Broadband Antireflection Coatings Research Articles

Characterization of sol-gel ORMOSIL antireflective coatings from phenyltriethoxysilane and tetraethoxysilane: Microstructure control and application

Chemical structure, microstructure and properties of organically modified silicate (ORMOSIL) antireflective (AR) coatings from phenyltriethoxysilane (PTES) and tetraethoxysilane (TEOS) were characterized. The reaction between PTES and TEOS was verified by means of FTIR, XPS, and 29Si NMR. N2 adsorption-desorption analysis and TEM were utilized to investigate the pore structure and microstructure control of ORMOSIL AR coatings. It was revealed that, by increasing PTES/TEOS molar ratio, the porosity of ORMOSIL AR coating decreased and then the refractive index increased gradually from 1.21 to 1.27. The hydrophobicity of ORMOSIL AR coatings was also improved significantly by increasing the water contact angle from 21.2° to 112°. Most importantly, the OIH mechanism between PTES and TEOS and hence the microstructure control of ORMOSIL AR coatings were induced in detail based on a deep discussion of results from 29Si MAS NMR, N2 adsorption-desorption analysis and TEM characterization. Finally, the feasibility of application of PTES/TEOS ORMOSIL thin films in preparation of double-layer broadband AR coatings was verified.

Broadband near-infrared antireflection coatings fabricated by three-dimensional direct laser writing.

Three-dimensional direct laser writing via two-photon polymerization is used to fabricate anti-reflective structured surfaces (ARSSs) composed of subwavelength conicoid features optimized to operate over a wide bandwidth in the near-infrared range from 3700 cm-1 to 6600 cm-1 (2.7-1.52μm). Analytic Bruggemann effective medium calculations are used to predict nominal geometric parameters such as the fill factor of the constitutive conicoid features of the anti-reflective structured surfaces (ARSSs) presented here. The performance of the ARSSs was investigated experimentally using infrared reflection and transmission measurements. An enhancement of the transmittance by 1.35%-2.14% over a broadband spectral range from 3700 cm-1 to 6600 cm-1 (2.7-1.52μm) was achieved. We further report on finite-element-based reflection and transmission data using three-dimensional (3D) model geometries for comparison. A good agreement between experimental results and the finite-element-based numerical analysis is observed once as-fabricated deviations from the nominal conicoid forms are included in the model. 3D direct laser writing is demonstrated here as an efficient method for the fabrication and optimization of ARSSs designed for the infrared spectral range.

超短紫外到近红外宽波段增透膜的研制

超短紫外到近红外宽波段增透膜的研制

Broad band antireflective coatings using novel in-situ synthesis of hollow MgF2 nanoparticles

A high performance antireflective (AR) should be a thin film with low refractive index, highly transparent, economical, and highly durable. In this paper, we present a novel synthesis of hollow Magnesium Fluoride (MgF2) nanoparticles, which is used to produce a high performance broadband antireflective coating. A formation–deformation-reformation route has been adopted to synthesize nanoparticles with hexagonal structure and cavities, which are crystalline and dispersible in any polar solvent. This facile synthesis route is a major breakthrough to achieve high performance broadband antireflective coating with an average transmittance of 98.3% in the visible range (400–800nm) and 96.2% in active solar range (300–1500nm). No significant drop in the efficiency was observed with antireflective-coated cover glass on the c-Si solar cell, whereas 6–7% efficiency loss was observed with uncoated glass. This can be a promising approach to develop high performance antireflective dielectric layer, which will enhance the overall performance of photovoltaic and solar thermal systems.

Preliminary study on broadband antireflection coatings for large aperture telescopes

A broadband anti-reflective (AR) coating design for astronomical large-aperture telescopes is proposed. We give simulations of two-, three- and four-layer silica sol-gel on fused silica and finally get the optimal optical constants. As a comparison, we discuss the traditional dielectric material that has been applied to broadband AR coatings. To better guide the following experiment, we also conduct error analysis and feasibility analysis, combining with the technological characteristics of sol-gel. The analytical method is suitable for other wavebands and substrates. It is also instructive for large area AR coatings in the field of solar cells.

