Graded Index Antireflection Coatings Research Articles

Microcell concentrating photovoltaics for space

Christian Ruud received his PhD in materials science and engineering at Penn State University under the guidance of Dr. Chris Giebink. His doctoral research was funded by a NASA Space Technology Graduate Research Opportunities award and covered microcell concentrating photovoltaics for space and graded-index antireflection coatings. He also holds a bachelor’s degree in chemistry from St. Olaf College in Northfield, Minnesota. Recently, Christian began a new role as a research engineer at PPG in Pittsburgh, Pennsylvania. Jeffrey Gordon is professor emeritus at Ben-Gurion University of the Negev's Department of Solar Energy and Environmental Physics in Israel, where he has been a faculty member for 45 years. He played a pivotal role in stewarding it to an internationally recognized center of research and graduate education. His research advances have led to over 250 publications in leading scientific journals and cover the fields of nanomaterial synthesis, solar energy conversion, advanced optical design, finite-time thermodynamics, biomedical science, and algae biotechnology, with several innovations having been translated into impactful commercial realities. Chris Giebink has been a professor of electrical engineering at Penn State University for 10 years and is presently moving to the Department of Electrical Engineering and Computer Science at the University of Michigan. His research on optoelectronic and photonic devices has led to more than 80 publications and 13 patents. He received his PhD in electrical engineering from Princeton University and holds undergraduate degrees in physics and engineering science from Trinity University. Christian Ruud received his PhD in materials science and engineering at Penn State University under the guidance of Dr. Chris Giebink. His doctoral research was funded by a NASA Space Technology Graduate Research Opportunities award and covered microcell concentrating photovoltaics for space and graded-index antireflection coatings. He also holds a bachelor’s degree in chemistry from St. Olaf College in Northfield, Minnesota. Recently, Christian began a new role as a research engineer at PPG in Pittsburgh, Pennsylvania. Jeffrey Gordon is professor emeritus at Ben-Gurion University of the Negev's Department of Solar Energy and Environmental Physics in Israel, where he has been a faculty member for 45 years. He played a pivotal role in stewarding it to an internationally recognized center of research and graduate education. His research advances have led to over 250 publications in leading scientific journals and cover the fields of nanomaterial synthesis, solar energy conversion, advanced optical design, finite-time thermodynamics, biomedical science, and algae biotechnology, with several innovations having been translated into impactful commercial realities. Chris Giebink has been a professor of electrical engineering at Penn State University for 10 years and is presently moving to the Department of Electrical Engineering and Computer Science at the University of Michigan. His research on optoelectronic and photonic devices has led to more than 80 publications and 13 patents. He received his PhD in electrical engineering from Princeton University and holds undergraduate degrees in physics and engineering science from Trinity University.

Fabrication and simulation investigation of zig-zag nanorod-structured graded-index anti-reflection coatings for LED applications

In this study, we report the fabrication, characterization, and modeling of a zigzag nanorod-structured graded-index antireflection coating fabricated by the oblique angle deposition (OAD) method. The optical properties of the reported coating were engineered by sculpting its composition and morphology using OAD, and the coating was designed to work with high-index, YAG:Ce ceramic phosphor plates. The coating enhances the normal-direction transmission of the phosphor plate over the broad visible-light spectrum. At 764 nm, the transmission is enhanced by 7.82%, compared to a potential maximum enhancement of 8.53%. For 633-nm light incident at 5°, the reported coating was measured to induce scattering loss of no greater than 1.27%. We developed a mathematical model that can use the OAD morphology information, developed from the film growth mechanism, as input parameters to simulate the optical properties of the nanostructured coatings. A finite-difference time-domain (FDTD) simulation was able to capture the polarization-dependent, antireflective performance of the coating, and the simulated transmission spectrum was in good agreement with both the measured spectrum and the spectrum calculated using the measured effective refractive indices in a transfer matrix formulation. In addition, the FDTD model was applied to predict the scattering losses. The simulation supports the experimental results and shows that the coating induces very weak scattering loss.

Silica-based antireflection coating by glancing angle deposition

ABSTRACTIn this study, silica-based multilayer graded-index antireflection coatings have been developed by RF magnetron sputtering exploiting the mechanism of glancing angle deposition (GLAD). GLAD is an extension of oblique angle deposition (OAD) where substrate rotation is also applied along with OAD. The optical properties of the films have been tailored by varying the oblique angle of deposition. Each subsequent layer of the multilayer coating has been deposited at a different oblique angle and has different thickness and refractive index. The refractive index of the films reduces successively as we move from the bottom to the top layer achieved by an increase in porosity with a subsequent increase in the flux incidence angle. These multilayer coatings when applied on glass substrates, exhibit a reduction in the average reflectance of glass from 8.75% to 1.6% in a wavelength range of 400–800 nm.

