Implantation as a tool for performing selective materials (application to solar energy conversion)

A selective solar material must absorb most of the solar spectrum, principally the visible light, and reflect the IR light.

Insulators are generally transparent in a wide part of the optical spectrum and the defects are revealed in these crystals by strong absorption bands. On the other hand, metals absorb much of the IR and near IR light and have a large reflection coefficient in the same region of the spectrum. In previous papers^1,2 it has been shown that metallic colloids, formed by precipitation of impurities in insulators, are responsible for a strong absorption band. Such metallic inclusions may be easily produced in most insulators by implantation. According to the nature of the implanted metal a selective absorption can be obtained. So a composite material (cermet) may be performed combining a colloidal absorption in the visible and a metallic reflection. We will discuss the different ways to achieve these properties using direct ion beam implantation.

Various cermets (LiF: Na, Au; MgO: Na, Au) have been studied as function of energy (0.1-1 MeV) and dose (10¹⁶-10¹⁷ ions/cm²). Colloids are completely developed by consecutive annealing.⁴

The modelization of these cermets requires a careful characterization by optical methods (spectrophotometry) and microscopic investigation (TEM, SEM, RBS, SIMS).⁷ These techniques are used to determine the filling factor and the concentration profile of metal in the insulating matrix.

With the help of the Maxwell-Garnett theory and using a single or multilayer model it is possible to suggest an interpretation of the optical properties.