Antireflection Coatings For Solar Cells Research Articles

Recent Applications of Antireflection Coatings in Solar Cells

The antireflection coating (ARC) suppresses surface light loss and thus improves the power conversion efficiency (PCE) of solar cells, which is its essential function. This paper reviews the latest applications of antireflection optical thin films in different types of solar cells and summarizes the experimental data. Basic optical theories of designing antireflection coatings, commonly used antireflection materials, and their classic combinations are introduced. Since single and double antireflection coatings no longer meet the research needs in terms of antireflection effect and bandwidth, the current research mainly concentrates on multiple layer antireflection coatings, for example, adjusting the porosity or material components to achieve a better refractive index matching and the reflection effect. However, blindly stacking the antireflection films is unfeasible, and the stress superposition would allow the film layer to fail quickly. The gradient refractive index (GRIN) structure almost eliminates the interface, which significantly improves the adhesion and permeability efficiency. The high-low-high-low refractive index (HLHL) structure achieves considerable antireflection efficiency with fewer materials while selecting materials with opposite stress properties improves the ease of stress management. However, more sophisticated techniques are needed to prepare these two structures. Furthermore, using fewer materials to achieve a better antireflection effect and reduce the impact of stress on the coatings is a research hotspot worthy of attention.

High Refractive Index Diphenyl Sulfide Photopolymers for Solar Cell Antireflection Coatings

The anti-reflection film can effectively reduce the surface reflectivity of solar photovoltaics, increase the transmittance of light, and improve the photoelectric conversion efficiency. The high refractive index coating is an important part of the anti-reflection film. However, the traditional metal oxide coating has poor stability and complicated processes. To address this issue, we prepared two organic high refractive index (HRI) photopolymers by modifying epoxy acrylic acid with 4,4′-thiodibenzenethiol, which can be surface patterned by nanoimprinting to prepare antireflection coatings. As a result, two modified photopolymers with high refractive index (n > 1.63), high optical transmittance (T > 95%), and thermal stability (Tg > 100 °C) are obtained after curing. In particular, the diphenyl sulfide photopolymer modified by ethyl isocyanate acrylate has a refractive index up to 1.667 cured by UV light. Our work confirms that the organic HRI photopolymer can be obtained by introducing high molar refractive index groups, with potential to be applied as a PV cell power conversion efficiency material.

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.

Effect of acetic acid concentration on optical properties of lead acetate based methylammonium lead iodide perovskite thin film

Abstract High absorption of perovskite thin films is crucial for efficient metal halide perovskite solar cells. In this study, methylammonium lead iodide (CH3NH3PbI3) perovskite thin films are prepared using lead (II) acetate Pb(Ac)2, acetic acid (HAc) and CH3NH3I through solution method. The effect of HAc volume (0 ml–100 ml at step size of 25 ml HAc) on optical properties and parameters of the CH3NH3PbI3 perovskite thin films developed are investigated using UV–Vis spectrophotometry and mathematical correlations. Interestingly, the increase of HAc amount to 75 ml leads to an increase in absorbance, refractive index, extinction co-efficient, dielectric properties and optical conductivity and a decrease in transmittance and optical band gaps of the CH3NH3PbI3 perovskite thin films. The optimized film is obtained at 75 ml HAc at which refractive index, film thickness and optical band gap is 1.45 at 485 nm, 355 nm and 1.47 eV, respectively. The enhanced absorption of the optimized thin film is attributable to an increase in structure ordering at HAc amount equals 75 ml. The fabricated CH3NH3PbI3 perovskite thin films have potential as ideal antireflection coatings for solar cells and optoelectronic applications.

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.

Optimization of broadband omnidirectional antireflection coatings for solar cells

Broadband and omnidirectional antireflection coating is generally an effective way to improve solar cell efficiency, because the destructive interference between the reflected and incident light can maximize the light transmission into the absorption layer. In this paper, we report the incident quantum efficiency ηin, not incident energy or power, as the evaluation function by the ant colony algorithm optimization method, which is a swarm-based optimization method. Also, SPCTRL2 is proposed to be incorporated for accurate optimization because the solar irradiance on a receiver plane is dependent on position, season, and time. Cities of Quito, Beijing and Moscow are selected for two- and three-layer antireflective coating optimization over λ = [300, 1100] nm and θ = [0°, 90°]. The ηin increases by 0.26%, 1.37% and 4.24% for the above 3 cities, respectively, compared with that calculated by other rigorous optimization algorithms methods, which is further verified by the effect of position and time dependent solar spectrum on the antireflective coating design.