The SiO2 and SiNx Multilayer Thin Film Deposition for Encapsulation of OLED Using a NSi-01 Single Precursor By Peald

OLEDs (Organic Light-Emitting Diodes) are already being used in many mobile devices and TVs, and the next generation of OLED displays will be flexible and foldable. These foldable and flexible OLED displays tend to reduce the radius of curvature under 2.5R and require excellent encapsulation characteristics at thin thicknesses. To satisfy these requirements, multilayer barrier films are considered to be indispensable for the OLEDs. Unlike a single thin film such as SiO2 and SiNx, which is currently applied, the multi-layer itself can produce an antireflection film effect due to a difference in refractive index of each thin film in laminated thin films, thereby reducing diffuse reflection. Also It is effective for blocking ultraviolet rays and loss stress.[1], [2]. Recently, The ALD (Atomic Layer Deposition) method is used for developing thin film encapsulation technology, due to advantages of self limiting surface reaction and controlling the fraction of reaction. In this paper, SiO2, SiNx and multi-layer thin films were deposited on PEN (polyethylene naphthalate) films by using the NSi-01 single precursor by PEALD (Plasma Enhanced Atomic layer Deposition) technique at low temperature (90℃), and thin film thickness and refractive index were measured using a M2000D Spectroscopic Ellipsometer from Woollam. Also, the WVTR (Water Vapor Transmission Rate) characteristics of the deposited films were confirmed using a MOCON Aquatran 2. The refractive index of the SiO2 and SiNx film was 1.47 and 1.85 respectively, which confirmed the possibility of antireflection effect through the multi-layer thin film. The WVTR characteristics were measured according to the thickness of SiO2 and SiNx film over 100hrs. The both deposited SiO2 and SiNx thin film below 150Å showed poor or breakdown encapsulation property (Fig.1). In order to overcome this, the SiO2 film and SiNx film were stacked in order. The deposited multi-layer film showed excellent WVTR characteristics with a breakdown prevention of encapsulation property at thin thickness. Also, 2 to 3% of the carbon in the deposited SiNx film is expected to lower film stress and maintain the flexibility of the entire film (Fig. 2). [3] From this research, we confirmed the possibility of deposit multi-layer thin film consist of silicon oxide film and nitride film in one chamber using a one precursor by PEALD. Especially, multilayer type thin film deposition suggested the possibility of preventing reflection and water vapor simultaneously. References Hemant Kumar Raut,*a V. Anand Ganesh,a A. Sreekumaran Nairb and Seeram Ramakrishna*acd “Anti-reflective coatings: A critical, in-depth review” Energy Environ. Sci.,3784, 2011S. Chhajed, M. F. Schubert, J. K. Kim, and E. F.Schubert, “Nanostructured Multilayer Graded-indexAntireflection Coating for Si Solar Cells with Broadbandand Omnidirectional Characteristics”, Applied Physics Letters, vol. 93, p. 251108, 2008KwanDo Kim, SeokHee Jang, JongMin Kim* and SangMok Chang, “Characterization of Thin Film Passivation for OLED by PECVD” , Korean Chem. 575, 2012 Figure 1

Standing Wave Field Distribution in Graded-Index Antireflection Coatings

Standing wave field distributions in three classic types of graded-index antireflection coatings are studied. These graded-index antireflection coatings are designed at wavelengths from 200 nm to 1200 nm, which is the working wavelength range of high energy laser system for inertial-fusion research. The standing wave field distributions in these coatings are obtained by the numerical calculation of electromagnetic wave equation. We find that standing wave field distributions in these three graded-index anti-reflection coatings are quite different. For the coating with linear index distribution, intensity of standing wave field decreases periodically from surface to substrate with narrow oscillation range and the period is proportional to the incident wavelength. For the coating with exponential index distribution, intensity of standing wave field decreases periodically from surface to substrate with large oscillation range and the period is also proportional to the incident wavelength. Finally, for the coating with polynomial index, intensity of standing wave field is quickly falling down from surface to substrate without an obvious oscillation. We find that the intensity of standing wave field in the interface between coating and substrate for linear index, exponential index and polynomial index are about 0.7, 0.9 and 0.7, respectively. Our results indicate that the distributions of standing wave field in linear index coating and polynomial index coating are better than that in exponential index coating for the application in high energy laser system. Moreover, we find that the transmittance of linear index coating and polynomial index coating are also better than exponential index coating at the designed wavelength range. Present simulation results are useful for the design and application of graded-index antireflection coating in high energy laser system.

