Chemical vapor deposition of spectrally selective surfaces for high-temperature photothermal conversion

Abstract

The surfaces intercepting the incident radiation in photothermal converters must satisfy three requirements. First, they must offer a spectral profile that is properly matched to the solar emission and thermal reradiation properties. Second, they must withstand elevated temperatures without harmful changes of their optoelectronic characteristics. Third, they must operate long enough to make the initial investment a reasonable economic proposition. Multilayer systems deposited by CVD offer attractive features. We report on an absorber–reflector tandem that satisfies all three requairements and is fabricated by a method easily expanded to large-scale economic manufacture. Although CVD technology is well developed for the fabrication of semiconductor devices, efforts to deposit the thin films used in solar energy converters are only in an early stage. A set of characteristic requirements makes it necessary to modify some aspects of existing technology. Attention must be paid to the optical and morphological properties of the deposit, and their dependence on the deposition parameters and the substrate. We discuss the difficulties encountered and describe some preliminary solutions that establish the feasibility of the concept. The silicon absorber can be deposited in satisfactory quality, reducing free-carrier absorption at elevated temperatures to an acceptable level. The layer is protected on either side against diffusion, and mismatches of thermal expansion are neutralized. The highly reflective silver film is stabilized against morphological changes bound to occur under the conditions of the subsequent CVD of the absorber. Recent developments include the CVD of amorphous silicon absorbers of improved temperature stability, and reflector films made of molybdenum. Stacks have been successfully tested at temperatures in excess of 500 °C.