Nanocomposite Coatings for Solar Energy Conversion: Large Opportunities for Small Structures

Due to their fascinating optical and electronical properties, nanometer-scaled structures play an important role in solar energy conversion [1]. Nanocomposite coatings consist typically of dielectric, semiconducting or metallic nanocrystals embedded in a host matrix. In this presentation it will be described how such nanostructures can result in interesting new material properties which might be exploited in solar energy conversion. This overview tries to illustrate the large variety of existing and envisaged solar energy applications of nanostructured thin films. Examples include antireflection coatings on solar collector glazing, colored coatings with high solar transmittance for novel glazing of solar thermal facades, photoluminescent quantum dot solar concentrators for photovoltaic energy conversion, nanostructured photovoltaic cells, and optical selective absorber coatings for thermal solar collectors. Dielectric nanostructured films are used as coatings on solar collector glazing. Nanoporous silicon dioxide films exhibit a low refractive index suitable for broad band anti-reflection, and such coatings enter the market now. Novel nanocomposite quaternary Mg-F-Si-O films exhibit also a low refractive index in the desired range (see Fig.2), and might be more resistant regarding gradual pore-filling by hydrocarbons [2]. Nanocomposite Ti-Si-O films are used in novel multilayered coatings on glazing of solar thermal facade collectors [3] (see Fig.3). Such coatings combine a colored reflection with a high solar transmittance and open new possibilities for the architectural integration of solar collectors in building facades [4]. The optical performance of the multilayered interference filters is optimized by computer simulations of thin film interference. Such thin film interference calculations are also used in order to design novel sun protection coatings with a better overheating protection at a comparable level of daylighting [5]. This novel glazing, developed by the research group of Prof. P. Oelhafen at the University of Basel, offers the possibility of tremendous energy savings due to the reduction of cooling needs in hot summers. One promising application of semiconductor nanocrystal containing composite films might be planar photoluminescent concentrators for photovoltaic solar energy conversion (see Fig.4). Quantum dot containing nanocomposite thin films have been synthesized by a low cost sol-gel process and characterised by photoluminescence spectroscopy. A tool for ray tracing simulations has been developed on the basis of Monte-Carlo methods that are applied to polarization-dependent reflection/transmission at the involved interfaces, photon absorption by the semiconductor nanocrystals and photoluminescent reemission [6]. Such simulations allow estimating the efficiency of the envisaged devices. Embedded nanometer-sized metal clusters play an important role for selective absorber coatings for solar thermal collectors. Metal doped amorphous hydrogenated silicon-carbon films (a-Si1-xCx:H/Me) can be deposited by a combined physical vapor deposition/plasma enhanced chemical vapor deposition process (PVD/PECVD). Multilayered solar absorber coatings have been developed on the basis of this promising material. The PVD/PECVD process has been upscaled from the laboratory to industrial production [1]. The highly durable coatings are extremely stable at elevated temperatures in air [7], and service lifetimes above 25 years are expected. The most recent research topics include the development of a low-cost sol-gel process for the deposition of nanocomposite selective solar absorber coatings, and novel thermochromic films for overheating protection of solar thermal systems.