Omnidirectional Antireflection Research Articles

Geometric design of antireflective leafhopper brochosomes

In nature, leafhoppers cover their body surfaces with brochosomes as a protective coating. These leafhopper-produced brochosomes are hollow, buckyball-shaped, nanoscopic spheroids with through-holes distributed across their surfaces, representing a class of deployable optical materials that are rare in nature. Despite their discovery in the 1950s, it remains unknown why the sizes of brochosomes and their through-holes consistently fall within the range of hundreds of nanometers across different leafhopper species. Here, we demonstrate that the hierarchical geometries of brochosomes are engineered within a narrow size range with through-hole architecture to significantly reduce light reflection. By utilizing two-photon polymerization three-dimensional printing to fabricate high-fidelity synthetic brochosomes, we investigated the optical form-to-function relationship of brochosomes. Our results show that the diameters of brochosomes are engineered within a specific size range to maximize broadband light scattering, while the secondary through-holes are designed to function as short-wavelength, low-pass filters, further reducing light reflection. These synergistic effects enable brochosomes to achieve a substantial reduction in specular reflection, by up to approximately 80 to 94%, across a broadband wavelength range. Importantly, brochosomes represent a biological example demonstrating short-wavelength, low-pass filter functionality. Furthermore, our results indicate that the geometries of natural brochosomes may have evolved to effectively reduce reflection from ultraviolet to visible light, thereby enabling leafhoppers to evade predators whose vision spectrum encompasses both ultraviolet and visible light. Our findings offer key design insights into a class of deployable bioinspired optical materials with potential applications in omnidirectional antireflection coatings, optical encryption, and multispectral camouflage.

Method for designing a broadband and omnidirectional hybrid antireflection coating for organic solar cells using the quarter-wavelength rule

The reflection losses are among the principal causes that limiting the performances of the solar cells. Indeed, the conventional organic solar cell (OSC) provides a relatively low photocurrent mainly due to light reflection at the front and back sides of the glass-substrate. To overcome this limitation we propose an optimized hybrid antireflective structure. The proposed design is a combination between multilayer antireflection coating (MARC) and moth eye structure (MES). The OSC with this antireflection coating, consisting of thin coherent multilayer stack and moth eye subwavelength structure, is modeled using transfer matrix method (TMM) and effective medium theory (EMT). In this work, several antireflection coating designs with different dielectric material films are investigated. The layer thicknesses of the MARC were tuned such that they obey to quarter-quarter-quarter (Q-Q-Q) and quarter-half-quarter (Q-H-Q) wavelength rules to obtain zero reflectance. Based on these configurations, we performed an optimization algorithm to design the antireflection coating that maximizes the short circuit photocurrent density (JSC). The optical analysis is applied to ITO/PEDOT:PSS/P3HT:PCBM/Al bulk heterojunction (BHJ) organic solar cell. The highest value of short circuit photocurrent density is obtained for OSC with hybrid MES/Glass-substrate/MARC(QHQ) antireflective structure using Al2O3/ZrO2/M-optm material films. In comparison with the conventional organic solar cell without antireflection coating, the short circuit photocurrent density was improved by 5% at normal incidence. Besides, the antireflection effect is maintained even at large incidence angle of 68° thanks to the omnidirectional optical propriety of the moth eye structure.

Leafhopper-inspired reversibly switchable antireflection coating with sugar apple-like structure arrays

Optical coatings with reversibly tunable antireflective characteristics hold a tremendous potential for next generation optical energy-related applications. Bioinpsired by the camouflage behavior of small yellow leafhoppers, silica hollow sphere/shape memory polymer composites are self-assembled using a non-lithography-based approach. The average visible transmittance of the as-patterned hierarchical structure array-covered substrate is increased by ca. 6.3% at normal incident, and even improved by more than 20% for an incident angle of 75°. Interestingly, the broadband omnidirectional antireflection performance can be reversibly erased and recovered by applying external stimuli under ambient conditions. To gain a better understanding, its reversibility, mechanical robustness, and the structure-shape effect on the antireflective properties are systematically investigated in this research.