Investigation of barium titanate thin films as simple antireflection coatings for solar cells

Barium titanate (BaTiO3, BTO) is a perovskite class material of remarkable dielectric, ferroelectric and ferromagnetic properties. Our previous studies on optical properties of BTO thin films proved high visible transmittance and sharp absorption edge at ∼300 nm. Therefore the usage of BTO as a UV blocker or an antireflection (AR) coating in visible region is straightforward. AR coatings are agreed to be important parts of many photonic devices, among them also of solar cells. In this paper, single layers of amorphous BTO are numerically and experimentally investigated as promising AR coatings for achieving increased light trapping in thin film silicon solar cells. Reduced reflections achieved by BTO thin films deposited using RF magnetron sputtering on a-Si:H/SiO2 compared with pristine a-Si:H/SiO2 system are clearly demonstrated. Antireflection effects are analyzed using simple AR systems comprising BTO.

Development of TiO2/PVA nanocomposites for application in solar cells

Thin films of TiO2/PVA nanocomposites were prepared by solvent casting method to be used as UV-masks for various applications and as anti-reflection coatings in solar cells. Doping of TiO2 nanoparticles into PVA matrix had effect on it’s semicrystallinity and electronic structure. In order to evaluate structural and optical properties of nanocomposite films, X-ray Diffractometer, UV-Vis Spectroscopy and Fourier Transform Infrared Spectroscopy were performed. XRD results of nanocomposite films were showing broadened peaks which were absent in that of the PVA.In absorption spectra, TiO2/PVA nanocomposite films were showing selective absorbance in UV region and no absorbance in visible region and intensity of absorbance was increased with increased wt% of TiO2 nanoparticles in TiO2/PVA nanocomposite.FTIR results confirmedformation of Ti-O-O bond which helps in suspension of TiO2 nanoparticles into PVA.Thermal stability of prepared nanocomposite films was determined using Differential Scanning Calorimetry (DSC). The presence of well-defined exothermic peak in nanocomposite films with 0.125 wt% and 0.062 wt% doping of TiO2 nanoparticles confirms crystalline nature of polymer nanocomposite films. Melting point temperature of TiO2/PVA nanocomposites was increased in compare to PVA.

Effects of thinning and heating for TiO2/AlInP junctions

TiO2/AlInP junctions are used to construct the antireflection coatings for solar cells and to passivate III–V nanostructure surfaces. The thickness of AlInP epilayer affects light absorption and appropriate Al composition determining further the energy barrier for carriers. We report on reducing the AlInP thickness by dry etching down to 10nm without introducing harmful defect states at TiO2/AlInP interface and AlInP/GaInP interface below, according to photoluminescence. Synchrotron-radiation photoelectron spectroscopy reveals that increased oxidation of phosphorus is not harmful to TiO2/AlInP and that post heating of the material enhances AlInP oxidation and group III element segregation resulting in decreased material homogeneity.

FDTD Analysis for Devices with Glass Substrates and Its Application to Antireflection Coating on Organic Solar Cells

Organic photovoltaics (OPVs) have been receiving growing interest, because they can provide large area and low cost solar cells for next generation. A power conversion efficiency (PCE) of more than 10 % under AM 1.5 illumination has been reported for the OPVs, although higher level of PCE is required for large-scale commercialization. Since the maximum thickness of light absorbing layer (around 100 nm) is determined by the low carrier mobility of organic semiconductors, it is important to develop antireflection technologies that can enhance light trapping in the thin absorbing layer. A popular method for analyzing light propagation within solar cells is the finite-difference time-domain (FDTD) simulation. In the FDTD method, the electric and magnetic field are spatially discretized on the grid and the temporal evolution of the field intensities is obtained through numerical integration. An advantage of the FDTD method is that it can obtain optical response for a wide wavelength range in a single simulation by Fourier transforming the results obtained by a brief input pulse. However, the FDTD simulation suffers from a considerable increase in calculation time when there is a glass substrate in the light path, which is much thicker than the other material layers within solar cells. To overcome this problem, in this study, we explore a new technique for FDTD simulation, to significantly reduce computational cost in the presence of a thick substrate, and apply this technique to the optical design of moth eye antireflection coatings on OPVs. In the proposed method, the absorption spectrum is averaged to eliminate the interference effect of light passing through a thick glass substrate and reproduce incoherent light addition within the substrate. We compare the proposed averaging method with the other two types of averaging methods, and show that the proposed algorithm gives the most accurate solution. In addition, our method is quite robust and independent of detailed values of control parameters. The validity of the results is checked by the comparison with the results obtained by the characteristic matrix method. In addition, we apply the proposed method to optimize the geometric pattern of moth eye array and maximize light trapping in OPVs. The results show that the combination of the FDTD simulation with our averaging method is quite useful to design broadband antireflection coatings for solar cells having thick glass substrates.

