Moth-eye Nanostructures Research Articles

Fabrication of moth-eye gold nanostructures by nanoimprint lithography using solvent-permeable porous cross-link molds derived from hydroxypropyl-cyclodextrin

This study aims to expand the possibility of nanoimprint lithography by fabricating a high-accuracy gold (Au) nanostructure, which holds great potential in broad functional applications, using solvent-permeable porous cross-linking molds derived from hydroxypropyl-cyclodextrin. Multiple functional Au moth-eye nanostructures were imprinted 30 times using Au nanopaste, which involved volatile dilution solvents in the adjusted nanoimprint processes. The proposed method using solvent-permeable porous cross-link molds having high oxygen transmission rate could avoid the nanopattern failure due to the trapping of solvent and/or gas in the solvent-impermeable molds.

Fabrication of Multiscale-Structure Wafer-Level Microlens Array Mold

The design and manufacture of cost-effective miniaturized optics at wafer level, usingadvanced semiconductor-like techniques, enables the production of reduced form-factor cameramodules for optical devices. However, suppressing the Fresnel reflection of wafer-level microlensesis a major challenge. Moth-eye nanostructures not only satisfy the antireflection requirementof microlens arrays, but also overcome the problem of coating fracture. This novel fabricationprocess, based on a precision wafer-level microlens array mold, is designed to meet the demandfor small form factors, high resolution, and cost effectiveness. In this study, three different kinds ofaluminum material, namely 6061-T6 aluminum alloy, high-purity polycrystalline aluminum, and purenanocrystalline aluminum were used to fabricate microlens array molds with uniform nanostructures.Of these three materials, the pure nanocrystalline aluminum microlens array mold exhibited auniform nanostructure and met the optical requirements. This study lays a solid foundation for theindustrial acceptation of novel and functional multiscale-structure wafer-level microlens arrays andprovides a practical method for the low-cost manufacture of large, high-quality wafer-level molds.

Multi-physics coupling effects of nanostructure characteristics on the all-back-contact silicon solar cell performances

Nanostructured front surface is an important method to improve the photovoltaic performances. Different nanostructure characteristics, which include the reflectance of nanostructure at a wavelength of 0.35–1.1 μm (Ra,0.35–1.1), the polarization sensitivity, the reflectance of nanostructure at a wavelength of 1.1–2.5 μm (Ra,1.1–2.5), and the surface area enhancement ratio (Ar), have different effects on photovoltaic performances. Therefore, the investigation of the coupling effects of the nanostructure characteristics on photovoltaic performances is an important way to optimize nanostructures. However, there is multi-physics coupling problem in this investigation. Hence, a multi-physics mathematical model is developed and applied in the physical model of an all-back-contact silicon solar cell with nanostructured front surface. Three types of nanostructures are chosen in this study. The grating with rectangle section and moth-eye nanostructures have their own advantages (easy processing and excellent anti-reflection). By combining the advantages of the two types of nanostructures, the grating with parabola section is proposed. The dimensions of the three nanostructures are determined by the height H, the bottom width (diameter), and the spacing L between the two adjacent nanostructures. Through analyzing nanostructure characteristics of each type of nanostructure with 22,386 different dimensions, it is found the grating with parabola section nanostructure not only has relatively lower Ra,0.35–1.1, but also has an advantage in the variation trend of the Ra,0.35–1.1 due to the polarization sensitivity. In addition, its Ar is the lowest, and it is also not sensitive to the variation of the dimension as same as its Ra,1.1–2.5. In this case, comparing to moth-eye with excellent anti-reflection, the grating with parabola section nanostructure not only has an absolute advantage in open circuit voltage and fill factor due to temperature, but also has comprehensive advantage in short circuit current, which make it have best performance in maximum output power density. Based on the analyses, a clear optimization proposal for nanostructures is proposed, and in the end, its effectiveness is verified in the actual environment through dynamic analysis.

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°.