Mesoporous Metal Fluoride Nanocomposite Films with Tunable Optical Properties Derived from Precursor Instability.

The precisely tailored refractive index of optical materials is the key to utilizing and manipulating light during its propagation through the matrix, thereby improving their application performances. In this paper, mesoporous metal fluoride films with engineered composition (MgF2 :LaF3 ) are demonstrated to achieve finely tunable refractive indices. These films are prepared using a precursor-derived one-step assembly approach via the simple mixing of precursor solutions (Mg(CF3 OO)2 and La(CF3 OO)3 ); then pores are formed simultaneously during solidification owing to the inherent instability of La(CF3 OO)3 . The mesoporous structures are realized through Mg(CF3 OO)2 and La(CF3 OO)3 ions, which interacted with each other based on their electrostatic forces, providing a wide range of refractive indices (from 1.37 to 1.16 at 633 nm). Furthermore, it is systematically several MgF2(1-x) -LaF3(x) layers with different compositions (x=0.0, 0.3, and 0.5) to form the graded refractive index coating that is optically consecutive between the substrate and the air for broadband and omnidirectional antireflection. An average transmittance of ≈98.03% (400-1100nm) is achieved with a peak transmittance of ≈99.04% (at 571nm), and the average antireflectivity is maintained at ≈15.75% even at an incidence of light of 65° (400-850nm).

Optical design of multilayer antireflection coatings for indoor solar cell applications.

Multilayer antireflection coatings (ARCs) for solar cells are conventionally designed to enhance the photocurrent level obtained at normal incidence. This is mainly because outdoor solar panels are usually placed such that they can receive strong midday sunlight at a nearly vertical angle. However, in the case of indoor photovoltaic devices, the direction of light changes considerably with changes in the relative position and angle between the device and light sources; therefore, it is often difficult to predict the incident angle. In this study, we explore a method to design ARCs suitable for indoor photovoltaics by essentially taking into account the indoor lighting environment, which is different from the outdoor conditions. We propose an optimization-based design strategy that aims to enhance the average level of the photocurrent generated when a solar cell receives irradiance randomly from all directions. We apply the proposed method to design an ARC for organic photovoltaics, which are expected to be promising indoor devices, and numerically compare the resultant performance with that obtained using a conventional design method. The results demonstrate that our design strategy is effective for achieving excellent omnidirectional antireflection performance and allows the realization of practical and efficient ARCs for indoor devices.

Bioinspired antireflective subwavelength nanostructures induced by femtosecond laser for high transparency glass

In nature, the Cicada wing exhibiting a high transparency at wide viewing angles can be bioinspired for the formation of broadband antireflective subwavelength nanostructures (ASS), which can significantly improve the transmittance of optical devices. In this study, a femtosecond laser micro-machining is developed to fabricate a biomimic nanostructure array on optical glass for antireflection. The underlying mechanism for the transmittance enhancement and geometry design of the artificial structure is thoroughly explored by applying a rigorous coupled-wave analysis method both in the visible and the near-infrared band. In addition, the height, diameter, and period of the ASS experimentally induced by the femtosecond laser are adjusted by controlling the laser-processing parameters of pulse energy and scanning velocity. Accordingly, the proposed bioinspired nanostructures featuring a wide-angle omnidirectional antireflection in the visible band, as well as hydrophobicity properties, are anticipated to be widely used in glass displays applications.

