm, which we call the envelope method. We compare this method with other possible methods, and demonstrate that the envelope algorithm is more accurate for estimating optical response and more robust against parameter variations than the other ones. We also apply this method to analyze the changes in the OPV performance associated with the changes in the properties of moth eye coating.

Light Absorption Enhancement in Laser-Crystallized Silicon Thin Films on Textured Glass

Liquid-phase crystallization of Si films on textured glass by line-focus diode laser produces a material with large high-quality grains of several hundred micrometers in width and up to centimeters in length, similar to the films crystallized on planar glass. A combination of glass texturing, rear Si texturing, white paint rear reflector, and a front moth-eye antireflection foil on a 7-μm absorber results in significant broadband absorption enhancement in the Si film with potential short-circuit current densities in solar cells of $\text{27.8} \;\text{mA/cm}^{\text{2}}$ or 36.3% enhancement compared with a planar reference film without light-trapping features.

Optimized design of moth eye antireflection structure for organic photovoltaics

To improve the power conversion efficacy of organic photovoltaics (OPVs), it is required to design antireflection structures that could realize efficient and broadband light trapping. In this article, we perform global optimization of the textured pattern of moth eye antireflection surfaces to maximize the short-circuit current density (J SC) of OPVs with a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)-based bulk heterojunction. We introduce an optimization algorithm consisting of two steps: in the first step, the simple grid search is conducted to roughly estimate a globally optimal solution, while in the second step, the Hooke and Jeeves pattern search is executed to refine the solution. By combining the optimization algorithm with the optical simulation based on the finite-difference time-domain method, we find the optimal period and height of moth eye array with which the level of J SC can be increased by 9.05% in the P3HT:PCBM-based organic solar cell. We also demonstrate that the optimized moth eye structure can significantly modify the light path at a long wavelength range to strengthen the electric field intensity and enhance energy absorption within the active layer.

Paraboloid Structured Silicon Surface for Enhanced Light Absorption: Experimental and Simulative Investigations.

In this paper, we present an optical model that simulates the light trapping and scattering effects of a paraboloid texture surface first time. This model was experimentally verified by measuring the reflectance values of the periodically textured silicon (Si) surface with the shape of a paraboloid under different conditions. A paraboloid texture surface was obtained by electrochemical etching Si in the solution of hydrofluoric acid, dimethylsulfoxide (DMSO), and deionized (DI) water. The paraboloid texture surface has the advantage of giving a lower reflectance value than the hemispherical, random pyramidal, and regular pyramidal texture surfaces. In the case of parabola, the light can be concentrated in the direction of the Si surface compared to the hemispherical, random pyramidal, and regular pyramidal textured surfaces. Furthermore, in a paraboloid textured surface, there can be a maximum value of 4 or even more by anisotropic etching duration compared to the hemispherical or pyramidal textured surfaces which have a maximum h/D (depth and diameter of the texture) value of 0.5. The reflectance values were found to be strongly dependent on the h/D ratio of the texture surface. The measured reflectance values were well matched with the simulated ones. The minimum reflectance value of ~4 % was obtained at a wavelength of 600 nm for an h/D ratio of 3.75. The simulation results showed that the reflectance value for the h/D ratio can be reduced to ~0.5 % by reducing the separations among the textures. This periodic paraboloidal structure can be applied to the surface texturing technique by substituting with a conventional pyramid textured surface or moth-eye antireflection coating.

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.

蛾の眼の構造に倣うモスアイ型反射防止 フィルムの連続的製造技術の開発

蛾の眼の構造に倣うモスアイ型反射防止 フィルムの連続的製造技術の開発

Self-assembled self-cleaning broadband anti-reflection coatings

Anti-reflection (AR) coatings are widely used in a spectrum of optical and optoelectronic devices, such as monitors, car dashboards, optical lenses, photodiodes, and solar cells. Narrowband quarter-wavelength single-layer and broadband multilayer dielectric AR coatings are typically fabricated by expensive and low-throughput vapor deposition processes (e.g., sputtering). Inspired by the subwavelength-structured cornea of some nocturnal moths, nanostructured broadband moth-eye AR coatings that can significantly suppress optical reflection over a wide range of wavelengths and light incident angles have been extensively exploited by both top-down and bottom-up approaches. Among many available bottom-up technologies, colloidal self-assembly is a promising approach as it is simple, fast, and inexpensive. In this review article, we will discuss two scalable colloidal self-assembly technologies based on Langmuir–Blodgett assembly and spin-coating for fabricating quarter-wavelength and moth-eye AR coatings with unique self-cleaning functionalities on transparent substrates (e.g., glass) and semiconductor wafers (such as crystalline silicon and GaAs).

Moth‐eye antireflection coating fabricated by nanoimprint lithography on 1 eV dilute nitride solar cell

ABSTRACTWe report on the performance of biomimicked antireflection coating applied to dilute nitride solar cell. The coating consists of nanostructures replicating the moth‐eye geometry and has been fabricated by nanoimprint lithography directly within the window layer covering the dilute nitride absorbing junction. The mean reflectivity within the spectral range of 320–1800 nm remains under 5% for incident angles up to 45°. The effect of the coating on the cell performance was assessed by measuring the current–voltage characteristics under simulated solar illumination. A clear performance increase was identified when comparing a solar cell with the moth‐eye coating with a solar cell having a standard SiNx/SiO2 coating. Copyright © 2012 John Wiley &amp; Sons, Ltd.