Antireflective and self-cleaning glass with robust moth-eye surface nanostructures for photovoltaic utilization

Omnidirectional and broadband antireflection (AR) self-cleaning coatings are crucial for the performance enhancement of photovoltaic (PV) modules. Herein, we demonstrate biomimetic moth-eye structures on glass through inductively coupled plasma (ICP) etching process using thermal dewetted copper nanoparticles as the masks. Moreover, the etched glasses are chemically treated to strengthen the nanostructures. The moth-eye glass exhibits excellent AR effects with a reflectance below 3% over the wavelength range from 570 to 950 nm. As the incident angles of the light increasing from 0o to 60o, the relative improvement of the power conversion efficiency (PCE) of PV module with moth-eye glass produces a dramatically increases from 4.6% to 9.9%, compared with that of PV modules with flat glass. Additionally, the moth-eye glass also possesses superhydrophilic self-cleaning and antifogging abilities, which are attractive for outdoor applications.

Theoretical perspective to light outcoupling and management in perovskite light-emitting diodes

Despite the rapid development of high-efficiency perovskite light-emitting diodes (PeLEDs) via material design and morphology control, a comprehensive evaluation of the power loss among various optical modes has seldom been conducted for the precise design of device architectures. Here, a quantitative analysis of light outcoupling efficiency (ηout) of PeLEDs is presented with the theoretical simulations by taking into account the key factors of refractive index and film thickness of various functional layers, and the emitter dipole orientation. For a planar PeLED, a rational design to enhance the ηout is predicted by constructing multilayer stack in series of transparent electrode/high-index transport layer/perovskite emitter/low-index transport layer/reflective electrode. Furthermore, the role of photonic structures on the outcoupling management is explored, and the potential of a substantial increase in ηout is demonstrated with an enhancement factor of over 1.75 by implementing a moth-eye nanostructure into the emitter. We anticipate that the findings shown here will provide the design guidelines for the optical optimization of PeLEDs.

Toward ultra-low reflectance semi-transparent organic photovoltaic cells with biomimetic nanostructured transparent electrode

Abstract Semi-transparent organic photovoltaic cells (STOPVs) hold the great potential as a solar conversion source installed on the glass curtain walls. The cell performance of STOPVs is to some extent balanced among the absorption, transmission and reflection of the incident solar radiation. For the practical application with a defined transparency, the efficiency of STOPVs can be optimized by maximizing the light absorption as well as minimizing the reflection that contribute to the light pollution. Herein, a low-reflectance STOPV is presented with relatively high efficiency, high transmittance, and suitable color rendering property. A 15.9% increase in photocurrent and a 28% decrease in reflection are realized with the implementation of biomimetic moth-eye nanostructures into the dielectric-metal-dielectric transparent electrodes. For an optimized PTB7:PC71BM-based STOPV on flexible plastic substrate, the average transmittance in the visible region is kept at 46.1% and the average reflectance is reduced to 10.3%, leading a power conversion efficiency of 3.4%. The pathway described here is promising for opening up opportunities of reliable STOPVs towards the future commercial applications.

Multifunctional Moth-Eye TiO2/PDMS Pads with High Transmittance and UV Filtering.

This work reports a facile fabrication method for constructing multifunctional moth-eye TiO2/polydimethylsiloxane (PDMS) pads using soft nano-imprinting lithography and a gas-phase-deposited thin sacrificial layer. Mesoporous TiO2 nanoparticles act as an effective UV filter, completely blocking high-energy UVB light and partially blocking UVA light and forming a robust TiO2/PDMS composite pad by allowing the PDMS solution to easily fill the porous TiO2 network. The paraboloid-shaped moth-eye nanostructures provided high transparency in the visible spectrum and also have self-cleaning effects because of nanoroughness on the surface. Furthermore, we successfully achieved a desired multiscale-patterned surface by partially curing select regions using TiO2/PDMS pads with partial UVA ray blockers. The ability to fabricate multifunctional polymeric pads is advantageous for satisfying increasing demands for flexible and wearable electronics, displays, and solar cells.

Al2O3 Encapsulated Teflon Nanostructures with High Thermal Stability and Efficient Antireflective Performance.

