Broadband Antireflection Properties Research Articles

Nanostructured encapsulation coverglasses with wide-angle broadband antireflection and self-cleaning properties for III–V multi-junction solar cell applications

We report the effect of nanocone arrays (NCAs) as an antireflection coating (ARC) of encapsulation coverglasses on the device performance of encapsulated III–V InGaP/GaAs/Ge triple-junction (TJ) solar cells. The NCAs were fabricated on the single-side surface of glasses using the gold nanopatterns (i.e., nanoclusters) prepared by the glancing angle deposition technique without additional thermal treatment and the subsequent dry etching. Their wetting behavior and optical properties, together with a theoretical prediction using the rigorous coupled-wave analysis method, were investigated. The NCAs ARC coverglass exhibited a much lower water contact angle (θCA) of <5° (i.e., superhydrophilic surface) and higher solar weighted transmittance (SWT) of ~95.9% over a wide wavelength region of 300–1800nm at normal incidence compared to the bare coverglass (i.e., θCA~63° and SWT ~92.8%). The use of the NCAs ARC coverglass in encapsulated III–V InGaP/GaAs/Ge TJ solar cells led to the higher short circuit current density (Jsc) of 14.22mA/cm2 and thus improved the conversion efficiency (η) to 32.07% (cf., Jsc=13.84mA/cm2 and η=30.6% for the cell with the bare coverglass). For incident angle-dependent solar cell characteristics, it also showed a superior solar power conversion property in wide incident light angles of 20–80°.

Performance Analysis of Reflection Reduction in Submicrometer Structures Using Rigorous Coupled Wave Analysis

The broadband antireflective (AR) properties of submicrometer structures (SMSs) have attracted considerable attention in recent years as a means to reduce the surface reflectance of optical and optoelectronic devices. This study employed polystyrene sphere lithography in conjunction with dry etching to fabricate SMSs on an Si substrate. We fabricated an array of conical structures 600 nm in diameter and 760 nm in height. Experimental results demonstrate that SMSs suppress the average reflectance to below 1% across a spectral range of 300-1200 nm. The AR performance of SMSs was then simulated using rigorous coupled wave analysis, the results of which are very close to those obtained in the experiment. To investigate the mechanism underlying the AR properties of SMSs, we calculated the reflectance spectra using structures of various heights, diameters, and refractive indices.

Design and fabrication of antireflective GaN subwavelength grating structures using periodic silica sphere monolayer array patterning

We designed and fabricated gallium nitride (GaN) subwavelength grating (SWG) structures on GaN/sapphire via patterning using the periodic silica sphere monolayer array as an etch mask and a subsequent dry etching for efficient antireflection coatings. Theoretical optimization of GaN SWG structures was performed in terms of their geometrical parameters by the rigorous coupled-wave analysis simulation using a theoretical structural model. The bullet-like parabola-shaped SWGs with a large height-to-diameter ratio (RH/D) yielded good broadband and wide-angle antireflective surface properties. Considering the RH/D, the GaN SWG structure using 320-nm silica spheres theoretically and experimentally exhibited the most efficient antireflection property because it provided a linearly graded effective refractive index profile with relatively long relaxation length. For various geometries of the fabricated GaN SWGs on GaN/sapphire, the calculated reflectance results showed a similar tendency with the experimental results.

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.

A bioinspired solution for spectrally selective thermochromic VO_2 coated intelligent glazing

We present a novel approach towards achieving high visible transmittance for vanadium dioxide (VO(2)) coated surfaces whilst maintaining the solar energy transmittance modulation required for smart-window applications. Our method deviates from conventional approaches and utilizes subwavelength surface structures, based upon those present on the eyeballs of moths, that are engineered to exhibit broadband, polarization insensitive and wide-angle antireflection properties. The moth-eye functionalised surface is expected to benefit from simultaneous super-hydrophobic properties that enable the window to self-clean. We develop a set of design rules for the moth-eye surface nanostructures and, following this, numerically optimize their dimensions using parameter search algorithms implemented through a series of Finite Difference Time Domain (FDTD) simulations. We select six high-performing cases for presentation, all of which have a periodicity of 130 nm and aspect ratios between 1.9 and 8.8. Based upon our calculations the selected cases modulate the solar energy transmittance by as much as 23.1% whilst maintaining high visible transmittance of up to 70.3%. The performance metrics of the windows presented in this paper are the highest calculated for VO(2) based smart-windows.