Bioinspired omnidirectional antireflective film with mechanical durability for efficient solar energy collection

•The internal structure of the dragonfly ommateum exhibits omnidirectional antireflection •Exquisite biomimetic structures could be prepared via a self-assembly strategy •Biomimetic antireflective film enables efficient omnidirectional antireflection •Biomimetic structure is the key to boosting the photovoltaic performance of solar cells Soaring developments in solar cells raise an urgent demand for superior antireflective (AR) materials to tackle the concern of their inadequate photovoltaic efficiency. Unfortunately, conventional AR films still suffer from a deficiency in omnidirectional antireflection and mechanical durability, significantly weakening their light-harvesting capability. Herein, a scalable biomimetic antireflective film (BARF) with ommateum-inspired multiscale hierarchical architecture was prepared using a method combining layer-by-layer thermal decomposition and in situ self-assembly. Remarkably, the BARF exhibited a transmittance of over 95% in light incident angles from −75° to 75°. Thereby, the output voltage of solar cells coated with BARF increased by 30.48% compared with commercial AR film. Furthermore, the BARF demonstrated excellent mechanical durability in falling-sand tests (∼2% transmittance loss). This work offers a general strategy for mass production of omnidirectional and durable AR films that are suitable for practical applications of protective AR film in solar cells, light pollution treatment, and video stealth technology. Soaring developments in solar cells raise an urgent demand for superior antireflective (AR) materials to tackle the concern of their inadequate photovoltaic efficiency. Unfortunately, conventional AR films still suffer from a deficiency in omnidirectional antireflection and mechanical durability, significantly weakening their light-harvesting capability. Herein, a scalable biomimetic antireflective film (BARF) with ommateum-inspired multiscale hierarchical architecture was prepared using a method combining layer-by-layer thermal decomposition and in situ self-assembly. Remarkably, the BARF exhibited a transmittance of over 95% in light incident angles from −75° to 75°. Thereby, the output voltage of solar cells coated with BARF increased by 30.48% compared with commercial AR film. Furthermore, the BARF demonstrated excellent mechanical durability in falling-sand tests (∼2% transmittance loss). This work offers a general strategy for mass production of omnidirectional and durable AR films that are suitable for practical applications of protective AR film in solar cells, light pollution treatment, and video stealth technology. Graphical Abstract View Large Image Figure Viewer Download Hi-res image

Tunable antireflective characteristics enabled by small yellow leafhopper-inspired soccer ball-shaped structure arrays

BackgroundNowadays, tunable antireflective characteristics have attracted considerable attention due to growing demands in next-generation optical energy-related fields. MethodBioinspired by the distinctive structural geometry on small yellow leafhopper wings, this study reports reversibly switchable antireflective characteristics enabled by ambient-temperature shape memory polymer-based soccer ball-shaped structure arrays with multilayer porous structures underneath. Significant findsThe resulting structures behave a broadband omnidirectional antireflection performance, where the average reflectance in the visible spectrum can even be reduced by 22% for an incident angle of 75º. Interestingly, the structures can be instantaneously deformed or recovered through applying external stimuli in ambient environments. The antireflective property switches associated with the shape memory transitions deliver vast opportunities in developing smart optical devices. Moreover, the structure-shape effect and reversibility on the antireflective properties are systematically assessed in the research.

Wide-angle broadband antireflection coatings based on boomerang-like alumina nanostructures in visible region

A novel boomerang-like alumina based antireflective coating with ultra-low reflectance has been produced for light incidence angles form 0 up to 45°. Boomerang-like alumina nanostructures have been fabricated on the BK7 glass substrates by dip-coating and surface modification via hot water treatment. To achieve the lowest residual reflectance, the effect of dip-coating rate and hot-water temperature in the treatment process has been investigated and optimized. To further investigate the boomerang-like alumina nanostructure and extract its graded refractive index profile by fitting the measured reflectance spectrum with the simulated one, a simulation based on the finite-difference time-domain (FDTD) method has been performed. The average reflectance measured at normal incidence for double-sided coated BK7 glass substrates is only 0.3% in the visible spectral region. Considering both sides, the average reflectance of the substrate decreased in the spectral range of 400–700 nm down to 0.4% at incidence angles of 45° by applying the boomerang-like alumina antireflection coatings. The optimized single layer boomerang-like alumina coating on the curved aspheric lens exhibited a low average reflectance of less than 0.14% and an average transmittance of above 99.3% at normal incidence. The presented process is a simple and cost-effective route towards broadband and omnidirectional antireflection coatings, which have promising potential to be applied on substrates having large scales with complex geometric shapes.