Multilayer silver nanoparticles embedded in graded-index dielectric layers

A pulsed laser deposited SiO2/Ag/ZnO/Ag/TiO2 multilayer structure is studied to enhance the light trapping capability of thin-film solar cell. Structural and optical properties of structure are studied with scanning electron microscopy, x-ray diffraction, photoluminescence and UV–visible spectroscopy. Proposed geometry improves the extinction spectra and quenches photoluminescence in comparison to TiO2/Ag and SiO2/Ag/ZnO geometry. Finite-difference time-domain (FDTD) simulations indicate a promising effect of the proposed geometries on thin-film solar cells. Twofold enhancement in total quantum efficiency of an optimized multilayer plasmonic graded-index thin-film solar cell is observed in comparison to the pristine solar cell. Results suggest a more concerted study of multilayer plasmonic nanostructures with graded-index anti-reflection coatings to improve the performance of thin-film photovoltaic devices.

Integrated three-dimensional photonic nanostructures for achieving near-unity solar absorption and superhydrophobicity

In this paper, we proposed and realized 3D photonic nanostructures consisting of ultra-thin graded index antireflective coatings (ARCs) and woodpile photonic crystals. The use of the integrated ARC and photonic crystal structure can achieve broadband, broad-angle near unity solar absorption. The amorphous silicon based photonic nanostructure experimentally shows an average absorption of ∼95% for λ = 400–620 nm over a wide angular acceptance of θ = 0°–60°. Theoretical studies show that a Gallium Arsenide (GaAs) based structure can achieve an average absorption of >95% for λ = 400–870 nm. Furthermore, the use of the slanted SiO2 nanorod ARC surface layer by glancing angle deposition exhibits Cassie-Baxter state wetting, and superhydrophobic surface is obtained with highest water contact angle θCB ∼ 153°. These properties are fundamentally important for achieving maximum solar absorption and surface self-cleaning in thin film solar cell applications.

Performance improvement of a triple-junction GaAs-based solar cell using a SiO<SUB align="right">2-nanopillar/SiO<SUB align="right">2/TiO<SUB align="right">2 graded-index antireflection coating

In this study, the enhanced performance of a triple-junction GaAs-based solar cell using a SiO 2 -nanopillar/SiO 2 /TiO 2 graded-index antireflection coating (GI-ARC) was demonstrated. The optical reflectance, photovoltaic current-voltage (I-V), and external quantum efficiency (EQE) of a cell with a SiO 2 /TiO 2 double-layer (DL) ARC and a cell with a GI-ARC were measured and compared. The cell with a GI-ARC exhibited the lowest optical reflectance. Thus, the average EQE enhancements (ΔEQE) of the cell with a GI-ARC compared to the cell with a DL-ARC were 5.88% for the top cell and –1.56% for the middle cell. In addition, a small difference in the photocurrent generated between the top cell and the middle cell was achieved. Finally, an additional 0.52% increase (from 24.99% to 25.51%) in conversion efficiency was obtained.

Ultra‐High Optical Transparency of Robust, Graded‐Index, and Anti‐Fogging Silica Coating Derived from Si‐Containing Block Copolymers

www.advopticalmat.de Inevitable Fresnel refl ection from the interface between two optical media with different refractive indices ( η ) can decrease light absorption in photovoltaic devices and can cause undesirable glare in fldisplays. Thus, an effective antirefl ection coating (ARC) for a higher optical transmittance can contribute to the accomplishment of improved contrast and brightness in display devices and higher levels of energy conversion effi ciency in solar panels. Traditional ARCs improve optical transmittance by introducing destructive interference between refl ected light beams from the air-fi lm and fi lm-substrate interfaces. An ideal ARC should satisfy the requirements of a quarter-wavelength fi lm thickness and a fi lm η of ( η s ) 0.5 , where η s denotes the η of the substrate. [ 1 ] However, an ARC with a homogeneous η can achieve high transmittance only in narrow wavelength and incidence angle ranges, which are fi xed by its thickness and refractive index. In contrast, an ARC with a gradually changing η , also known as a graded index, can avoid the formation of an interface with a sharp contrast in η . As a result, the Fresnel refl ection can be effectively minimized in a wide wavelength range and high optical transparency can be achieved. In an ideal gradedindex anti-refl ection coating (GIARC), the η should gradually vary from that of the substrate material, onto which the GIARC is coated, to the η of air ( ∼ 1), necessitating the use of a material with a widely tunable η . Theoretical studies showed that a quintic profi le of η varying from that of the substrate (typically 1.5‐2) to 1 is best for achieving the lowest refl ectance (<0.1% in the visible range). [ 2 ] While dense materials generally have an η larger than ∼ 1.35, the extensive variability of indices has been demonstrated by nanoporous materials for the applications for GIARC. [ 2‐6 ] Various methods, such as lithography, obliqueangle deposition, catalytic etching, chemical vapor deposition, and a sol-gel process, have been reported. [ 2‐5 ] However, these methods may lack cost-effectiveness and high-throughput capabilities due to the requirements of vacuum processing, multiple etching steps, or because of the limited controllability of η . Recently, highly porous block copolymer (BCP) thin fi lms were applied to the fabrication of ARC due to the excellent low-cost solution processability and facile tunability of η when controlling the fraction of the pores generated by the selective removal of one block. [ 7‐11 ] Kim and co-workers reported a wide tuning range of η between 1.17 and 1.42 in nanoporous polystyrene (PS) thin fi lms obtained by removing poly(methyl methacrylate) (PMMA) in PS-PMMA BCPs. The stacking of three PS layers with gradually changing porosity achieved a minimum refl ectance of less than 0.1%. Han and coworkers demonstrated that a mixture of PS-PMMA BCPs and PMMA homopolymers led to a graded distribution of PMMA along the vertical direction of the thin fi lms and that a gradient η can be obtained for the remaining PS nanostructures after the removal of PMMA, which can reduce the number of processing steps including spin-coating and acid etching. However, these approaches may be associated with challenges related to the low chemical and mechanical stability of the porous organic thin fi lms and the

