Enhanced absorption in silicon nanocone arrays for photovoltaics

Silicon nanowire arrays have been shown to demonstrate light trapping properties and promising potential for next-generation photovoltaics. In this paper, we performed systematic and detailed simulation studies on the optical properties of silicon nanocone arrays as compared to nanowires arrays. Nanocone arrays were found to have significantly improved solar absorption and efficiencies over nanowire arrays. Detailed simulations revealed that nanocones have superior absorption due to reduced reflection from their smaller tip and reduced transmission from their larger base. The enhanced efficiencies of silicon nanocone arrays were found to be insensitive to tip diameter, which should facilitate their fabrication. Breaking the vertical mirror symmetry of nanowires results in a broader absorption spectrum such that overall efficiencies are enhanced. We also evaluated the electric field intensity, carrier generation and angle-dependent optical properties of nanocones and nanowires to offer further physical insight into their light trapping properties.     

Lithography-free sub-100 nm nanocone array antireflection layer for low-cost silicon solar cell

A high-density and -uniformity sub-100 nm surface-oxidized silicon nanocone forest structure is created and integrated onto the existing texturization microstructures on a photovoltaic device surface by a one-step high-throughput plasma-enhanced texturization method. We suppressed the broadband optical reflection on chemically textured grade-B silicon solar cells for up to 70.25% through this nanomanufacturing method. The performance of the solar cell is improved with the short-circuit current increased by 7.1%, fill factor increased by 7.0%, and conversion efficiency increased by 14.66%. Our method demonstrates the potential to improve the photovoltaic device performance with low-cost and high-throughput nanomanufacturing technology.     

Ultraviolet antireflection coatings for use in silicon detector design

We report on the development of coatings for a charged-coupled device (CCD) detector optimized for use in a fixed dispersion UV spectrograph. Because of the rapidly changing index of refraction of Si, single layer broadband antireflection (AR) coatings are not suitable to increase quantum efficiency at all wavelengths of interest. Instead, we describe a creative solution that provides excellent performance over UV wavelengths. We describe progress in the development of a coated CCD detector with theoretical quantum efficiencies (QEs) of greater than 60% at wavelengths from 120 to 300 nm. This high efficiency may be reached by coating a backside-illuminated, thinned, delta-doped CCD with a series of thin film AR coatings. The materials tested include MgF(2) (optimized for highest performance from 120-150 nm), SiO(2) (150-180 nm), Al(2)O(3) (180-240 nm), MgO (200-250 nm), and HfO(2) (240-300 nm). A variety of deposition techniques were tested and a selection of coatings that minimized reflectance on a Si test wafer were applied to functional devices. We also discuss future uses and improvements, including graded and multilayer coatings.     

Absorption enhancement in ultrathin crystalline silicon solar cells with antireflection and light-trapping nanocone gratings

Enhancing the light absorption in ultrathin-film silicon solar cells is important for improving efficiency and reducing cost. We introduce a double-sided grating design, where the front and back surfaces of the cell are separately optimized for antireflection and light trapping, respectively. The optimized structure yields a photocurrent of 34.6 mA/cm(2) at an equivalent thickness of 2 μm, close to the Yablonovitch limit. This approach is applicable to various thicknesses and is robust against metallic loss in the back reflector.     

Design of porous silicon/PECVD SiOx antireflection coatings for silicon solar cells

The meso-porous silicon (PS) has become an interesting material owing to its potential applications in many fields, including optoelectronics and photovoltaics. PS layers were grown on the front surface of the n⁺ emitter of n⁺-p mono-crystalline Silicon junction. The thickness and the porosity of the PS layer were determined by an ellipsometer, as a function of time duration of anodization, and the variation law of the PS growth kinetics is established. Single layers PS antireflection coating (ARC) achieved around 9% of effective reflectivity in the wavelength range between 400 and 1000 nm on junction n⁺-p solar cells. To reduce the reflectivity and improve the stability and passivation properties of PS ARC, silicon oxide layers were deposited by PECVD on PS ARC. SiO_x layers of thickness of 105 nm combined with PS layer led to 3.8% effective reflectivity. V_oc measurements were carried out on all the samples by suns-V_oc method and showed an improvement of the quality of the passivation brought by the oxide layer. Using the experimental reflectivity results and taking into account the passivation quality of the samples, the PC1D simulations predict an enhancement of the photogenerated current exceeding 44%.     

Broadband optical antireflection enhancement by integrating antireflective nanoislands with silicon nanoconical-frustum arrays

Based on conventional colloidal nanosphere lithography, we experimentally demonstrate novel graded-index nanostructures for broadband optical antireflection enhancement including the near-ultraviolet (NUV) region by integrating residual polystyrene antireflective (AR) nanoislands coating arrays with silicon nano-conical-frustum arrays. This is a feasible optimized integration method of two major approaches for antireflective surfaces: quarter-wavelength AR coating and biomimetic moth's eye structure.     

