Transparent conductors as solar energy materials: A panoramic review

Because the Roman god Janus was usually represented with two heads placed back to back, the term Janus is used for the description of particles whose surfaces of both hemispheres are different from a chemical point of view. So, they could be used as building blocks for supraparticular assemblies, as dual-functionalized devices, as particular surfactants if one hemisphere is hydrophilic and the other hydrophobic, etc. If they could allow the segregation of negative charges on one hemisphere and positive charges on the other one, they would display a giant dipole moment allowing their remote positioning by rotation in an electric field as a function of field polarity. This review deals with the great and imaginative efforts which were devoted to the synthesis of Janus particles in the last fifteen years. A special emphasis is made on scalable techniques and on those which apply to the preparation of Janus particles in the nanometer range. Specific properties and applications of Janus particles are discussed.

Design and synthesis of Janus micro- and nanoparticles

The homogeneous combination of inorganic and organic moieties in a single-phase material provides unique possibilities to tailor the mechanical, electrical, and optical properties with respect to numerous applications. The synthetic pathway follows the well-known sol−gel procedure. Nevertheless, the establishment of defined hybrid products necessitates a basic understanding of the underlying chemistry as well as of the parameters influencing the processing techniques. Modern spectroscopy provides versatile tools, e.g., multinuclear NMR experiments and Raman scattering to investigate the sol−gel processing of organo(alkoxy)silanes in their co-condensation reactions with main group or transition metal alkoxides as well as in their copolymerization reactions with each other or with organic molecules. This almost infinite modular design concept has led to the industrial application of coatings for transparent plastics, glasses, and metals to prevent these substrates from mechanical abrasion, permeation, or co...

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Hybrid Sol−Gel-Derived Polymers: Applications of Multifunctional Materials

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

Millions of years before we began to generate functional nanostructures, biological systems were using nanometer-scale architectures to produce unique functionalities. For instance, moths use hexagonal arrays of nonclose-packed (ncp) nipples as antireflection coatings (ARCs) to reduce reflectivity from their compound eyes. The outer surface of the corneal lenses of moths consists of ncp arrays of conical protuberances, termed corneal nipples, typically of sub-300 nm height and spacing. These arrays of subwavelength nipples generate a graded transition of refractive index, leading to minimized reflection over a broad range of wavelengths and angles of incidence. Similar periodic arrays of ncp pillars have also been observed on the wings of cicada to render superhydrophobic surfaces for self-cleaning functionality. In this Communication, we report a simple and scalable bioinspired templating technique for fabricating broadband and superhydrophobic ARCs on technologically important silicon and glass substrates. Crystalline silicon is the most important material for solar cells. Unfortunately, due to the high refractive index of silicon, more than 30% of incident light is reflected back from the surface of crystalline silicon. ARCs are therefore widely utilized to reduce the unwanted reflective losses. Quarterwavelength silicon nitride (SiNx) films deposited by plasmaenhanced chemical vapor deposition (PECVD) are the industrial standard for ARCs on crystalline silicon substrates. However, the PECVD-deposited SiNx films are expensive to fabricate. Additionally, commercial SiNx ARCs are typically designed to suppress reflection efficiently at wavelengths around 600 nm. The reflective loss is rapidly increased for near-infrared and other visible wavelengths, which contain a large portion of the incident solar energy. In contrast, subwavelength-structured moth-eye ARCs directly patterned in the substrates are broadband and intrinsically more stable and durable than multilayer ARCs since no foreign material is involved. Nevertheless, current topdown lithographic technologies in creating subwavelength

J. Dreibholz

Papers

Subwavelength-structured antireflective surfaces on glass

Antireflective transparent covers for solar devices

Glazing with very high solar transmittance