Electrical and optical properties of Si-doped indium tin oxides as transparent electrode and anti-reflection coating for solar cells

We have studied the electrical and optical properties of Si-doped indium tin oxides (ITSOs) as transparent electrodes and anti-reflection coatings for Si-based solar cells. The ITSO thin films were obtained by co-sputtering of ITO and SiO2 targets under target power control. The resistivity of the ITSO thin films deposited at 0.625 in terms of power ratio (ITO/SiO2) were 391 Ωcm. In this condition, the ITSO thin films showed very high resistivity compared to sputted pure ITO thin films (1.08 × 10−3 Ωcm). However, refractive index of ITSO thin films deposited at the same condition at 500 nm is somewhat lowered to 1.97 compared to ITO thin films (2.06). The fabricated graded refractive index AR coatings using ITO, ITSO, and SiO2 thin films kept over 80% of transmittance regardless of their thickness varing from 97 nm to 1196 nm because of their low extinction coefficient. As the AR coating with graded refractive indices using ITO, ITSO, and SiO2 layers was applied to general silicon-based solar cell, the current level increased nearly twice more than that of bare silicon solar cell without AR coating.

Antireflective nanostructured zinc oxide arrays produced by pulsed electrodeposition

Conditions for the pulsed electrochemical deposition of nanostructured zinc oxide arrays with a certain morphology, crystal structure, and optical properties from aqueous electrolytes onto substrates of transparent electrically conducting tin dioxide and on single-crystal silicon wafers with built-in homojunctions are studied in order to develop antireflection coatings for solar cells. It is shown that it is possible to obtain single-layer planar antireflection coatings or arrays of nanorods of this material, both having the form of hexagonal prisms and exhibiting the moth-eye effect.

Nanowire-based multifunctional antireflection coatings for solar cells.

Organic (P3HT/PCBM) solar cells are coated with ZnO nanowires as antireflection coatings and show up to 36% enhancement in efficiency. The improvement is ascribed to an effective refractive index which results in Fabry-Perot absorption bands which match the polymer band-gap. The effect is particularly pronounced at high light incidence angles. Simultaneously, the coating is used as a UV-barrier, demonstrating a 50% reduction in the rate of degradation of the polymers under accelerated lifetime testing. The coating also allows the surface of the solar cell to self-clean via two distinct routes. On one hand, photocatalytic degradation of organic material on ZnO is enhanced by the high surface area of the nanowires and quantified by dye degradation measurements. On the other, the surface of the nanowires can be functionalized to tune the water contact angle from superhydrophilic (16°) to superhydrophobic (152°), resulting in self-cleaning via the Lotus effect. The multifunctional ZnO nanowires are grown by a low cost, low temperature hydrothermal method, compatible with process limitations of organic solar cells.

Transparent Electrodes Based on Three-Dimensional Nanohelixes Light Harvesting Applications

We present transparent electrodes based on 3-dimensional nanohelix array for light harvesting applications. Arrays of nanohelixes fabricated by electron-beam oblique-angle deposition (OAD) of various metal oxides including TiO2, WO3 and ITO have been used for improving the performance of dye-sensitized solar cells (DSSCs), Photoelectrochemical (PEC) cells for water splitting, and organic photovoltaics (OPVs), respectively. Such light harvesting devices with transparent electrodes based on metal-oxides nanohelixes array show enhanced performances such as higher light absorption and power conversion efficiency than devices with conventional thin-film and sintered nanoparticle electrodes. This is attributed to the unique optical, microstructural, and electrical properties of metal-oxides nanohelixes such as enhanced light scattering and large specific area enabled by 3-dimensional geometry, and improved separation and transportation of photo-generated carriers by the near-single-crystallinity of metal-oxides nanohelixes. In addition, a number of applications including dichroic filters for high efficiency white light-emitting diodes, near-perfect anti-reflection coatings for solar cells, and high performance chemical sensors will be discussed.