Scientific advancement is highly inspired and imitative of natural phenomenon's, which exhibits extremely developed and well-organized nanostructures to cope with challenges under different environmental circumstances, such as moth eyes protuberances for efficient antireflective (AR) performance. Innovative researches have been performed in the past to exterminate the undesirable reflectance in common optical components and optoelectronic industrial applications by biomimetic and replicating moth eye nanostructures creating gradient effect using metal oxides, composites, or polymers in multilayer AR coatings. However, in few multilayer AR designs, the properties mismatch at interfaces, high cost, low mechanical durability, wetting issues, or thermal stability bounds their practical applicability. Herein, we develop an approach for fabricating efficient, high-performance Teflon (polytetrafluoroethylene [PTFE]) AR nanostructures for glass-based supporting materials. Nanotailoring, the morphology and structure of PTFE, have been efficaciously carried out for fabricating high-performance AR coatings according to predicted optical simulation. The total reflectance from polymer AR coating lessens to <0.05% in a visible wavelength range which according to our best knowledge seems to be the superior AR performance by a polymer coating ever reported. Furthermore, the fabricated polymer AR coatings are omnidirectional, mechanically durable, and thermally stable up to 200 °C. Moreover, we modify and tune the refractive index of PTFE from 1.34 to 1.156 by inducing porosity and changing deposition angle.

Inverse Moth Eye Nanostructures with Enhanced Antireflection and Contamination Resistance.

Moth-eye-inspired nanostructures are highly useful for antireflection applications. However, block copolymer micelle lithography, an effective method to prepare moth eye nanopillars, can only be used on a limited choice of substrates. Another drawback of nanopillar substrates is that contamination is easily absorbed, thereby reducing transmittance. The production of antireflective surfaces that are contamination-resistant or that can be cleaned easily without the loss of optical properties remains challenging. Here, we describe an approach for creating inverse moth eye nanostructures on other optical substrates than the most commonly used fused silica. We demonstrate its feasibility by fabricating a borosilicate substrate with inverse nanostructures on both sides. The etching of nanoholes on both sides of the substrate improves its transmittance by 8%, thereby surpassing the highest increase of transmittance yet to be obtained with nanopillars on fused silica. More importantly, the substrate with inverse moth eye nanostructures is more robust against contaminations than the substrates with nanopillars. No significant decrease in performance is observed after five cycles of repeated contamination and cleaning. Our approach is transferable to a variety of optical materials, rendering our antireflection nanostructures ideal for applications in touch devices such as touch screens and display panels.

Fabrication of flexible silver nanowire conductive films and transmittance improvement based on moth-eye nanostructure array

Transparent conductive electrodes (TCEs) are widely used in optoelectronic devices, such as touch screens, liquid-crystal displays and light-emitting diodes. To date, the material of the most commonly used TCEs was indium-tin oxide (ITO), which had several intrinsic drawbacks that limited its applications in the long term, including relatively high material cost and brittleness. Silver nanowire (AgNW), as one of the alternative materials for ITO TCEs, has already gained much attention all over the world. In this paper, we reported a facile method to greatly enhance the transmittance of the AgNW TCEs without reducing the electrical conductivity based on moth-eye nanostructures, and the moth-eye nanostructures were fabricated by using a roll-to-roll ultraviolet nanoimprint lithography process. Besides, the effects of mechanical pressure and bending on the moth-eye nanostructure layer were also investigated. In the research, the optical transmittance of the flexible AgNW TCEs was enhanced from 81.3% to 86.0% by attaching moth-eye nanostructures onto the other side of the flexible polyethylene terephthalate substrate while the electrical conductivity of the AgNW TCEs was not sacrificed. This research can provide a direction for the cost-effective fabrication of moth-eye nanostructures and the transmittance improvement of the flexible transparent electrodes.

