This report describes the concentrating solar power (CSP) systems using solar absorbers to convert concentrated sunlight to thermal electric power. It is possible to achieve solar absorber surfaces for efficient photothermal conversion having high solar absorptance (a) for solar radiation and a low thermal emittance (e) at the operational temperature. A low reflectance (?'' 0) at wavelengths (?) 3 mm and a high reflectance (?'' 1) at l 3 mm characterize spectrally selective surfaces. The operational temperature ranges of these materials for solar applications can be categorized as low temperature (T 400 C). High- and mid-temperature applications are needed for CSP applications. For CSP applications, the ideal spectrally selective surface would be low-cost and easy to manufacture, chemically and thermally stable in air at elevated operating temperatures (T= 500 C), and have a solar absorptance= 0.98 and a thermal emittance= 0.05 at 500 C.

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Review of Mid- to High-Temperature Solar Selective Absorber Materials

Sol-gel thin films are formed by gravitational or centrifugal draining accompanied by vigorous drying. Drying largely establishes the shape of the fluid profile, the timescale of the deposition process, and the magnitude of the forces exerted on the solid phase. The combination of coating theory and experiment should define coating protocols to tailor the deposition process to specific applications.

/pdf/review-of-sol-gel-thin-film-formation-1qb6rlkbtz.pdf

Review of sol-gel thin film formation

Design and simulation of antireflection coating systems for optoelectronic devices : Application to silicon solar cells

Single-material TiO2 double-layer antireflection coatings

This paper compares the optical, electronic, physical and chemical properties of dielectric thin films that are commonly used to enhance the performance of bulk silicon photovoltaic devices. The standard buried-contact (BC) solar cell presents a particularly challenging set of criteria, requiring the dielectric film to act as: (i) an anti-reflection (AR) coating; (ii) a film compatible with surface passivation; (iii) a mask for an electroless metal plating step; (iv) a diffusion barrier for achieving a selective emitter; (v) a film with excellent chemical resistance; (vi) a stable layer during high-temperature processing. The dielectric coatings reviewed here include thermally grown silicon dioxide (SiO2), silicon nitride deposited by plasma-enhanced chemical vapour deposition (a-SiNx:H) and low-pressure chemical vapour deposition (Si3N4), silicon oxynitride (SiON), cerium dioxide (CeO2), zinc sulphide (ZnS), and titanium dioxide (TiO2).

While TiO2 dielectric coatings exhibit the best optical performance and a simple post-deposition surface passivation sequence has been developed, they require an additional sacrificial diffusion barrier to survive the heavy groove diffusion step. A-SiNx:H affords passivation through its high fixed positive charge density and large hydrogen concentration; however, it is difficult to retain these electronic benefits during lengthy high-temperature processing. Therefore, for the BC solar cell, Si3N4 films would appear to be the best choice of dielectric films common in industrial use. Copyright © 2004 John Wiley & Sons, Ltd.

C.S. Ashley

Papers

Sol-gel double-layer antireflection coatings for silicon solar cells