Antireflective Thin Films Research Articles

Studies on Preparation and Characterization of NIR Antireflection Thin Films

In this study, preparation of different designs of thin films by physical vapour deposition (PVD) with the help of MgF2 and YF3 materials of low and medium refractive index on the BK-7 glass substrate at Near Infra Region (NIR) with quarter wave thickness of 963nm (0.9 um) is presented. The structural and optical properties of MgF2/YF3 thin films produced by PVD technique have been investigated. Transmission is found to vary continuously with layer thickness. The optical transmittance at normal incidence was obtained in the wavelength range 500-2000nm by using UV/VIS/NIR spectrophotometer. The crystalline nature and orientation of the MgF2/YF3 thin films have been systematically investigated at room temperature by X-ray diffraction patterns (XRD). The surface morphology studies of Roughness (Ra), RMS, Grain size were investigated systematically by Atomic Force Microscope (AFM).

Fabrication of Hydrophobic Anti-Reflection Coating Film by Using Sol-gel Method

Anti-reflection coating films have used to increase the transmittance of displays and enhance the efficiency of solar cells. Hydrophobic anti-reflection coating films were fabricated on a glass substrate by sol-gel method. To fabricate an anti-reflection film with a high transmittance, poly ethylene glycol (PEG) was added to tetraethyl orthosilicate (TEOS) solution. The content of PEG was changed from 1 to 4 wt% in order to control the morphology, thickness, and refractive index of the SiO2 thin films. The reflectance and transmittance of both sides of the coated thin film fabricated with PEG 4 wt% solution were 0.3% and 99.4% at 500 nm wavelength. The refractive index and thickness of the thin film were n = 1.29 and d = 105 nm. Fluoro alkyl silane (FAS) was used for hydrophobic treatment on the surface of the anti-reflection thin film. The contact angle was increased from 13.2˚ to 113.7˚ after hydrophobic treatment.

Block copolymer self assembly for design and vapor-phase synthesis of nanostructured antireflective surfaces

The authors combine block copolymer self assembly with vapor-phase synthesis for design of antireflective thin film coatings. The nanometer-scale features in patterns formed by cylindrical phase block copolymers provide surface topography for vapor-phase growth of semiconductors and metals by oblique angle physical vapor deposition. The authors control the dimensions and density of the synthesized nanotextures through selection of copolymer molecular weight. A layer of aligned, densely packed germanium wire arrays with diameters much smaller than optical wavelengths acts as an effective optical medium, significantly reducing reflections and improving light coupling into a silicon substrate. A synthesized layer of uniformly sized silver nanoparticles provides antireflection instead through optical excitation of localized surface plasmons. The block copolymer-based synthesis approach allows control of particle shape anisotropy, tuning the frequency of plasmon resonances and expanding the spectral range of antireflection.

Fabrication of antireflective antifogging nano-porous silica thin film on glass substrate by layer-by-layer assembly method

In this research, silica nano-porous thin films were deposited on glass substrates by layer-by-layer assembly process. The structure, morphology, surface characteristics, surface roughness, and optical and hydrophilic properties of the thin film were investigated. The X-ray diffraction analysis shows that the silica nano-porous thin film has an amorphous structure. The transmittance spectra revealed that the silica nanoporous thin film increased transparency of the glass substrate from 91.8% to 97% at 550nm. In addition, the silica nanoporous thin film decreased water contact angle of glass surface from 67° to 3°. Therefore, silica nanoporous thin film showed high transparency and superhydrophilicity which greatly encourage the antireflection and antifogging functions of the thin film.

Anti-reflection porous SiO2 thin film deposited using reactive high-power impulse magnetron sputtering at high working pressure for use in a-Si:H solar cells

Porous SiO2 thin films with low reflectance and high transmittance were obtained using reactive high power impulse magnetron sputtering (HIPIMS) at a high working pressure of 6.67Pa (50mTorr). The average transmittance (450–600nm) of the SiO2 thin films was 94.45%. In comparison, SiO2 thin films deposited at a low working pressure of 0.27Pa (2mTorr) showed an average transmittance of 91.26%. The improvement in the transmittance was attributed to the lower refractive index resulting from the porous structure of the SiO2 thin films. To examine the effect of the anti-reflection SiO2 coating, an a-Si:H solar cell was produced on fluorine-doped tin oxide (FTO) glass. The initial energy conversion efficiency for cells using the anti-reflection, SiO2-coated FTO glass was 11.75%, higher than the 10.75% for the sample using the bare FTO glass. The increase in the short-circuit current density (Jsc) due to the decreased light reflectance was the largest contributor to the increase in the a-Si:H solar cell efficiency.

