Antireflective silicon nanostructures with hydrophobicity by metal-assisted chemical etching for solar cell applications

We present broadband antireflective silicon (Si) nanostructures with hydrophobicity using a spin-coated Ag ink and by subsequent metal-assisted chemical etching (MaCE). Improved understanding of MaCE, by conducting parametric studies on optical properties, reveals a design guideline to achieve considerably low solar-weighted reflectance (SWR) in the desired wavelength ranges. The resulting Si nanostructures show extremely low SWR (1.96%) and angle-dependent SWR (<4.0% in the range of 0° to 60°) compared to that of bulk Si (SWR, 35.91%; angle-dependent SWR, 37.11%) in the wavelength range of 300 to 1,100 nm. Relatively large contact angle (approximately 102°) provides a self-cleaning capability on the solar cell surface.     

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.     

Enhancement of antireflection property of silicon using nanostructured surface combined with a polymer deposition

Silicon (Si) nanostructures that exhibit a significantly low reflectance in ultraviolet (UV) and visible light wavelength regions are fabricated using a hydrogen etching process. The fabricated Si nanostructures have aperiodic subwavelength structures with pyramid-like morphologies. The detailed morphologies of the nanostructures can be controlled by changing the etching condition. The nanostructured Si exhibited much more reduced reflectance than a flat Si surface: an average reflectance of the nanostructured Si was approximately 6.8% in visible light region and a slight high reflectance of approximately 17% in UV region. The reflectance was further reduced in both UV and visible light region through the deposition of a poly(dimethylsiloxane) layer with a rough surface on the Si nanostructure: the reflectance can be decreased down to 2.5%. The enhancement of the antireflection properties was analyzed with a finite difference time domain simulation method.     

Fabrication of the periodic nanopillar arrays by heat-induced deformation of 2D polymer colloidal monolayer

A strategy based on heat-deformable monolayer polymer (polystyrene) colloidal template and precursor solution-dipping is presented to fabricate large-scale nanopillar arrays. In the method, the appropriate heating time of the polymer colloidal templates is a key factor. Fe₂O₃ and silica hexagonal periodic triangular prism nanopillar arrays with an aspect ratio of about 1.5:1 were successfully fabricated. This route is also applicable for synthesis of other material nanopillar arrays and opens a new way to create patterned 1D nanostructures for applications in sensor arrays, piezoelectric antenna arrays, optoelectronic devices, superhydrophobic and self-cleaning surfaces.     

Tuning the peak position of subwavelength silica nanosphere broadband antireflection coatings

Subwavelength nanostructures are considered as promising building blocks for antireflection and light trapping applications. In this study, we demonstrate excellent broadband antireflection effect from thin films of monolayer silica nanospheres with a diameter of 100 nm prepared by Langmuir-Blodgett method on glass substrates. With a single layer of compact silica nanosphere thin film coated on both sides of a glass, we achieved maximum transmittance of 99% at 560 nm. Furthermore, the optical transmission peak of the nanosphere thin films can be tuned over the UV-visible range by changing processing parameters during Langmuir-Blodgett deposition. The tunable optical transmission peaks of the Langmuir-Blodgett films were correlated with deposition parameters such as surface pressure, surfactant concentration, ageing of suspensions and annealing effect. Such peak-tunable broadband antireflection coating has wide applications in diversified industries such as solar cells, windows, displays and lenses.     

Antireflective coatings prepared by sol–gel processing: Principles and applications

Sol–gel processing is a powerful tool to prepare antireflective (AR) coatings on optical surfaces. In this paper the different strategies to obtain antireflective properties are reviewed: porous λ/4 layers, multilayer interference-type films and index-gradient materials such as “moth eye” structures. The processing of the respective films is described and evaluated; references to respective commercial products on glass substrates are given.AR coatings may have a particularly high importance for transparent ceramics as their index of refraction is significantly higher than that of common glass types. Reflective losses therefore are higher which is especially unpleasant for materials with a yet improvable intrinsic transparency.Recent studies indicate that specific porous λ/4 layers may exhibit pronounced anti-soiling features. Laboratory experiments as well as outdoor exposure tests were used to demonstrate the dust-repellant properties.     