Solar Cells: Paper‐Based Anti‐Reflection Coatings for Photovoltaics (Adv. Energy Mater. 9/2014)

In article number 1301804, Jeremy N. Munday and co-workers describe the use of a plant-based cellulose fiber paper as advanced antireflection coatings for solar cells. These coatings provide broadband, angle-independent optical response using a simple and inexpensive fabrication process. Additionally, these cellulose materials are lightweight, flexible, and recyclable, making them highly attractive materials for environmentally friendly production.

The versatile designs and optimizations for cylindrical TiO_2-based scatterers for solar cell anti-reflection coatings

The anti-reflection coating(ARC) based on dielectric nano-particles has been recently proposed as a new way to achieve the low reflectance required for solar cell front surfaces. In this scenario, the Mie modes associated with the dielectric nano-particles are utilized to facilitate photon forward scattering. In this work, versatile designs together with systematically optimized geometry are examined, for the ARCs based on dielectric scatterers. It is found that the Si3N4-TiO2 or SiO2-TiO2 stack is capable of providing low reflectance while maintaining a flat and passivated ARC-semiconductor interface which can be beneficial for reduced interface recombination and prevent V(OC) degradation associated with topography on the active materials. It is also confirmed that the plasmonic nano-particles placed at the front side of solar cells is not a preferred scheme, even with thorough geometrical optimization. At the ultimate design based on mixed graded index(GI) Mie-scattering, the averaged reflectance can be as low as 0.25%. Such a low reflectance is currently only achievable by ultra-long silicon nano-tips, but silicon nano-tips introduce severe surface recombination. On the other hand, the mixed GI Mie design preserves a flat and passivated ARC-silicon interface, with total thickness reduced to 279.8 nm, much thinner than 1.6 μm for silicon nanotips. In addition, the light trapping capability of mixed GI Mie design is much better than silicon nanotips. In fact, when compared to the state-of-art TiO2 light trapping anti-reflection coating, the mixed GI Mie design provides same light trapping capability while providing much lower reflectance.

Publisher's Note: “Effect of Ag nanoparticles integrated within antireflection coatings for solar cells” [J. Renewable Sustainable Energy 5, 033116 (2013)

First Page

Effect of Ag nanoparticles integrated within antireflection coatings for solar cells

Effect of Ag nanoparticles integrated within antireflection coatings for solar cells

Modeling and analysis of high-performance, multicolored anti-reflection coatings for solar cells

In this work solar cell anti-reflection coatings tuned to give a specific hue under solar illumination are investigated. We demonstrate that it is possible to form patterned coatings with large color contrast and high transmittance. We use colorimetric and thin film optics models to explore the relationship between the color and performance of bilayer anti-reflection coatings on Si, and predict the photocurrent generation from an example Si solar cell. The colorimetric predictions were verified by measuring a series of coatings deposited on Si substrates. Finally, a patterned Si sample was produced using a simple, low-cost photolithography procedure to selectively etch only the top layer of a bilayer coating to demonstrate a high-performance anti-reflection coating with strong color contrast.

Effect of Ag nanoparticles integrated within antireflection coatings for solar cells

The influence of the relative position of Ag metallic nanoparticles (Ag MNPs) embedded in a 100 nm SiOx Antireflection Coating (ARC) for specular polished c-Si substrates is studied. It is demonstrated that this Plasmonic ARC (PARC) can achieve lower average reflectivities than the optimised SiOx ARC. This has been done for different sizes of Ag nanoparticles. An alternative for PECVD to encapsulate Ag MNPs with SiOx is presented, avoiding the risk of metallic contamination in the reactor chamber as well as its effect on the size and shape of the self-aggregated Ag MNP. It is demonstrated, however, that this PARC is not suitable for silicon solar cells as a substitute for traditional ARC because it presents a high loss related with Fano destructive interference.