Broadband Antireflection Coatings Research Articles

Anti-reflection for monocrystalline silicon from diamond-like carbon films deposited by magnetron sputtering

In this work, diamond-like carbon (DLC) films used as anti-reflective coatings for monocrystalline silicon were deposited by magnetron sputtering for potential application in solar cells. The microstructural and optical properties of the films were investigated as a function of substrate temperature over a wide range during deposition. It showed that, when the substrate temperature increased from RT to 800 °C, the hybridized structures of the DLC films accordingly changed associated with a significant variation of refractive index between2.22 and 1.64 at a wavelength of 550 nm. Three types of coating systems, namely single-, three- and five-layer films on monocrystalline silicon substrates, were designed based on the anti-reflection principle and fabricated in terms of the relationships of refractive index and deposition rate with substrate temperature. In particular, a well-designed three-layer film, of which the refractive index gradually changed along the thickness, that is 1.8, 1.9 and 2.0, respectively, was successfully deposited at one step on monocrystalline silicon substrates by adjusting substrate temperature and deposition time, and featured a broadband anti-reflective characteristic with low average reflectivity of 8.7% at a wide solar spectrum of 400–1100 nm. This work demonstrates that the DLC film has a promising application potential as broadband anti-reflective coatings in silicon-based solar cells.

Broadband Antireflection Coatings Composed of Subwavelength-Sized SiGe Particles

Broadband Antireflection Coatings Composed of Subwavelength-Sized SiGe Particles

Broadband Anti-Reflection Coatings Fabricated by Precise Time-Controlled and Oblique-Angle Deposition Methods

Broadband anti-reflection (AR) coatings are essential elements for improving the photocurrent generation of photovoltaic modules and enhancing visibility in optical devices. In this paper, we report a hybrid-structured, anti-reflection coating that combines multi-layer thin films with a single top-oblique deposited layer. By simply introducing this low-refractive index layer, the broadband anti-reflection properties of optical thin films can be improved while simplifying the preparation. Precise time-controlled and oblique-angle deposition (OAD) methods were used to fabricate the broadband AR coating. By accurately measuring and adjusting the design errors for the thin and thick film layers, 22-layer and 36-layer AR coatings on a sapphire substrate with a 400–2000 nm wideband were obtained. This bottom-up preparation process and AR coating design have the potential to significantly enhance the broadband antireflective properties for many optical systems and reduce the manufacturing cost of broadband AR coatings.

Design and preparation of superhydrophobic, broadband and double-layer antireflective coatings

As one of the most widely used optical films, antireflective coating (ARC) is widely used in lasers, display screens and solar cells. In this article, the double-layer ARC coating consists of a bottom layer using SiO2-TiO2 composite nanomaterials and a top layer of SiO2 modified with Hexamethyldisilazane (HMDS). The optimal parameters of the double-layer ARC were obtained through the simulation software Macleod. Meanwhile, the aging time of SiO2 sol, the volume fraction of HMDS, the annealing temperature, the dip-coating speed, and the volume fraction of TiO2, which affect the refractive index of the bottom and top layers, were explored in order to match the best parameters. By adjusting the structure of the bottom and top layers, the double-layer broadband ARC coating has a maximum transmittance of 97.6% in the 400-1800 nm spectral range, and an average transmittance of 96.0%. In addition, HMDS was used to modify the SiO2 sol to obtain a double-layer ARC with superhydrophobic properties with a hydrophobic angle of 151.4°. After the high-temperature and high-humidity environment test, the antireflective performance of the double-layer ARC has not changed significantly, demonstrating the broad prospects for application in large outdoor solar photovoltaic power plants. Moreover, this work not only provides a new method for adjusting the refractive index of the coating, but also opens up a new way for the preparation of broadband ARC.

High-low refractive index stacks for broadband antireflection coatings for multijunction solar cells

Antireflection coatings are an interesting challenge for multijunction solar cells due to their broadband spectrum absorption and the requirement of current matching of each subcell. A new design for multijunction solar cell antireflection coatings is presented in this work in which alternative high and low index materials are used to minimize the reflection in a broadband (300–1800 nm). We compared the short circuit current density of high-low refractive index stacks designs with optimum double-layer antireflection coatings by considering two optical materials combinations (MgF2/ZnS and Al2O3/TiO2) for the AM0 and AM1.5D spectra. The calculations demonstrate that for lattice-matched triple-junction solar cells and inverted metamorphic quadruple-junction solar cells, high-low refractive index stacks outperform the optimum double-layer antireflection coatings. The new design philosophy requires no new optical materials because only two materials are used and exhibits excellent performance in broadband spectra. The angle performance of these antireflection coatings is slightly better than classical double-layers whereas the analysis for thickness sensitivity shows that small deviations from deposition targets only slightly impact the performance of antireflection coatings. Finally, some technical solutions for depositing these high-low refractive index multilayers are discussed.