Preparation of hydrophobic broadband antireflective SiO2 coating on flexible poly(methyl methacrylate) substrates

The double-layer broadband antireflective coatings is crucial for optical devices, e.g. plastic eyeglass lenses and the solar cell system. Among optical situation, the wide wavelength range of light is one of the most demanded. Here we designed a double-layer broadband antireflective (AR) coating with excellent average transmittance at visible region for poly (methyl methacrylate) (PMMA) using thin film design software. A template-free sol–gel process was developed to prepare the SiO2/PMHS coating with ultralow refractive index of 1.13 for top layer. By combining the colloidal silica and poly (methyl hydrogen) siloxane (PMHS), the coating with adjustable refractive index that vary between 1.13 and 1.37 was obtained. The formation mechanism of the ultralow refractive index was proposed. Meanwhile, using PMHS to partially substitute the hydrophilic groups in the coating, the hydrophobicity of the coatings was improved, and its static water contact angle was as high as 126°. Besides, the coating had excellent mechanical property and refractive index of 1.22–1.42 can be well prepared by combining base-catalyzed silica sol and acid-catalyzed silica sol together for bottom layer. Then, using the two raw materials base/acid -catalyzed mix SiO2 sol and the PMHS-SiO2 sol and optimizing the thickness, the prepared double-layer coatings with average transmittance of 99.30% in the 400nm to 800nm wavelength on PMMA substrate was successfully obtained. Those prepared hydrophobic broadband AR coatings represented excellent transmittance and had enormous potential application in the optical devices.

Fabrication of broadband antireflection coatings using wavelength-indirect broadband optical monitoring

Multi-layer optical coatings with complex spectrum requirements, such as multi-band pass filters, notch filters, ultra-broadband antireflection coating and etc., whose working wavelength is out of monitoring wavelength range, are difficult to be fabricated using direct broadband optical monitoring (BBOM). In this paper, a broadband antireflection (AR) coating in the wavelength range from 1300nm to 2000nm at 45° incident was designed and deposited by dual ion beam sputtering (DIBS). Ta2O5 and SiO2 were chosen as high and low refractive index coating materials, respectively. The optimized coating structure contains 4 non-quarter-wave (QW) layers. In order to obtain high transmittance, the most important is to realize the thickness accurate control. Due to the limitation of the monitoring wavelength range, which is only from 450nm to 1000nm, a wavelength-indirect broadband optical monitoring strategy was successfully employed to control the layers thickness during the deposition process. At last, a good agreement between theoretical and measured transmittance is obtained. The maximum error (the first layer) is only about 5.3% and the minimum error (the third layer) is about −0.25% base on the results of reverse engineering analysis.

Self-assembled nanotextures impart broadband transparency to glass windows and solar cell encapsulants

Most optoelectronic components and consumer display devices require glass or plastic covers for protection against the environment. Optical reflections from these encapsulation layers can degrade the device performance or lessen the user experience. Here, we use a highly scalable self-assembly based approach to texture glass surfaces at the nanoscale, reducing reflections by such an extent so as to make the glass essentially invisible. Our nanotextures provide broadband antireflection spanning visible and infrared wavelengths (450–2500 nm) that is effective even at large angles of incidence. This technology can be used to improve the performance of photovoltaic devices by eliminating reflection losses, which can be as much as 8% for glass encapsulated cells. In contrast, solar cells encapsulated with nanotextured glass generate the same photocurrent as when operated without a cover. Ultra-transparent windows having surface nanotextures on both sides can withstand three times more optical fluence than commercial broadband antireflection coatings, making them useful for pulsed laser applications.

Preparation and properties of five-layer graded-refractive-index antireflection coating nanostructured by solid and hollow silica particles

This study examined large-area omnidirectional broadband antireflective nanostructured double-sided silica coatings fabricated by dip-coating on glass substrates. The coating consisted of five layers of graded-refractive-index thin films, and the total thickness of the coating was 250nm. The refractive indices of the five layers of the coating changed from 1.33 to 1.11 with the thickness of each layer 50nm. We prepared a series of 50nm silica (SiO2) particles with hollow diameters ranging from 0nm to 38nm using the sol-gel method, and the sols for coating were composed of a mixture of 50nm SiO2 nanoparticles sol and SiO2 sol derived from acid-catalyzed tetraethyl orthosilicate. The separations of Fresnel reflection coefficients and polarized transmittances within a small range demonstrate a significant advantage of achieving omnidirectional broadband antireflective performance for the coated substrate. At a wavelength range of 380–1600nm, the average transmittance is 99.04% at normal incidence and the maximum unpolarized transmittances exceed 99% at wide angles of incident from 0° to 45°, and the increment in average unpolarized transmittances reach around 17.51% at an incident angle of 75°.

