BAND-GAP AND REFRACTIVE-INDEX DETERMINATION OF MO-BLACK COATINGS USING SEVERAL TECHNIQUES

The optical energy gap of molybdenum black solar selective absorbing c oatings has been estimated by various techniques and the results are c ompared. For thick absorbing coatings on metallic substrates the band gap has been estimated from reflectance spectra using the Kubelka-Munk function. The band gap of these coatings has also been estimated from transmission spectra obtained from a thin coating specially prepared on semitransparent aluminium coated glass substrates. The band gap is found to decrease from 1.96 to 1.67 eV with an increase of plating cur rent density from 1.4 to 2.8 mA/cm(2). This is interpreted as due to t he increase of disorder in the coating when depositing at relatively h igh rates resulting in increased encroachment of localised electronic states into the energy gap. The refractive indices of the coatings hav e been estimated. All the results obtained in the present work have be en correlated with the solar selective properties of the coatings.