All-Nanoparticle Monolayer Broadband Antireflective and Self-Cleaning Transparent Glass Coatings.

The vast majority of light-emitting diode and liquid-crystal displays, solar panels, and windows in residential and industrial buildings use glass panels owing to their high mechanical stability, chemical resistance, and optical properties. Glass surfaces reflect about 4-5% of incident light if no antireflective coating is applied. In addition to energy losses in displays, surface reflections diminish picture quality. Engineering of antireflective coatings can be beneficial for all types of glass screens, specifically for large screens and touch-screen devices when scratch-resistance and self-cleaning properties of the glass surface are also desired. A scalable and robust approach to produce antireflective coatings for glass surfaces with desired optical and mechanical properties is introduced in this work. The developed coating mimics the structure of a moth-eye cornea. The coating is a subwavelength-microstructured thin layer on the glass surface made of a monolayer of hemispherical silica nanoparticles obtained by hydrothermal fusion of spherical particles to the glass substrate. The sequence of the particle deposition in the layer-by-layer process is adjusted to balance attractive-repulsive interactions among nanoparticles and between the nanoparticles and the glass surface to generate coatings with a high surface coverage of up to 70%, which exceeds the 54.7% limit of the random sequential addition model. This level of surface coverage allows for a combination of properties beneficial for the described applications: (i) an average reflectance of 0.5 ± 0.2% for a visible and near-infrared optical spectrum, (ii) an improved mechanical stability and scratch resistance, and (iii) non-wetting behavior.

Broadband, millimeter-wave anti-reflective structures on sapphire ablated with femto-second laser

We designed, fabricated, and measured anti-reflection coating (ARC) on sapphire that has 116% fractional bandwidth and transmission of at least 97% in the millimeter wave band. The ARC was based on patterning pyramid-like sub-wavelength structures (SWS) using ablation with a 15 W femto-second laser operating at 1030 nm. One side of each of two disks was fabricated with SWS that had a pitch of 0.54 mm and a height of 2 mm. The average ablation volume removal rate was 1.6 mm3/min. Measurements of the two-disk sandwich show transmission higher than 97% between 43 and 161 GHz. We characterize instrumental polarization (IP) arising from differential transmission due to asymmetric SWS. We find that with proper alignment of the two-disk sandwich, RMS IP across the band is predicted to be 0.07% at normal incidence and less than 0.6% at incidence angles up to 20°. These results indicate that laser ablation of SWS on sapphire and on other hard materials such as alumina is an effective way to fabricate broadband ARC.

Tuning refractive indices of sol-gel silica coatings by ammonia treatment for broadband antireflection applications

Broadband antireflection coatings via sol-gel processes have find wide applications in a variety of optical areas. However, to adjust refractive indices of coatings, most sol-gel methods involved complicated processes that modify the microstructures of sol particles. Here, we find that ammonia treatment of the sol-gel silica coatings can significantly tailor their refractive indices. Based on the findings, we created broadband antireflection coatings through preparing two-layer coatings. The coatings comprised two layers: a bottom layer with a high refractive index via ammonia treatment of the sol-gel silica coating, and a top layer with a low refractive index from the normal sol-gel coating. The two-layer coatings demonstrated broadband antireflection characteristics, with transmittance higher than 98 % from 350 to 1400 nm. Further hexamethyldisilazane treatment led to formation of hydrophobic coatings, which exhibited high environmental stability. The study demonstrates a new and simple method to create broadband antireflection coatings, and the broadband antireflection coatings could be attractive in many areas, such as solar cells and high-powered laser system.

Enhancement of fluorescence from nitrogen-vacancy center ensemble in bulk diamond with broadband antireflection coatings

The negatively charged nitrogen-vacancy (NV−) center in diamond is a promising platform for quantum sensing. However, fluorescence from the NV− centers suffers large energy loss at the diamond–air interface. Here, we propose a broadband antireflection coating to enhance the fluorescence intensity by simultaneously reducing the energy loss of the excitation laser and the fluorescence. The reflectance for normal-incidence light decreases from nearly 17% for bared diamond to below 0.33% for coated diamond in the wavelength range 500 nm–800 nm. The reflectance averaged over the fluorescence bandwidth is below 3% for angles of incidence less than 20°. The measured emitted fluorescence for the coated diamond is 1.44 times that of uncoated diamond, corresponding to nearly 20% improvement in the measurement sensitivity. The proposed method is significant for enhancing the signal-to-noise ratio of NV−-based sensors.