Flat-top and patterned-topped cone gratings for visible and mid-infrared antireflective properties

Achieving a broadband antireflection property from material surfaces is one of the highest priorities for those who want to improve the efficiency of solar cells or the sensitivity of photo-detectors. To lower the reflectance of a surface, we are concerned with the study of the optical response of flat-top and patterned-topped cone shaped silicon gratings, based on previous work exploring pyramid gratings. Through rigorous numerical methods such as Finite Different Time Domain, we first designed several flat-top structures that theoretically demonstrate an antireflective character within the middle infrared region. From the opto-geometrical parameters such as period, depth and shape of the pattern determined by numerical analysis, these structures have been fabricated using controlled slope plasma etching processes. In order to extend the antireflective properties up to the visible wavelengths, patterned-topped cones have been fabricated as well. Afterwards, optical characterizations of several samples were carried out. Thus, the performances of the flat-top and patterned-topped cones have been compared in the visible and mid infrared range.

Large scale antireflective glass texturing using grid contacts in anodization methods

Antireflection coatings on glass are highly desirable, since glasses are widely used as a packaging material in the solar industry due to their transparency and durability. In this report, we propose a novel anodizing method using grid line contacts for antireflective nanostructures on a large area glass substrate. The nanoholes on the glass substrate exhibit broadband and omnidirectional antireflective properties, leading to an 11.34% improvement in the short-circuit current and enhanced power conversion efficiency from 7.9% to 8.57%. Moreover, the enhanced optical properties persist for 3 months in an outdoor environment. The proposed anodizing method can be considered as an alternative technique for the fabrication of large area nanostructures.

Fabrication of Anti-reflecting Si Nano-structures with Low Aspect Ratio by Nano-sphere Lithography Technique

Abstract Nano-structured photon management is currently an interesting topic since it can enhance the optical absorption and reduce the surface reflection which will improve the performance of many kinds of optoelectronic devices, such as Si-based solar cells and light emitting diodes. Here, we report the fabrication of periodically nano-patterned Si structures by using polystyrene nano-sphere lithography technique. By changing the diameter of nano-spheres and the dry etching parameters, such as etching time and etching power, the morphologies of formed Si nano-structures can be well controlled as revealed by atomic force microscopy. A good broadband antireflection property has been achieved for the formed periodically nano-patterned Si structures though they have the low aspect ratio (&lt;0.53). The reflection can be significantly reduced compared with that of flat Si substrate in a wavelength range from 400 nm to 1200 nm. The weighted mean reflection under the AM1.5 solar spectrum irradiation can be as low as 3.92% and the corresponding optical absorption is significantly improved, which indicates that the present Si periodic nano-structures can be used in Si-based thin film solar cells.

Robust and durable polyhedral oligomeric silsesquioxane-based anti-reflective nanostructures with broadband quasi-omnidirectional properties

Polymer-based anti-reflective coatings (ARCs) on glass pose major challenges for outdoor photovoltaic applications due to their incompatible mechanical and thermal properties. Here we demonstrate durable, chemically and thermally stable polyhedral oligomeric silsesquioxane-based (POSS) anti-reflective moth's eye nanostructures on glass fabricated by double-side nanoimprint lithography. These anti-reflective nanostructures exhibited excellent broadband and quasi-omnidirectional anti-reflective properties. An optimum resist composition for nanoimprinting was obtained by mixing a methacryl POSS cage mixture with 1,6-hexanediol diacrylate in a 1 : 12 molar ratio. Thermal free radical co-polymerization during nanoimprint lithography produced a uniform array of moth's eye nanostructures on both sides of a glass substrate with yields ∼90 to 100%. The transmittance of the resulting glass was enhanced to 98.2% (reflectance 1.26%) with excellent quasi-omnidirectional transmittance observed from −50° to +50° of angles of incidence. Furthermore, a series of ASTM-based tests on the imprinted ARC structures showed strong adherence to glass, better hardness and mechanical strength with superior chemical and thermal stability, thus suggesting their strong potential for commercial applications.