Ultrathin Film Broadband Terahertz Antireflection Coating Based on Impedance Matching Method

Broadband antireflection coatings for terahertz (THz) components are extremely important in the application of THz technology. We present an in-depth materials study on the broadband antireflection design based on the concept of impedance matching. Several selected conductive materials are tested and compared. They can all be used as THz broadband antireflection coatings regardless of their nature of conductivity. We provide a facile method to approximately predict the impedance matching condition for various conductive materials based on their dc real conductivity. The theoretical predications are in good agreement with experimental results and published data. Doped metal oxides or polymers are recommended as good candidates for fabricating antireflection coating in THz range. The thickness-independent dc real conductivity enables better control on individual tuning parameter—film thickness. Given a substrate, THz transmission in antireflection coating has a theoretical upper limit which is further reduced with non-negligible imaginary conductivity.

Hollow Rodlike MgF2 with an Ultralow Refractive Index for the Preparation of Multifunctional Antireflective Coatings.

Antireflective coatings with superhydrophobic, self-cleaning, and wide-spectrum high-transmittance properties and good mechanical strength have important practical value. In this research, hollow nanorod-like MgF2 sols with different void volumes were prepared by a template-free solvothermal method to further obtain hollow nanorod-like MgF2 crystals with an ultralow refractive index of 1.14. Besides, a MgF2 coating with an adjustable refractive index of 1.10-1.35 was also prepared by the template-free solvothermal method. Then through the combination of base/acid two-step-catalyzed TEOS and hydroxyl modification on the surface of nanosilica spheres, the SiO2 coating with good mechanical strength, a flat surface, and a refractive index of 1.30-1.45 was obtained. Double-layer broadband antireflective coatings with an average transmittance of 99.6% at 400-1400 nm were designed using the relevant optical theory. After the coating thickness was optimized by the dip-coating method, the double-layer antireflective coatings, whose parameters were consistent with those designed by the theory, were obtained. The bottom layer was a SiO2 coating with a refractive index of 1.34 and a thickness of 155 nm, and the top layer was a hollow rodlike MgF2 coating with a refractive index of 1.10 and a thickness of 165 nm. The average transmittance of the obtained MgF2-SiO2 antireflective coatings was 99.1% at 400-1400 nm, which was close to the theoretical value. The hydrophobic angle of the coating surface reached 119° at first, and the angle further reached 152° after conducting surface modification by PFOTES. In addition, because the porosity of the coating surface was only 10.7%, the pencil hardness of the coating surface was 5 H and the critical load Lc was 27.05 N. In summary, the obtained antireflective coatings possessed superhydrophobic, self-cleaning, and wide-spectrum high-transmittance properties and good mechanical strength.

Fabrication of broadband antireflection coatings using broadband optical monitoring mixed with time monitoring**Project supported by the National Natural Science Foundation of China (Grant Nos. 61308092 and 61505209).

Multi-layer optical coatings with complex spectrum requirements, such as multi-band pass filters, notch filters, and ultra-broadband antireflection coating, which usually contain very thin layers and sensitive layers, are difficult to be fabricated using a quartz crystal monitoring method or a single wavelength optical monitoring system (SWLOMS). In this paper, a broadband antireflection (AR) coating applied in the wavelength range from 800 nm to 1800 nm was designed and deposited by ion beam sputtering (IBS). Ta2O5 and SiO2 were chosen as high and low refractive index coating materials, respectively. The optimized coating structure contains 9 non-quarter-wave (QW) layers totally with ultra-thin layers and sensitive layers in this coating stack. In order to obtain high transmittance, it is very important to realize the thickness accurate control on these thin layers and sensitive layers. A broadband optical monitoring mixed with time monitoring strategy was successfully used to control the layer thickness during the deposition process. At last, the measured transmittance of AR coating is quite close to the theoretical value. A 0.6% variation in short wavelength edge across the central 180 mm diameter is demonstrated. A spectrum shift of less than 0.5% for 2 continuous runs is also presented.