Low-Temperature Preparation of SiO2/Nb2O5/TiO2–SiO2 Broadband Antireflective Coating for the Visible via Acid-Catalyzed Sol–Gel Method

Multilayer broadband antireflective (AR) coatings consisting of porous layers usually suffers poor functional durability. Based on a quarter-half-quarter multilayer structure, AR coatings with dense SiO2 film as the top layer are designed, and refractive index for each layer is optimized. After heat-treated at only 150 °C, refractive index of Nb2O5 film reaches to 2.072 (at 550 nm), which can meet design requirements of the middle layer. TiO2–SiO2 composites with controllable refractive indices are selected to be used as the bottom layer. The obtained triple-layer AR coating presents excellent performance, and the average transmittance at 400–800 nm attains 98.41%. Dense layers endow the multilayer structure good abrasion-resistance, and hexamethyldisilazane is further used to modify the surface of the AR coating, which can greatly improve the hydrophobicity of the coating. The proposed triple-layer broadband AR coating has potential value in practical applications of sol–gel deposition.

Embossed Mie resonator arrays composed of compacted TiO2 nanoparticles for broadband anti-reflection in solar cells

Mie resonator arrays formed by embossing titanium dioxide (TiO2) nanoparticles (NPs) from solution are investigated as optical coatings for anti-reflection applications. Compacted nanoparticle assemblies offer unique possibilities to tailor the effective refractive index (RI). Here, we demonstrate a simple table-top, low pressure, and low temperature method to fabricate structured optical coatings. TiO2 nanostructures in the form of nanodisks support Mie resonances in the visible wavelength spectrum and exhibit strong forward scattering into the high index substrates, making them suitable as broadband anti-reflection coatings for solar cells. TiO2 NP-based nanodisk arrays are designed, fabricated, and characterized regarding their anti-reflection properties on Si, GaAs, and InP substrates and solar cells. Detailed finite-difference time-domain simulations are performed to optimize the TiO2 NP-based Mie resonator arrays for the broadband anti-reflection as well as to explain the measured reflectance spectra. The solar-weighted reflectance is used as a figure of merit (FoM). TiO2 nanodisk arrays on Si show a FoM of ~ 7% in the 400–1,100 nm wavelength spectrum; similar values are obtained for GaAs and InP substrates. TiO2 nanodisk arrays embossed directly on prefabricated planar single-junction Si, GaAs, and InP solar cells result in an appreciable increase (~ 1.3 times) in the short-circuit current densities.

Spherical-chain silica with super-hydrophobic surface and ultra-low refractive index for multi-functional broadband antireflective coatings

We present a new type of ultra-low refractive index antireflective (AR) coating based on spherical-chain silica particle with super-hydrophobic surface. This AR coating is fabricated using methyltriethoxysilane (MTES) and ethylsilicate (TEOS) as co-precursors to direct silica particles to form a spherical-chain structure. Then hexamethyldisilazane (HMDS) was further used to modify the surface of spherical-chain silica particle, which greatly decreases the refractive index of the silica thin film from 1.139 to 1.098. The formation mechanism of the ultralow refractive index thin film is proposed. The spherical-chain silica particle surface modification with HMDS significantly improves the hydrophobicity of the coated film; the water contact angle of the film increases from 42° to 168°. Ultimately, using this hydrophobic ultralow-index coating as a top-layer, a high-performance double-layer antireflective coating was fabricated with self-cleaning, environmental resistance and a high average transmittance of 99.64% in the wavelength range of 400–800 nm. This multi-functional broadband antireflective coating has potential value in the field of solar cells.

Preparation of wide-angle and abrasion-resistant multi-layer antireflective coatings by MgF2 and SiO2 mixed sol

The wide-angle broadband antireflective coatings (ARCs) were fabricated by a sol–gel method and covered on a soda-lime glass substrate by using dip-coating process. The crystal structural, morphological and antireflection performance of ARCs were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), ultraviolet spectrophotometer (UV–vis), etc. In addition, the optimal experimental parameters of each coating were obtained by optical film design software combined with experimental characterization. The average transmittance of the wide-angle broadband ARCs is 9% higher than 88.90 % of the soda-lime glass substrate in the range of 400−1200 nm. Furthermore, the preliminary evaluation of wear resistance shows that the antireflective performance of the coating was stable before and after the friction test. All the excellent antireflective performance and abrasion-resistance afforded the ARCs with potential application in fields of construction, medical, new energy, military, etc.