Characterization and optimization of absorbing plasma-enhanced chemical vapor deposited antireflection coatings for silicon photovoltaics

We have optimized plasma-enhanced chemical vapor deposition (PECVD) of SiN-based antireflection (AR) coatings with special consideration for the short-wavelength (<600 nm) parasitic absorption in SiN. Spectroscopic ellipsometry was used to measure the dispersion relation for both the refractive index n and the extinction coefficient k, allowing a precise analysis of the trade-off between reflection and absorption in SiN-based AR coatings. Although we focus on photovoltaic applications, this study may be useful for photodetectors, IR optics, and any device for which it is essential to maximize the transmission of light into silicon. We designed and optimized various AR coatings for minimal average (spectrally) weighted reflectance (? R(w) ?) and average weighted absorptance (? A (w) ?), using the air mass 1.5 global solar spectrum. In most situations ? R (w) ? decreased with higher n, but ? A (w) ? increased because k increased with n. For the practical case of a single-layer AR coating for silicon under glass, an optimum refractive index of ~2.23 (at 632.8 nm) was determined. Further simulations revealed that a double-layer SiN stack with an n = 2.42 film underneath an n = 2.03 film gives the minimum total photocurrent loss. Similar optimization of double-layer SiN/SiO(2) coatings for silicon in air revealed an optimum of n = 2.28 for SiN. To determine the allowable tolerance in index and film thickness, we generated isotransmittance plots, which revealed more leeway for n values below the optimum than above because absorption begins to reduce photocurrent for high n values.     

Numerical study of antireflection coatings in waveguide structures with silicon nanoparticles interlayer

In this paper, the reflection properties of a multilayer structure containing silicon nanoparticles (Si-NPs), silicon nitride (Si₃N₄), and silicon (Si) is investigated theoretically and numerically. The structure is arranged and its main parameters are defined. The required equations for the propagation of electromagnetic plane waves are derived in detail to obtain the reflection coefficients in a closed form. The reflected power of the structure is determined using these coefficients. In the numerical results, the mentioned power is computed and illustrated as a function of wavelength of the incident radiation, angle of incidence, and Si-NPs parameters. Optimal Si-NPs parameters (particle’s diameter, space between particles, and layer thickness) for low reflection are proposed. These theoretical parameters could effectively be used to design new solar cells.     

Sputtered Ta2O5 antireflection coatings for silicon solar cells

Ta₂O₅ thin films were deposited onto monocrystalline silicon surfaces by magnetron reactive sputtering. When the Ta₂O₅ films are employed as antireflection coatings, the reflectance of the original silicon surface diminishes from about 30% to approximately 3%. At the same time, a very low absorption coefficient (less than 10³ cm^-1) for the Ta₂O₅ films below their fundamental absorption edge (hv < 4.2 eV) is obtained from reflectance and transmittance spectra of Ta₂O₅ deposited onto quartz substrates. Finally, the technique of reactive sputtering is considered from the point of view of preparing antireflection coatings for solar cells.     

Optimal single-band normal-incidence antireflection coatings

Mathematical and computational evidence that strongly suggests that optimal solutions exist to single-band, normal-incidence antireflection coating problems is presented. It is shown that efficient synthesis and refinement techniques can quickly and accurately find such solutions. Several visible and infrared antireflection coating examples are presented to support this claim. Graphs that show the expected optimal performance for different representative substrates, refractive-index ratios, wavelength ranges, and overall optical thickness combinations are given. Typical designs exhibit a pronounced semiperiodic clustering of layers, which has also been observed in the past. Explanations of this phenomenon are proposed.     

可见光聚合物基增透膜的研究进展

可见光增透膜是制造很多光学元件的必要材料之一。简要介绍了增透膜增透的原理,在使用单层和多层增透膜增透效果的计算基础上,重点综述了聚合物基增透膜技术的研究进展。这些技术主要包括:(1)在聚合物中引入低折射指数元素;(2)使用纳米孔聚合物薄膜调节折射指数;(3)使用高低折射指数组成的复合膜。此外还分析了国内外增透技术的发展趋势、市场需求及其发展前景。     

Realization of effective light trapping and omnidirectional antireflection in smooth surface silicon nanowire arrays

We have successfully fabricated well-ordered silicon nanowire (SiNW) arrays of smooth surface by using a low-cost and facile Ag-assisted chemical etching technique. We have experimentally found that the reflectance can be significantly suppressed (<1%) over a wide solar spectrum (300-1000 nm) in the as-grown samples. Also, based on our bundled model, we have used rigorous coupled-wave analysis to simulate the reflectance in SiNW arrays, and found that the calculated results are in good agreement with the experimental data. From a further simulation study on the light absorption in SiNW arrays, we have obtained a photocurrent enhancement of up to 425% per unit volume of material as compared to crystalline Si, implying that effective light trapping can be realized in the as-grown samples. In addition, we have demonstrated experimentally and theoretically that the as-grown samples have an omnidirectional high-efficiency antireflection property.     