Biomimetic artificial Si compound eye surface structures with broadband and wide-angle antireflection properties for Si-based optoelectronic applications

We report the biomimetic artificial silicon (Si) compound eye structures for broadband and wide-angle antireflection by integrating nanostructures (NSs) into periodically patterned microstructures (p-MSs) via thermal dewetting of gold and subsequent dry etching. The truncated cone microstructures with a two-dimensional hexagonal symmetry pattern were fabricated by photolithography and dry etching processes. The desirable shape and density of the nanostructures were obtained by controlled dewetting. The incorporation of p-MSs into the NS/Si surface further reduced the surface total reflectance over a wide wavelength range of 300-1030 nm at near normal incidence, indicating the average reflectance (Ravg) and solar weighted reflectance (RSWR) values of ~ 2.5% and 2%, respectively, compared to the only NSs on the flat Si surface (i.e., Ravg ~ 4.9% and RSWR ~ 4.5%). Additionally, the resulting structure improved the angle-dependent antireflection property due to its relatively omnidirectional shape, which exhibited the Ravg < 4.3% and RSWR < 3.7% in the wavelength region of 300-1100 nm even at a high incident light angle of 70° in the specular reflectance.

High-Performance, Single-Layer Antireflective Optical Coatings Comprising Mesoporous Silica Nanoparticles

Mesoporous silica nanoparticles are used to fabricate antireflectance coatings on glass substrates. The combination of mesoporous silica nanoparticles in conjunction with a suitable binder material allows mechanically robust single layer coatings with a reflectance <0.1% to be produced by simple wet processing techniques. Further advantages of these films is that their structure results in broadband antireflective properties with a reflection minimum that can tuned between 400 nm and 1900 nm. The ratio of binder material to mesoporous nanoparticles allows control of the refractive index. In this report, we discuss how control of the structural properties of the coatings allows optimization of the optical properties.

Robustly nano-tailored honeycomb structure for high-throughput antireflection polymer films

Although recently fabricated biomimetic nanostructures exhibit superior performance compared to corresponding nanostructures found in nature, their practical applications are limited due to the low mechanical stability of the nanostructures and the low productivity of the fabrication processes developed thus far. A nanoporous honeycomb structure that can alleviate any concentrated stress effectively was fabricated using roll-to-roll processable thermal nanoimprint lithography for high throughput and performance antireflection polymer films. A nanoscale positive (convex) antireflective structure of the moth eye, which shows broadband and omnidirectional antireflection properties, and a negative (concave) antireflective structure were produced using this process. The new nanoporous honeycomb structure exhibited high performance AR and enhanced mechanical stability. Moreover, a continuous process that can be used for industrial mass production was developed.

Oxidation Effects on Graded Porous Silicon Anti-Reflection Coatings

Efficient anti-reflection coatings (ARC) improve the light collection and thereby increase the current output of solar cells. By simple electrochemical etching of the Si wafer, porous silicon (PS) layers with excellent broadband anti-reflection properties can be fabricated. In this work, ageing of graded PS has been studied using Spectroscopic Ellipsometry, Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy. During oxidation of PS elements such as pure Si (Si0), Si2O (Si+), SiO (Si2+), Si2O3 (Si3+), and SiO2 (Si4+) are present. In addition both hydrogen and carbon is introduced to the PS in the form of Si3SiH and CO. The oxide grows almost linearly with time when exposed to oxygen, from an average thickness of 0–3.8 nm for the surface PS. The oxidation is then correlated to the optical stability of multi-layered PS ARCs. It is found that even after extensive oxidation, the changes in the optical properties of the PS structures are small.

Broadband and wide angle antireflection of sub-20 nm GaAs nanograss

GaAs nanograss with diameters of less than 20 nm has been fabricated using a simple, one-step, maskless plasma etching-based approach. GaAs nanograss exhibits remarkable broadband antireflection properties, which arise from the graded refractive index between air and the GaAs substrate by the nanograss layer. Moreover, GaAs nanograss shows less sensitivity to transverse electric polarized light and transverse magnetic polarized light over a wide range of incident angles compared to the strong variation in a bare GaAs substrate. These effects show that sub-20 nm GaAs nanograss with lengths of approximately 200 nm has an enhanced absorption of 98–100% when the incident energy is larger than the GaAs bandgap (λ = 240–873 nm) and an enhanced absorption of 72–98% when the incident energy is less than the bandgap (λ = 873–2400 nm). Our simple, one-step, and maskless plasma etching method opens a promising approach for the direct implementation of broad omnidirectional antireflective surfaces on solar cells and various optoelectronic devices to improve device performance.

Studying nanostructured nipple arrays of moth eye facets helps to design better thin film solar cells

Nipples on the surface of moth eye facets exhibit almost perfect broadband anti-reflection properties. We have studied the facet surface micro-protuberances, known as corneal nipples, of the chestnut leafminer moth Cameraria ohridella by atomic force microscopy, and simulated the optics of the nipple arrays by three-dimensional electromagnetic simulation. The influence of the dimensions and shapes of the nipples on the optics was studied. In particular, the shape of the nipples has a major influence on the anti-reflection properties. Furthermore, we transferred the structure of the almost perfect broadband anti-reflection coatings to amorphous silicon thin film solar cells. The coating that imitates the moth-eye array allows for an increase of the short circuit current and conversion efficiency of more than 40%.