Methylated Silica Surfaces Having Tapered Nipple-Dimple Nanopillar Morphologies as Robust Broad-Angle and Broadband Antireflection Coatings

In this work, mechanically, chemically and thermally resistant broad-band and broad-angle anti-reflection coatings were prepared on 10 cm diameter glass substrates combining sol-gel deposition with...

Aggregation of Silica Nanoparticles in Sol-Gel Processes to Create Optical Coatings with Controllable Ultralow Refractive Indices.

Optical coatings with controllable ultralow refractive indices are of profound significance in optical areas. However, it remains a challenge to fabricate such coatings using a simple method. Here we develop an effective and simple approach to create ultra-low-index coatings. This approach was based on a modified sol-gel process, with a key process that involved the aggregation of silica nanoparticles via the addition of a polymer surfactant (e.g., polyvinylpyrrolydone) in sols before coating. The approach involves three steps: the synthesis of silica sols under ammonia catalysis in ethanol (Stöber method), the addition of polyvinylpyrrolydone in the silica sols to induce the aggregation of the silica nanoparticles, and the formation of ultra-low-index coatings by depositing the aggregated silica sols on substrates. Through varying the aggregation extent, this approach produced coatings with controllable refractive indices ranging from 1.17 to 1.07. To the best of our knowledge, the minimum index value of 1.07 from our coating is among the lowest refractive indices ever reported. The ultra-low-index coatings demonstrated excellent optical properties, with which perfect quarter-wavelength antireflection coatings (maximum transmittance ∼100%) and broadband antireflection coatings (transmittance >98% from 400 to 1100 nm) can be prepared. One advantage of the antireflection coatings is that their transmission is less dependent on the refractive index and the thickness of the stacking layer, which make it promising in large-scale production. Moreover, the coatings can be made hydrophobic (water contact angle 136°) by exposing the coatings to a hexamethyldisilazane atmosphere, exhibiting high environmental stability in a humid environment. The aggregation of silica nanoparticles in sol-gel processes provides a scalable alternative to the current approaches for creating ultra-low-index coatings.

Preparation of MgF2/SiO2 coating with broadband antireflective coating by using sol–gel combined with electron beam evaporation

The broadband antireflective coatings (ARCs) can effectively reduce the optical loss in the transmittance process and improve energy efficiency in solar cells. MgF2/SiO2 bilayer films were prepared on the surface of soda–lime substrates via sol–gel spin-coating combined with electron beam (e-beam) evaporation. The bottom layer was the MgF2 film obtained from e-beam evaporation, whereas the top layer was the nanoporous SiO2 film via sol–gel spin-coating method. The refractive indices of MgF2 and SiO2 layers at 550 nm are 1.38 and 1.12, respectively. The structural, morphological, and optical properties of the films were characterized. The effects of annealing temperature, Triton X-100 concentration, and spin-coating speed on the properties of the film were investigated. The transmittance spectra of the coatings were investigated, and the broadband antireflective performance of the bilayer structure was examined. A bilayer coating with optical properties close to the theoretical design results was obtained by optimizing the refractive index and the thickness of the double coatings. The solar transmittances of the soda–lime glass coated with the MgF2/SiO2 bilayer antireflective film increased by 8.89% at 400–800 nm. The multilayer antireflective film exhibits promising applications in photoelectric devices, high power laser systems, and antidazzle glasses.

Nanoscale etching of microporous coatings for broadband antireflection coatings

Broadband antireflection coatings are of great scientific and technological significance in optical areas because of their high transmission in broad wavelength bands. Here, we report an effective and simple approach to create broadband antireflection coatings based on nanoscale etching. To acquire broadband antireflection coatings, coatings with homogenously distributed pores were prepared by thermal treatment of polymer doped silica coatings and then were subjected to aqueous etching. The aqueous etching had selectivity along the thickness of the coatings, resulting in a graded porous morphology with more porosity on the top and less porosity at the bottom. The coatings with graded porosity exhibited excellent broadband antireflection likely due to the graded refractive index. The transmission of the coatings could be controlled by varying the amount of the polymer additives and the coating thickness. The optimized coating showed transmittance higher than 98% from 350 to 1100 nm, which is comparable to current state-of-the-art broadband antireflection coatings. The aqueous etching based approach enables easy access to broadband antireflection coatings on a wide range of optical substrates, and presents a promising alternative to existing technologies for fabrication of broadband antireflection coatings.

