Anti-reflection (AR) coatings made by sol–gel processes: A review

doi:10.1016/S0927-0248(00)00365-2

Solar Energy Materials and Solar Cells

Volume 68, Issues 3–4, June 2001, Pages 313-336

https://doi.org/10.1016/S0927-0248(00)00365-2 Get rights and content

Abstract

Traditionally, various vacuum-based processes, depending on material systems and properties, and chemical etching process have been used for producing different types of anti-reflection (AR) coating on different substrate materials. In this paper, the development of sol–gel derived AR coating on different substrates for various applications in the past 40 years are reviewed. These coatings possess good uniformity in thickness and properties which have met requirements for various applications. The major approaches to fabricate AR coating and their characteristics have been discussed. This paper outlines the major solution coating processes and design principles of AR coatings. Major fabrication processes used in AR coating technologies have been compared. Different solution chemistries developed for producing different materials for AR coating preparation have been extensively reviewed. The optical performance of different types of sol–gel-derived AR coatings have been summarized and comparison to the commercial AR coating produced by traditional technologies have been discussed. The sol–gel AR coating has been shown to possess comparable or superior performance to those produced by vacuum-based processes.

Section snippets

Basic approaches to prepare anti-reflection (AR) coating

Optical reflection is a fundamental phenomenon occurring when light propagates across a boundary between two media, which have different refractive indices. There are two approaches to achieve low reflection:

(1) inhomogeneous layer (or called graded index layer); and

(2) interference-type multiple layer stack.

Sol–gel chemistry

Sol–gel process generally involves the use of inorganic salts or metal alkoxides as precursors. Hydrolysis and polycondensation reactions occur when the precursors are mixed with water and catalyst. The details of various chemistries have been reviewed by Brinker and Scherer [4]. In general, different metal alkoxides are used for preparing AR coatings. The general reactions may be expressed as

(1) Hydrolysis:

$M – (OR)_{x} +H – O – H → HO – M(OR)_{x−1} +H – OR,$ where M: Si, Ti, Zr, Hf, Ta, Nb, and Al, etc.

and R: CH₃, C

Dip coating

Dip coating is the most common and easiest way to deposit sol–gel film on a substrate. The substrates could be flat panels, cylinders or complex geometry. This process may be utilized to coat areas in the order of square meters, and can also operate in either continuous or batch modes. Scriven [6] divided a batch dip coating process into five stages: immersion, start up, deposition, drainage and evaporation. For a coating sol containing volatile solvent, evaporation accompanies the startup,

Multi-layer AR on glass plate

The first incidental observation of the sol–gel process dates back to 1846 [12], covering the hydrolysis and polycondensation of silicic acid under humidity to form a silicate glass. This process was investigated extensively on sol compositions and thin film physics on glass substrate by Geffcken and subsequent scientists in Schott Glaswerke [13], [14], [15]. The first AR coating was developed in the early 1960 s and had been in production since 1964 [16]. This interference-type AR coating is

Characteristics of sol–gel-derived AR coatings

The results of systematic evaluation of sol–gel-derived AR coatings are scarce in the literatures. Some of the characteristics, or the testing standards, are gathered and summarized in Table 2. The reflectance of these coatings varies, depending on the targeted application and sophistication of AR coating structures. In general, the optical performance is sufficient for different applications, and is able to reduce the reflectance of an uncoated surface to less than 0.6%, and even close to 0%

Conclusions

The sol–gel process has a long history since it was first found possible to produce thin oxide films with optical quality. The AR coating has been mainly deposited on a glass substrate for different applications. Many attempts have been focused on preparing low-cost simple AR coating for commercial applications. Development of novel organic–inorganic hybrid materials, interfusion of sol–gel science with polymer science, advancement in analytical capability and understanding of solution

Acknowledgements

The author is very grateful for the help and comments received from Dr. De-yin Jeng and Dr. Alex Moser while preparing the manuscript. This review paper also presents some experimental data from YTC America's internal R&D program on sol–gel AR coatings. The author would like to thank the management of YTC America Inc. and Yazaki Corporation for approval to publication of such data.

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¹: The author is currently working in Global Communication Semiconductors, Inc., 23155 Kashiwa Court, Torrance, CA 90505, USA.

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Anti-reflection (AR) coatings made by sol–gel processes: A review

Abstract

Section snippets

Basic approaches to prepare anti-reflection (AR) coating

Sol–gel chemistry

Dip coating

Multi-layer AR on glass plate

Characteristics of sol–gel-derived AR coatings

Conclusions

Acknowledgements

J. Non-Crystalline Solids

J. Non-Crystalline Solids

J. Non-Crystalline Solids

Sol. Energy Mater

Sol Energy Mater.

J. Opt. Soc. Am.

Ceramic Bulletin

Mater. Res. Soc. Symp.

Acta Physiochim

U.R.S.S.

J. Imaging Tech.

J. Appl. Phys.

Chem. Eng. Commun.

Thin Solid Films

Key Engineering Materials

Am. Ceram. Soc. Bull.

J. Sol–Gel Sci. Technol