Fabrication and performance of polymer–nanocomposite anti-reflective thin films deposited by RIR-MAPLE

Design of polymer anti-reflective (AR) optical coatings for plastic substrates is challenging because polymers exhibit a relatively narrow range of refractive indices. Here, we report synthesis of a four-layer AR stack using hybrid polymer:nanoparticle materials deposited by resonant infrared matrix-assisted pulsed laser evaporation. An Er:YAG laser ablated frozen solutions of a high-index composite containing TiO2 nanoparticles and poly(methyl-methacrylate) (PMMA), alternating with a layer of PMMA. The optimized AR coatings, with thicknesses calculated using commercial software, yielded a coating for polycarbonate with transmission over 97 %, scattering <3 %, and a reflection coefficient below 0.5 % across the visible range, with a much smaller number of layers than would be predicted by a standard thin film calculation. The TiO2 nanoparticles contribute more to the enhanced refractive index of the high-index layers than can be accounted for by an effective medium model of the nanocomposite.

Mechanically robust, thermally stable, broadband antireflective, and superhydrophobic thin films on glass substrates.

In this study, we developed a simple and versatile strategy to fabricate hierarchically structured lotus-leaf-like superhydrophobic thin films. The thin films are broadband antireflective, and the average transmittance of coated glass substrates reached greater than 95% in the wavelength range of 530-1340 nm, in contrast to 92.0% for bare glass substrate. The thin film surface shows a static water contact angle of 162° and a sliding angle less than 4°. Moreover, the thin film is thermally stable up to 300 °C, and shows remarkable stability against strong acid, strong alkali, water drop impact, and sand impact abrasion, while retaining its superhydrophobicity. Further, the thin film can pass the 3H pencil hardness test. The current approach may open a new avenue to a variety of practical applications, including windshields, eyeglasses, windows of high rise buildings and solar cells, etc.

Microstructures, optical and photoelectric conversion properties of spherical silicon solar cells with anti-reflection SnOx:F thin films

The microstructures and optical and photoelectric conversion properties of spherical silicon (Si) solar cells were investigated and discussed. The surface of the spherical Si with a pn junction provided high crystallinity, and the lattice constant of the center of Si spheres is larger than that of the surface, which would be due to the lattice distortion by defect structures at the center of Si. The conversion efficiencies of spherical Si solar cells coated with SnOx:F anti-reflection thin films were improved by annealing. The optical absorption and fluorescence of the solar cells increased, and the lattice constants of SnOx:F anti-reflection layers decreased after annealing. The mechanisms of chemical reactions at the Si/metal interface were also discussed.

Hydrophobic coating of silicate phosphor powder using atmospheric pressure dielectric barrier discharge plasma

A stable superhydrophobic coating was successfully deposited on commercial silicate‐based orange phosphor by using atmospheric pressure dielectric barrier discharge plasma with hexamethyldisiloxane (HMDSO) and HMDSO/toluene mixture as precursors. Owning to the good optical properties, the deposited film acts not only as a hydrophobic protective layer but also as an antireflection optical thin film capable of improving the phosphor photoluminescence efficiency. The plasma‐polymerized film based on SiOSi backbone containing methyl and phenyl nonpolar functional groups exhibited high‐water‐repellent characteristics. It was found that the water contact angle gradually increased with increasing the aging time and remained unchanged at about 140° after 1‐month aging. Besides, the thermal stability of the coated phosphor under high‐temperature condition was substantially enhanced by the aging. The findings of this work can contribute to improving the durability and reliability of the phosphor, eventually the long‐term stability of phosphor‐based light emitting diodes in practical applications. © 2014 American Institute of Chemical Engineers AIChE J, 60: 829–838, 2014

