**2. Fundamental properties of the key materials**

The definition of the thin-film given by Chopra et al. [12] provides a good starting point and also yields a criterion to discriminate the term 'thin film' from 'thick film'. According to their opinion, a thin film is defined as a material that is created ab initio by the random nucleation and growth processes of individually condensing/reacting atomic/ionic/molecular species on a substrate. Thin films may encompass a considerable thickness range, varying from a few nanometers to tens of micrometers and thus are best defined in terms of the production processes rather than by thickness. The layers for constructing the CIGS solar cells are back contact layer, absorber layer, buffer layer and window layer.
