Anti-reflective coatings (ARCs) have evolved into highly effective reflectance and glare reducing components for various optical and opto-electrical equipments. Extensive research in optical and biological reflectance minimization as well as the emergence of nanotechnology over the years has contributed to the enhancement of ARCs in a major way. In this study the prime objective is to give a comprehensive idea of the ARCs right from their inception, as they were originally conceptualized by the pioneers and lay down the basic concepts and strategies adopted to minimize reflectance. The different types of ARCs are also described in greater detail and the state-of-the-art fabrication techniques have been fully illustrated. The inspiration that ARCs derive from nature (‘biomimetics’) has been an area of major research and is discussed at length. The various materials that have been reportedly used in fabricating the ARCs have also been brought into sharp focus. An account of application of ARCs on solar cells and modules, contemporary research and associated challenges are presented in the end to facilitate a universal understanding of the ARCs and encourage future research.

Anti-reflective coatings: A critical, in-depth review

Optical reflection, or in other words the loss of reflection, from a surface becomes increasingly crucial in determining the extent of the light-matter interaction. The simplest example of using an anti-reflecting (AR) surface is possibly the solar cell that incorporates an AR coating to harvest sunlight more effectively. Researchers have now found ways to mimic biological structures, such as moth eyes or cicada wings, which have been used for the AR purpose by nature herself. These nanoscopic biomimetic structures lend valuable clues in fabricating and designing gradient refractive index materials that are efficient AR structures. The reflectance from a selected sub-wavelength or gradient index structures have come down to below 1% in the visible region of the spectrum and efforts are on to achieve broader bands of such enhanced AR regime. In addition to the challenge of broader bands, the performance of AR structures is also limited by factors such as omnidirectional properties and polarization of incident light. This review presents selected state-of-the-art AR techniques, reported over the last half a century, and their guiding principles to predict a logical trend for future research in this field.

Anti-reflecting and photonic nanostructures

Design, fabrication, and characterization of a broadband, omnidirectional, graded-index antireflection (AR) coating made using nanostructured low-refractive-index (n=1.05–1.40) silica deposited by oblique-angle deposition are reported. Averaged over wavelength range from 400 to 1100 nm and 0°–90° angle of incidence, polished Si reflects ∼37% of incident radiation. The reflection losses are reduced to only 5.9% by applying a three-layer graded-index AR coating to Si. Our experimental results are in excellent agreement with theoretical calculations. The AR coatings reported here can be optimized for photovoltaic cells made of any type of material.

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Nanostructured multilayer graded-index antireflection coating for Si solar cells with broadband and omnidirectional characteristics

Broadband antireflection schemes for silicon surfaces based on the moth-eye principle and comprising arrays of subwavelength-scale pillars are applicable to solar cells, photodetectors, and stealth technologies and can exhibit very low reflectances. We show that rigorous coupled wave analysis can be used to accurately model the intricate reflectance behavior of these surfaces and so can be used to explore the effects of variations in pillar height, period, and shape. Low reflectance regions are identified, the extent of which are determined by the shape of the pillars. The wavelengths over which these low reflectance regions operate can be shifted by altering the period of the array. Thus the subtle features of the reflectance spectrum of a moth-eye array can be tailored for optimum performance for the input spectrum of a specific application.

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Tunable reflection minima of nanostructured antireflective surfaces

A surface texture enhances the capacity of a solar cell to absorb incident radiation. In high efficiency and industry standard designs alike, pyramidal surface textures play the key role of reducing the reflectance of the cell surface. This reduction is achieved by ensuring that incident light rays suffer at least a double reflection in the various facets of the structure. In this work, we define a general expression for the reflectance of a pyramidal texture by identifying discrete paths of reflection and the fraction of reflected light that follows each of these paths. We apply the expression to analyse the reflection of normally incident light at textured surfaces. We examine three common morphologies, finding that a regular array of inverted pyramids just outperforms a random array of upright pyramids, with a regular array of upright pyramids showing poorer capacity to reduce front surface reflection. We extend the analysis to determine the transmittance of the various structures, thus permitting the calculation of a figure of merit that can be used to optimise the thickness of antireflection coatings (ARCs). Finally, by examining the angles at which light is reflected by the pyramidal textures, we find that an encapsulant of refractive index greater than 1.59 gives between 79 and 92% of the initially reflected light a second chance to enter the solar cell.

O. Nichiporuk

Papers

Pyramidal texturing of silicon solar cell with TMAH chemical anisotropic etching

Passivation of the surface of rear contact solar cells by porous silicon