Porous Nanomaterials: Porous Nanomaterials for Ultrabroadband Omnidirectional Anti‐Reflection Surfaces with Applications in High Concentration Photovoltaics (Adv. Energy Mater. 7/2017)

In article number 1601992, Ralph G. Nuzzo, John A. Rogers, and co-workers demonstrate broadband anti-reflection coatings that rely on porous nanomaterials formed by self-assembly. The image shows the use of such coatings (orange) on an array of lenses within a concentration photovoltaics architecture, where photons are efficiently concentrated onto underlying triple-junction solar cells, leading to an efficiency enhancement of ≈8%.

Durable broadband ultralow index fluoropolymer antireflection coatings for plastic optics

Durable broadband antireflection (AR) coatings remain an ongoing challenge for plastic optics used in a wide range of applications. Here, we show that glancing angle deposition of a commercial fluoropolymer can be used to fabricate extremely durable ultralow index AR coatings that reduce the solar spectrum-averaged (400<λ<1600 nm) reflectance of acrylic and polycarbonate plastic to <1% over a wide range of incidence angles up to ∼40°. The coatings feature strong adhesion and exhibit outstanding resistance to heat, humidity, dirt, ultraviolet light, outdoor exposure, solvents, acids, bases, abrasion, and repeated bend/compression cycling. They are successfully applied to f/1 curved lens surfaces as well as acrylic Fresnel lenses, where coating both sides increases the solar spectrum-averaged transmittance from 92% to 98%. These results represent a significant development for plastic optics commonly used in solar concentrators as well as more generally for broadband AR applications that demand extreme environmental, chemical, and mechanical durability.

Sequential Infiltration Synthesis for the Design of Low Refractive Index Surface Coatings with Controllable Thickness.

Control over refractive index and thickness of surface coatings is central to the design of low refraction films used in applications ranging from optical computing to antireflective coatings. Here, we introduce gas-phase sequential infiltration synthesis (SIS) as a robust, powerful, and efficient approach to deposit conformal coatings with very low refractive indices. We demonstrate that the refractive indices of inorganic coatings can be efficiently tuned by the number of cycles used in the SIS process, composition, and selective swelling of the of the polymer template. We show that the refractive index of Al2O3 can be lowered from 1.76 down to 1.1 using this method. The thickness of the Al2O3 coating can be efficiently controlled by the swelling of the block copolymer template in ethanol at elevated temperature, thereby enabling deposition of both single-layer and graded-index broadband antireflective coatings. Using this technique, Fresnel reflections of glass can be reduced to as low as 0.1% under normal illumination over a broad spectral range.

Investigate the effects of EG doping PEDOT/PSS on transmission and anti-reflection properties using terahertz pulsed spectroscopy

The conductivity of poly(3,4-ethylene dioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) is significantly enhanced on adding some organic solvent such as ethylene glycol (EG). In this paper, the optoelectronic properties of EG doped PEDOT/PSS on transmission and anti-reflection effects are investigated in detail by terahertz time domain spectroscopy (THz-TDS). The transmission line circuit theory gives us an insight into the THz transmission mechanisms of the main and second pulses. In particular, we show that the conductivities of 10% EG doped PEDOT/PSS are nearly frequency independent from 0.3 to 1.5 THz. To demonstrate applications of this property, we design and fabricate broadband terahertz neutral density filters and anti-reflection coatings based on 10% EG doped PEDOT/PSS thin films with varying thickness. Our measurements highlight the capability of THz-TDS to characterize the conductivity of EG doped PEDOT/PSS, which is essential for broadband optoelectronic devices in THz region.