Sol–gel preparation and characterization of nanoporous ZnO/SiO2 coatings with broadband antireflection properties

Nanoporous ZnO/SiO2 bilayer coatings were prepared on the surface of glass substrates via sol–gel dip-coating process. The structural, morphological and optical properties of the coatings were characterized. The refractive indices of ZnO layer and SiO2 layer are 1.34 and 1.21 at 550nm, respectively. The transmittance and reflectance spectra of the coatings were investigated and the broadband antireflection performance of the bilayer structure was determined over the solar spectrum. The solar transmittances in the range of 300–1200nm and 1200–2500nm are increased by 6.5% and 6.2%, respectively. The improvement of transmittance is attributed to the destructive interference of light reflected from interfaces between the different refractive-index layers with an optimized thickness. Such antireflection coatings of ZnO/SiO2 provide a promising route for solar energy applications.

Using One-Step, Dual-Side Nanoimprint Lithography to Fabricate Low-Cost, Highly Flexible Wave Plates Exhibiting Broadband Antireflection

In this study, we used dual-side nanoimprint lithography to prepare sub-wavelength periodical grating (SPG)–based wave plates on flexible substrates. Without employing any etching, we directly imprinted SPGs on both sides of a polycarbonate (PC) substrate through a one-step imprinting process. This dual-side nanoimprint lithography process dramatically decreases the typical imprinting pressure required in conventional resist-based nanoimprint lithography. With this low-cost formation of PC-based wave plates, any degree of phase retardation could be obtained merely by stacking a set of wave plates with designed depths and working wavelengths. Our SPG-based wave plates exhibited the excellent broadband antireflection properties of sub-wavelength structures possessing refractive index gradients on both sides of the PC substrates. The high flexibility, low cost, and low surface reflection of these dual-side SPG-based wave plates make them attractive alternatives to commonly used birefringent crystal–based wave plates, suggesting that this phase retardation technique has promising potential for use in the development of next-generation flexible optoelectronic devices and systems.

Ellipsometry, Reflectometry, and XPS Comparative Studies of Oxidation Effects on Graded Porous Silicon Antireflection Coatings

Efficient antireflection coatings (ARC) improve the light collection and thereby increase the current output of solar cells. By simple electrochemical etching of the Si wafer, porous silicon (PS) layers with excellent broadband antireflective properties can be fabricated. The close relation between porosity and refractive index, modeled using the Bruggeman effective medium approximation, allows PS multilayers to be tailored to fabricate ARCs optimized for use in solar cells. In this work, the effect of ageing on the reflection properties of multilayered PS ARCs is studied. The reflection is correlated to the oxidation of the structures through the use of x-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry (SE). It is found that even after extensive oxidation, very small changes in reflectivity are measured.

Broadband antireflection of block copolymer/homopolymer blend films with gradient refractive index structures

The factors that influence the broadband antireflection of block copolymer/homopolymer blend films were systematically investigated. The gradient refractive index structure, which was generated by removing the PMMA content from a film of PS-b-PMMA/PMMA blends, contributed to the broadband antireflection. The volume fraction of the PMMA block (fPMMA), the total degree of polymerization (N) and the weight percent of the homopolymer played a key role in the formation of the gradient porosity index inner structure. By increasing χN, the PS porous structure was kept from collapsing after removal of the PMMA domain, which led to a stable gradient porosity ratio structure and higher transmittance. While increasing fPMMA, the PMMA domains changed from a dispersed phase to a continuous phase, which caused the residual PS structure to collapse more easily and the transmittance decreased. The best broadband antireflective properties were achieved from the 45 mg mL−1 solution of PS-b-PMMA (Mn = 68 000–33 500, fPMMA = ∼25%) blended with 30 wt% PMMA: a mean transmittance of 98.4% at full spectrum, and >98.0% in the visible–NIR range between 600 nm and 2000 nm.

Light scattering by nanostructured anti-reflection coatings

The correlation between surface profiles of nanostructures and the behavior of scattered light, including specular and diffuse components, is demonstrated in detail. The nanorod arrays with large alignment variation exhibit broadband and omnidirectional anti-reflection properties due to the gradual index profile. This work provides insight into further structural design for nanostructured optoelectronic applications.