Omnidirectional / Unidirectional Antireflection-Switchable Structures Inspired by Dragonfly Wings

Randomly arranged irregular inclined conical structure-covered dragonfly wings, distinguished from periodic conical structure-covered cicada wings, are with high optical transparency for wide viewing angles. Bioinspired by the antireflective structures, we develop a colloidal lithography approach for engineering randomly arranged irregular conical structures with shape memory polymer-based tips. The structures establish a gradual refractive index transition to suppresses optical reflection in the visible spectrum. By manipulating the configuration of structure tips through applying common solvent stimulations or contact pressures under ambient conditions, the resulting unidirectional antireflection and omnidirectional antireflection performances are able to be instantaneously and reversibly switched. The dependences of structure shape, structure inclination, structure arrangement, and structure composition on the switchable antireflection capability are also systematically investigated in this study.

Characterization broadband omnidirectional antireflection ITO nanorod films coating

Three-dimensional (3D) nanostructure electrode based on the transparent conductive oxide (TCO) is an alternative and effective approach for increasing the performance of next-generation photovoltaic and optoelectronic devices. In this work, we prepared the vertically aligned indium tin oxide nanorods (ITO-NRs) film on silicon (100) wafer and commercial ITO thin-film coated glass substrate (ITO-TF/glass) by the glancing angle deposition (GLAD) technique. The morphologies of the ITO-NRs films were confirmed by field-emission scanning electron microscope (FE-SEM). The grazing-incident X-ray diffraction (GIXRD) observed that the ITO-NRs were formed as amorphous and nanocrystalline phase. The photoemission spectra revealed the atomic percentage of elements in ITO-NRs, and the work function of the ITO-NRs was found to increase as a function of layer thickness. To simply demonstrate their feasibility in TCO applications, the ITO-NRs films were deposited on ITO-TF/glass. We found that the resistivity of ITO-NRs on ITO-TF/glass was higher than that of a bare ITO-TF/glass substrate, which can explain through the surface recombination loss. Besides, by precisely tunning the layer thickness of ITO-NRs films, the optical transmittance was enhanced at 85.5–90.0% over the broad wavelength in the visible region with omnidirectional AR characteristic which is superior to that in conventional ITO-TF/glass. Our thickness dependence of the ITO-NRs properties discloses promising 3D TCO nanostructure in wide application.

Broadband and Omnidirectional Antireflection Surfaces Based on Deep Subwavelength Features for Harvesting of the Solar Energy

Broadband and omnidirectional antireflective surfaces are important to various solar energy technologies. The utilization of deep subwavelength features is reported for decrement in the broadband and omnidirectional reflection of the solar spectrum. The study commences with nanopillar (NP) arrays that are well known for their antireflection properties. Deep subwavelength features to the nanopillar arrays are introduced, which yield further decrease in reflection: quasinanolens (qNL), deep subwavelength sidewall scalloping (DSSS), and decreasing the nanopillar bottom diameter to reshape it into a light funnel (LF). Accordingly, surface silicon arrays are realized in a top−down fabrication process: nanopillar arrays, nanopillar arrays incorporated with DSSS, nanopillar arrays incorporated with qNL, nanopillar arrays incorporated with qNL and DSSS, and nanopillar arrays that are transformed into LF arrays and are complemented with qNL and DSSS. It is experimentally shown that utilization of deep subwavelength features produces a broadband reflection decrement with an almost 80% decrement for normal incidence and 60% decrement for an angle of incidence of 80°, for unpolarized illumination. The introduction of deep subwavelength features concludes a 20% decrement in the broadband diffused reflection compared with undecorated NP arrays. The effect of deep subwavelength features is also explored numerically, and the possible underlying light‐trapping mechanisms are discussed.