Transparent conductive oxide film with antireflective properties for Cu<sub>2</sub>ZnSnS<sub>4</sub> solar cells

At present, there are several kinds of broadband antireflection coatings (ARCs). For the flat multilayer ARC, it usually contains double, triple, or up to 4 layers. It has been demonstrated that the performance of a single layer coating is not good enough across the desired spectral range. Multiple layer ARCs have much better performance for broadband solar cells (SCs). When inspecting the antireflection structure of Cu<sub>2</sub>ZnSnS<sub>4</sub> solar cells (CZTSSCs), it is shown that the transparent conductive oxide (TCO) of traditional CZTSSCs does not have an satisfactory antireflective performance. This paper aims to investigate a way to increase the incident light transmitted into CZTSSCs, and thus improving the efficiency of solar cells by studying the use of the antireflective effect of a TCO film. It introduces a new type of TCO film with better antireflective properties across a wide wavelength range. An SiO<sub>2</sub>/ZnO antireflective TCO (ATCO) is designed under AM1.5 illumination. In order to measure the antireflective effect over the 300–800 nm wavelength range, an effective average reflectance method (EAR) is introduced. Considering the effect of the refractive index dispersion and the coupling of the TCO or ATCO films with the active layer, in this paper we use a multi-dimensional transfer matrix to optimize the thickness of each key layer to accurately confirm the best antireflective effect. In addition, the optimized TCO film and the optimized ATCO film in CZTSSCs are compared and analyzed by means of EAR. The result shows, through the comparison of the antireflection between conventional TCO CZTSSCs and ATCO CZTSSCs, that there are considerable differences in final optimal reflectivity between TCO layer and ATCO film. For the conventional CZTSSC, the optimal effective average reflectance of TCO layer is 5.6%, and the lowest reflectivity in the waveband from 400 nm to 500 nm is 6.9%. In addition, the corresponding values obtained in the new ATCO CZTSSC are 3.8% and 1.6% respectively. These apparent changes in reflectivity are appealing in that the new ATCO films can effectively reduce light loss and improve the efficiency of photovoltaic conversion.

Broadband antireflective coatings for high efficiency InGaP/GaAs/InGaAsP/InGaAs multi-junction solar cells

Design and fabrication of antireflection coatings (ARCs) was performed to maximize the photocurrent of Ⅲ-Ⅴ solar cells based on InGaP, GaAs, InGaAsP, and InGaAs. The current-limiting behavior of multi-junction solar cells allowed only the specific design of ARCs to improve the total current density of multi-junction solar cells, which enabled thin and efficient antireflection layers to achieve their full potential. ARCs were designed for InGaP/GaAs and InGaAsP/InGaAs double junction solar cells and InGaP/GaAs/InGaAsP/InGaAs quadruple junction solar cells. From the ARC design results, the best solar cell reflectance at the region of interest appeared to be 1.44% for InGaP/GaAs solar cells, 2.17% for InGaAsP/InGaAs, and 0.48% for the quadruple junction solar cells. To achieve the preferred refractive index for the ARC structures, nanostructures fabricated via glancing angle deposition were utilized. With ARC fabricated structures, InGaP/GaAs double junction solar cells yielded a power conversion efficiency of 13.3%, which was 1.34 times higher than that of cells without ARC (9.91%). Additionally, calculation results for the various ARC designs using dual and triple material systems were provided to increase their applicability.

Superhydrophilic antifogging broadband antireflective coatings with worm-like nanostructures fabricated by one dip-coating method and calcination

In this work, superhydrophilic antifogging antireflective coatings were prepared by the dip-coating of polyacrylate/polypropylsilsesquioxane core-shell nanocomposite particles’ sols followed by calcination at 500 °C for 4 h. The results showed that the transmittance of all the coatings in the band of 350–1000 nm was significantly higher than that of pure glass substrate, showing excellent broadband and antireflection performance. In particular, the maximum transmittance of the coatings with 8.0 g of propyltrimethoxysilane (PTMS-8) was 99.1% at the wavelength of 546 nm, and the average transmittance of the coatings reached 98.0% in the wavelength range of 400–700 nm and was enhanced by 9.3% compared to that of the bare glass substrate. Furthermore, the contact angle of the coatings decreased with the increase in the surface bumps structures of the coatings, and the minimum contact angle of the surface of the coatings with 10.0 g of propyltrimethoxysilane (PTMS-10) reached nearly zero within 0.5 s when the bumps evolved into a worm-like nanostructure, showing excellent superhydrophilic and antifogging properties. It was concluded that both roughness and special worm-like nanostructures give rise to the superhydrophilicity of the coatings.