Gold-silicon nanofiber synthesized by femtosecond laser radiation for enhanced light absorptance

In this study, we devised a new concept for the precise nanofabrication of Au-Si fibrous nanostructures using megahertz femtosecond laser irradiation in air and atmospheric pressure conditions. The weblike fibrous nanostructures of Au thin layer on silicon substrate, which are proposed for the application of solar cells, exhibit a specific improvement of the optical properties in visible wavelength. Varying numbers of laser interaction pulses were used to control the synthesis of the nanofibrous structures. Electron microscopy analysis revealed that the nanostructures are formed due to the aggregation of polycrystalline nanoparticles of the respective constituent materials with diameters varying between 30 and 90 nm. Measurement of the reflectance through a spectroradiometer showed that the coupling of incident electromagnetic irradiation was greatly improved over the broadband wavelength range. Lower reflectance intensity was obtained with a higher number of laser pulses due to the bulk of gold nanoparticles being agglomerated by the mechanism of fusion. This forms interweaving fibrous nanostructures which reveal a certain degree of assembly.

PACS

81.05.Zx; 81.07.-b     

Fabrication of Large Area Broadband and Omnidirectional Antireflective Transparent Foils by Roll‐to‐Roll Extrusion Coating

This study investigates the fabrication and performance of broadband and omnidirectional antireflective polymer foils, in the visible spectrum (400–800 nm), consisting of subwavelength inverted moth‐eye structures. The foils are fabricated by a high throughput roll‐to‐roll extrusion coating process allowing structuring on both sides at a rate of 60 m min⁻¹, with web width 45 cm. The highest average transmittance obtained in the visible spectrum is (98 ± 1) %; compared with (92 ± 1) % for the unstructured foil. The antireflective foil shows no significant difference in transmittance between normal incidence and incidence up to at least 60°. The foil performance is also investigated for different depths (Dp ) and shapes of structures. The transmittance initially increases with Dp and reaches a maximum at Dp ≈ 120 nm. For process parameters yielding greater depths, other shape factors also play a critical role in the foil's antireflective properties.

     

Rapid replication and facile modulation of subwavelength antireflective polymer film using injection nanomolding and optical property of multilayer coatings

A rapid, cost-effective and high-throughput process for nanotexturing subwavelength structures with high uniformity using the polycarbonate (PC) is realized via injection nanomolding. The process enables the precise control of nanohole array (NHA) surface topography (nanohole depth, diameter, and periodicity) over large areas thereby presenting a highly versatile platform for fabricating substrates with user-defined, functional performance. Specifically, the optical property of the PC substrates were systematically characterized and tuned through the modulation of the depths of NHA. The aspect ratio submicron holes can be easily modulated and experimentally proven by simply adjusting the molding temperature. The nanotextured depths were reliably fabricated in the range of 200 to 400 nm with a period of approximately 700 nm. The fabricated PC films can reduce the reflectivity from an original bare film of 10.2% and 8.9% to 1.4% and 2.1% with 400-nm depth of nanoholes at the wavelength of 400 and 550 nm, respectively. Compared with conventional moth-like nanostructures with nanopillar arrays with heights adjustable only by an etching process, this paper proposes a facile route with submicron holes to achieve a similar antireflective function, with a significantly reduced time and facile height modulation capability. Furthermore, the effects of multilayer coatings of dielectric and metallic layers on the nanomolded NHA have been performed and potential sensing application is explored.     

Hot embossing of moth eye-like nanostructure array on transparent glass with enhanced antireflection for solar cells

Antireflection (AR) silicon and glass surfaces are necessarily required for solar cells, because a reflective silicon solar cell with a glass covering will reflect a percentage of sunlight. In this work, we demonstrate a universal and scalable net-shape nanofabrication method for broadband nanostructured AR surface on transparent glass, intended for solar cell applications. Moth eye-like glass nanopillars with various diameters were successfully fabricated by a combination of precision hot embossing and ultrasonic vibration demolding process. The morphologies of nanopillars were detected to characterize different profiles formed by glass flow at elevated temperatures. Facile optical experiments were designed and conducted to measure the AR performance at varying wavelengths and angle of incidences and the proposed nanostructures exhibit excellent AR property. Additionally, a feasible optical modeling is developed and compared with the measurement to evaluate the theoretical optical behaviors of glass nanostructures based on their embossed shapes. The inexpensive and environmental hot embossing method with ultrasonic vibration demolding is expected to create AR glass nanostructured surfaces for widespread applications such as solar cells, displays and laser systems.     