Porous anodic alumina with low refractive index for broadband graded-index antireflection coatings

Materials with very low refractive index are essential to prepare broadband graded-index antireflection (AR) coatings. However, the availability of such materials is very limited. In this study, large-area (4 cm×4 cm) low refractive index porous anodic alumina (PAA) coatings on glass substrate were prepared successfully by electron-beam evaporation, electrochemical oxidation, and chemical etching method. The nanopore size of PAA film is smaller than 40 nm, and the refractive index of PAA film is n=1.08. Besides, five-layered graded-index broadband PAA coatings with refractive indices following the Gaussian profile were also prepared to noticeably eliminate the reflectance of glass over a broadband wavelength, and the lowest reflectivity is 0.64% at the wavelength of 534 nm at normal incidence. The PAA AR coatings having an omnidirectional nature are likely to have practical applications in photovoltaic cells and optical devices.

Hollow Silica Nanoparticles in UV−Visible Antireflection Coatings for Poly(methyl methacrylate) Substrates

We have demonstrated the utility of hollow silica nanoparticles in fabricating conformal thin film nanoporous antireflection (AR) coatings on both poly(methyl methacrylate) (PMMA) and glass substrates. Layer-by-layer (LbL) assembly was successfully used to produce ultrathin AR coatings on planar and textured surfaces. Hollow silica nanoparticles were synthesized to extend the range of apparent refractive indices possible in an AR coating, enabling the design of both single index and graded index AR coatings on PMMA substrates. The diameter and shell thickness of the silica nanoparticles are the two independent, controllable parameters that we manipulated to tune the refractive index of the coating. The AR coatings reduced the minimum reflection of PMMA from 7% to 0.5%, while the maximum transmission increased from 92% to 98% at the optimized wavelength region that could be adjusted from the near UV into the visible. Cross sectional SEM showed that conformal coatings can be achieved on grooved PMMA Fresnel lenses. AFM was used to study surface topography on flat substrates.

Pulse magnetron sputtering in a reactive gas mixture of variable composition to manufacture multilayer and gradient optical coatings

Optical interference coatings have been produced by pulse magnetron sputtering a silicon target in a variable mixture of the reactive gases oxygen and nitrogen. Conventional H/L multilayer stacks with sharp transition of refractive index and gradient layer systems with inhomogeneous refractive index profile have been designed and deposited on glass and plastic substrates. Design examples and results of deposited films will be presented for multi-band rugate filters, edge and dichroic filters as well as for graded index antireflection coatings. All films were grown at a single sputtering station only, without interruption of the plasma process. The avoidance of substrate movement and process interruption, together with accurate process control, guarantees a very stable and highly efficient process with high reproducibility of the coating on 8″ substrates with a film thickness uniformity of ± 1%.

Deposition of gradient-index antireflective coating: An approach based on ultrastructure processing

A new concept, based on ultrastructure processing, has been developed to deposit graded-index antireflective coatings. According to this concept the substrate is coated, in proper sequence, by a series of solutions (or sols) of identical composition but of differing particle size. The microporous gel coating deposited in this way is characterized by a gradation of porosity due to the deposition of larger particles at the top surface and smaller particles in the substructure. On thermal consolidation, the gel coating forms a single-layer, graded-index coating; thus the chemical leaching/etching steps presently followed for porosity grading could be avoided. In this communication the results of some preliminary experiments which demonstrate the technical feasibility of the concept are reported.