Multi-scale investigation on the absorbed irradiance distribution of the nanostructured front surface of the concentrated PV-TE device by a MC-FDTD coupled method

Photovoltaic-thermoelectric (PV-TE) hybrid device is one of the most representative ways for the full-spectrum solar energy utilization. The concentrator and nanostructured front surface have become important approaches to improve the conversion efficiency of the PV-TE device by enhancing the solar energy absorption. However, the concentrator causes badly non-uniform absorbed irradiance distribution of the PV-TE device surface, which has a great influence on the conversion efficiency of the PV-TE device. In addition, due to the multi-scale problem, it is hard to study the combined effects of the concentrator and nanostructured surface on the absorbed irradiance distribution of the PV-TE hybrid device surface. In this paper, a 3D model for a concentrated PV-TE hybrid system that employs the PV-TE hybrid device with moth-eye nanostructures and a linear Fresnel reflective solar concentrator is established. For the multi-scale problem, a Monte Carlo-Finite Difference Time Domain (MC-FDTD) coupled method is presented. At first, three parameters including duty ratio, height, and diameter are used to analyze the influences of the moth-eye nanostructure dimension on the reflectance. Then, taking the effects of the sun shape, the slope error, and the polarization of incident electromagnetic wave into considerations, the comparison and analysis on the absorbed irradiance distributions of PV-TE surface under four different conditions (with plane surface, with nanostructured front surface, with concentrator and plane surface, with concentrator and nanostructured front surface) were conducted by the MC-FDTD coupled method. Eventually, based on the precious investigation, a novel way of using different dimensional nanostructures is proposed to improve the uniformity of the absorbed irradiance distribution. As a result, this approach not only improve the uniformity of absorbed irradiance distribution very well, but also can let the mean absorbed irradiance be raised 1.6 times to reach 7644.14W/m2 compared to plane surface.

Multi-scale Investigation on the Optical Performance of a Concentrated Photovoltaic Thermoelectric Hybrid System by a MC-FDTD Coupled Method

Abstract Taking photovoltaic-thermoelectric (PV-TE) hybrid system with the full-spectrum solar energy utilization as application background, a 3D model for a PV-TE hybrid system that employs nanostructure surface and a linear Fresnel reflective solar concentrator was developed in this paper. In this model, a Monte Carlo-Finite Difference Time Domain (MC-FDTD) coupled method is presented to solve the multiscale optical problems. On the premise of taking the effects of the sun shape, the slope error and the polarization of incident electromagnetic wave into consideration, the combined effects of the concentrator and nanostructure surface on the absorbed irradiance distribution of the hybrid system surface were studied. Three parameters of the dimensions for the moth-eye nanostructure, including duty ratio, height and diameter, are optimized, and the optimized duty ratio is 100%, the height is 0.9 μm and the diameter is 0.3 μm. The absorbed irradiance of the moth-eye surface with concentrator is increased at least fourteen-fold compared to plane surface without concentrator. The results from present work can offer a reference for the study supplies theory, means and data bases for the optimization of the nanostructure surface, homogenization of the absorbed irradiance distribution and efficient utilization of the solar energy.

Optimization and continuous fabrication of moth-eye nanostructure array on flexible polyethylene terephthalate substrate towards broadband antireflection.

Reflection loss can cause harmful effects on the performance of optoelectronic devices, such as cell phones, notebooks, displays, solar cells, and light-emitting diode (LED) devices. In order to obtain broadband antireflection (AR) properties, many researchers have utilized surface texture techniques to produce AR subwavelength structures on the interfaces. Among the AR subwavelength structures, the moth-eye nanostructure is one of the most promising structures, with the potential for commercialization in the near future. In this research, to obtain broadband AR performance, the optimization of moth-eye nanostructures was first carried out using the finite difference time domain method within the spectral ranges of 400-800nm, including the optimization of shape, height, pitch, and residual layer thickness. In addition, the continuous production of moth-eye nanostructure array upon a flexible polyethylene terephthalate substrate was demonstrated by using the roll-to-roll ultraviolet nanoimprint lithography (R2R UV-NIL) process and anodic aluminum oxide mold, which provided a solution for the cost-effective fabrication of moth-eye nanostructure array. The AR performance of moth-eye nanostructure array obtained by the R2R UV-NIL process was also investigated experimentally, and good consistence was shown with the simulated results. This research can provide a beneficial direction for the optimization and cost-effective production of the moth-eye nanostructure array.