Preparation and Characterization of Porous Silica Antireflective Thin Film on Glass Substrates by Chemical Etching

In this paper, porous silica antireflective thin film with special nanostructure has been successfully established using glass-cover board as a single precursor through a novel chemical etching route. The process mainly involves selective leaching of active component on the surface of glass such as sodium oxide, calcium oxide. The effects of etching time, acid concentration and temperature on the porous silica antireflective thin film were demonstrated. Surface microstructure and optical properties of the porous silica antireflective thin film were characterized in detail by scanning electron microscopy (SEM), UV-visible (UV-Vis) spectroscopy. The results indicate the porous silica antireflective thin film were composed of granular with diameter less than 30nm, and the thickness of film was in the range of 80~100nm. Enhanced transparency (up to 94.6%) was achieved in a wide wavelength range.

SURFACE MORPHOLOGY OF MGF2/YF3 MULTI LAYER THIN FILMS BY THERMAL EVAPORATION METHOD

Magnesium fluoride and Yttrium fluoride antireflec tion multilayer thin films have been deposited on t o well cleaned BK-7 Glass substrate in Vacuum of 2x10 -5 mbar using physical vapour deposition technique. S urface morphology analysis by AFM of different multilayer structures deposited by thermal evaporat ion method i.e., Single layer, double layer, three layer and four layer thin films were presented in this study. The main object ive of this study was to investigate the influence of thickness of various antireflection multilayer thin films on the effect of surface roughness and grain size of the particle s. The thickness of the films has been determined by quartz crystal monitor method. T he surface morphology studies are performed using A tomic force Microscopy (AFM) techniques. AFM was the best tool to investig ate the surface smoothness and to find the grain si ze of the particles. The grain size is calculated for all the films of different thickness. Index Terms: Multilayer’s, AFM, MgF 2/YF 3, Roughness, Grain size, BK-7, Antireflection

Fabrication of Broadband Antireflective Plasmonic Gold Nanocone Arrays on Flexible Polymer Films

Flexible broadband antireflective and light-absorbing nanostructured gold thin films are fabricated by gold vapor deposition onto Teflon films modified with nanocone arrays. The nanostructures are created by the oxygen plasma etching of polystyrene bead monolayers on Teflon surfaces. The periodicity and height of the nanocone arrays are controlled by the bead diameter and the overall etching time. The gold nanocone arrays exhibit a reflectivity of less than 1% over a wide spectral range (450-900 nm) and a wide range of incident angles (0-70°); this unique optical response is attributed to a combination of diffractive scattering loss and localized plasmonic absorption. In addition to nanocones, periodic nanostructures of nanocups, nanopyramids, and nanocavities can be created by the plasma etching of colloidal bilayers. This fabrication method can be used to create flexible nanocone-structured gold thin films over large surface areas (cm(2)) and should be rapidly incorporated into new technological applications that require wide-angle and broadband antireflective coatings.

The Properties of ITO/Arc-ZnO Passivating Layer for TCO Conducting Substrate Deposited by RF Magnetron Sputtering

Abstract. In this work, a thermally stable multilayered transparent conducting oxide (TCO) utilizing ZnO antireflective thin film (arc-ZnO) has been prepared by RF magnetron sputtering. The novel ITO/arc-ZnO conducting substrate with blocking layer capabilities has been designed using step-down interference coating structure based on double layer antireflection coating (DLAR). The XRD result revealed a mixed-oriented type of crystalline structure between ITO and ZnO with preferred [222] and [002] orientations, respectively. The antireflection behaviour of multilayer ITO/arc-ZnO is evidence with the presence of two maximum peaks overshoots at 440 nm and 750 nm range while eliminating approximately 92% reflectance, hence contributes to higher overall transmittance (≥ 87%) in the mid-wavelength region. The ITO/arc-ZnO blocking layers conserves the low resistivity of ITO at 5.71 x 10-4 Ω cm, even after oxidizing during air annealing process above 400 °C. These results demonstrate that the multilayered ITO/arc-ZnO with tailored refractive index substrate is a realistic approach for higher overall transmittance with good stability in electrical properties, prolonged with an added capability of suppressing back electron transfer that is foreseen suitable for dye sensitized solar cell application.