Preparation and characterization of transparent fluorocarbon emulsion doped with antimony tin oxide and TiO2 as thermal-insulating and self-cleaning coating

A transparent thermal insulation and self-cleaning coating was prepared from a fluorocarbon emulsion doped with antimony tin oxide (ATO) and anatase TiO₂ nanoparticles. The thermal insulation and self-cleaning properties of the coating film were optimized by adjusting the amount of ATO and anatase TiO₂ nanoparticles in the fluorocarbon emulsion. The fluorocarbon coating containing 2.0 wt% ATO and 0.1 wt% TiO₂ possessed good comprehensive properties of thermal resistance, self-cleaning, weathering resistance, etc. Compared with the blank glass substrate, the mean light transmittance of the coating film only decreased by about 12% in the visible range. The temperature in the chamber covered with the coated glasses decreased 7°C lower than the common glass chamber without coating. The methyl red painted on the coating was completely faded after three days of ultraviolet irradiation, so the coating film exhibited an excellent self-cleaning property. The transparent coating with excellent thermal insulation and good self-cleaning will be developed for a potential building glass paint used for energy saving and environmental protection.     

Fabrication of polymer antireflective coatings by self-assembly of supramolecular block copolymer

We demonstrated a method of fabricating antireflective coatings based on the self-assembly of supramolecular block copolymer formed by polystyrene terminated with carboxyl (PS-COOH) and poly(methyl methacrylate) terminated with amine (PMMA-NH₂) via hydrogen bonding. Different porous films were generated by selectively removing PS-COOH from the spin-coated films with a selective solvent, cyclohexane, under different conditions. The refractive index of such porous film can be tuned from 1.49 down to 1.26 by controlling the thickness of the porous film. For the porous layer with n ˜ 1.26, the light transmittance of the glass about 97.93% was achieved in the visible range (λ ˜ 574 nm). By varying the solution concentration and exposing time in cyclohexane, inhomogeneous three-layered porous films were generated: top and bottom layers with high porosities and the middle layers with lower porosities, respectively. The light transmittance of the glass coated with this inhomogeneous film was about 98.00% in the near-infrared region corresponding to wavelength between 800 and 1400 nm. The wavelength region of the broadband antireflective films with high transmittance more than 99.00% can be fine tuned to 1200-2000 nm with increasing the film thickness.     

Antireflective grassy surface on glass substrates with self-masked dry etching

Although recently developed bio-inspired nanostructures exhibit superior optic performance, their practical applications are limited due to cost issues. We present highly transparent glasses with grassy surface fabricated with self-masked dry etch process. Simultaneously generated nanoclusters during reactive ion etch process with simple gas mixture (i.e., CF₄/O₂) enables lithography-free, one-step nanostructure fabrication. The resulting grassy surfaces, composed of tapered subwavelength structures, exhibit antireflective (AR) properties in 300 to 1,800-nm wavelength ranges as well as improved hydrophilicity for antifogging. Rigorous coupled-wave analysis calculation provides design guidelines for AR surface on glass substrates.     

溶胶-凝胶法制备梯度折射率玻璃涂层的研究

本文讨论了用溶胶-凝胶法制备Na_2O-B_2O_3-SiO_2系统梯度折射率减反射膜的实验方法。这类玻璃膜具有多孔的微观结构,拥有比基玻璃更小的密度和折射率,膜中微孔结构经过热处理和化学处理后,使薄膜的折射率呈现梯度变化,玻璃涂层在宽的波长范围内起到减反射作用。将它应用于太阳能电池系统中可提高电池效率10%。 文中讨论了膜的厚度影响因素。应用热分折、红外吸收光谱研究了凝胶转化为玻璃的过程,应用开口气孔模型讨论了烧结过程.     

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.     

纳米SiO2复合粒子/有机硅低聚物透明超疏水复合涂层的制备及其性能

本文以Stober法制备的胶体SiO2粒子与粉体SiO2粒子结合的SiO2复合粒子在玻璃基底构建粗糙表面,以三乙氧基甲基硅烷（MTES）与正硅酸乙酯（TEOS）为前聚体制备的酸性有机硅低聚物作为粘接剂,使用偶联剂KH540与氟硅烷PFDT进行改性,通过喷涂法在玻璃基底上制备出SiO2复合粒子/酸性有机硅低聚物复合透明超疏水涂层,然后探究SiO2复合粒子、酸性有机硅低聚物、偶联剂KH540以及氟硅烷PFDT对复合涂层的影响。研究表明：当SiO2复合粒子由粒径为110 nm的胶体SiO2粒子与粒径为50 nm的粉体SiO2粒子两种粒子组成,SiO2复合粒子溶液与酸性有机硅稀释液的混合质量比为4:1,添加偶联剂KH540与氟硅烷PFDT的质量比为混合液的1%时,复合涂层在可见光波长范围内透光率可达88%,静态接触角能达155°,在800目砂纸上磨损60 cm后仍能保持超疏水性能,具有良好的自清洁性,为透明超疏水涂层的制备提供一种简便、低成本方案。     

Optical properties of Ni and Cu nanowire arrays and Ni/Cu superlattice nanowire arrays