Flexible silver-mesh electrodes with moth-eye nanostructures for transmittance enhancement by double-sided roll-to-roll nanoimprint lithography

A double-sided R2R NIL system is established and the novel Ag-mesh electrodes with moth-eye nanostructures have been fabricated. An increase of 4.5% in transmittance has been achieved while remaining the sheet resistance at 22.8 ± 1.3 Ω sq−1.

Optimized moth-eye anti-reflective structures for As2S3 chalcogenide optical fibers.

We computationally investigate moth-eye anti-reflective nanostructures imprinted on the endfaces of As2S3 chalcogenide optical fibers. With a goal of maximizing the transmission through the endfaces, we investigate the effect of changing the parameters of the structure, including the height, width, period, shape, and angle-of-incidence. Using these results, we design two different moth-eye structures that can theoretically achieve almost 99.9% average transmisison through an As2S3 surface.

Wide angle light collection with ultralow reflection and super scattering by silicon micro-nanostructures for thin crystalline silicon solar cell applications

Conventional c-Si solar cells employ micron-sized pyramids for achieving reduced reflection (∼10%) and enhanced light trapping by multiple bounces (maximum 3) of the incident light. Alternatively, bio-mimetic, moth-eye sub-wavelength nanostructures offer broadband antireflection properties (∼3%) suitable for solar cell applications in the optical regime. However, such structures do not provide any advantage in the charge carrier extraction process as radial junctions cannot be formed in such sub-wavelength dimensions and they have high surface area causing increased charged carrier recombination. The choice of the geometry for achieving optimum photon–electron harvesting for solar applications is therefore very critical. Cross-fertilization of the conventional solar cell light-trapping techniques and the sub-wavelength nanostructures results in unique micro-nanostructures (structures having sub-wavelength dimensions as well as dimensions of the order of few microns) which provide advanced light management capabilities along with the ability of realizing radial junctions. It is seen that an ultralow reflection along with wide angle light collection is obtained which enables such structures to overcome the morning, evening and winter light losses in solar cells. Further, super-scattering in the structures offer enhanced light trapping not only in the structure itself but also in the substrate housing the structure. Ray and wave optics have been used to understand the optical benefits of the structures. It is seen that the aspect ratio of the structures plays the most significant role for achieving such light management capabilities, and efficiencies as high as 12% can be attained. Experiments have been carried out to fabricate a unique micro-nanomaze-like structure instead of a periodic array of micro-nanostructures with the help of nanosphere lithography and the MacEtch technique. It is seen that randomized micro-nanomaze geometry offers very good antireflection properties (∼1%) which are close to the expected optical behaviour of the periodic array at a much lower surface area.

Multiscale Micro-Nano Nested Structures: Engineered Surface Morphology for Efficient Light Escaping in Organic Light-Emitting Diodes.

Various micro-to-nanometer scale structures are extremely attractive for light escaping in organic light-emitting diodes. To develop and optimize such structures, an innovative approach was demonstrated for the first time to fabricate multiscale micro-nano nested structures by photolithography with a well-designed mask pattern followed by a controllable thermal reflow process. The experimental and theoretical characterizations verify that these unique nested structures hold the capability of light concentration, noticeable low haze, and efficient antireflection. As a proof-of-concept, the incorporation of this pattern onto the glass substrate efficiently facilitates light escaping from the device, resulting in current efficiency 1.60 times and external quantum efficiency 1.63 times that of a control flat device, respectively. Moreover, compared to a hexagonally arranged microlens array and quasi-random biomimetic moth eye nanostructures, the nested structures proposed here can magically tune the spatial emission profile to comply with the Lambertian radiation pattern. Hence, this novel structure is expected to be of great potential in related ubiquitous optoelectronic applications and provide scientific inspiration to other novel multiscale micro-nanostructure research.

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.

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

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