Millimeter-wave antireflection coating for cryogenic silicon lenses

We have developed and tested an antireflection (AR) coating method for silicon lenses used at cryogenic temperatures and millimeter wavelengths. Our particular application is a measurement of the cosmic microwave background. The coating consists of machined pieces of Cirlex glued to the silicon. The measured reflection from an AR-coated flat piece is less than 1.5% at the design wavelength. The coating has been applied to flats and lenses and has survived multiple thermal cycles from 300 to 4 K. We present the manufacturing method, the material properties, the tests performed, and estimates of the loss that can be achieved in practical lenses.     

The theory of double-layer antireflection coatings

The theory of double-layer antireflection coatings using non-quarter-wave films is reviewed and extended. Normal incidence only is considered.The spectral reflectance of a double-layer coating depends on the refractive indices of the layers and the adjacent media, and on Ø₁ and Ø₂, the phase thickness of the layers. It is known that the reflectance is theoretically zero if the refractive indices satisfy certain inequalities and if the phase thicknesses are those calculated from certain equations.To obtain a fuller appreciation of double-layer coatings, the reflectance throughout the whole (Ø₁, Ø₂) plane is studied analytically. The pattern formed by the lines of constant reflectance exhibits translational symmetry, and the unit pattern which generates the whole pattern has a twofold axis of symmetry. The shape of the unit pattern depends on the refractive indices of the four media. Numerical examples are given to illustrate the similarities and differences between the unit patterns of some double-layer systems.     

Multilayer antireflection coatings for the visible and near-infrared regions

With a high-refractive-index mixed-oxide dielectric material of ZrTiO(4) and ZrO(2) [Substance H2 (Sub2) from E. Merck, Darmstadt, Germany], in combination with magnesium flouride (MgF(2)), design optimization and experimental production of low-loss antireflection (AR) coatings are carried out. Design-optimization studies that make use of these materials as constituents of a seven-layer coating system demonstrate that when the useful bandwidth of an AR coating is extended to cover a wider spectral range, the designs are in general found to have increased integrated reflection loss, higher ripple, and increased spectral instability. The experimental studies on Sub2 material show that the films have excellent optical performance over a wider process window, the advantage of which is demonstrated in the production of different AR coatings on a variety of glasses with refractive indices that range from 1.45 to 1.784 and different mechanical, thermal, and chemical properties. The manufacturing process of AR coatings shows a consistency better than 99% with respect to optical properties and durability.     

Universal antireflection coatings for substrates for the visible spectral region

It is possible to design normal-incidence antireflection coatings that reduce the reflectance of any substrate with a refractive index that lies in the range of 1.48 to 1.75. The performance of such coatings depends on the width of the spectral region over which the reflectance is to be suppressed, on the coating materials used for their construction, and on the overall optical thickness of the layer system. For example, the calculated average spectral reflectance of a set of six different substrates with refractive indices 1.48, 1.55, 1.60, 1.65, 1.70, and 1.75, when coated with a 0.56-μm-thick, eight-layer antireflection coating designed for the 0.40-0.80-μm spectral region, was 0.34%. This is higher than the average reflectance that is attainable with a conventional antireflection coating of similar optical thicknesses designed for a particular refractive index. However, it is an acceptable value for most applications. With the universal type of antireflection coating described, it is thus possible to coat a number of different refractive-index substrates in one deposition run, and this can result in considerable cost and time savings.     

Triple-layer broad band antireflection coatings using homogeneously mixed dielectric coatings

Mixtures of ZnS and MgF₂ of different compositions were successfully used for the innermost and intermediate layers in triple-layer (quarter-half-quarter) antireflection coatings. The suitability of this mixture for optical interference coatings is discussed. The transmittance of the sample was found to be 99–99.25% over a broad range in the visible region (490–725 nm). This is in good agreement with theoretical predictions.     

Design of optical path for wide-angle gradient-index antireflection coatings

What we believe to be a new principle is introduced for the design and selection of gradient-index antireflection profiles that are effective over a wide range of incident angles as well as wavelengths at a given physical film thickness. It is shown that at oblique incidence the smoothness of the optical path of incident light inside a gradient-index film has a crucial effect on the overall reflection. Thus the smoothness of variations in refractive angle (rather than that of the index profile itself) needs to be maximized for wide-angle operation. As an example, the performance of Gaussian and Quintic profiles at large incident angles are considered in light of this point of view.