In this study, Ni and Cu nanowire arrays and Ni/Cu superlattice nanowire arrays are fabricated using standard techniques such as electrochemical deposition of metals into porous anodic alumina oxide templates having pore diameters of about 50 nm. We perform optical measurements on these nanowire array structures. Optical reflectance (OR) of the as-prepared samples is recorded using an imaging spectrometer in the wavelength range from 400 to 2,000 nm (i.e., from visible to near-infrared bandwidth). The measurements are carried out at temperatures set to be 4.2, 70, 150, and 200 K and at room temperature. We find that the intensity of the OR spectrum for nanowire arrays depends strongly on the temperature. The strongest OR can be observed at about T = 200 K for all samples in visible regime. The OR spectra for these samples show different features in the visible and near-infrared bandwidths. We discuss the physical mechanisms responsible for these interesting experimental findings. This study is relevant to the application of metal nanowire arrays as optical and optoelectronic devices.     

Nanorainforest solar cells based on multi-junction hierarchical p-Si/n-CdS/n-ZnO nanoheterostructures

Solar cells based on one-dimensional nanostructures have recently emerged as one of the most promising candidates to achieve high-efficiency solar energy conversion due to their reduced optical reflection, enhanced light absorption, and enhanced carrier collection. In nature, the rainforest, consisting of several stereo layers of vegetation, is the highest solar-energy-using ecosystem. Herein, we gave an imitation of the rainforest configuration in nanostructure-based solar cell design. Novel multi-layer nanorainforest solar cells based on p-Si nanopillar array/n-CdS nanoparticles/n-ZnO nanowire array heterostructures were achieved via a highly accessible, reproducible and controllable fabrication process. By choosing materials with appropriate bandgaps, an efficient light absorption and enhanced light harvesting were achieved due to the wide range of the solar spectrum covered. Si nanopillar arrays were introduced as direct conduction pathways for photon-generated charges' efficient collection and transport. The unique strategy using PMMA as a void-filling material to obtain a continuous, uniform and low resistance front electrode has significantly improved the overall light conversion efficiency by two orders of magnitude. These results demonstrate that nanorainforest solar cells, along with wafer-scale, low-cost and easily controlled processing, open up substantial opportunities for nanostructure photovoltaic devices.     

A simple and fast fabrication of a both self-cleanable and deep-UV antireflective quartz nanostructured surface

Both self-cleanability and antireflectivity were achieved on quartz surfaces by forming heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane self-assembled monolayer after fabrication of nanostructures with a mask-free method. By exposing polymethylmethacrylate spin-coated quartz plates to O₂ reactive ion etching (RIE) and CF₄ RIE successively, three well-defined types of nanopillar arrays were generated: A2, A8, and A11 patterns with average pillar widths of 33 ± 4 nm, 55 ± 5 nm, and 73 ± 14 nm, respectively, were formed. All the fabrication processes including the final cleaning can be finished within 4 h. All nanostructured quartz surfaces exhibited contact angles higher than 155° with minimal water droplet adhesiveness and enhanced transparency (due to antireflectivity) over a broad spectral range from 350 to 900 nm. Furthermore, A2 pattern showed an enhanced antireflective effect that extends to the deep-UV range near 190 nm, which is a drawback region in conventional thin-film-coating approaches as a result of thermal damage. Because, by changing the conditions of successive RIE, the geometrical configurations of nanostructure arrays can be easily modified to meet specific needs, the newly developed fabrication method is expected to be applied in various optic and opto-electrical areas.PACS codes: 06.60.Ei; 81.65.Cf; 81.40.Vw.     

Enhanced Optical Transmittance by Reduced Reflectance of Curved Polymer Surfaces

Subwavelength nanostructure arrays on surfaces improve their optical transmittance by reducing the reflection of light over a wide range of wavelengths and angles of incidence. A method to imprint a sub‐100 nm nanostructure array on a large surface (Ø 20 mm) made from thermoplastic materials is reported. Transmittance through the flat polymer is improved by ≈6.5%, reaching values of up to 97.5%, after imprinting. The optical properties of the nanostructured samples are highly reproducible. After eight repeated imprinting operations with the same stamp, the transmittance of the nanostructured surface is decreased by less than 0.2%. Moreover, the nanostructures can also be imprinted on curved polymethylmethacrylate surfaces, achieving a maximum transmittance of 97%. This method to prepare large‐scale antireflective nanostructures on flat and flexible curved polymer surfaces is of interest for the production of antireflective screens, optical devices, and biomedical devices such as contact lenses and intraocular lenses.