Fabrication of broadband anti-reflective sub-micron structures using polystyrene sphere lithography on a Si substrate

Anti-reflective coatings are widely used on the surfaces of solar cells to increase the efficiency of photoelectric conversion. Sub-wavelength structures have gradually replaced conventional anti-reflective (AR) thin films due to their broadband AR properties. This paper successfully fabricated structures with a variety of surface morphologies on Si substrate using polystyrene sphere lithography in conjunction with two-step inductive coupling plasma (ICP) and high density plasma (HDP) etching processes. We successfully fabricated various sub-micron structures with heights of 700nm and above. Experimental results show that the sub-micron pyramidal structure has the best anti-reflection performance with the average reflectance effectively suppressed to below 1% across the spectral range of 300–1200nm.

Enhanced Light Absorption and Minority Carrier Lifetime for Pn&lt;sup&gt;+&lt;/sup&gt; Multicrystalline Silicon by Treatment with HF/H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;-Based Solutions

A novel method was proposed to form porous silicon (PS) antireflection layers and thin SiO2 films at the same time by HF/H2O2 treatment of acid-textured pn+ multicrystalline silicon. Porous silicon structures formed inside the cavities and the porosity became large with an increase of the treated time resulting in a dramatical decrease of reflectance. The reflectance decreased to less than 5% within the wavelength range of 420-970 nm after 5 min HF/H2O2 treatment. Furthermore, the minority-carrier lifetime showed an increase of about 42% for a short treated time because of the formation of a thin silicon oxide layer resulting the reduction of dangling silicon bonds in the interface between PS/Si.

Improvement in power conversion efficiency of organic photovoltaic devices by using excimer ultraviolet-radiation induced mesoporous silica anti-reflection coating

An anti-reflection thin film with a mesoporous structure is demonstrated to improve the light-collection and photocurrent-generation abilities of organic photovoltaic cells. The mesoporous film was prepared by sol–gel-derived silica with an excimer ultraviolet-radiation removable self-assembling triblock copolymer as the nanostructured template. The method for fabricating this film was much easier than conventional processes used to prepare anti-reflection materials as they involve vacuum or high-temperature conditions. By using this mesoporous film as an anti-reflection coating on the glass side of an indium tin oxide-coated glass substrate, an enhancement of about 6% in the photocurrent generation could be achieved in a device with a poly(3-hexylthiophene):(6,6)-phenyl-C61-butyric acid methyl ester bulk-heterojunction layer. A glass substrate with a coating of the mesoporous anti-reflection film showed an obvious improvement in transmittance for wavelengths extending from the visible-light region to the near-infrared region. The result also indicates that the prepared anti-reflection material is not only compatible with the photoactive materials with wide band gaps but works very well with semiconductors with small band gaps as well, e.g. thieno(3,4-b)-thiophene/benzodithiophene copolymer (PTB7). In this work, ~3.6% photocurrent enhancement can also be obtained in PTB7-based solar cells, indicating the wide compatibility of thus-prepared mesoporous film.

Transmittance Enhanced Properties of Novel Encapsulated ITO/Arc-TiO&lt;sub&gt;2&lt;/sub&gt; Antireflective TCO Substrate Prepared by RF Magnetron Sputtering

In this work, a thermally stable multilayered transparent conducting oxide (TCO) utilizing TiO2 antireflection thin film (arc-TiO2) encapsulated under indium tin oxide (ITO) glass has been prepared by RF magnetron sputtering. The novel tri-functional conducting substrate with blocking layer capabilities has been designed via step-down interference coating structure of double layer antireflection coating (DLAR). The mixed-oriented type between the strongest ITO peak at (222) and a weak TiO2 peaks at (101) orientations have been observed under XRD analysis. The antireflection properties of double-layer ITO/arc-TiO2 is evidence with the existence of two maximum peaks around 410 nm and 750 nm. While, the corresponding reduction in reflectance of about 8% and 2% compared to bare ITO was achieved. The ITO/arc-TiO2 blocking layers conserves the low resistivity of ITO at 2.05 x 10-4 Ω cm, even after oxidizing during air annealing process above 400 °C. These results demonstrate that the multilayered ITO/arc-TiO2 with tailored refractive index by means of annealing treatment is a promising approach to realize a substrate which (a): electrically and thermally stable against processing temperature, (b): sustains the higher transmittance of the substrate even there is increase in total substrate thickness and (c): prevents electron recombination process occurring at the interface between the redox electrolytes and the TCO surface. The stable properties are found to be beneficial for use as TCOs in DSSCs.