Super-regular femtosecond laser nanolithography based on dual-interface plasmons coupling

Abstract Advances in femtosecond laser-material interaction facilitate the extension of maskless optical processing to the high efficiency and deep-subwavelength scale. Here, a hybrid plasmon lithography technique has been demonstrated by irradiating near-infrared femtosecond laser pulses onto the Si material coated with thin Cr films in a vacuum chamber, and superior nanograting structures are found to deeply penetrate through the thin Cr film into the underlying Si substrate. In stark contrast to the common ripple structures formed on the Si surface, the Cr-layer mediated Si nanograting structures not only exhibit the spatially super-regular arrangements with a deep-subwavelength period of 355 nm but also present the nonsinusoidal sharp-edged groove geometry with a large depth-to-width aspect ratio of 2.1. Theoretical analyses and calculations reveal that the anomalous structure characterizations are physically ascribed to the excitation of dual-interface coupled plasmons in the thin metal layer, which possess the squeezed spatial wavelength and the periodic columnar intensity distributions. Moreover, the further deepening of periodic nanostructures into the Si substrate is also elucidated by the simulation of electric field enhancements at the bottom of shallow grooves under irradiation of subsequent laser pulses. In combination with a wet etching process, the Si nanograting structures can be modified into the smooth and narrow-mouthed V-profiles, whose optical measurements show a near omnidirectional antireflection especially in the visible range of 565–750 nm, which is expected for the design of advanced photonic devices.

Tunable Omnidirectional Antireflection Coatings Inspired by Inclined Irregular Nanostructures on Transparent Blue-Tailed Forest Hawk Dragonfly Wings.

Randomly arranged inclined irregular nanostructure-covered blue-tailed forest hawk dragonfly wings are highly transparent for wide viewing angles. Inspired by the dragonfly wings, monolayer silica colloids are self-assembled on shape memory polymer-coated substrates and utilized as plasma etching masks to pattern disorderly arranged inclined irregular conical structures. The structures build gradual refractive index transitions at various angles of incidences, resulting in omnidirectional antireflection performance over the whole visible wavelength region. In comparison with a bare substrate, the optimized structure-covered substrate presents 10% higher optical transmission at 0° and even 41% higher optical transmission at an angle of incidence of 75°. Importantly, by manipulating the structural configuration of the shape memory polymer-based structures, the corresponding antireflection characteristics can be instantaneously and reversibly eliminated and recovered after drying out of common household liquids or applying contact pressures in ambient environments. The tunable omnidirectional antireflection coatings are prospective candidates for realizing optical modulation, which exhibits an enormous application value in smart windows, intelligent display screens, optical components, and novel optoelectronic devices.

The broadband and omnidirectional antireflective performance of perovskite solar cells with curved nanostructures

Various shaped nanostructures have been continually used in the antireflection of organic-inorganic hybrid perovskite solar cells. A full understanding of the shape-dependent light trapping behavior is the basis for finding the excellent broadband and omnidirectional photovoltaic performance. Hence, three-dimensional models of silica sphere, hemisphere, moth-eye and cone nanostructured perovskite solar cells were performed at omnidirectional angle in this work. We proposed electric field intensity per volume as a key parameter, verifying the transition of light trapping modes of several curved nanostructures from normal incidence to oblique incidence. As the incident angle increases, the antireflection effect of four curved structures becomes increasingly crucial, and the differences among them are also greater. Consequently, the moth-eye nanostructured perovskite solar cell has achieved the best photovoltaic performance considering omnidirectional incidence. The short-circuit current density is increased by 8.4% at normal incidence and increased by 36.4% at 60° incidence compared to the planar reference. Our work offers the theoretical analysis for omnidirectional antireflection structure optimization, and provides guidance for the experimental research and production of potential antireflection structures of solar cell.

Omnidirectional and Broadband Antireflection Effect with Tapered Silicon Nanostructures Fabricated with Low-Cost and Large-Area Capable Nanosphere Lithography.