Performance and durability of broadband antireflection coatings for thin film CdTe solar cells

Light reflection from the glass surface of a photovoltaic (PV) module is a significant source of energy loss for all types of PV devices. The reflection at the glass and air interface accounts for ∼4% of the total energy. Single layer antireflection coatings with sufficiently low refractive index have been used, such as those using magnesium fluoride or porous silica, but these are only effective over a narrow range of wavelengths. In this paper, the authors report on the design, deposition, and testing of multilayer broadband antireflection coatings. These coatings reduce the weighted average reflection over the wavelength range used by thin film CdTe devices to just ∼1.22%, resulting in a 3.6% relative increase in device efficiency. The authors have used multilayer stacks consisting of silica and zirconia layers deposited using reactive magnetron sputtering. Details of the stack design, sputter deposition process parameters, and the optical and microstructural properties of the layers are provided. Antireflection coatings on glass exposed to the outdoors must not degrade over the lifetime of the module. A comprehensive set of accelerated environmental durability tests has been carried out in accordance with IEC 61646 PV qualification tests. The durability tests confirmed no damage to the coatings or performance drop as a result of thermal cycling or damp heat. All attempts to perform pull tests resulted in either adhesive or substrate failure, with no damage to the coating itself. The coatings also passed acid attack tests. Scratch resistance, abrasion resistance, and adhesion tests have also been conducted. The optical performance of the coatings was monitored during these tests, and the coatings were visually inspected for any sign of mechanical failure. These tests provide confidence that broadband antireflection coatings are highly durable and will maintain their performance over the lifetime of the solar module. All dielectric metal-oxide multilayer coatings have better optical performance and superior durability compared with alternative single layer porous sol–gel coatings. Thin film CdTe devices are particularly problematic because the antireflection coating is applied to one side of the glass, while device layers are deposited directly on to the opposite glass surface in the superstrate configuration. In thin film CdTe production, the glass is exposed to high temperature processes during the absorber deposition and the cadmium chloride activation treatment. If glass precoated with a broadband antireflection coating is to be used, then the coating must withstand temperatures of up to ∼550 °C. Surprisingly, our studies have shown that multilayer silica/zirconia antireflection coatings on soda lime glass remain unaffected by temperatures reaching 600 °C, at which point mild crazing is observed. This is an important observation, demonstrating that low cost glass, which is preprocessed with a broadband antireflection coating, is directly useable in thin film CdTe module production.

Preparation of mechanically stable triple-layer interference broadband antireflective coatings with self-cleaning property by sol–gel technique

Mechanically stable broadband antireflective coatings with self-cleaning property were fabricatedviasol–gel method.

The design of broad band anti-reflection coatings for solar cell applications

The design of broadband anti-reflection coatings (ARCs) for solar cell applications using multiobjective differential evolutionary (MODE) algorithms is reported. The effect of thickness and refractive index contrast within the layers of the ARC on the bandwidth of reflectance is investigated in detail. In the case of the hybrid plasmonic ARC structures the effect of size, shape and filling fraction of silver (Ag) nanoparticles on the reflectance is studied. Bandwidth is defined as the spectral region of wavelengths over which the reflectance is below 2%. Single, two and three layers ARCs (consisting of MgF 2 , Al 2 O 3 , Si 3 N 4, TiO 2 and ZnS or combinations of these materials) were simulated for performance evaluation on an a-Si photovoltaic cell. It is observed that the three layer ARC consisting of MgF 2 /Si 3 N 4 /TiO 2 (ZnTe) of 81/42/36 nm thicknesses, respectively, exhibited a weighted reflectance of 1.9% with a bandwidth of 450 nm over the wavelength range of 300–900 nm. The ARC bandwidth could be further improved by embedding randomly distributed Ag nanoparticles of size between 100 and 120 nm on a two layer ARC consisting of Al 2 O 3 /TiO 2 with thickness of 42 nm and 56 nm respectively. This plasmon-dielectric hybrid ARC design exhibited a weighted reflectance of 0.6% with a bandwidth of 560 nm over the wavelength range of 300–900 nm.

A hydrophobic and abrasion-resistant MgF2coating with an ultralow refractive index for double-layer broadband antireflective coatings

A novel sol–gel route was proposed in order to prepare an ultralow-index MgF2coating with both hydrophobicity and good abrasion-resistance.