Novel encapsulated ITO/arc-ZnO:TiO2 antireflective passivating layer for TCO conducting substrate prepared by simultaneous radio frequency-magnetron sputtering

Graphical abstractDisplay Omitted Highlights? We fabricated ITO/arc-ZnO:TiO2 antireflective TCO with blocking layer capability for DSSCs. ? Step-down interference structure based on double layer antireflection coating was used. ? Two maximum transmittance peaks around 440nm and 760nm could be seen. ? About 11% and 3% reduction in reflectance compared to bare ITO alone were achieved. ? Stable properties of antireflective TCO against annealing temperature achieved. A thermally stable multilayered transparent conducting oxide utilizing nanocomposite ZnO:TiO2 antireflection thin film (arc-ZnO:TiO2) on an indium-tin oxide (ITO) substrate has been prepared by radio frequency-magnetron sputtering. The effects of post-deposition annealing on the morphological, structural, optical and electrical properties were investigated with the aim to find the best conditions for the antireflection substrate. The X-ray diffraction behaviors of mixed type crystalline structures preferred at 222] and 101] between ITO and ZnO were observed, respectively. The grain size increased from 37.9nm to 46.9nm due to the aggregation of smaller grain during the annealing process. Average transmittances of approximately 82% were observed for all the samples in the range between 440nm and 760nm. A corresponding reduction in the reflectance of about 11% and 3% compared to bare ITO was achieved. The refractive index increased from 1.82 up to 2.0 due to the change in the packing density. For all ranges of annealing conditions, the arc-ZnO:TiO2 layers conserved the low resistivity of ITO at 1.96i?10-4?. In other words, the oxygen consumed in the annealing process improved the crystallinity of the film without much change to the resistivity of the ITO-based substrate. The increment of the root mean square roughness from 2.12nm to 3.21nm evidenced by the atomic force microscopy analysis was supported by the increment of grain growth.

A novel precursor-derived one-step growth approach to fabrication of highly antireflective, mechanically robust and self-healing nanoporous silica thin films

A novel precursor-derived one-step growth (POG) approach has been developed to fabricate highly antireflective, mechanically robust and self-healing nanoporous silica thin films. The nanoporous silica thin films were uniform and smooth with low roughness. Their transmittance decreased with the increase of exposure time to wet air, but it could completely self-heal after exposure to dry air. The key to achieving the antireflective nanoporous silica thin films is to control the growth time, CTAB/TEOS ratio and reaction temperature in the Stober solution. The maximum transmittance of coated substrates reached as high as 99.2%. The thin film coated glass substrates also exhibited excellent reduced reflection at high incident angles in addition to a significant reduction in broadband reflection. Meanwhile the thin films have excellent mechanical strength and stability (passed the 6H pencil hardness test, sand flow test and washability test). The current approach is facile, effective and versatile, and may open a new avenue to advanced multifunctional optical films of varied compositions and other 2D nanoarchitectures, and provide many new and exciting opportunities in the fabrication of functional devices for optics, electronics, optoelectronics, electrochemistry, molecule separation, and so on.

Atmospheric pressure chemical vapor deposition of high silica SiO2–TiO2 antireflective thin films for glass based solar panels

The atmospheric pressure chemical vapor deposition (APCVD) of SiO2–TiO2 thin films on glass by employing a [[(tBuO)3SiO]2–Ti(OiPr)2] single-source precursor, which can be prepared from commercially available materials, results in antireflective thin films with high silica SiO2 : TiO2 ratio (i.e. SiO2 > 1) on float glass under industrially relevant manufacturing conditions.