In this report, we present a process for the fabrication and tapering of a silicon (Si) nanopillar (NP) array on a large Si surface area wafer (2-inch diameter) to provide enhanced light harvesting for Si solar cell application. From our N,N-dimethyl-formamide (DMF) solvent-controlled spin-coating method, silica nanosphere (SNS in 310 nm diameter) coating on the Si surface was demonstrated successfully with improved monolayer coverage (>95%) and uniformity. After combining this method with a reactive ion etching (RIE) technique, a high-density Si NP array was produced, and we revealed that controlled tapering of Si NPs could be achieved after introducing a two-step RIE process using (1) CHF3/Ar gases for SNS selective etching over Si and (2) Cl2 gas for Si vertical etching. From our experimental and computational study, we show that an effectively tapered Si NP (i.e., an Si nanotip (NT)) structure could offer a highly effective omnidirectional and broadband antireflection effect for high-efficiency Si solar cell application.

Highly Efficient Anti-Reflection Film for Solar Cells

When light transmits into the solar cell, reflection occurs at the interface between air and an absorptive medium lowers solar cell efficiency. In this study, we present micro-dome structured anti-reflection (AR) films fabricated via simple imprinting technique. The functionality of the film in photovoltaic devices is evaluated, revealing reduced reflection caused by gradient refractive index effect and omnidirectional antireflection behavior, which lead to enhance the photovoltaic performance under both outdoor and indoor lighting. In addition, the AR film does not require any manipulation of solar cells as using a simple strategy of attaching the film on the surface, and its hydrophobic property affords self-cleaning effect, which should be beneficial for building integrated photovoltaics. Combined with these properties, our detachable AR films provide a practical guideline to boost solar cell performance.

Facile fabrication and optical characterization of nanoflake aluminum oxide film with high broadband and omnidirectional transmittance enhancement

Abstract In this study, the non-lithography fabrication of nanoflake aluminum oxide (Al2O3) film with omnidirectional broadband antireflection properties was demonstrated. The nanoflake Al2O3 films on glass slide and silicon wafer substrates were sequentially prepared by reactive magnetron sputtering and surface modification with alkali-treatment and hot water treatment processes. The morphologies, chemical compositions and optical properties of the prepared samples were characterized by field-emission scanning electron microscope (FE-SEM), photoelectron spectroscopy (PES), variable angle spectroscopic ellipsometry (VASE) and UV-VIS spectrophotometry with angle-dependent transmittance measurements, respectively. The results indicated that the thickness of the sputtered Al2O3 film was a crucial parameter for the growth of nanoflake for the nanostructure formed by alkali treatment with 0.1 M of potassium hydroxide (KOH) at room temperature and hot water treatment in deionized (DI) water at 80 °C for 3 min. The best optical performance was achieved with the single nanoflake film layer, resulting in enhanced optical transmittance exceeding 90% over the broad range of visible region at wide incident angle. This facile fabrication process of the nanoflake layer was offering the potential benefits for inexpensive omnidirectional anti-reflection (AR) coating in large area.

Broadband omnidirectional antireflection coatings inspired by embroidered ball-like structures on leafhoppers

Leafhopper (Thaia rubiginosa) wings exhibit antireflection behaviors for wide viewing angles, resulting from leafhopper-generated brochosomal coatings on the wing surface. The embroidered ball-like brochosomes establish a gradual transition in the refractive index at air/wing interface, leading to the broadband omnidirectional antireflection performance. Inspired by the brochosomal coatings, this study develops a non-photolithography-based approach for assembling embroidered ball-like structure arrays exploiting a modified Langmuir-Blodgett technology. The average visible transmittance of a polymer film can be improved by ca. 8% at normal incidence, and improved by even ca. 24% as the incident angle reaches 75° by introducing the artificial brochosome arrays. The dependence of the structure shape on the antireflection capabilities is also systemically studied in this research.

Mie-Type Surface Texture-Integrated Visible and Short-Wave Infrared InGaAs/InP Focal Plane Arrays

Semiconductor nanostructures with high refractive indexes are highly promising to serve as a broadband omnidirectional antireflection layer for optoelectronic device applications. InP nanopillar ar...